Infrared Spectroscopy
Overview:
Infrared spectroscopy involves intramolecular vibrations. The atoms within a molecule move in
relation to each other. The change in dipole moment associated with the vibrations, interact with the
electric component of electromagnetic radiation.
The frequency of IR contains information about bond strength and atomic masses
The strength of the absorbance contains information about abundance and polarity
Example: CO
d+
C
d-
O
In the simple classical model this molecule would be expected to contract and expand along the C-O
axis. This is because the bond between the two atoms is not rigid; it is a flexible attraction between
the two atoms. There is a difference in electron density around the carbon and oxygen due to their
differing electronegativities. The oscillation of the two partial charges sets up an oscillating
electromagnetic field. This vibration occurs at about 2140 cm-1.
• Intra molecular vibrations with natural frequencies within the observable IR range
• Atomic motion must produce a change in the molecular dipole moment to be observed
In molecules more complex than diatomic CO, many modes of vibration are possible. Only those
that result in a changing dipole moment will be observed in the IR spectrum. In a strict sense each
vibration mode is a property of the whole molecule, every atom and bond affects its frequency and
strength. In practice however the general frequency and strength of many vibrational modes can be
explained in the first approximation by examining small groups of atoms within the total structure.
It is this feature that makes IR spectroscopy so useful in organic chemistry; IR spectra contain
information about the presence of functional groups within a molecule.
Types of Vibrations
Stretching: changes in length of bonds between atoms within a molecule
Bending:
changes in angle between bonds
Both types of vibration can be complicated by coupling if similar bonds are closely connected.
CO2:
Symmetrical stretch 1340 cm-1
Asymmetrical stretch 2350 cm-1
• A "normal" C=O stretch occurs around 1700 cm-1. Coupling between the two carbonyls in CO2
produces two stretching frequencies; the symmetrical stretch is IR inactive.
Bending 666 cm-1
• Bending vibrations involve a change in bond angle
Methylenes
A CH2 group:
Stretching Modes
Symmetrical stretch
2926 cm-1
Asymmetrical stretch
2853 cm-1
Bending Modes
In the HCH plane:
Scissoring 1465 cm-1
Rocking 720 cm-1
Out of the HCH plane:
Wagging 1350-1150 cm-1
Twisting 1350-1150 cm-1
Overtone Bands
The resonant frequency of a system of masses and springs is known as the fundamental frequency.
The system can also be forced to oscillate at multiples of the resonant (fundamental) frequency.
Harmonic oscillations (overtones) do occur in IR spectra but are usually very weak. When an
overtone of one vibration coincides with the fundamental frequency of another, coupling can occur;
this is known as Fermi resonance.
Spectral Interpretation:
Rigorous vs Pragmatic Approach
From this very brief description of molecular vibration, it becomes obvious that even simple
molecules can give rise to very complex spectra. In a rigorous treatment of molecular vibration each
transition is a characteristic of the entire molecule and involves all atoms and bonds within it. Each
transition can give rise to overtones and can couple to other transitions. All of these aspects must be
considered to completely assign or predict the total IR spectrum of a molecule. A small structural
change will effect many transitions in the spectrum. It is very unlikely that two different molecules
will have the same IR spectrum. For these reasons IR spectra are very useful for checking the
identity of compounds by comparison.
Luckily many functional groups and some substructures of organic molecules have characteristic
vibrations (usually stretches) that are not effected to any great extent by the rest of the molecule. By
concentrating on these characteristic absorbencies, we can gain useful information about the
structures present in an organic molecule. To gain this useful information the spectroscopist looks
for the presence of diagnostic absorbencies, ignoring most of the peaks present in the spectrum. A
knowledge of which frequencies to consider and which to ignore is very important.
In the past when the IR spectrum was the major source of spectroscopic information readily available
to a chemist, as much information as possible was gleaned from the spectrum. More recently, 1H and
13C NMR spectroscopy which provide structural detail more readily have become widely available .
The interpretation of IR spectroscopy has become less vigorous, concentrating on functional group
information.
Spectral Regions: a tool for analysis:
In general the IR spectrum is divided into three regions:
The High Frequency Region:
The Fingerprint Region:
4000 - 1300 cm-1
1300 - 900 cm-1
The Low frequency Region:
900 - 400 cm-1
The high frequency range is known as the functional group region as here most absorbencies can be
attributed to the stretching of specific functional groups: (OH, NH, CH, CN, CC, C=O, C=C, and
C=N).
The fingerprint region contains a large variety of bending absorbencies. All but the simplest of
molecules will have many peaks in this region. The frequency and strength of these bending
vibrations are sensitive to the overall structure of the molecule and are therefore not of diagnostic
use. Some characteristic functional group absorbencies such as C-O and C-N stretches occur in the
fingerprint region. These functional group absorbencies can be confused with the many bending
absorbs and are of lessened diagnostic value.
The low frequency region is where halogen C-X stretching and aromatic C-H bending occur. Both of
these types of absorbencies are useful.
Bands of Specific Interest
The O-H Stretch:
OH stretching is very strong and is usually very broad due to intermolecular hydrogen bonding. It
typically occurs from 3500 - 3200 cm-1. It is strong due to the polarity (difference in electronegativity
between oxygen and hydrogen). The high frequency is due in part to the light mass of hydrogen. A
non hydrogen bonded OH stretch is much sharper and at higher frequency but is usually seen only
in the vapour phase or in dilute solution in non polar solvents.
O
R
O
H
Hydrogen bonding pulls some electron
density away from the OH bond, lowering
the force constant of the bond and decreases
the frequency of the stretch. The broadening
is due to coupling and the diversity of
hydrogen bonding in a dissordered
environment.
Alcohols
-
O H
Carboxylic Acids
-
O H
C=O
N-H Stretch
H
1° R N
H
H
2° R N
R
Symmetrical and Asymmetrical modes
\ Two bands between3500 and 3400 cm-1
amines and amides
One band 3300 - 3100 cm-1
amines
-
N H
C-H Stretch
-
C H
1-hexene
The C=O Stretch
The C=O stretch of carbonyls is very strong and is usually unmistakable in an IR
spectrum. Its frequency ranges from 1870 to 1540 cm-1, but is usually between 1750 1650 cm-1. The functionality of the carbonyl, ring size, and unsaturation all effect the
position of the absorption in a predictable manner.
Ketones
• Normal aliphatic ketones absorb at 1715 cm-1, this is considered a standard C=O
stretch
Aldehydes
• Substitution of H for an alkyl side chain increases the ketone frequency slightly,
aliphatic aldehydes absorb near 1740 - 1720 cm-1.
Carboxylic Acids
Carboxylic acids often form strong intermolecular hydrogen bonded pairs (dimers)
unless in very dilute solution. As monomers the C=O stretch is typically near 1760
cm-1, while dimers absorb near 1710 cm-1. Substitution with the electronegative
oxygen increases the C=O bond strength, raising the absorbence frequency. Hydrogen
bonding reduces this effect.
Esters
Like acids, esters are effected by the second oxygen which inductively pulls electrons
towards it. Normal esters absorb in the region of 1750 - 1735 cm-1.
Amides
Substituting the carbonyl with a nitrogen reduces the carbonyl stretching frequency,
this is thought to be due to a resonance effect where the oxygen of the carbonyl bares a
partial +ve charge.
solid phase
dilute solution
1° amides 1650 cm-1
1690 cm-1
2° amides 1640 cm-1
1680 cm-1
3° amides 1680-1630 cm-1
The C=C Stretch
C=C stretching occurs in the region 1680 - 1580 cm-1. The absorbence is weak as the
dipole is due only to different substituents. The absorbence is strengthened by
substitution by polar groups such as the carbonyl.
The C-O Stretch
C-O stretches occur in the upper fingerprint region where they overlap with many
bending vibrations. The C-O stretch is usually very strong and may often be
distinguished in simple molecules.
Alcohols
1050 - 1200 cm-1
• Aliphatic
Ethers
1150 -1085 cm-1
• Aliphatic
Acids 1320 -1210 cm-1
Esters 1300 - 1000 cm-1
Alkene & Aromatic C-H bending
Out of plane C-H bending occurs in the region of 1000 - 650 cm-1. These are usually
very intense (often the strongest absorbences for alkenes). The pattern of bending
bands for alkyl substituted aromatic rings is often diagnostic.
C-X Stretching
Carbon - halogen stretching occurs in the low frequency region of the IR spectrum.
C-F
C- Cl
C-Br
C-I
1400 - 730 cm-1
850 - 550 cm-1
690 - 515 cm-1
600 - 500 cm-1