Infrared and raman spectroscopy : Principles and spectral interpretation

9 7 8 01 2 8 0 4 1 67 8

SECOND EDITION

I nf rared

and Raman

Spectroscopy

Principles and Spectral Interpretation

5nr*

Wavenumber (orr1)

PETER J. LARKIN

INFRARED AND RAMAN

SPECTROSCOPY

INFRARED AND

RAMAN

SPECTROSCOPY

PRINCIPLES AND SPECTRAL

INTERPRETATION

SECOND EDITION

P e t e r J . L a r k in

Spectroscopy and Materials Characterization, Solvay,

Stamford, C T , United States

ELSEVIER

Elsevier

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T y p e s e t b y TN Q B ook s an d Jo u rn a ls

To my wife Donna and my children Elizabeth and Matthew

My thanks and appreciation for mentoring provided by Norman B. Colthup

Contents

Preface ix

1. Introduction: Infrared and Raman

Spectroscopy 1

1. Historical Perspective: Infrared and Raman

Spectroscopy 3

References 5

2. Basic Principles 7

1. Electromagnetic Radiation 7

2. Molecular Motion/Degrees of Freedom 8

3. Classical Harmonic Oscillator 10

4- Quantum Mechanical Harmonic Oscillator 12

5. Infrared Absorption Process 14

6. The Raman Scattering Process 15

7. Classical Description of the Raman Effect 18

8. Symmetry: Infrared and Raman Active

Vibrations 19

9. Selecting the Raman Excitation Wavelength 22

10. Calculating the Vibrational Spectra of

Molecules 25

References 28

3. Instrumentation and Sampling

Methods 29

1. Instrumentation 29

2. Sampling Methods for Infrared Spectroscopy 34

3. Quantitative Analysis 51

References 61

4- Environmental Dependence of

Vibrational Spectra 63 12

1. Solid, Liquid, and Gaseous States 63

2. Hydrogen Bonding 66

3. Crystalline Lattice Vibrations 68

4. Fermi Resonance 70

References 73

5. Origin of Group Frequencies 75

1. Coupled Oscillators 75

2. Rules of Thumb for Various Oscillator

Combinations 79

References 84

6. IR and Raman Spectra—Structure

Correlations: Characteristic Group

Frequencies 85

1. X—H Stretching Group ( X = 0 , S, P, N, Si, B) 85

2. Aliphatic Groups 87

3. Conjugated Aliphatics and Aromatics 92

4. Carbonyl Groups 113

5. C —O and C —N Stretches of Alcohols, Ethers,

and Amines 118

6. N = 0 and Other Nitrogen-Containing

Compounds 122

7. C-Halogen and C —S Containing

Compounds 123

8. C = S , S = 0 , P = 0 , B -O /B -N , and S i- O

Compounds 125

9. Inorganics 130

References 13 3

7. General Outline for IR and Raman

Spectral Interpretation 135

L Tools of the Trade 135

2. Infrared Sample Preparation Issues 136

3. Overview of Spectral Interpretation 139

4. Interpretation Guidelines and Major

Spectra—Stmcturc Correlations 142

CONTENTS

8. Illustrated IR and Raman Spectra

Demonstrating Important Functional

Groups 153

1. Aliphatic 153

2. Cyclic Ether and Amine 153

3. C = C Double Bonds 154

4. Triple Bonds 154

5. Aromatic Rings 154

6. SiX'Membered Ring Heterocycle 154

7. Five^Membered Ring Heterocycle 154

8. Ketones, Esters, and Anhydrides 155

9. Amides, Ureas, and Related Compounds 155

10. Thiocarhonyls 155

11. C = N Compounds 156

12. Alcohols 156

13. Ethers 156

viii

14. Amines and Amine Salts 156

15. N = 0 Compounds 157

16. Azo Compound 157

17. Boronic Acid Compound 157

18. Chlorine, Bromine, and Fluorine

Compounds 157

19. Sulfur Compounds 157

20. Phosphorus Compounds 157

2 1. Inorganic Compounds 158

22. Polymers and Biopolymers 158

9. Unknown IR and Raman Spectra 211

Appendix: IR Correlation Charts 261

Index 265

Preface

IR and Raman spectroscopy have

tremendous potential to solve a wide variety

of complex problems. Both techniques are

completely complementary providing char￾acteristic 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. The successful application of both

techniques, however, 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. The second edition of

this book is a response to the continued

rapid growth in the field of vibrational

spectroscopy. This has resulted in a corre￾sponding need to educate new users on the

value of both IR and Raman spectral inter￾pretation skills.

To begin with, the end user must have a

suitable knowledge base of the instrument

and its capabilities. Furthermore, they

must develop an understanding of the

sampling options and limitations, available

software tools, and a fundamental under￾standing of important characteristic group

frequencies for bolh IR and Raman spec￾troscopy. A critical skill set that an analyst

may require to solve a wide variety of

chemical questions and problems using

vibrational spectroscopy is depicted in

Fig. 1 below.

FIGURE 1 Skills required c , . ,

Application Notebook; F e b t ^ o W vlb™t,onal sp ectro sco p e. Adapted from McDowall, R. D. Spectroscopy

PREFACE

Selecting the optimal spectroscopic We have attempted to provide an rnte￾tectoou e to solve complex chemical prob- grated approach to the important group

1 e n c o u n t e r e d by the analyst requires the frequency of both IR and Raman spectros￾user to develop a skill set outlined in Fig. 1. A copy. An extensive use of graphics is used to

krmwledge of spectral interpretation enables describe the basic principals of vibrational

the user to select the technique with the most spectroscopy and the origins of group fre￾favorable selection of characteristic group quencies. The book includes sections on

frequencies, optimize the sample options, basic principles in Chapters 1 and 2, mstru￾rincluding accessories if necessary), and use mentation, sampling methods, and quanh￾suitable software tools (both instrumental tative analysis in Chapter 3, a discussion of

d chemometric) to provide a robust, important environmental effects m Chapter 4

sensitive analysis that is easily validated and a discussion of the origin of group

In this book we provide a suitable level of frequencies in Chapter 5. Chapters 4 and 5

information to understand instrument capa- provide the essential background to under￾hilities sample presentation, and selection of stand the origin of group frequencies to

arious accessories. The main thrust of this assign them in a spectra and to explain why

text is to develop a high level of spectral group frequencies may shift. Selected prob￾terpretation skills. A broad understanding lems are included at the end of some of these

2 the bands associated with functional groups chapters to help highlight important points,

for both IR and Raman spectroscopy is the Chapters 6 and 7 provide a highly detailed

basic spectroscopy necessary to make the most description of important characteristic group

of the potential and set realistic expectations frequencies and strategies for interpretation

for vibrational spectroscopy applications in of IR and Raman spectra. £ u u ,

both academic and industrial settings. Chapter 8 is the culmination of the book

° A rimary goal of this book has been to and provides 156 interpreted paired IR and

fnllv integrate the use of both IR and Raman Raman spectra arranged in groups. The

1 n dv as spectral interpretation tools, selected compounds are not intended to

spectroscopy H inlecrated the discussion provide a comprehensive spectral library but

f IRS and Raman group frequencies into rather to provide a significant selection of

A fferent classes of organic groups. This is interpreted examples of functional group

° , w i with naired generalized IR and frequencies. This resource of interpreted IR

supplem entedw ithp^ nu^ erQUS tables that and Raman spectra should be used help

^ T c u ^ s e d in the text, and finally referenced verify proposed assignments that the user

art> , u™ f„iiv interpreted IRand Raman will encounter. The final chapter is comprised

103 n-^This fullv^integrated approach to IR of the paired IR and Raman spectra of 54

SPHRnman interpretation enables the user to different unknown spectra with a corre￾uttiize the strengths of both techniques while spending answer key.

also recognizing their weaknesses.

CHAPTER

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 spec￾troscopy 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 instrumenta￾tion, 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 vibra￾tions 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 nonpolar groups, while IR spectroscopy is best at the asymmetric

vibrations of polar groups. Table 1.1 below briefly summarizes some of the differences

between the techniques.

IR and Raman spectroscopy involves the study of the interaction of radiation with molec￾ular vibrations but differs in the manner in which photon energy is transferred to the mole￾cule 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 dipole￾mediated 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

Infrared arul Raman S/vctri isco/iy, Sea >rul F.dilian

http://dx.doi.oni/10.1016/B978-0-12-804162-8.00001 -X 1 © 2018 F.lsevicr Inc. All rights reserved.

T A B L E 1.1 Comparison of Raman, Midrinfrared (1R), and NeardR Spectroscopy

Ease of sample preparation

Liquids

Powders

Polymers

Gases

Fingerprinting

Best vibrations

Group frequencies

Aqueous solutions

Quantitative analysis

Low frequency modes

R am an Infrared N ear-IR

Very simple Variable Simple

Very simple Very simple Very simple

Very simple Simple Simple

Very simple11 Simple Simple

Simple Very simple Simple

Excellent Excellent Very good

Symm etric Asym metric C om b/overton e

Excellent Excellent Fair

Very good Very difficult Fair

Good Good Excellent

Excellent Fair N o

'True for FT-Raman at 1064 nm excitation to minimize possible fluorescence interference.

troscopy, the interaction

Raman Pola" z^bl^ ° f^ raJ^nalCbands are characterized by their frequency (energy), inten￾The IR and Ram poiarizability), and band shape (environment of bonds). Since the

sity (polar character or p ^ ^ each moiecuie, the IR and Raman spectrum provide

vibrational energy ^ molecule. The frequencies of these molecular vibrations depend

a " fingerprint' o fa their geometric arrangement, and the strength of their chemical

on the masses of e a • deinformation on molecular structure, dynamics, and environment,

bonds. ^ ^ SPeCtt^app^oaches are used for interpretation of vibrational spectroscopy and eluci￾dahonof1 molecular structure:

theory with mathematical calculations of the forms and frequencies of the

" ' ^ C T ^ p i S d ^ ’racteristic frequencies for chemical functional groups.

2' Se . _ fr^mipncies have been explained and refined using the mathemat- Many empirical group reliabmty).

ical theoretical appr° , tification problems are solved using the empirical approach. Certain

In general, many 1 characteristic vibrations in which only the atoms in that particular

functional groups s ow ^ ege vibrations are mechanically independent from the rest of

group are ‘¡*lsP ao~ _ vibrations will have a characteristic frequency, which remain rela￾the molecule, tnesc 8 R f hat moiecule the group is in. Typically, group frequency

tively unchanged regardless

1. HISTORICAL PERSPECTIVE: INFRARED AND RAMAN SPECTROSCOPY 3

OH str

alcohols

phenols

NH str

amines

amides

1 =CH and aromatic CH str 1

aliphatic CH str |

P-OH or SH str |

X=Y=Z

or X=Y

-N=C=0,

-C=N

c = o

acid

ester

ketone

amide

C=C olefinic, aromatic

NH2 def, R-C02- salt, C=N

c h 2

c h 3

c -o -c

ethers

esters

C-OH

alcohols

phenols

S=0

p=o

C-F

=CH

aromatic

C-CI

C-Br

1 I 1 I l H 1 1 1 1 1 i i i 1 i i ■

3000 2000 1500 1000 500

Wavenumbers (cm-1)

FIG U R E 1.1 Regions of the fundamental vibrational spectrum with some characteristic group frequencies.

str, stretch.

analysis is used to reveal the presence and absence of various functional groups in the mole￾cule 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:

• X—H stretch (str) highest frequencies (3700—2500 cm-1)

• X = Y stretch and cumulated double bonds X = Y = Z asymmetric stretch

(2500-2000 cm-1)

• X = Y stretch (2000-1500 cm "1)

• X—H deformation (def) (1500—1000 cm-1)

• X -Y stretch (1300-600 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: INFRARED 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 chal￾lenging with only a few users. However, with conceptual and experimental advances, IR

gradually became a more widely used technique. Important early work developing IR spec￾troscopy 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 via coupling to an eccentric variable speed motor that enabled

------------- r t

1

------------1------------- 1------------- 1 r

C H 3, C H 2. C H , stretch

------------T

O C - H 1

1 1

1

,:.r Arom -H

\

L c = c h 2

I C - C H

R -C H 3 * a

A ro m -C H 3 a ^ 23

R - C H r R u: .

R -O -C H j | r

R 2-N -C H 3

1

1 1

0 = C -H

\ _____________ 1_____________ ! _____________ 1________

■ ____

FIGURE 1.2 The correlation chart for CH3, CH2, and CH stretch infrared bands,

studies of the internal vibrations of molecules. When the osrillnHr.r,

of one of the natural frequencies of vibration for the mechanical model matched thai

and the model responded by exhibiting one of the internal vibrations of th e T o T e ^ ri!

normal mode). muietute (i.e.,

In the 1940s both Dow Chemical and American Cyanamid corrman.^o u •„ .

prism based, single beam, meter focal length instruments primarily to s m d v ^ H 0^ ^ 0

The development of commercially available IR instruments had t ^ hydrocarbons.

American Cyanamid Stamford laboratories contracting with a small ^ ^

Perkin-Elmer (PE). The Stamford design produced by PE was a short f comPany

spectrometer. With the commercial availability of instrumentatio °th ength Prism IR

benefited from the conceptual idea of a correlation chart of important b e then

summarize where various functional groups can be expected to abs h3 ™?-that concise,y

of the correlation chart enabled chemists to use the IR spectrum to det *** * “E d u c tio n

The explosive growth of the IR technique in the 1950s and 1960s was ermUle structure.3'4

of commercially available instrumentation as well as the conceptual b deve^0Pment

lation chart. The appendix shows some selected IR group frequencv ^ , r° U&h a corre￾variety of important functional groups. Shown below in Fig i 2 ation charts for a

CH3/ CH2/ and CH stretch IR bands. * e correlation chart for

The subsequent development of double beam IR instrumentation and IR

resulted in widespread use of IR spectroscopy as a structural techni a Corre^at*on charts

base resulted in a great increase in available IR interpretation tools and t h ^ widespread user

of Fourier transform infrared (FT-IR) instrumentation. More recently development

benefited from dramatic improvements in instrumentation a n d T ^ ? ' ” sPectrosc°Py has

used than in the past. becoming much more widely

REFERENCES 5

References

1. Kettering, C. F.; Shultz, L. W .; A ndrew s, D. H. Phys. Rev. 1930; 36, 531.

2. Colthup, N. B. /. Chem. Educ. 1961, 38 (8), 3 9 4 -3 9 6 .

3. Colthup, N. B. /. Opt. Soc. Am. 1950, 40 (6), 3 9 7 -4 0 0 .

4. Socrates, G. Infrared Characteristic Group Frequencies, 3rd ed.; John W iley & Sons: New York, 2001.

CHAPTER

Basic Principles

1. ELECTROMAGNETIC RADIATION

All light (including infrared (IR)) is classified as electromagnetic (EM) radiation and con￾sists of alternating electric and magnetic fields that is described classically by a continuous

sinusoidal wavelike 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 compo￾nent. Fig. 2.1 depicts the electric field amplitude of light as a function of time.

The important parameters are the wavelength (A, length of one wave), frequency (v, num￾ber cycles per unit time), wavenumbers (T, number of waves per unit length) and are related

to one another by the following expression

v _ 1

" ~ (c/n) “ A

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

Time

FIGURE 2.1 The amplitude of the electric vector of electrom agnetic radiation as a function of time. The w ave￾length is the distance between two crests.

Infrared and Hainan S/vcrrnst'u/rv. Second Edition

http://dx.doi.Org/l 0 .1 0 1 6/B978-0-1 2-804162-8.00002-1 7 <i) 2018 Elsevier Inc. All rights reserved