Interpretation of Infrared Spectra, A Practical Approach

John Coates

in Encyclopedia of Analytical Chemistry R.A. Meyers (Ed.) pp. 10815–10837  John Wiley & Sons Ltd, Chichester, 2000

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

Interpretation of Infrared absorptions in the spectrum. This information can indicate

whether there is backbone to the structure and, if so,

Spectra, A Practical Approach whether the backbone consists of linear or branched chains.

Next it is possible to determine if there is unsaturation and/or aromatic rings in the structure. Finally, it is possible

John Coates

to deduce whether specific functional groups are present. Coates Consulting, Newtown, USA

If detected, one is also able to determine local orientation of the group and its local environment and/or location in the structure. The origins of the sample, its prehistory, and

1 the manner in which the sample is handled all have impact Introduction 1

on the final result. Basic rules of interpretation exist and,

2 The Origins of the Infrared Spectrum

2 if followed, a simple, first-pass interpretation leading to

3 Spectral Interpretation by Application of

material characterization is possible. This article addresses

Vibrational Group Frequencies

6 these issues in a simple, logical fashion. Practical examples

3.1 The Hydrocarbon Species and are included to help guide the reader through the basic Molecular Backbone

6 concepts of infrared spectral interpretation.

3.2 Simple Functional Groups

3.3 The Carbonyl Group

3.4 Other Functional Groups Associated with Heteroatoms

13 1 INTRODUCTION

3.5 Simple Inorganics

4 The Practical Situation – Obtaining the

The qualitative aspects of infrared spectroscopy are one

Spectrum and Interpreting the Results

15 of the most powerful attributes of this diverse and

4.1 Sample History

16 versatile analytical technique. Over the years, much has

4.2 Physical Characteristics of the been published in terms of the fundamental absorption Sample

17 frequencies (also known as group frequencies) which are

4.3 The Chemistry of the Sample

17 the key to unlocking the structure – spectral relationships

4.4 The Infrared Sampling Method

18 of the associated molecular vibrations. Applying this knowledge at the practical routine level tends to be

5 An Overview to Infrared Spectral

a mixture of art and science. While many purists will

Interpretation – Some Simple Rules and

argue against this statement, this author believes that it

Guidelines

19 is not possible to teach a person to become proficient as

5.1 A Quick Diagnostic Assessment of an interpretive spectroscopist by merely presenting the an Infrared Spectrum

20 known relationships between structure and the observed

Abbreviations and Acronyms

22 spectra. Instead, the practical approach, which has been

Related Articles

22 adopted in this text, is to help the reader appreciate the

References

23 visual aspects of the spectroscopy and how to interpret these relative to the structure and chemistry of the sample.

This is achieved by recognizing characteristic shapes and patterns within the spectrum, and by applying the

The vibrational spectrum of a molecule is considered to information obtained from published group frequency

be a unique physical property and is characteristic of the data, along with other chemical and physical data from molecule. As such, the infrared spectrum can be used as

the sample.

a fingerprint for identification by the comparison of the Included in the text is a discussion of the interrelation- spectrum from an ‘‘unknown’’ with previously recorded

ships that exist between the practical side of acquiring reference spectra. This is the basis of computer-based

the spectrum, the chemistry and physics of the sample spectral searching. In the absence of a suitable reference

under study, the physical interactions of the sample with database, it is possible to effect a basic interpretation of the

its environment, and the impact of the structure on the spectrum from first principles, leading to characterization,

spectrum. In essence, the interpretation of infrared spec- and possibly even identification of an unknown sample.

tra is much more than simply assigning group frequencies. This first principles approach is based on the fact that

The spectrum is rich in information, and this article is structural features of the molecule, whether they are the

intended to help the reader to extract the maximum backbone of the molecule or the functional groups attached

using the knowledge available for the sample and the to the molecule, produce characteristic and reproducible

acquired spectral data. One important factor to bear in

INFRARED SPECTROSCOPY

mind is that a successful interpretation is based not only bromide, KBr). For this reason, when a spectral region is on the presence of particular bands within the spectrum,

quoted in the text, the higher value will be quoted first, but also the absence of other important bands. Complete

consistent with the normal left-to-right (high to low cm 1 ) classes of compounds can be rapidly excluded during the

representation of spectra. Also, the terms infrared band, interpretation by the use of no-band information.

peak and absorption will be used interchangeably within It must be understood that this article addresses

the text to refer to a characteristic spectral feature. the issue of infrared spectral interpretation from the

The spectral group frequencies provided in this text perspective of the average operator of an infrared

were obtained from various literature sources published instrument. It is not a detailed treatise on the theory

over the past 30 years, and most of these are included of infrared spectroscopy where the modes of vibration

in the cited literature. Every attempt to ensure accuracy are discussed in terms of group theory, and where

has been taken; however, there will be instances when mathematical models are used to compare theoretical

individual functional groups may fall outside the quoted and observed values for the fundamental vibrations of a

ranges. This is to be expected for several reasons: the molecule. There are many excellent texts that cover this

influences of other functional groups within a molecule, subject. ⊲ 1 – 4⊳ Instead, this article focuses on the day-to-

the impact of preferred spatial orientations, and environ- day problems associated with characterizing a material or

mental effects (chemical and physical interactions) on the attempting to perform some form of identification. One

molecule.

of the main challenges in presenting a text on spectral The preferred format for presenting spectral data for interpretation is to form a balance between the theory

qualitative analysis is in the percentage transmittance that is needed to appreciate the links between molecular

format, which has a logarithmic relationship ( log 10 ) with structure and the observed spectrum and the practice.

respect to the linear concentration format (absorbance). For this reason, a minimum amount of relevant theory

This format, which is the natural output of most is included in the next section, which provides a basic

instruments (after background ratio), provides the best understanding of why the spectrum exists, how it is

dynamic range for both weak and intense bands. In this formed, and what factors contribute to the complexity

case, the peak maximum is actually represented as a of observed spectra. It has been assumed that the reader

minimum, and is the point of lowest transmittance for a has a fundamental knowledge of molecular theory and

particular band.

bonding, and that there is an understanding of basic structures, in particular for organic compounds.

Infrared spectral interpretation may be applied to both organic and inorganic compounds, and there are

2 THE ORIGINS OF THE INFRARED

many specialized texts dealing with these compounds, in

SPECTRUM

combination and as individual specialized texts. There are too many to reference comprehensively, and the

In the most basic terms, the infrared spectrum is formed reader is directed to a publication that provides a

as a consequence of the absorption of electromagnetic bibliography of the most important reference texts. ⊲ 5⊳

radiation at frequencies that correlate to the vibration of However, the most informative general reference texts

specific sets of chemical bonds from within a molecule. are included, ⊲ 6 – 14⊳ with books by Socrates ⊲ 10⊳ and Lin-

First, it is important to reflect on the distribution of energy Vien ⊲ 11⊳ being recommended for general organics, and

possessed by a molecule at any given moment, defined as by Nakamoto ⊲ 13⊳ and Nyquist et al. ⊲ 14⊳ for inorganics

the sum of the contributing energy terms (Equation 1): (salts and coordination compounds). There are numerous

E total D E electronic specialized texts dealing with specific classes of materials, C E vibrational C E rotational C E translational and undoubtedly polymers and plastics form the largest

⊲ 1⊳ individual class. ⊲

15 – 17⊳ In this particular case, texts by The translational energy relates to the displacement of Hummel and Scholl ⊲ 16⊳ and Koenig ⊲ 17⊳ provide a good

molecules in space as a function of the normal thermal basic understanding.

motions of matter. Rotational energy, which gives rise The following comments are made relative to the con-

to its own form of spectroscopy, is observed as the ventions used within this article. The term frequency

tumbling motion of a molecule, which is the result of is used for band/peak position throughout, and this is

the absorption of energy within the microwave region. expressed in the commonly used units of wavenum-

The vibrational energy component is a higher energy ber (cm 1 ). The average modern infrared instrument

term and corresponds to the absorption of energy by a records spectra from an upper limit of around 4000 cm 1 molecule as the component atoms vibrate about the mean

center of their chemical bonds. The electronic component optics of the instrument (commonly based on potassium

(by convention) down to 400 cm 1 as defined by the

is linked to the energy transitions of electrons as they

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

are distributed throughout the molecule, either localized of a bond between two atomic centers. The fundamental, within specific bonds, or delocalized over structures, such

which involves an energy transition between the ground as an aromatic ring. In order to observe such electronic

state and the first vibrational quantum level, is essen- transitions, it is necessary to apply energy in the form of

tially unaffected by the anharmonicity terms. However, visible and ultraviolet radiation (Equation 2):

transitions that extend beyond the first quantum level (to the second, third, fourth, etc.), which give rise to

E D hn

frequency/energy

weaker absorptions, known as overtones, are influenced by anharmonicity, which must be taken into account

The fundamental requirement for infrared activity, lead- when assessing the frequency of these higher frequency ing to absorption of infrared radiation, is that there must

vibrations.

be a net change in dipole moment during the vibration Having defined the basis for the simple vibration of for the molecule or the functional group under study.

an atomic bond, it is necessary to look at the molecule Another important form of vibrational spectroscopy is

as a whole. It is very easy to imagine that there is an Raman spectroscopy, which is complementary to infrared

infinite number of vibrations, which in reality would spectroscopy. The selection rules for Raman spectroscopy

lead to a totally disorganized model for interpretation. are different to those for infrared spectroscopy, and in

Instead, we describe the model in terms of a minimum this case a net change in bond polarizability must be

set of fundamental vibrations, based on a threefold set of observed for a transition to be Raman active. The remain-

coordinate axes, which are known as the normal modes ing theoretical discussion in this article will be limited to a

of vibration. All the possible variants of the vibrational very simple model for the infrared spectrum. The reader

motions of the molecule can be reduced to this minimum

is encouraged to refer to more complete texts ⊲ 2 – 4⊳

set by projection on to the threefold axes. It can be shown detailed discussion of the fundamentals.

for

that the number of normal modes of vibration for a given While it was stated that the fundamental infrared

molecule can be determined from Equations (4) and (5): absorption frequencies are not the only component to be

evaluated in a spectral interpretation, they are the essence number of normal modes D 3N

6 (nonlinear) ⊲4⊳ and foundation of the art. For the most part, the basic

model of the simple harmonic oscillator and its modifica- D 3N

5 (linear) ⊲ 5⊳ tion to account for anharmonicity suffice to explain the

where N is the number of component atoms in the origin of many of the characteristic frequencies that can

molecule.

be assigned to particular combinations of atoms within In practice, apart from the simplest of compounds, most

a molecule. From a simple statement of Hooke’s law we molecules have nonlinear structures, except where a spe- can express the fundamental vibrational frequency of a

cific functional group or groups generate a predominant molecular ensemble according to Equation (3):

linear arrangement to the component atoms. If we calcu-

late the number of modes for a simple hydrocarbon, such nD

2pc µ

as methane (nonlinear, tetrahedral structure), a value of nine is obtained. This would imply that nine sets of absorp-

where n D fundamental vibration frequency, k D force tion frequencies would be observed in the spectrum of constant, and µ D reduced mass. The reduced mass, µ D methane gas. In reality, the number observed is far less,

corresponding to the asymmetric and symmetric stretch- masses for the chemical bond under consideration.

m 1 m 2 /⊲ m 1 C m 2 ⊳ , where m 1 and m 2 are the component

ing and bending of the C H bonds about the central This simple equation provides a link between the

carbon atom. The reason for the smaller than expected strength (or springiness) of the covalent bond between

number is that several of the vibrations are redundant or two atoms (or molecular fragments), the mass of the inter-

degenerate, that is, the same amount of energy is required acting atoms (molecular fragments) and the frequency of

for these vibrations. Note that although a small number vibration. Although simple in concept, there is a rea-

of vibrational modes is predicted, and in fact observed, sonably good fit between the bond stretching vibrations

the appearance of the methane spectrum at first glance predicted and the values observed for the fundamentals.

is far more complex than expected, especially at higher This simple model does not account for repulsion and

spectral resolutions (<1 cm 1 ). At relatively high resolu- attraction of the electron cloud at the extremes of the

tions, a fine structure is superimposed, originating from vibration, and does not accommodate the concept of

rotational bands, which involve significantly lower energy bond dissociation at high levels of absorbed energy. A

transitions. Each of the sets of vibrational – rotational model incorporating anharmonicity terms is commonly

absorptions manifest this superimposed fine structure used to interpret the deviations from ideality and the

for low-molecular-weight gaseous compounds, methane overall energy – spatial relationship during the vibration

being a good example. Several medium-molecular-weight

INFRARED SPECTROSCOPY

compounds may also show evidence of some fine struc- combination of the bands associated with these group ture when studied in the vapor state. For example, it is

frequencies and the skeletal frequencies are used to common to observe the sharp feature (or spike) assigned

identify a specific compound. The latter forms the basis of to the Q-branch of the vibrational – rotational spectrum,

the use of reference spectra for spectral matching by visual as indicated by the vapor spectrum of acetone (Figure 1).

comparison or by computer-based searching, for the If we proceed up the homologous series from methane

identification of an unknown from its infrared spectrum.

The group frequencies may be viewed quantitatively, atoms, which would imply 54 normal modes. In this case

(CH 4 ) to n-hexane (C 6 H 14 ), there are 20 component

as well as qualitatively. A given absorption band assigned the picture is slightly more complex. Methane is a unique

to a functional group increases proportionately with the molecule, and only contains one type of C H group – no

number times that functional group occurs within the other types of bond exist in this molecule. In hexane there

molecule. For example, in the spectrum of n-hexane, the are several types of bond and functionality. For reference,

intensities measured for the group frequency absorptions

a simple two-dimensional representation of the structure assigned to methyl and methylene correspond to four is provided in Figure 2(a).

methylene groups and two methyl groups on a rel- As we can see, there are two terminal methyl groups

ative basis, when compared with other hydrocarbon

compounds within a homologous series. For example, Each of these groups has its corresponding C H

(CH 3 ) and four connecting methylene groups (CH 2 ).

if we examine the C H stretching (or bending) band stretching and bending vibrations (see later text for the

intensities for CH 3 and CH 2 , we will observe that the actual absorption frequencies). Also, the methyl groups

relative intensities of CH 3 to CH 2 decrease with increase are linked to a neighboring methylene group, which is

in chain length. Restated, there is less methyl contribu- in turn linked to neighboring methylene groups, and so

tion and more methylene contribution with increase in on. These linkages feature carbon – carbon bonds. For

chain length/molecular weight. The reverse holds true if interpretation, we view the C H groups as functional

we examine the spectra of linear hydrocarbons with chain groups, giving rise to the common group frequencies,

lengths shorter than that of hexane. and the C C linkages as the backbone, producing the

If we apply these ideas to a different hexane iso- skeletal vibrations. As a rule, a group frequency may

mer, such as isohexane (2-methylpentane), we would

be applied generally to most compounds featuring the see significant differences in the spectrum. These can be corresponding functional group. In contrast, the skeletal

explained by evaluating the structure (Figure 2b), which vibrations are unique to a specific molecule. The group

contains three methyl groups, two methylene groups, and frequencies help to characterize a compound, and the

a group that contains a single hydrogen attached to car- bon (the methyne group). This adds a new complexity

100 to the spectrum: the main absorptions show differences in appearance, caused by the changes in relative band

80 Q

intensities, splittings of absorptions occur (originating 60

from spatial/mechanical interaction of adjacent methyl Q

groups), and changes are observed in the distributions of 40

the C C skeletal vibrations, in part due to the splitting by the methyl side chain. Further discussions concerning the

20 impact of chain branching are covered later in this article.

T ransmittance (%)

0 Comparison of Figures 3 and 4 provides a graphical repre- sentation of the aspects discussed for the hexanes of struc-

turally similar compounds, i.e. n-heptane and isooctane.

From a first-order perspective, the idea of the quan- titative aspects of the group frequencies carries through Figure 1 Vapor spectrum of acetone with characteristic

Wavenumber (cm − 1 )

for most functional groups, and the overall spectrum is Q-branch slitting, denoted by Q. Copyright Coates Consulting.

essentially a composite of the group frequencies, with band intensities in part related to the contribution of each

H functional group in the molecule. This assumes that the

HH H H

HC C C H C C H functional group does give rise to infrared absorption HC C C H H C C CH frequencies (most do), and it is understood that each

H HH HH H H H H H

group has its own unique contribution based on its extinc- tion coefficient (or infrared absorption cross-section).

(a)

(b)

Figure 2 Structures for hexane isomers: (a) n-hexane and Returning to the fundamental model, we should now (b) isohexane (2-methylpentane). Copyright Coates Consulting.

look at the larger picture. In reality, we assign the

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

symmetric vibration. While this might be less intuitive, it )

% 80 is still a rational concept, and therefore easy to understand (

e and accept. Again, we see a good example with the C H

c n 60 stretch of an aliphatic compound (or fragment), where it ta

we observe the asymmetric C H stretch of the methyl

m CH 3 (CH 2 ) 5 CH 3 and methylene groups (2960 and 2930 cm 1 , respectively) s

n 20 occurring at slightly higher frequency than symmetric ra

vibrations (2875 and 2855 cm 1 , respectively for methyl T 0

0 0 0 0 and methylene). For the most part, this simple rule

0 0 6 0 0 0 2 0 0 0 0 8 0 4 0 0 0 8 0 0 0 0 0 2 0 0 6 0 4 0 0 0 8 6 0 4 holds true for most common sets of vibrations. Naturally 3 3 2 2 2 1 1 1 1 1 there are always exceptions, and a breakdown of the

Wavenumber (cm − 1 )

rationale may occur when other effects come into play, such as induced electronic, spatial or entropy-related

Figure 3 Attenuated total reflectance (ATR) spectrum of n -heptane. Copyright Coates Consulting.

effects.

There are many other spatially related scenarios that

tend to follow well-orchestrated patterns, examples being )

in-plane and out-of-plane vibrations, the differences %

( 80 between cis and trans spatial relationships, and a variety

e c of multicentered vibrations that are defined as twisting n 60

or rocking modes. Many of these are exhibited with the ta it 40 CH 3 CH 3 C H vibrations that occur in saturated, unsaturated and

m s CH 3 C CH 2 CH CH 3 aromatic compounds. Molecular symmetry of the static n 20

ra CH 3 or the dynamic (during vibration) molecule has a large

T 0 impact on the spectrum, in addition to factors such as

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 relative electronegativity, bond order and relative mass

4 3 6 2 3 8 2 4 0 2 2 0 0 0 0 0 0 8 1 6 8 0 6 1 1 4 2 1 1 0 of the participating atoms.

Wavenumber (cm − 1 )

Finally, while discussing the vibrational origins of infrared spectra, it is worth commenting that further

Figure 4 ATR spectrum of 2,2,4-trimethylpentane (isooctane). complexity may be noted in the spectrum, beyond Copyright Coates Consulting.

what is expected based on the fundamentals. As noted, transitions to higher energy levels, although theoreti-

observed absorption frequencies in the infrared spectrum cally not allowed, can occur and these give rise to to much more that just simple harmonic (or anharmonic)

overtone bands, which in the mid-infrared region occur stretching vibrations. In practice, we find that various

at approximately twice the fundamental frequency for other deformation motions (angular changes), such as

the first overtone. Higher overtones exist, typically the bending and twisting about certain centers within a

second (3 ð fundamental) and third (4 ð fundamental), molecule, also have impact, and contribute to the overall

and sometimes higher, and these are observed, with absorption spectrum. By rationalizing the effort needed to

extremely low intensity, relative to the fundamental in the move the atoms relative to each other, one can appreciate

near-infrared spectral regions, between 800 and 2500 nm that it takes less energy to bend a bond than to stretch

(12 500 and 4000 cm 1 ). Other types of bands that can add it. Consequently, we can readily accept the notion that

complexity to a spectrum are combination bands (sum and the stretching absorptions of a vibrating chemical bond

difference), bands due to transitions from energy states occur at higher frequencies (wavenumbers) than the

higher than the ground state or ‘‘hot bands’’, and bands corresponding bending or bond deformation vibrations,

due to interactions between a weaker overtone or com- with the understanding, of course, that energy and

bination band and a fundamental of the same or similar frequency are proportionally related. A good example is

frequency, known as Fermi resonance bands. In the lat- the C H set of vibrations, observed in the hydrocarbon

ter case, two relatively strong absorptions are observed, spectra, and in virtually all organic compounds. Here, the

where normally only a single absorption is expected for simple C H stretching vibrations for saturated aliphatic

the fundamental. ⊲ 3⊳

As additional functional groups are added to a basic corresponding simple bending vibrations nominally occur

species occur between 3000 and 2800 cm 1 , and the

backbone structure, forming a more complex molecule,

additional bands are observed, either directly associated Next in our understanding is that it can take slightly

between 1500 and 1300 cm 1 .

with the fundamental vibrations of the functional groups, more energy to excite a molecule to an asymmetric than a

or indirectly related to interactions between component

INFRARED SPECTROSCOPY

functional groups or the basic substructure. Such interac- arrangement of the groups involved is reflected in tions can be severe, and result in overwhelming distortions

the infrared spectrum as additional bands and added in the appearance of the spectrum, a good example

complexity.

being hydrogen bonding. This will be dealt with in depth As we move on to simple organic compounds, where later.

one or more functional groups or heteroatoms are added to the molecule, we see many changes occurring in the spectrum. These result from the bonding associated with

3 SPECTRAL INTERPRETATION BY

the functional group, and also local disturbances to the

APPLICATION OF VIBRATIONAL

basic backbone spectrum that relate again to spatial

GROUP FREQUENCIES

changes and also to local and neighboring electronic effects. Examples of such functionalities are halogens,

This section includes tabulated data relative to the simple oxygen species, such as hydroxy and ether groups, most significant group frequencies for the most common

and amino compounds. Carbonyl compounds, where the functional groups and structural components found in

added functional group includes the CDO bond, also organic compounds. Brief reference is also made to simple

provide very profound contributions to the spectrum, inorganic compounds, in the form of simple ionic species.

and because of the wide diversity of these compounds More detailed listings can be found in published literature,

they are best dealt with as a separate class. and the reader is encouraged to acquire one or more of

A very characteristic group of compounds, from these reference texts. ⊲ 9 – 13⊳ As already indicated, the use

a spectral point of view, are the multiple-bonded of tabulated data is only a part of the interpretation

nitrogen compounds, such as cyanides and cyanates. process, and other facets of the spectrum must be taken

These typically have very characteristic absorptions, into account.

which are easy to assign, and are free from spectral To help gain an understanding of infrared spectral

interferences. The same can be said for some of interpretation, it is instructive to start at the root of most

the hydrides of heteroatoms, such as sulfides (thiols), organic compounds, namely the fundamental backbone

silanes, and phosphines. Finally, there are other, oxygen- or the parent hydrocarbon structure. We shall start with

containing functional groups, as encountered in the the simple, aliphatic hydrocarbon, which is at the root

nitrogen-oxy (NO x ), phosphorus-oxy (PO x ), silicon-oxy of most aliphatic compounds. Aliphatic hydrocarbons

(SiO x ), and sulfur-oxy (SO x ) compounds. These are exist in simple linear chains, branched chains and in

sometimes more difficult to identify from first principles, cyclic structures – examples of the linear and branched

and a knowledge of the presence of the heteroatom chain scenarios were provided earlier for hexane isomers.

is helpful. The spectra are characteristic, but many of Any one molecule may exist with one or more of these

the oxy absorptions occur within a crowded and highly component structures. The infrared spectrum can provide

overlapped region of the spectrum, mainly between information on the existence of most of these structures,

1350 and 950 cm 1 . Also, many of these compounds either directly or by inference.

feature C O bonding, which is common in other The introduction of unsaturation in the form of a

frequently encountered functionalities such as ethers double or triple bond has a profound impact on the

and esters.

chemistry of the molecule, and likewise it has a significant influence on the infrared spectrum. Similarly, the same is

3.1 The Hydrocarbon Species and Molecular Backbone

observed when an aromatic structure is present within

a molecule. Infrared spectroscopy is a powerful tool In this section we include the characteristic absorption for identifying the presence of these functionalities. It

frequencies encountered for the parent hydrocarbon provides information specific to the group itself, and

species and the associated backbone or substituent group. also on the interaction of the group with other parts

This includes aliphatic and aromatic structures. The of the molecule and on the spatial properties of the

spectral contributions are characterized, as previously group. Examples of these include conjugation between a

noted, as C H stretching and bending vibrations and double bond and another unsaturated center, an aromatic

C C vibrations (stretching and bending), which for ring or a group, such as a carbonyl (CDO), and the

the most part are unique for each molecule, and are orientation or location of the double bond within the

generally described as skeletal vibrations. In the case molecule, such as cis or trans and medial or terminal. It

of aromatic compounds, ring CDC C stretching and should be noted that cis/trans relationships are not specific

bending vibrations are highly characteristic, and are to unsaturated hydrocarbons, and the terminology is

diagnostic. Likewise, the same can be said for the referenced elsewhere, such as with secondary amide

unsaturated carbon – carbon multiple bonding in alkene structures. Again, the associated changes in the spatial

and alkyne structures.

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

Table 1 Saturated aliphatic (alkane/alkyl) group frequencies Table 2 Olefinic (alkene) group frequencies Group frequency (cm 1 )

Functional group/assignment

Origin

Group frequency,

Assignment

wavenumber (cm 1 )

Methyl (−CH 3 )

Alkenyl CDC stretch 1470 – 1430/1380 – 1370 Methyl C H asym./sym. bend

2970 – 2950/2880 – 2860 Methyl C H asym./sym. stretch

CDC

Aryl-substituted CDC 1385 – 1380/1370 – 1365 gem -Dimethyl or ‘‘iso’’- (doublet)

Conjugated CDC 1395 – 1385/1365

Trimethyl or ‘‘tert-butyl’’ (multiplet)

CH 3095 – 3075

Terminal (vinyl) C H stretch

Methylene ( CH 2 )

C 3040 – 3010 3095 – 3075

Pendant (vinylidene) C H 2935 – 2915/2865 – 2845 Methylene C H asym./sym. stretch

stretch 1485 – 1445

Medial, cis- or trans-C H 750 – 720

Methylene C H bend

Methylene (CH 2 ) n rocking

Cyclohexane ring vibrations

CH 1420 – 1410

Vinyl C H in-plane bend

Vinylidene C H in-plane

bend 2900 – 2880

Methyne ( CH−)

CH 995 – 985 C 915 – 890 Vinyl C H out-of-plane bend 1350 – 1330

Methyne C H stretch

Vinylidene C H out-of-plane 1300 – 700

Methyne C H bend

Skeletal C C vibrations

bend

trans -C H out-of-plane bend 2850 – 2815

Special methyl (−CH 3 ) frequencies

CH 970 – 960

Methoxy, methyl ether O CH 3 ,

700 (broad)

cis -C H out-of-plane bend

C H stretch

Methylamino, N CH 3 , C H stretch

3.1.1 Saturated Aliphatic and Alicyclic Compounds See Table 1. The C H stretch vibrations for methyl 60

and methylene are the most characteristic in terms

of recognizing the compound as an organic compound

containing at least one aliphatic fragment or center. The 20 H C C (CH 2 ) 3 CH 3 bending vibrations help to tell more about the basic

T ransmittance (%) 0

structure. For example, a strong methylene/methyl band (1470 cm 800 1 ) and a weak methyl band (1380 cm 1 ), plus 600 4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000

a band at 725 – 720 cm 1 (methylene rocking vibration)

Wavenumber (cm − 1 )

is indicative of a long-chain linear aliphatic structure (note that splitting may be observed for the 1470 and

Figure 5 ATR spectrum of 1-hexene. Copyright Coates Con- 720 cm 1 bands, which is indicative of a long-chain

sulting.

compound, and is attributed to a crystallinity and a high degree of regularity for the linear backbone structure).

unsaturated hydrocarbons featuring CDC, with attached In contrast, strong methyl bands, showing significant

hydrogens, usually occur as either a single or a pair splitting, and a comparatively weaker methylene/methyl

of absorptions, in the ranges indicated in Table 2. As band indicate chain branching, and the possibility of

noted, the number of bands and their positions are isopropyl or tert-butyl substituents (depending on the

indicative of the double bond location and the spatial amount of splitting, and the relative band intensities).

arrangement around the double bond. The position of the

A comparison between linear and branched chain CDC stretching frequency does vary slightly as a function hydrocarbons can be seen in Figures 3 and 4, where in the

of orientation around the double bond, but it is less case of isooctane, both isopropyl and tert-butyl groups

informative than the C H information. The C H out-of- are present.

plane bending is typically the most informative relative to the location and spatial geometry of the double bond,

3.1.2 Unsaturated Compounds where terminal and medial double bonds may be clearly

differentiated. Figure 5 provides a good example with the See Table 2. As already commented upon, the saturated

spectrum of 1-hexene, which contains the terminal vinyl hydrocarbon C H stretching absorptions all occur below

group. Note that a fully substituted, medial double bond 3000 cm 1 . Any band structures observed between 3150

has only the CDC as the sole indicator of the presence and 3000 cm 1 are almost exclusively indicative of

of the double bond, unless the bond is conjugated with a unsaturation (CDC H) and/or aromatic rings. The

second unsaturated site.

INFRARED SPECTROSCOPY

Table 3 Aromatic ring (aryl) group frequencies

Origin Group frequency,

Assignment

wavenumber (cm 1 )

a CDC C CH 1615 – 1580 Aromatic ring stretch 3 CDC C a 1510 – 1450

Aromatic ring stretch

40 H CH 3

CH 3130 – 3070

Aromatic C H stretch

CH 1225 – 950 (several)

Aromatic C H in-plane

20 H H

bend

CH 900 – 670 (several) Aromatic C H

1600 1400 1200 1000 800 770 – 735 (a) 1,2-Disubstitution (ortho)

out-of-plane bend

770 – 730 C 710 – 690 Monosubstitution (phenyl)

810 – 750 C 900 – 860 1,3-Disubstitution (meta)

1,4-Disubstitution (para)

‘‘Combi’’ b 2000 – 1660 (several) Aromatic combination

bands 60

a CDC C used as an approximation of the unique aromatic ring

‘‘Combi’’ denotes assignment to combination bands.

20 H CH 3

3.1.3 Aromatic Compounds

ransmittance (%) T H

See Table 3. The existence of one or more aromatic rings in a structure is normally readily determined

from the C H and CDC C ring-related vibrations. The

C H stretching occurs above 3000 cm 1 and is typically

exhibited as a multiplicity of weak-to-moderate bands, compared with the aliphatic C H stretch. The structure 80

of the bands is defined by the number and positions of the

60 CH

C H bonds around the ring, which in turn are related to

40 H the nature and number of other substituents on the ring. H Note that the same applies to the C H out-of-plane bend-

20 H H

ing vibrations, which are frequently used to determine the

CH 3

degree and nature of substitution on the ring – examples are provided in Figure 6(a – c), with the comparison of the three xylene isomers. This picture often becomes

Wavenumber (cm − 1 ) in a compound. The other most important set of bands

more complex if multiple- or fused-ring structures exist

(c)

are the aromatic ring vibrations centered around 1600 Figure 6 ATR spectra of xylene isomers: (a) o-xylene, 1,2- and 1500 cm 1

, which usually appear as a pair of band dimethylbenzene; (b) m-xylene, 1,3-dimethylbenzene; (c) p- xylene, 1,4-dimethylbenzene. Copyright Coates Consulting.

structures, often with some splitting. The appearance and ratio of these band structures is strongly dependent on the position and nature of substituents on the ring.

Table 4 Acetylenic (alkyne) group frequencies Origin

Group frequency,

Assignment

3.1.4 Acetylenic Compounds

wavenumber (cm 1 )

See Table 4. Although acetylenic compounds are not very

Terminal alkyne (monosubstituted)

Medial alkyne on the carbon – carbon bond stretching as a function of (disubstituted)

can be characteristic. It is instructive to note the impact

Alkyne C H stretch increase in bond order for the series of single-, double-,

CH 3320 – 3310

Alkyne C H bend and triple-bonded carbon:

CH 680 – 610

CH 630 (typical)

Alkyne C H bend

C C stretch: ¾1350 – 1000 cm 1 (skeletal vibrations)

CDC stretch: 1680 – 1620 cm 1 This increase in bond order produces a corresponding 2260 – 2100 cm 1 increase in bond strength, which in turn increases the force

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

constant, k (see Equation 3), supporting the Hooke’s law assigned to the halogen – carbon stretching. When a single model described earlier.

halogen is present, the determination of this group is straightforward. However, if more than one halogen is

influenced by whether the group is terminal or medial. present, the interpretation is usually more complex. In The single hydrogen of the terminal acetylene itself is

such cases, the result varies depending on whether the very characteristic, reflecting the labile nature of the

halogens are on the same or different carbon atoms, and, if acetylenic C H.

on different atoms, whether the atoms are close neighbors. This is particularly the case with small molecules, and

3.2 Simple Functional Groups

the resultant spectral complexity arises from the fact that there is restricted rotation about the carbon – carbon

Obviously, there is a potentially broad number of

bond.

molecular fragments that can be considered to be Single bonds usually exhibit free rotation, which would functional groups attached to an organic structure or

normally mean that there are no preferred spatial backbone. This section features the most simple and

orientations for the molecules. However, owing to the most common of the functional groups,

size of the halogen atom, relative to the carbon and halogens (X D F, Cl, Br and I), hydroxy (X D OH),

C X, i.e. the

hydrogen that form the backbone, the molecules tend oxy or ether (X D OR, where R D alkyl), and amino

to exhibit certain specific conformations, where the (X D NH 2

spatial interaction between neighboring halogen atoms carbonyl functionality, these three basic functional groups

is minimized, and each conformation provides its own cover most of the common occurrences in simple organic

contribution to the overall spectrum. It is important compounds. Note that for the oxy/hydroxy and amino

to appreciate that this issue of spatial orientation has functionalities, these are molecular fragments, and they

an impact even on high-molecular-weight compounds, contribute their own set of characteristic absorptions to

such as the polyhalogenated polymers, e.g. poly(vinyl the spectrum of the compound. In fact, the bonding

chloride). Here preferred orientations have an impact on between the functional group and the backbone is only

the crystallinity of the polymer, and this in turn has a one part of the overall picture used for the spectral

significant impact on both the spectrum and the physical interpretation.

properties of the material.

Another important issue to consider with halogen

3.2.1 Halogenated Compounds substituents is the high electronegativity of the halogen See Table 5. In principle, the interpretation of the spectra

atom. This can have a noticeable impact on the spectrum of molecules containing one or more halogens would

of neighboring group frequencies, including adjacent seem to be straightforward. The functionality is simple,

hydrogen atoms. In such cases, significant shifting of with just a single atom linked to carbon to form the group.

the C H frequencies can occur – the direction of the With the polar nature of this group, one would expect the

shift being dependent on the location of the C H, spectral contribution to be distinctive. In reality, this is

and whether the halogen adds or extracts electron not always the case.

density from the C H bond – adding strengthens (higher In aliphatic compounds, the C X bond typically

frequency) and extracting weakens (lower frequency). possesses a unique group frequency, which may be

The same influences can be observed with halogen- substituted carbonyl compounds, such as acyl halides

Table 5 Aliphatic organohalogen compound group and a-substituted acids, where the bond strength of the frequencies

carbonyl group is increased (see section 3.3). In most cases, both a shift to higher frequency and an increase in

Origin Group frequency,

absorption strength for the band are observed. Table 5 only presents the group frequencies for the CF 1150 – 1000

Assignment

wavenumber (cm 1 ) a

Aliphatic fluoro compounds,

aliphatic compounds, because no well-defined C X

absorptions are observed for halogen-substituted aro- C Cl

C F stretch

Aliphatic chloro compounds,

matic compounds. The presence of a halogen on an

C Cl stretch

aromatic ring can be detected indirectly from its elec-

C Br 700 – 600

Aliphatic bromo compounds,

tronic impact on the in-plane C H bending vibrations. CI 600 – 500

C Br stretch

Aliphatic iodo compounds,

Normally, we do not consider the in-plane bending bands

C I stretch

to be of use because, as pointed out earlier, these occur in

a spectral region that is crowded by other important group

Note that the ranges quoted serve as a guide only; the actual ranges are influenced by carbon chain length, the actual number of halogen

frequencies. However, in the case of a halogen-substituted

substituents, and the molecular conformations present.

ring, the intensity of these vibrations is enhanced relative

INFRARED SPECTROSCOPY

to other absorptions by as much as three to four times. Table 6 Alcohol and hydroxy compound group frequencies For reference, it is informative to compare the intensities

for these bands, between 1150 and 1000 cm 1 , for the

Origin

Group frequency,

Assignment

wavenumber (cm 1 )

spectra of toluene and chlorobenzene (Figure 7a and b).

OH

3570 – 3200 (broad)

Hydroxy group, H-bonded OH stretch

3.2.2 Hydroxy and Ether Compounds

Normal ‘‘polymeric’’ OH stretch

See Table 6 for alcohols and hydroxy compounds.

Dimeric OH stretch The hydroxy function is probably one of the most

Internally bonded OH stretch dominant and characteristic of all of the infrared group

3645 – 3600 (narrow) Nonbonded hydroxy group, frequencies. In most chemical environments, the hydroxy

OH

OH stretch group does not exist in isolation, and a high degree

Primary alcohol, OH stretch of association is experienced as a result of extensive

Secondary alcohol, OH stretch

hydrogen bonding with other hydroxy groups. These Tertiary alcohol, OH stretch 3640 – 3530 a Phenols, OH stretch

hydroxy groups may be within the same molecule

Primary or secondary, OH (intramolecular hydrogen bonding) or they most likely

OH

in-plane bend exist between neighboring molecules (intermolecular

Phenol or tertiary alcohol, OH hydrogen bonding). The impact of hydrogen bonding

bend is to produce significant band broadening and to lower

Alcohol, OH out-of-plane the mean absorption frequency. The lowering of the

bend frequency tends to be a function of the degree and b C O ¾1050

Primary alcohol, C O stretch ¾ 10 strength of the hydrogen bonding. In compounds such as b

Secondary alcohol, C O stretch

carboxylic acids, which exhibit extremely strong hydrogen ¾ 1150 b Tertiary alcohol, C O stretch bonding, forming a stable dimeric structure, a highly

¾ 1200 b Phenol, C O stretch characteristic, large shift to lower frequencies is observed.

a Frequency influenced by nature and position of other ring substituents. b Approximate center of range for the group frequency.

In special circumstances, where the hydroxy group is isolated – either because of steric hindrance effects

60 Cl

or because the sample is in the vapor state or in

a dilute solution of a nonpolar solvent – the band is characteristically narrow, and is observed at the natural,

20 H H higher frequency. This absorption is important for the

T H ransmittance (%)

characterization of certain hindered phenol antioxidants,

0 a commercially important class of compounds in the food,

4000 600 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 polymer, and formulated oil industries. It must be appreciated that while the hydroxy absorp-

(a)

Wavenumber (cm − 1 )

tion is singly one of the most important bands in the 100

infrared spectrum, other vibrations are also important for the actual characterization of the compound. Alco-

80 hols exist as three distinct classes – primary, secondary and tertiary – distinguished by the degree of carbon sub-

60 CH

3 stitution on the central hydroxy-substituted carbon, a

40 H H single substitution being primary, double substitution being secondary, and triple substitution being tertiary.

20 H H This is an important fact, because the chemistry and

ransmittance (%) T

H oxidation stability of the alcohol are strongly influenced

0 by the degree of substitution. Whether an alcohol is pri- 4000 800 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000

mary (1 ° ), secondary (2 ° ) or tertiary (3 ° ), may be reflected (b)

Wavenumber (cm − 1 )

in the position of the OH stretch absorption, but typically this is determined by the other absorptions, in particu-

Figure 7 Comparison of ATR spectra of (a) chlorobenzene lar the C O stretching frequency. Another absorption and (b) toluene. Copyright Coates Consulting.

of lower importance, but often characteristic, is assigned

INTERPRETATION OF INFRARED SPECTRA, A PRACTICAL APPROACH

to another form of bending vibration, the out-of-plane compounds, where the hydrogen of the hydroxy group bend or wagging vibration of the O H. The OH bend-

is replaced by an aliphatic (alkyl) or aromatic (aryl) ing vibrations are broadened by hydrogen bonding as is

molecular fragment. Having stated that, the overall the stretching absorption, but often to a lesser extent.

appearance of an ether spectrum is drastically different The differences between primary and secondary alcohols

from that of a related alcohol. This is due to the can be appreciated from Figure 8(a) and (b), where the

overwhelming effect of hydrogen bonding on the hydroxy spectra of 1- and 2-octanol are presented.

group. However, many of the relationships that exist for See Table 7 for ethers and oxy compounds. In some

the C O component of the alcohol carry over to the respects, ethers are related to alcohol and hydroxy

corresponding ether. The relationships that pertain to primary, secondary, and tertiary structures remain intact.

100 The main difference is that one now considers the bonding on both sides of the oxygen, because if carbon is on both sides, then two ether bonds exist. Ethers can exist as simple ethers (same group both sides) and mixed ethers

50 (different groups both sides). Infrared spectroscopy is fairly sensitive for differentiating these ether functions, especially when the structures are mixed aliphatic or aliphatic/aromatic.

T ransmittance (%) 0

3.2.3 Amino Compounds

Wavenumber (cm − 1 )

(a)

See Table 8. In some respects, the infrared spectra and the characteristic group frequencies of amines tend to

100 Table 8 Amine and amino compound group frequencies

Origin

Group frequency,

Assignment 50

wavenumber (cm 1 )

Primary amino

NH

Aliphatic primary amine,

ransmittance (%)

T NH stretch

Aromatic primary amine,

NH stretch 4000

C 3415 – 3380

Primary amine, NH bend (b)

Wavenumber (cm − 1 )

CN

Primary amine, CN stretch

Secondary amino

Figure 8 ATR spectra of (a) primary (1-octanol) and (b) sec-

Aliphatic secondary amine, ondary (2-octanol) alcohols. Copyright Coates Consulting.

NH

NH stretch

Aromatic secondary amine, Table 7 Ether and oxy compound group frequencies

NH

NH stretch

Heterocyclic amine, Origin

NH

NH stretch 1 wavenumber (cm D ) NH

Group frequency,

Assignment

Imino compounds, NH stretch

CH 2820 – 2810

Methoxy, C H stretch

NH

Secondary amine, NH bend

Secondary amine, COC

Alkyl-substituted ether,

CN stretch

C O stretch

Tertiary amino

COC 1140 – 1070

Cyclic ethers, large

CN

Tertiary amine, CN stretch

rings, C O stretch

f OH 1270 – 1230

Aromatic ethers,

Aromatic amino

Aromatic primary amine, CO

aryl O stretch

CN

CN stretch COOC

¾ 1250 C 890 – 800 a Epoxy and oxirane rings

890 – 820 a Peroxides, C O O

CN

Aromatic secondary amine,

stretch

CN stretch