Infrared and Raman Spectroscopy: Principles and Spectral Interpretation [1st ed.] 0123869846, 9780123869845

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INFRARED AND RAMAN SPECTROSCOPY PRINCIPLES AND SPECTRAL INTERPRETATION PETER LARKIN

AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Elsevier 225 Wyman Street, Waltham, MA 02451, USA 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands Copyright Ó 2011 Elsevier Inc. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier. com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein Library of Congress Cataloging-in-Publication Data Larkin, Peter (Peter J.) Infrared and raman spectroscopy: principles and spectral interpretation/Peter Larkin. p. cm. ISBN: 978-0-12-386984-5 (hardback) 1. Infrared spectroscopy. 2. Raman Spectroscopy. I. Title. QD96.I5L37 2011 535’.8’42ddc22 2011008524 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-386984-5 For information on all Elsevier publications visit our web site at elsevierdirect.com Printed and bound in The USA 11 12 13 14 10 9 8 7 6 5 4 3 2 1

To my wife and family

Preface IR and Raman spectroscopy have tremendous potential to solve a wide variety of complex problems. Both techniques are completely complementary providing characteristic fundamental vibrations that are extensively used for the determination and identification of molecular structure. The advent of new technologies has introduced a wide variety of options for implementing IR and Raman spectroscopy into the hands of both the specialist and the nonspecialist alike. However, the successful application of both techniques has been limited since the acquisition of high level IR and Raman interpretation skills is not widespread among potential users. The full benefit of IR and Raman spectroscopy cannot be realized without an analyst with basic knowledge of spectral interpretation. This book is a response to the recent rapid growth of the field of vibrational spectroscopy. This has resulted in a corresponding need to educate new users on the value of both IR and Raman spectral interpretation skills. To begin with, the end user must have a suitable knowledge base of the instrument and its capabilities. Furthermore, he must develop an understanding of the sampling options and limitations, available software tools, and a fundamental understanding of important characteristic group frequencies for both IR and Raman spectroscopy. A critical skill set an analyst may require to solve a wide variety of chemical questions and problems using vibrational spectroscopy is depicted in Figure 1 below. Selecting the optimal spectroscopic technique to solve complex chemical problems encountered by the analyst requires the user to develop a skill set outlined in Figure 1. A knowledge of spectral interpretation enables the user to select the technique with the most favorable selection Software knowledge Instrument capability

Instrument accessories

Chemometric models

Spectroscopist

Sampling options

Spectral interpretation

Method validation Method development

FIGURE 1

Skills required for a successful vibrational spectroscopist. (Adapted from R.D. McDowall, Spectroscopy Application Notebook, February 2010).

ix

x

PREFACE

of characteristic group frequencies, optimize the sample options (including accessories if necessary), and use suitable software tools (both instrumental and chemometric), to provide a robust, sensitive analysis that is easily validated. In this book we provide a suitable level of information to understand instrument capabilities, sample presentation, and selection of various accessories. The main thrust of this text is to develop high level of spectral interpretation skills. A broad understanding of the bands associated with functional groups for both IR and Raman spectroscopy is the basic spectroscopy necessary to make the most of the potential and set realistic expectations for vibrational spectroscopy applications in both academic and industrial settings. A primary goal of this book has been to fully integrate the use of both IR and Raman spectroscopy as spectral interpretation tools. To this end we have integrated the discussion of IR and Raman group frequencies into different classes of organic groups. This is supplemented with paired generalized IR and Raman spectra, use of numerous tables that are discussed in text, and finally referenced to a selection of fully interpreted IR and Raman spectra. This fully integrated approach to IR and Raman interpretation enables the user to utilize the strengths of both techniques while also recognizing their weaknesses. We have attempted to provide an integrated approach to the important group frequency of both infrared and Raman spectroscopy. Graphics is used extensively to describe the basic principles of vibrational spectroscopy and the origins of group frequencies. The book includes sections on basic principles in Chapters 1 and 2; instrumentation, sampling methods, and quantitative analysis in Chapter 3; a discussion of important environmental effects in Chapter 4; and a discussion of the origin of group frequencies in Chapter 5. Chapters 4 and 5 provide the essential background to understand the origin of group frequencies in order to assign them in a spectra and to explain why group frequencies may shift. Selected problems are included at the end of some of these chapters to help highlight important points. Chapters 6 and 7 provide a highly detailed description of important characteristic group frequencies and strategies for interpretation of IR and Raman spectra. Chapter 8 is the culmination of the book and provides 110 fully interpreted paired IR and Raman spectra arranged in groups. The selected compounds are not intended to provide a comprehensive spectral library but rather to provide a significant selection of interpreted examples of functional group frequencies. This resource of interpreted IR and Raman spectra should be used to help verify proposed assignments that the user will encounter. The final chapter is comprised of the paired IR and Raman spectra of 44 different unknown spectra with a corresponding answer key. Peter Larkin Connecticut, August 2010

C H A P T E R

1 Introduction: Infrared and Raman Spectroscopy Vibrational spectroscopy includes several different techniques, the most important of which are mid-infrared (IR), near-IR, and Raman spectroscopy. Both mid-IR and Raman spectroscopy provide characteristic fundamental vibrations that are employed for the elucidation of molecular structure and are the topic of this chapter. Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes) and is an excellent technique for rapid, accurate quantitation. All three techniques have various advantages and disadvantages with respect to instrumentation, sample handling, and applications. Vibrational spectroscopy is used to study a very wide range of sample types and can be carried out from a simple identification test to an in-depth, full spectrum, qualitative and quantitative analysis. Samples may be examined either in bulk or in microscopic amounts over a wide range of temperatures and physical states (e.g., gases, liquids, latexes, powders, films, fibers, or as a surface or embedded layer). Vibrational spectroscopy has a very broad range of applications and provides solutions to a host of important and challenging analytical problems. Raman and mid-IR spectroscopy are complementary techniques and usually both are required to completely measure the vibrational modes of a molecule. Although some vibrations may be active in both Raman and IR, these two forms of spectroscopy arise from different processes and different selection rules. In general, Raman spectroscopy is best at symmetric vibrations of non-polar groups while IR spectroscopy is best at the asymmetric vibrations of polar groups. Table 1.1 briefly summarizes some of the differences between the techniques. Infrared and Raman spectroscopy involve the study of the interaction of radiation with molecular vibrations but differs in the manner in which photon energy is transferred to the molecule by changing its vibrational state. IR spectroscopy measures transitions between molecular vibrational energy levels as a result of the absorption of mid-IR radiation. This interaction between light and matter is a resonance condition involving the electric dipolemediated transition between vibrational energy levels. Raman spectroscopy is a two-photon inelastic light-scattering event. Here, the incident photon is of much greater energy than the vibrational quantum energy, and loses part of its energy to the molecular vibration with the

1

2

1. INTRODUCTION: INFRARED AND RAMAN SPECTROSCOPY

TABLE 1.1

Comparison of Raman, Mid-IR and Near-IR Spectroscopy Raman

Infrared

Near-IR

Very simple

Variable

Simple

Liquids

Very simple

Very simple

Very simple

Powders

Very simple

Simple

Simple

Polymers

Very simple*

Simple

Simple

Gases

Simple

Very simple

Simple

Fingerprinting

Excellent

Excellent

Very good

Best vibrations

Symmetric

Asymmetric

Comb/overtone

Group Frequencies

Excellent

Excellent

Fair

Aqueous solutions

Very good

Very difficult

Fair

Quantitative analysis

Good

Good

Excellent

Low frequency modes

Excellent

Difficult

No

Ease of sample preparation

* True for FT-Raman at 1064 nm excitation.

remaining energy scattered as a photon with reduced frequency. In the case of Raman spectroscopy, the interaction between light and matter is an off-resonance condition involving the Raman polarizability of the molecule. The IR and Raman vibrational bands are characterized by their frequency (energy), intensity (polar character or polarizability), and band shape (environment of bonds). Since the vibrational energy levels are unique to each molecule, the IR and Raman spectrum provide a “fingerprint” of a particular molecule. The frequencies of these molecular vibrations depend on the masses of the atoms, their geometric arrangement, and the strength of their chemical bonds. The spectra provide information on molecular structure, dynamics, and environment. Two different approaches are used for the interpretation of vibrational spectroscopy and elucidation of molecular structure. 1) Use of group theory with mathematical calculations of the forms and frequencies of the molecular vibrations. 2) Use of empirical characteristic frequencies for chemical functional groups. Many empirical group frequencies have been explained and refined using the mathematical theoretical approach (which also increases reliability). In general, many identification problems are solved using the empirical approach. Certain functional groups show characteristic vibrations in which only the atoms in that particular group are displaced. Since these vibrations are mechanically independent from the rest of the molecule, these group vibrations will have a characteristic frequency, which remains relatively unchanged regardless of what molecule the group is in. Typically, group frequency

3

3000

X=Y=Z or X≡Y N=C=O, C≡N

2000

C=O acid ester ketone amide

C=C olefinic, aromatic NH2 def, R CO2 -salt, C=N

P OH or SH str

NH str amines amides

aliphatic CH str

OH str alcohols phenols

=CH and aromatic CH str

Intensity

HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY

CH2 CH3

1500 Wavenumbers(cm–1)

C O C ethers esters C OH alcohols phenols S=O P=O C F

=CH aromatic C Cl C Br

1000

500

FIGURE 1.1 Regions of the fundamental vibrational spectrum with some characteristic group frequencies.

analysis is used to reveal the presence and absence of various functional groups in the molecule, thereby helping to elucidate the molecular structure. The vibrational spectrum may be divided into typical regions shown in Fig. 1.1. These regions can be roughly divided as follows: • • • • •

XeH stretch (str) highest frequencies (3700e2500 cm 1) XhY stretch, and cumulated double bonds X¼Y¼Z asymmetric stretch (2500e2000 cm 1) X¼Y stretch (2000e1500 cm 1) XeH deformation (def) (1500e1000 cm 1) XeY stretch (1300e600 cm 1)

The above represents vibrations as simple, uncoupled oscillators (with the exception of the cumulated double bonds). The actual vibrations of molecules are often more complex and as we will see later, typically involve coupled vibrations.

1. HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY IR spectroscopy was the first structural spectroscopic technique widely used by organic chemists. In the 1930s and 1940s both IR and Raman techniques were experimentally challenging with only a few users. However, with conceptual and experimental advances, IR gradually became a more widely used technique. Important early work developing IR spectroscopy occurred in industry as well as academia. Early work using vibrating mechanical molecular models were employed to demonstrate the normal modes of vibration in various molecules.1, 2 Here the nuclei were represented by steel balls and the interatomic bonds by helical springs. A ball and spring molecular model would be suspended by long threads attached to each ball enabling studies of planar vibrations. The source of oscillation for the ball and spring model was through coupling to an eccentric variable speed motor which enabled studies of the internal vibrations of molecules. When the oscillating frequency matched that of one of the natural frequencies of vibration for the mechanical model

4

1. INTRODUCTION: INFRARED AND RAMAN SPECTROSCOPY

3400

3300

3200

3100

CH3, C≡C H

3000

2900

2800

2700

CH2, CH, stretch Arom-H C=CH2 C=CH R CH3 Arom-CH3 R CH2 R R O CH3 R2 N CH3 O=C H

FIGURE 1.2

The correlation chart for CH3, CH2, and CH stretch IR bands.

a resonance occurred and the model responded by exhibiting one of the internal vibrations of the molecule (i.e. normal mode). In the 1940s both Dow Chemical and American Cyanamid companies built their own NaCl prism-based, single beam, meter focal length instruments primarily to study hydrocarbons. The development of commercially available IR instruments had its start in 1946 with American Cyanamid Stamford laboratories contracting with a small optical company called PerkineElmer (PE). The Stamford design produced by PE was a short focal length prism IR spectrometer. With the commercial availability of instrumentation, the technique then benefited from the conceptual idea of a correlation chart of important bands that concisely summarize where various functional groups can be expected to absorb. This introduction of the correlation chart enabled chemists to use the IR spectrum to determine the structure.3, 4 The explosive growth of IR spectroscopy in the 1950s and 1960s were a result of the development of commercially available instrumentation as well as the conceptual breakthrough of a correlation chart. Appendix shows IR group frequency correlation charts for a variety of important functional groups. Shown in Fig. 1.2 is the correlation chart for CH3, CH2, and CH stretch IR bands. The subsequent development of double beam IR instrumentation and IR correlation charts resulted in widespread use of IR spectroscopy as a structural technique. An extensive user base resulted in a great increase in available IR interpretation tools and the eventual development of FT-IR instrumentation. More recently, Raman spectroscopy has benefited from dramatic improvements in instrumentation and is becoming much more widely used than in the past.

HISTORICAL PERSPECTIVE: IR AND RAMAN SPECTROSCOPY

References 1. 2. 3. 4.

Kettering, C. F.; Shultz, L. W.; Andrews, D. H. Phys. Rev. 1930, 36, 531. Colthup, N. B. J. Chem. Educ. 1961, 38 (8), 394e396. Colthup, N. B. J. Opt. Soc. Am. 1950, 40 (6), 397e400. Infrared Characteristic Group Frequencies G. Socrates, 2nd ed.; John Wiley: New York, NY, 1994.

5

C H A P T E R

2 Basic Principles 1. ELECTROMAGNETIC RADIATION All light (including infrared) is classified as electromagnetic radiation and consists of alternating electric and magnetic fields and is described classically by a continuous sinusoidal wave like motion of the electric and magnetic fields. Typically, for IR and Raman spectroscopy we will only consider the electric field and neglect the magnetic field component. Figure 2.1 depicts the electric field amplitude of light as a function of time. The important parameters are the wavelength (l, length of 1 wave), frequency (v, number cycles per unit time), and wavenumbers (n, number of waves per unit length) and are related to one another by the following expression: n 1 ¼ n ¼ ðc=nÞ l where c is the speed of light and n the refractive index of the medium it is passing through. In quantum theory, radiation is emitted from a source in discrete units called photons where the photon frequency, v, and photon energy, Ep, are related by Ep ¼ hn where h is Planck’s constant (6.6256  10 27 erg sec). Photons of specific energy may be absorbed (or emitted) by a molecule resulting in a transfer of energy. In absorption spectroscopy this will result in raising the energy of molecule from ground to a specific excited state E

+ –

+ –

Time

FIGURE 2.1 The amplitude of the electric vector of electromagnetic radiation as a function of time. The wavelength is the distance between two crests.

7

8

2. BASIC PRINCIPLES

E2 Molecular energy levels

Ep

(

) Ep = E2 – E1

E1

FIGURE 2.2 Absorption of electromagnetic radiation.

as shown in Fig. 2.2. Typically the rotational (Erot), vibrational (Evib), or electronic (Eel) energy of molecule is changed by 6E: DE ¼ Ep ¼ hn ¼ hcn In the absorption of a photon the energy of the molecule increases and DE is positive. To a first approximation, the rotational, vibrational, and electronic energies are additive: ET ¼ Eel þ Evib þ Erot We are concerned with photons of such energy that we consider Evib alone and only for condensed phase measurements. Higher energy light results in electronic transitions (Eel) and lower energy light results in rotational transitions (Erot). However, in the gas-state both IR and Raman measurements will include Evib þ Erot.

2. MOLECULAR MOTION/DEGREES OF FREEDOM 2.1. Internal Degrees of Freedom The molecular motion that results from characteristic vibrations of molecules is described by the internal degrees of freedom resulting in the well-known 3n 6 and 3n 5 rule-of-thumb for vibrations for non-linear and linear molecules, respectively. Figure 2.3 shows the fundamental vibrations for the simple water (non-linear) and carbon dioxide (linear) molecules. The internal degrees of freedom for a molecule define n as the number of atoms in a molecule and define each atom with 3 degrees of freedom of motion in the X, Y, and Z directions resulting in 3n degrees of motional freedom. Here, three of these degrees are translation, while three describe rotations. The remaining 3n 6 degrees (non-linear molecule) are motions, which change the distance between atoms, or the angle between bonds. A simple example of the 3n 6 non-linear molecule is water (H2O) which has 3(3) 6 ¼ 3 degrees of freedom. The three vibrations include an in-phase and out-of-phase stretch and a deformation (bending) vibration. Simple examples of 3n 5 linear molecules include H2, N2, and O2 which all have 3(2) 5 ¼ 1 degree of freedom. The only vibration for these simple molecules is a simple stretching vibration. The more complicated CO2 molecule has 3(3) 5 ¼ 4 degrees of freedom and therefore four vibrations. The four vibrations include an in-phase and outof-phase stretch and two mutually perpendicular deformation (bending) vibrations. The molecular vibrations for water and carbon dioxide as shown in Fig. 2.3 are the normal mode of vibrations. For these vibrations, the Cartesian displacements of each atom in molecule

9

MOLECULAR MOTION/DEGREES OF FREEDOM

(a)

Water O H

H

O

O H

H

νip

νop

H

H

νdef

(b) CO2 O

C

O

νop

O

C

O

νip

O

C

νdef

O

O

C O

νdef

FIGURE 2.3 Molecular motions which change distance between atoms for water and CO2.

change periodically with the same frequency and go through equilibrium positions simultaneously. The center of the mass does not move and the molecule does not rotate. Thus in the case of harmonic oscillator, the Cartesian coordinate displacements of each atom plotted as a function of time is a sinusoidal wave. The relative vibrational amplitudes may differ in either magnitude or direction. Figure 2.4 shows the normal mode of vibration for a simple diatomic such as HCl and a more complex totally symmetric CH stretch of benzene. (a) Diatomic Stretch

Equilibrium position of atoms

Displacement

Time

(b) Totally symmetric CH stretch of benzene

FIGURE 2.4

Normal mode of vibration for a simple diatomic such as HCl (a) and a more complex species such as benzene (b). The displacement versus time is sinusoidal, with equal frequency for all the atoms. The typical Cartesian displacement vectors are shown for the more complicated totally symmetric CH stretch of benzene.

10

2. BASIC PRINCIPLES

3. CLASSICAL HARMONIC OSCILLATOR To better understand the molecular vibrations responsible for the characteristic bands observed in infrared and Raman spectra it is useful to consider a simple model derived from classical mechanics.1 Figure 2.5 depicts a diatomic molecule with two masses m1 and m2 connected by a massless spring. The displacement of each mass from equilibrium along the spring axis is X1 and X2. The displacement of the two masses as a function of time for a harmonic oscillator varies periodically as a sine (or cosine) function. In the above diatomic system, although each mass oscillates along the axis with different amplitudes, both atoms share the same frequency and both masses go through their equilibrium positions simultaneously. The observed amplitudes are inversely proportional to the mass of the atoms which keeps the center of mass stationary X1 m2 ¼ m1 X2 The classical vibrational frequency for a diatomic molecule is: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 1 n ¼ Kð þ Þ 2p m1 m2 where K is the force constant in dynes/cm and m1 and m2 are the masses in grams and n is in cycles per second. This expression is also encountered using the reduced mass where 1 1 1 m1 m2 þ or m ¼ ¼ m1 þ m2 m m1 m2 In vibrational spectroscopy wavenumber units, n (waves per unit length) are more typically used m1

K

m2

X1

–X2

Time

Displacement

FIGURE 2.5 Motion of a simple diatomic molecule. The spring constant is K, the masses are m1 and m2, and X1 and X2 are the displacement vectors of each mass from equilibrium where the oscillator is assumed to be harmonic.

CLASSICAL HARMONIC OSCILLATOR

11

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   1 1 1 K n ¼ þ 2pc m1 m2 where n is in waves per centimeter and is sometimes called the frequency in cm 1 and c is the speed of light in cm/s. If the masses are expressed in unified atomic mass units (u) and the force constant is ˚ ngstro¨m then: expressed in millidynes/A sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   1 1 þ n ¼ 1303 K m1 m2 where 1303 ¼ [Na  105)1/2/2pc and Na is Avogadro’s number (6.023  1023 mole 1) This simple expression shows that the observed frequency of a diatomic oscillator is a function of 1. the force constant K, which is a function of the bond energy of a two atom bond (see Table 2.1) 2. the atomic masses of the two atoms involved in the vibration. TABLE 2.1 Approximate Range of Force Constants for Single, Double, and Triple Bonds Bond type

˚ ngstro¨m) K (millidynes/A

Single

3e6

Double

10e12

Triple

15e18

Table 2.1 shows the approximate range of the force constants for single, double, and triple bonds. Conversely, knowledge of the masses and frequency allows calculation of a diatomic force constant. For larger molecules the nature of the vibration can be quite complex and for more accurate calculations the harmonic oscillator assumption for a diatomic will not be appropriate. The general wavenumber regions for various diatomic oscillator groups are shown in Table 2.2, where Z is an atom such as carbon, oxygen, nitrogen, sulfur, and phosphorus. TABLE 2.2 General Wavenumber Regions for Various Simple Diatonic Oscillator Groups Diatomic oscillator

Region (cmL1)

ZeH

4000e2000

ChC, ChN

2300e2000

C¼O, C¼N, C ¼C

1950e1550

CeO, CeN, CeC

1300e800

CeCl

830e560

12

2. BASIC PRINCIPLES

4. QUANTUM MECHANICAL HARMONIC OSCILLATOR Vibrational spectroscopy relies heavily on the theoretical insight provided by quantum theory. However, given the numerous excellent texts discussing this topic only a very cursory review is presented here. For a more detailed review of the quantum mechanical principles relevant to vibrational spectroscopy the reader is referred elsewhere.2-5 For the classical harmonic oscillation of a diatomic the potential energy (PE) is given by 1 KX2 2 A plot of the potential energy of this diatomic system as a function of the distance, X between the masses, is thus a parabola that is symmetric about the equilibrium internuclear distance, Xe. Here Xe is at the energy minimum and the force constant, K is a measure of the curvature of the potential well near Xe. From quantum mechanics we know that molecules can only exist in quantized energy states. Thus, vibrational energy is not continuously variable but rather can only have certain discrete values. Under certain conditions a molecule can transit from one energy state to another (Dy ¼ 1) which is what is probed by spectroscopy. Figure 2.6 shows the vibrational levels in a potential energy diagram for the quantum mechanical harmonic oscillator. In the case of the harmonic potential these states are equidistant and have energy levels E given by   1 Ei ¼ yi þ hv yi ¼ 0; 1; 2. 2 PE ¼

Here, n is the classical vibrational frequency of the oscillator and y is a quantum number which can have only integer values. This can only change by Dy ¼ 1 in a harmonic oscillator model. The so-called zero point energy occurs when y ¼ 0 where E ¼ ½ hn and this vibrational energy cannot be removed from the molecule. Probability

=2

E =1

=0

Xe

X

E = ( i = 1/2) h c υ o

Δυ = ± 1

FIGURE 2.6 Potential energy, E, versus internuclear distance, X, for a diatomic harmonic oscillator.

13

IR ABSORPTION PROCESS

Potential energy (E)

Anharmonic oscillator Harmonic oscillator

Do

i=

De

0

1/2 h ν

Internuclear distance (X)

FIGURE 2.7 The potential energy diagram comparison of the anharmonic and the harmonic oscillator. Transitions originate from the y ¼ 0 level, and Do is the energy necessary to break the bond.

Figure 2.6 shows the curved potential wells for a harmonic oscillator with the probability functions for the internuclear distance X, within each energy level. These must be expressed as a probability of finding a particle at a given position since by quantum mechanics we cannot be certain of the position of the mass during the vibration (a consequence of Heisenberg’s uncertainty principle). Although we have only considered a harmonic oscillator, a more realistic approach is to introduce anharmonicity. Anharmonicity results if the change in the dipole moment is not linearly proportional to the nuclear displacement coordinate. Figure 2.7 shows the potential energy level diagram for a diatomic harmonic and anharmonic oscillator. Some of the features introduced by an anharmonic oscillator include the following. The anharmonic oscillator provides a more realistic model where the deviation from harmonic oscillation becomes greater as the vibrational quantum number increases. The separation between adjacent levels becomes smaller at higher vibrational levels until finally the dissociation limit is reached. In the case of the harmonic oscillator only transitions to adjacent levels or so-called fundamental transitions are allowed (i.e., 6y ¼  1) while for the anharmonic oscillator, overtones (6y ¼  2) and combination bands can also result. Transitions to higher vibrational states are far less probable than the fundamentals and are of much weaker intensity. The energy term corrected for anharmonicity is   1 Ey ¼ hne y þ 2

hce ne



1 yþ 2

2

where ce ne defines the magnitude of the anharmonicity.

5. IR ABSORPTION PROCESS The typical IR spectrometer broad band source emits all IR frequencies of interest simultaneously where the near-IR region is 14,000e4000 cm 1, the mid-IR region is

14

2. BASIC PRINCIPLES

4000e400 cm 1, and the far-IR region is 400e10 cm 1. Typical of an absorption spectroscopy, the relationship between the intensities of the incident and transmitted IR radiation and the analyte concentration is governed by the LamberteBeer law. The IR spectrum is obtained by plotting the intensity (absorbance or transmittance) versus the wavenumber, which is proportional to the energy difference between the ground and the excited vibrational states. Two important components to the IR absorption process are the radiation frequency and the molecular dipole moment. The interaction of the radiation with molecules can be described in terms of a resonance condition where the specific oscillating radiation frequency matches the natural frequency of a particular normal mode of vibration. In order for energy to be transferred from the IR photon to the molecule via absorption, the molecular vibration must cause a change in the dipole moment of the molecule. This is the familiar selection rule for IR spectroscopy, which requires a change in the dipole moment during the vibration to be IR active. The dipole moment, m, for a molecule is a function of the magnitude of the atomic charges (ei) and their positions (ri) X m ¼ ei r i

The dipole moments of uncharged molecules derive from partial charges on the atoms, which can be determined from molecular orbital calculations. As a simple approximation, the partial charges can be estimated by comparison of the electronegativities of the atoms. Homonuclear diatomic molecules such as H2, N2, and O2 have no dipole moment and are IR inactive (but Raman active) while heteronuclear diatomic molecules such as HCl, NO, and CO do have dipole moments and have IR active vibrations. The IR absorption process involves absorption of energy by the molecule if the vibration causes a change in the dipole moment, resulting in a change in the vibrational energy level. Figure 2.8 shows the oscillating electric field of the IR radiation generates forces on the

Time

+

+

Forces generated by the photon electric field

+

+

+

–

–

Dipole

–

–

–

One photon cycle

FIGURE 2.8 The oscillating electric field of the photon generates oscillating, oppositely directed forces on the positive and negative charges of the molecular dipole. The dipole spacing oscillates with the same frequency as the incident photon.

15

THE RAMAN SCATTERING PROCESS

molecular dipole where the oscillating electric field drives the oscillation of the molecular dipole moment and alternately increases and decreases the dipole spacing. Here, the electric field is considered to be uniform over the whole molecule since l is much greater than the size of most molecules. In terms of quantum mechanics, the IR absorption is an electric dipole operator mediated transition where the change in the dipole moment, m, with respect to a change in the vibrational amplitude, Q, is greater than zero.   vm s0 vQ 0 The measured IR band intensity is proportional to the square of the change in the dipole moment.

6. THE RAMAN SCATTERING PROCESS Light scattering phenomena may be classically described in terms of electromagnetic (EM) radiation produced by oscillating dipoles induced in the molecule by the EM fields of the incident radiation. The light scattered photons include mostly the dominant Rayleigh and the very small amount of Raman scattered light.6 The induced dipole moment occurs as a result of the molecular polarizability a, where the polarizability is the deformability of the electron cloud about the molecule by an external electric field. Figure 2.9 shows the response of a non-polar diatomic placed in an oscillating electric field. Here we represent the static electric field by the plates of a charged capacitor. The negatively charged plate attracts the nuclei, while the positively charged plate attracts the least tightly

Electron and proton center in homogeneous diatomic molecule

Time Photon electric field

–

+ – =

=

+

=

+

=

+

– Field turned off

FIGURE 2.9

Dipole induced by charged capacitor plates

– Forces on charges induced by the external photon field

Induced dipole moment resulting from electron displacement

Induced dipole moment of a homonuclear diatomic originating from the oscillating electric field of the incident radiation. The field relative to the proton center displaces the electron center. The charged plates of a capacitor, which induces a dipole moment in the polarizable electron cloud, can represent the electric field.

16

2. BASIC PRINCIPLES

Wavelength (nm)

1042

1087

1064

Rayleigh Scattering

hc ν

Stokes Raman Scattering

hc (νL – ν m)

Energy

Anti-Stokes Raman Scattering hc (νL + νm)

1

1 0

Intensity

0

200

100

0

–100

–200

(νL + νm)

FIGURE 2.10 Schematic illustration of Rayleigh scattering as well as Stokes and anti-Stokes Raman scattering. The laser excitation frequency (nL) is represented by the upward arrows and is much higher in energy than the molecular vibrations. The frequency of the scattered photon (downward arrows) is unchanged in Rayleigh scattering but is of either lower or higher frequency in Raman scattering. The dashed lines indicate the “virtual state.”

bound outer electrons resulting in an induced dipole moment. This induced dipole moment is an off-resonance interaction mediated by an oscillating electric field. In a typical Raman experiment, a laser is used to irradiate the sample with monochromatic radiation. Laser sources are available for excitation in the UV, visible, and near-IR spectral region (785 and 1064 nm). Thus, if visible excitation is used, the Raman scattered light will also be in the visible region. The Rayleigh and Raman processes are depicted in Fig. 2.10. No energy is lost for the elastically scattered Rayleigh light while the Raman scattered photons lose some energy relative to the exciting energy to the specific vibrational coordinates of the sample. In order for Raman bands to be observed, the molecular vibration must cause a change in the polarizability. Both Rayleigh and Raman are two photon processes involving scattering of incident light ðhcnL Þ, from a “virtual state.” The incident photon is momentarily absorbed by a transition from the ground state into a virtual state and a new photon is created and scattered by a transition from this virtual state. Rayleigh scattering is the most probable event and the scattered intensity is ca. 10 3 less than that of the original incident radiation. This scattered photon results from a transition from the virtual state back to the ground state and is an elastic scattering of a photon resulting in no change in energy (i.e., occurs at the laser frequency).

CLASSICAL DESCRIPTION OF THE RAMAN EFFECT

17

Raman scattering is far less probable than Rayleigh scattering with an observed intensity that is ca. 10 6 that of the incident light for strong Raman scattering. This scattered photon results from a transition from the virtual state to the first excited state of the molecular vibration. This is described as an inelastic collision between photon and molecule, since the molecule acquires different vibrational energy ðnm Þ and the scattered photon now has different energy and frequency. As shown in Fig. 2.10 two types of Raman scattering exist: Stokes and anti-Stokes. Molecules initially in the ground vibrational state give rise to Stokes Raman scattering hcðnL nm Þwhile molecules initially in vibrational excited state give rise to anti-Stokes Raman scattering, hcðnL þ nm Þ. The intensity ratio of the Stokes relative to the anti-Stokes Raman bands is governed by the absolute temperature of the sample, and the energy difference between the ground and excited vibrational states. At thermal equilibrium Boltzmann’s law describes the ratio of Stokes relative to anti-Stokes Raman lines. The Stokes Raman lines are much more intense than anti-Stokes since at ambient temperature most molecules are found in the ground state. The intensity of the Raman scattered radiation IR is given by:  2 va 4 IR fn Io N vQ where Io is the incident laser intensity, N is the number of scattering molecules in a given state, n is the frequency of the exciting laser, a is the polarizability of the molecules, and Q is the vibrational amplitude. The above expression indicates that the Raman signal has several important parameters for Raman spectroscopy. First, since the signal is concentration dependent, quantitation is possible. Secondly, using shorter wavelength excitation or increasing the laser flux power density can increase the Raman intensity. Lastly, only molecular vibrations which cause a change in polarizability are Raman active. Here the change in the polarizability with respect to a change in the vibrational amplitute, Q, is greater than zero.   va s0 vQ The Raman intensity is proportional to the square of the above quantity.

7. CLASSICAL DESCRIPTION OF THE RAMAN EFFECT The most basic description of Raman spectroscopy describes the nature of the interaction of an oscillating electric field using classical arguments.6 Figure 2.11 schematically represents this basic mathematical description of the Raman effect. As discussed above, the electromagnetic field will perturb the charged particles of the molecule resulting in an induced dipole moment: m ¼ aE where a is the polarizability, E is the incident electric field, and m is the induced dipole moment. Both E and a can vary with time. The electric field of the radiation is oscillating

18

2. BASIC PRINCIPLES

(a)

(c)

(b)

(d)

νo Rayleigh

νo + νm

νo

νm

νo ± ν m

Raman anti-Stokes

νo – νm Raman Stokes

FIGURE 2.11 Schematic representing Rayleigh and Raman scattering. In (a) the incident radiation makes the induced dipole moment of the molecule oscillate at the photon frequency. In (b) the molecular vibration can change the polarizability, a, which changes the amplitude of the dipole moment oscillation. The result as shown in (c) is an amplitude modulated dipole moment oscillation. The image (d) shows the components with steady amplitudes which can emit electromagnetic radiation.

as a function of time at a frequency n0, which can induce an oscillation of the dipole moment m of the molecule at this same frequency, as shown in Fig. 2.11a. The polarizability a of the molecule has a certain magnitude whose value can vary slightly with time at the much slower molecular vibrational frequency nm, as shown in Fig. 2.11b. The result is seen in Fig. 2.11c, which depicts an amplitude modulation of the dipole moment oscillation of the molecule. This type of modulated wave can be resolved mathematically into three steady amplitude components with frequencies n0, n0 þ nm, and n0 nm as shown in Fig. 2.11d. These dipole moment oscillations of the molecule can emit scattered radiation with these same frequencies called Rayleigh, Raman anti-Stokes, and Raman Stokes frequencies. If a molecular vibration did not cause a variation in the polarizability, then there would be no amplitude modulation of the dipole moment oscillation and there would be no Raman Stokes or anti-Stokes emission.

8. SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS The symmetry of a molecule, or the lack of it, will define what vibrations are Raman and IR active.5 In general, symmetric or in-phase vibrations and non-polar groups are most easily studied by Raman while asymmetric or out-of-phase vibrations and polar groups are most easily studied by IR. The classification of a molecule by its symmetry enables understanding of the relationship between the molecular structure and the vibrational spectrum. Symmetry elements include planes of symmetry, axes of symmetry, and a center of symmetry. Group Theory is the mathematical discipline, which applies symmetry concepts to vibrational spectroscopy and predicts which vibrations will be IR and Raman active.1,5 The symmetry elements possessed by the molecule allow it to be classified by a point group and vibrational analysis can be applied to individual molecules. A thorough discussion of Group Theory is beyond the scope of this work and interested readers should examine texts dedicated to this topic.7

19

SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS

Totally symmetric vibration

Center of symmetry

H

O

H

C

O

O

H

H

C

O

Raman active

Asymmetric vibration

O

C

O

IR active

FIGURE 2.12

The center of symmetry for H2, CO2, and benzene. The Raman active symmetric stretching vibrations above are symmetric with respect to the center of symmetry. Some IR active asymmetric stretching vibrations are also shown.

For small molecules, the IR and Raman activities may often be determined by a simple inspection of the form of the vibrations. For molecules that have a center of symmetry, the rule of mutual exclusion states that no vibration can be active in both the IR and Raman spectra. For such highly symmetrical molecules vibrations which are Raman active are IR inactive and vice versa and some vibrations may be both IR and Raman inactive. Figure 2.12 shows some examples of molecules with this important symmetry element, the center of symmetry. To define a center of symmetry simply start at any atom, go in a straight line through the center and an equal distance beyond to find another, identical atom. In such cases the molecule has no permanent dipole moment. Examples shown below include H2, CO2, and benzene and the rule of mutual exclusion holds. In a molecule with a center of symmetry, vibrations that retain the center of symmetry are IR inactive and may be Raman active. Such vibrations, as shown in Fig. 2.12, generate a change in the polarizability during the vibration but no change in a dipole moment. Conversely, vibrations that do not retain the center of symmetry are Raman inactive, but may be IR active since a change in the dipole moment may occur. For molecules without a center of symmetry, some vibrations can be active in both the IR and Raman spectra. Molecules that do not have a center of symmetry may have other suitable symmetry elements so that some vibrations will be active only in Raman or only in the IR. Good examples of this are the in-phase stretches of inorganic nitrate and sulfate shown in Fig. 2.13. These are Raman active and IR inactive. Here, neither molecule has a center of symmetry but the negative oxygen atoms move radially simultaneously resulting in no dipole moment change.

20

2. BASIC PRINCIPLES

Raman active, IR inactive symmetric vibrations

(a)

(b) O

O

O

N

O

S

O

O

O

Sulfate, in-phase SO4 stretch

Nitrate, in-phase NO3 stretch

(c) X

X X

1,3,5-trisubstituted benzene, 2,4,6 C-radial in-phase stretch

FIGURE 2.13 Three different molecules, nitrate, sulfate, and 1,3,5-trisubstituted benzene molecules that do not have a center of symmetry. The in-phase stretching vibrations of all three result in Raman active, but IR inactive vibrations.

Another example is the 1,3,5-trisubstituted benzene where the C-Radial in-phase stretch is Raman active and IR inactive. In Figure 2.14 some additional symmetry operations are shown, other than that for a center of symmetry for an XY2 molecule such as water. These include those for a plane of symmetry, a two-fold rotational axis of symmetry, and an identity operation (needed for group theory) which makes no change. If a molecule is symmetrical with respect to a given symmetry element, the symmetry operation will not make any discernible change from the original configuration. As shown in Fig. 2.14, such symmetry operations are equivalent to renumbering the symmetrically related hydrogen (Y) atoms. Figure 2.15 shows the Cartesian displacement vectors (arrows) of the vibrational modes Q1, Q2, and Q3 of the bent triatomic XY2 molecule (such as water), and shows how they are modified by the symmetry operations C2, sv, and s /v. For non-degenerate modes of vibration such as these, the displacement vectors in the first column (the identity column, I) are multiplied by either (þ1) or ( 1) as shown to give the forms in the other three columns. Multiplication by (þ1) does not change the original form so the resulting form is said to be symmetrical with respect to that symmetry operation. Multiplication by ( 1) reverses all the vectors of the original form and the resulting form is said to be anti-symmetrical with respect to that symmetry element. As seen in Fig. 2.15, Q1 and Q2 are both totally

21

SYMMETRY: IR AND RAMAN ACTIVE VIBRATIONS

(a)

(b)

After

Before 1

1

2

3

1

1

2

3

Identity operation, I (no change)

3

2

3

2

Two-fold axis of rotation, C2 (rotate 180o on axis)

(c)

(d) 2

3

Plane of symmetry,

V

2

3

1

1

1

(reflection in mirror plane)

2

3

Plane of symmetry,

2

3

/ (reflection in molecular plane) V

FIGURE 2.14 Symmetry operations for an XY2 bent molecule such as water in the equilibrium configuration.

I

C2

σV

σ /V

Q1 (1)

(1)

(1)

(1)

(1)

(1)

(1)

(1)

(1)

(–1)

Q2

Q3 (1)

(–1)

FIGURE 2.15 The bent symmetrical XY2 molecule such as H2O performing the three fundamental modes Q1, Q2, and Q3. The vectors in column one (identity I) are transformed by the C2, sv, and s /v, operations into the forms in the remaining columns, where the vectors are like those in column one multiplied by (þ1) symmetrical or ( 1) antisymmetrical.

symmetric modes (i.e., symmetric to all symmetry operations), whereas Q3 is symmetric with respect to the s /v operation but anti-symmetric with respect to the C2 and sv operations. The transformation numbers (þ1 and 1) are used in group theory to characterize the symmetries of non-degenerate vibrational modes. From these symmetries one can deduce that Q1, Q2, and Q3 are all active in both the IR and Raman spectra. In addition,

22

2. BASIC PRINCIPLES

the dipole moment change in Q1 and Q2 is parallel to the C2 axis and in Q3 it is perpendicular to the C2 axis and the sv, plane. Doubly degenerate modes occur when two different vibrational modes have the same vibrational frequency as a consequence of symmetry. A simple example is the CeH bending vibration in Cl3CeH molecule where the CeH bond can bend with equal frequency in two mutually perpendicular directions. The treatment of degenerate vibrations is more complex and will not be discussed here.

9. CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES The basis of much of the current understanding of molecular vibrations and the localized group vibrations that give rise to useful group frequencies observed in the IR and Raman spectra of molecules is based upon extensive historical work calculating vibrational spectra. Historically, normal coordinate analysis first developed by Wilson with a GF matrix method and using empirical molecular force fields has played a vital role in making precise assignments of observed bands. The normal coordinate computation involves calculation of the vibrational frequencies (i.e., eigenvalues) as well as the atomic displacements for each normal mode of vibration. The calculation itself uses structural parameters such as the atomic masses and empirically derived force fields. However, significant limitations exist when using empirical force fields. The tremendous improvements in computational power along with multiple software platforms with graphical user interfaces enables a much greater potential use of ab initio quantum mechanical calculational methods for vibrational analysis. The standard method for calculating the fundamental vibrational frequencies and the normal vibrational coordinates is the Wilson GF matrix method.8 The basic principles of normal coordinate analysis have been covered in detail in classic books on vibrational spectroscopy.1,4,5,8 In the GF matrix approach a matrix, G, which is related to the molecular vibrational kinetic energy is calculated from information about the molecular geometry and atomic masses. Based upon a complete set of force constants a matrix, F, is constructed which is related to the molecular vibrational potential energy. A basis set is selected that is capable of describing all possible internal atomic displacements for the calculation of the G and F matrices. Typically, the molecules will be constructed in Cartesian coordinate space and then transformed to an internal coordinate basis set which consists of changes in bond distances and bond angles. The product matrix GF can then be calculated. The fundamental frequencies and normal coordinates are obtained through the diagonalization of the GF matrix. Here, a transformation matrix L is sought: L

1

GFL ¼ L

Here, L is a diagonal matrix whose diagonal elements are li’s defined as: li ¼ 4p2 c2 vi

2

Where the frequency in cm 1 of the ith normal mode is n2i . For the previous equation, it is the matrix L 1 which transforms the internal coordinates, R, into the normal coordinates, Q, as: Q ¼ L

1

R

CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES

23

In this equation, the column matrix of internal coordinates is R and Q is a column matrix that contains normal coordinates as a linear combination of internal coordinates. In the case of vibrational spectroscopy, the polyatomic molecule is considered to oscillate with a small amplitude about the equilibrium position and the potential energy expression is expanded in a Taylor series and takes the form: ! ! 3N 3N X 3N X vV 1X v2 V dqi þ dqi dqj þ ,,, V ¼ V0 þ vq i 2 i ¼ 1 j ¼ 1 vqi vqj i¼1 e

e

expressed in internal coordinates, qi,, which are directly connected to the internal bond lengths and angles. The above expression is simplified: 1. The first term Vo ¼ 0 since the vibrational energy is chosen as vibrating atoms about the equilibrium position. 2. At the minimum energy configuration the first derivative is zero by definition. 3. Since the harmonic approximation is used all terms in the Taylor expansion greater than two can be neglected. This leaves only the second order term in the potential energy expression for V. Using Newton’s second law, the above is expressed as: !   3N X d2 q i vV v2 V ¼ ¼ qj vqi vqi vqj dt2 j¼1 e

The above equation of motion is solved to yield a determinant whose eigenvalues (mu2) provides the vibrational frequencies (u). The eigenvectors describe the atomic displacements for each of the vibrational modes characterized by the eigenvalues. These are the normal modes of vibration and the corresponding fundamental frequencies. The force constant fij is defined as the second derivative of the potential energy with respect to the coordinates qi and qj in the equilibrium configuration as: ! v2 V fij ¼ vqi vqj In order to obtain the molecular force field with the force constants given by the above equation, a variety of computational methods are available. In general, the calculations of the vibrational frequencies can be accomplished with either empirical force field method or quantum mechanical methods.9,10 The quantum mechanical method is the most rigorous approach and is typically used for smaller to moderately sized molecules since it is computationally intensive. Since we are examining chemical systems with more than one electron, approximate methods known as ab initio methods utilizing a harmonic oscillator approximation are employed. Because actual molecular vibrations include both harmonic and anharmonic components, a difference is expected between the experimental and calculated vibrational frequencies. Other factors that contribute to the differences between the calculated and experimental frequencies include neglecting electron correlation and the limited size of the basis set. In order to obtain a better match with the experimental frequencies, scaling factors are typically introduced.

24

2. BASIC PRINCIPLES

Quantum mechanical ab initio methods and hybrid methods are based upon force constants calculated by Hartree-Fock (HF) and density functional based methods. In general, these methods involve molecular orbital calculations of isolated molecules in a vacuum, such that environmental interactions typically encountered in the liquid and solid state are not taken into account. A full vibrational analysis of small to moderately sized molecules typically takes into account both the vibrational frequencies and intensities to insure reliable assignments of experimentally observed vibrational bands. The ab initio Hartree-Fock (HF) method is an older quantum mechanical based approach.9,10 The HF methodology neglects the mutual interaction (correlation) between electrons which affects the accuracy of the frequency calculations. In general, when using HF calculations with a moderate basis set there will be a difference of ca. 10e15% between the experimental and calculated frequencies and thus a scaling factor of 0.85e0.90. This issue can be resolved somewhat by use of post-HF methods such as configuration interaction (CI), multi-configuration self-consistent field (MCSF), and Møller Plessant perturbation (MP2) methods. Utilizing configuration interaction with a large basis set leads to a scaling factor between 0.92 and 0.96. However, use of these post-HF methods comes with a considerable computational cost that limits the size of the molecule since they scale with the number of electrons to the power of 5e7. The ab initio density functional theory (DFT) based methods have arisen as highly effective computational techniques because they are computationally as efficient as the original HF calculations while taking into account a significant amount of the electron correlation.9,10 The DFT has available a variety of gradient-corrected exchange functions to calculate the density functional force constants. Popular functions include the BLYP and B3LYP. The scaling factors encountered using a large basis set and BLYP or B3LYP often approach 1 (0.96e1.05). Basis set selection is important in minimizing the energy state of the molecule and providing an accurate frequency calculation. Basis sets are Gaussian mathematical functions representative of the atomic orbitals which are linearly combined to describe the molecular orbitals. The simplest basis set is the STO-3G in which the Slater-type orbital (STO) is expanded with three Gaussian-type orbitals (GTO). The more complex split-valence basis sets, 3-21G and 6-31G are more typically used. Here, the 6-31G consists of a core of six GTO’s that are not split and the valence orbitals are split into one basis function constructed from three GTO’s and another that is a single GTO. Because the electron density of a nucleus can be polarized (by other nucleus), a polarization function can also be included. Such functions include the 6-31G* and the 6-31G**. Accurate vibrational analysis requires optimizing the molecular structure and wavefunctions in order to obtain the minimum energy state of the molecule. In practice, this requires selection of a suitable basis set method for the electron correlation. The selection of the basis set and the HF or DFT parameters is important in acquiring acceptable calculated vibrational data necessary to assign experimental IR and Raman spectra.

CALCULATING THE VIBRATIONAL SPECTRA OF MOLECULES

25

References 1. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962. 2. Pauling, L.; Wilson, E. B. Introduction to Quantum Mechanics with Applications to Chemistry; McGraw-Hill: New York, NY, 1935. 3. Kauzmann, W. Quantum Chemistry, An Introduction; Academic: New York, NY, 1957. 4. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993. 5. Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; D. Van Nostrand Company: New York, NY, 1945. 6. Long, D. A. Raman Spectroscopy; McGraw-Hill: New York, NY, 1977. 7. Cotton, F. A. Chemical Applications of Group Theory; Wiley (Interscience): New York, NY, 1963. 8. Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra; McGraw-Hill: New York, NY, 1955. 9. Handbook of vibrational spectroscopy, vol. 3, Sample Characterization and Spectral Data Processing; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley: 2002. 10. Meier, R. J. Vib. Spectrosc. 2007, 43, 26e37.

C H A P T E R

3 Instrumentation and Sampling Methods 1. INSTRUMENTATION Raman scattering and IR absorption are significantly different techniques and require very different instrumentation to measure their spectra. In IR spectroscopy, the image of the IR source through a sample is projected onto a detector whereas in Raman spectroscopy, it is the focused laser beam in the sample that is imaged. In both cases, the emitted light is collected and focused onto a wavelength-sorting device. The monochromators used in dispersive instruments and the interferometers used in Fourier transform instruments are the two basic devices. Historically, IR and Raman spectra were measured with a dispersive instrument. Today, almost all commercially available mid-IR instrumentation are (Fourier Transform) FT-IR spectrometers, which are based upon an interferometer (see below). Raman instruments include both grating-based instruments using multi-channel detectors and interferometer-based spectrometers.

1.1. Dispersive Systems A monochromator consists of an entrance slit, followed by a mirror to insure the light is parallel, a diffraction grating, a focusing mirror, which directs the dispersed radiation to the exit slit, and onto a detector.1, 2 In a scanning type monochromator, a scanning mechanism passes the dispersed radiation over a slit that isolates the frequency range falling on the detector. This type of instrument has limited sensitivity since at any one time, most of the light does not reach the detector. Polychromatic radiation is sorted spatially into monochromatic components using a diffraction grating to bend the radiation by an angle that varies with wavelength. The diffraction grating contains many parallel lines (or grooves) on a reflective planar or on a concave support that are spaced a distance similar to the wavelength of light to be analyzed. Incident radiation approaching the adjacent grooves in-phase is reflected with a path length difference. The path length difference depends upon the groove spacing, the angle of incidence (a), and the angle of reflectance of the radiation (b).

27

28

3. INSTRUMENTATION AND SAMPLING METHODS

Incident radiation

1 2

Exiting radiation from adjacent grooves has path length difference

FIGURE 3.1 Schematic of a diffraction grating. Wavetrains from two adjacent grooves are displaced by the path length difference. Constructive interference can occur only when the path length difference is equal to the wavelength multiplied by an integer (first, second, third order). The polychromatic radiation will be diffracted at different angles resulting in spatial wavelength discrimination.

Figure 3.1 shows a schematic of a diffraction grating with the incident polychromatic radiation and the resultant diffracted light. When the in-phase incident radiation is reflected from the grating, radiation of suitable wavelength is focused onto the exit slit. At the exit slit, the focused radiation will be in-phase for only a selected wavelength and its whole number multiples, which will constructively interfere and pass through the exit slit. Other wavelengths will destructively interfere and will not exit the monochromator. Thus, each of the grooves acts as an individual slit-like source of radiation, diffracting it in various directions. Typically, a selective filter is used to remove the higher-order wavelengths. When the grating is slightly rotated a slightly different wavelength will reach the detector.

1.2. Dispersive Raman Instrumentation Raman instrumentation must be capable of eliminating the overwhelmingly strong Rayleigh scattered radiation while analyzing the weak Raman scattered radiation. A Raman instrument typically consists of a laser excitation source (UV, visible, or near-IR), collection optics, a spectral analyzer (monochromator or interferometer), and a detector.1, 2, 3 The choice of the optics material and the detector type will depend upon the laser excitation wavelength employed. Instrumental design considers how to maximize the two often-conflicting parameters: optical throughput and spectral resolution. The collection optics and the monochromator must be carefully designed to collect as much of the Raman scattered light from the sample and transfer it into the monochromator or interferometer. Until recently, most Raman spectra were recorded using scanning instruments (typically double monochromators) with excitation in the visible region. An example of an array-based simple single grating-based Raman monochromator is depicted in Fig. 3.2. Use of highly sensitive array detectors and high throughput single monochromators with Rayleigh rejection filters have dramatically improved the performance of dispersive Raman systems. The advent of highly efficient Rayleigh line filters to selectively reject the Rayleigh scattered radiation enables the instrument to use only one grating, thereby greatly improving

29

INSTRUMENTATION

Slit

Source

M1

RF

G

AD

M2

FIGURE 3.2 Schematic of a simple array-based high throughput, single monochromator-based Raman instrument. For the source, laser light is scattered at 90 or 180 and collection optics direct light to a Rayleigh filter, RF, to remove the Rayleigh scattered radiation. G is the grating, M1 and M2 are spherical mirrors, and AD is the array detector.

the optical throughput. Two commonly used filters include the “holographic notch” and dielectric band filters.3 Use of an array detector results in dramatically improved signal-to-noise ratio (SNR) as a result of the so-called multi-channel advantage. The array detectors (AD) are often photodiode arrays or CCD’s (charge-coupled-devices), where each element (or pixel) records a different spectral band resulting in a multichannel advantage in the measured signal. However, only a limited number of detector elements (typically 256, 512, or 1064 pixels) are present in commercially available detector elements compared to the number of resolvable spectral elements. Thus, to cover the entire spectral range (4000e400 cm 1) requires either low-resolution spectrum over the entire spectral range or high resolution over a limited spectral range. One solution to this is to scan the entire spectral range in sections using a multi-channel instrument and adding high resolution spectra together to give the full Raman spectrum.

1.3. Sample Arrangements for Raman Spectroscopy Although the Raman scattered light occurs in all directions, the two most common experimental configurations for collecting Raman scattered radiation typically encountered are 90 and 180 backscattering geometry. Various collection systems have been used in Raman Spectroscopy based upon both reflective and refractive optics. 1, 2, 3Figure 3.3 shows 90 and 180 collection geometries using refractive and reflective optics, respectively. The 180 collection optics are typically used in FT-Raman spectrometers and in Raman microscopes. The 180 collection geometry is the optimum sample arrangement for FT-Raman as a result of narrow band self-absorption in the near-IR region. Raman spectroscopy is well known to be a technique requiring a minimum of sample handling and preparation. Typical Raman accessories include cuvette and tube holders, solids holders, and clamps for irregular solid objects. Often NMR or capillary tubes are used and many times the Raman spectra can be measured directly on the sample in their container.

30

3. INSTRUMENTATION AND SAMPLING METHODS

(a) To spectrometer

Laser

(b) Laser To spectrometer

FIGURE 3.3 Two different collection optics for Raman spectroscopy. In (a) a simple 90 collection geometry using refractive optics found in older dispersive instruments. In (b) 180 collection geometry using reflective optics typical of FT-Raman instrumentation.

1.4. Interferometric Spectrometers Both FT-IR and FT-Raman spectroscopy use an interferometer to separate light into individual components.4, 5 The FT-Raman measurements use a Nd:YAG 1064 nm near-IR excitation laser and the Raman scattered light falls in the near-IR region. In the IR spectral region, significant improvements in instrumental performance are realized since the instrument measures all wavelengths of IR light simultaneously without use of an entrance slit. The simultaneous measurement of light results in a multiplex (or Felgett’s) advantage while the latter results in a throughput (or Jacquinot) advantage. The relative weakness of the source (both IR and the Raman scattered light) along with the poor detector sensitivity make the Fourier transform measurements attractive in this spectral region. Figure 3.4 shows the schematic of the Michelson interferometer. Use of an interferometer along with computation using the Fast Fourier Transform enables the generation of IR and Raman spectra from the time-dependent interferogram. The components in a Michelson interferometer include a beam splitter, and a fixed and moving mirror. The collimated light from the source incident on an ideal beamsplitter will be divided into two equal intensity beams where 50% is transmitted to the moving mirror and the other 50% is reflected to the fixed mirror. The light is then reflected off both mirrors back to the beamsplitter where 50% is sent to the detector and the other 50% is lost to the source. As the moving mirror scans a defined distance (Dl), the path difference between the two beams is varied and is called the optical retardation and is two times the distance traveled by the moving mirror (2Dl). The interferometer records interferograms caused by phase-dependent interference of light with varying optical retardation. The principle of operation can be easily described by first considering the source to contain only a single monochromatic wavelength, l. When the position of the moving mirror with

31

INSTRUMENTATION

Fixed mirror l

Lens

Source er

itt

am

Be

l sp

Movable mirror Lens

Detector

FIGURE 3.4 Schematic of a Michelson interferometer. The displacement of the moving mirror is indicated by Dl.

respect to the beamsplitter is identical to that of the fixed mirror, the optical retardation is zero (zero path difference) and the two beams will combine at the beamsplitter in-phase resulting in constructive interference for the beam passing through to the detector. The detector response will reach a maximum whenever the optical retardation is an integral number of the wavelengths (0, l, 2l.). Similarly, a minimum detector response will result from destructive interference at optical retardation values with intervals of l/2 (l/2, 3l/ 2.). As the mirror scans at constant velocity, a simple sine wave will result as the two beams move in and out of phase. The Fourier transform of the sinusoidal interferogram will give rise to a single band with a characteristic frequency and intensity of the monochromatic source. If a polychromatic source is used, the interferogram will consist of a summation of all the different cosine functions corresponding to all of the wavelengths and of the intensities in the source. Only at zero path difference will all the wavelengths be in-phase. Thus, the resulting interferogram in FT-IR and FT-Raman spectra have a very strong center-burst and rapidly damped intensity in the wings of the interferogram. It is necessary to precisely know the optical path differences in the interferometer and this is accomplished using a heliumeneon laser. A computer is used to perform the fast Fourier transform to generate the spectrum, which can then be further processed. The computer interface in the FT-IR and FT-Raman spectrometers is used for setting measurement parameters, spectral processing, library searching, and for quantitation. 1.4.1. FT-IR SPECTROMETERS In a FT-IR spectrometer, sampling occurs just prior to the detector and its collimating optics. Various sampling techniques employed with IR spectroscopy are discussed below.

32

3. INSTRUMENTATION AND SAMPLING METHODS

Commercial FT-IR spectrometers typically operate in a single beam mode requiring sequential measurements of the single beam background and sample spectra, which are then ratioed to provide the final absorbance or %Transmittance IR spectrum. The single beam background spectrum provides a wavelength-dependent throughput of the instrument and is a function of the source emission, detector response, beamsplitter properties, and residual atmospheric absorptions from carbon dioxide and water vapor. 1.4.2. FT-RAMAN SPECTROMETERS Commercial FT-Raman spectrometers use mostly a 1064 nm Nd:YAG laser, notch filters at the laser wavelength to reduce Rayleigh scattered light entrance into the interferometer, high quality interferometers, and sensitive detectors which peak in the near-IR region. For FT-Raman spectrometers, the source depicted in Fig. 3.4 is the Raman scattered light. Presently, by switching only a few optical elements it is now possible to utilize one instrument to collect both the IR and Raman data. A major advantage of near-IR excitation based-FT-Raman spectroscopy is the greatly reduced fluorescence interference encountered for many compounds with visible excitation. This allows the Raman spectra of many compounds to be measured which was previously impossible. This occurs because the near-IR photons normally do not have sufficient energy to access the vibronic states that cause fluorescence. However, fluorescence problems are not completely eliminated for all samples and limitations due to sensitivity and absorption also exist. The y4 dependence shown by the Raman intensity is an intrinsic limitation to the sensitivity using near-IR excitation and can only be addressed by continued improvements in detector sensitivity. The inherent near-IR absorption profile of the sample itself can also pose significant problems. Many compounds have multiple absorption bands due to XeH type bonds in the near-IR spectral region which can attenuate the incident laser and/or the Raman lines. The unequal absorption throughout the Raman spectrum can affect the relative Raman band intensities and is termed as narrow band self-absorption. Thermal decomposition can sometimes occur due to absorption at the laser wavelength.

2. SAMPLING METHODS FOR IR SPECTROSCOPY 2.1. IR Transmitting Materials Infrared spectroscopy can be used to study a wide range of sample types either in bulk or in microscopic amounts over a wide range of temperatures and physical states. Quite often, an IR transmitting material is needed to aid sampling. Since quartz strongly absorbs in the near-IR region, front-silvered or gold-coated reflective optics are often employed rather than refractive lenses. However, transmitting materials are needed for other materials including the beamsplitter. Table 3.1 summarizes some important infrared transmitting materials. For the majority of applications, NaCl or KBr windows are used and ZnSe is frequently used for aqueous or wet samples. The alkali metal halides fuse under pressure to give IR and optically transparent windows, but they are hygroscopic and fog if not treated properly.

SAMPLING METHODS FOR IR SPECTROSCOPY

TABLE 3.1

33

Selected Infrared Transmitting Material

Material

Wavenumber range (cmL1)

Refractive index

Comments

NaCl

5000e625

1.52

Common, low cost, hygroscopic

KBr

5000e400

1.54

Common, low cost, very hygroscopic

BaF2

5000e870

1.45

Water insoluble, easily cracked

CaF2

5000e1100

1.40

Water insoluble, good at high pressures

KRS-5

5000e275

2.38

Water insoluble, good ATR, deforms, poisonous

ZnSe

5000e550

2.41

Water insoluble, good ATR

Diamond

4500e2500, 1800e200

2.4

Very hard, inert, diamond Anvil cell also good for ATR

2.2. Sampling Techniques The measured IR spectra are defined by the sample-preparation procedures and the optical spectroscopic techniques employed.6, 7 However, the quality of the spectra often depends upon the spectroscopist’s sample-preparation skills. A wide variety of sampling techniques exists; these are necessary to insure that the optimum spectrum will be measured for the varied physical sample states encountered by the analyst. For any given sample, there will usually be several different sampling techniques that can be used to obtain the spectrum. The available accessory equipment, personal preference of the analyst, or the nature of the particular sample will often dictate the selection of the technique. In general, the IR radiation incident upon a sample can be reflected, absorbed, transmitted, or scattered. Io ¼ Ir þ Ia þ It þ Is Here Io is the intensity of the incident radiation; Ir, Ia, It, and Is are the intensities of the reflected, absorbed, transmitted, and scattered radiation, respectively. Experimentally, any of the above intensity components can be used to measure the IR spectrum of the sample. The extent that light is transmitted, reflected, scattered, or refracted is dependent upon the sample morphology, crystalline state, the angle of the incident light, and the difference in refractive indices of the sample and surrounding matrix. Figure 3.5 depicts the angular dependence of the transmission, refraction, and reflected light. Transmitted light has a 0 angle of incidence while reflectance (which includes specular, external, internal, and diffuse) is observed at larger angles. The critical angle (Qc) beyond which refraction becomes imaginary and all light is reflected is also shown. In general, measurement of acceptable quality FT-IR spectra requires development of requisite sample-preparation skills necessary to measure photometrically accurate spectra. General criteria for suitable IR sample preparations include the following: 1. Suitable band intensities in the spectrum. The strongest band in the spectrum should have intensity in the range of 5e15% transmittance. The resulting preparation must be uniformly thick and homogenously mixed without holes or voids to insure a good quality spectrum.

34

3. INSTRUMENTATION AND SAMPLING METHODS

Transmitted Refracted

n2 n1 c

Total Reflection

FIGURE 3.5 The angle of incidence of the light and the refractive index difference help define how much light is transmitted, refracted, and reflected.

2. Baseline. The baseline should be relatively flat. The highest point in the spectrum should lie between 95 and 100% transmittance. 3. Poor atmospheric compensation. Water vapor and carbon dioxide bands should be minimized. Since the sample scan is ratioed against a previously measured background spectral subtraction of water vapor/carbon dioxide, reference spectrum should be used. 4. X-axis scale. Plots should employ a 2X-axis expansion and not a simple linear X-axis. This provides an expansion of the important fingerprint region. Figure 3.6 shows the FT-IR spectrum of starch with two different sample preparations. In the top spectrum (A), the starch was dissolved in water and prepared as a cast film on ZnSe resulting in a good quality, photometrically accurate spectrum. In the case of starch, this is one of the preferred sample-preparation techniques. The Nujol mull preparation of powdered starch provides an unacceptably poor quality FT-IR spectrum and a so-called false spectrum. A broadening of the most intense bands and a strengthening of the weak bands characterize false IR spectra. In general, this is caused by a non-uniform sample film or distribution of sample in a Nujol of KBr matrix. In the case of starch, good quality IR spectrum cannot be obtained by either Nujol mulls or KBr discs. Starch demonstrates that the nature of the particular sample dictates what sample-preparation technique is appropriate.

2.3. Transmission IR Simple transmission IR measurements are commonly made of gas, liquid, and solid-state samples. The most accurate information is obtained if the path length or analyte concentration is adjusted such that the peaks of interest have an absorbance between 0.3 and 0.9. The two simplest types of samples that can be prepared are gas phase samples and neat (low volatility) liquids. Gas phase measurements require only the introduction of the sample into a gas

35

SAMPLING METHODS FOR IR SPECTROSCOPY

851 762 708

934

1240

1447 1410 1364

2928

60

20

1152 1081 1026

40 3380

%Transmittance

80

1643

2152

100

(A): Water cast film/ZnSe

0 100

2156

60

3500

3000

2000 1800 1600

1400 1200

993 930

1081

1153

850

1244

2500

1373

(B): Nujol mull

1457

3291

0

2853

N

20

1000

760 718

N

1643

40

2922

%Transmittance

80

800

600

Wavenumber (cm–1)

FIGURE 3.6 The FT-IR spectrum of starch prepared as (A) water cast film on a ZnSe plate and (B) Nujol Mull. The N marks the Nujol bands.

cell, which can vary in path length from 10 cm to 8 meters. Highly accurate quantitation of single and multiple components is possible because the path length is well defined and no solid state effects are present (see below). 2.3.1. Liquids and Solutions Liquids and solutions are easily measured by FT-IR spectroscopy. Neat liquids can be easily prepared as a capillary film between two plates. The liquid film should be uniformly thick without holes or voids to insure a good, quantitative IR spectrum. Some common sample-preparation techniques for liquids include the following: 1. Capillary film. Neat liquids can be easily prepared as a capillary film. A drop of liquid sample is simply sandwiched between the cell windows. 2. Demountable cell. Here the technique is identical to the capillary film except a spacer (usually lead or teflon) is used to define the thickness of the liquid film. Disassembly allows access to the windows for easy cleaning or repolishing. The cell thickness is easily changed by selecting a different spacer. For a neat liquid sample, a thickness of about 0.01 mm is suitable.

36

3. INSTRUMENTATION AND SAMPLING METHODS

3. Sealed amalgam cell. This cell is constructed using a lead spacer which is amalgamated with mercury to create a very tight seal. This type of cell provides very accurate path length and is suitable for very volatile liquids. The filling and cleaning of the cell are done through the use of a pair of hypodermic syringe fittings. Fixed path length cells are commercially available in path lengths from 15 mm to 500 mm and can be used to measure the analyte dissolved in a suitable solvent. Both pure liquids and solutions can be measured using the above. Sealed amalgam cells are commonly used for solutions. Additional sample preparations include smears, cast films, and analytes in solution. More viscous liquids can be prepared as a smear on a single window. 2.3.2. Cast Films Samples that have been dissolved in a volatile solvent can be prepared by casting a thin film and allowing the solvent to evaporate. The sample is dissolved into a moderate to highly volatile solvent, and a film is prepared by evaporating the solution directly onto an IR transmitting window. The non-volatile sample remains behind on the plate as a thin film. This sample-preparation technique is widely used, but not limited to, obtaining the IR spectra of polymers Typically, a uniform film can be obtained by spreading the film repeatedly on a window using a spatula or the edge of a capillary pipet. Ideally, the resulting film should be amorphous to eliminate scattering and crystallinity effects. In the case of smaller molecules, evaporation of the solvent can result in formation of a thin crystalline film. It is often important that identical solvents and drying conditions are employed for both the sample and the reference material. This can be important since in many cases, significant differences can be observed in the IR spectrum as a function of the crystalline form. When preparing a cast film, first select a suitable solvent and IR window. The solvent must not react with the sample, but should be volatile and dissolve the sample. For the majority of applications, NaCl or KBr windows are used and ZnSe is frequently used for aqueous solutions. Dissolve the sample in the selected solvent. The concentration is not critical, however, higher concentrations will result in faster sample preparation. Now add a few drops of the solution on the central portion of the infrared window. Gentle heating using a hot plate and a flow of nitrogen can be used to hasten the solvent evaporation. Care should be taken since some materials will decompose even under gentle heat. With more volatile solvents, the solution can be allowed to spread out over the surface of the window and as the solvent evaporates a thin film will remain. However, some solutions will not spread out evenly by themselves and will require manipulation to obtain a thin uniform film (see below). With less volatile solvents (for example water) or solutions that will not spread out evenly on the surface of the window, a flexible spatula or glass pipet should be used to spread a thin film and facilitate the solvent evaporation. For a uniform film, the solution may require a thin film to be stroked on. Examine the resultant film visually to confirm that a thin uniform film free of holes and voids has been prepared. The thickness of the film for FT-IR spectroscopy should be approximately 5 mm. There is only limited control over the thickness of a cast film. In general, the

SAMPLING METHODS FOR IR SPECTROSCOPY

37

more concentrated the solution and the more drops placed on the window, the thicker the film. The process of obtaining the correct thickness is simply trial and error. Lastly, confirm from the IR spectrum that all of the solvent has evaporated leaving only the material of interest. The analyst should be familiar with the spectrum of the pure solvent. 2.3.3. SOLID-POWDERED SAMPLES: KBR DISCS AND NUJOL MULLS

Solid-powdered samples can be prepared as fine particles dispersed in NUJOL or as a disk in a transparent KBr matrix. A NUJOL mull preparation simply mixes mineral oil and finely ground sample to form a paste, which is then sandwiched, between two IR transmitting windows. A KBr disc uses a mixture of dried KBr powder and finely ground sample. The KBr/sample mixture forms a clear disc when pressed under high pressure using a hydraulic press. Both preparations require that the sample must be sufficiently ground and mixed with the support matrix to insure a good, artifact free IR spectrum. Insufficiently mixed and ground sample particles result in undesirable spectral artifacts. Most obvious is a sloping baseline due to scattering. A distorted band shapes as a result of anomalous dispersion of the refractive index in an absorption band (Christiansen effect). False spectra due to non-uniform sample thickness are characterized by broadening of the bandshape of the strong bands and relative intensification of weak bands. NUJOL MULL

A Nujol mull involves mixing of mineral oil and finely ground sample to form a paste, which is then sandwiched, between two IR transmitting windows (typically KBr or NaCl). In general, 30e50 mg of sample is needed for this sample preparation. The sample must be ground to an average particle size of at least 0.5 mm using an agate mortar and pestle. Larger particles approaching the wavelength of light being used will scatter light resulting in a sloping background. A suitably ground sample will be evenly distributed over the mortar and takes on a smooth, glossy appearance. A very small amount of the mulling agent (mineral oil) is added to the mortar, and grinding is continued until the sample is suspended in the mulling agent to form a smooth, creamy paste. A small amount of the mull is transferred to the center of an infrared window. A second window is placed on top of the sample and the two are pressed together. The mull should be spread out into a thin translucent film, free of holes or voids. The representative IR spectra of Nujol and Fluorolube is shown in Fig. 3.7. Nujol has bands characteristic of a long chain hydrocarbon. If detailed information is needed in the CH stretching region and the deformation region, a Fluorolube (perfluorinated hydrocarbon) mull should be used which has no appreciable absorption bands from 4000 to 1350 cm 1. If necessary, a split-mull spectrum can be obtained by merging a Nujol and Fluorolube mull spectra. A significant advantages of Nujol mull sample preparation is that no water is introduced into the sample. This can be particularly important when trying to confirm the presence of an OH or NH type species or when analyzing a particularly hydroscopic material. When faced with analyzing particularly toxic solid materials, the analyst can limit potential exposure by simply mulling the sample between the salt plates.

38

3. INSTRUMENTATION AND SAMPLING METHODS

598

50

650

1042

1273

60

902

2955

40 30

511

1377 1463

70

2854

%Transmittance

80

438

90

723

2297

100

3500

3000

2500

2000 1800

1600 1400 1200

970

1126

10

1199

2925

20

1000

800

600

Wavenumber (cm–1)

FIGURE 3.7 The FT-IR spectrum of Nujol and Fluorolube. The Nujol spectrum is shown as a solid line and the Fluorolube spectrum is shown as a dotted line.

KBR DISC SAMPLE PREPARATION

A KBr disc involves mixing of dried KBr powder and finely ground sample. The KBr/ sample mixture forms a clear disc when pressed in a die under high pressure using a hydraulic press. Only 5 mg of sample is needed for this sample preparation. The sample must be ground to an average particle size of at least 0.5 mm using an agate mortar and pestle to minimize a scatter-induced sloping background. The sample concentration varies from 0.1% to 3% (by weight). In general, start with ca. 5 mg of sample and add 500 mg of KBr powder a little at a time and mix well. Transfer the thoroughly mixed KBr preparation into a 13 mm die. Press the disc in a bench top press. With a hydraulic press, anywhere from 2 to 8 tons is sufficient. Solids are more frequently run as KBr discs than as mulls. This preference probably arises because KBr has no absorptions of its own over the entire transmission range and requires far less sample than a mull. However, the KBr disc method does have some disadvantages compared to a mull. KBr is very hygroscopic and some water is almost always introduced in the sample preparation. Figure 3.8 shows a KBr disc prepared without any organic sample. The water introduced from the sample grinding results in bands at ca. 3440, 1630, and 560 cm 1, from the OH stretch, OH bend, and OH wag that can complicate interpretation of the spectrum. KBr discs of crystalline materials are also sometimes less reproducible than mulls. The large pressure along with the addition of water can cause changes in hydration state, crystallinity, or polymorphism. In some cases, the sample itself can ion exchange or react with KBr. 2.3.4. Melts Melts are the last commonly used IR sample technique that is particularly useful for polymers. This method involves melting the sample on a salt plate and allowing it to resolidify. This method is not recommended for crystalline materials because of possible orientation

39

SAMPLING METHODS FOR IR SPECTROSCOPY

100

60

1632 3434

%Transmittance

80 70

558

90

50 40 30 20 10 0 3500

3000

2500

2000 1800 1600 1400 1200 1000 Wavenumber (cm–1)

800

600

FIGURE 3.8 The FT-IR spectrum of a KBr disc prepared by mixing KBr powder using a mortar and pestle into a fine powder and preparing a clear disc using a hydraulic press. Characteristic bands from water are observed at 3434, 1632, and ca. 560 cm 1.

effects. If a sample is known to decompose under heat, this method should not be employed. The technique involves using a hot plate, heat the sample above its melting point, and allowing the sample to become a liquid. Next, using a pipet take one drop of the melt and place it on an IR transmitting window such as KBr. Use the pipet to draw down the liquid into a uniform film and allow it to form a solid.

2.4. Reflection Techniques Two of the more commonly used FT-IR sampling techniques that capitalize on the spectral information that can be obtained from reflection (and refraction) properties are ATR (attenuated total reflection or internal reflection) and DRIFTS (diffuse reflectance FT-IR spectroscopy). 2.4.1. Attenuated Total Reflectance (ATR) ATR is a contact sampling method that involves a crystal with a high refractive index and excellent IR transmitting properties. ATR is one of the more popular sampling techniques used by FT-IR Spectroscopists because it is quick, non-destructive and requires no sample preparation. It is a contact sampling method that involves an IRE (Internal Reflection Element) with a high refractive index and excellent IR transmitting properties. The sampling technique capitalizes on the spectral information that can be obtained from reflection phenomena. The technique is used to measure the IR spectra of surfaces or of material that is too thick or strongly absorbing to be analyzed by more traditional transmission methods. A variety of ATR accessory designs exist and are available from almost all manufacturers of accessories for infrared spectrometry. In the case of ATR, the angle of incidence must be greater than the critical angle so that total internal reflectance occurs. Figure 3.5 depicted the dependence of light transmission,

40

3. INSTRUMENTATION AND SAMPLING METHODS

refraction and reflection upon both the material refractive indices and the angle of incidence. However, the reflected light does contain spectral information about the sample at the sampleecrystal interface and the ATR technique capitalizes on this. A single reflection crystal is commonly used in micro-ATR work and a multiple reflectance ATR crystal is often used for bulk samples. An ATR accessory provides an IR spectrum of a sample because the radiation at the reflection point probes the sample with an “evanescent wave.” At a frequency within an absorption band, the reflection will be attenuated while at frequencies well away from an absorption band, all light is reflected. Internal reflectance occurs when the angle of incidence exceeds the “critical” angle. This angle is a function of the real parts of the refractive indices of both the sample and the ATR crystal   1 h2 qC ¼ sin h1 Here, h2 is the refractive index of the sample and h1 is the refractive index of the crystal. Typical crystals such as Ge, ZnSe, or diamond have high refractive indices much higher than that of organics resulting in internal reflections at moderate angles of incidence. Figure 3.9 depicts some of the basic principals of ATR measurements. The refractive index of the sample is a complex with both a real and imaginary component: h ¼ n þ ik The real part of this expression n in the regular refractive index holds when there is no absorption. The imaginary component of the refractive index, ik, holds within an absorption

Sample z

e–z/dp dp IRE

FIGURE 3.9

O1 O2

Diagram of a single reflection ATR crystal depicting the basic principles of the technique.

SAMPLING METHODS FOR IR SPECTROSCOPY

41

band resulting in an effect known as anomalous dispersion. The parameter k is directly related to the extinction coefficient found in the LamberteBeer law. The total absorption intensity, A, as a function of q can be expressed as: Z N aðzÞe z=dp dz AðqÞ ¼ 0

where z is the depth into the sample and a(z) is the absorption coefficient of the sample as a function of depth. As depicted in Fig. 3.9, the reflected radiation penetrates the sample boundary as a so-called evanescent wave. The electric field amplitude of this evanescent wave exhibits an exponential dependence. The parameter, dp, the effective penetration depth is expressed as: l dp ¼  1=2 2pnp sin2 q n2sp Where, l equals the wavelength of the radiation, q is the angle of incidence, np is the refractive index of the IRE, and nsp ¼ n2/n1 the ratio of the refractive indices of the sample and IRE. This depth of penetration is defined as the distance from the IRE-sample boundary where the intensity of the evanescent wave decays to 1/e (37%) of its original value. There are a number of important consequences to the above expression. The sampling depth of the ATR method is approximately 2e15 mm, is wavelength dependent, and increases with decreasing wavenumber. Experimentally, varying the angle of incidence of the light and the refractive index of the crystal used can control the sampling depth. Increasing either results in a decreased sampling depth. Careful comparison with classic transmission spectrum also indicates that ATR measurements result in small differences in the peak frequency and bandshape as a result of refractive index effects. ATR requires excellent contact between the sample and the crystal and is therefore an excellent method for liquids or soft, easily deformed solids. Use of micro ATR crystals with a protective ATR coating enable much greater pressure to be used to insure good contact between the ATR crystal and harder materials. Table 3.2 summarizes some of the commonly used ATR crystal materials. 2.4.2. Diffuse Reflectance Diffuse reflectance (DRIFTS, Diffuse Reflectance Infrared Fourier Transform Spectroscopy) is applied to analyze powders and rough surface solids. Typically, the technique is applied to powders since the technique relies upon scattering of radiation within the sample. The incident light on a sample may result in a single reflection from the surface (specular reflectance) or be multiply reflected giving rise to diffusely scattered light over a wide area which is used in DRIFTS measurements. Collection optics in the DRIFTS accessory are designed to reject the specularly reflected radiation and collect as much of the diffuse reflected light as feasible. In general, theories to describe the diffuse reflectance phenomena consider plane-parallel layers within the sample with light scattering in all directions. The observed reflectance

42

3. INSTRUMENTATION AND SAMPLING METHODS

TABLE 3.2

Commonly Used ATR)Crystal Materials and Characteristics

Material

Wavenumber range (cmL1)

Refractive index

Comments

Germanium

5000e850

4.0

Hard and brittle, good ATR material but temperature sensitive.

KRS-5

5000e275

2.38

Water insoluble, conventional ATR material. Relatively soft, deforms, poisonous. Reacts with complexing agents.

ZnSe

5000e550

2.41

Water insoluble, hard and brittle, good ATR material. Attacked by acids and strong alkalines.

Diamond

4500e2500

2.4

Very hard, inert, used in micro-ATR crystals. Often used as a protective film for ATR elements such as ZnSe

3.4

Limited wavelength range for ATR. Material is inert, hard and brittle.

1800e200 Silicon

8300e1500

* ATR is attenuated total reflectance.

spectrum using the KubelkaeMunk equation is a ratio of the single beam spectra of the sample diluted by KBr powder against pure powder KBr and is defined as: fðRN Þ ¼

ð1

RN Þ2 K C ¼ ¼ 2RN s k

where K is the absorption coefficient and s is the scattering coefficient of the sample material. The reflectance, RN, indicates the sample is thick enough that no radiation reaches the back surface. The above predicts a linear relationship between K and the maximum value of f(RN) for each peak provided s is constant. The scattering coefficient is dependent on the particle size and packing and must remain constant for reproducible results. Lastly, C is the concentration of the sample and k is related to the particle size and molar absorptivity of the sample, k ¼ s/2.303e. The DRIFTS spectrum can show a strong dependence on the refractive index of the sample, the particle size and size distribution, packing density, and sample homogeneity. For quantitative analysis, the particle size, sample packing, and dilution must be carefully controlled. To obtain the highest quality DRIFTS spectrum, the following precautions are taken. 1. Particle size should be small and uniform. This will result in narrower bandwidths and more accurate relative intensities. The sample/matrix particulates size should be 50 mm or less. 2. Dilute sample in a non-absorbing matrix powder such as KBr or KCl. The finely powdered sample is ~5% relative to the matrix powder. This insures a deeper penetration of the incident light into the sample, which will increase the diffuse scattered light. 3. Homogeneity. The samples should be well mixed. 4. Packing. The sample should be loosely packed in the cup to maximize the IR beam penetration.

SAMPLING METHODS FOR IR SPECTROSCOPY

43

For quantitative analysis, the particle size, sample packing, and dilution must be carefully controlled. Despite compliance with the precautions listed above, the ratioed diffuse reflectance spectrum will appear different from that obtained with a classic transmission IR measurement. In general, low intensity bands will be increased relative to intense bands and the strong intensity bands will have broader, rounder peak shapes. A KubelkaeMunk conversion available in most FT-IR software packages can compensate for some of these effects.

2.5. Microscopy Both IR and Raman spectroscopy have been very successfully coupled to microscopy with widespread applications in heterogeneous samples in industrial, forensic, and biological sciences. Vibrational microscopy is used for particle and contaminant analysis, studies of laminar structures, characterization of fibers, and domain structure and chemical compositional mapping. IR microspectroscopy involves coupling a microscope to an infrared (typically FT-IR) spectrometer and provides the capability to obtain spectra of extremely small samples (as small as 10 mm in diameter in certain specialized applications). The technique has widespread applications in the study of heterogeneous samples and is used for particle and contaminant analysis, studies of laminar structures, domain structures, and chemical composition mapping. The microscope system allows one to focus IR radiation onto the sample, to collect and image transmitted or reflected IR radiation from the sample onto a sensitive detector and to view and position the sample. The system includes an optical microscope system for viewing the sample and reflective optics for imaging the IR radiation. The system also includes remote apertures for defining the spatial area of the sample to be examined. Commercial FT-IR microscopes are capable of a variety of sampling techniques including: specular reflectance, diffuse reflectance, micro-ATR, grazing angle, and conventional transmission measurements. Just as when studying materials in bulk, it is the sample itself that determines the most appropriate sampling technique. Several commercial Raman microspectrometers are available with excitation sources that span the UV to the near-IR. The ease of sampling makes Raman spectroscopy an excellent choice for microscopy. However, fluorescence remains a considerable obstacle to application of Raman microscopy to many samples. More recently, IR and Raman imaging have been successfully applied to a variety of heterogeneous systems. 2.5.1. Transmission IR Microscopy The ideal sample for transmission IR microscopy is a thin (5e10 mm), smooth, flat sample that does little to alter the optical path of the IR radiation. This requires the sample to also be large enough (ca. 25 mm) to minimize diffraction of light. Sample-preparation techniques can include: a) slicing and microtoming b) flattening sample with a roller c) squeezing sample between salt plates or a diamond compression cell As with macroscopic IR measurements, sample preparation plays an important role in obtaining quality IR spectra when using an IR microscope. Important experimental

44

3. INSTRUMENTATION AND SAMPLING METHODS

parameters include the diffraction light limit, light scattering, and focal shifts due to high refractive index samples. When the sample or aperture size is on the order of the wavelength of light being used, diffraction plays a major role in the measured IR spectrum. Significant distortion of the measured IR spectrum occurs. Diffraction effects can be minimized by the: a) use of widest possible apertures. Keep aperture closed within the sample image but not closed enough to reduce energy throughput significantly. b) use of the same aperture setting in the background and sample scan. c) flattening of sample which will enlarge the sample area. Surface irregularities and fine particles can scatter light. Remedies include adding KBr or KI and applying pressure to obtain a clear plate or adding NUJOL oil to the a roughened surface. Samples or substrates with very high refractive index can give rise to focal shifts. This can be corrected by adjusting the focal system of the microscope by observing either the sharpness of the aperture edge or the energy throughput. At times unwanted internal reflections can also interfere. An example of this is interference fringes that can occur when using a diamond anvil cell. 2.5.2. Reflection IR Microscopy Infrared reflectance measurements are desirable because of the reduced sample preparation required. The IR microscope will be set to reflectance rather than transmittance mode and if necessary, to an additional attachment placed on the IR objective. Commonly used reflectance techniques include specular reflectance and ATR measurements. The most commonly used reflectance technique is ATR. A micro-ATR accessory is employed that attaches to the IR microscope objective. Once contact is established between the sample and the ATR element, the IR spectrum is measured. The resultant spectrum is typical of that obtained using any ATR accessory. Specular reflectance is a less commonly used technique for IR microscopy and involves a front surface mirror-like reflection from the exterior of a material where the angle of incidence is equal to the angle of reflection. The measured spectrum depends on the angle of incidence, the sample index of refraction, absorption properties, and sample surface morphology. The technique requires no sample preparation and is best suited for smooth, highly reflective surfaces and can include samples such as single crystals and thin films on a reflective metal surfaces. Not surprisingly, the resultant specular reflectance IR spectra are very different from typical transmission IR data. The spectra show derivative-like shape within the absorption bands. A KramerseKronig transformation which is often available in FT-IR software is often used to deconvolute the specular reflectance data and provide more absorbancelike spectra.

3. QUANTITATIVE ANALYSIS Any quantitative analyses involve measurement of a test sample and comparison with standards of known concentration. The resulting calibration plot reflects the relationship

QUANTITATIVE ANALYSIS

45

between the measured quantity (such as infrared absorbance or the Raman intensity) and the analyte concentration. Provided appropriate standards for calibration of an analytical method selected, vibrational spectroscopy is a valuable potential technique for quantitative analysis.1 Quantitative analysis methods can include both classical and chemometric PLS (principal least squares) analysis. Only classical analysis techniques will be discussed here. For an understanding of chemometric applications to spectroscopy the reader should consult a reference dedicated to this subject.8, 9 Advantages of using vibrational spectroscopy include the following: 1. 2. 3. 4. 5.

Wide variety of sample types (gases, liquids, solids) Clearly separated peaks are often found in the IR and Raman spectra Unique bands frequently allow multicomponent analyses of mixtures Typically not much effort is needed to acquire or prepare sample for analysis Sample quantity is in milligram quantity

Both the analyte and non-analyte components will be observed by both IR and Raman spectroscopy. Consequently, the main components of the sample must be confirmed prior attempting quantitation. In general, Raman and IR spectroscopy of solids can be performed at concentrations of 1e100%. When strong isolated bands are available, this can be extended to lower concentrations (~0.1 %). IR spectroscopy can be employed to quantitate much lower concentrations (down to ppm levels) by greatly increasing the sample path length for solutions and for gases.

3.1. Relationship of IR and Raman Signal to Analyte Concentration The basis of quantitative analyses in absorption spectroscopy is the LamberteBeer law. log

I0 1 ¼ log ¼ A ¼ abC I T

where I0 is the incident light, I is light after it has passed through the sample, T is the transmittance, A is the absorbance, a is the absorption coefficient, b is the sample thickness, and C is the sample concentration. The absorption coefficient is a constant specific for the material at a particular wavelength. Where the LamberteBeer law holds, the measured absorbance is a linear function of the concentration (and path length). Both single component and multicomponent analysis is possible. Analytical methods based upon IR and Raman spectroscopy often use either peak heights or peak areas. Commercial instrument software presently allows both peak height and areas to be used when developing analytical methods. The integrated peak areas may provide a more robust calibration if the analyte band changes with increasing concentration or is influenced by molecular interactions. Deviations from the LamberteBeer law resulting in a non-linear relationship between the measured absorbance and the analyte concentration often occur. At the high concentrations typically encountered in IR measurements of solids, the molecular interactions of the sample itself will often cause non-linearity. However, if the calibration takes such non-linearity into account, it will not affect the quantitative accuracy of the analytical method.

46

3. INSTRUMENTATION AND SAMPLING METHODS

As discussed in the previous chapter, the intensity of the Raman scattered radiation is given by:  2 va IR fn4 Io N vQ where Io is the incident laser intensity, N is the number of scattering molecules in a given state, n is the frequency of the exciting laser, a is the polarizability of the molecules, and Q is the vibrational amplitude, and the parameter ðva=vQÞis the Raman cross section. Analogous to infrared spectroscopy, the Raman cross section is a constant specific for the material at a particular wavelength. Thus, if the Raman cross section and the incident laser intensity (I0) remain constant, the intensity of the Raman band (IR) is directly proportional to the sample concentration. Once again, at the high concentrations often encountered in Raman measurements interactions of the components in the sample itself often result in non-linearity. To develop a quantitative analysis method for either IR or Raman spectroscopy, well-characterized standards covering a broad range of compositions are required. In the case of the IR spectra of solid-state samples, neither the sample thickness nor the concentration of the sample in the matrix is known. With Raman spectroscopy, the incident laser intensity can be very difficult to precisely control. For this reason, a ratio of an analyte band is often normalized against another component to eliminate these sources of variation.6 The selection of appropriate peaks in the spectrum are chosen as the inputs for the quantitative analysis based on the spectral separation and knowledge of the underlying vibrational modes.

3.2. Quantitative Analysis: Ratio Method A calibration curve defines the relationship between an analytical signal produced by the analyte and its concentration. In the common case of linear calibration, a linear regression will be used to fit the analytical signal y to the concentration x of the analyte in calibration samples. y ¼ a þ bx For determination of an unknown, the calibration equation is inverted. a y þ b b A different form of calibration equation is often applicable in spectroscopy. The simplest case involves only two components (C1 þ C2 ¼ 100%) with two isolated analytical bands with absorbances A1 and A2. Here, we assume a linear relationship between the concentration of the analytes and the observed signal with a slope that differs depending upon the analytes response function. For IR spectroscopy, this response will depend upon the molar absorptivities of the bands and for the Raman cross section. This simple case is commonly used when employing the ratio method of IR analysis that relies upon Beer’s law and enables quantitation of components without knowledge of the sample path length. x ¼

A1 b1 a1 C1 C1 ¼ ¼ k A2 b2 a2 C2 C2

QUANTITATIVE ANALYSIS

47

With simple rearrangement, we then obtain the following: %C1 ¼

100 1 þ kA2 =A1

In general, the analysis is more complex since the calibration curves are often affected by overlapping bands, additional interfering components, molecular interactions effecting the absorption coefficient and Raman cross sections and in the case of FT-Raman the selfabsorption phenomena discussed below. Thus, the expressions for both IR and Raman are more complex than that suggested by the simple two component case. For IR, we could simply expand the original expression as: A1 ¼ a1 b1 C1 þ a2 b2 C2 For FT-Raman spectroscopy, the expression is more complex and below we add an additional term (1 e ax) which takes into account near-IR self-absorption of the Raman scattered light.   vs 1 e ax Þ þ K IR eNIo vU

The resultant calibration curves will often be non-linear. Most instrumental software will enable the data to be fit with empirical equations such as quadratic or higher-order polynomial fits. An alternative is the use of a class of semi-empirical equations and linear fractional transformations. The equation form is suitable for the peak ratio method. Unlike quadratic equations, linear fractional transformations are easily inverted, which facilitates the development of calibration equations in a manner similar to the usual practice encountered in the linear calibration case. 3.2.1. Fractional Linear Calibration Equations Fractional linear transformation can be applied to account for typically encountered variation from the simple two-component case discussed above.10 For a mixture of two components A and B, a spectral response (absorbance or Raman intensity) in which there may be interference with the two analytical bands is R1 ¼ k1A CA þ k1B CB R2 ¼ k2A CA þ k2B CB

The ratio of concentrations CA/(CA þ CB) (mole fraction) as a function of the ratio of the spectral response R1/(R1 þ R2) has the form of a fractional linear transformation CA aR1 þ bR2 ¼ ðCA þ CB Þ R1 þ dR2 where coefficients a, b, and d are constants which can be determined empirically. In our case, b will often equal zero and the above simplifies to an expression with only two variables: CA aR1 ¼ R1 þ dR2 ðCA þ CB Þ

48

3. INSTRUMENTATION AND SAMPLING METHODS

The above equation can be employed for the non-linear calibration curves typically encountered in many IR and Raman quantitation procedures. CA R1 1 ¼ ¼ R1 þ dR2 ðCA þ CB Þ 1 þ dR2 =R1 In a specialized case where a ¼ 1 and b ¼ 0, the above expression simplifies to the standard expression for the ratio method of a linear system that we started with.

3.3. Example of Quantitation Using Ratio Method of Analysis Below is an example of quantitation of the components in PAMeAETAC water-soluble copolymers using both FT-IR and FT-Raman spectroscopy. In this example, a broad selection of nine well-characterized PAMeAETAC standards are used to develop calibration curves for both IR and FT-Raman spectroscopy. Since the calibration curves are non-linear, a linear fractional transformation equation is used to model the non-linear calibration curves. The structure of poly AMDeAETAC is shown below. poly (AMD—AETAC) CH2

CH C

CH2 O

CH C

NH2

O

O n

m CH2 CH2

N(CH3)3 Cl Copolymers of acrylamide (AMD) and 2-(acryloyloxy) ethyltrimethylammonium chloride (AETAC)

Figure 3.10 shows the FT-IR and FT-Raman spectra of pure PAM, and two PAMeAETAC copolymers with 53 and 90% AETAC(Q9) respectively. These spectra illustrate the many strong characteristic bands that exist for the polymer backbone, primary acrylamide, the ester carbonyl, and the positively charged trimethylammonium chloride groups. Table 3.3 lists assignments of some of the important IR and Raman bands for PAM and AETAC. Figure 3.11 shows the FT-IR and FT-Raman spectra of 47% AMD: 53% AETAC copolymers along with the selected baselines and peak heights used for this analysis. Bands characteristic of both groups are isolated and intense enough to provide quantitative bands for both components. In the IR spectrum, the quantitation method uses the AETAC ester carbonyl and the AMD amide carbonyl at 1734 and 1671 cm 1, respectively. In the FT-Raman spectrum, the quantitation method uses the AETAC trimethylammonium chloride band at 719 cm 1 and the AMD primary amide NH2 rock band at ca. 1105 cm 1.

49

QUANTITATIVE ANALYSIS

Figure 3.12 shows the FT-IR calibration plot for AMDeAETEC copolymers using a peak height ratio and plotting the calculated fit using nine calibration standards. The FT-IR calibration plot uses the peak height ratio of the AETAC ester carbonyl relative to the total carbonyl (ester þ amide). The non-linearity observed in the above IR calibration curve is a result of deviations in Beer’s law and overlapping contributions of the two carbonyl bands. The two variable functions chosen to model the calibration curve provides an excellent semi-empirical method to take into account these sources of non-linearity. The mole fraction AETAC with can be calculated from the FT-IR spectrum using the following expression: Mole fraction AETAC ¼

1:088ðAbs:1734 cm 1 Þ ðAbs: 1734 cm 1 Þ þ 0:874ðAbs:1670 cm

1Þ

(a)

783

1124

1621

1663

3344

PAM

100 80

954

630

1055 1262

1482 1450 1414 1340

1166

1734

1629

2955

1671

20

3018

40

3169

60 3342

%Transmittance

3199

40 20

1177

60

1451 1419 1349 1323

2932

80

2858

100

PolyAMD—AETAC (52.5%)

100

3500

3000

2500

2000

1800

1600

955 1166

1400

877

1057

1341 1256

1453 1390 1485

1733

20

3379

40

1671

60

3013 2960

80

1200

PolyAMD—AETAC (89.6%)

1000

800

600

Wavenumber (cm–1)

FIGURE 3.10 The FT-IR and FT-Raman spectra of the following: pure PAM and two PAMeAETAC copolymers (47/53 and 10/90). The FT-IR spectra are shown on the left and the FT-Raman spectra on the right. A cast film sample preparation on ZnSe of all three samples after dissolving the powders in water was used for the FT-IR data. For the FT-Raman data, the samples were in the supplied crystalline form using an NMR tube for sampling.

50

3. INSTRUMENTATION AND SAMPLING METHODS

(a)

783

1124

1621

1663

3344

PAM

100 80

954

630

1055 1262

1482 1450 1414 1340

1166

1734

1629

2955

1671

20

3018

40

3169

60 3342

%Transmittance

3199

40 20

1177

60

1451 1419 1349 1323

2932

80

2858

100

PolyAMD—AETAC (52.5%)

100

3500

3000

2500

2000

1800

1600

955 1166

1400

877

1057

1341 1256

1453 1390 1485

1733

20

3379

40

1671

60

3013 2960

80

1200

PolyAMD—AETAC (89.6%)

1000

800

600

Wavenumber (cm–1)

FIGURE 3.10 (continued).

TABLE 3.3

Some Band Assignments for the Poly AMD-AETEC Samples

Group

PAM Bands (cmL1)

Assignments

NH2

3350, 3200 (IR,R)

NH2 i.ph. str. and o. ph. str.

NH2

1608e1621 (IR,R)

NH2 bend

NH2

1107 (R)

NH2 rock

C¼O

1663e1656 (IR,R)

C¼O str.

CH2eCH

2936 (IR)

CH2 o. ph. str.

CH2eCH

2925 (R)

CH(C¼O) str.

CH2eCH

1455 (IR,R) 1324 (IR,R)

CH2 bend

CeN

1424 (IR,R)

Involves CeN str.

51

QUANTITATIVE ANALYSIS

TABLE 3.3

Some Band Assignments for the Poly AMD-AETEC Samplesdcont’d

Group

PAM Bands (cmL1)

Assignments

C¼O

1608e1621 (IR,R)

NH2 bend

Ester C¼O

1734 (IR,R)

C¼O str.

Ester C¼O

1166 (IR)

Involves CeO str.

CH2eN(CH3)3

3010e3022 (IR,R)

CH3 o. ph. str.

CH2eN(CH3)3

955 (IR,R)

Involves NC4 o. ph. str. þ CH3 rock

CH2eN(CH3)3

720 (R)

Involves NC4 i. ph. str.

Ester C¼O

1734 (IR,R)

C¼O str.

Peak Ht AETAC C=O

0.8

1671

0.9

1734

AETEC

Poly AMD(47%) — AETAC(53%) Peak Ht AMD C=O 1166

0.6

0.3

954

0.4

1262

1482

0.5

1450 1416

Absorbance

0.7

0.2 0.1

719

0.0 30

20

953 1036

1107 1089

1673

5

1620

1732

10

1340

15

1242

Peak Ht AMD NH2 rk

874 846

1450

25

Raman intensity

Peak Ht AETAC NC4 i.ph str.

0 1800

1700

1600

1500

1400

1300

1200

1100

1000

900

800

700

600

Wavenumber (cm–1)

FIGURE 3.11 The baseline and peak height measurements used for quantitation. The top FT-IR spectrum shows the peak heights used to quantitate the ester and amide carbonyl bands. The bottom FT-Raman spectrum shows the peak heights used to quantitate the amide NH2 rock and the cationic ammonium NC4 in-phase stretch bands.

52

3. INSTRUMENTATION AND SAMPLING METHODS

1.0 A (1733 cm ) [A (1733 cm ) + A (1670 cm )]

C=O Absorbance ratio

0.8

1.088 (A 1733 cm ) (A 1733 cm ) + 0.874 (A 1670 cm )

0.6

0.4

0.2

0.0 0.0

0.2

0.4 0.6 Mole fraction AETAC

0.8

1.0

FIGURE 3.12 FT-IR calibration plot for AMDeAETAC copolymers. The form of the fractional linear transformation used to fit this data is only very slightly non-linear.

1.0

Raman intensity ratio

0.8

0.6 I (719 cm )

0.4

[I (719 cm ) + I (1105 cm )]

2.39 (IR719 cm )

0.2

[(I 719 cm ) +15.96 (I 1105 cm )]

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Mole fraction AETAC

FIGURE 3.13 The FT-Raman calibration plot for powdered AMDeAETAC copolymers. The form of the fractional linear transformation used to fit this data is quite non-linear.

Figure 3.13 shows the FT-Raman calibration plot for AMDeAETEC copolymers using a peak height ratio and plotting the calculated fit for the same nine standards. The form of the fractional linear transformation used to fit this data is quite non-linear. Despite this, the FT-Raman technique provides excellent quantitation of PAMeAETAC copolymers. The mole fraction AETAC with can be calculated from the FT-Raman spectrum using the following expression:

53

QUANTITATIVE ANALYSIS

0.6

Absorbance

0.5

500 1000

2000

3000

4000

0.4 0.3 0.2 0.1 Poly AMD–AETAC (90%) PAM

0 1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

Wavelength (nm)

FIGURE 3.14 The diffuse reflectance near-IR spectra of pure PAM and poly AMDeAETAC (90%) copolymer. Note the different absorbances at 1152 and 1206 nm where the analytical Raman bands are located.

Mole fraction AETAC ¼

2:39ðIR 719 cm 1 Þ ½ðIR 719 cm 1 Þ þ 15:96ðIR 1105 cm

1 ފ

The significant non-linearity in the calibration curve above is due largely to narrow band self-absorption phenomena that are typical of 1064 nm excitation-based Raman spectra.1 Such self-absorption is defined as the absorption of the Raman scattered light by the sample near-IR absorption bands themselves before the light can escape the sample in the NMR tube used in this experiment. Where there is a near-IR absorption band, the Raman band intensity will be decreased. The magnitude of this effect will vary significantly in powdered samples because of the variation in the path lengths the scattered light will follow. In a liquid sample, the magnitude of the Raman band suppression will follow a simple the LamberteBeer law. Figure 3.14 shows the diffuse reflectance near-IR spectra of pure PAM and poly AMDeAETAC (90%) copolymer. The 719 cm 1 AETAC band and the 1105 cm 1 PAM band both have different near-IR absorbances for these two components. Much of the observed curvature in the correlation plot is a consequence of this selfabsorption process and not molecular environmental induced changes in the Raman cross sections.

54

3. INSTRUMENTATION AND SAMPLING METHODS

Chapter 3: Questions 1. Outline the dispersive unit used in an FT-IR spectrometer to separate light into individual components. 2. Summarize the four guidelines for quality FT-IR spectrum. 3. Good sample-preparation skills are necessary to acquire photometrically accurate, high quality IR spectra. How can one differentiate between a good and a false IR spectrum? What causes a false IR spectrum? 4. Describe general regions of the fundamental vibrational spectrum with some characteristic group frequencies. 5. Outline one type of dispersive unit used in a Raman spectrometer to separate light into individual components. 6. Describe components of a Raman Fiber optic Probe. 7. List advantages and disadvantages of Raman spectrometers. Consider wavelength selection. 8. What is the difference between Rayleigh and Raman scattered light? 9. Summarize the advantages and disadvantages to KBr disc and Nujol mull sample preparations. 10. Does Raman or IR spectroscopy differentiate between amorphous and different crystalline materials? 11. Illustrate how light can be transmitted, refracted, and reflected through a crystal. What are the important variables? 12. What samples will provide the best quality IR spectrum using an ATR accessory and why? The worst? 13. How does an ATR measured spectrum differ from a classically measured transmission spectrum? What important parameters can the experimenter control? 14. Define the evanescent wave described by ATR theory. How does it define the sampling depth? 15. Identify three important criteria for selecting suitable ATR crystals. 16. What are the ideal sample characteristics for transmission IR microscopy? 17. List unwanted optical effects encountered in IR microscopy. 18. What is the diffraction light limit and what limits does it impose on IR microscopy?

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993. Analytical Applications of Raman Spectroscopy; Pelletier, M. J., Ed.; Blackwell Science: London, 1999. Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; John Wiley: New York, NY, 1986. Chase, D. B.; Rabolt, J. F. Fourier Transform Raman Spectroscopy, From Concept to Experiment; Academic: 1994. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. Coleman, P. B. In Practical Sampling Techniques for Infrared Analysis; CRC: Boca Raton, 1993. Beebe, K. R.; Pell, R. J.; Seasholtz, M. B. Chemometrics A Practical Guide; John Wiley: New York, NY, 1998. Martens, H.; Naes, T. Multivariate Calibration; John Wiley: New York, NY, 1989. Peter Fortini, personal communication.

C H A P T E R

4 Environmental Dependence of Vibrational Spectra 1. SOLID, LIQUID, GASEOUS STATES Vibrational spectroscopy shows a strong dependence on the environment of the molecule being examined. The IR and Raman spectra are significantly different for molecules in the gaseous, liquid, or solid state. In the gaseous state, the molecules can be considered to be non-interacting and can rotate freely with a characteristic angular momentum. (We are ignoring pressure broadening.) Coupling of the vibrational transition with the rotational degrees of freedom of the molecule occurs resulting in band structure that is highly characteristic of the molecular structure and rotationalevibrational interactions.1,2 This band structure is highly dependent upon the molecular symmetry and falls into four basic types: linear (e.g., diatomics, CO2) spherical top (e.g., CH4, SF6), symmetric top (oblate e.g. CHCl3 prolate e.g., CH3Cl), and asymmetric top (e.g., H2O). Numerous qualitative and quantitative gas phase applications exist using both IR and Raman techniques but will not be covered here. Rotational fine structure can be observed in the vibrationalerotational band cluster if the molecular moment of inertias are sufficiently low as it is typical for simpler molecules. Fine structure occurs because angular momentum is also quantized. For larger molecules, the fine structure is typically unresolved resulting in broad bands. However, the band contour reveals the molecule symmetry and the direction of the dipole moment change during the vibration. Figure 4.1 shows the classical description of the IR gas phase contours of carbon dioxide, a linear type rotator. In Fig. 4.1a, CO2 is bending and rotating about an axis parallel to the change in the dipole moment which is not affected by the rotation. As seen in Fig. 4.1b, this results in a single frequency depicted in Fig. 4.1c. In Fig. 4.1d, this rotation changes the orientation of the dipole moment vector, the vertical component which shown in Fig. 4.1e interacts with the radiation oscillating in the vertical plane. This complex wave function can be resolved trigonometrically into two steady amplitude waves which give rise to two lines in the spectrum at V R and V þ R (see Fig. 4.1f). Although the molecule vibrates at only one frequency, the rotational frequency varies, giving rise to two broad wings on the central band shown without fine structure. The total band cluster is shown in Fig. 4.1g as the sum of Fig. 4.1c and Fig. 4.1f. As shown in Fig. 4.1h and Fig. 4.1j, the CO2 molecule is

55

56

4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA

Change in dipole moment

(a)

Time

(b)

(c)

Vibrational

(g) Vertical component of dipole moment

CO2bending and rotating

(d)

(e)

V–R

Total band

V+R

Dipole moment change is perpendicular to the bond axis

(f) Vibration + – Rotation R is variable

(h) Change in dipole moment CO2 Out-of-phase stretching and rotating

(j)

(k) Total band

Dipole moment change is parallel to the bond axis

FIGURE 4.1

Classical treatment describing the IR gas phase contours of CO2. Although the molecule vibrates at only one frequency, the rotational frequency varies, giving rise to two broad wings on the central band shown without fine structure. In this case the CO2 molecule cannot rotate without rotating the dipole moment vector and the total band contour shown in (k) is a broad doublet with no central peak.

stretching out-of-phase. In this case, the CO2 molecule cannot rotate without rotating the dipole moment vector and the total band contour shown in Fig. 4.1k is a broad doublet with no central peak. Figure 4.2 shows the characteristic band clusters for three different vibrational modes of a para-substituted aromatic, an asymmetric top rotator. In larger molecules such as these, the rotational fine structure is typically not observed. The white arrows in the first two inplane vibrations and a white plus sign for the last out-of-plane vibration indicate the dipole moment changes. These vectors are each parallel to one of the axes of rotational moments of inertia as shown in the figure. The rotational moments in the condensed phase state molecules can and do interact and these interactions strongly effect the vibrational spectrum. In the liquid state, molecules may have any orientation relative to the spectrometer. Further, a molecule may be found in several conformations, all of which may have different spectra (e.g., rotational isomers). No rotational fine structure is observed and the resulting overall spectrum may consist of a large number of fairly broad bands. Here, hydrogen bonding can have a dramatic effect on the IR and Raman spectra. Typically, if the molecule is associated, the bands will also be broader than for the non-associated state.

57

HYDROGEN BONDING

A

O

O o

o

o

o

+o

o

o

+o

B o

o

O ~ 1013 cm–1

(a)

O

O - +-

+

-

o o

O

~1117 cm–1

~ 817 cm–1

(b)

(c)

+ +

FIGURE 4.2 Asymmetric top gas phase band contour (para-substituted benzenes). inertia are at a minimum at (a), intermediate at (b), and at a maximum for (c). In the bottom row are typical gas phase IR spectra showing the gas phase contour for a, b, and c+ type bands. The b type bands have no central peak. In a and c type bands, the relative intensities of the central peaks to the band wings vary with changes in the relative a, b, and c moments of inertia.

Lastly, in the solid state, the crystallinity (or lack of it) of the molecule can be quite important and the nature of the sample preparation techniques employed can also be important. Figure 4.3 shows the IR spectra of common table sugar (sucrose) prepared as a KBr disc and as a cast film from water and clearly illustrate the differences observed in the IR spectrum of a crystalline and amorphous material. The large number of sharp bands evident in the upper spectrum is characteristic of a crystalline solid. When sugar is prepared as a water cast film, it becomes an amorphous material resulting in broader less distinct bands seen in the lower spectrum. Both spectra have some water present. In general, crystalline solids can be frozen into one or more configurations fixed in a lattice resulting in sharper bands. Often, crystalline splitting of degenerate vibrations can occur and forbidden vibrations may result in weak bands. Vibrational spectroscopy can distinguish between different crystalline forms and quantitatively assay mixtures. This ability has been exploited for various applications.

2. HYDROGEN BONDING Both IR and Raman spectroscopies are very sensitive to the molecular interactions involved in hydrogen bonding.3 The hydrogen bond itself involves an association of a hydrogen atom from an XeH group of a molecule with the Y atom of another group (XeHeY).4,5 The X type atoms involved often have high electronegativities resulting in an

58

4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA

583 552

729 683

866 1124

3388

2124

Cast film, amorphous

3500

3000

2500

587

928 1053

3351

20

996

1134

40

1336 1265

1420

60

864 833

1648

80

2932

%Transmittance

100

1069 991 910

1347

2940

40 20

1461 1431

60

1279 1239

2723

80

1628

2476

KBr disc, crystalline solid

3563

%Transmittance

100

2000 1800 1600 1400 1200 1000

800

600

Wavenumber (cm–1)

FIGURE 4.3 The FT-IR spectra of common table sugar. The upper spectrum was prepared as a KBr disc while the lower spectrum was prepared as a water cast film on ZnSe.

XeH bond which is partially ionic while the HeY bond is suggested to be mostly electrostatic with some weak covalent characteristics. In general, hydrogen bonding to an XeH group results in a decrease in the XeH stretching frequency accompanied by a broadening and intensification of this band. Both OH and NH groups are found to show highly characteristic changes with hydrogen bonding. Figure 4.4 shows the OH stretches for two different phenolic OH stretches. The steric hindrance provided in 2,6-di-tert-butylphenol results in a strong sharp band at 3645 cm 1 characteristic of a non-hydrogen bonded OH group. Conversely, the easily accessible OH group of p-creosol results in a broad strong band at 3330 cm 1 characteristic of intermolecular hydrogen bonding between different phenolic OH groups. Intermolecular hydrogen bonding in chemical systems are very common. Examples of the condensed phase hydrogen bonded OH and NH stretching frequencies for carboxylic acid dimer and isocyanuric acid are shown below. Carboxylic Acid Dimer O R

H

O C

C O H

O

R OH Str. ~ 3000 cm–1

59

HYDROGEN BONDING

3024

60

O H O H R R

3083

80 OH (CH3)3C

C(CH3)3

40 3330

OH 3645

%Transmittance

100

20 “Free” OH

4000

Hydrogen-bonded OH

3800

3600

3400

CH3

3200

Wavenumber (cm–1)

FIGURE 4.4 The solid state IR spectrum of para-creosol and 2,6-di-tert-butylphenol illustrating the effect of hydrogen bonding on the OH stretching frequency.

Isocyanuric acid: Note that all of the NH and C¼O groups are involved in hydrogen bonding but only one set is shown for simplicity. O H

O

N C

C

N H

H N C

H

O

O

H

C N

N

C

C N

O NH Str. ~ 3209, 3080 cm–1 H

O

The carbonyl C¼O stretching frequency for the carboxylic acid dimer and the isocyanuric acid molecules also have a lower frequency and broader bandwidth as a result of hydrogen bonding. Additional changes in the vibrational spectrum of molecules involved in hydrogen bonding also occur. Bands involving XeH bending will usually increase in wavenumber upon formation of the hydrogen bond. The resultant changes in frequency and bandwidth are not as pronounced as observed in the XeH stretching region. Lastly, new low frequency bands can often be seen involving the stretching and bending of the HeY bond. The OHeO out-of plane hydrogen bend of the carboxylic acid dimer (960e875 cm 1) is a classic wellknown example of this. Intramolecular hydrogen bonding can have a dramatic effect on the IR and Raman spectra and typically favors formation of five- and six-membered rings. Conjugated intramolecular hydrogen bonding systems in particular can result in very strong hydrogen bonds. Examples of this include the keto-enol conjugated system which involves conjugated ketone and hydroxy groups. As shown below, a resonance structure can be drawn that places the

60

4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA

negative charge on the carbonyl oxygen and a positive charge on the oxygen carrying the hydrogen bonding proton. H O C

H O

O

C

C

CH

O C CH

Some examples of intramolecularly hydrogen bonded systems are shown below. 2-Hydroxy-4-(methoxy)-benzophenone H O

O C

OH Str. ~2900 cm–1 OC H3

2-(4,6eDiphenyle1,3,5etriazinee2eyl)e5 alkyloxy phenol

N N

N

H O

OH Str. ~ 2900 cm–1

OR

3. FERMI RESONANCE The vibrational spectrum of a molecule typically is considered to have isolated vibrational coordinates resulting fundamental vibrations. For example, carbon dioxide which has four total vibrations (3n 5) has two IR active vibrations, an out-of-phase stretch and a doubly degenerate deformation and a Raman active in-phase stretch. However, under certain conditions the vibrational levels are not well separated and can interact leading to phenomena such as Fermi resonance.6 Such interaction requires suitable symmetry for the two vibrations, an anharmonic nature of the vibration, and a similar energy for the two states (typically within 30 cm 1). The vibrations cannot be significantly separated within distinctly different parts of the molecule and they must mechanically interact in order for the two vibrations to mix. In most cases, Fermi resonance involves a fundamental that interacts with a combination or overtone band resulting in two relatively strong bands where only one nearly coincident fundamental is expected. Figure 4.5 shows the idealized vibrational spectrum for a stretching and bending vibration before (Fig. 4.5a) and after Fermi resonance along with the idealized energy level diagram. The idealized Fermi resonance shown involves the overtone of

61

FERMI RESONANCE

1.8 s

1.6

+2

δ

+ –

x

1.4

s

(b)

2

Intensity

1.2

δ

s

1.0 0.8 0.6 0.4

s

0.2

(a)

δ

0.0 3500

3000

2500

2000

Wavenumber

1500

1000

(cm–1)

FIGURE 4.5 Schematic diagram illustrating Fermi resonance. The idealized vibrational spectrum for a stretching and bending vibration are shown before (a) and after Fermi resonance (b). To the left of the idealized spectra is the energy level diagram.

2360

20 0.20

CO2 o. ph. str.

Fermi resonance doublet: CO2 i. ph. str. + CO2 bend overtone

0.15

1278

Raman intensity

CO2 bend

669

40

CO2 gas phase

Combination band: CO2 i.ph. str. + CO2 o. ph. str.

1385

60

2324

80

3600

100 3728

%Transmittance

a bending vibration (nd) with a symmetric stretching vibration (nS) that results in a classic Fermi resonance doublet that straddles the original symmetric band frequency. The Raman spectrum of carbon dioxide shown in Fig. 4.6 provides a classic example of Fermi resonance.6 Only one band for the CO2 in-phase stretch fundamental is expected (1332 cm 1) but a classic Fermi resonance doublet is observed at 1385 and 1278 cm 1 instead. The two Raman bands are a linear combination of the fundamental in-phase stretch and the CO2 bend overtone.

0.10

2 Χ 669 = 1338

1385 + 1278 = 1331 2

0.05 0.00 3500

3000

2500

2000 1800 1600 1400 1200 1000

800

600

Wavenumbers (cm–1)

FIGURE 4.6 1278 cm

1

The IR and Raman spectra of carbon dioxide. Note the Fermi resonance doublet at 1385 and resulting from interaction of the y ¼ 1 CO2 i.ph. str. and the y ¼ 2 (overtone) of the 669 cm 1 CO2 bend.

62

4. ENVIRONMENTAL DEPENDENCE OF VIBRATIONAL SPECTRA

TABLE 4.1 Selected Groups that Display Fermi Resonance Bands in their Vibrational Spectrum. Active Assignment

IR

Raman

1385, 1278

CO2 i. ph. str þ 2x CO2 bend

No

Yes

ReC(¼0)eH

~2820, ~2710

CH str. þ 2x CH i.ph. bend

Yes

Yes

N¼C¼O

~1300, ~1200

NCO i.ph. str. þ 2x NCO bend

Yes

Yes

ReCOOH

~3000e2650

OH str. þ 2x OH i. pl bend

Yes

Yes

~3000e2550

OH str. þ 2x C-O str.

Yes

Yes

CH2

~2920e2890

CH2 str. þ 2 x CH2 bend

Yes

Yes

AryleC(¼O)eCl

~1780, ~1730

C¼O str. þ 2x 880 aryl-C str.

Yes

Yes

Group

Bands (cm

CO2

L1

)

1

NhCeN¼C(NH2)2

2200, 2170

ChN str. þ (1250 þ925 cm ) Combination band

Yes

e

ReChCeR

2300, 2235

ChC str þ CeChC bend

No

Yes

C¼O

1810, 1780

C¼O str. þ 2x ~900 ring bend

Yes

e

AryleNH2

3355, 3205

NH2 i. ph. str. þ 2x NH2 bend

Yes

Yes

NH4Cl

2825

NH4 i. ph. str. þ 2x NH4 bend

Yes

Yes

e C(¼O)eNHeC

3300 (s), 3100(m)

MH str. þ 2x CNH str.-bend

Yes

Yes

Also noteworthy is that the combination band deriving from the bands involving the CO2 inphase str. and o. ph. str. are observed as two doublets in the 3728e3600 cm 1 region because of the Raman active CO2 in-phase str. Fermi resonance doublet. In general, Fermi resonance has several effects on the vibrational spectrum. As illustrated by the carbon dioxide case, the resultant bands are shifted from their expected frequency. Furthermore, the intensity is greater than expected for a simple overtone band. Lastly, because the frequencies of the fundamentals themselves are typically environmentally sensitive, changes in solvents or crystalline state can affect the observed Fermi resonance band intensities. Fermi resonance can be found in the IR and Raman spectra of a significant amount of functional groups.3 Table 4.1 summarizes some groups that display Fermi resonance in the vibrational spectrum, with their band frequencies, assignments, and activities.

References 1. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962. 2. Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. 3. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. 4. Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, California, 1960. 5. Bene, J. E.; Jordan, M. J. T. Int. Rev. Phys. Chem. 1990, 18 (1), 119e162. 6. Diem, M. Introduction to Modern Vibrational Spectroscopy; John Wiley: New York, NY, 1993.

C H A P T E R

5 Origin of Group Frequencies When a molecule contains a bond or group with a vibrational frequency significantly different from adjacent groups, then the vibrations are localized primarily at this group. Such characteristic IR and Raman bands for particular bond or group are called “group frequencies” and are extremely useful for both qualitative and quantitative analysis. In order to understand the origin of these group frequencies, coupled oscillator systems are presented here and related to actual chemical groups. These model systems are based upon a classical description of different general types of mechanical coupling of bond vibrations.1,2,3 This overview enables us to provide a framework to understand the sensitivity of vibrational spectra to molecular structure. Lastly, some general rules concerning mechanical coupling of vibrations are presented.

1. COUPLED OSCILLATORS The simplest coupled oscillator system to consider is the stretchestretch interaction of a linear triatomic molecule.1,2 Here, only two bonds with two stretching vibrations are present which will mechanically couple. The two-coupled oscillators exert forces on each other when they oscillate, resulting in both in-phase and out-of-phase stretching vibrations (see Fig. 5.1). The out-of-phase stretch is of higher frequency than the in-phase stretch. The key difference between the two vibrations is the displacement of the central atom. Here we label the central atom as Mc and the two equivalent external atoms as M. The out-of-phase (asymmetric) stretch involves movement of all three atoms (see Fig. 5.1a) and the two vectors associated with the central mass reinforce one another (i.e., cooperate), resulting in larger movement of the central atom during the vibration. Characteristic phasing of the out-of-phase stretch is that one bond stretches while the other contracts. We can regard the middle atom as split into two halves moving with the same frequency and phase. Hence we consider the central atom, Mc, as 2  (M/2). Thus, the two oscillators of M e (M/2) vibrate at 180 out-of-phase with each other. The result of this is that the frequency of this type of vibration is strongly dependent on the mass of the central atom. We can extend the classical vibrational frequency definition presented earlier to the out-of-phase stretching vibration by splitting the central mass in half and the simple expression is: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffi   1 2 K pffiffiffi 3 ¼ 1303 nop ¼ 1303 K þ m1 m2 m

63

64

5. ORIGIN OF GROUP FREQUENCIES

(a)

(b)

FIGURE 5.1 The stretching vibrations of a linear triatomic. The in-phase and out-of-phase stretches have been resolved into diatomic Me(N) and Me(M/2) components. The out-of-phase stretch is shown in diagram a and the in-phase stretch in diagram b.

where K is the force constant and gives the restoring force for unit displacement from the equilibrium position, and m1 and m2 are the masses for the linear triatomic in Fig. 5.1. For three equivalent masses nop simplifies to the last term in the above equation. In the case of the in-phase (symmetric) stretch, the central atom does not move (see Fig. 5.1b). This is equivalent to the two exterior masses being connected to an infinite mass (N) where both mdN systems oscillate with same frequency and phase. In this case, the two spring force vectors exerted on the central mass oppose one another, resulting in no movement of the central atom during the vibration. Here, the frequency of this vibration is independent of the mass of the central atom in the symmetrical molecule and can be expressed classically as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffi   1 1 K pffiffiffi 1 nip ¼ 1303 K þ ¼ 1303 m1 N m

Comparison of the above simple expressions predicts that the out-of-phase vibrations will be higher in frequency than the in-phase vibration. For a bent triatomic molecule, we find that the frequencies of the in-phase and out-ofphase stretching vibrations are a function of the bond angle, a, between the three atoms.1,2 We again consider the force vectors for the central atom m2 relative to the two exterior atoms, m1 and m3 for the in-phase and out-of-phase stretches. Figure 5.2 (lower left) shows the total restoring force tending to move the central atom along the right-hand bond. The restoring force vectors will by definition be opposite in sign from the Cartesian displacement vectors. For the in-phase mode, the forces act in opposition so there is less restoring force, while for the out-of-phase mode the forces cooperate resulting in more restoring force. The same analysis holds for the left-hand bond. The

65

COUPLED OSCILLATORS

Restoring forces for in-phase and out-of-phase vibrations m2 C

m2

C

X axis m3

S

m1

C m3

m1 Vector diagram for m2 atom

Cooperation

Opposition In-phase KT, ip= K (1 + cos )

Out-of-phase

KT, op = K (1 – cos )

FIGURE 5.2 The force diagrams for the central atom in a bent triatomic. The nature of the interaction of the two bond oscillators is a function of the bond angle, a. S and C indicate stretching and contraction of a bond and the bold arrows are the restoring forces. Both the in-phase (contract-contract) and the out-of-phase (stretch-contract) modes are shown.

resultant mechanical coupling tends to lower and raise the in-phase and out-of-phase frequencies, respectively relative to that of the m2em3 diatomic model. Taking into account the bond angle dependence, we can calculate the frequency for the inphase and out-of-phase stretching vibrations for bent triatomics. The out-of-phase stretch can be approximately calculated by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   1 1 cosa þ nop ¼ 1303 K m1 m2 where the bending force constant can be assumed to be zero. The in-phase stretch can be approximately calculated by: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   1 1 þ cosa nip ¼ 1303 K þ m1 m2 Here, we again neglect the bending force constant, but in this case it does contribute slightly. Figure 5.3 demonstrates the frequency dependence of triatomic in-phase and out-of-phase stretches on the bond angle, a. When a is greater than 90 , then the in-phase stretch is lower in frequency than the out-of-phase stretch, while a is less than 90 , then the in-phase stretch is higher in frequency than the out-of-phase stretch. Lastly, when a ¼ 90 the in-phase and outof-phase stretches are equal because there is no mechanical interaction (vectors are orthogonal). An important extension of the above discussion is to consider the effects of mechanical coupling as a function of chain length.1,2 This is important since it applies to

66

5. ORIGIN OF GROUP FREQUENCIES

linear aliphatic systems. Shown in Fig. 5.4 is a plot of vibrational frequencies for coupled CeC stretches of linear aliphatics as a function of increased chain length (i.e., 2,3,4. atoms) showing in-phase and out-of-phase vibrations. Standing waves are used to help describe the vibrations and their relative frequencies. For clarity, a plane (or node) separates the molecular segments, which vibrate out-of-phase with each other. In-phase (symmetric) Out-of-phase (asymmetric) Cooperation: raises frequency Opposition: lowers frequency

180

Bond angle ( )

opp

coop

90 opp coop

0

opp

coop

Wavenumber (cm–1)

FIGURE 5.3 The frequency dependence of the in-phase and out-of-phase stretches of a triatomic molecule as a function of the bond angle, a. The arrows shown are displacement vectors.

C

Wavenumbers (cm–1)

1200

S C

S C

C

S

1100

uncoupled

1000

900

S

S

S

S S 800

1 bond

2 bonds

3 bonds

Number of bonds

FIGURE 5.4 Plot of vibrations for coupled CeC stretch of linear aliphatics, as a function of increased chain length.

67

COUPLED OSCILLATORS

B

Wavenembers (cm–1)

B

B B

B

400

N B

B

O

Cooperation

300

O B

O 200

Opposition

2 bonds

3 bonds

B

B 4 bonds

Number of bonds

FIGURE 5.5 Plot of vibrations for coupled CeC bending vibrations of linear aliphatics, as a function of increased chain length. Note the out-of-phase vibration (opposition) is lower in frequency than the in-phase vibration (cooperation). Bending and open vibrations are indicated with B and O. No change in the bond angle is indicated by N.

Usually, the greater the number of nodes for stretching vibrations, the higher is the frequency. Figure 5.4 shows that a connection of several similar oscillators in linear chains results in as many chain frequencies as there are bonds. Thus, similar CeC, CeO, and CeN bonds will interact resulting in prominent IR and Raman bands. The relatively constant mechanical interaction gives results in group frequencies. Fortunately, for longer chains only a few of these vibrations provide important bands. We must also consider bending vibrations. The skeletal single-bond bending vibrations in aliphatic molecules vibrate at much lower frequencies than CeC stretches, CH stretch, or CH bending vibrations and generally fall in the far-IR region. The vibration involves mostly a deformation of the CeCeC bonds and the out-of-phase vibration is lower in frequency than the in-phase vibration. Figure 5.5 plots the vibrations for coupled CeC bending vibrations of linear aliphatics, as a function of increased chain length.

2. Rules of Thumb for Various Oscillator Combinations For oscillator combinations of equal or nearly equal frequency both in-phase and out-ofphase oscillator combinations are involved.1,2 This simplest case considers equal oscillator combinations and includes stretchestretch and bendestretch vibrations. Some examples are shown in Fig. 5.6 and Table 5.1.

68

5. ORIGIN OF GROUP FREQUENCIES

CH2 = C = CH2

H

1071 cm–1

CH2 = C = CH2

H

H N

3300 cm–1

1980 cm–1

H N

3370 cm–1

R

R

FIGURE 5.6 Examples of equal oscillator combinations. TABLE 5.1 XY2 Group

Examples of XY2 Type Identical Oscillators Out-of-phase stretch

In-phase stretch

ReNH2

3370 cm

1

3300 cm

1

ReCH2eR

2925 cm

1

2850 cm

1

ReCO2

1600 cm

1

1420 cm

1

ReNO2

1550 cm

1

1375 cm

1

ReSO2eR

1310 cm

1

1130 cm

1

Examples of some two identical oscillator groups (non-linear M1eM2eM1) shown in Table 5.1 include: two hydrogen atoms on groups such as a methylene or primary amine and two oxygen atoms on groups such as carboxylate, nitro or sulfones. The XY2 group in the above can be considered approximately as an isolated oscillator since it has stretching frequencies significantly higher than the frequencies of the CeC bond oscillators connected to it. As a result only the XY2 group atoms move appreciably during the stretching vibration. Because of this the above groups result in good group frequencies. The out-of-phase stretch is also an important IR group frequency for linear alkane alcohols, ethers, and primary and secondary amines.2 The vibration involves stretches of the CeC, CeO, or CeN bonds, which have very similar force constants. The frequencies shown below support this. CH3 992 cm 1 OH 1033 cm 1 NH2 1036 cm 1

CH3 CH3 CH3

Consequently, stretches involving CeC, CeO, and CeN will strongly couple giving rise to skeletal out-of-phase stretching vibrations in the following general spectral regions: C C C C

C O C N

Ow1050 cm Cw1125 cm Nw1080 cm Cw1140 cm

1 1 1 1

69

COUPLED OSCILLATORS

B

H

B X

S

Y

Amide CNH str./bend 1550 cm–1

S Cooperation: higher frequency

H

O

O X

S

Y

Amide CNH str./open 1250 cm–1

S

Opposition: lower frequency

FIGURE 5.7 The stretchingebending interaction for the HeXeY group. S stands for stretch, B for bend, and O for open. The approximate band position is also included for the amide group CNH stretch/bend and stretch open vibrations.

The CeCeO and CeOeC species in particular give good strong IR group frequencies as a result of a relatively constant interaction with other CeC bonds. Figure 5.7 shows the second type of important equal oscillator vibration, the bend-stretch vibration.2 Mechanical coupling of bending and stretching vibrations can occur if both are of nearly equal frequency. In this case, we consider a simple bent HeXeY oscillator where both X and Y are heavier than the hydrogen. The HeX bending frequency may be nearly identical to that of the XeY stretching frequency, allowing the two oscillators to couple. Two vibrations result involving the XeY stretch and the HeXeY bend which differ only in the relative phases. An example of this is the amide group where X ¼ nitrogen and Y ¼ carbon. The HeX stretching oscillator has much higher frequency than the XeY stretching oscillator and is therefore an isolated vibration (see below for rules for unequal oscillators). As shown above, there are three components for this molecular vibration: the rotation of the XeY bond, the stretching of the XeY bond, and the rotation of the HeX bond. Here, the rotation of the XeY bond has little effect since the vectors involved are orthogonal (i.e., 90 ) with respect to both the XeY stretch and the HeX rotation. However, rotation of the HeX bond results in movement of the X atom either in the same or in opposite direction that it moves during the XeY stretch. The bend/stretch involves cooperation of the X atom vectors

70

5. ORIGIN OF GROUP FREQUENCIES

1. High frequency: only atoms of high frequency component move appreciably. Cl C C C C C C N C O C H Os H s s s c C

s

C

C

2. Low frequency: only the bond length or angle of low frequency component changes appreciably. O C H C

s

C

O

C

C

H

s

s

C

N

O

s

C

Cl

s

C

FIGURE 5.8 Examples of unequal oscillator combinations.

and thus higher frequency while the bend/open involves opposition of the X atom vectors and thus lower frequency. Oscillators with different frequencies have two important approximate rules of thumb: 1. In high frequency vibration of the combination, only atoms of the high frequency component move. 2. In low frequency vibration of the combination, only the bond length or angle of the low frequency component changes. Some simple examples of typical unequal oscillator combinations encountered are shown in Fig. 5.8. Lastly, for systems containing more than two equal oscillators, the resulting vibrations can become much more complex and standing waves can be used to simplify the description of the vibrations.2 An excellent example of this is the stretching vibrations for a benzene ring. Figure 5.9 shows the standing vibrational waves employed to describe the stretching vibrations of six-membered rings.

S

C S C

S C

C

C S

C S

3a

C S

S

S C

1

2a

2b

S C

3b

FIGURE 5.9 Standing vibrational waves for stretching vibrations of six-membered rings.

S

4

71

COUPLED OSCILLATORS

Chapter 5: Questions 1. Using the mathematical expression for a harmonic oscillator, calculate the force constant, K, for the following species: CH 3000 cm 1, NH 3400 cm 1, OH 3600 cm 1. Note that H ¼ 1 amu, C ¼ 12 amu, O ¼ 16 amu, and N ¼ 14 amu. What are the possible sources of the observed variation in the frequencies for the above? 2. The following ethers have bands associated with the in-phase and out-of-phase CeO stretching vibrations: 1100, 810 cm 1 1080, 890 cm 1 1020, 990 cm 1 1220, 890 cm 1. Using a scatter plot of frequency versus bond angle, explain the origin of the observed trends. 1100, 810 cm-1

1080, 890 cm-1 O

O

1020, 990 cm-1 1220, 890 cm-1 O

O

3. Calculate the in-phase and out-of-phase stretching frequencies for six and four membered cyclohexane, cyclopropane, and cyclobutane rings. Here k ¼ 4.2 mdynes/ A and M ¼ 14 amu for the methylene group. The in-phase ring stretch frequency equation is: yip ¼ 1303½kð1 þ cosaÞðM1 þ 1=M2 ފ1=2 ¼ 1303½k=MŠ1=2 2cosa=2 Ring Size

a (deg)

In-phase stretch (exp)

N¼6

109.5

802

N¼5

104

886

N¼4

90

1003

Calculated

4. Calculate the in-phase and out-of-phase stretching frequencies for the following XY2 groups: ReNH2 (6.4 mdynes/A), ReCH2eR (4.9 mdynes/A). 5. Provide a summary of the general types of vibrations encountered. In general, where in the vibrational spectrum will these vibrations be found? 6. The bending vibration for propane is observed at 375 cm 1 while for butane it is observed at 427 and 271 cm 1. Draw the vibrations and explain the frequency differences. 7. Suggest other possible functional groups beyond those listed in table 5.1 that would fit the XY2 type identical oscillators.

72

5. ORIGIN OF GROUP FREQUENCIES

References 1. Infrared and Raman Spectroscopy, Methods and Applications; Schrader, B., Ed.; VCH: New York, NY, 1995. 2. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. 3. Barrow, G. M. Introduction to Molecular Spectroscopy; McGraw-Hill: New York, NY, 1962.

C H A P T E R

6 IR and Raman Spectra-Structure Correlations: Characteristic Group Frequencies In the following chapter we include generalized Infrared and Raman spectra of selected regions which illustrate the generalized appearance of bands for selected characteristic group frequencies. In each case, the upper spectrum shows the transmission infrared spectra with bands pointing downward and the lower spectrum shows the Raman spectrum with bands pointing up. The vertical axes are expressed in percent infrared transmission and Raman intensity. We also describe the approximate forms of selected vibrations and include tables summarizing important bands and their assignments.

1. XeH STRETCHING GROUP (X[O, S, P, N, SI, B) The set of spectra in Fig. 6.1, illustrates some useful XeH stretching bands. The large differences in XeH stretching frequencies are mostly due to differences in bonding and therefore the force constants. Quite often these bands are more intense in the IR than in the Raman spectra.1-4 As shown in the top two spectra in Fig. 6.1, the OH stretch is particularly weak in the Raman but strong in the IR. The hydrogen-bonded alcoholic OH stretch vibration gives rise to a broad band near 3350 cm 1 in the IR spectrum. A more strongly hydrogen-bonded OH in a carboxylic acid dimer has a much broader band centering about 3000 cm 1. An exception to this is the SH group of a mercaptan which gives rise to a sharp weak IR band and a strong Raman band at about 2560 cm 1. Aliphatic primary amines have a weak NH2 doublet in the IR near 3380 and 3300 cm 1 which derive from the out-of-phase and in-phase NH2 stretching vibrations, respectively. The weak shoulder observed near 3200 cm 1 is the Fermi-resonance enhanced overtone of the 1620 cm 1 NH2 bend. Typically, the in-phase NH2 stretch is stronger in the Raman spectra than the out-of-phase stretch. In primary amides, O¼CeNH2, bands involving the NH2 stretches are found at 3370 and 3200 cm 1, and are typically stronger in the IR than in the Raman spectra. The PH2 group gives rise to a mediumeweak single band in both the IR and Raman spectra near 2280 cm 1.

73

74

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

FIGURE 6.1 The generalized IR and Raman spectra of selected XeH stretching bands, where X ¼ oxygen, sulfur, phosphorus, nitrogen, or silicon.

Secondary aliphatic amines have a NH stretch band near 3300 cm 1 which is quite weak in the IR but stronger in the Raman spectrum. Monosubstituted amides, O¼CeNHe, have a stronger IR band near 3300 cm 1 which derives from the NH stretch. An additional weak band near 3100 cm 1 is the overtone of the 1550 cm 1 CNH stretch/bend. The last group shown is SiH2 species that has a single band at ca. 2130 cm 1 which is quite strong in the IR but weak in the Raman. Although not shown in Fig. 6.1, BeOH and BH/BH2 are characterized by moderate to strong IR OH and BH stretching bands but weak Raman bands. The BeOH group has a strong broad band between 3300 and 3200 cm 1 from the OH stretching vibration. The BH and BH2 groups have moderate to strong IR bands in the 2630e2350 cm 1 region. The BH2 group has both an out-of-phase (2640e2570 cm 1) and in-phase (2532e2488 cm 1) BH2 stretching bands. Fully annotated IR and Raman spectra of selected carboxylic acids (#24e27), amides (#30e32), alcohols (#40e47), boronic acid (64), and amines (#51e53) are included in the last section for comparison.

2. ALIPHATIC GROUPS The set of spectra in Fig. 6.2 illustrate some selected group frequencies for alkane groups.1-4 The group frequencies are also summarized in Table 6.1. Figure 6.3 illustrates the vibrations of the methylene (CH2)1-4 group. For the non-cyclic (CH2)n chain, the CH2 out-of-phase and the in-phase stretching bands are near 2930 and 2855 cm 1, respectively, and the CH2 bend (deformation) band is near 1465 cm 1. For the methylene segments (CH2)n in the trans zig-zag form,

ALIPHATIC GROUPS

75

FIGURE 6.2 The generalized IR and Raman spectra of linear and branched alkane groups. The spectral components from methyl and methylene groups have been separated in the top two rows of spectra. The bottom row illustrates the sensitivity of IR and Raman spectra to branching in alkane group.

the in-phase CH2 twist is observed near 1300 cm 1 in the Raman spectrum, and the in-phase (CH2)n rock is found near 723 cm 1 in the IR. Figure 6.4 illustrates the vibrations of the CH3 group. For the CH3 group on an alkane carbon, bands near 2960 and 2870 cm 1 derive from the out-of-phase and in-phase stretches and the out-of-phase bend results in a band at ca. 1465 in the IR. The in-phase bend of a single CH3 on an alkane carbon results in a band near 1375 cm 1 in the IR. The presence of an adjacent group such as a N or O atom can result in a significant frequency shift in the methyl (CH3) in-phase stretch to lower frequency between 2895 and 2815 cm 1. When there are two methyl groups on one carbon such as an iso-propyl or a gem-dimethyl group, this splits into two nearly equal intense bands near 1385 and 1365 cm 1 in the IR. When there are three methyl groups on one carbon, such as a tertiary-butyl group, there is further splitting into a weak band at ca. 1395 cm 1 and a stronger band near 1365 cm 1 in the IR spectrum. In addition, for these groups there are strong Raman bands at ca. 800 cm 1 which involves the in-phase stretch of the C(C3) group and at ca. 700 cm 1 for the in-phase stretch of the C(C4) group. However, although intense, this Raman band will vary considerably in frequency due to strong mechanical coupling with attached CeC, CeO, CeN or CeS groups. Table 6.2 summarizes this variation for a few selected

76

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.1 Selected Group Frequencies: Aliphatics Intensities1 Group CH3

CH2

CH

Assignment 2

Frequency (cmL1)

IR

R

ReCH3

o.ph. str.

2975e2950

vs

vs

”

i.ph. str.

2885e2860

vs

vs

”

o.ph. bend

1470e1440

ms

ms

”

i.ph. bend

1380e1370

m

vw

ReCH(CH3)2

bend-bend

1385e1380

m

vw

”

bend-open

1373e1365

m

vw

R(CH3)3

bend-bend

1395e1385

m

vw

”

bend-open

1373e1365

ms

vw

aryl-CH3

i.ph. str. þ

2935e2915

ms

ms

”

bend overtone

2875e2855

m

m

ReOeCH3

i.ph. str.

2850e2815

m

m

”

i.ph. bend

1450e1430

ms

m

R2NeCH3

i.ph. str

2825e2765

s

s

O¼CeCH3

i.ph. bend

1380e1365

s

w

ReCH2eR

3

o. ph. str.

2936e2915

vs

vs

”

i. ph. str.

2865e2833

vs

vs

”

Fermi resonance

2920e2890

w

m

”

bend

1475e1445

ms

ms

(CH2)>3

i.ph. twist

1305e1295

e

m

(CH2)>3

i.ph. rock

726e720

m

e

O¼CeCH2

bend

1445e1405

m

m

NhCeCH2

bend

1445e1405

m

m

OeCH2

wag

1390e1340

m

m

CleCH2

wag

1300e1220

m

m

R3CH

CH bend

1360e1320

w

w

O¼CeH

CH str. þ

2900e2800

m

m

”

rk overtone

2770e2695

m

m

”

CH rock

1410e1380

m

m

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero o.ph. str. ¼ out-of-phase stretch, i.ph. str. ¼ in-phase stretch, rk ¼ rock 3 Note: In the trans zig-zag segments of the long chain (CH2)>6 group, the Raman active CH2 out-of-phase stretch band is at 2900e2880 cm (2883 cm 1 in polyethylene). The gauche segments have this at 2936e2916 cm 1 in the Raman. 2

1

77

ALIPHATIC GROUPS

Nodal plane

S

O

C O

O

CH2 Out-of-phase stretch

O

O O

OO

O O

OO S

O

O

S

O

O

CH2 In-phase stretch

O –

O

CH2 Wag

O

O

CH2 Twist

B

O

CH2 Bend

O + O

O

O

O

O

O

O O

CH2 Rock

FIGURE 6.3 Vibrations of the CH2 group. C and S stand for contract and stretch, and B and O stand for bend and open. Movement of atoms are depicted by arrows or by þ and Nodal planes are used to help differentiate the vibrations.

signs if the atom movement is out of the page.

compounds. Fully annotated IR and Raman spectra of selected aliphatic compounds are included in the last section of fully interpreted spectra for comparison (#1e3). In Fig. 6.5, the relative intensities in the IR and Raman spectra in CH stretching region are illustrated for some selected alkanes. The vibrations involving the methyl and methylene stretches (3000e2800 cm 1) give rise to bands that are strong and characteristic in both the IR and Raman spectra but show greater complexity particularly in the Raman spectra due to Fermi-resonance effects.5,6,7 The CH stretching region of the IR and Raman spectra shows additional bands due to Fermi resonance between the methylene CH stretching vibrations and the overtone of the methylene bending (deformation) vibrations (1480e1440 cm 1). Bands observed in the 2900 cm 1 region as a weak shoulder in the IR spectrum and as more isolated and obvious bands in the Raman spectra derive from Fermi resonance. This occurs in the IR and Raman spectra of aliphatics when: 1. There are at least two almost equivalent neighboring methylene groups. 2. The symmetry and frequency of the methylene CH2 stretch and the CH2 deformation overtone is suitable for Fermi resonance. The above results in multiple Fermi-resonance doublets associated with the methylene group including the broad bands observed in the 2920e2898 cm 1 region and results in significantly more complicated Raman spectra of n-alkanes as observed in Fig. 6.5.

78

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

Nodal plane

O

c

O

c

O

O

O O

s

o O

o

O

O O

O

O

B O

O O

o

B

CH3 In-phase bend

CH3 Out-of-phase bend

O o

O O

B

o

O

O

O

O

O

O

O O

O O

O

O

O

o O

s

o

O

O

s

CH3 In-phase stretch

O

O

O

s

CH3 Out-of-phase stretch

B

s O

O

O

O

s O

O

O

O

B

CH3 Rock

CH3 Torsion

FIGURE 6.4 Vibrations of the CH3 group. Nodal planes are used to differentiate between doubly degenerate vibrations. C and S stand for contract and stretch, and B and O stand for bend and open.

The conformational equilibrium of n-alkanes due to rotation about the CeC bond of the long chain aliphatic can also affect the IR and Raman spectra in the gas phase or in pressure induced transitions. Thus, for n-Pentane these conformations would include methylene in the all-trans (TT a planar zig-zag form), single gauche (TG), and double gauche (GG) conformers.8 The methylene and methyl stretching bands are sensitive markers of these different conformers. The all trans conformation is typically found in the condensed state under ambient room temperature conditions such as shown in Fig. 6.5. The relative IR intensities of the methyl and methylene bands can be used to estimate the relative concentration of the two species. In the case of the IR spectra in Fig. 6.5, the methyl (CH3) out-of-phase stretch at ca. 2960 cm 1 is doubly degenerate and is therefore twice as intense at the methylene (CH2) out-of-phase stretch at 2925 cm 1. Using this relationship, the relative amount of methyl and methylene group can be rapidly estimated from the IR spectrum.

79

CONJUGATED ALIPHATICS AND AROMATICS

TABLE 6.2

Summary of the Raman In-Phase CeC/CeX Stretching Band for Selected Compounds, where X is Carbon, Nitrogen, Oxygen or Sulfur

X-Element

CeCeCeX (i.ph.str.)

Carbon (12)

827 cm

(C)2CeX (i.ph. str.)

1

800 cm

1

(C)3CeX (i. ph. str.) 733 cm

CH3

CH3

CH3CH2CH2CH3 H3C

868 cm

1

802 cm

CH3

H3 C

CH

CH3

CH3

Nitrogen (14)

1

1

748 cm

1

CH3

NH2

CH

H2N

CH3 H3C

NH2

CH3

Oxygen (16)

822 cm

1

819 cm

1

CH3

750 cm

CH3

CH3 OH

HO

CH

1

H3C

OH

CH3 CH3

Sulfur (32)

670 cm

1

631 cm

1

CH3 SH

HS

CH CH3

3. CONJUGATED ALIPHATICS AND AROMATICS Selected IR and Raman characteristic frequencies of conjugated aliphatics and aromatics are summarized in Table 6.3.1-4 Included are triple and cumulated double bond species, olefinics, and aromatics. Three sets of generalized IR and Raman spectra also illustrate some of the characteristic bands for these species. Below, we discuss olefinics, triple and cumulated double bonds and aromatics in more detail.

80

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

100 CH3

CH

80

CH2

CH2

CH3

2852 2930

2925

2853

2875 2853

Cyclohexane CH2 2938 2924

2936 2898

2937 2904

2962

2897

2962

2963

1.0

CH2

2863

2875

2875 2863

2938 2917

2876 2935 2914

CH2

n-Octane

CH2

2963

2.0

n-Heptane

n-Hexane

n-Pentane

3.0

2959 2926

2874 2860 2962 2926

2938 2966

20

2959

2874 2858

60

2856

2873

CH2

2879

%Transmittance

CH2

40

Relative intensity

CH3

CH3

CH3

CH3

CH3

CH3

CH2 0.0

Wavenumber (cm–1)

FIGURE 6.5 The infrared and Raman spectra in the CH stretching region are illustrated for n-pentane, n-hexane, n-heptane, n-octane, and cyclohexane. Additional bands due to Fermi resonance are clearly observed in the Raman spectra. TABLE 6.3 Selected Group Frequencies: Triple Bonds, Cumulated Double Bonds, Conjugated Aliphatics and Aromatics Intensities1 Group

Assignment

Frequency (cmL1)

IR

R

Triple

CeChCeH

CH str.

3340e3267

s

w

Bonds

”

ChC str.

2140e2100

w

vs

”

CH wag

710e578

s,br

w

CeChCeC

ChC str.

2245e2100

e

s

CH2eChN

ChN str.

2260e2240

m

vs

”

CH2 bend

1440e1405

m

m

conj-ChN

ChN str.

2235e2185

var.

s

SeChN

ChN str.

2170e2135

ms

s

81

CONJUGATED ALIPHATICS AND AROMATICS

TABLE 6.3

Selected Group Frequencies: Triple Bonds, Cumulated Double Bonds, Conjugated Aliphatics and Aromaticsdcont’d Intensities1

Group

Assignment

Frequency (cmL1)

IR

R

Cumulated

>C¼C¼CH2

CCC o.ph. str.

2000e1900

vs

vw

Double

eN¼C¼O

NCO o.ph. str.

2300e2250

vs

vw

Bonds

eN¼C¼S

NCS o.ph. str.

C¼C Alkane Subst.

C¼C mono, cis, 1,1

1

vs

mw

1660e1630

m

s

2

C¼C str.

C¼C trans, tri, tetra

C¼C str.

1680e1665

weo

s

C¼CHeR mono, cis, trans

CH str.

3020e2995

m

m

C¼CH2 mono, 1,1

CH2 o.ph. str.

3090e3075

m

m

”

CH2 i.ph. str.

3000e2980

m

s

”

CH2 bend

1420e1400

w

m

ReCH¼CH2

trans CH i.ph. wag

”

Aromatics

2200e2000

2

995e985

s

w

3

910e905

s

w

3

¼CH2 wag

R2C¼CH2

¼CH2 wag

900e885

s

w

ReCH¼CHeR trans

¼CH i.ph. wag

980e965

s

e

ReCH¼CHeR cis

¼CH i.ph. wag

730e650

ms

e

ReCH¼CR2

¼CH wag

840e790

m

aryl CH

CH str.

3100e3000

mw

s

ring

quadrant str.

1620e1585

var

m

”

quadrant str.

1590e1565

var

m

”

semicircle str.

1525e1470

var

vw

”

semicircle str.

1465e1400

m

vw

mono, meta, 1,3,5

2,4,6 radial i.ph str.

1010e990

vw

vs

meta, 1,2,4 & 1,3,5

lone H wag

935e810

m

e

para and 1,2,4

2 adj. H wag

880e795

s

e

meta and 1,2,3

3 adj. H wag

825e750

s

e

ortho and mono

4 & 5 adj. H wag

800e725

s

e

mono, meta, 1,3,5

ring out-of-plane bend

710e665

s

e

para

ring in-plane bend

650e630

e

m

mono

ring in-plane bend

630e605

w

m

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero Conjugation lowers C¼C frequencies 10e50 cm 1 (e.g., NhCeCH¼CH2 1607 cm 1) 3 Electron donor substituents lower the ¼CH2 wag frequencies (e.g., ReOeCH¼CH2 813 cm 1) and electron withdrawers raise them (e.g., NhCeCH¼CH2 960 cm 1) 2

82

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

3.1. Alkyl-Substituted Olefinic Groups The set of IR and Raman spectra in Fig. 6.6 illustrate selected group frequencies for the ethylene C¼C groups with alkane group substituents.1-4 Particularly, useful Raman bands for the olefinic group include the ¼CH stretch, in-phase ¼CH2 stretch, and the C¼C stretch. In the IR spectra, the olefinic out-of-plane CH and CH2 wag are quite important. Figure 6.7 illustrates the in-plane olefinic vibrations and Fig. 6.8 summarizes the observed and estimated C¼C stretching frequencies for fifteen different compounds containing an olefinic group. The out-of-plane CH wag vibrations are shown in Fig. 6.9. The in-plane vibrations of the olefin group are depicted in Fig. 6.7. The vinyl and 1,1-disubstituted ethylenes have a ¼CH2 group with ¼CH2 out-of-phase and in-phase stretching bands near 3080 and 2985 cm 1, respectively. The ¼CH group in vinyls, cis, trans 1,2-disubstitituted ethylenes, and trisubstituted ethylenes all have a ¼CH stretching band at ca. 3010 cm 1. The vinyl, 1,1-disubstituted, and 1,2-cis disubstituted ethylenes all have a C¼C stretch with strong intensity in the Raman and medium intensity in the IR. In the case of 1,2 trans disubstituted, tri-substituted, and tetra-substituted ethylene groups, the C¼C stretch at ca. 1670 cm 1 is quite strong in the Raman but weak or absent in the IR. The frequency of the C¼C stretch will vary depending on the nature of the substituent. Conjugation effects tend to be particularly important, usually lowering the C¼C stretching frequency. The selected compounds in Fig. 6.8 vary in the cyclic olefinic structures and most importantly the CeC¼C bond angle. Mechanical coupling of the stretching vibrations of bonds attached to olefinic groups is the critical variable in determining C¼C stretching fequencies in cyclic olefin species.1 The calculated frequencies are from an adapted form of the triatomic model discussed earlier and includes mechanical coupling of the CeC stretch

FIGURE 6.6 Generalized IR and Raman spectra of alkyl-substituted olefins.

83

CONJUGATED ALIPHATICS AND AROMATICS

O

O

O

O

C=C str

O

1662–1631 cm–1

1692–1665 cm–1

O

O O

CH2 Out-of-phase str.

3080 cm–1

CH2 In-phase str.

2990 cm–1

CH Str.

3020 cm–1

CH2 Bend

1415 cm–1

Cis CH rock

1415 cm–1

O

O O

O

O

O

O

O O

O

O O

R–CH=CH-R Trans R2C=CH–R R2C=CR2

O

O

R–CH=CH2 R2C=CH2 R–CH=CH-R Cis

O O

FIGURE 6.7 The olefin in-plane vibrations are shown above. Arrows depict the movements of atoms.

with the C¼C stretch. The model uses force constants for the C¼C and attached CeC of 8.9 and 4.5 mdynes/A, respectively, neglects the bending force constants, and includes the CeC¼C bond angle. The approximate form of the model describing the total forces on the olefinic group for the external and internal C¼C group are shown on the left side of Fig. 6.8. Despite their simplicity, these calculations are successful in predicting trends in the frequencies in the top two rows. This data demonstrates that little or no change in the C¼C force constant is required to explain the observed frequency shifts when one or both of the olefinic carbon atoms are part of a ring. No calculations are shown for the bottom row, but the experimental data indicates that for ring sizes between four and six, the effects of the indicated changes in both the external and internal CeC¼C bond angles approximately cancel each other, resulting in similar C¼C stretching frequencies. The critical importance that mechanical effects has in determining the C¼C stretching frequencies in cyclic olefin structures has similarly been demonstrated using vibrating ball bearing and coil spring molecular models.9

84

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

F2

H

F1

H

H C

H

H

C

120

H

H

C 135

126

H

H

H C

C 150

180 C

F2

C H Calc (cm–1) 1639

1661

1696

1756

Obs (cm–1) 1651

1657

1678

1736

H 120

F1 F2

H

H 108

H

H

H

H

90

1957

H H C C H 0

60

F2 Calc (cm–1) 1636

1606

1587

1636

1776

Obs (cm–1) 1646

1611

1566

1656

1974

R 120 R 120

R 126 R 108

Obs (cm–1) 1681

1678

R 135 R

150

R

90

H

R

60 1675

1883

180 R

C C R 0 2250

FIGURE 6.8 The cyclic C¼C stretching frequencies as a function of the CeC¼C bond angles. The calculated frequencies used force constants F1 and F2 values of 8.9 and 4.5 mdynes/A, respectively. The simple model used to calculate the frequencies is summarized on the left-hand side of the figure.

+

–

O

O

–O

O

Vinyl trans CH wag

+–

O

O

O

+

O

O

+

1,2-Trans-disubst trans CH wag

+–

O

+

+O

O

O

O

O

O

+ – O

O

O

O

+

O+ O

–

Vinyl cis CH wag

Vinyl =CH2 wag

+O

O–

+ – O

+O

O–

O

–O

O

1,1-disubst =CH2 wag

+

+– O

+

O

O

O

O–

+

O+

1,2-Cis-disubst Cis CH wag

FIGURE 6.9 The out-of-plane CH wag vibrations of the olefinic group are shown above. Movement of atoms are depicted by arrows or by þ and signs.

The CH wag bands (see Fig. 6.9) are strong in the IR and highly characteristic of the alkyl substitution on the ethylene C¼C group. These alkyl-substituted olefinic CH wag bands are observed near 990 and 910 cm 1 for vinyl groups, near 890 cm 1 for 1,1-disubstitution, near 970 cm 1 for trans 1,2-disubstitution, and roughly at 680 cm 1 for cis 1,2-disubstitution. The nature of the substituent can result in large but predictable frequency shifts of the olefinic CH and CH2 wag. The ¼CH2 wag is sensitive to electronic changes and shows a linear frequency dependence based upon experimentally determined substituent Hammet constant as well as

85

CONJUGATED ALIPHATICS AND AROMATICS

0.8

–COOH –CHO –CN

O s–trans –C–CH3 s–cis –CF3 –phenyl O –C–OCH3

O –O–C–CH3

0.2 0.0 – 0.2

–F

–CH3

–O–CH3 Cis Gauche

– 0.4 800

820

840

=

=

0.4

=

Hammet constant

0.6

860

880

900

920

940

960

980

Wavenumber (cm–1)

FIGURE 6.10 Correlation of the Hammet constant of substituents of mono-substituted ethylene with the CH2 wag frequency.

the calculated substituent electronegativity and the electron density on the carbon atom.10,11 Figure 6.10 shows the linear dependence for the mono-substituted ethylene ¼CH2 wag as a function of the intrinsic electronic contribution to the Hammet values.11 These well-defined trends of the frequency dependence of the CH wag vibrations on the substituent properties are also observed in more highly substituted olefinic groups.1 For example, substitution of an electron withdrawing ester group (eCOeOeR) in the XYC ¼CH2 species will raise the frequency of the CH2 wag. Similarly, substitution of an electron donating ether group (eOR) will lower the frequency. The trans CH wag shows a predictable frequency dependence upon the substituent electronegativity. An increase in the substituent electronegativity relative to an aliphatic will lower the CH wag frequency. Fully annotated IR and Raman spectra of selected olefinic compounds are included for comparison in the last section (#4e6, and 31e32).

3.2. Triple and Cumulated Double Bonds Figure 6.11 illustrates some of the characteristic bands for ChC, ChN, and the X¼Y¼Z groups.1-4 The CeChCeH group has a CH stretch band near 3300 cm 1 and a hCeH wag band near 630 cm 1 which are both strong in the IR and weak in the Raman. The ChC stretch band is found at ca. 2125 cm 1, which is weak in the IR but strong in the Raman. The symmetrically substituted ChC stretching vibration is IR inactive but Raman allowed. For the dialkyl acetylenes, the Raman spectra usually has two bands near 2300 and 2230 cm 1 resulting from Fermi resonance splitting of the ChC stretch. For the diaryl acetylenes, only a single ChC stretch band is observed in the Raman near 2220 cm 1. The ReChN group results in IR and Raman ChN stretch bands near 2240 cm 1 which can be lowered by roughly 20 cm 1 by conjugation with a suitable substituent. The eSeChN

86

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

FIGURE 6.11

Generalized IR and Raman spectra of selected triple and cumulated double bond compounds.

group is characterized by IR and Raman bands near 2150 cm 1 which involve the ChN stretch. The eN¼C¼O, eN¼C ¼S, and eN¼C¼N e groups all have very strong IR bands in the 2280e2050 cm 1 region resulting from an out-of-phase stretch of the X¼Y¼Z bonds. Fully annotated IR and Raman spectra of two selected nitrile containing compounds as well as an acetylene compound are included in the last section for comparison (#7e9).

3.3. Aromatic Benzene Rings The twenty different normal modes of benzene are shown in Fig. 6.1212. A simple substitution of only one substituent results in a significant increase in the modes of vibration to thirty. Table 6.4 provides the calculated and experimental frequencies along with the unique Wilson numbers and accompanying symmetry for the modes of vibration for benzene.13 As shown in Fig. 6.12, the benzene CeH stretching vibrations are observed in the 3100e3000 cm 1 region. In all other vibrations below 3000 cm 1 the CeH bond length remains relatively unchanged. Many of the bands observed in the 1600e1000 cm 1 region involve in-plane CeH bending vibrations that interact with various ring C¼C vibrations. Out-of-plane CeH bending (i.e., wag) vibrations give rise to bands below ca. 1000 cm 1. In general, many of the aromatic ring vibrational modes are insensitive to the substituent groups and can be regarded as good group frequencies for the benzene ring. Some of the vibrations are substituent sensitive and result in mechanical coupling with the substituent group vibrations and can provide important structural information. However, not all of these

87

CONJUGATED ALIPHATICS AND AROMATICS

20a

2

3064 cm-1

3073 cm-1

1350 cm-1

1482 cm-1

9b

9a

1309 cm-1

19b

1482 cm-1

1599 cm-1

14

3

3056 cm-1

19a

8b

1599 cm-1

3057 cm-1

7b

3056 cm-1

3064 cm-1

8a

13

7a

20b

15

1146 cm-1

1178 cm-1

1178 cm-1

+

-

18b

18a

5

1

12

+

+

-

+

-

+

17a

+

-

+

967 cm-1

1010 cm-1

1037 cm-1

1037 cm-1

-

+ 17b

-

+

-

967 cm-1

-

+ 10a +

+

993 cm-1

-

-

-

+ 10b

+

-

+

+

+

+

6a

11

-

-

6b

673 cm-1

606 cm-1

606 cm-1

+

-

-

+

-

-

+

16a

- + - + -

+

4

+

+

-

707 cm-1

846 cm-1

846 cm-1

-

990 cm-1

- + -

398 cm-1

+ - + - 16b +

+

-

398 cm-1

FIGURE 6.12 Approximate vibrational modes of benzene adapted from (M.A. Palafox, Int. J. Quant. Chem., 77, 661e684, 2000). The electronic structure was determined using density functional method with a 6e31G* basis set. Out-of-plane molecular vibrations are depicted using þ and to indicate out of and into the page, respectively.

88

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.4 Calculated and Experimental Frequencies for the Normal Ring Modes of Benzene Frequency (cmL1) Wilson no.

Symmetry

Calc

IR (Liquid)

IR (Gas)

Description

1

a1g

1026

993

993.1

d (CCC)

2

a1g

3218

3073

3073.9

n (CeH)

3

a2g

1375

1350

1350

d (CeH)

4

b2g

719

707

707

g (CCC)

5

b2g

1015

990

990

g (CeH)

6

e2g

618

606

608.1

g (CCC)

7

e2g

3192

3056

3056.7

n (CeH)

8

e2g

1664

1599

1600.9

n (C¼C)

9

e2g

1203

1178

1177.8

d (CeH)

10

e1g

866

846

847

g (CeH)

11

a2u

694

673

673.9

g (CeH)

12

b1u

1013

1010

1010

d (CCC)

13

b1u

3182

3057

3057

n (CeH)

14

b2u

1378

1309

1309.4

n (C¼C)

15

b2u

1178

1146

1149.7

d (CeH)

16

e2u

412

398

398

g (CCC)

17

e2u

978

967

967

g (CeH)

18

e1u

1071

1037

1038.3

d (CeH)

19

e1u

1524

1512

1483.9

n (C¼C)

20

e1u

3208

3064

3064.4

n (CeH)

The data is from (International J of Quantum Chemistry, 77, 661e684, 2000) and includes calculated frequencies used density functional method with a 6e31G* basis set and experimental values

vibrations are of use since many are either weak or absent in either the IR and Raman spectra. The vibrational modes of the aromatic benzene ring that are sensitive to the mass and electronic properties of ring substituents include the following: 1. 1300e1050 cm 1 region, involving in-plane movement of the ring carbons and the substituents. 2. 850e620 cm 1 region, involving the CH wag vibrations (intense in IR but weak in Raman) and ring out-of-plane vibrations. Some of the characteristic group frequencies for mono- and di-substituted benzenes are illustrated in the generalized IR and Raman spectra depicted in Fig. 6.13.1-4,14 Many different types of aromatic ring vibrations are illustrated which demonstrate how IR and

CONJUGATED ALIPHATICS AND AROMATICS

89

FIGURE 6.13 Generalized IR and Raman spectra of mono- and di-substituted benzenes. The complex ring vibrations can be greatly reduced using a standing wave description. This technique allows easy visualization of the form of the ring vibrations.

Raman spectra can be used to characterize aromatic substitution types. Useful group frequencies include the following: • • • • •

CH stretch bands Ring stretch bands Ring bend bands CH rock and CH wag bands Aromatic summation bands

Figure 6.14 shows the form of the 12 elementary vibrations for a six-membered ring with equal bond lengths, masses, and no substituents. The vibrational standing waves for these vibrations are also shown.1,4 The top row illustrates the six ring stretching vibrations and the bottom row the six bending vibrations for this simple six-membered ring. The dashed lines show the nodal lines used to define the sextant , quadrant, semicircle, or whole ring vibrations. These lines separate molecular segments which vibrate out-of-phase with each other. The doubly degenerate ring modes are labeled with subscripts “a” and “b”. Bands originating from ring vibrations in the IR and Raman spectra shown in Fig. 6.13 are annotated using the above description. The bottom row indicates the accompanying motion of the attached hydrogens in benzene which mechanically couple with the above ring modes. The aryl CH stretch bands are observed in the 3100e3000 cm 1 region. The IR spectrum usually has several bands here while the Raman usually has only one. The ring quadrant stretch vibration has two components observed in the IR and Raman spectra near 1600

90

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

S

C S S C C S

C

4

C S S C

C S

3a

3b

2a

O

B O O B

– + – + – +

S

C

OBO B O B

BO B

6

5a

5b

S C

S

2b

1

– –

8

+ +

– –

+ – – +

7a

7b

Modes 1 & 6 mix in mono, meta- and 1,3,5 substituted ring

1-6 Attached hydrogen motions in benzenes

1+6

_

H

H

H

H

H

+H

C

C

C

C

C +

C +

Above ~3000 Below ~3000 CH stretch Carbon radial

Above ~1250 Below ~1250 CH bend Carbon tangential

Above ~710 Below ~710 CH wag Carbon out-of-plane

FIGURE 6.14

The twelve elementary ring vibrations for a six-membered ring with equal bond lengths, masses, and no substituents are illustrated with vibrational standing waves. The elementary stretching vibrations are shown in the top row and the in-plane and out-of-plane bending vibrations are shown in the second row. For rings of suitable symmetry, mixing of modes 1 and 6 results in vibrations shown in row three, which provide very strong Raman bands. The fourth row indicates the attached hydrogen motions in benzene which couple with the ring modes. Dotted lines are used to indicate nodal lines.

and 1580 cm 1. Identical para substituents provide an exception, where no quadrant stretching bands are seen in the IR spectrum since they are forbidden by symmetry. The ring semicircle stretch has two bands in the IR near 1500 and 1450 cm 1 except in the para-substituted benzene where they are observed near 1515 and 1415 cm 1 in the IR. In mono and meta isomers, there is a very strong Raman band near 1000 cm 1. This ring vibration involves movement of the 2,4,6 carbons radially in-phase and is a result of mixing of the whole ring stretch with the sextant in-plane ring bend. In the mono- and meta- substituted benzenes, there is a strong IR band near 690 cm 1 called the sextant out-of-plane deformation or ring pucker, where the 2,4,6 and 1,3,5 sets of carbons move oppositely out-of-plane. 3.3.1. Aryl CH Wag The infrared spectrum in the region from ca. 900 to 600 cm 1 have intense bands that are highly characteristic of the number of hydrogen atoms present on the aromatic ring. The number of IR bands and their frequencies of vibrations involving the CH wag are significantly determined by the number of adjacent hydrogen atoms present on the benzene ring. To a lesser extent, there is some dependence of the CH wag frequency on the electron

91

CONJUGATED ALIPHATICS AND AROMATICS

TABLE 6.5 Selected IR Bands Useful for Substitution Determination of Aromatic Rings in the 950e680 cm Spectral Region

1

Substituent st

No. of Subst

Pattern

1 CH Wag

2nd CH Wag

Ring sextant out plane def

1

Mono

780e720

5 Adj CH wag

NA

710e690

2

Ortho (1,2)

820e720

4 Adj CH wag

NA

inactive

2

Meta (1,3)

950e880

lone H wag

870e730

2 Adj H wag

740e690

ring pucker

ring pucker

2

Para (1,4)

860e790

2 Adj H wag

NA

inactive

3

1,2,3

805e760

3 adj H wag

NA

720e680

ring pucker

3

1,2,4

900e870

lone H wag

800e760

720e680

ring pucker

3

1,3,5

865e810

lone H wag

NA

730e680

ring pucker

4

1,2,3,4

860e800

2 adj H wag

NA

inactive

4

1,2,3,5

850e840

lone H wag

NA

720e680

4

1,2,4,5,

870e860

lone H wag

NA

inactive

6

1,2,3,4,5,6

NA

NA

inactive

2 Adj H wag

ring pucker

donating or withdrawing characteristics of the substituent which effects the total electron density of the aromatic ring carbon atoms. Table 6.5 summarizes some of the more important and intense IR bands in this region. Complications in this spectral region can occur due to mechanical coupling of the CH wag with the bending vibrations of substituents such as eNO2, carboxylic acids, and acid salts.1-4 Some of the less intense CH wag bands are not included in table 6.5 despite the fact that they are found in a relatively small frequency range. The mono-substituted vibrational mode shown in Fig. 6.15 is found at ca. 900 cm 1 and is an example of a weaker band that remains useful but is not included in Table 6.4. The range of this band is between 940 and 860 cm 1 and the frequency varies as a function of the substituent. Although this band can be used as a group frequency for structural determination, it is of greater use to demonstrate the linear frequency dependence of the band upon the calculated carbon electron density.15 The frequency of the mono-substituted benzene ring CH wag (900 cm 1) shows a welldefined linear dependence on the experimentally determined Hammet values and the calculated electron density of the ring carbon atoms. The linear frequency dependence of this band upon the calculated electron density of the benzene ring para carbon atom for monosubstituted benzene ring is shown in Fig. 6.16. In many cases, the band positions for other aromatic CH wag vibrational modes listed in Table 6.5 can be estimated using an empirical expressions and a table of the mono-substituted aromatic band values (w900 cm 1 mode). Detailed examples of this type of analysis can be found in the literature (N.B. Colthup, Applied Spectrosc., 30, 589, 1976).1,4,15 Fully annotated IR and Raman spectra of selected aromatic compounds are presented in the last section for comparison and include #10e15, 27, 45, 57, 64e69, 72, and 77.

92

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

+

-

+

+

FIGURE 6.15

Approximate vibrational mode of the 900 cm ring. The solid circle represents the substituent.

1

band CH wag for a mono-substituted benzene

6.03 6.02

–OCH3

O

–OH

–NMe2

=

–NH–C–CH3

–NH2

–F

–C(CH3)3

6.01 –CH3

6.00 5.99

–CH2OH –NH3+ –C=CH2

–CCH

–C–OCH3

O

=

–C–H O

880

890

–C–CH3 & –CF3

900

910

920

O

=

Mono-substituted aryl CH wag

870

–NO 2

=

5.96 860

O

=

O –C–NH2

5.98 5.97

–CN

=

Calculated electron density (para carbon atom)

6.04

–C–Cl

930

940

Wavenumber (cm–1)

FIGURE 6.16 (w900 cm

1

Correlation of the benzene ring para carbon electron density as a function of substituent with the mode) CH2 wag frequency. Data adapted from N.B. Colthup, Appl. Spectrosc., 30, 589, 1976.

3.4. Fused Ring Aromatics Polynuclear fused ring aromatics such as naphthalenes, anthracenes, and phenanthrenes have characteristic IR and Raman bands in the same general regions as benzene derivatives.1-4 The aryl-CH wag vibrations of these polynuclear aromatic compounds provide strong IR bands that provide important substitution information. Characteristic IR and Raman bands involving ring stretching vibrations are typically observed in the 1630e1500 cm 1 region. In general, alkyl-substituted naphthalenes have a doublet near 1600 cm 1, and additional bands in the 1520e1505 cm 1 and 1400e1390 cm 1. In the Raman spectra, a very strong characteristic band is observed between 1390 and 1370 cm 1 and another strong band between 1030 and 1010 cm 1. Some of the characteristic CH wag IR bands of naphthalenes are summarized in Table 6.6.

93

CONJUGATED ALIPHATICS AND AROMATICS

TABLE 6.6

Selected IR bands Useful for Substitution Determination of Naphthalene Rings in the 900e700 cm Spectral Region

1

Substituent CH Wag

2nd (cmL1)

CH Wag

3rd (cmL1)

810e775

3 Adj CH wag

780e760

4 adj H wag

NA

2 (mono)

875e823

lone H wag

825e800

2 adj H wag

760e735

1,2 (di)

835e800

2 adj H wag

800e726

4 adj H wag

NA

No. of Subst

Pattern

1st (cm

1

1 (mono)

1 2

)

L1

CH Wag

4 adj H wag

3.5. Heterocyclic Aromatic Six-Membered Ring Compounds Six-membered heterocyclic aromatic compounds are benzene-like rings where one or more of the ring carbons are replaced with nitrogen atoms. Important examples of these compounds include pyridine which involves replacement with one nitrogen atom and triazine which involves substitution with three nitrogen atoms. Because, the six-membered heterocyclic rings typically have an aromatic double bonds in the ring analogous to benzenes, the resultant IR and Raman spectra have many of the same type of group frequencies. However, substitution of an OH or SH group in the ortho or para position relative to the ring nitrogen atom typically results in tautomerization and the formation of a C¼O or C¼S bond, respectively.1,3,4 Such tautomerization also results in characteristic changes in the bands associated with the ring vibrations. Structural examples of these type of tautomerization is shown in Fig. 6.17 for 2-hydroxypyridine and cyanuric acid.

FIGURE 6.17 The resultant keto form resulting from tautomerization of 2-hydroxypyridine and Cyanuric Acid. The keto tautomer of 2-hydroxypyridine results in a carbonyl band between ca. 1650 and 1680 cm 1. The keto tautomer of cyanuric acid results in a carbonyl band cluster between 1700 and 1780 cm 1.

94

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

3.5.1. Pyridines The IR and Raman spectra of pyridine compounds have characteristic bands due to the CH stretch, ring stretching and deformation, and substituent informative CH wag vibrations. A fully annotated IR and Raman spectrum of pyridine (#15) is included in the last section for comparison. Analogous to benzene, the CH stretching bands of pyridine compounds is found in the 3100e3000 cm 1 region. The quadrant ring stretch vibrations typically give rise to moderate to strong IR and Raman bands in the 1620e1555 cm 1 region. The ring semi-circle stretch also results in characteristic IR and Raman bands in the 1500e1410 cm 1 region. The Raman spectra of pyridine compounds is typically dominated by a very strong band in the 1050e980 cm 1 region that involves the 2,4,6 carbon radial stretch. The quadrant ring in-plane bend also results in a moderate Raman band in the 850e750 cm 1 region which is correlated with the substituent position. Similarly, the strong adjacent CH wag observed in the IR in the 850e740 cm 1 region is correlated with the pyridine substitution. Some of the characteristic bands for pyridine compounds are listed in Tables 6.7 and 6.8.1-4 Protonation of the pyridine nitrogen to form the pyridinium salt results in multiple bands from the NþeH stretch between 3300 and 1900 cm 1 that have moderate IR intensity but are weak in the Raman spectrum. The multiple band structure derives from Fermi resonance. Potentially useful Raman bands with variable intensities include the NH in-plane bend at ca. 1250e1240 cm 1 and the NH wag at 940e880 cm 1. The N-oxides of pyridines have a strong IR band in the 1300e1200 cm 1 region involving the NeO stretch. This band is moderate to weak in the Raman spectra. IR and Raman bands involving the NeO stretch and the ring in-plane bend occurs between 880 and 830 cm 1. 3.5.2. Triazines and Melamines The triazine ring in s-triazine, alkyl- and aryl-substituted triazines, and melamine compounds have characteristic IR and Raman bands due to ring stretch and deformation vibrations.1-4,16,17 The quadrant ring stretching vibrations result in strong IR and moderate Raman bands in the 1600e1500 cm 1 region. The semi-circle ring stretching vibrations result in multiple strong IR and weak Raman bands in the 1450e1350 cm 1 region. Particularly, intense Raman bands of triazine compounds include the N-Radial in-phase stretch in the 1000e980 cm 1 region and the quadrant in-plane bend in the 690e660 cm 1 region. Lastly, in the IR there is a moderately intense band in the 860e775 cm 1 region that involves the ring sextant out-of-plane deformation. Melamine consists of a triazine ring with substitution of an amino group on all three ring carbon atoms. The amino group has multiple IR and Raman bands between 3500 and 3100 cm 1 involving the NH2 stretch and a doublet between 1650 and 1620 cm 1 from the NH2 deformation. The added complexity is due to differences in hydrogen bonding in the solid crystalline state. Measurements of melamine in DMSO solution results in the expected doublet from the NH2 out-of-phase and in-phase stretch. The triazine ring itself exhibits the expected IR and Raman bands at 1550 cm 1 deriving from the quadrant ring stretch and 1470e1430 cm 1 from the semi-circle ring stretch. The unique characteristic of Raman bands include the strong band at 984 cm 1 from

95

CONJUGATED ALIPHATICS AND AROMATICS

TABLE 6.7 Selected In-plane Stretching and Bending Vibrations for Pyridine and Mono-Subsituted Pyridine Intensities1 Pyridine

Assignment

Region (cmL1)

IR

R

Pyridine

Aryl CH str.

3100e3000

m

m

Quadrant str.

1615e1570

s

m

Semi-circle str

1490e1440

s

mw

2,4,6 carbon radial str.

1035e1025

m

vs

Ring breath/str.

995e985

m

s

Quadrant in-plane bend

660e600

e

m

Quadrant str.

1620e1570

s

m

Quadrant str.

1580e1560

s

m

Semi-circle str.

1480e1450

s

m

Semi-circle str.

1440e1415

s

w

CH in-plane rk

1050e1040

m

s

2,4,6 carbon radial str.

1000e985

m

vs

Quadrant in-plane bend

850e800

w

ms

Quadrant str.

1595e1570

m

ms

Quadrant str.

1585e1560

s

m

Semi-circle str.

1480e1465

s

m

Semi-circle str.

1430e1410

s

w

2,4,6 carbon radial str.

1030e1010

m

vs

Quadrant in-plane bend

805e750

w

m

Quadrant str.

1605e1565

m

ms

Quadrant str.

1570e1555

m

m

Semi-circle str.

1500e1480

m

m

Semi-circle str.

1420e1410

s

w

2,4,6 carbon radial str.

1000e985

m

vs

Quadrant in-plane bend

805e785

w

ms

2-Mono- substituted

3-Mono- substituted

4-Mono- substituted

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, var. ¼ variable, e ¼ zero

the N-Radial in-phase stretch and the quadrant in-plane ring bend at 675 cm 1. A sharp, moderate IR band is observed between 825 and 800 cm 1 which involves the traizine ring sextant out-of-plane deformation. This vibration involves an out-of-plane movement of all three carbon atoms and the three nitrogen atoms in opposite directions.

96

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.8 Selected Out-of-plane CH wag and Out-of-plane Sextant Deformation Vibrations for Pyridine and Mono-Subsituted Pyridine Intensities1 IR

R

760e74

s

e

out-of-plane sextant def.

710e700

s

e

2-Mono- substituted

4 adjacent H wag

780e740

s

e

3-Mono- substituted

3 adjacent H wag

820e770

s

e

out-of-plane sextant def

730e690

m-s

w

2 adjacent H wag

850e790

s

e

Pyridine

Assignment

Region (cm

Pyridine

5 adjacent H wag

4-Mono- substituted

)

L1

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, var. ¼ variable, e ¼ zero

Tautomerizing the triazine ring by making one of the double bonds external to the ring results in the iso form in which there are fewer than three double bonds in the ring. As discussed above, substitution of the carbon atom with an OH or SH results in formation of the iso form of the triazine ring. In addition to the formation of a C¼O or C¼S bond, the IR active band that derives from the sextant out-of-plane bend will be found in the 795e750 cm 1 region. Thus, the sextant out-of-plane bend vibration found near 800 cm 1 can be used to discriminate between normal triazine rings and the iso form of the triazine ring. A fully annotated IR and Raman spectrum of ammeline (#34) and trimethyl isocyanurate (#35) are included in the last section for comparison. 3.5.3. Fiveemembered Ring Heterocyclic Compounds Pyrroles, furans, and thiophenes are five-membered ring aromatics containing two conjugated double bonds and a single nitrogen, oxygen or sulfur atom in the ring respectively. Additional compounds exist in which an additional carbon atom (position 2 or 3) are replaced with nitrogen or oxygen atoms. The double bonds are somewhat delocalized in these five-membered rings and the ring vibrations can be described using a standing wave description. Figure 6.18 depicts selected approximate ring vibrations for a five-membered ring containing one heteroatom.1 However, the ring carbon atoms are not identical and changing the substituent can significantly affect the IR and Raman spectra. Table 6.9 lists some of the characteristic ring stretching vibrations for pyrroles, furans, and thiophene compounds.1-4,18 The quadrant and semi-circle ring stretching vibrations typically mix with the CH rock vibrations. The quadrant stretching vibration consists primarily of the C¼C stretch while the first semi-circle stretch consists mainly of the CeXeC in-phase stretch with some CeC contraction. The second semi-circle stretch consists mainly of the CeXeC out-of-phase stretch. Important five-membered ring heteroaromatic vibrations not depicted in Fig. 6.18 include the ring CH stretch, the eCH¼CHe ring double bond cis CH wag and NH stretch in pyrrole type compounds. The NH stretch of pyrroles are typically observed between 3450 and

97

CONJUGATED ALIPHATICS AND AROMATICS

Contr

Str Str

Contr

Contr

Str

Str

Str

Contr

In

Out Str Str

In

Contr

Out

Down –

Up

Up +

+

+ Down

In

In

Out

+ Up

–

–

–

Down

Down

+ Up

Out

FIGURE 6.18 Approximate ring vibrations for a five-membered ring containing one heteroatom (solid circle) and two double bonds. The quadrant stretch predominantly involves the double bond stretching vibration.

TABLE 6.9

Selected In-plane Stretching Vibrations for Selected Pyrroles, Furans and Thiophenes Intensities1

Pyridine

Assignment

Region (cmL1)

IR

R

Pyrrole

NH str.

3500e3000

s

mew

¼CH str.

3135, 3103

m

s

Quadrant str. þ CH rk

1530

s

e

Quadrant str. þ CH rk

1468

m

s

Semi-circle str þ CH rk

1418

m

e

Semi-circle str. þ CH rk

1380

s

m

Ring in-phase str.

1143

m

vs

Quadrant str.

1560e1540

wem

m

Quadrant str.

1510e1490

m

vs

Semi-circle str.

1390e1380

sem

s

1-subst pyrrole

(Continued)

98

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.9 Selected In-plane Stretching Vibrations for Selected Pyrroles, Furans and Thiophenesdcont’d Intensities1 L1

Pyridine

Assignment

Region (cm

2-subst pyrrole

Quadrant str.

1570e1545

3-subst pyrrole

Furan

2-subst furan

Thiophene

2-subst thiophene

)

IR

R

wem

m

Quadrant str.

1475e1460

m

vs

Semi-circle str.

1420e1400

sem

s

Quadrant str.

1570e1560

wem

m

Quadrant str.

1490e1480

m

vs

Semi-circle str.

1430e1420

sem

s

¼CH str.

3156, 3121, 3092

m

sem

Quadrant str. þ CH rk

1590

s

w

Quadrant str. þ CH rk

1483

s

vs

Semi-circle str þ CH rk

1378

s

s

Ring in-phase str.

1138

e

vs

Quadrant str.

1560e1540

wem

m

Quadrant str.

1510e1490

m

vs

Semi-circle str.

1390e1380

sem

s

¼CH str.

3110, 2998

m

sem

Quadrant str. þ CH rk

1588

s

e

Quadrant str. þ CH rk

1408

s

vs

Semi-circle str þ CH rk

1357

s

s

Ring in-phase str.

1031

e

vs

Quad bend

832

s

vs

Quadrant str.

1540e1514

wem

m

Quadrant str.

1455e1430

m

vs

Semi-circle str.

1361e1345

sem

s

3200 cm 1. However, for five-membered rings with more than one nitrogen atom in the ring such as imidazoles, a strongly hydrogen-bonded NHeN ¼ complex occurs resulting in a broad complex set of bands in the 3000e2600 cm 1 observed in both the IR and Raman spectra. Pyrroles and furans which have nitrogen or oxygen atom in the ring, respectively, have moderate to strong IR and Raman bands involving the ¼CH stretch between 3180 and 3090 cm 1. In the case of thiophenes which has a sulfur atom in the ring, the ¼CH stretching vibration is somewhat lower in frequency at 3120e3060 cm 1. In general, the ¼CH stretching vibrations of five-membered ring heterocycles are slightly higher than

CARBONYL GROUPS

99

observed for their six-membered ring analogues. Five-membered ring heterocyclic compounds with a unsubstituted eCH ¼CHe group are characterized by a strong IR band in the 800e700 cm 1 region which derives from the in-phase cis CH wag.

4. CARBONYL GROUPS The generalized IR and Raman spectra of the carbonyl region are shown in Fig. 6.19 and the carbonyl group frequencies are summarized in Table 6.10.1-4 The C¼O stretching vibration responsible for these bands typically give rise to strong IR bands and weak Raman bands. In addition to the carbonyl band itself, much useful information can be obtained from the bands of the attached groups. The column of spectra on the left side shows examples of unconjugated carbonyl species including ketones (near 1715 cm 1), aldehydes (near 1730 cm 1), esters (near 1740 cm 1), and acid chlorides (near 1800 cm 1). The central column of spectra shows examples of carboxylic acid, carboxylic acid salt, anhydride, and cyclic anhydride while the third column of spectra shows examples of various amides.

4.1. Review of Selected Carbonyl Species Carboxylic acid dimers are characterized by an IR band near 1700 cm 1, and a Raman band near 1660 cm 1 which derive from the out-of-phase and the in-phase C¼O stretch in the dimer, respectively. Weak, characteristic bands from the CeOH bend are also observed

FIGURE 6.19 1400 cm 1.

The generalized IR and Raman spectra of selected carbonyl compounds between 1900 and

100

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.10

Selected Group Frequencies: Carbonyls Intensities1 Assignment

Frequency (cmL1)

IR

R

ReCOeH

C ¼O str.

1740e1720

s

m

conj-COeH

C ¼O str.

1710e1685

s

w

ReCOeR

C ¼O str.

1725e1705

s

m

conj-COeR

C ¼O str.

1700e1670

s

m

conj-CO-conj

C ¼O str.

1680e1640

s

m

HeCOeOeR

C ¼O str.

1725e1720

s

m

ReCOeOeR

C ¼O str.

1750e1735

s

m

Conj-COeOeR

C ¼O str.

1735e1715

s

m

g lactones

C ¼O str.

1795e1760

s

m

ReCOeOH dimer

C ¼O o.ph. str.

1720e1680

s

e

”

C ¼O i.ph. str.

1670e1630

e

m

ReCOeN

C ¼O str.

1695e1630

s

m

ReCOeCl

C ¼O str.

1810e1775

s

m

ReCOeOeCOeR

C ¼O i.ph. str.

1825e1815

s

m

”

C ¼O o.ph. str.

1755e1745

ms

m

Cyclic anhydride

C ¼O i.ph. str.

1870e1845

m

s

””

C ¼O o.ph. str.

1800e1775

s

mw

ReCO2

CO2 o.ph. str.

1650e1540

s

w

”

CO2 i.ph. str.

1450e1360

ms

s

Group C¼O

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero

near 1420 cm 1. Carboxylic acid salts give rise to two bands involving the out-of-phase and in-phase CO2 stretching vibrations. The out-of-phase stretch band is observed near 1570 cm 1 and is typically quite strong in the IR but weak in Raman spectrum. The inphase stretch band is observed near 1415 cm 1 and is typically strongest in the Raman spectrum. Anhydrides are characterized by two bands which involve the stretching of both C¼O groups in- and out-of-phase with respect to each other. Unconjugated, non-cyclic anhydrides result in bands near 1820 cm 1 and 1750 cm 1, where the higher frequency, C¼O in-phase stretch is more intense than the out-of-phase stretch for both the IR and the Raman spectra. Five-membered ring cyclic anhydrides give rise to bands near 1850 cm 1 and 1780 cm 1, where the lower frequency out-of-phase stretch band is strongest in the IR spectrum, while the higher frequency in-phase band is strongest in the Raman spectrum.

101

CARBONYL GROUPS

This significant difference between the IR intensity of the in-phase and out-of-phase C¼O stretch for the non-cyclic anhydrides and the cyclic anhydrides is directly related to the relative orientation of the two carbonyl groups. An integrated IR absorbance ratio of the two carbonyl bands can be used to predict the dihedral angle of the CeO bonds.1 Amides are characterized by strong IR bands from the C¼O stretch near 1670 cm 1. The Raman bands for the amide carbonyl stretch are typically quite weak. The primary amide group (O¼CeNH2) also has bands near 1610 cm 1 involving the NH2 bend as well as near 1410 cm 1 involving the CeN stretch. The non-cyclic secondary amide group (O¼CeNHeC) not only has a C¼O stretch at 1670 cm 1, but another strong band in the IR spectrum near 1550 cm 1 involving the CNH bend and the CeN stretch. In contrast, the cyclic secondary amide (O¼CeNHeC) group does not have a 1550 cm 1 band. Amides with no NH have only the C¼O stretch in this region. Fully annotated IR and Raman spectra of selected carbonyl compounds are included in Chapter 8 for comparison (see #16e29 for ketones, esters and anhydrides, 30e39 for amides, ureas, and related compounds).

4.2. Factors that Effect Carbonyl Frequencies The carbonyl stretching frequencies are dependent upon mass, mechanical, and force constant effects. The mass effect is typically the least important factor in understanding carbonyl frequency dependence upon structure. This arises because the carbonyl stretching vibration involves mostly a change in the C¼O bond length with only a slight movement of the attached atoms (bending). This is an excellent example of a high frequency C¼O oscillator attached to a low frequency CeX oscillator (see unequal oscillators discussed earlier). The effect of changing mass on the carbonyl frequency when replacing a carbon atom with a heavier atom is quite slight due to the mass effect alone. The mass effect is more important when decreasing the substituent mass from carbon (12) to hydrogen (1) and should decrease the C¼O frequency about 17 cm 1 when comparing a ketone to an aldehyde. However, even in this simple example, the higher C¼O frequency of the aldehyde is counter to that expected by the mass effect, and demonstrates the importance of changes in the force constants.

O

O

C CH2

CH2

CH2

C

H

The above shows the approximate carbonyl stretching vibration for a ketone and an aldehyde. The very slight movements of the carbon atoms which results in changes in the carbonyl carbon bond angle are not shown. The aldehyde CH bend and C¼O stretch are similar in energy and therefore couple more strongly than either the CeC stretch or the CeC bend. The characteristic frequencies found in strained ring carbonyls is an excellent example of the importance of mechanical effects in carbonyl compounds and is analogous to that observed in cyclic olefin structures. Once again, the bond angle is the critical variable in

102

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

determining the carbonyl stretching frequency. Shown below are the structures, important bond angles, as well as the observed and calculated frequencies based upon a simple mechanical model for the carbonyl stretching frequencies of four cyclic carbonyl compounds.

O

O 120°

126°

C

C

O

O 135°

C

150°

C

Calc (cm–1)

1714

1737

1774

1836

–1)

1715

1740

1782

1822

Obs (cm

The cyclic C¼O stretching frequencies as a function of the CeC¼O bond angles is shown above. The same mechanical triatomic model is used for the above calculations as was used for the cyclic olefin structures earlier, where FCeC and FC¼O are 4.5 and 11.1 mdynes/A, respectively. The good agreement between the calculated and observed frequencies indicates that most of the shifts derive from mechanical effects and not from changes in force constants. Force constant effects on carbonyl stretching frequencies include mesomeric, inductive, and field effects.1-4 Carbonyl inductive effects derive from the polar nature of the carbonyl C¼O bond. As shown below, the more the oxygen atom can attract electrons, the weaker the carbonyl C¼O force constant becomes. O

O

C

C

O

O

O

R C R

R C Cl

Cl C Cl

1715 cm–1

1800 cm–1

1828 cm–1

The polarity of the carbonyl bond is a consequence of the greater electronegativity of the oxygen relative to the carbon atom. Electronegative substituents on the carbon atom will compete with the oxygen for electrons and thereby raise the carbonyl frequency. Substitution with strongly electron-withdrawing chlorine atoms provides a classic example of this. Field effects are another example of inductive effects in the carbonyl group. In this case, the presence of an electron rich chlorine spatially near to the oxygen atom favors the non-polar C¼O group and raises the frequency. Thus, IR can discriminate between the cis and gauche rotational isomers in a-chlorocarbonyl compounds (CleCH2eC(¼O)eX). Mesomeric and conjugation effects can be understood by examining resonance structures. Mesomeric effects derive from substituting the carbonyl group with lone pair electron containing heteroatoms such as chlorine, oxygen, or nitrogen.

103

CeO AND CeN STRETCHES

O

O

C Cl

C Cl

1800 cm–1

O

O

C O CH2

C O CH2

O

O

C NH2

C NH2

1740 cm–1

1660 cm–1

As shown above, the non-bonding electrons rearrange, resulting in donation of electrons to the oxygen atom and thereby weakening the carbonyl C¼O bond. The mesomeric effect is not important for acid chlorides since the resonance structure placing a positive charge on the chlorine atom is not favored. Conversely, the mesomeric effect is the dominant determinant of the carbonyl frequency in amides. Conjugation of a carbonyl with either a vinyl or a phenyl group typically lowers the carbonyl frequency by 20e30 cm 1. This is also a result of electron rearrangement, analogous to the mesomeric effect discussed above and can easily be understood by use of resonance structures. Shown below are the approximate frequencies for an unconjugated ketone, single, and double conjugated ketone. O CH2

C CH2

1715 cm–1

O CH2

C CH CH2

1685 cm–1

O CH2 CH C CH CH2

1655 cm–1

Lastly, interaction effects are key in understanding the source of two bands in anhydrides and carboxylic acid salts; these have their origin in both mechanical interactions and electron redistribution. Anhydrides have two carbonyl groups that vibrate in- and out-of-phase to each other resulting in two bands in the IR and Raman spectra. Comparison of the generalized IR spectra of cyclic and non-cyclic anhydrides shows that the relative intensity easily enables rapid identification. Similarly, carboxylic acid salt have two bond-and-a-half CeO bonds which vibrate in- and out-of-phase to give two bands (see Fig. 6.19). The different relative intensities for these two vibrations derive from the total dipole moment change. The electron redistribution that occurs during the in- and out-of-phase vibrations for these compounds is termed as an interaction force constant and also contributes to the observed frequencies.

5. CeO AND CeN STRETCHES The IR and Raman spectra in Fig. 6.20 illustrate some group frequencies for vibrations involving CeO or CeN stretches interacting with connected CeC vibrations.1-4 Table 6.11

104

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

FIGURE 6.20 The generalized IR and Raman spectra of selected CeO and CeN stretching bands.

TABLE 6.11 Selected Group Frequencies: Alcohols and Ethers Intensities1 Group OH

Assignment

Frequency (cmL1)

IR

R

ReOH..O

OH str.

3400e3200

sbr

vw

O¼CeOH dimer

OH str.

3200e2600

sbr

e

”

OH wag

960e875

m

e

O

OH str.

2800e2100

s br

vw

O

OH str.

3100e2200

sbr

vw

CH2eOeCH2

COC o.ph. str

1270e1060

s

w

“

COC i.ph. str.

1140e800

m

s

C¼CeOeCH2

COC o.ph. str.

1225e1200

s

w

AreOeCH2

AreO str.

1310e1210

s

m

”

OeCH2 str.

1050e1010

m

m

CH2eOH

CeO str.

1090e1000

s

mw

“

CeO str.

900e800

mw

s

..

PeOH

..

SeOH CeO

105

N¼O AND OTHER NITROGEN CONTAINING COMPOUNDS

TABLE 6.11

Selected Group Frequencies: Alcohols and Ethersdcont’d Intensities1

Group R2CHeOH

L1

Assignment

Frequency (cm

CeO str.

1150e1075

)

IR

R

m

mw

“

CeO str.

900e800

mw

s

R3CeOH

CeO str.

1210e1100

s

mw

“

CeO str.

800e750

mw

s

AreOH

CeO str.

1260e1180

s

w

O¼CeOeC

CeO str.

1300e1140

s

w

O¼CeOH

CeO str.

1300e1200

s

w

Epoxy

ring i.ph. str.

1270e1245

m

s

”

ring o.ph. str.

935e880

s

m

”

ring o.ph. str.

880e830

s

m

1

s ¼ strong, m¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero

summarizes some of the important group frequencies for alcohols and ethers. Typically, vibrations involving these groups tend to give band clusters rather than single bands. The bands involving the CeO and CeN stretches of alcohols and amines show a systematic frequency dependence on their substitution (i.e., primary, secondary.). Primary alcohols and amines have bands at approximately 1050 cm 1 and near 1075 cm 1 respectively. Secondary alcohols have bands at roughly 1100 cm 1, while aliphatic secondary amines have IR bands near 1135 cm 1. Tertiary alcohols usually have IR bands near 1200 cm 1. Aliphatic ethers have characteristic IR bands near 1120 cm 1 while alkylearyl ethers have two sets of bands near 1250 cm 1 involving aryl-O stretch and 1040 cm 1 involving the OeCH2 stretch. Phenols have IR bands near 1240 cm 1 and aromatic-NH2 aniline species have IR bands roughly at 1280 cm 1 involving aryl-O and aryl-N stretches, respectively. Fully annotated IR and Raman spectra of selected alcohol and ethers are included in Chapter 8 (alcohols #40e47 and ethers 48e50) for comparison.

6. N[O AND OTHER NITROGEN CONTAINING COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for some N¼O type compounds are shown in Fig. 6.21.1-4 Table 6.12 summarizes the selected group frequencies for selected nitrogen containing compounds. As shown in Fig. 6.21 and in Table 6.12, NOx containing compounds provide strong IR and Raman strong bands that have excellent group frequencies. The unconjugated nitro group (CeNO2) are illustrated in the top spectra in Fig. 6.21 and have NO2 stretching bands near 1560 and 1375 cm 1. The out-of-phase stretch at w1560 cm 1

106

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

FIGURE 6.21

The generalized IR and Raman spectra of important bands for nitroalkanes, organic nitrates, and

organic nitrites.

TABLE 6.12

Selected Group Frequencies: Nitrogen Compounds Intensities1

Group

Assignment

Frequency (cmL1)

IR

R

CH2eNH2

NH2 o.ph. str.

3500e3300

m

vw

“

NH2 i.ph. str.

3400e3200

m

m

“

NH2 bend

1630e1590

m

vw

“

NH2 wag

900e600

sbr

w

CH2eNHeCH2

NH str.

3450e3250

vw

w

“

CeNeC o.ph. str.

1150e1125

m

mw

AreNH2

CeN str.

1380e1260

sbr

m

.. X CeNHþ 3

NH3 str.

3200e2700

s

vw

”

NH3 o.ph. bend

1625e1560

mw

vw

”

NH3 i.ph. bend

1550e1505

w

vw

107

N¼O AND OTHER NITROGEN CONTAINING COMPOUNDS

TABLE 6.12

Selected Group Frequencies: Nitrogen Compoundsdcont’d Intensities1 L1

Group

Assignment

Frequency (cm

C2NHþ.. X 2

NH2 str.

3000e2700

)

IR

R

sbr

w

”

NH2 bend

1620e1560

mw

w

C3NHþ..X

NH str.

2700e2300

s

w

O ¼CeNH2

NH2 o.ph. str.

3475e3350

s

w

”

NH2 i.ph. str.

3385e3180

s

w

”

NH2 bend

1650e1620

ms

w

O ¼CeNHeC

NH str.

3320e3270

ms

w

”

CNH str. bend

1570e1515

ms

w

O ¼CeNHeC cyclic

NH str.

3300e3100

ms

w

>C ¼N

C ¼N str

1690e1630

mw

ms

>C ¼NeOH

OH str.

3300e3150

s

w

”

NeO str.

1000e900

s

s

CH2eNO2

NO2 o.ph. str.

1600e1530

s

mw

”

NO2 i.ph. str.

1380e1310

s

vs

AreNO2

NO2 o.ph. str.

1555e1485

s

e

”

NO2 i.ph. str.

1357e1318

s

vs

CeOeNO2

NO2 o.ph. str.

1640e1620

s

mw

”

NO2 i. ph. str.

1285e1220

s

s

”

NeO str.

870e840

s

s

CeOeN ¼O

s-trans N ¼O str.

1681e1640

s

s

”

s-cis N ¼O str.

1625e1600

s

s

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero

is more intense in the IR spectrum while the in-phase stretch at w1375 cm 1 is more intense in the Raman spectrum. In nitroalkanes, the CeN stretch gives rise to bands in the 915e 865 cm 1 region that are intense in the Raman and mediumeweak in the IR spectra. These bands are sensitive to the rotational isomer present in the CH2eCH2eNO2 group. When the CeC group is trans to the CNO2 species the CeN band is at w900 cm 1 and when the gauche orientation is present, it is at w 880 cm 1. The covalent nitrate group (ReOeNO2 ) shown in the middle spectra in Fig. 6.21 has bands near 1620 and 1275 cm 1 involving the NO2 stretch. The out-of-phase stretch at w1620 cm 1 is more intense in the IR spectrum while the in-phase stretch at w1275 cm 1 is more intense in the Raman spectrum. The NeO stretch also gives rise to a prominent band near 870 cm 1.

108

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

The covalent nitrite group (ReOeN¼O) is characterized by N¼O stretching bands between 1600 and 1640 cm 1 and a NeO stretch near 800 cm 1. The N¼O bands are sensitive to the rotation isomer present in the CH2eOeNO group. The N¼O stretch bands are typically observed near 1640 and 1600 cm 1 for the s-trans and s-cis isomers, respectively. Here s-cis indicates the single (OeC) bond is cis to the N¼O bond and s-trans indicates the single (OeC) bond is trans to the N¼O bond. The fully annotated IR and Raman spectra of selected nitrogen containing compounds including amines (#51e56), C¼N type groups (#57e59), N¼O type groups (60e62), and an azo group (63) are found in Chapter 8 for comparison.

7. C-HALOGEN AND CeS CONTAINING COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for the stretching vibrations of CeCl, CeBr, CeSeC, and CeSeSeC groups is shown in Fig. 6.22.1-4 Table 6.13 shows some selected group frequencies of halogenated compounds. All of these have rotational isomers where the atom that is trans to the halogen or sulfur is either

FIGURE 6.22 The generalized IR and Raman spectra for selected vibrations involving the CeCl, CeBr, CeSeC, and CeSeSeC groups.

109

C-HALOGEN AND CeS CONTAINING COMPOUNDS

TABLE 6.13

Selected Group Frequencies: Halogen Compounds Intensities1

Group

Assignment

Frequency (cmL1)

IR

R

CF2 and CF3

CF str.

1350e1120

vs

mw

AreF

CF str.

1270e1100

s

m

CeCl

CeCl str.

830e560

s

s

CeCH2eCH2eCl

trans CeCl str.

730e720

s

s

“

gauche CeCl str.

660e650

s

s

CH2eCl

CH2 wag

1300e1240

s

s

CeBr

CeBr str.

700e515

s

vs

CeCH2eCH2eBr

trans CeBr str.

650e640

s

vs

“

gauche CeBr str.

565e560

s

vs

CH2eBr

CH2 wag

1250e1220

s

m

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero.

a carbon or a hydrogen atom. Both the IR and Raman spectra have isolated bands CeS and C-Halogen stretching bands which are due to the rotational isomers. For chlorinated aliphatics such as CeCH2eCH2eCl, the bands involving the CeCl stretch appear near 730 and 660 cm 1. The aliphatic sulfide group (eCH2eCH2eSeCH2eCH2) is quite similar with CeS stretching bands near 720 and 640 cm 1. For brominated aliphatics such as CeCH2eCH2eBr, the bands involving the CeBr stretch appear at much lower frequency near 635 and 560 cm 1. The CeF stretch of organic fluorine compounds results in strong IR bands but weak to moderately intense Raman bands. Monofluorinated compounds have a strong IR and a weak to moderate Raman band between 1100 and 1000 cm 1. Difluorinated compounds (eCFe2) have two very strong IR bands between 1250 and 1050 cm 1 involving the CF2 out-of-phase stretch. These bands are weak to moderate in the Raman spectrum. Trifluorinated compounds (eCF3) have multiple strong bands between 1350 and 1050 cm 1 involving the CF3 out-of-phase stretch. Both the IR and Raman have some characteristic bands such as the CH2 wags and the disulfide SeS stretch, which are not sensitive to rotational isomers. The disulfide group (ReSeSeR) is characterized by a very strong Raman band near 500 cm 1 that involves the SeS stretch. In addition, the CH2eCl group has a CH2 wag band cluster near 1285 cm 1, whereas the CH2eBr and CH2eS groups have these bands near 1240 cm 1. In aryl-halides, the CeX stretching vibration couples with the aryl ring vibrations resulting in strong IR and moderate Raman bands. Fluorine-substituted aromatics (ArdF) have a moderately strong IR band between 1270 and 1100 cm 1 involving the aryl ring CeF stretch. This same band is found between 1100 and 1030 cm 1 for the aryl-Cl species and between 1075 and 1025 cm 1 for aryl bromo species. These band regions can be further defined from high to lower frequency for para, meta, and ortho substitution of the Cl and

110

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

Br aryl halides. Iodobenzenes (para) have a characteristic aryl-I band between 1061 and 1057 cm 1. The fully annotated IR and Raman spectra of selected chlorine, bromine, and fluorine containing compounds are included in Chapter 8 for comparison (#62e69, 99).

8. S[O, P[O, BeO/BeN AND SIeO COMPOUNDS The generalized IR and Raman spectra illustrating group frequencies for some S¼O, SO2, P¼O, and SieO type compounds are shown in Fig. 6.23. Table 6.14 summarizes the selected group frequencies for the chosen S¼O, P¼O, BeO, and SieO containing compounds.1-4 Compounds containing S¼O generally have strong characteristic IR and Raman bands. The characteristic IR and Raman bands of a sulfoxide (R2SO), sulfone (R2SO2), and a dialkyl sulfate (ROeSO2eOR) are shown in the first column of spectra in Fig. 6.23. The frequency of these bands can be correlated with the substituent electronegativity. In general, electronegative substituents tend to raise the S¼O frequency since they favor the S¼O resonance structure. As illustrated in the top left spectra, the intensity of the S¼O stretch near 1050 cm 1 is more intense in the IR than the Raman spectrum. For both sulfones and dialkyl sulfates, the SO2 group results in two bands due to the in- and out-of-phase stretches. The higher frequency out-of-phase SO2 stretch is stronger in the IR and the in-phase SO2 stretch is more intense in the Raman spectra. Table 6.14 also summarizes

FIGURE 6.23 The generalized IR and Raman spectra illustrating group frequencies for some S ¼O, P ¼O, and SieO type compounds.

111

S¼O, P¼O, BeO/BeN AND SIeO COMPOUNDS

important bands for sulfonamides (ReSO2eN), sulfonates (ReSO2eOR), sulfonyl chlorides, sulfonic acids (anhydrous: ReSO2eOH), and sulfonic acid salts (ReSO3 ). The characteristic IR and Raman bands of a phosphine oxide (R3P ¼O) and phosphorus ester ((ReO)3P ¼O) are shown in the second column of spectra in Fig. 6.23. The P¼O stretching frequency depicted have been shown to correlate with the sum of the substituent TABLE 6.14

Selected Group Frequencies: Sulfur, Phosphorus, Silicon, and Boron Compounds Intensities1

Group Sulfur Cpds

Assignment

Frequency (cmL1)

IR

R

SH

SH str.

2590e2540

w

s

CeSO2eC

SO2 o.ph. str.

1340e1290

s

w

“

SO2 i.ph. str.

1165e1120

s

s

CeSO2eN

SO2 o.ph. str.

1380e1310

s

m

“

SO2 i.ph. str.

1180e1140

s

s

CeSO2eOeC

SO2 o.ph. str.

1375e1335

s

ms

“

SO2 i.ph. str.

1195e1165

s

s

CeSO2eCl

SO2 o.ph. str.

1390e1361

s

w

“

SO2 i.ph. str.

1181e1168

s

s

CeSO2eOH anhydrous

OH str.

3100e2200

sbr

w

““

SO2 o.ph. str.

1352e1342

s

w

SO2 i.ph. str.

1165e1150

s

s

CeSO3 H3O (hydrate)

OH str.

2800e2100

sbr

e

““

SO3 o.ph. str.

1230e1120

s

e

““

SO3 i.ph. str.

1085e1025

m

e

C2S ¼O

S ¼O str.

1065e1030

s

w

PH

PH str.

2450e2270

ms

mw

““ þ

Phosphorus Cpds

Silicon Cpds

2

P ¼O

P ¼O str.

1320e1140

s

m

PeOH

OH str.

2800e2100

sbr.

vw

PeOeCH2

PeOeC str.

1050e970

s

mw

PeOeAr

OeAr str.

1240e1160

s

mw

“

PeO str.

995e855

s

w

SiH

SieH str.

2250e2100

vs

m

SieO

SieO str.

1100e1000

vs

w

SieCH3

CH3 i.ph. bend

1270e1250

vs

w (Continued)

112

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

TABLE 6.14

Selected Group Frequencies: Sulfur, Phosphorus, Silicon, and Boron Compoundsdcont’d Intensities1

Group Boron Cpds

BH

Assignment

Frequency (cm

BeH str

2640e2350

L1

)

IR

R

s

mw

BH (complete octet)

BeH str

2400e2200

s

mw

BeHeB (Bridge)

BeHeB str.

2220e1540

s

mw

BeOH

OH str

3300e3200

s

w

BeO

BeO str

1380e1310

s

var

BeN

BeN str.

1465e1330

s

var

B-Phenyl

CeBeC/ring str.

1440e1430

s

1

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, br ¼ broad, var. ¼ variable, e ¼ zero Electronegative substituents raise the P ¼O frequency (example: R3P ¼O w1160 cm 1, (ReO)3P ¼O w 1270 cm 1)

2

electronegativities. In general, the IR spectrum provides a better selection of strong characteristic bands. Table 6.14 summarizes selected important group frequencies for phosphorus compounds including the PH stretch, the OH stretch of the PeOH group, and the PeOeC stretch of the PeOeCH2 group. The PeO-phenyl group gives rise to two bands involving the phenoxy CeO stretch and the PeO stretch. The characteristic IR and Raman bands of a siloxane polymer is shown in the bottom right hands corner in Fig. 6.21. In general, IR spectroscopy provides a much better selection of strong, characteristic group frequency bands of silicon compounds. For silicones, the SiCH3 group has a good IR in-phase CH3 bending band near 1265 cm 1 and the SieO stretch has a strong, broad IR band in the 1100e1000 cm 1 region. Table 6.14 also summarizes selected important group frequencies for boron containing compounds. The BeO stretch results in a strong characteristic IR band between 1380 and 1310 for both organic and inorganic compounds containing the BeO species. Compounds containing BeN species also result in strong characteristic IR bands between 1465 and 1330 cm 1. Little information on the characteristic Raman bands BeO and BeN containing species are available in the literature. The interpreted IR and Raman spectra for sulfur containing compounds (70e76, 83, 84), phosphorus containing compounds (77e80, 88, 89), silicon containing compounds (81, 93, 108), and boron containing compounds (64, 90) are included in Chapter 8 for comparison.

9. INORGANICS Both IR and Raman spectroscopy provides important structural information on a multitude of ionic inorganic compounds.19 Both techniques are sensitive to the crystalline form of the inorganic species. Table 6.15 summarizes selected IR and Raman bands for a few common inorganic compounds. The interpreted IR and Raman spectra of several selected inorganic compounds (#82e94) are included in Chapter 8 for comparison.

113

INORGANICS

TABLE 6.15

Selected Group Frequencies: Common Inorganic Compounds

Inorganics

IR bands (cmL1)

Raman bands (cmL1)

NH4 þ

3100 s, 1410 s

3100 w, 1410 w

NCO

2170 vs, 1300 m, 1210 m

2170 m, 1300 s, 1260 s

NCS

2060 vs

2060 s

CN

2100 m

2080 s

CO3

1450 vs, 880 m, 710 w

1065 s

HCO3

1650 m, 1320 vs

1270 m, 1030 s

NO3

1390 vs, 830 m, 720 w

1040 s

1270 vs, 820 w

1320 s

1130 vs, 620 m

980 s

HSO4

1240 vs, 1040 m, 870 m

1040 s, 870 m

SO3

980 vs

980 s

1030 vs, 570 m

940 s

(eSiO2 )x

1100 vs, 470 m

e

TiO2

660 vs, 540 vs

e

NO2 SO4

PO4

2

3

s ¼ strong, m ¼ medium, w ¼ weak, v ¼ very, e ¼ zero.

114

6. IR AND RAMAN SPECTRA-STRUCTURE CORRELATIONS: CHARACTERISTIC GROUP FREQUENCIES

Chapter 6: Problems 1. Explain the frequency dependence observed for CH3 in-phase bending band in the following series: BeCH3 ¼ 1310 cm 1, CeCH3 ¼ 1380 cm 1, NeCH3 ¼ 1410 cm 1, OeCH3 ¼ 1445 cm 1, and FeCH3 ¼ 1475 cm 1. 2. Explain the observed splitting in the CH3 in-phase bend IR band in isopropyl or gem-dimethyl groups and t-butyl groups. 3. Explain the observed frequency dependence of the C¼CH2 wag vibration: (NhC)2eC¼CH2 ¼ 985 cm 1, NhCeCH ¼CH2 ¼ 960 cm 1, ReC¼CH2 ¼ 910 cm 1, ReOeC¼CH2 ¼ 813 cm 1. Hint draw possible resonance structures and consider mesomeric electron donation to the ¼CH2 carbon. How would you expect the IR intensity to vary? 4. Trans-substituted olefins (ReC¼CeR) show the following systematic increase in the observed CH wag. An isolated trans alkene (T) has a CH wag at 965 cm-1, a conjugated transetrans diene (TT) at 986 cm 1, the transetransetrans alkene (TTT) at 994 cm 1, and the transetransetransetrans alkene (TTTT) at 997 cm 1. This is a result of vibrational interaction with conjugated diene groups. Using diagrams of the CH wag explain this progression. Hint: mechanical interaction (see N.B. Colthup, Appl Spectrosc., 25, 368, 1971). 5. The P¼O, S¼O, and SO2 stretching frequencies are relatively unaffected by conjugation or strained ring. How does the geometry relate to this? The inductive effect has a significant impact on the P¼O, S¼O, and SO2 stretching frequencies. Where the inductive effect is: X3P¼O 4 X3PþeO How could changing the substituents change the frequency (i.e., what simple parameter would explain the frequency shifts)? 6. Draw the five expected ring stretching modes for a five-membered ring. Use standing wave to help. Use your vibrations to assign IR bands observed for the following ring stretch vibrations:

Rings 1-Substituted 2-Substituted 2-Substituted 2-Substituted

pyrrole pyrrole Furans Thiophenes

Region 1 (IR:var, R:var)

Region 2 (IR:m, R:vs)

Region 3 (IR:ms, R:s)

1560e1540 1570e1545 1605e1560 1535e1514

1510e1490 1475e1460 1515e1460 1454e1430

1390e1380 1420e1400 1400e1370 1361e1347

7. The aromatic sextant stretch typically is IR and Raman forbidden and is therefore not observed. This band has been assigned for certain Cl-substituted aromatics to occur between 1330 and 1305 cm 1 which is below that observed for either the quadrant or semi-circle ring stretches. Draw the two different extremes of the sextant ring stretch. The lower frequency is due to an interaction force

INORGANICS

115

constant. Explain this concept as it relates to the observation for the aromatic sextant stretch. 8. How does the interaction force constant explain the observed differences between 2,4-pentanedione (CH3eCOeCH2eCOeCH3 at 1725 and 1707 cm 1) and an anhydride (CH3eCOeOeCOeCH3 at 1833 and 1764 cm 1).

References 1. Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic: New York, NY, 1990. 2. Bellamy, L. J.; The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: New York, NY, 1975; vol. 1. 3. Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic: San Diego, 1991. 4. Socrates, G. Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley: New York, NY, 1994. 5. Jordanov, B.; Tsankov, D.; Korte, E. H. J. Mol. Struct. 2003, 651e653, 101e107. 6. Atamas, N. A.; Yaremko, A. M.; Seeger, T.; Leipertz, A.; Bienko, A.; Latajka, Z.; Ratajczak, H.; Barnes, A. J. J. Mol. Struct. 2004, 708, 189e195. 7. Amorim da Costa, A. M.; Marques, M. P. M.; Batista de Carvalho, LA. E. Vib. Spectrsoc. 2002, 29, 61e67. 8. Balabin, R. M. J. Phys. Chem. A 2009, 113, 1012e1019. 9. Colthup, N. B. J. Chem. Educ. 1961, 38 (8), 394e396. 10. Colthup, N. B.; Orloff, M. K. Spectrochim. Acta, Part A 1971, 27A, 1299. 11. Determined by the author using Hammet values from Domingo, L. R.; Perez, P.; Contreras, R. J. Org. Chem. 2003, 68, 6060e6062. 12. Tasumi, M.; Urano, T.; Nakata, M. J. Mol. Struct. 1986, 146, 383e396. 13. Palafox, M. A. Int. J. Quantum Chem. 2000, 77, 661e684. 14. Versanyi, G. Vibrational Spectra of Benzene Derivatives; Academic: NY, 1969. 15. Colthup, N. B. Appl. Spectrosc. 1976, 30, 589. 16. Larkin, P. J.; Makowski, M. P.; Colthup, N. B. Spectrochim. Acta A 1999, 55, 1011e1020. 17. Larkin, P. J.; Makowski, M. P.; Colthup, N. B.; Flood, L. Vibr. Spectrosc. 1998, 17, 53e72. 18. El-Azhary, A. A.; Hilal, R. H. Spectrochim. Acta, Part A 1997, 53, 1365e1373. 19. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry, Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, 5th ed.; John Wiley: New York, NY, 1997.

C H A P T E R

7 General Outline and Strategies for IR and Raman Spectral Interpretation Spectroscopists must develop an analytical toolbox suitable for their needs. This includes the ability to measure a suitable spectrum, understand the basic chemistry of the compound, and systematically interpret the spectrum. In this chapter, the IR and Raman spectrum will be discussed briefly in terms of spectral regions rather than functional groups. Both Raman and FT-IR spectroscopies provide important molecular structural information. However, the successful application of both techniques relies heavily on the skill of the spectroscopist. There are common skills used for solving structural problems using vibrational spectroscopy. In general, tools and skills that can be called upon include the following components: • Interpersonal and communication skillsdDefine the problem and get background information on the sample. • InstrumentationdSelect most appropriate technique and sample preparation. • Data analysis techniquesdExperience is important in selecting the most appropriate tool for your application. • Spectral interpretation skills. • Spectral database tools (spectral libraries, interpretation software). • Ab initio-based normal mode analysis (ultimate structural verification).

1. TOOLS OF THE TRADE IR and Raman spectroscopies are used to study a very wide range of sample types and may be examined either in bulk or in microscopic amounts over a wide range of temperatures and physical states (e.g., gases, liquids, latexes, powders, films, fibers, or as a surface or embedded layer). Because IR and Raman spectroscopies have a very broad range of applications, it is important to be knowledgeable about the advantages and limitations of various available techniques in order to select the most appropriate technique for the problem facing the spectroscopist. Often a broad-based knowledge of spectroscopy is required including an understanding of the instrument, the data analysis as well as spectral interpretation skills.

117

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

Basic instrumental techniques for bulk analysis include mid-IR (most commonly FT-IR) as well as Raman spectroscopy. Raman spectroscopy has various laser excitation sources and instrument designs (such as FT-Raman or grating-based instruments). When examining smaller samples micro-analysis techniques must be employed such as microscopy (IR and Raman, imaging, and confocal), and micro-ATR for IR. Once a suitable sampling technique has been selected to provide the spectrum (or spectra) needed, a data analysis technique to provide information needed to solve the problem is required. For identification and structural verification purposes, this can include spectral databases and interpretation. Chemometric data analysis may be employed for developing quantitative models or analysis of large spectral data sets such as those encountered in imaging techniques.

723

394 357 310

776 720

902 840

1138 1081 1046

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1123

1070

1298

891

1063

1130

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548

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CH rk Trans chain C–C–C i. ph.str

375

Trans chain C–C–C o. ph. str

671

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twist

1175 1129 1099 1063

1296

720

941 891

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1348 1295 1229 1188

1472 1431

3

O CH3—(CH2)14—C—OH

C–O str. CH

1462 1438 1422

1703

2955

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1466 1451

1408 1463

1473

2638

CO o. ph. str

CH chain i. ph. rk .

CO i. ph. str . CH chain i. ph. twist

CH chain C–C–C i. ph.str

720

1113

1302

1421

CH bend

4 [CH3—(CH2)14—CO2—]2 Ca++

CH chain C–C–C o. ph. str .

891

CH i. ph. str .

2724 2610

CH o. ph. str

CH3—(CH2)15—OH Trans chain C–C–C o. ph str. .

Dimer O… H…O o. pl. wag

1176 1130 1103 1063

0.40 Ra I

CH o. ph. str

CH chain i. ph. rock crystal splitting

Trans chain C–C– C i. ph str .

1370 1296

20

H O bend

1542 1470

60

C–CC–O o. ph. str . .

CH chain wag

H O OH. str

1569

IR %T

C=O i. ph. str . in dimer

CH bend

1461 1441

0.0 100

Trans chain CH o. ph. str .

C=O o. ph. str . in dimer

1634

0.4

CH i. ph. str

1622 1576

Ra I

CH o. ph. str .

2846 2850

0.8

CH o. ph.str

2918

20

2959 2924

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3429

IR %T

C–C–C o. ph. str

CH wag

OH str .

1

CH i. ph. rk .

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CH o. ph. bend

CH i. ph. str .

2724

0.00 100

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1459 1437 1369

CH o. ph. str . CH o. ph. str str.

2846 2849 2725 2882 2676

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.

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CH , CH str .

2733

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3303

IR %T

2846 2850

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2881 2917 2957

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2875 2859

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Ra I

1379

100 IR %T

0.00 3500

3000

2500

2000

1800

1600 Wavenumber

1400 (cm )

1200

1000

800

600

400

FIGURE 7.1 The FT-IR and FT-Raman spectra of n-heptane, 1-hexadecanol, palmitic acid, and calcium palmitic acid salt.

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Success will often depend on your selection of the most suitable technique for the problem at hand. As a rule, mid-IR provides a better general workhorse technique than Raman spectroscopy because it contains a greater selection of bands characteristic of functional groups. This is illustrated in the Fig. 7.1 of the IR and Raman spectra for several organic species. The Raman spectra have characteristic bands for the aliphatic backbone, but do not have strong bands associated with important functional groups of the alcohol, carboxylic acid, and carboxylic acid salt. IR has strong characteristic bands for the aliphatic methyl and methylene groups as well as the alcohol, carboxylic acid, and carboxylic acid salt groups. The Raman spectrum of each of the four species is very similar with bands characteristic of the methyl, methylene, and skeletal carbon backbone. In the four compounds selected (n-heptane, 1-hexadecanol, palmitic acid, and calcium palmitic acid salt), the Raman spectrum does not provide strong bands for the hydroxyl, carbonyl, or carboxylic acid. Moderate-to-strong Raman bands can occur for carbonyl groups that are more polarizable (e.g., amide carbonyls or the very strong carbonyl observed in isocyanurate). There are functional groups for which Raman is the preferred method while some functional groups provide useful bands in both types of spectra. In general, IR provides a broader representation of characteristic bands for a greater variety of functional groups. The Raman spectra have characteristic bands for the aliphatic backbone, but do not have strong bands associated with important functional groups of the alcohol, carboxylic acid, and carboxylic acid salt.

2. IR SAMPLE PREPARATION ISSUES The quality of the information that can be derived from a FT-IR spectrum is directly related to the quality of the spectrum itself. Therefore selection of the appropriate sample technique and the resultant spectrum quality is critical. Operators must develop requisite sample-preparation skills necessary to measure photometrically accurate spectra. In the case of neat liquids, the liquid film should be uniformly thick (ca. 5e8 mm) without holes or voids to insure a good, quantitative IR spectrum. Similarly, films (cast or pressed) should also be uniformly thick without holes or voids. Lastly, solid-powdered samples can be prepared as a uniform dispersion in either a Nujol or a KBr matrix. In both cases, the sample powder must be sufficiently grounded and mixed with the support matrix to insure a good, artifact-free IR spectrum. A wide variety of IR sample preparation techniques exist because there is not a universal sampling technique that will provide high-quality IR spectra for all samples. The nature of the particular sample will often dictate the selection of the technique. As an example, the IR spectra of potato starch measured using a Nujol mull, KBr disc, water cast film, and ATR sampling are shown in Fig. 7.2. Starch provides an excellent example of the importance of selecting an appropriate IR sample-preparation technique suitable for the sample of interest. Both the Nujol mull and KBr disc sample preparations are inappropriate for starch and result in poor-quality IR spectra. The Nujol mull provides an extremely false spectrum and results in very limited spectral information that could be used for identification and interpretation. The KBr disc

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

Water cast film/ZnSe

1152 1081 1025

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N

984 932 858 764 719

N

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1364

60

40

3500

3000

2500

2000

1800 1600 1400 1200 1000

Wavenumber

1150 1077

764 709 800

600

ATR

3500

3000

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2000

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576 525

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(d) 1456 1416 1337 1244

(b)

100

Transmittance

932 852

2154

Transmittance

80

762 708

2149

(c) 1642

(a)

100

1800 1600 1400 1200 1000

800

600

Wavenumber

FIGURE 7.2 IR Spectra of potato starch: Comparison of (a) Nujol mull (N ¼ Nujol signal), (b) KBr disc, (c) water cast film on ZnSe sample preparation, and (d) ATR sampling.

spectrum, while false, is a significant improvement over the Nujol mull preparation and could be used for identification. Dissolving the starch in water and preparing a cast film on ZnSe results in a photometrically accurate IR spectrum. The resultant IR spectrum is of an amorphous film. Lastly, using ATR, an excellent IR spectrum is obtained of the starch powder. Note that the ATR technique results in changes in peak position, shape, and also results, as is typical of ATR, in a wavelength dependence of the band intensities. Five different general criteria should be kept in mind for suitable IR sample preparation for both samples and library reference spectrum. 1. Suitable band intensities in the spectrum. The strongest band in the spectrum should have intensity in the range of 5e15% transmittance. The resulting preparation must be uniformly thick and homogenously mixed without holes or voids to insure a good quality spectrum. 2. Baseline. The baseline should relatively be flat. The highest point in the spectrum should lie between 95 and 100% transmittance. 3. Atmospheric compensation. Water vapor and carbon dioxide bands should be minimized. Since the sample scan is ratioed against a previously measured background, spectral subtraction of water vapor/carbon dioxide, reference spectrum should be used.

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4. Spurious bands. Any band that does not belong to the sample is spurious and must be accounted for or better yet eliminated. Use of an IR window contaminated with a previous sample is a very common source of spurious bands. 5. X-axis scale. Plots should employ a 2X-axis expansion and not a simple linear X-axis. This provides an expansion of the important fingerprint region. Some spectral databases such as the Aldrich collection utilize a 4X-axis expansion.

2.1. Select Suitable Sampling and Know the Limitations The sample and problem at hand will determine if a bulk or microscopy measurement is used. The sampling by Raman spectroscopy is straightforward with fluorescence interference representing the main limitation. In the case of IR spectroscopy, sample preparation choices must be made to produce the most photometrically accurate spectrum. Sample preparation for bulk analysis includes KBr disc, Nujol mull, ATR, DRIFTS, and cast films. For a KBr disc preparation, the entire spectrum is available, but KBr is hydroscopic. Water will be introduced in the KBr disc preparation and this will affect the ability to interpret the OH/NH stretching region. For a Nujol mull sample preparation, there will be interferences from the mulling oil aliphatic groups. ATR can be considered a more universal sampling choice but the IR band intensities exhibit a wavelength dependence. Some band shape distortions will occur. Less commonly used techniques include DRIFTS where the analyst must be concerned about the photometric accuracy of the spectrum. Lastly, a solvent cast films can be used. Issues with this technique include preparing a uniform film, eliminating much or all of the solvent, and the resultant crystalline state of the material.

3. OVERVIEW OF SPECTRAL INTERPRETATION Spectral interpretation requires a systematic, knowledge-based approach for optimal success. Both the sample IR and Raman spectrum and any reference spectrum must be of good quality. Before beginning to interpret the IR and Raman spectrum, the analyst should get a suitable amount of background information. Spectral interpretation is a critical skill that is learned with practice and experience. Resources to use include electronic and paper libraries, empirical characteristic frequencies for chemical functional groups as well as outside vendor and interior libraries. The quality of the available reference library is critical. The IR and Raman libraries ideally should capture knowledge of the spectroscopy of the band assignments of molecules included in the database in order to facilitate interpretation of other molecules.

3.1. Hierarchy of Quality for Spectral Reference Libraries Spectral reference libraries have a hierarchy of quality. The usefulness of a spectral library is defined in part by how accurate it is and how much known information is captured in it. For example, the resolution of both the sample and reference libraries should match. In the case of IR spectrum, the sample preparation should always be indicated. In the case of Raman spectral reference libraries, a non-resonant excitation frequency should be employed and the final processed spectrum should include a background intensity correction.

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

The following is a general hierarchy of spectral library quality: • Spectrum incorrectly identified with wrong structure. (This is the worst case scenario.) An example of this can include an IR spectrum dominated by water vapor and carbon dioxide. • A false IR spectrum with trade name only (useless). • A photometrically accurate IR or Raman spectrum with trade name only (almost useless). • A photometrically accurate IR or Raman spectrum with structure (Best case scenario of commercial libraries). • A fully interpreted photometrically accurate IR and Raman spectrum with structure (Best case scenario of in-house libraries).

3.2. Computer-Based Libraries and Software Tools Computer-based systems that provide guidance in spectral identification or group frequency analyses are a useful tool, but have limitations that must be understood. The usefulness of a spectral library is defined in part by how accurate it is and how much known information is captured in it. The spectral library must contain structures similar to the unknown in order to be useful. The critical decision to be made is whether the measured spectrum can be considered consistent with the reference spectrum. A common first step for structural identification is to employ a spectral library search. A fundamental limitation is that libraries contain no knowledge of the chemistry of the system and are limited by the size and scope of spectral libraries. Libraries are most useful if the compound of interest, with suitable sample preparation/presentation, exists in the database. A reference spectrum and sample of the same species, but different crystallinity, in the library will negatively affect the accuracy of the identification. In general, a number of automatic search methodologies exist that can be used to compare the spectrum of an unknown sample against many different spectra in the library. These include a simple peak matching to discrimination analysis using Euclidean distance, wavelength correlation (or other algorithms). A search simply reports the best-known match from the spectral library. The unknown may be from an entirely different class of materials. Another approach is to use an empirically based, computer-assisted structural interpretation to aid in a group frequency analysis of the IR and Raman spectrum. The analyst should also use their background knowledge of the chemistry of the sample to help screen such results. One approach involves software that incorporates numerous correlation tables organized by functional group fragments. The analysis of spectrum results in suggested fragments with highlights of important bands for that fragment. These fragments can be combined to provide a hypothetical structure for confirmation. Some software allows the addition of functional group band assignments by the analyst. This can enable the software to focus on the type of molecules commonly encountered in the particular lab. Approximate intensities are often included and include the frequency range. This is a more flexible analysis tool than the “brute force” spectral library approach. The most rigorous approach to structural verification involves using ab initio determined structure with normal mode analysis. The calculations are based on an isolated molecule and result in the theoretical IR and Raman band frequencies, relative intensities, and corresponding vibrations. Interactions encountered in the condensed phase such as hydrogen bonding

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and various environments from different crystalline structures are not taken into account. Therefore, the computational results are most appropriate for gas-phase spectra. The size of the molecule limits this approach because of the computationally intensive analysis employed. Analysis of the Cartesian displacement vectors describing the vibrations from the normal mode analysis can provide important insight into group frequencies and mechanical interactions of the vibrations in the molecule. This approach provides the most rigorous structural verification approach.

3.3. Define the Problem that Needs to be Solved The first step involves defining the problem. Ideally, this should be done before the spectrum of a sample is measured. A complete unknown is rare. Get all the background chemistry information available so that you have a good sample history. Obviously, the analyst should be clear if the sample is research or product related. If this is a research sample, obtain the reaction scheme. Ask the submitter what are the expected products and impurities and if this is an intermediate or final product. If the sample is product related then the analyst should determine the product area/general area of chemistry. Examples can include paper industry related, polymer additives, polymers, and pharmaceutical API’s. IR and Raman spectroscopy are commonly employed for contaminant analysis. In this case, find out what the source matrix is and if this is believed to be a contaminant. In some instances, an identification of extracted materials is needed. In this case, determine the matrix from which the material was extracted. What solvent was used? What filters were used? The analyst must know what spectral signals might be expected from these possible contaminants. Questions to be asked to the submitter can include the following: Where did the sample come from exactly? Was there any sample manipulation prior to analysis? Has there been similar work done in the past? What other techniques are being used in the analysis?

3.4. Examine the IR and Raman Spectrum The first step of interpreting the IR and Raman spectrum is to identify any common impurities that could be present. Some common impurities include silicone oil, water, plasticizers such as phthalates, flow aids such as Mg stearate and commonly encountered polymers (e.g., PE, Nylon, Teflon), and long-chain aliphatics. Look for possible solvents that may have been introduced by any extraction procedures and not fully removed from an isolated reaction product. The analyst should also look for inorganic sulfates, sulfites, or nitrates that may be present as a result of treatment of a reaction mixture with sulfuric acid or nitric acid. Next, determine if the spectrum represents single component or multiple components. Almost all organic and many inorganic species have IR and Raman bands. A complex spectrum can represent either a complex molecule with a wide range of functional groups or multiple components. Multiple component identification is challenging and is a skill that improves with practice. If you have assigned multiple species, review if that is consistent with the chemical background. If possible estimate relative amounts of components present.

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

Be aware of orientation/crystallinity effects since vibrational spectroscopy has excellent environmental sensitivity. This can result in very different IR and Raman spectra for amorphous versus crystalline states. Most of the observed spectral differences are a result of changes in hydrogen-bonding environments. Because of this the analyst should try to ensure that the reference spectra followed the same sample-preparation method and has the same crystalline state as the unknown. Some sample preparations (e.g., cast films) can result in a different crystalline state from the sample of interest. The last step for structural identification and verification is to assign all main peaks in the IR and Raman spectrum. A standard systematic way to begin analyzing the IR/Raman spectrum is to begin at the high wavenumber end of the spectrum (4000 cm 1) and look for the presence and absence of characteristic absorptions as you move to lower wavenumbers. The most common and distinctive bands can then be organized into several spectral regions. Whenever possible, assignments should utilize confirming bands. If possible, measuring a full spectral region (4000e400 cm 1) is important because, for example, the confirming bands for many inorganic species occurs below 650 cm 1 (vide infra). When possible use the basic knowledge about the sample and the chemistry associated with it. Check for group frequencies associated with suspected structural components. Computer-based spectral libraries and structural interpretation software may be valuable tools. Remember to keep in mind the limitations and spectral contributions from any sample-preparation techniques used. Try to assign the most intense bands first. In most cases, you will not be able to fully and accurately assign group frequencies to all of the bands in the spectrum.

4. INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS This section is to be used as a general guide to interpret IR and Raman spectra systematically. As a first check, examine the overall spectral appearance. If the spectrum is very simple with only a few vibrational bands then focus on a low molecular weight organic or inorganic species. Look at inorganic salts such as sulfate, carbonate, or nitrate or simple common organic solvents and water. For organics and hydrocarbons look for bands in the IR and Raman spectra in the region between 3200 and 2700 cm 1, because these are excellent group frequencies associated with CeH-type stretches. After a quick evaluation of the spectral appearance and determining, if hydrocarbons are present, begin to interpret the vibrational spectrum. Start from 4000 cm 1 and work systematically to lower wavenumbers. The presence and absence of functional groups needs to be noted.

4.1. Hydroxy (OH), Amino (NH), and Acetylene (hCH) Groups: 3700e3100 cmL1 Bands in this region are typically due to various OH and NH stretching vibrations. Hydrogen bonding will have a large effect on the amino and hydroxyl stretching vibrations. The OH stretch is medium to strong in intensity in the IR spectrum but is generally weak in the Raman spectrum. IR spectroscopy will be excellent for NH stretching vibrations. Raman

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spectroscopy will be poor for OH and acetylene CH stretching vibrations but moderate for NH stretching vibrations. In the condensed phase, the hydroxyl OH stretching band is generally intermolecularly hydrogen bonded with a broad band appearing between 3550 and 3230 cm 1. In some cases due to crystalline structure or steric hindrance, the hydroxyl group is not hydrogen bonded and resultant sharp and moderately strong free OH stretching bands are observed between 3670 and 3580 cm 1. The OH stretches for water alcohols and phenols are generally observed at 3400 cm 1 but can be found between 3700 and 3300 cm 1. The observed OH stretch depends upon the strength of the hydrogen bond. Acidic protons for the hydroxyl species of carboxylic acid dimers and phosphoric acids result in very broad bands centered at ca. 3000 cm 1. IR bands for hydrogen-bonded amino groups are generally not as broad as those observed for hydrogen-bonded hydroxyl groups. Primary amine (NH) groups result in a single weak band between 3500 and 3300 cm 1 while a secondary amine (NH2) group results in a weak doublet between 3550 and 3250 cm 1 with a spacing of about 70 cm 1. A secondary amide NH results in a stronger band between 3470 and 3200 cm 1 while a primary amide (NH2) group results in a strong doublet between 3550 and 3150 cm 1 with a spacing of about 100e200 cm 1. The acetylene hCH group has a moderate-to-strong sharp IR band between 3340 and 3280 cm 1. The band is quite weak in the Raman spectrum. Lastly, weak overtones from carbonyl bands can occur between 3500 and 3400 cme1 region. Typically, the overtone band frequencies will be ca. 20 cm 1 less than twice the carbonyl stretching band wavenumber and the intensity will be significantly less intense than the carbonyl stretching band.

4.2. Unsaturated Aryl and Olefinic Groups: 3200e2980 cmL1 Both IR and Raman spectroscopy provide moderately strong bands for these groups. If sharp bands are observed between 3000 and 3200 cm 1 then it is likely an unsaturated group is present. Aromatics have multiple weak bands from CH stretch vibrations between 3100 and 3000 cm 1. Most olefins have characteristic CH stretching bands in the 3100e2980 cm 1 region. Observation of isolated bands at 3010 cm 1 and 3040 cm 1 most likely indicates a simple olefinic group. Pyroles and furans will typically have bands in the 3180 and 3090 cm 1 region. Three-membered rings such as cyclopropyl and epoxy rings will also have bands in 3100e2990 cm 1 and 3060e2990 cm 1 regions, respectively.

4.3. Aliphatic Groups: 3000e2700 cmL1 Well-defined, strong characteristic IR and Raman bands for CeH stretching vibrations characteristic of aliphatics are typically observed. Both methyl and methylene groups (CH3, CH2) result in doublets at slightly different frequencies. Methyl CH stretching bands typically have bands between 2975e 2950 cm 1 and 2885e2865 cm 1. Methylene CH stretching bands typically have bands between 2940e2915 cm 1 and 2870e2840 cm 1. The relative intensities of the bands can be used to estimate the relative amount of methyl to methylene groups. Heteroatoms adjacent to these groups result in characteristic group frequencies:

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

The methoxy group has one band at ca. 2835 cm 1, methyl and methylene groups next to the nitrogen atom in tertiary amines result in bands at ca. 2800 cm 1, and aldehydes result in two peaks near 2730 cm 1 (due to Fermi resonance).

4.4. Acidic Protons: 3100e2400 cmL1 Broad bands in this region are typically due to acidic and strongly hydrogen-bonded hydrogen atoms. In general, IR is a much better technique than Raman spectroscopy for these groups. Hydrogen bonding is an important determinant in the frequencies for these groups. The main OH stretching band for carboxylic acid dimers is found at ca. 3000 cm 1 with additional overtone/combination bands at ca. 2650 cm 1 and 2550 cm 1. Primary, secondary, and tertiary amine salts absorb in this region and have distinctive band structure. The NeHþ stretch for these groups are characterized by moderate-to-strong IR and Raman bands. Primary amine salts for the eNHþ 3 have an absorption band(s) between 3350 and þ , >NH , and C¼NHþ have bands between 2700 and 3100 cm 1. Amines with > NHþ 2 1 2250 cm . Phosphoric acids have broad absorption bands in this region.

4.5. SH, BH, PH, and SiH: 2600e2100 cmL1 Both IR and Raman can be very useful for these species. The SH stretch for mercaptans and thiophenols occurs in the region 2590e2540 cm 1 and is generally quite strong in the Raman spectrum, but weak in the IR. BH stretches occur in the region 2630e2350 cm 1 and are moderate to strong in the IR spectrum, but are generally weak in the Raman spectrum. PH stretches occur in the region 2440e2275 cm 1 and are moderately intense in the IR spectrum and moderate to weak in the Raman spectrum. SiH stretches occur in the region 2250e2100 cm 1 and are strong in both the IR and Raman spectrum.

4.6. Triple Bonds and Cumulated Double Bonds: 2300e1900 cmL1 Both IR and Raman can be very useful for these species. The IR and Raman intensities for the XhY and X¼Y¼Z stretching vibrations are variable. Refer to chapter 6 (section 3.2) to use the table and idealized spectra (Figure 6.11 and table 6.3). Contributions from atmospheric carbon dioxide generate a false signal in this region of the IR spectrum.

4.7. Carbonyl-Containing Species: 1900e1550 cmL1 IR is excellent for carbonyl species while Raman is quite variable. The carbonyl C¼O stretching vibration results in strong characteristic IR bands. Raman bands for this vibration are typically moderate to weak with some structures resulting in a strong C¼O stretch. This band is easily identified in the IR spectrum because of its intensity and its lack of interference from most other group frequencies. Carbonyl groups are present in many different compounds such as ketones, carboxylic acids, aldehydes esters, amides, acid anhydrides, acid halides, lactams, lactones, urethanes, and carbamates. Bands above 1775 cm-1 are often due to carbonyl compounds such as anhydrides (doublet), acid halide, or strained ring carbonyl (such as lactones and carbonates in five-membered rings). Bands between

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1750 and 1700 cm 1 fall in the middle of the carbonyl range. Carbonyl compounds in this region can include aldehydes, esters, carbamates, ketones, and carboxylic acids. Bands below 1700 cm 1 fall in the low end of the carbonyl range. Such carbonyl compounds can include amides, ureas, and carboxylic acid salts. Since conjugation can lower carbonyl frequencies, this range will also include conjugated aldehydes, ketones, esters, and carboxylic acids. In general, bands below 1600 cm 1 are due to the out-of-phase stretch of carboxylic acid salts. The remaining carbonyl signals are due to C¼O stretches and occur above 1600 cm 1. Fermi resonance sometimes gives rise to two bands in the C¼O stretching region even though only one C¼O bond is present. See Chapter 2 for an explanation of this effect.

4.8. Olefinic (C[C), Imino (C[N), and Azo (N[N) Compounds: 1690e1400 cmL1 The stretching vibrations of these groups are characterized by strong Raman band which makes Raman an excellent technique for these types of compounds. The intensities of the IR absorptions are variable. The C¼C stretching vibrations result in variable IR intensities. Most olefin C¼C stretching bands occur between 1680 and 1600 cm 1. In IR this results in a narrow, weak absorption band. Conjugation with another double bond lowers the frequency and often increases the IR band intensity. Imino (C¼N) stretching vibrations result in strong Raman and moderately strong IR bands. Most C¼N stretching vibrations occur between 1690 and 1630 cm 1. Azo (N¼N) stretching vibrations give rise to strong, well-defined Raman bands, but a very poor quality IR bands. Most Azo N¼N stretching bands occur between 1580 and 1400 cm 1. The nature of the compound is very important in analyzing spectra of azo species. Symmetrically substituted trans azo compounds give rise to strong Raman bands. Although the signal is forbidden in the IR, non-symmetrically substituted trans azo compounds can give rise to strong IR signals.

4.9. Organic Nitrates (N[O): 1660e1450 cme1 These species include nitro (ReNO2), organic nitrates (ReNO2), organic nitrites (ReOeN¼O), and nitroso groups. The N¼O stretching vibrations result in strong IR bands and generally moderate Raman bands. Aliphatic and aromatic nitro compounds have two IR strong bands between 1580e1475 cme1 and 1390e1320 cme1. Organic nitrites have two strong IR bands between 1660e1615 cme1 and 1300e1270 cme1. Only the lower band is observed by Raman spectroscopy. Inorganic nitrate salts (NO3 ) provide a broad band associated with the out-of-phase NO3 between 1410 and 1350 cme1 (IR strong, Raman moderate). Organic nitrites provide strong IR and Raman bands with generally two bands resulting because of rotational isomers. The cis form exhibits a band in the region 1625e1610 cme1 while the trans form exhibits a band in the region 1680e1650 cme1. Nitroso groups are characterized by strong IR and Raman bands. In the solid and liquid state, nitroso groups exist as dimers in either the cis or trans state. The aliphatic cis form results in two bands: 1425e1330 cme1 and 1345e1320 cme1. The trans form results in a single band in the region 1290e1175 cme1. The aromatic cis form results in two bands: 1400e1390 cme1 and 1410 cme1. The trans form results in a single band in the region 1300e1250 cme1.

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7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

4.10. Amine NH Deformation Vibrations for Amines, Amine Salts, and Amide Compounds: 1660e1500 cme1 NH deformation bands result in moderate IR bands and weak Raman bands. Primary amine NH2 in-phase deformation occurs in the region 1660e1590 cme1. Amine salts result e1 in a NHþ while a NHþ 2 in-phase deformation band found between 1620 and 1560 cm 3 out-of-phase deformation results in two sets of bands between 1635e1585 cme1 and 1585e1560 cme1. In secondary carbamates and mono-substituted amides, the CNH group results in a strong IR band between 1570 and 1510 cme1. Note that water also has a band near 1640 cme1 from the OH deformation.

4.11. Aromatic and Hetero-aromatic Rings: 1620e1420 cme1 Sharp bands in this region involve the aromatic ring quadrant and semi-circle stretching vibrations. The IR intensities for these bands are variable in intensity. The Raman intensities are moderate for the quadrant stretch and quite weak for the semi-circle stretching bands. Typically two sets of bands are observed in this region due to the quadrant and semi-circle ring stretch vibrations. These may occur as single bands or as a multiple component envelop of bands. In general, the quadrant stretch will result in bands near 1600 and 1580 cme1 while the semi-circle stretch will result in bands near 1500 and 1460 cme1 The relative intensity of the IR bands will vary with the aromatic substituent pattern and type of substituent. Sixmembered ring heteroaromatics resulting from substituting an aromatic carbon atom with a nitrogen atom result in similar characteristic ring-stretching bands between 1600 and 1500 cme1. These include single nitrogen-substituted aromatic molecules such as pyridines, quinolines, and isoquinolines, two nitrogen-substituted aromatic molecules such as pyrimidines and quinazolines, and three nitrogen-substituted molecules such as triazine species. Simple aromatic compounds will also generate a series of weak IR absorption bands between 2000 and 1700 cme1 which derive from multiple combination bands.

4.12. Methyl and Methylene Deformation Vibrations: 1500e1250 cme1 These alkane deformation vibrations results in moderate IR bands and weak to moderate Raman bands. When these groups have identical substituents then the CH2 “scissors” deformation and CH3 out-of-phase deformation are found in the same frequency region, ca. 1480e1430 cme1. When the CH2 and CH3 are on hydrocarbons, this band is near 1460 cme1. When the CH2 (methylene) is near an unsaturated group, the deformation band is found near 1440 cme1. If the methylene is adjacent to chlorine, bromine, iodine, sulfur, phosphorus, nitrile, nitro, or carbonyl, this band occurs between 1450 and 1405 cme1. The CH3 in-phase deformation is a useful moderately intense IR band, but provides only a weak Raman band. The CH3 in-phase (“umbrella”) deformation is strongly dependent upon the electronegativity of the adjacent atom. The more electronegative the atom, the higher the frequency with a range of 1470e1250 cme1. Examples include OeCH3 at ca. 1450 cme1, CeCH3 at ca. 1450 cme1, and SieCH3 at ca. 1265 cme1. In aliphatic groups, steric hindrance to the vibrations due to the presence of two or three adjacent methyl

INTERPRETATION GUIDELINES AND MAJOR SPECTRAeSTRUCTURE CORRELATIONS

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groups can result in highly characteristic bands. Here CeCH3 results in a single band at 1378 cme1, while the isopropyl C(CH3)2 has an equal intensity doublet at 1385 and 1368 cme1, and the t-butyl C(CH3)3 has a non-equally intense doublet at 1395 and 1368 cme1.

4.13. Carbonate, Nitrate, Ammonium, and BeO Type Compounds: 1480e1310 cme1 Inorganic carbonate, nitrate, and ammonium ions provide a broad, intense IR band near 1400 cme1 but only a weak Raman band. IR bands result from the out-of-phase XY3 stretch for carbonate- and nitrate-containing compounds. Strong sharp Raman bands in the 1040e1100 cme1 region result from the in-phase XY3 stretch. The ammonium ion deformae1 region. tion band is due to a NHþ 4 bend and is found in the 1480e1390 cm The BeO stretch provides a strong characteristic IR band. Inorganic boric acid has a strong broad band near 1410 cme1 due to the BO3 out-of-phase stretch. Organic borates [B(RO)3], boronates [PhB(RO)2], metaborates (cyclic six-membered BeO ring), boronic acid esters, and anhydrides are all characterized by strong IR bands in the region 1390e1310 cme1 involving the BeO stretch.

4.14. Organic and Inorganic SOx Type and Thiocarbonyl (C[S) Compounds: 1400e900 cme1 In general, both IR and Raman provide strong characteristic bands associated with SOx and C¼S stretching vibrations. Inorganic SOx type compounds include sulfites (SO2e 3 ), sulfates (SO2e 4 ) as well as more complex SXOY type compounds (thiosulfate, pyrosulfite, dithionate). Sulfites have a strong broad IR band and a strong doublet Raman band between 1010 and 900 cme1 involving the SO3 out-of-phase and in-phase stretching vibrations. Sulfates are characterized by a very strong IR band between 1130 and1080 cme1 involving e1 the SOe2 4 out-of-phase stretch and a strong Raman band between 1065 and 955 cm e2 involving the SO4 in-phase stretch. SXOY type compounds are characterized by strong IR and Raman bands between 1250 and 1000 cme1 involving complex S¼O stretching vibrations. Organic SOX type compounds include >S¼O, >SO2 as well as sulfonic acids (eSO3H), and þ sulfonic acid salts (eSOe 3 M ) (See chapter 6, section 8). In general, the stretching frequencies involving the S¼O group of these types of compound show a dependence on the electronegativity of the substituents. Sulfoxides (R2>S¼O) are characterized by a strong IR band and a moderate-to-weak Raman band between 1070 and 1030 cme1 from the S¼O stretch. Similarly, sulfinic acids (eS(¼O)eOH) are characterized by a very strong IR and moderate-tostrong Raman band in the region 1090e990 cme1 due to the S¼O stretch. The S¼O stretch is shifted to higher frequencies for sulfinic acid esters (RdS(¼O) dOeR) at 1140e1125 cme1 (strong IR; moderate intensity Raman) and dialkyl sulfites ((RO)2S¼O) in the region 1220e1170 cme1 (strong IR; strong, polarized Raman). Sulfones (Rd SO2dR) are characterized by two vibrations, an out-of-phase and in-phase stretch of the SO2 group. In most cases, strong IR bands result for both in-phase and out-of-phase vibrations while a strong Raman band only occurs for the in-phase SO2 stretch. Sulfones (>SO2) have strong characteristic bands at 1360e1290 cme1 and 1200e1120 cme1. Sulfanamides (RdSO2dNh)

130

7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

have strong bands at 1360e1315 cme1 and 1180e1140 cme1. The higher frequencies are due to the influence of the more electronegative nitrogen atom. Sulfonyl halides (dSO2dX) have strong bands at 1385e1360 cme1 and 1190e1160 cme1. The out-of-phase SO2 stretch results in a moderate-to-strong Raman band. Covalent sulfonates (RdSO2dOR) and organic sulfates (eOdSO2dOe) have bands at 1420e1330 cme1 and 1200e1145 cme1. Organic sulfate salts (ROdSO2dOe Mþ) are characterized by a strong IR band in the region 1315e1210 cme1 and a strong Raman band in the region 1140e1050 cme1. þ Organic sulfonic acids (dSO3H) and salts (dSOe 3 M ) are characterized by strong IR and Raman bands involving the SO2 and SO3 stretching vibrations. Sulfonic acids (dSO3H), if not dried, are easily ionizable with small amounts of water resulting in a hydrated form (dSOe 3 H3Oþ). Anhydrous sulfonic acids (ReSO2dOH) have strong IR and Raman bands at 1355e1340 cme1 and 1200e1100 cme1 from the SO2 out-of-phase and in-phase stretches, þ e1 and respectively. Hydrated sulfonic acids (RSOe 3 H3O ) have bands at 1230e1120 cm e1 1120e1025 cm from the SO3 out-of-phase and in-phase stretches, respectively. Both SO3 vibrations result in strong IR bands. However, a moderate-to-strong Raman band is observed þ only for the SO3 in-phase stretch. Sulfonic acid salts (dSOe 3 M ) have bands at e1 e1 1250e1140 cm and 1070e1030 cm . The out-of-phase SO3 stretch results in very strong IR bands but weak Raman bands. However, the in-phase SO3 stretch results in strong IR and Raman bands. Thiocarbonyl compounds (>C¼S) are generally characterized by a strong IR and Raman band between 1250 and 1200 cme1 that involve the C¼S stretch.

4.15. P[O-Containing Compounds: 1350e1080 cme1 The P¼O stretching vibration for organic phosphorus compounds results in a strong IR band between 1350 and 1140 cme1 while the Raman band intensity is moderate to weak. The frequency of the P¼O stretch shows a strong dependence on the substituent electronegativity and hydrogen bonding effects. Inorganic POx type compounds include dibasic e 3e phosphate (HPO2e 4 ), monobasic phosphate (H2PO4 ), phosphate (PO4 ). The phosphate outof-phase stretch has strong IR and Raman bands between 1180 and 1000 cme1. The dibasic e1 from phosphate (HOdPO2e 3 ) has strong IR and Raman bands between 1150 and 1000 cm e the PO3 out-of-phase stretch. The monobasic phosphate ((HO)2 PO2 ) has strong IR and Raman bands between 1200 and 950 cme1 from the PO2 out-of-phase stretch. The P¼O band can appear as a doublet, possibly due to Fermi resonance or, in some cases, rotational isomerism.

4.16. Fluorinated Alkane Groups: 1350e1000 cme1 The CeF stretch of organic fluorine compounds results in strong IR bands but weak to moderately intense Raman bands. Monofluorinated compounds have a strong IR and a weak to moderate Raman band between 1100 and 1000 cme1. Difluorinated compounds (dCF2d) have two very strong IR bands between 1250 and 1050 cme1 involving the CF2 out-of-phase stretch. These bands are weak to moderate in the Raman spectrum. Trifluorinated compounds (dCF3) have multiple strong bands between 1350 and 1050 cme1 involving the CF3 out-of-phase stretch. Fluorine substituted aromatics (ArdF) have a moderately strong IR band between 1270 and 1100 cme1 involving the aryl ring CeF stretch.

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4.17. CeO Stretching Vibrations: 1300e750 cme1 Bands in the region 1300e750 cme1 are associated with the CeO stretch of species such as alcohols, ethers, esters, carboxylic acids, and anhydrides. Ethers are characterized by CdOdC out-of-phase and in-phase stretches. In general, the out-of-phase CeOeC stretch results in strong IR bands in the range 1270e1060 cme1 but weak Raman bands. The in-phase CeOeC stretch results in strong Raman bands and moderate IR bands at 1140e800 cme1. Alcohols are characterized by CeO type stretch vibrations in the region 1200e750 cme1. Primary alcohols are characterized by a strong IR band between 1090 and 1000 cme1 and a strong Raman band between 900 and 800 cme1. Secondary alcohols are characterized by multiple strong IR bands between 1150 and 1075 cme1 and a strong Raman band between 900 and 800 cme1. Tertiary alcohols are characterized by multiple strong IR bands between 1210 and 1100 cme1 and a strong Raman band between 800 and 750 cme1. Carbonyl compounds such as esters, carboxylic acids, and anhydrides include a CeO group that is often used to confirm the functional group. The CeO stretch of carboxylic acids is characterized by moderately intense IR bands in the range 1380e1210 cme1 with the corresponding Raman bands having highly variable intensity. Similarly, esters result in strong IR bands between 1300 and 1100 cme1 and corresponding Raman bands of variable intensity. Anhydrides are characterized by strong IR and Raman bands between 1310 and 980 cme1 involving the CeOeC stretching vibrations.

4.18. SieO and PeO Containing Compounds: 1280e830 cme1 The SieO group is characterized by strong IR bands and variable Raman bands. Compounds containing PeOeC group are characterized by bands in the range 1090e920 cme1 which involve the out-of-phase stretching vibration. These characteristic bands are typically strong in the IR and moderate to weak in the Raman spectrum. The PeOeP and PeOH groups are characterized by strong IR bands but only moderate-toweak Raman bands. The PeOeP group is characterized by a strong IR band between 1025 and 870 cme1 involving the PeOeP out-of-phase stretch. The PeOH group is characterized by bands involving the PeO stretch between 1000 and 900 cme1 with strong IR intensity, but moderate-to-weak Raman intensity. The siloxane SieOeSi and SieOH groups are characterized by strong IR bands at 1090e1010 cme1 and 955e830 cme1, respectively, involving the SieO stretch. Raman spectra of siloxanes do not provide useful bands in this region. Alkoxysilanes, SieOeR, are characterized by an out-of-phase stretch (1110e1000 cme1) which results in a strong IR band and an in-phase stretch (1070e990 cme1) which results in moderate-to-strong intensity IR and Raman bands.

4.19. CH Wag of Olefinic and Acetylenic Compounds: 1000e600 cme1 The CH wag vibration provides important medium-to-strong IR bands for characterizing alkenes and acetylenes. These bands are typically weak in the Raman spectrum. The position of these bands is dependent on the properties (electron withdrawing) of the substituents.

132

7. GENERAL OUTLINE AND STRATEGIES FOR IR AND RAMAN SPECTRAL INTERPRETATION

Vinyl groups, eCH¼CH2, have strong IR bands between 1000e940 cme1 and 960e810 cme1 (995e980 cme1 and 915e905 cme1, respectively, for hydrocarbons). Vinylidene groups, >C¼CH2, exhibit strong IR bands between 985 and 700 cme1 (895e885 cme1 for hydrocarbons). Trans-di-substituted ethylenes, eCH¼CHe, have strong IR bands between 980 and 890 cme1 (1000e955 cme1 for hydrocarbons). Vibrational interaction with conjugated diene groups results in a systematic increase in the observed CH wag. An isolated trans alkene (T) has a CH wag at 965 cme1, a conjugated transetrans diene (TT) at 986 cme1, the transetranse trans alkene (TTT) at 994 cme1, and the transetransetransetrans alkene (TTTT) at 997 cme1. Cis-di-substituted ethylenes have strong IR bands between 800 and 600 cme1 (730e650 cme1 for hydrocarbons). Tri-substituted alkenes, >CH¼CHe, absorb between 850 and 790 cme1. The CH wag of acetylenic, ChCH, have moderate-to-strong IR bands between 700 and 578 cme1.

4.20. Aromatic In-plane 2,4,6 Radial Carbon In-phase Stretch: 1290e990 cme1 Very strong Raman bands are observed from aromatic ring vibrations involving the 2,4,6 radial carbon in-phase stretch. A very strong Raman band between 1010 and 990 cme1 is observed for mono-, 1,3 di-, and 1,3,5 tri-substituted benzenes. Only a weak to moderately intense sharp IR band is observed.

4.21. Aromatic CH Wag: 900e700 cme1 The aromatic ring CH wag vibrations results in strong IR absorption bands but only weak Raman bands. The frequencies of the aromatic CH wag are largely dependent upon the number of adjacent hydrogen atoms since the adjacent CH wag vibrations are mechanically coupled to one another. For this reason, the IR bands from the in-phase CH wag vibrations are quite useful in distinguishing aromatic ring substitutions. Mono-substituted benzenes have a band at ca. 750 cme1 involving the five adjacent CH wag (overall region between 820 and 728 cme1). Ortho di-substituted benzenes also have a band at ca. 750 cme1 involving the four adjacent CH wag (overall region between 790 and 728 cme1). The three adjacent CH wag vibrations of meta- or 1,2,3 tri-substituted benzenes have a band at ca. 782 cme1 (overall region between 825 and 750 cme1). The two adjacent CH wag vibrations of para, 1,2,4 tri-substituted, or 1,2,3,4 substituted benzenes have a band at ca. 817 cme1 (overall region between 880 and 795 cme1). The lone CH wag vibration (meta, or 1,2,4,. or 1,3,5,. or 1,2,3,5,. or 1,2,4,5,. or penta) substituted benzenes have a band at ca. 860 cme1 (overall region between 935 and 810 cme1). The aryl CH adjacent classification can also be extended to pyridines and napthalenes.

4.22. Halogen-Carbon Stretch: 850e480 cme1 Strong IR and Raman bands result for the halo-alkanes involving the CeX stretching vibrations (X¼F, Cl, Br, I). Fluoroalkanes are summarized above. The CeCl stretching

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vibrations occur between 860 and 505 cme1. Compounds with more than one chlorine atom will have multiple bands deriving from the in-phase and out-of-phase CeClx (X ¼ 2,3,4) which may also couple with other groups. The CeBr stretching vibrations occur between 680 and 485 cme1. Compounds with more than one bromine atom will have multiple bands deriving from the in-phase and out-of-phase CeBr2 which may also couple with other groups. The CeI stretching vibrations occur between 610 and 485 cme1.

4.23. OH, NH, NH2 Wag: 900e500 cme1 In the condensed state, moderately intense (but broad) IR bands are found between 900 and 500 cme1 due to the XeH wag of hydrogen-bonded water, amines, amides, and alcohols. Hydrogen bonding significantly influences the frequency for these characteristic IR bands. No significant Raman bands from these vibrations are observed. Alcohols exhibit bands that are generally centered around 650 cme1. Water absorbs below 800 cme1. Primary amines have bands between 900 and 770 cme1 and secondary amines between 750 and 680 cme1. Amides have bands between 750 and 550 cme1.

4.24. Metal Oxides: 800e200 cme1 Metal oxides of various types have strong IR and Raman bands from the metal-O-metal group.

C H A P T E R

8 Illustrated IR and Raman Spectra Demonstrating Important Functional Groups The IR and Raman spectra of each compound are presented in a stacked format with the spectrum number given in the upper right-hand corner of each IR spectrum. The sample preparation and type of crystal used for the IR measurement is listed in parenthesis after the compound name in the spectral index below. The IR measurements were made using a BioRad FTS-575C spectrometer. All Raman measurements were made using 180o backscattering on a BioRad FT-Raman spectrometer (575C with Raman III accessory) and background corrected using a KBr white light reflectance measurement. A melting point or NMR tube was used for the Raman measurements. A single bounce diamond ATR accessory was used for the IR measurements of the polymers. The assignment shown on all of the spectra are consistent with that discussed in the text. Page numbers of spectra are shown on the right hand side.

1. ALIPHATIC 1. Heptane, [IR: capillary film/ZnSe], 2. Cyclohexane, [IR: capillary film/ZnSe], 3. Isooctane (2,2,4-Trimentyl pentane), [IR: capillary film/KBr],

140 140 140

2. C[C DOUBLE BONDS 4. 1-Hexene, [IR: capillary film/KBr], 5. cis-3-Heptene, [IR: capillary film/KBr], 6. trans-3-Heptene, [IR: capillary film/KBr],

141 141 141

3. TRIPLE BONDS 7. Acetonitrile, [IR: capillary film/KBr], 8. Acrylonitrile, [IR: capillary film/KBr], 9. 1-Octyne, [IR: capillary film/KBr],

142 142 142

135

136

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

4. AROMATIC RINGS 10. 11. 12. 13. 14. 15.

Toluene, [IR: capillary film/ZnSe], o-Xylene, [IR: capillary film/KRS5], m-Xylene, [IR: capillary film/KRS5], p-Xylene, [IR: capillary film/KBr], m-Tetramethyl xylene diisocyanate, [IR: smear/KBr], Pyridine, [IR: capillary film/KRS5],

143 143 143 144 144 144

5. KETONES, ESTERS, AND ANHYDRIDES 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

Acetone, [IR: capillary film/KBr], n-Butyl acetate, [IR: capillary film/KBr], Dimethyl carbonate, [IR: capillary film/KBr], Dimethyl malonate, [IR: capillary film/KBr], Dimethyl terephthalate, [IR: KBr disc], Butyric anhydride, [IR: capillary film/KBr], Maleic anhydride, (Furan-2,5-dione) [IR: KBr disc], 4-Hydroxy benzaldehyde, [IR: KBr disc], Glycolic acid, (2-Hydroxyacetic acid) [IR: KBr disc], Palmitic acid, [IR: KBr disc], Oleic acid, (Z-Heptadec-9-enoic acid): [IR: capillary film/KBr], Benzoic acid, [IR: KBr disc], Stearic acid, calcium salt, [IR: KBr disc], Glycine (Zwitterion), (2-Aminoacetic acid) [IR: KBr disc],

145 145 145 146 146 146 147 147 147 148 148 148 149 149

6. AMIDES, UREAS, AND RELATED COMPOUNDS 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

Acetamide, [IR: methanol cast film/KRS5], Acrylamide, [IR: KBr disc], N-butyl methyl acrylamide, [IR: smear/KRS5], 1,3-Dimethyl urea, [IR: KBr disc], Ammeline, (4,6-Diamino-1,3,5-triazin-2(1H)-one) [IR: KBr disc], Trimethyl isocyanurate, (1,3,5-Trimethyl-1,3,5-triazinane-2,4,6-trione) [IR: KBr disc], 2-Imidazolidone, (Imidazolidin-2-one) [IR: methylene chloride cast film/KRS5], Dimethyl formamide, [IR: capillary film/ZnSe], Ethyl carbamate, [IR: methanol cast film/KRS5], Acetohydroxamic acid, (N-Hydroxyacetamide) [IR: KBr disc],

149 150 150 150 151 151 151 152 152 152

COMPOUND INDEX

137

7. ALCOHOLS 40. 41. 42. 43. 44. 45. 46. 47.

Methanol, [IR: capillary film/KBr], Isopropyl alcohol, (Propan-2-ol) [IR: capillary film/KBr], t-Butyl alcohol, (2-Methylpropan-2-ol) [IR: capillary film/KBr], 1-Butanol, [IR: smear/KRS5], 1-Hexadecanol, [IR: methylene chloride cast film/ZnSe], Phenol, [IR: melt/KRS5], 2,6-Di-t-butyl phenol, [IR: methylene chloride cast film/ZnSe], Diethylene glycol, (2,2’-oxydiethanol) [IR: smear/ZnSe],

153 153 153 154 154 154 155 155

8. ETHERS 48. n-Dibutyl ether, (1-Butoxypentane) [IR: capillary film/ZnSe], 49. Tetrahydrofuran, [IR: capillary film/ZnSe], 50. Epichlorohydrin, (2-(chloromethyl)oxirane) [IR: capillary film/ZnSe],

155 156 156

9. AMINES AND AMINE SALTS 51. 52. 53. 54. 55. 56.

Isobutyl amine, (2-Methylpropan-1-amine) [IR: capillary film/ZnSe], Aniline, [IR: capillary film/KRS5], n-Dibutyl amine, (n-Butylpentan-1-amine) [IR: capillary film/ZnSe], n-Tributyl amine, (N,N-Dibutylpentan-1-amine) [IR: capillary film/ZnSe], Trimethyl amine HCl, [IR: KBr disc], Tetramethyl ammonium chloride, [IR: KBr disc],

156 157 157 157 158 158

10. C[N COMPOUNDS 57. Benzamidine HCl, [IR: KBr disc], 58. Acetamide oxime, (Z)-N’-Hydroxyacetimidamide [IR: KBr disc], 59. Pipenidindione dioxime, (2E,6E)-Piperidine-2,6-dione dioxime [IR: KBr disc],

158 159 159

11. N[O COMPOUNDS 60. Isoamyl nitrate, [IR: capillary film/KBr], 61. 2-nitropropane, [IR: capillary film/KBr], 62. Isopropyl nitrate, [IR: capillary film/KBr],

159 160 160

138

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

12. AZO COMPOUND 63. Azo-tert-butane, (E)-1,2-di-tert-butyldiazene [IR: capillary film/KBr],

160

13. BORONIC ACID COMPOUND 64. meta-tolyl boronic acid, [IR: Nujol mull/KBr],

161

14. CHLORINE, BROMINE, AND FLUORINE COMPOUNDS 65. 66. 67. 68. 69.

1-Chlorooctane, [IR: capillary film/KBr], 1-Bromohexane, [IR: capillary film/KBr], 3-Fluorotoluene, (1-Fluoro-3-methylbenzene) [IR: capillary film/KBr], 3-Chlorotoluene, (1-Chloro-3-methylbenzene) [IR: capillary film/KBr], 3-Bromotoluene, (1-Bromo-3-methylbenzene) [IR: capillary film/KBr],

161 161 162 162 162

15. SULFUR COMPOUNDS 70. 71. 72. 73. 74. 75. 76.

Dimethyl sulfoxide, (Methylsulfinylmethane) [IR: capillary film/KBr], Methane sulfonic acid, sodium salt, [IR: Nujol/Fluorolube mixed mull on KBr], para-Toluene sulfonic acid hydrate, [IR: KBr disc], Dodecyl sulfate, sodium salt, [IR: Nujol/Fluorolube mixed mull on KBr], Sodium isopropyl xanthate, [IR: KBr disc], Zinc dimethyl dithiocarbamate, [IR: KBr disc], 2-Imidazolidinethione, (Imidazolidine-2-thione) [IR: KBr disc],

163 163 163 164 164 164 165

16. PHOSPHORUS COMPOUNDS 77. 78. 79. 80.

Triphenyl phosphine, [IR: KBr disc], Tributyl phosphite, [IR: capillary film/KBr], n-Dibutyl phosphite, [IR: capillary film/KBr], Sodium di-isobutyl dithiophosphate, [IR: water cast film/KRS5],

165 165 166 166

17. SILOXANE COMPOUNDS 81. Siloxane polymer (silicone grease), [IR: smear/KBr],

166

COMPOUND INDEX

139

18. INORGANIC COMPOUNDS 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94.

Ammonium chloride, [IR: KBr disc], Sodium sulfate, [IR: KBr disc], Sodium sulfite, [IR: KBr disc], Sodium carbonate, [IR: KBr disc], Sodium bicarbonate, [IR: KBr disc], Potassium nitrate, [IR: KBr disc], Tribasic sodium phosphate, [IR: KBr disc], Dibasic sodium phosphate, [IR: KBr disc], Boric acid, [IR: KBr disc], Sodium perchlorate, [IR: KBr disc], Sodium molybdate, [IR: KBr disc], Silica gel, [IR: KBr disc], Talc, [IR: ATR],

167 167 167 168 168 168 169 169 169 170 170 170 171

19. POLYMERS AND BIOPOLYMERS 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110.

Isotactic polyethylene, [IR: ATR], Polybutadiene, [IR: ATR], Polyvinyl alcohol (PVA), [IR: ATR], Polyethylene oxide, [IR: ATR], Polytetrafluoroethylene (Teflon), [IR: ATR], Poly acrylic acid, [IR: ATR], Polyvinyl pyrrolidone, [IR: ATR], Nylon 6,6, [IR: ATR], Cellulose, [IR: ATR], Starch, [IR: ATR], Sodium carboxymethyl cellulose, [IR: ATR], Hydroxypropylmethyl cellulose, [IR: ATR], Ethyl cellulose, [IR: ATR], Siloxane polymer, [IR: cap film/KBr], Poly acrylamide (PAM), [IR: water cast film/ZnSe], Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC), [IR: water cast film/ZnSe],

171 171 172 172 172 173 173 173 174 174 174 175 175 175 176 176

140

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100 1379

(CH2) 5 B

B

CH 3

B

B B

776

394 357

310

C-C-C o. ph. str

902 840

1451

Ra I

CH 3

CH2 i. ph. twist

1302

2959

2875

2937

CH2 bend + CH3 o. ph. bend

CH2, CH3 str.

CH2 i. ph. rk.

CH3 i. ph. bend

2733

0.1

CH3 o.ph. str

2963

Heptane

20

2859

40

1138 1081 1046

60 1466

IR %T

1

723

80

0.0 100

2 904 862

1039

1257

60

2660

2793

80 IR %T

40

1029

Ring i. ph. str.

427

1157

1445

0.4

1267

CH2 bend

2665

Ra I

CH2 i.ph. str.

2938 2853

CH2 o.ph. str.

1348

0.8

802

2853

2925

Cyclohexane

1450

S

20

0.0 isopropyl

980 C C C

C

i. ph. str.

303

513

C

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumbers(cm–1)

1200

1000

CH3

C C C

827

929

1248

1169 1098

1353

1452

C

CH3

i. ph. str.

2715

0.1

CH3

C

2873

2908

2957

0.2

CH3 o. ph. bend.

CH3 CH CH2

746

CH3 i. ph. str.

3 CH3

CH3 i. ph. bend.

t-butyl

CH3 o. ph. str.

0.3

1366

2902

2956

Isooctane

20

Ra I

1469 1393

2871

60 40

1248

80 IR %T

1169

100

800

600

400

141

C¼C DOUBLE BONDS

100 553

739

632

1103

CH(CH 2)3CH 3

360 311

629

915 875 820

1105 1058

1-Hexene

CH 2

910

C

CH2 chain twist

1297

1642

993

1459

1642

2870 2877

H H

H

1219

Ra I

C

H C C

C

H

1443 1417

=CH2 o. ph. str.

2735

0.1

H

H

CH3 i. ph. bend

=CH2 CH2 bend bend

C=C str.

3081

=CH str.

1380

1822 3078

CH3 o. ph. str.

2 x 910

2961

20

=CH2 o. ph. str.

3000

IR 60 %T 40

1297

4

80

0.0

100

H

711

870

968

1069

C 3H 7

312

709

486

389

852 897

1069 1020 970

2731

1269

CH2 bend + CH3 o. ph. bend 1450

CH3 i. ph. str.

C2H5

cis-3-Heptene

1460

1378

1655 2932 2874

H C C

1655

=CH str.

Ra 0.1 I

H C C

C=C str.

3009

0.2

CH3 i. ph. str.

H

cis o. pl. wag

CH3 i. ph. bend =CH rk

CH2 o. ph. str. 2876

CH3 o. ph. str.

2963 3007

20

=CH str.

2936

IR 60 %T 40

1304

5

80

0.0

772

1050

1291

C H

trans o. pl. wag

C2H5

H C C

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumbers(cm–1)

1200

1000

800

600

400

326

H 467

908 866 808

1067 1030

1248

C3H7

trans-3-Heptene

C

966

1460 1441 1379

2931 2875 2730

2997

Ra 0.1 I

2965

C=C str. HC=CH i. ph. str.

6

H

CH3 i. ph. bend

CH2 bend + CH3 o. ph. bend 1441

CH3 o. ph. str.

1670

0.2

CH3 o. ph. str.

2876

20

HC=CH o. ph. str.

2936 2963

80 IR 60 %T 40

3027

100

142

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

750

918

1039

2293

1444 1376

N

C

C-C str,

C-C-N bend 380

920

CH3 i. ph. bend.

CH3

1443 3715

3003 2884

CH3 o. ph. str.

C≡N str.

2 X 380 bend

CH3 rk

CH3 o. ph. bend.

2253

0.3 0.2 Ra I 0.1

2254

CH3 i. ph. str.

2293

20

CH3 o. ph. str.

2943

Acetonitrile

40

7

918 + 1376

2733

60

918 + 2254

3165

80 IR %T

3003 2945

100

0.0

870 805

=CH2 bend

C

C

C H

cis CH wag

H H

H C

687

1094

H

=CH2 wag + trans CH wag

H

H

C

C H

H2C

C

C H

C

N

241

1610

0.3

1287

1655 1609

C=C str.

2230

Acrylonitrile

2 x 969

C≡N str.

8

969

=CH2 str. =CH str.

1413

40 20

1939

60

2229

2990

IR %T

3119 3071

80

2280

100

568

689

1094

872

C-C str. 1287

3120

0.1

2991

0.2 Ra I

1414

3035

i. pl. bend

0.0

C≡C str.

527

885

1113

725

9

CH3 rk

629

2119

CH2 i. ph. str. CH2 o. ph. str.

CH2 bend + CH3 o. ph. bend

≡C-H wag 339

≡C-H str.

2x ≡C-H wag

≡C-CH 2 bend

2958 2934 2861

3314

20

8

1467 14601433 1380 1327

60 40

1-Octyne

1687

2120

80 IR %T

1242

CH3 i. ph. bend

100

292

632

885 843 810

1114 1077 1014

1439

1327 1304

2

2851 2731

4 3314

Ra I

2930

6

0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

143

AROMATIC RINGS

10 896

464

1005

695

729

O

523

1032

219

787

O

623

2 x 1381

CH3

Toluene

1080

1030

1379 1461

Aryl CH3 i. ph. bend

1606

2983 2921

Aryl CH str.

1605

1858 1803

1943

3056

0.6 Ra 0.4 I 0.2

O

Ring Quad. Str.

Aryl CH3 i. ph. str. + 2x 1461 cm-1

1211 1158

0.8

CH3 o. ph. bend 5 adj Aryl CH i. ph. wag Ring Semi. Str.

1438 1381

Aryl CH str.

2737

20

Ring CH wag summation bands

3028

40

CH3 i. ph. bend

1496

60

2921

3086

80 IR %T

2872 2735

2 x 1379

1735

100

0.0 100

o-Xylene

435 CH3

Aryl CH i. pl. bend

986

506

257

583

1053

CH3

1158

1223

505

582 735 743

Aryl CH3 i. ph. bend

1291

2 x 1385

1450 1385

3045 2920

0.2

4 adj aromatic CH wag

CH3 o. ph. bend

Ring semi-circle CH3 o. ph. bend

Ra 0.4 I

932 1120 1053 1020 985

1467

1496

1383

1606 Ring Quad. str.

CH3 i. ph. str. + overtone

1223

1291

1788

CH3 o. ph. str.

2940

1608 1583

0.6

Aryl CH str.

CH3 i. ph. str. + overtone

2733

0.8

3017

20

2878

3063

40

2 x 1383

1900

60

2731

3105

IR %T

1942

11

80

0.0 100

432

691

769

3 adj CH wag

O

538

726

1001 3

227

O

278

516

1095 1035

1251

1171

CH3

1379

i. ph. bend

0.0 3500

3000

2500

2000

m-Xylene

515 876

905

1095

1040

1250 1460

1492

1612 CH3

1451

Ra 0.3 I

Lone aryl CH wag

Ring Ring Quad. str. semi-circle Aryl CH

Aryl CH3 i. ph. str. + 2x 1460 cm-1

1612 1593

Aryl CH str.

3051 2919 2865 2733

0.5

3017 2921

20

2864

40

1170

1377

1772

2x 1377

1653

60

1932

2732

IR %T

1854

12

80

1800

1600

1400

Wavenumbers(cm–1)

1200

1000

800

400

600

144

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

1043

1220

1119

1379

483

795

Ring semi-circle str.

CH3

313

O

812

Aryl CH3 i. ph bend

460

1206

830

O

646

388

0.2

1630

Ring Quad. str

3055 3014 2922 2866 2735

Ra I

1455

Aryl CH3 i. ph. str. + 2x 1455

0.4

2 adj. H wag

1184

0.6

1517

Aryl CH str.

CH3

CH3 o. ph. bend

1619 1580

p-Xylene

20

13

1448 1380 1314

3020 2923

40

1891

60

2x 1379

2869

IR %T

3095

80

1793

2733

100

0.0

1003

500

633 580

797

706

1086

1157

1273

NCO

O

604 749

705

N

N

Ring Quad. i. pl. bend 654 606

1219 1149

1486

1575

0.2

2957

3147

0.4

406

885 992 993

1031

1439

N

5 adj. H wag

3059

0.6

Ring semi-circle str.

1032

Ring Quad. str

1068

1217 1148

1483

1.0 0.8

0.0 3500

3000

2500

2000

1800

262

343

565

685

NCO

795

931

1489

1634

1873

1923

Aryl CH str.

Ra I

O

15

1582

20

Pyridine

1232

2259

3054 3027

3079

60 40

1988

3002

H2O

3410

IR %T

3 adj. H wag

H2O 3660

80

1423 1368

1606

2879

0.0 100

NCO i. ph. str

1173 1117 1088

0.1

Aryl lone CH wag

CH3 i. ph bend

1276

0.2

3071 2987 2941 2868 2769 2724

Ra I

Ring Quad. str

NCO o. ph. str.

CH3 str.

1463 1443 1416

0.3

Ring semi-circle str.

1608

Aryl CH str.

CH3 o. ph. str.

2256

20

NCO Comb. 2259 + 1416

2936

40

3058

60

2983

IR %T

929 863

14

80 3655

m-Tetramethyl xylene diisocyanate

100

1600

1400 –1

Wavenumber(cm )

1200

1000

800

600

400

145

KETONES, ESTERS AND ANHYDRIDES

16 530

Acetone

390

899

1067

1222

1429

1358

531

C-C-C i. ph. str.

C CH3

C=O i. pl. rk.

788

CH3 o. ph. bend

1710

2698

3005

2848

0.2

903

1093

1283 1716

C=O str.

0.4 Ra I

O CH3

CH3 i. ph. bend

2924

CH3 o. ph. str.

C-C-C o. ph. str.

1222

CH3 i. ph. str.

20

0.6

1422

2 x 1716 (C=O)

1363

40

530 + 1222 1748

60

2925

IR %T

3005

80

3414

100

0.0

634 607

739

951 1066 1244

1367

C

1032

1464

C

O

O

C=O o. pl. wag

O

C=O i. pl. rk.

O-CH2 str.

O CH 3

C O nC 4H9

480 430

308

635 808

1124 1065 1033

0.04

Chain i. ph. CH2 twist 1303

1739

0.08

920 881 839

C-C-O o. ph. str. C=O str.

2739

Ra I

CH3 (CH2) o. ph. str.

1452

0.12

CH3 (CH2) i. ph. bend CH3 (C=O) i. ph. bend

17 O

n-Butyl acetate

2940

20

1387

2 x 1743 (C=O) 2962

40

2876

60

2877

IR %T

1743

80

3463

100

0.00

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

256

692

367

517

O C O CH 3

C-O-C-O-C i. ph. str.

860

916 970

CH 3

0.0 3500

Dimethyl carbonate

915 970

795

1095

1214 1277

(O) CH3 o. ph. bend

1754

O

O-C-O o. ph. str.

1212 1163 1119

2964

0.2

C=O str.

2888 2848

3035

0.4 Ra I

1757

(O) CH3 str.

20

1435

CH3 i. ph. bend

40

1455

60

1458

IR %T

18 1605

80

2857

3009 2963

100

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS involves C-C-O str.

0.04

580

687

851 785

954 1026

1343 1278 1206 1154

1740

C OCH3

382 308

574

688

(O)CH3 o. ph. bend

787

2851

3000

Ra 0.08 I

1751

0.12

O

CH2 (C=O) bend

C=O o. ph str.

C=O i. ph str.

O

CH3O C CH2

916 847

(O) CH3 o. ph. str.

involves O-C str.

1022 994 951

0.16

1757

CH3 i. ph. str. + 2x bend

20

1439

(O)CH3 i. ph. bend

40

1460 1415

60

19

1414

2850

3006 2959

IR %T

2958

Dimethyl malonate

80

3473

100

1190 1157

146

0.00 Semicircle str.

498 814

C

O

708 635

272

CH3

843

961

o

354

1282

O

730

1108

1192

1016 955

876

1408 1387

C

O

o

798

2845

3042

0.4

C-O-C str.

1450

0.8

Ra I

C-C-O o, ph. str. Ring Quad str.

2961

3086

Aryl CH str.

O

2 adj. Aromatic CH wag + C=O wag

1108

C=O str.

1280

1726 1720

(O) CH3 str.

1435

(O) CH3 i. ph. bend

20

1.2

1535

1961 1905

1504

2 x 1720

CH3

1610

40

3017 2960 2844

60

3422

IR %T

20

=

Dimethyl terephthalate

80

2102

100

0.0 COOH impurity 563

750

792

873 930

C H 3 (C H 2 ) 3

1030

1051

O

C

(C H 2 ) 3 C H 3

353 293

460

570

C-O str.

749

1413

1296 1240 1140 1117

C

901 869 829

0.02

CH2(C=O) bend

O

O

1099 1043

2746

Ra 0.04 I

1812 1750

0.06

CH3 i. ph. bend

1453 1415

C=O C=O o. ph. str. i. ph. str.

2880

0.08

CH2 bend + CH3 o. ph. bend

1301

1819

20

21

1712

2970

40

1750

2940

CH3 o. ph. str.

2918

60

2941

Butyric anhydride

IR %T

2880

80

1462

100

0.00

3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

147

KETONES, ESTERS AND ANHYDRIDES

697

896 839

559

H

O

CC wag

C-O-C i. ph. str.

410 549

1240

271

872

1594

O H

643

1061

C-C-O o. ph. str.

O

768

1787

638

1108 1060

1242

1460 1434 1290 1399 1268

1633

1842

C=O o. ph. str

1857

3122 3060

1.0

C=C str. =

=CH str.

2.0

Ra I

1783

C=O i. ph. str

1634

20

1856

40

1591

1933

3188

22

3123

60

3593

IR %T

H2O 3435

80

Maleic anhydride

100

0.0

H

604

C

OH

Para Quad. i. pl. bend

338 266 224

416 453

837 793

646 604

862

o

4-Hydroxy benzaldehyde

507

708 640

789

835

859

1113

O

o

1164

1286

1218 1160

1388 1314

1516 1666

1453

Ar. Ar-O str. Semi circle str.

23

Ar. 2 adj. H wag

1015

1.0

2881 2791

2.0 3169 3066

Ra I

C=O str.

1586

3.0

Ar. Quad str.

Ar CH str.

1452 1389 1315 1286 1219

OH str.

AldehydeCH str. + 2 x bend

1598

20

1665

3171

40

1906

60

2789

2969 2880 2831

80 IR %T

OH wag

AldCH bend

100

0.0

533

617

679

885

935

999

889

1088

O CH 2

493

1086

530

O-C-C-O i. ph. str.

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

C O

677

1250 1231

1429 1360

1713

C-C-O o. ph. str.

1726

3281

0.1

2975 2947

CH2 str.

Ra I

CH2 bend

H

H O

O-C-C-O o. ph. str.

=

C=O str. (acid OH bonded to alcO)

0.2

1362 1432

2565

2647

2764

3294

2938

40 20

1560

(CH2)OH str.

1730

60

(O=C)OH str. 2238

IR %T

24

OH wag

80

600

400

Glycolic acid

100

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS CH2 wag

100

0.0 100

548

688

375

671

893

1711

723

938

1285 1247

1462 1413

2674

CH2 twist

CH 3(CH 2) 7

C

Cis CH wag

C

C

226

H

724

857

1656

(CH2 ) 7 C OH

H

1119 1083 1022 972

2855

Dimer CH2 rk O…H…O o. pl. wag

1303 1267

0.1

C=C str.

2726

3007

0.2

C

O

C=O o. ph. str. in dimer

0.3

H

H

C-O str. CH2 bend

1441

0.4

CH2 chain i. ph. str.

2854

0.5

Ra I

720

941 891

1099 1129 1063 1099

26

3005

CH2 o. ph. str.

2898 2926

Oleic acid

20

=CH str.

OH

Trans chain C-C-C o. ph. str.

OH str.

60 40

C

Trans chain C-C-C i. ph. str.

1296

2882 2849

CH2 twist

80 IR %T

O CH3 (CH2)1 4

1175

0.2

C=O i. ph. str. in dimer

1634

0.4

CH2 rk

Dimer O…H…O o. pl. wag

C-O str. CH2 bend

2725

2959 2924

0.6 Ra I

C=O o. ph. str. in dimer

2846

0.8 Trans chain CH2 o. ph. str.

CH2 i. ph. str.

1703

2955

20

CH3 o. ph. str. CH2 o. ph. str.

2918

Palmitic acid

40

2676

60

1472 1431 1348 1295 1229 1188

80 IR %T

25

OH str.

1462 1438 1422

148

0.0

C=O o. ph. str. in dimer

5.0 Aryl CH str.

550

805

O

707 665

934

1180 1126 1292

1326

OH C-O str. i. pl. bend

C OH

o

o

o

421

618

1028

1180 1132

1.0

1290

C=O i. ph. str. in dimer

1443

2.0

1635 1603

3.0 3073

Ra I

Ring quad. str.

Dimer O…H…O o. pl. wag

27

796

4.0

1687

3071

20

3007 2834 2674 2559

Benzoic acid

40

1582

60

1454 1424

1496

80 IR %T

5 adj Ar-H wag + C=O wag

1002

Ring semi. str. OH str. + combination bands

1027

100

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

149

AMIDES, UREAS AND RELATED COMPOUNDS CH2 chain wag

720

1113

1421

CH3(CH2)16CO2

CH2 chain i. ph. twist

1130 1103 1063

1569

CH2 chain C-C-C i. ph. str. CH2 chain C-C-C o. ph. str. 959 937 891

CH2 bend

Ca 2

Stearic acid

1576

CH2 i. ph. str.

0.20 0.10

CO2 i. ph. str.

CO2 o. ph. str.

2724 2610

Ra I

CH2 chain o. ph. str.

CH2 chain i. ph. rk.

1176

0.30

1542 1470

2957

0.40

2850

CH2 o. ph. str.

2917

20

2881

CH3 o. ph. str.

2846

40

1622

H2O bend

3429

60

1302

80 IR %T

28

H2O str.

1461 1441 1370 1296

100

0.00 100

29

NH3+ str.

503 359

495

602

696

Glycine (Zwitterion)

608 698

911 893

1032

893

CH2 CO2

N-C-C i. ph. str.

1139 1109 1035

1668 1631 1568 1515 1440 1410

NH3+ str. 2615

3143

0.2

3007 2972 2904

0.4

NH3 1325

CO2 NH3 i.ph. i.ph. str. bend

0.6

Ra I

1333

1444

1112

2122

CH2 str.

1412

20

CO2 o.ph. str. + NH3 o. ph. bend

1611

3169

3008 2970 2895

40

CH2 bend

2614

60

3383

IR %T

+ summation bands

1522

80

0.0 100

1006

458

C-C-N i. ph. str.

0.00 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

Acetamide

587

O=C-N bend 583

878

CH3 NH2 i. ph. rock bend 1151

C-N str.

467

876 826

1048 1007 1154

1354

C-C=O bend

1396

1466

2934

1679 1639 1591

3353

CH3 str.

NH2 bend

NH2 wag

=

C=O str.

NH2 i. ph. str.

Ra 0.20 I

1647

3167

3355

C NH2

NH2 o. ph. str.

0.30

0.10

CH3

3159

0.40

O

1405 1356

40 20

1744

60

2820

IR %T

30

CH3 o. ph. bend

80

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS H

702 665

841 810

1049

NH2

306

845

1145

=CH2 wag

1053

C-C-N o. ph. str.

505

1136

1281

1352

1429

1433

1284

1636

C

C=O wag 616

0.2

C=C str.

=CH2 i. ph. str.

=CH rk

O

CH2 CH

1681

0.4

=CH str.

3334 3162 3104 3033 3011

Ra I

=CH2 o. ph. str.

31

NH2 wag

NH2 rk

NH2 bend =CH2 bend

C=O str.

0.6

1613

2 x 961

2 x 1429

1673

20

3354

Acrylamide

40

1589

60

wag

C C

1922

IR %T

NH2 str. o. ph. i. ph.

3186 3101 3039 2813 2737

80

C-C-N H o. ph. str.

988 961

100

960

150

0.0 2X 1542

C-N-H str-bend

705

620

871 808

986 959 1087

1233

1311

C-O-C o. ph. str.

CH2 bend

32 H

H C C

H

=CH2 wag H C C wag H

CH 2

CH

O C O(C 4H 9)

N H

949 873 833

1633

1541

1628

1668

C=C str.

1119 1064

=CH2 o. ph. str.

0.04

2935

=CH str.

1453 1411

Ra I

0.08

NH str.

1670

0.12

C=O str.

3104 3030 2938 2876

20

2960

3297

40

2872

60

NH wag

1459 1404

3066

IR %T

3310

N-butyl methyl acrylamide

80

1301 1237

100

0.00

C=O str.

509 675

1175

775

N

C

N

H

H

N C

N

(N) CH3 i. ph. bend

NH bend + C-N str.

H

N-C-N i. ph. str.

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

CH 3

511

H

1624 1552 1473 1423

2811

3346

0.4

2945 2904

0.8

O

CH 3

240

NH str.

Ra I

1269

1043

CH3 o. ph. str.

1.2

NH wag

932

20

33

1178

CH3 i. ph. str.

3343

40

1420

60

1543

1,3-Dimethyl urea

IR %T

1627 1591

80

932

2 x 2 x 1543 1591

3176 3050 2947 2902 2812

100

1000

800

600

400

151

AMIDES, UREAS AND RELATED COMPOUNDS

417

685 635 582

O

389 235

580

O

N

864

653

N

428

N

555

N

H

NH 2

698 994

1175

1278

N

Ammeline

869 793

994

1169

1283 1412

1447

N

O N

N

1445

2878

3186 3059

3459

0.2

1617

Ra I

1707 1668

0.4

N

Ring quad str. + NH2 bend

C=O str.

H-bonded NH2 str.

1548

“free” N-H str.

H 2N N

1514

0.6

Ring N i. ph. radial N

1719

20

34

3185 3089

3464

40

O

1684

2865

60

….

1615

80 IR %T

N-H

NH2 str.

2661

100

0.0 100

Trimethyl isocyanurate

424

483

940

1136

1056

484

700 752

940

0.2

1685

3007

0.4

1136

2965

0.6

CH3 Ring C-N str. i. ph. bend + CH3 o. ph. bend

C=O i. ph. str.

1279

CH3 i. ph. str.

1387

0.8

579

C=O o. ph. str.

O

CH3

1477

N

CH3

1460

N

1678

O

N

1751

20

H 3C

750

O

40

Ra I

1277 1391

60

35

2543

2967

IR %T

3429

80

0.0 CH2 bend

1275

N-C-N o. ph. str.

C N

511 710

1204

1379

1518 1488

1659

2905

2964

0.8 3304

O C

N

N

5 membered ring i. ph. str.

NH bend + C-N str.

0.4

H

498

N

698

C N

H

H

770

N

934

C=O str.

1.2 Ra I

1105 1038 991 1510 1451

1675

CH2 N-H str. o. ph. str.

H

H

1109 1043 999

1.6

CH2 i. ph. str.

3308

20

2902

40

1576

3005

60

2957

IR %T

36

NH wag

80

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

400

600

H

2-Imidazolidone

100

152

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

658

865 1093

659

867

405 355 319 CH 3

226

513

397

O CH 2

674

C

858

522

679

626

857 792

996 1080

1275

1690 1625

1460 1443

NH2 rk

O

NH 2

38

998

C-N str. NH2 bend

O-CH2 str

1128

(O ) C O str

1077

1131

1342

1487

1427 1382

1616

1701

2936 2902

C=O str.

3206

3407

0.10

2983

NH2 NH2 o. ph. i. ph. str. str.

N

C

bend

NH2 wag

1926

2916

CH3 o. ph. str.

0.20 Ra I

2986

3337 3214

3422

Ethyl carbamate

40

O

1093

1662

CH2 CH3 bend

60

0.30

NC3 i. ph. str. 1441 1408

CH (=O) str.

80

20

N C H CH3

C=O str.

0.00 100

IR %T

O

CH3

(N)CH3 bend

1500

2931

Ra 0.08 I 0.04

37

CH i. pl bend

0.16 0.12

1257

1503

CH3 i. ph. str.

CH3 o. ph. str.

20

1439 1388

2 X 1676

40

1484

60

1676

IR %T

2861

Dimethyl formamide

80

2930 2860

3336

100

0.00 100

OH wag 444 641

590

964

752

1090

1378

O

OH

C N

588

349 309 273 225

449

647

993 964

H

1346

1522

993

CH3

C-N-O str.

1086

0.10

C=O str.

1040

1192 1321

Ra I

CH3 i. ph. str.

CH3 o. ph. CNH bend str/bend

1620 1566

0.20

39

CH3 i. ph. bend

1623

2937 2870 2944

CH3 o. ph. str. NH str.

2996

0.30

3207

20

3040

40

1450

60

1438 1390

IR %T

OH str.

3202

Acetohydroxamic acid

80

0.00 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

153

ALCOHOLS

100

40 664

OH

C-O str.

1152 1103

1458

3366

OH-CH bend

41

80 655

wag

1159 1128

818

O

CH 3 953

CH

OH

CH 3 820

C-C-O o. ph. str. + CH3 rk

C

491 430

953

C O C i. ph. str.

1132

1343

2722

1454

2976 3376

0.2

2924

0.4 Ra I

CH3 i. ph. bend

CH3 o. ph. str. 2883

OH str.

CH3 o. ph. bend

CH3 i. ph. str.

2972

3347

20

2933

40

1466 1409 1380 1310

60 2885

IR %T

H

Isopropyl alcohol

0.00 100

CH3

438

CH3 bend

wag

Methanol

1030

CH3 rk

CH3 i. ph. str.

OH str.

0.04

1115

bend

0.08 Ra I

O

O

1036

0.12

H

H C

CH3 o. ph. str. 2945

3350

20

2945 2833

40

1421

60

2837

IR %T

1450

80

C C O

i. ph. str.

348

C

0.00 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

t-Butyl alcohol

642

749

1023

1237

752

CH 3 C

474

0.10

42

C OH

C3-C-O str.

916

2714

0.30 Ra I 0.20

CH3

CH3 o. ph. bend

CH3 i. ph. str.

H O wag

CH 3

914

2972

CH3 OH str. o. ph. str.

1455

0.40

CH3 i. ph. bend

2977 2920

0.50

3368

20

1200

CH3 o. ph. bend

40

1209

60

1472 1383 1365

2882

80 IR %T

1659

2567

0.0 100

154

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

100

656

1043

C-C-C-O i. ph. str.

1114 1070 1030 964 901 880

CH3

(CH 2)3

487

811

828

(CH2)n i. ph. twist

wag

OH

398 355

738

953

847

1215

1113

H

C-C-O o. ph. str.

1300 1254

OH str.

O

1070

2872

CH2 bend, CH3 o. ph. bend 1451

2938

2877

2959

CH3 i. ph. str.

CH2 i. ph. str.

2737

0.08 Ra I 0.04

CH2 o. ph. str.

3231

0.12

3331

20

2934

40

1-Butanol

1379 1340

60 1462

IR %T

515

43 80

0.00

1473 1463

CH2 o. ph. bend

878

935

996

891

618 534 503 691

811 754

1001

O

245

O

536

620

814

1026

1169

885

1024 1070

1169

1238

1370

1499

1501

4

45

wag

OH

Ar. 5 adj CH wag

O

Aryl Ar. C-O str. Semicircle str

1598

3060

8

1474

1597

Ar. Ring Quad str.

Aryl CH str.

1254

12

3048

3258

Phenol

20

H

980

1773 1709

1846

2051

60

O

40

Ra I

1063

1130

1459 1437 1369

2724

OH str.

(CH2 )15 OH

Trans chain C-C-C o. ph str.

C-O-H bend 1934

IR %T

CH2 chain i. ph. rock crystal splitting

Trans chain C-C-C i. ph str. 1296

CH2 i. ph. str.

0.00 100 80

C-CC-O o. ph. str.

CH3

0.16 Ra I 0.08

44 720

C-O-H bend

1123 1070

2638

CH2 o. ph. str.

2846 2850

0.24

CH3 o. ph. str.

2918

20

2881

1-Hexadecanol

40

2956

60

3303

IR %T

1298

OH str.

80

1408

100

0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

155

ALCOHOLS AND ETHERS

623

Aryl-O str.

291 326 447

808 747

528

845 932

1116 1096 1020

1272 1231 1198

C(CH 3) 3

2,6-Di-t-butyl phenol

747

(CH3 )3C

T-Butyl CH3 i. ph. bend

1471

449

588

880

843 795

1094 1171

1231

OH

CH3 o. ph. bend 1586

3 adj H wag

1143

1391 1362

1481

1315

1645

1798

1584 1426

2914 2959

3644

Aryl CH str.

3639

4

Ring semicircle. str.

CH3 o. ph. str.

Free OH str.

8 Ra I

Ring Quad. str.

3081 2959 2914

12

46

2874

60 IR %T 40 20

1695

3083

80

1854

1912

100

0 H

(CH2)2

649

812 1062

896

922

896 824

354

1236

OH

Diethylene glycol

O

529

(CH2)2

1241

HO

C-C-O(H) o. ph. str.

1119 1081 1059

1463

C-O-C o. ph. str.

2704

0.2

CH2 bend

1279

2930

0.4 3349

Ra I

CH2 o. ph. str.

CH2 i. ph. str.

2875

0.6

2936

3370

20

2875

40

1131

H2O (impurity)

1354

OH str

60

47

O bend

1456 1423

IR %T

1657

C

80

392

100

0.0

291

839 879

960

1117 1058

1300

1484 1449

2797 2735

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

1200

1000

800

600

400

n-Dibutyl ether

739

839

962

1041

1234

1302 1375

CH2 i. ph. twist

0.8 Ra I 0.4

O nC4H9

C-O-C o. ph. str.

1124

2864

CH2 bend + CH3 o. ph. bend

2872

2935 2913

CH2 o. ph. str.

CH2 i. ph. str.

nC4H9

CH3 i. ph. bend

2938

1.2

CH3 o. ph. str.

2960

60 IR %T 40

1464

2796

80

20

48

2734

100

156

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

577

656

787 1183

1290

O

40 912 1071

CH2 i. ph. str.

CH2 bend.

1232

1070 1029

1486

2719 2657

0.1

1450

2962

0.2

5 membered ring i. ph. str. 914

CH2 o. ph. str.

0.3

C-O-C o. ph. str.

2861

2975

20

Ra I

49

2875

Tetrahydrofuran

60

IR %T

1365

80

1461

2682

1971

100

0.0 100

50 1193 1137 1092

724

CH CH2

374

C-Cl str.

723

761

855

962 927

CH2

Ring

223

516

962

443

Ring o. ph. str. i. ph. str. 844

1398

1255

Ring CH2 wag

O Cl

CH2-(Cl) wag

1206 1137 1092

3066

0.1

CH2-Cl bend

CH2-Cl o. ph. str.

1479 1433

Ra I

1433 1398 1267

Ring CH2 bend

Ring CH2 i. ph. str.

0.2

1481

2926 2964 3003

Ring CH2 o. ph. str.

2965

20

3007

Epichlorohydrin

40

3063

60

2927

IR %T

1518

80

0.0

0.04

CH3 i. ph. bend C-N str.

CH2 bend + CH3 o. ph. bend

NH2 CH3 CH CH2 NH2 wag

C

0.00 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

1200

1000

800

600

400

280

i. ph. str.

483

C

953

1173 1132 1063

1338

C C

362

798

CH3

1458

2719

778

840

1065

940

51

1279

1616 CH3 i. ph. str.

1469 1388

2872

NH2 bend

2870

2956

3329

0.12

Ra I 0.08

2931

NH2 i. ph. str.

3382

Isobutyl amine

20

NH2 o. ph. str.

2957

60 40

3377 3300

80

IR %T

2737

2 x 1618

100

157

AMINES AND AMINE SALTS 2 x 1621

52

Aniline

501

691

753

1277

881

1027 993

1467

757

390

533

622

Ar-N str.

816

1029

1602

1175

998

1929

1840

O

1280

0.2

O

1174 1156

0.4

5 adj. H wag

O

Ring SemiRing Quad. str. circle str.

3356 3209

Ra I

NH2 bend

1604

0.6

Aryl NH2 o. ph. NH CH str. 2 str. i. ph. str. 3053

0.8

1499

20

NH 2

1622

3433

40

NH2 wag

3034

60

3356

IR %T

3214 3071

80

235

100

0.0

735 NH wag

C-N-C o. ph. str. Chain CH2 twist

NH

C4 H9 278

C4 H9

447

n-Dibutyl amine

961

1302

1234 1132

1462

CH2 bend + CH3 o. ph. bend

53

1058 1027 896 878 834

0.4

CH2 i. ph. str.

1132

0.8 3327

Ra I

2964

1.2

CH2 o. ph. str.

NH str.

CH2 (N) i. ph. str.

CH3 i. ph. bend

1298

1.6

2870 2810

20

2875 2929

CH3 o. ph. str.

2810 2735

40

2958

60

2934

IR %T

1445

80

1378

3287

100

0.0

100

C 4 H9

C 4 H9

267

903 881 828 947

0.00 3500

3000

2500

2000

n-Tributyl amine

901

1086

940

1251 1180

1304 1377

732

N

Chain CH2 twist 1301

1464

C 4 H9

C-N str.

1179 1109 1050

0.05

CH2 bend + CH3 o. ph. bend

CH2 i. ph. str.

1380

0.10

CH3 i. ph. bend

1445

2960 2934

Ra 0.15 I

CH2 (N) i. ph. str.

1645

CH2 o. ph. str.

0.20

2933

CH3 o. ph. str.

2874

20

2957

40

2863

2798

60

54

1690

2680

80

2799 2734

IR %T

1800 1600 1400 Wavenumber (cm–1)

1200

1000

800

600

400

158

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

55 466

818

1167

1067

1371

989

NC3 i. ph. str.

CH3 CH3

N H Cl

819

CH3

CH3 rk

1071

468 410

990

(N) CH3 i. ph. bend

1243

0.1

NC3 o. ph. str.

1464 1435 1404

2875 2817

Ra I

1260

1414 1485

2480

NH str. (+ summation bands)

3012

(N) CH3 o. ph. str.

(N) CH3 o. ph. bend

2599 2471

0.2

(N) CH3 i. ph. str.

2701

20

2947

Trimethyl amine HCl

40

2629

3012 2960

IR 60 %T

1732

3419

80

2139 2054

100

0.0

457

1406

457

NC4 i. ph. str.

387

1287

1454 1401

2 x 1401

Cl

758

951

CH3 rk

948

(N) CH3 o. ph. bend

N CH3 CH3

1181

0. 4

(N) CH3 i. ph. bend

1492

(N) CH3 i. ph. str.

2878 2787

Ra 0. 8 I

CH3

1481

1.2

1071

1298

1753

1832

2347

2573 2469

2919

3094

2952

(N) CH3 o. ph. str.

56

CH3

1529

20

3018

IR 60 %T 40

3018 2950

80

3478 3397

Tetramethyl ammonium chloride

100

0.0

522

691

610

927

1028

839

Ring o. pl. sextant bend

1003

O

617 692

394

765

1607

O

1168 1097 1032

3178 3065

0.4

Cl NH2

1368 1296

1605

NH2 C

5 adj. H wag

NCN i. ph. str.

1531 1483

Aryl CH str.

Ra 1.2 I 0.8

57

Ring Semicircle str.

Ring Quad str.

1.6

NH2 o. pl. wag

1175 1110

1807 2361

1526 1482 1446

3236

20

NCN o. ph. str.

3063

40

2 x 700 wag

2758

60

3420

Benzamidine HCl

IR %T

NH2 str.

1676

80

785

100

0.0

3500

3000

2500

2000

1800

1600 1400 1200 Wavenumber(cm–1)

1000

800

600

400

100 O-H….N str.

NH str.

2 x 1644

O-H….N str.

CH2 o. ph. str

2 x 1604

CH3 o. ph. str

CH

CH3 i. ph. str.

CH2 i. ph. str

CH3 CH3

2294 CH3 i. ph. str

C¼N AND N¼O COMPOUNDS

C=N str.

C=N str.

(C 2H 4) O

2000

N O

N=O str.

NH CH3 2 bend o. ph. bend

s-cis

s-trans

1647

1600

CH2 bend

C-N str.

CH3 i. ph. bend

C-N-C o. ph. str.

CH3 i. ph. bend CH2 bend + CH3 o. ph. bend

1400 Wavenumber (cm–1)

1800

=

CH3 o. ph. str. NH2 NH2 o. ph. i. ph. str. str.

3163

80

IR 60 %T 40 20

0.3

0.1

Ra 0.2 I

0.0 100 80

20 0.6

0.4

0.2

0.0 100 80 60 40 20

1.6 1.2 0.8

2500

1200

N-O str

C C

C

i. ph. str.

800

475 CH 3

NH/OH wag

C-C-N i. ph. str.

NH/OH wag

HO N

N-O str.

N-O str.

C

1000

600

H

N

C

58

NH 2

N OH

413

59

N OH

400

60

250

IR 60 %T 40

Ra I

IR %T

Ra I 0.4 0.0 3000

352

Acetamide oxime

159

Isoamyl nitrate

297 265

Pipenidindione dioxime

337 416 358 303 409 411 423

3492

3500

603

511 511 461 513 513

654 692

921 902 917

620 603 620 700 645 592 600

982

809 800 833

1045 1063

894 821 777 708 651 589 782 763

1102 1048 1021 1016 983

1184 1171 1136

974 942 874 957 911

1387 1371

1117 1139

1365 1399

1421 1362 1452

1053

1289 1252 1255 1184 1237 1169

1432 1468

1657 1676

1587 1604

1483 1427 1465 1430 1386 1331 1337 1308

1821

1593 1650

1646 1605

2449

1665 1670 1644

2770 2766

2943 2886 2886

2793

2941

2983 2965 2926 2934

3397 3396

3088 3076 2960 2963

2726 2875 2874 2726

3372

3490 3367 3266 3181

160

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

721

524

621

851 C

CH

N

NO 2

CH 3

851

i. ph. str.

620

953 902

318

525

NO2 i. ph. str. + CH3 bend

1102

1551

0.1

NO2 o. ph. str.

1448 1400

2997

0.2

1179

1552

2950

0.3

Ra I

1102 C

Isopropyl CH str + NO2 str. combination

61

CH 3

C

1179

20

1360 1306

40

1305 1359

60

2874 2735

2-nitropropane

IR %T

1468

80

1400

2996 2945 2900

100

0.0

853

CH O NO2

880

NO2 i. ph. str.

C C

O

333

852

561

CH 3

459

908

1102

706

562

760

1037

1183 1144

N

i. ph. str. 706

257

C

409

1378 1279

CH 3

941 908

0.4

62

N-O str.

1183 1146 1101

0.8

CH3 o. ph. bend

1624

2877 2742

1.2

NO2 o. ph. str.

2929

1.6

1277

2948

2.0

CH3 i. ph. bend

1626

Isopropyl CH str + NO2 str. combination

1351

20

1450

40

Ra I

1470 1390

2994

60

2996

Isopropyl nitrate

IR %T

2551

80

2944 2887

3229

100

0.0

CH3 str.

544

263 364

594

503

928

715

1400

842

1600

1240 1207

1450

1366

2707

t-Butyl CH3 i. ph. bend

1572

N=N str.

553

Involves (C)3 C-N o. ph. str.

CH3 o. ph. bend

5 2874

Ra I

Involves C-N=N bend + C-C str.

796

N

(H3C)3C

1027

10

C(CH3)3

N

1362

2974

20

Involves N-C(C)3 i. p. str. 1241 1211

1477 1453 1385

2867

40

2931

IR %T

2976 2923

Azo-tert-butane

60

63

715

80

868

1026

100

0 3500

3000

2500

2000

1800

Wavenumber(cm–1)

1200

1000

800

600

400

161

BORONIC ACID, CHLORINE AND BROMINE COMPOUNDS

727

226

1001

Involves B-OH bend

381 335

505

584

665

528

702

O

827 801

3060 3036 2918 2865

1583

OH

1202 1168 1089 1045

1605

B

Aryl CH3 str.

Involves O-B-O str.

meta-tolyl boronic acid (3-Methylphenylboronic acid)

492

652 592 708

917 1042

1224 1199 1181 1121

1311

3 adj H wag

O

OH

10

3206

64

H3C

Aryl CH str.

Ra I

Aryl Ring Quad Str.

1348

OH str.

1419

20

1414 1379 1346 1310

40

2924 2854

3266

1456

60

1604 1582

3032 Nujol

80 IR %T

841 788 765

B-OH wag

100

0

65

C-Cl str. H trans to Cl

246

H

457

1-Chlorooctane

654

1306

725

Cl

C

656

C

CH2 i. ph. twist 1304

Cl

730

2857

CH2 i. ph. str.

H

C-Cl str. C trans to Cl

893 846

2857

1462 CH2 (Cl) o. ph. str.

CH2 bend + CH3 o. ph. bend

(Cl) CH2 wag

CH3 i. ph. bend

1170 1119 1080 1039

0. 2

CH3

2731

0. 4

(CH 2) 7

1445

0. 6

2928

CH2 o. ph. str.

2874

0. 8

CH3 o. ph. str.

2958 2936

20

2958

40

Ra I

Cl

60

2996

IR %T

1377

80

915

100

0. 0

66 726

646 564

852

1047

1204

565

Br

H

485

801 754

962

1111 1067

646

CH2 i. ph. twist

C-Br str H trans to Br

0.00 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

1200

1-Bromohexane

C-Br str C trans to Br C

CH2 bend + CH3 o. ph. bend

CH2 o. ph. str.

C

1000

800

600

400

249

1279 1241

953

H Br

380

CH2 i. ph. str.

CH2 i. ph. rk

(Br) CH2 wag

1306

0.04

1465

2859 3011

Ra I 0.08

CH3 i. ph. bend

1440

0.12

CH3 o. ph. str.

CH 3

2875

0.16

(C H 2) 5

2732

20

CH2 (Br) o. ph. str.

Br

2930

40

2959

60

2963 2936

IR %T

3003

80

1379

100

162

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

528

728 682

925

444

1004

887 857

1079

777 729

1004

1251

528

245

3 adj H wag O

514

1265

1142

Lone H wag

O

557

1620 1596

10

Aryl CH3 i. ph. bd.

1265 1251 1160 1142 1079

20

67

Involves Aryl Ring Ar-F str. Quad Str. Ring semi-circle

Aryl CH3 str.

Ra I

1382

1518 F

1489

H3C

Aryl CH str.

1490 1446 1381

20

3080 3061 2924 2870

3-Fluorotoluene

IR 60 %T 40

1620 1590

80

1460

3040 2924 2867

100

0

434 682

236

521

O

416

684

1217

1384

1078

856

773

3 adj H wag

Involves Ar-Cl str.

856

1602 1577

388

O

2924 2862

10

Ring semi-circle Aryl Ring Quad Str.

Aryl CH3 str.

Ra 20 I

Aryl CH3 i. ph. bend

1217 1165 1099 1079

Aryl CH str.

Cl

Lone H wag 999

H3C

1477

1603 1577

20

3064

3-Chlorotoluene

40

1164 1100 1042 998

68

1452 1383

IR 80 %T 60

1862

3063 2924 2861

1936

100

0 100

30

Ring semi-circle Aryl Ring Quad Str.

Aryl CH str.

Ra 20 I

Aryl CH3 str.

Involves Ar-Br str.

308

667 3 adj H wag

O

522

834

1212 1165 1093 1071 1028

1380

1450

1601 1570

2922 2861

3061

O

10

429

869

1040 995

Lone H wag

681

Aryl CH3 i. ph. bend

667

Br

834 770

H3C

1071

20

996

1601 1569

40

1212 1165

1452 1381

1936

69

1473

IR 60 %T

3-Bromotoluene

2922 2859

3061

80

0 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

1200

1000

800

600

400

163

SULFUR COMPOUNDS

669 670

1053

335 307

384

699 984

564 535

790

908

SO3 bend. 353

577 544

968

1204 1170

0.2

1062

SO3 o. ph. str.

1453 1412

3021

0.4

C-S str. 799

(S) CH3 o. ph. bend

SO3 i. ph. str.

1052

(S) CH3 o. ph. str.

1184

(S) CH3 i. ph. bend

(S) CH3 i. ph. str.

71

1078

Na

2940

0.6

SO3

1249 1220

20

Ra I

CH 3

40

1337

60

1435

3018 2940

IR %T

1415

2258

100 80

Dimethyl sulfoxide

955

700

1311

1408

CH 3

Methane sulfonic acid, sodium salt

0.0

S

1421

2998

CH 3

C-S-C o. ph. str.

S=O str.

1045

Ra 0.4 I 0.2

(S) CH3 o. ph. bend

O

(S) CH3 rock

954

(S) CH3 o. ph. str.

C-S-C i. ph. str.

(S) CH3 i. ph. bend

(S) CH3 i. ph. str.

2914

0.6

H2O OH str.

2819

20

1437

1659 H2O bend

40

900

70

2094 1996

2995 2911 2822

60

3442

80 IR %T

2336

100

CH3 o. ph. bend

(Sulfonic acid Hydrate)

493 566

801

270 234

553

396

635

O

703 680

1000

677

816

706

846

1200

SO3bend O

818

1007

1181 1124 1037

1045 1014

H3 O

Aryl 2 adj. H wag

SO3i. ph. str. + p-aryl ring mode

1215 1188

SO3

1379 1308

CH 3

1454

3066 2986

0.1

Aryl-S

1599

0.3 Ra I 0.2

Ring Quad. str.

(aryl) CH3 i. ph. str.

2932

Aryl CH str.

1401

Ring semi-circle str.

20

0.4

1455

1597 1495

1659

1850

2243

40

2670

60 2930

IR %T

72

SO3o. ph. str.

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

800

600

400

316

H3O+ str.

80

1126

100

para-Toluene sulfonic acid hydrate

0.0

164

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

422 407

920 992

836 763 722 635 592 637 599 575

1250 1221

1150 1085

1086 1064

S-O-C i. ph. str.

891 838

O SO3 Na

SO3 bend

C-C-C chain o. ph. str.

993

CH3 (CH2 )11

C-C-C chain i. ph. str.

1371

0.1

2729

2954

1454 1437

0.2 Ra I

1379

1468

CH2 i. ph. twist

SO3 i. ph. str.

73

CH2 i. ph. rk.

S-O-C o. ph. str.

1130

CH2 o. ph. str.

0.3

CH2 i. ph. str.

1297

CH3 o. ph. str.

CH3 i. ph. bend

CH2 bend + CH3 o. ph. SO3 bend o. ph. str.

2848

20

H2O bend

2955

40

H2O str.

2918 2850

60

2881

IR %T

3458

Dodecyl sulfate, sodium salt

80

1658

100

0.0 CH bend

1627

812

507 449

608

905

465

1089

1037

1140 1189

664

C-O-C, CCC i. pl. bend

393 364 292

S

2731

1320

1463 1442 1373

C-C-C O-CC2 o. ph. str. i. ph. str. C-O-C CS2 o. ph. str. o. ph. str.

xH2O

815

Na

CS2 i. ph. str.

907

CH3

2873

0.2

H2O str.

-

74

943

CH O C 2978 2936

0.4

CH3 o. ph. bend CH3 i. ph. bend

S

CH3 0.6 Ra I

1630 1609

2107

2976

H2O bend

CH3 o. ph. str.

H2O wag hindered rotation

CH3 rk

1345 1327 1189 1168 1140 1104 1052

0.8

H2O

1459 1448

20

3503

40

3203

60

3381

IR %T

3349

Sodium isopropyl xanthate

80

2929 2872

H2O str.

2358

100

669

445

1049 1149

571

974

CS2 i. ph. str.

C-N str. + CH3 bend

223 371 299

885

2

965

Zn S

1146

CH3

1245

S N C

1512 1445 1391

2929

0.1

CH3

2859 2800

3013

0.2

438

CS2 o. ph. str.

0.3

979

0.4

Ra I

567

1625

1935

2091

CH3 o. ph. str.

1244

H2O str.

1392

20

1439

40

75 H2O bend

1522

60 IR %T

2928 2857

80

3435

Zinc dimethyl dithiocarbamate

0.0 100

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber (cm–1)

1200

1000

800

600

400

165

SULFUR AND PHOSPHORUS COMPOUNDS CH2 bend

2-Imidazolidinethione

509

681

592

H N

510

923 5 membered ring i. ph. str.

330

1500 1465

N

665

2966 2894

3285 3245

N2C=S str

NH bend + CN str.

Ra I 0.8 0.4

H

N C N

1109 1050 1001 979

1.2

H

CH2 NH str. o. ph. str.

920

1372

1.6

S C

H

1520 1483 1467 1378 1310 1284 1212

3249

1521

CH2 i. ph. str.

20

1047 1002

2574

40

1310 1277 1207

60

2885

IR %T

2961

80

76

1714

100

0.0

(CH 2)3C H3

(CH 2)3C H 3

914

252

3000

2500

2000

1800

1600

1400

Wavenumber (cm-1)

1200

213

1000

800

600

400

Tributyl phosphite

964

0.0 3500

272

552 878

774 721

1148 1062 1023 CH2 twist

545

O

O

PO3 i. ph. str.

833 817 772 718

P

1120 1064 1036 967 906 884

0.4

1264

1464 O

2737

H 3(CH 2)3

PO3 o. ph. str. + P-O-C i. ph. str.

P-O-C o. ph. str. + C4O chain str.

1300 1262 1232

Ra 1.2 I 0.8

1450

1.6

CH2 bend + CH3 o. ph. bend

1381

CH2 o. ph. str.

CH3 i. ph. bend

CH2 i. ph. str.

2875

2.0

CH3 o. ph. str.

2936 2910

20

2936 2874

40

1381

2426

2735

78

60

2960

IR %T

3461

0.0 100 80

Triphenyl phosphine

492 697 500

1571

3

1.0

853 743

O

683 619

P

O

1178 1158 1095 1028

Ra I 2.0

O

1584

3049

3.0

1000

Ring Semicircle str.

4.0

5 adj. H wag

P-phenyl str.

Ring Quad. Str.

Aryl CH str.

77

409

1476 1435

40 20

1024 996

1090

Phenyl CH wag summation bands

1178

1308 1281

1815 1768

1667 1582

60 IR %T

1958 1888

3063 3001

80

3443

100

166

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

554

732

901 832 795

1150

1464 1383

1069 1033

P-O-C o. ph. str. + PH wag .

976

1261

(CH2)3 CH3

H

294

395

475

832 775 720

958 907 886

0.4

O P O

P PO2 O i. ph. str. O

1150 1121 1063

0.8

CH3 (CH 2)3

CH2 bend + P=O str. CH3 o. ph. CH2 bend twist 1302 1259

1.2

2734

Ra I

P-H str.

1453

1.6

1391

2.0

2430

CH3 o. ph. str.

O

CH3 i. ph. bend

CH3 i. ph. str.

2913 2962

40 20

H2O bend

2876

H2O str.

79

1640

2425

60

2875

IR %T

2963 2937

n-Dibutyl phosphite

80

3478

100

80 468

282

564

367 319

455 523

830

965

702 687 662

995

417

994

686

950

841 812

570

910

1161 1130 1179 1130

1372 1343 1301 1260

CH3

CH3

2000

1800

710

491

800 Si CH3 rk

Si-O-Si o. ph. str.

688

Si CH3 i. ph def.

Si

1020

1092

1261 CH3 O

790

0.4

Si

867

0.8

O

1265

CH3

2966

1.2

1413

2907

1.6

81

Si CH3 o. ph def.

Si CH3 i. ph str.

0

PS2 o. ph. str.

864

1721

1445 1412 1385

2723

Si CH3 o. ph str.

2809

20

2963

60 40

PS2 i. ph. str.

797

Na

926

1468

1263

1392 1368

P-O-C i. ph. str.

1464

-

H2O bend

2906 2851

80

Ra I

P-O-C o. ph. str.

S

O

1632

(CH3)2CH CH2

0.0 100

IR %T

S P

3374

0.4

O

1604

(CH3)2CH CH2

1.2 0.8

1621

2725

1.6

1945

Ra I

2963 2901

2.0

CH3 i. ph. bend

2875

2.4

H2O str. CH3 o. ph. str.

CH2 bend + CH3 o. ph. bend

CH3 i. ph. str.

2960

40 20

H2O bend

2875

3409

80 60

2087

100

IR %T

3114

Siloxane polymer (silicone grease)

Sodium di-isobutyl dithiophosphate

0.0

0.0 3500

3000

2500

1600

1400

1200

1000

800

600

400

167

INORGANIC COMPOUNDS

671 1647

1402

NH4 i. ph. str.

H

Cl

H

NH4 bend

1759

1405

2 x 1405

2822

0.4

N

474

3047

0.8 3140

Ra I

NH4 o. ph. str.

H

2 x 1402

3042

3136

20

1.2

82

H

2806

40

1755

60

2006

IR %T

H2O bend H2O str.

2004

80

Ammonium chloride

100

0.0

83

1622 H2O bend

H2O str.

639 SO4 i. ph. str.

3.0

SO4 o. ph. str.

2.0

SO 4

SO4 i. ph. bend

2 Na 1153 1132 1101

1.0

648 633 621

Ra I

SO4 o. ph. bend

994

1130

20

466 451

40

Sodium sulfate

60 617

IR %T

3434

80

2096

100

0.0

1214

988 SO3 i. ph. str..

1.6 1.2

SO3 o. ph. str..

SO3 i. ph. bend

2 Na

SO3 o. ph. bend

639

498

SO3

0.8

950

2.0

Ra I

970

SO3 i. ph. + o. ph str.

0.4 0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

Sodium sulfite

20

2 x 632

84

496

H2O bend

632 + 496

1137

2 x 970 H2O str.

1435

60 40

1622

1931

IR %T

3435

80

496 + 970

632

100

168

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

695 CO3 i. ph. str.

0.5

CO3 o. ph. str.

0.4

Ra 0.3 I 0.2

CO3 o. pl. bend CO3 i. pl. bend

1081

1442

1081 + 702

85

880

1615

1442 + 1081

H2O str.

20

1778

2495

3436

IR 60 %T 40

H2O bend

1429

0.1

702

CO3 2 Na 1769

Sodium carbonate

80

2855

2955

100

0.0 100

86

657 567

87

826 NO3 o. ph. str.

1051

NO3 i. ph. str.

2.0

NO3 o. pl. bend

NO3 i. pl. bend

1345

1360

716

NO3 2 K

1.0

226 205

992

1051 + 716

3.0

Ra I

696

1046

1309

1621

1764

3435

1385 + 1051

H2O str.

1385

20

2398

2741

H2O bend

80

IR 60 %T 40

1457 1437

1684 1623

1913

0 100

698 686 660 647

Na O

834

C

O

CO2 bend

O-CO2 o. ph. str. + OH def. 1268

CO2 o. ph. str.

O

2535

Ra 60 I 40

H

2906

80

CO2 i. ph. str.

837

1452

100

20

Potassium nitrate

1400

1696 1661 1618

20

1046 1033 1000

1845

2045

1922

2545

3070

IR 60 %T 40

2919

OH str. 3431

Sodium bicarbonate

80

0.0 3500

3000

2500

2000

1800 1600 1400 1200 Wavenumber (cm–1)

1000

800

600

400

169

INORGANIC COMPOUNDS 100

0.0 100

500

B HO

OH

1200

600

1165

1400

811

1378

3254 3178

4 2

800

600

0 3500

3000

2500

234

505 462 399

882

BO3 o. ph. str.

OH str.

6

Boric acid

OH

BO3 def.

209

10 8

1194

1449

3217

12

Ra I

560

BO3 i. ph. str.

20

90

546

40

647

884

60

OH wag + PO3 o. pl. bend

OH wag + BO3 o. pl. bend

2095 2031

80

2362 2262

2631 2516

0 100

547 524

462 1181

1358

1790

2852

2402

4 2

IR %T

549 590

2

O

999

HO

PO3 i. ph. str.

1131 1066

Na

P

6

Ra I

PO3 o. ph. str.

O

588 577

P-OH bend

2 O

860

1156

10

1073

20

8

89

1392 1359

1796

40

2454

60

3430

IR %T

Involves P-O(H) str.

P-OH str + P-OH bend

H2O

2853

80

Tribasic sodium phosphate

12 H20

416

3

O

861

0.8

Na

O

551

H2O

816

1.6

O P

Dibasic sodium phosphate

O

PO4 i. ph. str.

1072 1031 1004 993

2.4

938

PO4 o. ph. str.

3

3311

Ra I

1423 1384

1013

3.2

936 948

3545

3345

20

692

40

1566

H2O

60

1664

IR %T

88

H2O

2279

80

2000

1800

1600

Wavenumber (cm–1)

1000

400

170

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS 100

MoO4 i. ph. str.

483 445

MoO4 o. ph. str.

60

285

3296

20

324

Na2MoO4 • H2O

40

843 808 834

Ra I

674 639 600 547

904

1402

1560

H2O

80

92

MoO4 o. ph. str.

896 858 899 834 822

20

3305

40

1698 1677

2223

H2O

60 3453

Sodium molybdate

80

1801

0 100

IR %T

657 629 621

NaClO4 • H2O

626

1085

ClO4 o. ph. str.

891

40 20

ClO4 i. pl. bend.

ClO4 o. pl. bend.

1149 1098 1089

Ra 80 I 60

953

1135

ClO4 i. ph. str.

120 100

659

941

H2O

20

1108

40

1632

60 3536 3438

Sodium perchlorate

91

H2O 2060

80

IR %T

0

100

468

800

479

1.6 1.2

399

1183

20

969

1872

1970

H2O

1092

40

Si-O-Si o. ph. str.

O

Ra I 0.4

O

793

• x H2O

Si

977

0.8

1000

800

1078

Silica gel

IR %T

1636

60

93

H2O

3443

80

x

0.0 3500

3000

2500

2000

1800

1600

Wavenumber (cm–1)

1400

1200

600

400

171

INORGANICS AND POLYMERS

100

60

94

1065

IR %T

involves MgO/SiO str. 3675 3661

80

40

Talc

676 665

H2O (hydrate) OH str 944 1051 1015

1.0

3MgO 4SiO2 H2O (Talc) •

792

Ra I •

468 432

362

2.0

291

20

0.0

321 251

529

456

399

842 809

3000

2500

2000

1800

1600 1400 1200 Wavenumber (cm–1)

1000

466

800

600

400

*

N

Polybutadiene

726

911 964

CHCH2

Cis, Trans

0.0 3500

Isotactic polyethylene

998 973 899 841 809 1037 999 973 899 993

1077

1219 1152 1239 1209

3078

1.0

CH2CH

1011

1668

Ra 2.0 I

96

CH2 def 1438

3.0

1655

3006

4.0

*

Trans CH wag

2908

50

Cis CH wag

C=C str

=CH str. CH2 str

2844

60

*

N

1079

70

1304

80

1312

3073 3006 2915 2844

IR %T

1653 1640

0.0 100 90

1167

1455 1460 1436 1360 1330

1.0

CH

CH def.

1447 1436 1404

2.0

H2C

CH2 def. CH3 o. ph. def

CH2 o. ph str.

Ra 4.0 I 3.0

*

CH3 i. ph. def.

CH3 o. ph str.

5.0

CH3

1376

CH3 i. ph str.

80

95

1277

90

2953 2950 2924 2904 2884 2917 2867 2838 2723 2840

IR %T

1359 1304

100

172

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

*

N

916 1087

841

1023

1142

620 841

626 637 CF2 def.

CF2 o. ph.str.

6

387

290

8

*

N 3500

3000

2500

2000

1800 1600 1400 Wavenumber (cm–1)

1200

599 575

1299

CF2CF2

1218

*

2 0

734

0 10

1147

20

1202

40

4

279

363

844 858

99

IR 60 %T

Ra I

483 421

856 915 961 947

1240

1148 1094 1028 1141

1281 1295

80

1381

Polytetrafluoroethylene (Teflon)

0.0 100

582 536

OCH2CH2

1237

1479 *

Involves C-O-C i. ph. str.

Involves C-O-C o. ph. str.

929

2885

CH2 Def.

2940

1.0

1093 1059

20

1062

CH2 str.

98 1145

2880

40

1.8

Ra I

1238

1419 1370 1322

60

1467

2946

2807 2695

*

N

1417 1360 1341 1279 1241

3386

0.0 100

CH OH

1607

CH2

2720

*

1367

2.0

Ra I

IR %T 80

Involves C-O str.

231

1568 CH2 Def. 1444

OH str.

1.0

Polyethylene oxide

CH str.

2909

CH2 o.ph.str.

80

1447 1396

IR 90 %T

97

CH def.

2906

2937

1710

OH def.

3280

Polyvinyl alcohol (PVA)

98

1000

800

600

400

IR %T

3450

3429 3296

3069

2977

2949 2882

2925

2938

OH str.

3077 2932 2859

CH2 o. ph. str.

CH2 o. ph. str.

*

3032 2936 2650

CH2

HO

O N

O

CH

2000

*

N

N

1228 1167

1182 1102

1073 1017

NH wag

C

O

H N

600

648 606 554

1231 1167 1105

1004

933 895 844

912

904 819

795

733 646

607

Poly acrylic acid

173

Polyvinyl pyrrolidone

316

100

364

607

506

101

*

N

102

H N

400

(CH2)6

N-C=O bend

(CH2)4

756

1452 1412 1377

1339 1318

Involves C-O str.

C

O

800

Nylon 6,6

1456

1372 1315

898 852

1697 CH2 def.

*

1000 601

728

1284 1270

1492 1460

934

936

687

1370 1347 1311 1231

1421

1107 1076 1023

1043

952

1426

C=O str.

1274 1200 1180 1141

1494 1456

CH def

1200

1128 1063

1473 1464 1416 1371

1673 1609

1655 CH2 def.

1298 1235

CH2 Involves def. CNH str/open

1400

1382

C=O str.

1535 1441

CNH str/bend

1600

C=O str.

1800

Wavenumber (cm–1)

1550 1475

1671

1631

POLYMERS AND BIOPOLYMERS

C

CH2

CH

CH2 i. ph. str.

2500 1637

100

90

80

70

60 5.0

4.0

1.0

90

0.0 100

80

70

Ra 3.0 I 2.0

IR %T 60

50

40

3.0

1.0

0.0

100

NH str.

2732

90

80

70

3.0

2.0

1.0

3000 2867

2919

Ra 2.0 I

IR %T

Ra I

0.0 3500

3304

174

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

CH def

2730

479

992 OH

N

578 765 717

*

O

1262 1211

3395

OH

Starch

1380 1339

O O HO

1459

2908

OH

864

O O HO

939

4.0 Ra I 3.0

441

Involves Ring in-phase and C-O str of Semi-circle str. COH/C-O-C

OH

361 308

OH str.

CH2 def

1.0

927 856

CH def

5.0

2.0

761 706

1241

OH def

60

Starch

1337

1639 H2O

1148 1077

70

104 1410

3309

80

2927

90

50

457 437

Involves C-O str. of COH/C-O-C

0.0 100

IR %T

519 491

1474

N

OH

Cellulose

568

O O

HO

996 970 897

*

1149

1379 1339 1294

HO O

O

OH

3381

1.0

OH

2895

Ra 3.0 I 2.0

898 Ring semi-circle str.

OH

4.0

252

5.0

1120 1095 1054 1030

OH def

OH str.

1123 1085 1054

Cellulose

40

379

CH2 def

50

663

H2O

60

103

1427 1369 1315 1281 1202 1160 1105

70

2939 2894

80

3334 3274

IR %T

1642

Involves C-O str. of COH/C-O-C

90

0.0

CH2CO2

Sodium Carboxyl Methyl Cellulose

3000

2500

2000

699

Involves C-O str.

0.0 3500

579

N

OR

R = H or

700

Ra I 1.0

916

1418 1374 1328 1263

* O

*

1451

2900

RO O

Ring semi-circle str. 477 430 356 302

CO2 i.ph.str.

OR

105

904 1115 1100 1048 1021

1413

1590 CO2 o.ph.str.

OH str.

1611

80 2.0

1322 1266

Involves C-O str. of COH/C-O-C

2878

3223

IR %T 90

3392

Sodium carboxymethyl cellulose

98

1800

1600

1400

Wavenumber (cm–1)

1200

1000

800

600

400

175

POLYMERS AND BIOPOLYMERS 100

1051

290 458

701

946 891 854

1267 1201

3438

1030

1455

1369

1.0

Hydroxypropylmethyl cellulose

1100

Ring in-phase and Semi-circle str.

Involves C-O str. of COH/C-O-C

OH

1152 1122

2935 2893

CH3

CH2CH

2837

2.0

CH3 or

R = H or

CH def

N

OR

60

Ra I

CH2 def

O

*

OH str.

1191

*

O

70

1453

H2O

OR RO

944

80

1373 1313

2967 2899 2836

3451

IR %T

1636

106

90

CH2 def N

438 388

882 823

699

1458 1486

2815 2716

251

Involves C-O str. of COH/C-O-C

CH2CH3

915

OR

Ring in-phase and Semi-circle str.

1115

2932 2876

1101 1054

* O

*

R = H or

Ra 1.0 I

917 882 817

1484 1444

OR RO O

2975

2.0

107

Ethyl cellulose

CH2 (O) i.ph. str.

1375 1310 1280 1203

H2O

1159

CH3 o.ph. str. of OCH2CH3

OH str.

1403 1371 1354 1270

IR 90 %T 80

1636

2974 2928 2871

3471

0.0 100

0.0 Si-O-Si o. ph. str.

Si CH3

*

3113

Siloxane polymer

N

1411

2.0

O

643

Si CH3

709

O

790

*

867

3.0

Si CH3 rk

CH3

1263

2965

CH3

1.0

842

Si CH3 i. ph. def.

Ra 4.0 I

492

2906

1261

CH3 str. o. ph. str.

754 687

1413 Si CH3 o. ph. def.

o. ph. str.

20

1089 1029

40

108

800

2962

1593

2904

80 IR %T 60

5.0

1942

100

0.0

3500

3000

2500

2000

1400 1800 1600 1200 Wavenumber (cm–1)

1000

800

600

400

176

8. ILLUSTRATED IR AND RAMAN SPECTRA DEMONSTRATING IMPORTANT FUNCTIONAL GROUPS

100

109 783

1177 1124

483

639

843 774

999

1325

1456

1621

O

454

953 1270

NH2 str.

1341

1730 1673

Ester C=O str.

NC4 o.ph. str. + CH3 rk

1452

1733

CH3 (N+) o.ph. str.

NC4 i.ph. str.

379

1485

Amide C=O str. CH3 (N+) o. ph. bend

720

1671

3013 2960

3379 NH2 str. + H2O OH str.

CH2 o. ph. str.

877

H 2N

955

O

874 849

O

110

C

CH 3

0.20 Ra I 0.10

1108

1451 1419 1349 1323

C

2830

0.30

O

1

H3 C

60

20

H 2N

Involves Ester C-O str.

CH

1057

N

CH 2

1166

C 2H 4

9

1144 1084 1039

H3 C

CH

1453 1390 1341 1256

CH 2

Cl

80

40

CH C

C-C-N i. ph. str.

1210

3359

NH2 bend

100

IR %T

CH 2

C-N str.

0.00

3022 2970 2931

Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC)

0.10 Ra I 0.05

C=O str.

NH2 bend

NH2 wag

NH2 rock

CH2 bend CH rk

1424

NH2 i. ph. str.

0.15

1663

NH2 o. ph. str.

CH (C=O) str.

1658 1610

0.20

CH2 o. ph. str.

2925

20

3199

3344

40

3208

Poly acrylamide (PAM)

60 IR %T

2858

2932 2858

80

0.00 4000

3400

3000

2500

2000

1800

1600

1400

Wavenumber(cm-1)

1200

1000

800

600

400

200

C H A P T E R

9 Unknown IR and Raman Spectra On the following pages, 44 unknown spectra are presented without labels or interpretation to help test and develop the readers knowledge and understanding of functional group analysis and spectral band assignment. The spectra are found on pages 178 to 199. The sample preparation used to acquire the spectra is provided and where appropriate hints are included to help guide the interpretation. For interpretation and an explanation of the key features useful to help identify each compound an answer key is included at the end of the unknown spectra starting on page 200.

177

178

9. UNKNOWN IR AND RAMAN SPECTRA

903 862

1257

80

2793 2660 2612

3154

100

Neat/cap film

60

1450

IR %T 40 20

801

2917 2852

1

5.0 4.0

1157

1266

1347

2665

1.0

1444

2.0

426 384

3.0

1028

2938 2852

Ra I

0.0

811

984 919

1168

80

2720

3182

100

1468

40 2957 2903 2871

20

2

2873

0

809

8.0

2959

6.0

469

0.0 3500

3000

2500

2000

1800

1600

1400

Wavenumber(cm–1)

1200

1000

800

600

400

249

420

956 921 870

1036

1158

2.0

1346 1320 1252

2719

1464 1452

Ra I 4.0

306

IR %T

Neat/cap film

1386 1367

2840

60

10

5

3500 3000 2500 2000 1800

Wavenumber (cm–1)

0

1600 1400 1200 1000 800 413

100

600 302

545

722

938

531

768

855

400

259

418 376

495

693

887

1059 1019 971

1211 1160

886

1446

1649

1374

433

565

1057

1220

1299

1782

2728

522

60

220

796 816

1020

1111

1202

1293

1429 1413

1651

2970 3075 2938 2853

100

503

655 644

847

925

1111

1268

1476 1461 1393 1363

1646

1788

6.0

1269 1218 1194

20 1516

60 2728

2985 2913

Ra I 4.0

1897

2.0 3077

20

1463 1449 1378

40 2905 2869

80

3057 3094 3024

80

1618 1575

2963

40

2923 2876 2907 2779 2713

IR %T

3065

IR %T

3016 2963

UNKNOWN IR AND RAMAN SPECTRA

179

Neat/cap film

3

0.0

Neat/cap film

4

15

Ra I

1.0

3500 3000 2500

2.0

2000 1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 379

529

1386 1343

558

912 872 840 767 741

1096 1049

1261 1232

1627

1581

0.0

400

252

478 437 409

860

567 511

1076 1021 978

1163

1464 1428

1623

701

1376

771

790

408

1268 1214 1165

566

732 700

855

1076 1020 977

533

1398

1509 1463 1439

1597

1800 1745 1690

1924

100

559

766

841

1427

40

870

4.0 1464

60

916

3.0 2294

0 15

1097 1049

5.0 2250

40

1329 1262 1232

20 2750

60

3098 3068 3048 2968 2944 2861

80

1453 1431

Ra I 2250

IR %T

2880

80

2841 2751

5 3012 3059 2909 2860

Ra I

2973 2931 2941 2881

IR %T

3635 3544

180 9. UNKNOWN IR AND RAMAN SPECTRA

Neat/cap film

20

5

10

100 0

Neat/cap film

6

1.0

3500 3000 2500 2000

1800 1600 1400 1200 Wavenumber (cm–1) 1000 800

0.0 600 400 276

398

0.0 100 289

513 480

888 854 806 755 686

1143 1094 1033

2235

374

Ra 1.5 I

355

488

660

738

952 901 847

1073 1046 1030 1011992

1114

1449 1436 1381 1332 1260

2301

804 739

887

1181 1150 1072 1032

1464 1456 1436 1380 1339 1279

2053

2472

2738 2669

3177

3456

100

454

515

Ra 2.0 I

749

2.8

828

40

1114 1072 1030 966 902 880 810

1.0 2874

1.8 2964 2920 2935 2873

IR 60 %T

1298

80

1433 1379 1339 1295 1252 1216

IR 60 %T

1466

2738

0.5 2962

20

2960 2937 2908 2876 29342874

40

1253 1222

1450

20 3332

80

2736

3330

UNKNOWN IR AND RAMAN SPECTRA

181

Neat/cap film

7

Neat/cap film

8

182 100

80

60

IR %T 40

20

3.0

1.0

0.0

100

80

60

40

20

28

20

Ra 2.0 I

IR %T

Ra I 10

0 3500

3548 3327

3326

3000

3169

30643088 3030 2931 2873

3059 3006 2938

3004

2959 2930 2873

3003 2733

2635

2500

2427

2415

2000 1800 1600 1400 1200 Wavenumber (cm–1)

9. UNKNOWN IR AND RAMAN SPECTRA

2736

2937 2913 2875

1952 1876 1810 1751 1671

1672 1607 1586

1602

1461

1454

1496

1458 1438 1379

1457 1440 1369 1303

1209 1177 1157

1000

1030

1301 1289 1233

1209

1003

1021

1094 1043 1011

1080

1038

912

800

815 799 752

780

735 698

409

738 648

641

533

508 468 420

9

10

375 337

Neat/cap film

400

461

Neat/cap film

600

595

485

1003

1131

916 890

892 859

816

621 595

969

1091 1043

1347 1300 1232

183

UNKNOWN IR AND RAMAN SPECTRA

60

H2O

3433

IR %T

809

80

KBr disc

1467

H2O 1630

100

474

40

1111

20 0 2.0

11

Ra I 1.0

0.0 100

864

80

702 687 662

2906

1445 1412 1385

Smear

800

12

1.8

491

2907

1261

20

1092

40

1020

IR %T

2963

60

688

790

867

2809

1413

0.5

1265

2966

1.0

3114

Ra I

710

1.5

0.0 3500

3000

2500

2000

1800

1600

1400

1200

Wavenumber (cm–1)

1000

800

600

400

1.0

3500 3000 2500 2000

1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 488 435

853

1015

1128

1217

1305

Ra I 835

874

1121

40 1255

889

614

1083

1048

1290

1367

1455

1977

2 288

1071 1029

1227

1366 1332

1488 1451

2719

4 2963 2876

914

1070

2975 2860

20

1444

80 2754 2694

40

1399

60

2962 2916

911

658

1241 1181

1365

1460

1967

2682

80

2719

IR %T 2855

Ra I

2967 2855

IR %T

2889

184 9. UNKNOWN IR AND RAMAN SPECTRA

100 Neat/cap film

60

13

8

6

100 0

Neat/cap film

20

14

3.0

2.0

0.0 600 400

0.4

0.2

3500

3000

0.6

2500

2000

1800

1600

1.0

Wavenumber (cm–1)

1400

1200 436 378

711

897

1000

100

800

600

261 212

443

614 553

758

785

Ra I

656

871

1105

996

1248

755 692

1041

1601

40

0.0

400 279

508

60 1050 1026 1003 987

70

712

1087

1303 1248 1174 1154 1038 1022

20

898

1.2

1118

80 1337 1316 1250 1205 1160

1422

IR %T

1379 1339 1260

1498

Neat/cap film

1456

1601 1588

3072

20

1642

1794 1733

3007 2944 2837

3172

1077

784

1173

1303

553 511

883

1336

1701

1840

2030

2548 2489

3092 3063 3003 2956 2836

1468 1442

1588

60

1470

0.8

1604

Ra I 2895

90 2914 2899 2857

80

3296

3331

IR %T

3416

UNKNOWN IR AND RAMAN SPECTRA

185

100

15

10

0

ATR of paper: ID Components

16

1.4

1.0

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

1000

800 478 431 411 340

3.0 1258

831

60

942 883

1131 1113 1044 1004

477 431

794 765 733

1258

1465 1432 1410 1380

1483

386

846

1010

1202 1159

1296

1380

1451

2.0 988

1664

Nujol

831 795 765 734

2.0 883

Neat/Cap film Nujol

1167 1132 1111 1043 1007 942

20 2256

1456

1626

1716 1662

1838

2636

2720

921 772 722 538 517

851

1306 1245 1209 1156 1097 1071

1378 1362

60

1303

40

1451 1412

Ra I 3277 3187 3024 3000

80

1484

80 2736 2607

Nujol mull

2925 2934 2875 2876

2997 2916 2867

20

2739

0

2998

IR %T 3046

Ra I 40

2963

IR %T

3049

186 9. UNKNOWN IR AND RAMAN SPECTRA

100

17

1.0

100 0.0

18

Hint: Ether

0.0

600

400

3500

3000

2500

2000

Wavenumber (cm–1)

1800

1600

1400

1200

1000 577

Ra I 440

478

1155 1080 1017

538

896

1071

1156

1261

800

600

253

415

689

2.0

438

575 528

861 766 709

930

1243

Ra I

357 309

1126

940 865

1049

1264

1607 1591

672

764

880

1000

1604

1153 1089 1065 997

1320 1297 1243

1489 1474 1441

838

1203

1587

20

766 709

2.0 1461 1420 1367

60 1641

80 2081

3339 3262

1538

1703

1924

2794 2674

3064

60

1381 1338

40 2930

3078 3063

80

1460

20

2910

IR %T 40

3421

IR %T

3423

UNKNOWN IR AND RAMAN SPECTRA

187

100 ATR

19

4.0

100 0.0

KBr disc

20

4.0

0.0

400

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

1000 626

5 655

650 622 462

923

1046 1012

2.0

926

80

800

474

100

600

323

590

873 818 772 724

960

1113 1057

1169

1302

1453 1415

1716

2875

1168

1358

29212960 2935

1466 1412

60

1011

1444

H2O

1452

4.0 2876

Neat/Cap film

1714

2736

20

3003 2934 2854

40

1415 1343

1580

60

2929

IR %T

3011

IR %T 589 540 475

723

896

994

1111

1259 1231

3628 3417

80

3433

188 9. UNKNOWN IR AND RAMAN SPECTRA

100

21

Ra I

0.0

KBr disc

40

20

22

Ra I 10

0

400

2.0

3500

3000

2500 339

6.0

2000

1800

Wavenumber (cm–1)

1600

1400

1200

1000

800 415

Ra I 4.0 631

20

892 857

Neat, cap film 1247

630

929 893 861

1084 1060

1159

1368

273 211

1066 1045 981

1321 1265

1445 1428

353

726 678

655

2969 2916

1265

20

953

60

1462 1451 1384

2915

1451 1437

2872

1061 970 955

1375

2835

654

784 758

1320

80

1161 1126 1061

40 2556

4.0 2928

6.0

2875 2835 2730

40

2573

2.0 2969

8.0

2809 2722

60

1449 1385 1316 1247

2721

IR %T 2925 2866

Neat, cap film

2963

80 3204

Ra I

2927 2866 2912

IR %T

2968

UNKNOWN IR AND RAMAN SPECTRA

189

100

23

Note: compound contains S

100 0.0

600

24

Note: compound contains S

0.0

400

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

10 282

1071

810

60

1000

700 679

1309

911 831

800

600

514

297

557

1079

1160

1265 1251

1490 1445 1381

245

528

Note: compound contains F

391 349

649

751

1134

1182

40

977

1004

Neat/Cap film 1280

2

1456

777 728

1004

1142

1265

1489

Neat/Cap film 682

925

1251

1619 1590

20

1180 1137 1101 1071

Ra I 8

1619 1595

444

887

1079 1041 1004

1743

1930

3040 2956 2923 2866

528

728

857

1459

40

1353

20 1354

IR %T 60

1627

4

3079 3061 2967 2923 2869

80

1628 1607

80 3086

Ra I

3090

IR %T

3047

190 9. UNKNOWN IR AND RAMAN SPECTRA

100

25

10

6

100 0

26

Note: compound contains F

0

400

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

1000 800

431

800

600 280

545

Note: compound contains Cl 458

597

885

Neat/Cap film 381

596 544

859 800

1075

1204 1147

457 430

720

1434

815 751

1025

1195 1163

338 271

530

665 617

1072

1433 1372

1595

1007

1217 1194

1765

750 692

926

1163

1371

1493

1594

1743

1431

3071 3044 2939

664 594 530 499

815

892

1070 1026 1013

60

885

1076

10 1204

20

1149

40 1369

60

1264

Ra I 1764

Neat/Cap film

1575 1547

IR %T 1767

3071

40

1575 1548

1796

80 2939

5

1797 1765

3184

Ra I

2530

3082

IR %T

3146 3083

80 3470

100

3550

UNKNOWN IR AND RAMAN SPECTRA

191

H2O

20

27

10

0

100

28

0

400

1.0

2.0

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

0.0

1000

800

600 351 243

1058

1056

1213

535 477

607

1246

725 662

795

100

446

534

1200 612

5.0 1201 1187

Note: Compound contains S and is a monohydrate

798

Nujol

893

20 1377 1309 1272

1450

1601

1272

237

400

652

960 894 859 834

1164 1124

1384

1580

1.0

1022

3.0 1727

2.0

1625

1040

1123 1073

1287 1274

1729

3071

3439

705

771

959

652

1600 1580 1488

1040

1381

743

1463

2873 2861

60

1304 1290 1195 1178 1116

60 1461

80 2233 2096

2734

3.0 2898 2960 2937 2931 2875

Neat/Cap film

1459 1420 1441

Ra I 40

2855

Ra I 3073

20

2731

40

2929 2889

IR %T

2974

IR %T

3544 3484

80

3475

192 9. UNKNOWN IR AND RAMAN SPECTRA

100

29

0.0

Nujol mull

Nujol

30

4.0

400

2

3500

3000

2500

4

2000

1800 1600 1400 Wavenumber (cm–1)

1200

1000

800 288 232

649

600 440 362

842

405

542

996

622

1296

727

558

839 776

916

1225

1460 1445 1382 1341

580

905

983

1153

1380 1358 1280

845 773

753

1055 1040

463

1445

397

1457

2922

2876

3367 3291 3193

1045 975

1181 1150

1377 1356

1611

921 861 809 769

1461

60

772

6

914

1025

KBr disc

1021

20 1150 1112 1051

40

1119 1074

60

1148

80

1272

1345

Ra I 0.8 2936 2962

1.2

2735

2963

Neat cap film

2877

20

1460 1403 1371

IR %T 40

1630

0.4

2945 2914 2892 2797 2695 2542

3314

Ra I 3378

IR %T

2960 2946 2892

3410 3315

80

3398 3285

UNKNOWN IR AND RAMAN SPECTRA

193

100

31

0.0

100

32

8

0

400

3500

3000

2500

2000

1800 1600 1400 Wavenumber (cm–1)

1200

1000

800 410

695

ATR

622

794 761

1002

3.0

1032

754

906

1181 1154 1070 1028

1372

631

389

1033

1152

1296

100 0

40

34

Ra I 2.0

0.0

600

400

224

322

813 775

934

949

1198

285

566 470

1450

866 770

580

888

1027

619

811

935

1603

1759

3067

1307

1482 1396 1363 1231 1246 1214 1152 1120

1432

Nujol mull

1183 1156

Impurity 1493 1452

1604 1590 1472 1451 1379

40

1331

60 1601

1727

2915

60

1452

1.0 2715

2968

6

1603

IR %T 4

3070 3001

80

1584

80 3060 3026 2923 2851

2

3054

Ra I

2907 2854

20 3626

IR %T

3626

194 9. UNKNOWN IR AND RAMAN SPECTRA

100

33

8

1.0

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

1000 881

800 731

837

600 281

621

Smear

1032

20

1082 1004

40

1218

H2O 728 705

1457

781

932 883

621

1008

1084

1218

1419 1378

403

896 879 834

1059

1135

1299

280

2963 2937 2874

2958

1444

2928

2873

1131

1464

2809

735

1206

1337 1302

961 898

1096

1378

60

1159

60

1302

80

1466 1488

1.0

1709

2.0 2811 2734

Ra I

1442

3002

Neat cap film

2854

3.0

2924 2955

40

1605 1586

2732

2925 2891 2852

Ra I 3061

IR %T 3287

80

3062

20

3326

IR %T

3378

UNKNOWN IR AND RAMAN SPECTRA

195

100

35

0.0

100

36

2.0

Note: Quaternary amine salt

0.0

400

3500

3000

2

2500

2000

1800

1600

Wavenumber (cm–1)

1400

1200

1000

800

6 250

538

722 697

755

458

862

996

600

297

466

777

1003

1298

1452

936

384

891

1076

1152

1373

620

4.0

292

Note: Contains P 1003

H2O

685 618

1030

1312

1485

1651

1654 1556

1296

1440 1373

602

1152

3086 2976 2936 2877

465

721

1041 1003 936

80

392

4 1180

20 1191 1121

40 1439

60

1096

KBr disc 1624 1591

2.0 2879

2935

Neat cap film

1593

Ra I 2976

3290

40

1575

Ra I 2741

3289

20

3056 2991

60

2963

80

3060

IR %T

3145

IR %T

3435

196 9. UNKNOWN IR AND RAMAN SPECTRA

100

37

0.0

100

H2O

38

0

400

2.0

3500 3000 2500 2000 1800

Wavenumber (cm–1) 1600 1400 1200 1000 800 237

768 715

815

Neat cap film

832

100

666

1073 1029

741 692

968

600

439

532

670

336

825 805 752

1003

1167

1222 1091

1284

0.0

1297

Note: Contains Boron

1149 1120 1070 1031 962 904

80

1260 1231 1150 1118

60 1591

959

1555

3081 3042 2926

511

877 835 772 750 695 626

1223 1171 1129 1071

1615

1425 1359 1310 1283

KBr disc

1299

6.0 1417

0.4 1616 1589 1557 1483 1444 1378

1.6

1485 1431 1466

20

1337

0.8

1450

40 2910 2874

2926

1.2

2909

20 2959 2935

1926

2145

80

1262

Ra I 3279

60

1483

2736

2937 2875

IR %T

2963

Ra I 40

3076 3048 2963

IR %T 3416

UNKNOWN IR AND RAMAN SPECTRA

197

100

39

40

Note: Contains Boron

4.0

0.0 400

3500 3000 2500 2000

4

1800 1600 1400 1200 Wavenumber (cm–1) 1000 800 600 429

707 656

346

856

559

636

750

1118

420

223

661 619

800

1181 1160

1341 1314

1367

1603

Ra I 10

1330

1574

2925

703

996

1442 1368 1381 1350 1309

Nujol Mull

1119

1367

1477 1408

3057

20

1337

60 1635

3144 2994

689

1603

2854

580

1494

3078 3025

1088 1026

760

1180

1456

60

961

8 1573

KBr disc 1738 1726

80 2920 2852 2752

40

1710 1660

Ra I 40 3175

IR %T

1801

20 3331

IR %T

3317

80

3162

198 9. UNKNOWN IR AND RAMAN SPECTRA

100

41

Note: Purchased as Phenyl Boronic Acid with an HPLC assay of 97%

0 100

42

12

0 400

4

3500

3000

2500

2000

1800

Wavenumber (cm–1)

1600

1400

1200

8

1000

800

600 388 378

583

676 617 584 463

814

767 732

1027

1195

758

927

348

998

1089

1193

1569 1509 1447 1384

2921

284

1265 1236

6

780

984

1650 1632 1550 1467 1437

60 2199

2865 2747

661

626

827

1383 1232

762

926

669

1302

1087 995 978

1304 1265

1535

1824

1579

2486

2745 2646

3091 3029

1473 1444

Nujol mull

1443

16 3120 2981

40

1556

Ra I 4

1659

20 2684

IR %T 40

2925 2853

2

3130

20

3123

Ra I

3470 3419 3334

IR %T

3469 3419 3330

UNKNOWN IR AND RAMAN SPECTRA

199

100

80

60

43

8

Note: Nitrogen 5 member ring aromatic heterocycle

0

100

KBr disc

80

44

Note: Nitrogen 6 member ring aromatic heterocycle

12

0

400

200

9. UNKNOWN IR AND RAMAN SPECTRA

1. Cyclohexane: 2930, 2855 CH2 (note no 2960 CH3), 1452 CH2 (note no 1378 CH3), (note no 723 CH2 chain).

2. 2,4-dimethyl pentane: 2965 CH3 o.ph. str., 2940e2850 CH3 and CH2 str., 1387, 1369 C(CH3)2 i.ph def, (note no 723 CH2 chain). CH3

CH3

CH3CHCH2CHCH3

3. 2,5-dimethyl-1,5-hexadiene: 3080, 2980 ¼CH2 str., 2950-2860 CH3 and CH2 str., 1780 2x890, 1650 C¼C str., 1450 CH2 and CH3 def, 890 R2C ¼CH2 CH wag. CH3 H3C

H2C C

C

CH2

CH2

H2C

4. 4-tert-butyltoluene: Aliphatic group: 2965, 2876 cm 1 CH3 str. 1470e1460 cm 1 CH3, 1392, 1363 cm 1 C(CH3)3, 1375 cm 1 CH3. Aromatic group: 3100e3000 cm 1 aromatic CH str., 1517, 1407 cm 1 para aromatic ring, 817 cm 1 2 adj aryl H.

H3C

C(CH3)3

5. 1-methyl naphthalene: 3100e3000 arom CH, 2975e2860 CH3, 1597, 1510, 1398 aromatic ring, 1462, 1378 CH3, 790 3 adj aryl H, 772 4 adj aryl H. CH3

UNKNOWN IR AND RAMAN SPECTRA

201

6. n-Butyronitrile: 2970 CH3, 2250 ChN str., 1460 CH3, CH2 def, 1425 CH2(eChN) def 1375 CH3 i.ph. def. H2C H3C

C

N

CH2

7. 2-Hexyne: Aliphatic group 2964/2873 cm 1 CH3 str., 2935, 2920 cm 1 CH2 str., 1464e 1450 cm 1 CH3/CH2 def, 1380 CH3 i. ph. def., 1339 CH2 wag. Acetylene group, ChC str in Raman at 2301 and 2235 cm 1 (Fermi resonance), 375 cm 1 CeChC bend (Raman). H2C H3C

C

C

CH3

CH2

8. n-Butanol: Aliphatic group: 2960, 2874 cm 1CH3 str., 2935 CH2 str., 1470e1450 CH3, CH2 def, 1378 CH3 i.ph def. Hydroxy group (primary alcohol): 3300 cm 1 OH str., 1420 broad OH bend, 1070e1025 involves CeO str of CH2eOH, w660 cm 1 (broad) OH wag. H2C

HO H2C

CH3

CH2

9. Trans-2-Hexen-1-ol: Aliphatic group: 2959, 2873 cm 1CH3 str., 1458 CH3, CH2 def, 1379 CH3 i.ph def. Olefinic group: 3004 cm 1 CH str., 1672 cm 1 C ¼C str., 1300 trans CH rk, 969 cm 1 (IR) trans CH wag. Hydroxy group (primary alcohol): 3327 cm 1 OH str., 1091, 1043, 1091 cm 1 involves CeO str., w650 cm 1 (broad) OH wag.

H2 C H3C

C H2

H C

CH2OH

C H

10. Benzyl alcohol: 3330 OH, 3100e3000 arom CH, 2925, 2870 CH2, 1600, 1493 arom ring, 1452 CH2/arom ring, 1420 broad OH, 1015 CH2eOH, 735 5 adj H, 696 arom ring. OH CH2

202

9. UNKNOWN IR AND RAMAN SPECTRA

11. Silicon dioxide: SiO2 1130e1000 IR strong (involves OeSieO o. ph. str.). Note:featureless Raman spectrum. 12. Dow Corning vacuum grease, dimethyl siloxane polymer: 2966e2963 CH3 o. ph. str., 2907e2906 CH3 i. ph. str., w1412 CH3 o. ph. def, 1261 CH3 i. ph. def, 1090e1020 IR involves SieOeSi str. CH3 *

O

CH3

Si

O

Si

CH3

CH3

*

N

13. Tetrahydrofuran: Aliphatic 2975e2860 suggests only methylene. Two bands at 1070 and 911 cm 1 suggest ether (CeOeC out-of-ph. and in-ph. str., respectively).

O

14. 1,4-Dioxane: Aliphatic 2955e2850 CH2, 1450e1442 CH2, 1362 O CH2 wag, 1120 cm involves CeOeC o. ph. str. 900e830 cm 1 involves CeOeC in-phs. str.

1

O

O

15. Anisole: 3100e3000 aromatic CH, 3000e2838 CH3(O), 1602, 1588, 1498 cm 1 aromatic ring, 1468, 1452, 1440 cm 1 CH3(O) þ aromatic ring, 996 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 1247, 1040 arom-OeCH3, 752 cm 1 5 adjacent aryl H, 690 cm 1 aromatic ring. CH3 O

16. Paper: Mostly cellulose and carbonate, some water, with a trace of organic sizing: carbonyl and olefinic. 17. Trans, trans-2,4-Hexadien-1-ol: Olefinic group, 3024, 3003 cm 1 CH str., 1672 cm 1 C ¼C str., 1300 cm 1 trans CH rk, 988 cm 1 (IR) trans CH wag (note higher frequency due to adjacent trans, trans olefin). Hydroxy (primary alcohol) group. 3277, 3187 cm 1 OH str., 1097, 1071, w1000 (shoulder) cm 1 involves CeO str.

UNKNOWN IR AND RAMAN SPECTRA

203

OH

18. 1,2-Epoxypentane: Aliphatic CH str with epoxy CH2 o. ph. str at ca. 3050 cm 1 and CH2 def. at 1483 cm 1. Epoxy CeOeC in-ph str. At 1258 cm 1 (both IR and Raman) and CeOeC out-of-ph str at 831 cm 1. Aliphatic peak intensity of methylene (2935 cm 1) to methyl (2875 cm 1) consistent with ca. 2 methylene to 1 methyl group. CH2CH2CH3

O

19. Meta (3) chlorophenol: 3360 cm 1 OH str., 3100e3000 aryl CH str (note no aliphatic (CH2, CH3) 3000e2800 cm 1, 1605, 1592, 1478, 1448 cm 1 aryl ring, 1000 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 1245e1190 cm 1 involves aryl-OH str, 852 lone H, 772 3 adj H, 680 aryl ring. Cl

HO

20. Dextrin: Polysaccaride/carbohydrate: 3421 cm 1 OH str., 1338 cm 1 OH def., 1155, 1080, 1017 cm 1 involves CeO str. of CeOH/CeOeC polysaccharide ring system. Axial connection similar to starch result in characteristic IR bands 930, 861, 766, and 709 involving the ring in-phase and semi-circle str. The cluster of Raman bands centered around the intense 478 cm 1 band also is diagnostic of the axial type connection. Presence of water is evident from bands at 2081 (combination) and 1641 cm 1 (def.). Biopolymeric aliphatic: CH2 and CH str bands between 2930 and 2820 cm 1 and CH2 def at 1460 cm 1. HO

O O

*

*

OH

OH

n

204

9. UNKNOWN IR AND RAMAN SPECTRA

21. 2-Hexanone: Ketone C ¼O str 1716 cm 1and C ¼O overtone at 3417 cm 1. 1412 cm 1 from CH2e(C ¼O) and 1358e1380 cm 1 from CH3 i.ph def. Aliphatic group: CH2 and CH3 stretches at 2935, 2875 , and 2960 cm 1, respectively. Peak intensity of methylene (2935 cm 1) to methyl (2875 cm 1) consistent with ca. 3 methylene to 2 methyl. No evidence of branching in Raman spectra. O

22. Sodium acetate: Simple IR and Raman spectra. Dominant IR bands at 1580 and 1444 cm indicate a carboxylate salt (CO2 out-of-phase and in-phase stretch, respectively).

1

O

O

Na

23. Ethyl methyl sulfide: Aliphatic group. The CH2eS has bands at 2928 cm 1 from the CH2 o.ph.str., 2872e875 cm 1 from the CH2 i.ph.str., 1265 cm 1 (IR) from the CH2 wag and 1437e1428 cm 1 from the CH2 def. The CH3eS has bands at 2969 cm 1 from the CH3 o.ph.str., 2915 cm 1 from the CH3 i.ph.str., the 1375 cm 1 is from the CH3 i.ph.def. and bands between 1070 and 950 cm 1 involve the CH3 rock þ CeC str. Bands involving the CeSeC str are observed between 780 and 650 cm 1. H3C

CH2CH3 S

24. 2-Propanethiol: SH group: SH str results in characteristic band at 2550e2580 cm 1. Aliphatic group: The isopropyl group has characteristic bands at 2963 cm 1 (CH3 o.ph.str), 2866 cm 1 (CH3 i.ph.str), 1462e1451 cm 1 (CH3 o. ph. def.), and in the IR spectrum at 1384, 1368 cm 1 (CH3 i.ph. def). The CHeS group has bands at 2912e2930 cm 1 (CH str.) and 1247 cm 1 (CH wag). The isopropyl CeC/CeS str results in a strong Raman band at 631 cm 1 involving the in-phase (C)2CeS str.

H3C CH H3C

SH

UNKNOWN IR AND RAMAN SPECTRA

205

25. 3-Fluorotoluene (1-fluoro-3-methylbenzene): Aliphatic (CH3) 2967e2866 cm 1 methyl CH3 str., 1459 cm 1 CH3 o. ph. def, 1381 cm 1 CH3 i. ph. def. Aromatic group: 3100e3000 cm 1 aryl CH str., 1619e1590 cm 1 aryl ring quadrant str. (IR/Raman), 1489 cm 1 (IR) aryl ring semi-circle str., 1004 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 857 cm 1 lone H wag, 777 cm 1 (IR) 3 adj H wag, 682 cm 1 (IR) ring pucker. Aryl-F results in IR/Raman bands at 1265, 1251 cm 1 involving the Aryl-F str. F

CH3

26. 1,3-Bis(trifluoromethyl)benzene: Note no aliphatic observed. Aromatic group: 3100e3000 cm 1 aryl CH str., 1628e1607 cm 1 aryl ring quadrant str. (IR/Raman), 1004 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 911 cm 1 lone H wag, 810 cm 1 (IR) 3 adj H wag, 700 cm 1 (IR) ring pucker. CF3 group: 1354, 1309, 1280 involves CF3 o. ph. str., 1181, 1134 cm 1 involves CF3 i. ph. str. F3C

CF3

27. Phenyl acetate: Aromatic group: 3100e3000 cm 1 aryl CH str., 1594 cm 1 aryl ring quadrant str. (IR/Raman), 1493 cm 1 (IR) aryl ring semi-circle str 1007 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 750 cm 1 (IR/Raman) 5 adjacent H wag, 692 cm 1 aryl ring pucker. Acetate group: 1765e1740 cm 1 C ¼O str., 1431 cm 1 (O)CH3 o. ph. def., 1371 cm 1 (O)CH3 i. ph. def., 1217e1163 cm 1 involves feOeCeC o. ph. str., 815 cm 1 involves feOeCeC i. ph. str.. 2939 cm 1 CH3 o. ph. st.

O C H3C

O

206

9. UNKNOWN IR AND RAMAN SPECTRA

28. 2,4,6-Trichlorobenzoyl chloride: Aromatic group: 3146e3082 cm 1 aryl CH str., 1575e1547 cm 1 aryl ring quadrant str. (IR/Raman), 1434 cm 1 (IR) aryl ring semi-circle str. The band at 800 cm 1 derives from the lone H wag and the band at 720 cm 1 involves the aryl ring pucker. The band at 1075 cm 1 (IR/Raman) involves the aryl-Cl str. Aromatic acid chloride group: 1796 cm 1 C ¼O str. and the weaker band at 1767 cm 1 involves the overtone of the 885 cm 1 that is Fermi resonance enhanced with the carbonyl. The 885 cm 1 and 1204 cm 1 involves the (aryl)CeC(¼O) str. Cl

O

Cl

Cl

Cl

29. Dioctyl phthalate: Aliphatic group: CH2 stretches at 2931, 2873 cm 1 and CH3 stretches at 2960 and 2861 cm 1, respectively. Peak intensity of methylene (2931 cm 1) to methyl (2960 cm 1) consistent with ca. 4 methylene to 2 methyl. Methylene and methyl deformation bands are observed at 1463 and 1381 cm 1. Phthalate Carbonyl Group: C ¼O str (IR/Raman) at 1727 cm 1, band cluster between 1290 and 1270 cm 1 involves CeCeO o. ph. str. and the band at 1040 cm 1 involves the CeCeO i. ph. str. The band at 1123 involves the OeC str of the aliphatic group. Aromatic group: 3100e3000 cm 1 aryl CH str., 1601e1580 cm 1 aryl ring quadrant str. (IR/Raman), 1488 cm 1 (IR) aryl ring semi-circle str. The band at 743 cm 1 derives from the 4 adj H wag.

O O O

O

UNKNOWN IR AND RAMAN SPECTRA

207

30. 1-Hexane sulfonic acid sodium salt (sodium 1-hexanesulfonate): Note sample was prepared as a nujol mull which interferes with IR aliphatic bands. Raman aliphatic bands indicates both methyl and methylene groups. No aromatic observed. The water results in bands in the IR spectrum at 3544 and 3484 cm 1 from the OH str., 1625 cm 1 from the OH def., a combination bands at 2233 and 2096 cm 1 and the broad OH bend at ca. 622 cm 1. Sulfonic acid salt group: Strong IR bands from 1213 to1187 cm 1 derive from the SO3 o. ph. str. The strong IR/Raman band at ca. 1058 cm 1 derives from the SO3 i. ph. str. The band at ca. 795 cm 1 (IR/Raman) involves the CeS str. O H3 C

(H2C)5

S

O

Na

O

31. 3-Amino pentane: Aliphatic group: CH3 stretches at 2962, 2876 cm 1 and CH2 stretch at 2922 cm 1, respectively. The peak intensity of methylene (2922 cm 1) to methyl (22962 cm 1) is consistent with ca. 1 methylene to 1 methyl. Aliphatic bands from methyl and methylene deformation vibrations are observed at 1461, 1377 and 1356 cm 1. Primary Amine group: The NH2 stretching vibrations result in a doublet between 3380 and 3290 cm 1 with a shoulder at 3193 cm 1 which derives from an overtone (Fermi-resonance enhanced) of the 1611 cm 1 NH2 deformation band. The band cluster from 921 to 769 cm 1 derives from the NH2 wag and twist vibrations. Bands involving the CeN stretch occur at w1150 and w1045 cm 1.

NH2

32. a-D-Glucose: Water: The band at 1630 cm 1 indicates some water is present in the KBr disc preparation. Aliphatic group: CH2 and CH str vibrations are observed between 2945 and 2892 cm 1. The CH2 deformation has a strong characteristic band at 160 cm 1. Hydroxyl group: OH stretch at 3410, 3315 cm 1. Note different hydrogen bonding environments. OH def at ca. 1345 cm 1. Bands between 1150 and 996 cm 1 involve the CeO str of primary and secondary alcohols. Primary alcohols (eCH2eOH) are mostly the lower frequency 1025e1000 cm 1 band cluster and the secondary alcohols (>CHeOH) are mostly the 1150, 1112 cm 1 bands. The cyclic ether has bands at w 1120e1110 involving the CeOeC o. ph. str. and at w840 cm 1 from the CeOeC i. ph.str. H OH H

O

HO H

HO H H

OH OH

208

9. UNKNOWN IR AND RAMAN SPECTRA

33. Butylated hydroxy toluene: Note Nujol mull aliphatic interference. Aliphatic group: 2968, w2876 cm 1 CH3 str. 1472e1450 cm 1 CH3 o.ph. def., t-butyl 1396, 1363 cm 1 C(CH3)3 i. ph. def., 1375 cm 1 CH3 i. ph. def. Aromatic group: 3100e3000 cm 1 aromatic CH str., 1604e1590 cm 1 (IR/R) quadrant ring str., 1482, 1432 cm 1 (IR) semi-circle str., 866 cm 1 (IR) aryl lone H wag. Phenol hydroxyl group: 3626 (IR/R) OH str., 1246e1198 cm 1 (IR/R) involves aryl-O str. CH3

OH

CH3

H3C

CH3

H3C

CH3

CH3

34. Polystyrene: Aliphatic group: 2923e2854 cm 1 CH2 str. 1452 CH2 def. Aromatic group: 3100e3000 cm 1 aromatic CH, 1603, 1584, 1493 cm 1 aromatic ring quad and semi-circle str., 1002 cm 1 (Raman) 2,4,6 C Radial i. ph.str., 754 cm 1 (IR) 5 adjacent aryl H, 695 cm 1 (IR) aromatic ring pucker.

*

H2 C

CH

*

35. Di-n-butyl amine: Aliphatic group: 2964/2958 cm 1 and 2873 cm 1 CH3 str., 2928 cm 1 (IR) CH2 o. ph. str., Note multiple bands in Raman from trans and gauche methylene groups. 1298 cm 1 (Raman) CH2 twist of chain. 1464/1445 cm 1 involves methyl and methylene deformation. 1378 cm 1 CH3 i. ph. def. Secondary amine group: 3287 cm 1 NH str., 2809 cm 1 CH2 (N) i. ph. str., 1131 cm 1 CeNeC o. ph. str., 735 cm 1 NH wag. (CH2)3CH3

HN (CH2)3CH3

UNKNOWN IR AND RAMAN SPECTRA

209

36. Benzalkonium chloride: Aromatic group: 3100e3000 aromatic CH, w1600, 1488 cm 1 aromatic ring quad and semi-circle str., 1002 cm 1 (Raman) 2,4,6 C Radial i. ph.str., 728 cm 1 (IR) 5 adjacent aryl H, 705 cm 1 (IR) aromatic ring pucker. Quaternary amine salt: (Nþ) CH3 has CH3 o. ph. str. at 3002 cm 1, the CH3 i. ph. str. at 2852 cm 1, the CH3 o. ph. bend/def at 1488 cm 1. The NC4 i. ph. str. is expected between 800 and 950 cm 1 and may possibly be assigned to the band at 837 cm 1. Cl N C7H15

37. n-Ethyl acetamide: Aliphatic group: 2976, 2877 cm 1 CH3 str., 2936 CH2 str., 1452 cm 1 CH3/CH2 def., 1373 CH3 i. ph. def. Amide group: 3290 cm 1 NH str., 1654 cm 1 C ¼O str., 1556 cm 1 CNH str./bend, 3086 cm 1 overtone of 1556 cm 1 band (2  1556), 1298 cm 1 CNH str./open, 721 cm 1 NH wag. H N

O

C

CH2CH3

CH3

38. Triphenyl phosphine oxide: Aromatic group: 3100e3000 aromatic CH str., 1593, 1575, 1485/ 1439 cm 1 aromatic ring quad and semi-circle str., 1121 cm 1 involves P-aryl str., 1003 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 755 cm 1 (IR) 5 adjacent aryl H, 697 cm 1 (IR) aromatic ring pucker. Phosphine oxide (P ¼O) group: 1191/1180 cm 1 (IR/Raman) P ¼O str.

P

O

210

9. UNKNOWN IR AND RAMAN SPECTRA

39. 4-Methyl-pyridine-3-boronic acid: Boronic acid group: 3420e3200 cm 1 (broad) OH str., 1359 cm 1 involves BeO str., 1071 cm 1 BeOH def., 772 (B)OH wag. Pyridine group: 3080e3040 cm 1 aryl CH str., 1615, 1591, 1555 aryl quadrant str., 1480e1425 cm 1 involves aryl semi-circle str., 1091 cm 1 involves 2,4,6 C Radial i.ph.str., 1003 cm 1 involves the whole ring breath (str). 877 cm 1 lone H wag, 835 cm 1 2 adj H wag. CH3

OH B OH

N

40. Tributyl borate: Aliphatic group: 2959 and 2874 cm 1 CH3 str., 2935 CH2 o. ph. str., 1485 cm 1 (O)CH2 def., 1466e1458 cm 1 CH3, CH2 def. Borate ester group: 1337e1297 cm 1 involves the B(O)3 o. ph. str., 832, 815 cm 1 (Raman) most likely involves the B(O)3 i. ph. str., while the 666 cm 1 band (IR) is from the B(O)3 out-of-plane deformation. 1407 cm 1 involves the OCH2 wag þ BeO str. Bands at 1073 and 1029 cm 1 involve mostly the CeO str. C4H9 O

B

C4H9 O

C4H9 O

41. Phenyl boroxole: Phenyl boronic acid is a labile compound that in the solid state readily forms a Boron ether. The Boron ether when put in an acidic medium readily hydrolyzes to form the phenyl boronic acid. Raman spectrum indicates no aliphatic group is present (IR Nujol mull preparation interferes). No bands associated with the BeOH group are observed including the (B)OH str (w3250 cm 1), the BeOH bend (w1000 cm 1), and the BeOH wag (w800 cm 1) (see interpreted IR/Raman spectra # 64 of m-tolylboronic acid). Aromatic group: 3100e3000 cm 1 aromatic CH str., 1603, 1574, 1494/1442 cm 1 aromatic ring quad and semi-circle str., 1180 cm 1 involves B-aryl str., 996 cm 1 (Raman) 2,4,6 C Radial i.ph.str., 760 cm 1 (IR) 5 adjacent aryl H, 703 cm 1 (IR) aromatic ring pucker. BeO linkage: Strong characteristic IR bands involving the BeO str. are observed between 1381 and 1310 cm 1 in both the IR (strong) and Raman (moderate).

211

UNKNOWN IR AND RAMAN SPECTRA

B

OH

3

O

O

B

B B

OH

+

H2O

O

42. Azodicarbonamide: NH2 group: NH2 o. ph. and i. ph. str at 3331 and 3175 cm 1, respectively (IR and Raman). NH2 bend/def at 1635 cm 1 (IR). NH2 rk (Raman and IR) at 1118 cm 1 and the NH2 wag at 750 cm 1 (IR) with an overtone 2  NH2 wag at 1477 cm 1. Amide C ¼O group: C ¼O str. in IR at 1738/1726 cm 1. The IR and Raman bands at 1367, 1337, and 1300 cm 1 involve the CeN str. Azo N ¼N group: Strong Raman band at 1573 cm 1 involves the N ¼N str. O C

NH2

N

H2N

N

C O

43. 4-Methyl-imidazole: NeH group: This is characterized by complex hydrogen bonding involving NeH$$$N ¼ due to p-type hydrogen bonding of the unsaturated N atom in the heterocyclic ring. Thus a broad envelope of NH str vibrations are observed from 3400 to2700 cm 1 in the IR spectrum. The NH wag occurs at 926 cm 1 and the broad band at 1824 cm 1 is due to the Fermi-resonance enhanced overtone of the NH wag. Imadazole ring: Ring ¼CH str. bands at 3120, 3091, and 3029 cm 1. The out-of-phase C ¼C/C ¼N str. (Quadrant str.) results in bands between 1580 and 1509 cm 1 and the in-phase C ¼C/C¼N str. (semi-circle str.) results in bands at w1440 cm 1. The band cluster at 1304, 1265 cm 1 may involve the imidazole ring in-phase breathing mode. Methyl group: Bands at 2921 and 2865 involve the CH3 str. and the band at 1384 cm 1 involves the CH3 i. ph. def. N

N H

212

9. UNKNOWN IR AND RAMAN SPECTRA

44. Melamine: Amino group: 3500e3100 cm 1 involves NH2 str. Note differences in hydrogen bonding (i.e., strongly hydrogen-bonded versus weakly hydrogen-bonded NH). 1650e1620 cm 1 doublet from NH2 deformation. w582 cm 1 possibly NH2 wag. Triazine group: 1550 cm 1 quadrant ring str., 1470e1430 cm 1 semi-circle ring str., 984 cm 1 (Raman) N-Radial i. ph. str., 811 cm 1 ring pucker (sextant out-of-plane bend), 675 cm 1 quadrant in-plane ring bend.

H2N

NH2

N

N

N

NH2

A P P E N D I X

IR Correlation Charts Infrared group frequency correlation charts 1700

3400 3300 3200 3100 3000 2900 2800 2700

CH3, C≡C–H

1600

1500

1400

1300

CH3, CH2, CH, bend

CH2, CH, stretch

R–CH3 Arom-H

–CH(CH3)2

C=CH 2 –C(CH3)3 C=CH R–CH2–R

R–CH3

R–O–CH3

Arom-CH3 R–CH2–R

O=C–CH2–

R–O–CH3 R2–N–CH3

N≡C–CH2– >C=CH2

O=C–H

O=C–H

3600

3400

3200

3000

2800

2600

2400

2200

O=C–CH3 Si–CH3

X-H stretch R–OH …..O O=C–OH ……O S–OH ….. O P–OH ….. O R–NH2 R–NH–R R–NH3+ Cl− R2–NH2+ Cl− R3–NH+ Cl− O=C–NH2 O=C–NH–C O=C–NH–C cyclic S–H P–H Si–H

213

1200

214 2400

IR CORRELATION CHARTS

2300

2200

2100

2000

1900

1900 1800

1800

1700

C≡C C≡N, X=Y=Z str.

1600

1500

1400

C=O str. Cyclic anhydride

CH2–C≡N

R–CO–O–CO–R R–CO–Cl

Conj C≡N S–C≡N

R–CO–O–R

C–C≡C–H

CONH–CO–O–R

–N=C=O

H–CO–O–R R–CO–H

–N=C=S >C=C=CH2

Conj-CO–H R–CO–R Conj-CO–R Conj-CO-Conj R–CO–OH dimer R–CO–N R–CO2−

1700

1600

1500

C=C s tr.

1400

1000

900

800

700

Olef CH wag

C=C

R–CH=CH2 R2C=CH2

C=C

R–CH=CH–R trans C=C

R–CH=CH–R trans RCH=CR2

Ar Ring str. Ring

Ar-H wag Ring mono, 1,3 or 1,3 5 Lone Ar-H 2 adj Ar-H 3 adj Ar-H 4 & 5 adj Ar-H

600

1300

215

IR CORRELATION CHARTS

1400

130

1100

120

1000

900

800

1500 1400

1300

C–O str.

1200

1100

1000

SO, PO, SiO str.

O=C–OH

C–SO2–C

O=C–O–C

C–SO2–N

Ar-OH

C–SO2–O–C

R C–OH

C–SO2–Cl

3

R2CH–OH

C–SO2–OH anhydride C–SO3 − H3O+

–CH2 –OH

C–SO3 − Na+

Epoxy Ar-O–CH2

C2S=O

C=C–O–CH2

P=O

–CH2–O–CH2

P–O–CH2 Si–O

1700

1600

900

1500

1400

1300

1200

1100

1000

900

NH bend, CN str., NO str. C–NH2

C–NH2

CH2–NH–CH2 Ar-NH2 C–NH3 + C2 –NH2 + O=C–NH2 O=C–NH–C C=N C=N–O–C CH2–NO2 Ar-NO2 C–O–NO2 C–O–N=O

800

700

800

Index A A. See Absorption intensity a. See Angle of incidence Absorbed radiation, 33 Absorption intensity (A), in attenuated total reflectance, 41 Acetamide, 136, 149f Acetamideoxime, 137, 159f Acetohydroxamic acid, 136, 152f Acetone, 136, 145f Acetonitrile, 135, 142f Acetylene group, spectra-structure correlations of, 124e125 Acetylenic compounds, spectra-structure correlations of, 131e132 Acid halides, spectra-structure correlations of, 126e127 Acidic protons, spectra-structure correlations of, 126 Acrylamide, 136, 150f Acrylonitrile, 135, 142f AD. See Array detector AETAC. See Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride Alcohols group frequencies of, 104e105t IR and Raman spectra of, 136e137, 153f, 154f, 155f spectra-structure correlations of, 103e105, 104e105t, 104f Aldehydes spectra-structure correlations of, 126 stretching vibration for, 101 Aliphatic groups IR and Raman spectra of, 135, 140f spectra-structure correlations of, 74e78, 75f, 76t, 77f, 78f, 79t, 80f, 125e126 conjugated, 79e99, 80e81t Aliphatic primary amines, IR and Raman spectra of, 73, 74f Aliphatic secondary amines, IR and Raman spectra of, 73, 74f Alkane groups, spectra-structure correlations of, 74, 75f, 76t fluorinated, 130 Alkoxysilanes, spectra-structure correlations of, 131

AMD band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f quantitative analysis of, 48e53 spectra of, 48, 49e50f Amide compounds IR and Raman spectra of, 136, 149f, 150f, 151f, 152f spectra-structure correlations of, 101, 128 Amines IR and Raman spectra of, 137, 156f, 157f, 158f spectra-structure correlations of, 126 deformation vibrations, 128 Amine salts IR and Raman spectra of, 137, 156f, 157f, 158f spectra-structure correlations of, 128 Amino (NH) group correlation chart for, 215 environmental dependence of vibrational spectra and, 58e59 IR and Raman spectra of, 74, 74f spectra-structure correlations of, 124e125, 133 3-Amino pentane, 193f, 207 Ammeline, 136, 151f Ammonium, spectra-structure correlations of, 129 Ammonium chloride, 138, 167f Amplitude in classical harmonic oscillator, 10, 10f dipole moment modulation by, 18, 18f vibrational, dipole moment and, 15 Analyte concentration, IR and Raman signal and, 45e46 Angle of incidence (a) for attenuated total reflectance, 39e40, 40f in dispersive systems, 27, 28f refractive index and, 33, 34f triatomic molecule stretches and, 65, 66f Angle of radiation (b), in dispersive systems, 27, 28f Anharmonic oscillator, potential energy of, 13, 13f Anhydrides IR and Raman spectra of, 136, 145f, 146f, 147f, 148f, 149f spectra-structure correlations of, 100e101, 126e127

217

218

INDEX

Aniline, 137, 157f Anisole, 185f, 202 Anti-Stokes Raman scattering, 16f, 17 classical description of, 17e18, 18f Aromatic rings correlation chart for, 214 IR and Raman spectra of, 136, 143f, 144f spectra-structure correlations of, 128, 132 benzene, 86e91, 87f, 88r, 89f, 90f conjugated, 79e99, 80e81t fused, 92, 93t heterocyclic, 93e99, 93f Array detector (AD), signal-to-noise ratio and, 29 Aryl CH wag, 90e91, 91t, 92f Aryl groups, spectra-structure correlations of, 125 Asymmetric vibrations, IR spectroscopy for, 1 Atmospheric compensation, for sample preparation, 34, 120 Atomic masses in diatomic oscillator, 11 in GF matrix method, 22 ATR. See Attenuated total reflectance Attenuated total reflectance (ATR), 39e41, 40f, 42t with microscopy, 44 of starch, 119e120, 120f Azo compounds IR and Raman spectra of, 137, 160f spectra-structure correlations of, 127 Azodicarbonamide, 198f, 211 Azo-tert-butane, 137, 160f

B b. See Angle of radiation B3LYP function, 24 BaF2, in IR transmission, 32, 33t Band assignments, for PAM-AETAC samples, 48, 50e51t Band intensities, for sample preparation, 33, 120 Band shape, in IR and Raman vibrational bands, 2 Baseline, for sample preparation, 34, 130 3-21G Basis set, 24 6-31G Basis set, 24 Basis set, in vibrational spectra assessment, 24 Beam splitter, in Michelson interferometer, 30e31, 31f Bending vibration. See Deformation vibrations Bent triatomic molecule, as coupled oscillators, 64e65, 65f, 66f Benzalkonium chloride, 195f, 209 Benzamidine HCl, 137, 158f Benzene. See also 1,3,5-Trisubstituted benzene Cartesian coordinate displacements for, 9, 9f center of symmetry of, 19, 19f elementary ring vibrations of, 89, 90f

mono- and di-substituted, 88e89, 89f normal ring modes of, 86, 87f, 88t spectra-structure correlations of, 86e91, 87f, 88t, 89f, 90f stretching vibration of, 70, 70f Benzoic acid, 136, 148f Benzyl alcohol, 182f, 202 BH stretch, spectra-structure correlations of, 126 Bipolymers, IR and Raman spectra of, 139, 171f, 172f, 173f, 174f, 175f, 176f 1,3-Bis(trifluoromethyl)benzene, 190f, 205 BLYP function, 24 BeN type compounds group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112 BeO type compounds group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 129 Boltzmann’s law, in Raman scattering, 17 Bond angle, stretching frequency and, 101e102 Boric acid, 139, 169f Boronic acid compound, IR and Raman spectra of, 138, 161f 3-Bromo toluene, 138, 162f 1-Bromohexane, 138, 161f 1-Butanol, 137, 154f n-Butanol, 181f, 201 n-Butyl acetate, 136, 145f t-Butyl alcohol, 137, 153f N-Butyl methyl acrylamide, 136, 150f Butylated hydroxy toluene, 194f, 208 Butyric anhydride, 136, 146f n-Butyronitrile, 180f, 201

C CaF2, in IR transmission, 32, 33t Calcium palmitic acid salt, IR spectroscopy v. Raman spectroscopy, 118f, 119 Calculating, vibrational spectra, 22e24 Capillary film, 35 Carbon dioxide (CO2) center of symmetry of, 19, 19f Fermi resonance and, 60e62, 61f, 62t fundamental vibrations for, 8, 9f IR gas phase contours of, 55, 56f Carbonates, spectra-structure correlations of, 126e127, 129 Carbonyl groups environmental dependence of vibrational spectra and, 59 group frequencies for, 100t

219

INDEX

factors that effect, 99f, 101e103 IR and Raman spectra of, 99f review of, 99e101 spectra-structure correlations of, 99e103, 99f, 100t, 125, 126e127 Carboxylic acid dimers hydrogen bonding in, 58 IR and Raman spectra of, 73, 74f spectra-structure correlations of, 99e100 Cartesian coordinate displacements for harmonic oscillator, 9, 9f for XY2 molecules, 20e22, 21f Cast films preparation of, 36e37 of starch, 119e120, 120f CeBr group frequencies of, 108, 109t IR and Raman spectra of, 108, 108f, 138, 161f, 162f spectra-structure correlations of, 108e110, 108f, 109t CeCl group frequencies of, 108, 109t IR and Raman spectra of, 108, 108f, 138, 161f, 162f spectra-structure correlations of, 108e110, 108f, 109t wavenumber regions for, 11, 11t Cellulose, 139, 174f Center of symmetry, spectroscopy and, 19, 19f CH bend, correlation chart for, 213 CH stretch, correlation chart for, 4, 4f, 213 CH2. See Methylene CH3. See Methyl Chain length, vibrational frequency and, 66e67, 66f Chemometric data analysis, 118 3-Chloro toluene, 138, 162f 1-Chloro-octane, 138, 161f CI. See Configuration interaction method Classic harmonic oscillator, 10e11, 10f, 11t potential energy of, 12 C]N compounds, IR and Raman spectra of, 137, 158f, 159f CeN stretching vibrations correlation chart for, 215 group frequencies of, 104e105t IR and Raman spectra of, 103, 104f spectra-structure correlations of, 103e105, 104e105t, 104f CeO stretching vibrations correlation chart for, 215 group frequencies of, 104e105t IR and Raman spectra of, 103, 104f spectra-structure correlations of, 103e105, 104e105t, 104f, 131 C]O stretching vibrations, correlation chart for, 214

CO2. See Carbon dioxide Combination bands, anharmonic oscillators and, 13 Communication skills, for spectra interpretation, 117 Computer-based libraries, for spectral interpretation, 122e123 Configuration interaction (CI) method, 24 Conjugation effects, in carbonyl groups, 102e103 Coordinate analysis, with GF matrix method, 22 Correlation charts for CH3, CH2, and CH, 4, 4f development of, 4 group frequency, 213e215 Coupled oscillators bent triatomic molecule, 64e65, 65f, 66f equal combinations, 67e68, 68f, 68t group frequencies of, 63e67, 64f, 65f, 66f, 67f linear aliphatic systems, 65e67, 66f, 67f linear molecules, 63e64, 64f rules of thumb for, 67e70, 68f, 68t, 69f, 70f unequal combinations, 70, 70f Crystallinity, in IR spectra, 57, 58f CeSeC compounds IR and Raman spectra of, 108, 108f spectra-structure correlations of, 108e110, 108f, 109t CeSeSeC compounds IR and Raman spectra of, 108, 108f spectra-structure correlations of, 108e110, 108f, 109t Cyanuric acid, spectra-structure correlations of, 93, 93f Cyclohexane, 135, 140f, 178f, 200 n-Cyclohexane, IR and Raman spectra for, 80f Cyclopropyl rings, spectra-structure correlations of, 125

D Data analysis techniques, for spectra interpretation, 117 Deformation vibrations. See also X-H deformation of amines, spectra-structure correlations of, 128 of CO2, 8, 9f Fermi resonance and, 60e61, 61f of H2O, 8, 9f of linear aliphatic systems, 67, 67f of methyl groups, spectra-structure correlations of, 128e129 of methylene groups, spectra-structure correlations of, 128e129 Degenerate vibrations, 22 Degrees of freedom calculating, 8 molecular motion and, 8e9, 9f

220

INDEX

Demountable cell, 35 Density functional theory (DFT) methods, 24 Dextrin, 187f, 203 DFT. See Density functional theory methods Dialkyl acetylenes, spectra-structure correlations of, 85 Dialkyl sulfates, spectra-structure correlations of, 110e111, 110f, 111e112t Diamond, for IR transmission, 32, 33t Diatomic molecule Cartesian coordinate displacements for, 9, 9f as classic harmonic oscillator, 10e11, 10f classical vibrational frequency for, 10 dipole moments of, 14e15, 14f potential energy of, 12, 12f wavenumber regions for, 11, 11t Dibasic phosphate, spectra-structure correlations of, 130 Dibasic sodium phosphate, 139, 169f n-Dibutyl amine, 137, 157f n-Dibutyl ether, 137, 155f n-Dibutyl phosphite, 138, 166f Diethylene glycol, 137, 155f Diffraction grating, in dispersive systems, 27e28, 28f Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), 41e43 Dimethyl carbonate, 136, 145f Dimethyl formamide, 136, 152f Dimethyl malonate, 136, 146f 2,4-Dimethyl pentane, 178f, 200 Dimethyl siloxane polymer, 183f, 202 Dimethyl sulfoxide, 138, 163f Dimethyl terephthalate, 136, 146f 1,3-Dimethyl urea, 136, 150f 2,5-Dimethyl-1,5-hexadiene, 179f, 200 Di-n-butyl amine, 195f, 208e209 Dioctyl phthalate, 192f, 206 1,4-Dioxane, 184f, 202 2-(4,6eDiphenyle1,3,5etriazinee2eyl)e5 alkyloxy phenol, hydrogen bonding in, 60 Dipole moment amplitude modulation of, 18, 18f in IR absorption, 14e15, 14f in Raman scattering, 15e16, 15f vibrational amplitude and, 15 Dispersive Raman instrumentation, 28e29, 29f Dispersive systems, 27e28, 28f Dissociation limit, anharmonic oscillator and, 13 2,6-Di-t-butyl phenol, 137, 155f Dodecyl sulfate, 138, 164f Double bond correlation chart for, 214 cumulated, spectra-structure correlations of, 80e81t, 85e86, 86f, 126

force constant for, 11, 11t IR and Raman spectra of, 135, 141f wavenumber regions for, 11, 11t Double bond stretch, vibrational spectrum for, 3, 3f dp. See Effective penetration depth DRIFTS. See Diffuse Reflectance Infrared Fourier Transform Spectroscopy

E Eel. See Electronic energy Effective penetration depth (dp), in attenuated total reflectance, 41 Electric fields dipole moments and, 14e15, 14f in electromagnetic radiation, 7, 7f in Raman scattering, 15e18, 15f, 18f Electromagnetic radiation, 7e8, 7f, 8f absorption of, 7e8, 8f parameters of, 7, 7f in Raman scattering, 15, 17e18, 18f Electronic energy (Eel), 8 Empirical characteristic frequencies, for vibrational spectroscopy interpretation, 2e3, 3f Empirical force fields in coordinate analysis, 22 in GF matrix method, 23 for molecular force field, 23 Energy. See also Potential energy anharmonic oscillator and, 13 photon absorption and, 7e8, 8f Environmental dependence, of vibrational spectra, 55e62 Fermi resonance, 60e62, 61f, 62t hydrogen bonding, 57e60, 59f solid, liquid, gaseous states, 55e57, 56f, 57f, 58f Epichlorohydrin, 137, 156f Epoxy rings, spectra-structure correlations of, 125 1,2-Epoxypentane, 186f, 203 Erot. See Rotational energy Esters, IR and Raman spectra of, 136, 145f, 146f, 147f, 148f, 149f Ethers group frequencies of, 104e105t IR and Raman spectra of, 137, 155f, 156f spectra-structure correlations of, 103e105, 104e105t, 104f n-Ethyl acetamide, 196f, 209 Ethyl carbamate, 136, 152f Ethyl cellulose, 139, 175f Ethyl methyl sulfide, 189f, 204 Ethylenes, spectra-structure correlations of, 132 Evib. See Vibrational energy

221

INDEX

F Far-infrared (Far-IR) spectroscopy, frequencies of, 13e14 Far-IR spectroscopy. See Far-infrared spectroscopy Fermi resonance idealized, 60e61, 61f selected groups and, 62t vibrational spectra and, 60e62, 61f Fluorinated alkane groups, spectra-structure correlations of, 130 Fluorine compounds, IR and Raman spectra of, 138, 161f, 162f 3-Fluoro toluene, 138, 162f Fluorolube, FT-IR spectrum of, 38f 3-Fluorotoluene, 190f, 205 Force constant (K) for bond types, 11, 11t in classical harmonic oscillator, 10 in GF matrix method, 23 stretching frequencies and, 102 Fractional linear calibration equations, 47e48 Frequency of electromagnetic radiation, 7, 7f in IR absorption, 14 in IR and Raman vibrational bands, 2 of photons, 7, 7f vibrational, classical, 10e11, 10f FT-IR calibration plot for AMD-AETAC copolymers, 49, 52f of PAM-AETAC samples, 49, 52f FT-IR spectroscopy interferometer for, 30e32, 31f of PAM-AETAC samples, 48, 49e50f ratio method for, 46e47 FT-Raman calibration plot for AMD-AETAC copolymers, 52, 52f of PAM-AETAC samples, 52, 52f FT-Raman spectroscopy interferometer for, 30e32, 31f of PAM-AETAC samples, 48, 49e50f ratio method for, 47 Fundamental transitions, harmonic and anharmonic oscillators and, 13 Fundamental vibrational spectrum, 3, 3f Furans in-plane vibrations of, 96, 97e98t Furans, spectra-structure correlations of, 96e99, 97e98t, 97f, 125

G Gaseous state, environmental dependence of vibrational spectra in, 55e57, 56f, 57f

Gaussian-type orbitals (GTO), in STO-3G basis set, 24 GF matrix method, coordinate analysis with, 22e23 a-D-Glucose, 193f, 207e208 Glycine, 136, 149f Glycolic acid, 136, 147f Groove spacing, in dispersive systems, 27, 28f Group frequencies of alcohols and ethers, 104e105t of boron compounds, 110, 111e112t of carbonyl groups, 100t factors that effect, 99f, 101e103 correlation charts of, 213e215 coupled oscillators, 63e67, 64f, 65f, 66f, 67f of halogen compounds, 108, 109t of inorganic compounds, 112, 113t of nitrogen containing compounds, 105, 106e107t origin of, 63e71 oscillator combination rules of thumb, 67e70, 68f, 68t, 69f, 70f of phosphorus compounds, 110, 111e112t of silicon compounds, 110, 111e112t of sulfur compounds, 110, 111e112t X-H stretching group, 73e74, 74f Group Theory symmetry and, 18 for vibrational spectroscopy interpretation, 2e3, 3f GTO. See Gaussian-type orbitals

H H2. See Hydrogen H2O. See Water Halogen-carbon stretch, spectra-structure correlations of, 132e133 Hammet constant, methylene group correlation with, 84e85, 85f Harmonic oscillator Cartesian coordinate displacements for, 9, 9f classic, 10e11, 10f, 11t potential energy of, 12, 12f, 13f quantum mechanical, 12e13, 12f, 13f Hartree-Fock (HF) method, 24 Heisenberg’s uncertainty principle, harmonic oscillator and, 12f, 13 Heptane, 135, 140f n-Heptane IR and Raman spectra for, 80f IR spectroscopy v. Raman spectroscopy, 118f, 119 cis-3-Heptene, 135, 141f trans-3-Heptene, 135, 141f Hetero-aromatic rings, spectra-structure correlations of, 128

222

INDEX

Heterocyclic aromatic five-membered rings, spectrastructure correlations of, 96e99, 97e98t, 97f Heterocyclic aromatic six-membered rings, spectrastructure correlations of, 93e99, 93f pyridines, 94, 95t, 96t triazines and melamines, 94e96 Heteronuclear diatomic molecule, dipole moments of, 14 1-Hexadecanol IR and Raman spectra of, 137, 154f IR spectroscopy v. Raman spectroscopy, 118f, 119 n-Hexane, IR and Raman spectra for, 80f 1-Hexane sulfonic acid sodium salt, 192f, 207 2-Hexanone, 188f, 204 1-Hexene, 135, 141f 2-Hexyne, 181f, 201 HF. See Hartree-Fock method Homonuclear diatomic molecule, dipole moments of, 14 Hydrogen (H2), center of symmetry of, 19, 19f Hydrogen bonding in acidic proton vibrational frequencies, 126 in amino and hydroxy stretching vibrations, 124 environmental dependence of vibrational spectra and, 57e60, 59f intramolecular, 59e60 4-Hydroxy benzaldehyde, 136, 147f Hydroxy (OH) group environmental dependence of vibrational spectra and, 58, 60f IR and Raman spectra of, 73, 74f spectra-structure correlations of, 124e125, 133 2-Hydroxy-4-(methoxy)-benzophenone, hydrogen bonding in, 60 Hydroxypropylmethyl cellulose, 139, 175f 2-Hydroxypyridine, spectra-structure correlations of, 93, 93f

I Identification, spectral databases and interpretation for, 118 2-Imidazolidinethione, 138, 165f 2-Imidazolidone, 136, 151f Imino compounds, spectra-structure correlations of, 127 Impurities, identifying, 123 Infrared (IR) absorption process, 13e15, 14f dipole moment in, 14e15, 14f frequencies in, 13e14 Infrared (IR) gas phase contours, of CO2, 55, 56f Infrared (IR) spectroscopy. See also FT-IR spectroscopy; Mid-infrared spectroscopy; Near-infrared spectroscopy

analyte concentration and, 45e46 application of, 1 center of symmetry and, 19, 19f dispersive systems for, 27e28, 28f historical perspective on, 3e4 instrument development for, 4 interpretation of, 2, 117e133 Raman spectroscopy v., 1e2, 2t, 118f, 119 symmetry and, 18 tools for, 117e119, 118f transmission, 34e39 Infrared (IR) transmitting materials, 32, 33t Inorganic compounds IR and Raman spectra of, 138e139, 167f, 168f, 169f, 170f, 171f spectra-structure correlations of, 112, 113t In-phase stretch of bent triatomic molecule, 64e65, 65f, 66f of CO2, 8, 9f Fermi resonance and, 61, 61f of H2O, 8, 9f of linear molecules, 63e64, 64f In-plane vibrations of olefinic groups, 82, 83f of pyridines, 94, 95t of pyrroles, furans, and thiophenes, 96, 97e98t Instrumentation, 27e32 dispersive Raman instrumentation, 28e29, 29f dispersive systems, 27e28, 28f interferometric spectrometers, 30e32, 31f Raman spectroscopy sample arrangements, 29, 30f for spectra interpretation, 117 Intensity in IR and Raman vibrational bands, 2 of Raman scattering, 17, 46 wavenumber v., 14 Interaction effects, in carbonyl groups, 103 Interferometric spectrometers, 30e32, 31f Internal reflectance, in attenuated total reflectance, 40, 40f Internal reflection element (IRE), in attenuated total reflectance, 39 Interpersonal skills, for spectra interpretation, 117 Intramolecular hydrogen bonding, 59e60 IR absorption process. See Infrared absorption process IR gas phase contours. See Infrared gas phase contours IR spectroscopy. See Infrared spectroscopy IR transmitting materials. See Infrared transmitting materials IRE. See Internal reflection element Isoamyl nitrate, 137, 159f Isobutyl amine, 137, 156f Isocyanuric acid, hydrogen bonding in, 59

223

INDEX

Isooctane, 135, 140f Isopropyl alcohol, 136, 153f Isopropyl nitrate, 137, 160f Isoquinolines, spectra-structure correlations of, 128 Isotactic polyethylene, 139, 171f

K K. See Force constant KBr disc preparation of, 38, 39f KBr windows, in IR transmission, 32, 33t Ketones, stretching vibration for, 101 KRS-5, in IR transmission, 32, 33t Kubelka-Munk equation, in DRIFTS, 41e43

L Lactones, spectra-structure correlations of, 126e127 Lambert-Beer law in absorption spectroscopy, 14 in attenuated total reflectance, 41 in quantitative analysis, 45e46 Linear molecules in-phase stretch of, 63e64, 64f molecular motions for, 8, 9f out-of-phase stretch of, 63e64, 64f Liquids environmental dependence of vibrational spectra in, 55e57 preparation for, 35e36

M Magnetic fields, in electromagnetic radiation, 7 Maleic anhydride, 136, 147f MCSF. See Multi-configuration self-consistent field method Melamines, 199f, 212 spectra-structure correlations of, 94e96 Melts, preparation of, 38e39 Mesomeric effects, in carbonyl groups, 102e103 Meta (3) chlorophenol, 187f, 203 Metal oxides, spectra-structure correlations of, 133 Meta-tolyl boronic acid, 138, 161f Methane sulfonic acid, 138, 163f Methanol, 136, 153f Methoxy group, spectra-structure correlations of, 126 Methyl (CH3) bend, correlation chart for, 213 Methyl (CH3) groups deformation vibrations of, spectra-structure correlations of, 128e129 spectra-structure correlations of, 75, 76t, 78f, 125e126 1-Methyl naphthalene, 180f, 200 Methyl (CH3) stretch, correlation chart for, 4, 4f, 213

Methylene (CH2) bend, correlation chart for, 213 Methylene (CH2) groups deformation vibrations of, spectra-structure correlations of, 128e129 Hammet constant correlation with, 84e85, 85f spectra-structure correlations of, 74e75, 76t, 77f, 125e126 Methylene (CH2) stretch, correlation chart for, 4, 4f, 213 4-Methyl-imidazole, 199f, 211e212 4-Methyl-pyridine-3-boronic acid, 197f, 210 Michelson interferometer, 30e31, 31f Microscopy, 43e44 reflection IR, 44 transmission IR, 43e44 Mid-infrared (Mid-IR) spectroscopy application of, 1 frequencies of, 13e14 Raman spectroscopy v., 1e2, 2t Mid-IR. See Mid-infrared spectroscopy Mirror, in Michelson interferometer, 30e31, 31f Molecular force field, 23 Molecular geometry, in GF matrix method, 22 Molecular motion degrees of freedom and, 8e9, 9f of linear molecules, 8, 9f of non-linear molecules, 8, 9f Molecular vibrational kinetic energy, in GF matrix method, 22 Møller Plessant perturbation (MP2) method, 24 Monobasic phosphate, spectra-structure correlations of, 130 MP2. See Møller Plessant perturbation method Multi-configuration self-consistent field (MCSF) method, 24

N NaCl windows, in IR transmission, 32, 33t Napthalenes, spectra-structure correlations of, 92, 93t Near-infrared (Near-IR) spectroscopy application of, 1 frequencies of, 13e14 Raman spectroscopy v., 1e2, 2t Near-IR. See Near-infrared spectroscopy Newton’s second law, with GF matrix method, 23 NH group. See Amino group NH2 group IR and Raman spectra of, 73, 74f spectra-structure correlations of, 133 Nitrates in-phase stretching of, 19, 20f organic, spectra-structure correlations of, 127 spectra-structure correlations of, 129 Nitrites, organic, spectra-structure correlations of, 127

224

INDEX

Nitrogen containing compounds group frequencies of, 105, 106e107t IR and Raman spectra of, 105, 106f spectra-structure correlations of, 105e108, 106e107t, 106f 2-Nitropropane, 137, 160f Nitroso groups, spectra-structure correlations of, 127 N]O compounds group frequencies of, 105, 106e107t IR and Raman spectra of, 105, 106f, 137, 159f, 160f spectra-structure correlations of, 105e108, 106e107t, 106f N-O stretching vibrations, correlation chart for, 215 Non-linear molecules, molecular motions for, 8, 9f Non-polar groups, Raman spectroscopy for, 1 Nujol Mull FT-IR spectrum with, 34, 35f, 38f preparation of, 37, 38f of starch, 119e120, 120f Nylon, 6, 6, 139, 173f

O n-Octane, IR and Raman spectra for, 80f 1-Octyne, 135, 142f OH group. See Hydroxy group Olefinic compounds correlation chart for, 214 spectra-structure correlations of, 127, 131e132 Olefinic groups in-plane vibrations of, 82, 83f out-of-plane vibrations of, 84, 84f Olefinic groups, spectra-structure correlations of, 82e85, 82f, 83f, 84f, 85f, 125 Oleic acid, 136, 148f Out-of-phase stretch of bent triatomic molecule, 64e65, 65f, 66f of CO2, 8, 9f of H2O, 8, 9f of linear aliphatic systems, 66e67, 66f of linear groups, 68e69, 69f of linear molecules, 63e64, 64f Out-of-plane vibrations of olefinic groups, 84, 84f of pyridines, 94, 96t Overtones, anharmonic oscillators and, 13

P Palmitic acid, IR spectroscopy v. Raman spectroscopy, 118f, 119 Palmitic acid, 136, 148f PAM. See Poly acrylamide PAM-AETAC band assignments for, 48, 50e51t

diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f quantitative analysis of, 48e53 spectra of, 48, 49e50f Paper, 185f, 203 PE. See Potential energy n-Pentane, IR and Raman spectra for, 80f PH stretch, spectra-structure correlations of, 126 Phenol, 137, 154f Phenyl acetate, 191f, 205e206 Phenyl boronic acid, 198f, 210e211 Phosphate, spectra-structure correlations of, 130 Phosphine oxide. See also P]O Phosphine oxide, spectra-structure correlations of, 110f, 111e112t Phosphoric acids, spectra-structure correlations of, 126 Phosphorus compounds, IR and Raman spectra of, 138, 165f, 166f Phosphorus ester, spectra-structure correlations of, 110f, 111e112t Photon energy, parameters of, 7 Photon frequency, parameters of, 7, 7f Photons, 7 absorption of, 7e8, 8f parameters of, 7, 7f in Raman scattering, 16e17, 16f Pipenidindione dioxime, 137, 159f P]O compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 130 PeO containing compounds, spectra-structure correlations of, 131 Polar groups, IR spectroscopy for, 1 Polarizability, in Raman scattering, 15e16, 16f, 18, 18f Poly acrylamide (PAM) band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f IR and Raman spectra of, 139, 176f quantitative analysis of, 48e53 spectra of, 48, 49e50f Poly acrylic acid, 139, 173f Poly-2-(acryloyloxy)-Ethyltrimethyl Ammonium Chloride (AETAC) band assignments for, 48, 50e51t diffuse reflectance of, 53, 53f FT-IR calibration plot of, 49, 52f FT-Raman calibration plot of, 52, 52f

225

INDEX

IR and Raman spectra of, 139, 176f quantitative analysis of, 48e53 spectra of, 48, 49e50f Polybutadiene, 139, 171f Polyethylene oxide, 139, 172f Polymers, IR and Raman spectra of, 139, 171f, 172f, 173f, 174f, 175f, 176f Polystyrene, 194f, 208 Polytetrafluoroethylene, 139, 172f Polyvinyl alcohol, 139, 172f Polyvinyl pyrrolidone, 139, 173f Potassium nitrate, 139, 168f Potential energy (PE) of anharmonic oscillator, 13, 13f of harmonic oscillator, 12, 12f, 13f Primary amine salts, spectra-structure correlations of, 126 2-Propanethiol, 189f, 204e205 Pyridines in-plane vibrations of, 94, 95t IR and Raman spectra of, 136, 144f out-of-plane vibrations of, 94, 96t spectra-structure correlations of, 94, 95t, 96t, 128 Pyrimidines, spectra-structure correlations of, 128 Pyrroles in-plane vibrations of, 96, 97e98t Pyrroles, spectra-structure correlations of, 96e99, 97e98t, 97f, 125

Q Quantitative analysis, 44e53 analyte concentration and, 45e46 examples of, 48e53 of PAM-AETAC samples, 48e53 ratio method, 46e48 Quantum mechanical harmonic oscillator, 12e13, 13f potential energy of, 12, 12f Quantum mechanical method, for molecular force field, 23 Quantum mechanics, vibrational energy in, 12 Quinazolines, spectra-structure correlations of, 128 Quinolines, spectra-structure correlations of, 128

R Raman scattering center of symmetry and, 19, 19f classical description of, 17e18, 18f intensity of, 17, 46 process of, 15e17, 15f, 16f Raman spectroscopy. See also FT-Raman spectroscopy analyte concentration and, 45e46 application of, 1 dispersive Raman instrumentation for, 28e29, 29f

historical perspective on, 3e4 interpretation of, 2, 117e133 IR spectroscopy v., 1e2, 2t, 118f, 119 sample arrangements for, 29, 30f symmetry and, 18 tools for, 117e119, 118f Ratio method, 46e48 fractional linear calibration equations for, 47e48 Rayleigh light filters, 28e29, 29f Rayleigh scattering classical description of, 17e18, 18f dispersive Raman instrumentation and, 28e29, 29f oscillating dipoles in, 15 process of, 16, 16f Reflected radiation, 33 Reflection IR, 39e43 ATR, 39e41, 40f, 42t DRIFTS, 41e43 microscopy with, 44 Refractive index angle of incidence and, 33, 34f for attenuated total reflectance, 40e41, 40f Ring vibrations, of benzene, 89, 90f Rotational energy (Erot), 8 3n-5 Rule-of-thumb, 8, 9f 3n-6 Rule-of-thumb, 8, 9f

S Sample preparation, 33e34, 34f, 35f for attenuated total reflectance, 39 of cast films, 36e37 issues with, 119e121, 120f for liquids, 35e36 of melts, 38e39 of solid-powdered samples, 37e38, 38f, 39f Sampling methods, 32e44 IR transmitting materials, 32, 33t microscopy, 43e44 reflection IR, 39e43 ATR, 39e41, 40f, 42t DRIFTS, 41e43 techniques for, 33e34, 34f, 35f, 119e121, 120f transmission IR, 34e39 cast films, 36e37 liquids and solutions, 35e36 melts, 38e39 solid-powdered samples, 37e38, 38f, 39f Scattered radiation, 33 Sealed amalgam cell, 36 SH stretch, spectra-structure correlations of, 126 Signal-to-noise ratio (SNR), array detector and, 29 SiH stretch, spectra-structure correlations of, 126 Silica gel, 139, 170f

226

INDEX

Silicon dioxide, 183f, 202 Siloxane compounds, IR and Raman spectra of, 138, 166f Siloxane polymer, 138e139, 166f, 175f Single bond force constant for, 11, 11t wavenumber regions for, 11, 11t Single bond stretch, vibrational spectrum for, 3, 3f SieO containing compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112, 131 Six-membered rings, stretching waves for, 70, 70f Slater-type orbital (STO), in STO-3G basis set, 24 SNR. See Signal-to-noise ratio S]O compounds correlation chart for, 215 group frequencies of, 110, 111e112t IR and Raman spectra of, 110, 110f spectra-structure correlations of, 110e112 Sodium acetate, 188f, 204 Sodium bicarbonate, 139, 168f Sodium carbonate, 139, 168f Sodium carboxymethyl cellulose, 139, 174f Sodium di-isobutyl dithiophosphate, 138, 166f Sodium isopropyl xanthate, 138, 164f Sodium molybdate, 139, 170f Sodium perchlorate, 139, 170f Sodium sulfate, 138, 167f Sodium sulfite, 138, 167f Software tools, for spectral interpretation, 122e123 Solid state, environmental dependence of vibrational spectra in, 55e57, 58f Solid-powdered samples, preparation of, 37e38, 38f, 39f Solutions, preparation for, 35e36 Solvents, identifying, 123 Sox compounds, spectra-structure correlations of, 129e130 Spectral databases, for identification and structural verification, 118 Spectral database tools computer-based, 122e123 for spectra interpretation, 117 Spectral interpretation, 121e124 computer-based libraries and software tools for, 122e123 examine the spectrum, 123e124 problem definition, 123 spectral reference library hierarchy of quality, 121e122 Spectral reference libraries computer-based, 122e123

hierarchy of quality of, 121e122 Specular reflectance, with microscopy, 44 Spurious bands, sample preparation and, 121 Starch IR and Raman spectra of, 34, 35f, 139, 174f sample preparation of, 119, 120f Stearic acid, 136, 149f STO. See Slater-type orbital STO-3G basis set, 24 Stokes Raman scattering, 16f, 17 classical description of, 17e18, 18f Stretch. See Double bond stretch; In-phase stretch; Out-of-phase stretch; Single bond stretch; Triple bond stretch Stretching frequencies bond angle and, 101e102 force constant and, 102 Stretching vibration for aldehydes, 101 of carbonyl groups, 101e103 CeO, spectra-structure correlations of, 131 Fermi resonance and, 60e61, 61f hydrogen bonding in, 124e125 for ketone, 101 of six-membered rings, 70, 70f Stretch-stretch interaction, group frequencies of, 63e64, 64f Structural verification, spectral databases and interpretation for, 118 Structure correlations, with spectra, 124e133 800-200 cm 1, 133 850-480 cm 1, 132e133 900-500 cm 1, 133 900-700 cm 1, 132 1000-600 cm 1, 131e132 1280-830 cm 1, 131 1300-750 cm 1, 131 1350-1000 cm 1, 130 1350-1080 cm 1, 130 1400-900 cm 1, 129e130 1480-1310 cm 1, 129 1500-1250 cm 1, 128e129 1620-1420 cm 1, 128 1660-1450 cm 1, 127 1660-1500 cm 1, 128 1690-1400 cm 1, 127 1900-1550 cm 1, 126e127 2300-1900 cm 1, 126 2600-2100 cm 1, 126 3000-2700 cm 1, 125e126 3100-2400 cm 1, 126 3200-2980 cm 1, 125 3700-3100 cm 1, 124e125

227

INDEX

Sulfanamides, spectra-structure correlations of, 129e130 Sulfates in-phase stretching of, 19, 20f spectra-structure correlations of, 129e130 Sulfinic acids, spectra-structure correlations of, 129e130 Sulfonamides, spectra-structure correlations of, 110f, 111e112t Sulfonates, spectra-structure correlations of, 110f, 111e112t Sulfones, spectra-structure correlations of, 110e111, 110f, 111e112t, 129e130 Sulfonic acids, spectra-structure correlations of, 110f, 111e112t, 129e130 Sulfonic acid salts, spectra-structure correlations of, 110f, 111e112t, 129e130 Sulfonyl chlorides, spectra-structure correlations of, 110f, 111e112t Sulfonyl halides, spectra-structure correlations of, 129e130 Sulfoxide. See also S]O Sulfoxides, spectra-structure correlations of, 110e111, 110f, 111e112t, 129e130 Sulfur compounds, IR and Raman spectra of, 138, 163f, 164f, 165f Symmetric vibrations, Raman spectroscopy for, 1 Symmetry center of, 18e19, 19f Group Theory and, 18 of in-phase stretch of linear molecules, 64, 64f IR and Raman active vibrations, 18e22, 19f, 20f, 21f of XY2 bent molecule, 20, 20f

T Talc, 139, 171f Taylor series, for GF matrix method, 23 4-Tert-butyltoluene, 179f, 200 Tetrahydrofuran, 137, 156f, 184f, 202 Tetramethyl ammonium chloride, 137, 158f m-Tetramethyl xylene diisocyanate, 136, 144f Thiocarbonyl compounds, spectra-structure correlations of, 129e130 Thiophenes in-plane vibrations of, 96, 97e98t Thiophenes, spectra-structure correlations of, 96e99, 97e98t, 97f Toluene, 136, 143f para-Toluene sulfonic acid hydrate, 138, 163f Trans, trans-2,4-hexadien-1-ol, 186f, 203 Trans-2-hexen-1-ol, 182f, 201 Transmission IR, 34e39 of cast films, 36e37 of liquids and solutions, 35e36

of melts, 38e39 microscopy with, 43e44 of solid-powdered samples, 37e38, 38f, 39f Transmitted radiation, 33 Triazines, spectra-structure correlations of, 94e96 Triazine species, spectra-structure correlations of, 128 Tribasic sodium phosphate, 139, 169f n-Tributyl amine, 137, 157f Tributyl borate, 197f, 210 Tributyl phosphite, 138, 165f 2,4,6-Trichlorobenzoyl chloride, 191f, 206 Trimethyl amine, 137, 158f Trimethyl isocyanurate, 136, 151f Triphenyl phosphine, 138, 165f Triphenyl phosphine oxide, 196f, 209e210 Triple bond correlation chart for, 214 force constant for, 11, 11t IR and Raman spectra of, 135, 142f spectra-structure correlations of, 80e81t, 85e86, 86f, 126 wavenumber regions for, 11, 11t Triple bond stretch, vibrational spectrum for, 3, 3f 1,3,5-Trisubstituted benzene, in-phase stretching of, 20, 20f

U Ureas, IR and Raman spectra of, 136, 149f, 150f, 151f, 152f

V Vibrating mechanical molecular models, 3 Vibrational amplitude, dipole moment and, 15 Vibrational energy (Evib), 8 in quantum mechanics, 12 Vibrational frequency for bent triatomic molecule, 64e65, 65f, 66f classical, 10e11, 10f for linear aliphatic systems, 66e67, 66f, 67f for linear molecules, 63e64, 64f Vibrational quantum number, anharmonic oscillator and, 13 Vibrational spectroscopy advantages of, 45 application of, 1 calculating, 22e24 environmental dependence of, 55e62 Fermi resonance, 60e62, 61f, 62t hydrogen bonding, 57e60, 59f solid, liquid, gaseous states, 55e57, 56f, 57f, 58f interpretation of, 2 techniques for, 1 Vibrational spectrum, 3, 3f Vinyl groups, spectra-structure correlations of, 132

228

INDEX

W Water (H2O), fundamental vibrations for, 8, 9f Wavelength, of electromagnetic radiation, 7, 7f Wavenumbers in classical harmonic oscillator, 10e11 of electromagnetic radiation, 7, 7f intensity v., 14 Wavenumber regions, for diatomic oscillator groups, 11, 11t

X X-axis scale, for sample preparation, 34, 121 X-H deformation, vibrational spectrum for, 3, 3f X-H stretch correlation chart for, 213 vibrational spectrum for, 3, 3f

X-H stretching group, IR and Raman spectra of, 73e74, 74f wavenumber regions for, 11, 11t XY2 bent molecule Cartesian coordinate displacements, 20e22, 21f symmetry of, 20, 20f XY2 type identical oscillators, 68, 68f, 68t Xylenes, 136, 143f

Z Zero point energy, 12 Zinc dimethyl dithiocarbamate, 138, 164f ZnSe FT-IR spectrum with, 34, 35f in IR transmission, 32, 33t