INFRARED
SPECTROSCOPY
IR spectroscopy is the study of interaction
between infrared radiations and matter.
 Infrared radiations refers broadly to that
part of electromagnetic spectrum between
visible and microwave region.

IR spectrum of ethanol
PRINCIPLE
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The principle of IR spectroscopy is related to
the vibrational and rotational energy of a
molecule.
When the frequency of the IR radiation is
equal to the natural frequency of vibration, the
molecule absorb IR radiation.
Absorption of IR radiation causes an
excitation of molecule from a lower to the
higher vibrational level.
Each vibrational level is associated with a
number of closely placed rotational level.
Therefore the IR spectroscopy is also called
as “vibrational-rotational spectroscopy”
All the bonds in a molecule are not capable
of absorbing IR energy but those bonds
which are accompanied by a change in
dipole moment will absorb in the IR region
and such transitions are called IR active
transitions.
 The transitions which are not accompanied
by a change in dipole moment of the
molecule are not directly observed and are
considered as IR inactive.
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In IR spectroscopy the changes in the
vibrational energy depends upon
i. Mass of the atoms present in a
molecule
ii. Strength of the bonds
iii. Arrangement of atoms within the
molecule

No two compounds except the enantiomers
can have the similar IR spectra.
THEORY
When a molecule absorb radiation with a
frequency less than 100 cm-1 ,molecular
rotation takes place and if a molecule
absorb more energetic radiation in the
region of 104 to 102 cm-1
, molecular
vibration takes place.
 A single vibrational energy change is
accompanied by a large number of
rotational energy changes and thus the
vibrational spectra appear as vibrational
rotational bands.
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FATE OF ABSORBED RADIATION
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There are 3 main process by which a molecule
can absorb radiation, each of these route
involve an increase of energy which is
proportional to the light absorbed.
i. First route occurs when absorption of
radiation leads to a higher rotational energy
level in a rotational transition.
ii. Second occurs when absorption of radiation
leads to a higher vibrational energy level in a
vibrational transition.
iii. Third occurs when absorption of radiation
leads to a higher electronic energy level in its
electronic transitions.
Energy level diagram
Two criteria must be satisfied by a
molecule for the absorption of IR radiation:
i. The molecule
should
possess
vibrational and rotational frequency.
ii. The molecule must give rise to
asymmetrical charge distribution.
 Three main type of absorption bands
occur in IR spectra:
i.
Fundamental
ii. Overtone
iii. combinational
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FERMI RESONANCE
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Interactions which occur between fundamental and
overtone or combinational bands are known as
Fermi resonance.
This phenomenon can be observed whenever two
fundamental or a fundamental and overtone bands
have nearly the same energy.
Here molecule transfer its energy from fundamental
to overtone and back again and so that the each
level become partially fundamental or partially
overtone in character.
As a result, two strong bands are observed in the
spectrum, instead of the expected strong and weak
bands.

E.g.: CO2
◦ It normally shows fundamental band at 1337
cm-1 and overtone at 1334.6 cm-1 .
◦ But due to the effect of Fermi resonance the
first band shift towards higher frequency and
give rise to two bands at 1285.5 cm-1 and
1388.3 cm-1 .
FINGERPRINT REGION
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In IR, the region from about 1500 to 500 cm-1 usually contains a very
complicated series of absorptions. These are mainly due to all manner
of bending vibrations within the molecule. This is called the fingerprint
region.
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Here the number of bending vibrations are usually
number of stretching vibrations.
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It is much more difficult to pick out individual bonds in this region than it
is in the "cleaner" region at higher wave numbers.
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The importance of the fingerprint region is that each different
compound produces a different pattern of troughs in this part of the
spectrum.
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Many compounds show unique absorption bands in this region and
which is very useful for the identification of the compound.

Compare the infra-red spectra of propan-1-ol and propan-2-ol. Both
compounds contain exactly the same bonds. Both compounds have
very similar troughs in the area around 3000 cm-1 - but compare them
in the fingerprint region between 1500 and 500 cm-1.
more than the
Spectrum of propan-1-ol and propan-2-ol
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Fingerprint region can be sub-divided into
three:
i. 1500-1350 cm-1
– Here doublet near 1380 cm-1 and 1365 cm-1 shows
the presence of tertiary butyl group in the
compound.
ii. 1350-1000 cm-1
All classes of compounds having groups like
– alcohols, esters , lactones, acid anhydrates show
characteristic absorptions (s) due to C – O
stretching.
iii. Below 1000 cm-1
– Distinguishes between cis and trans alkenes and
mono and disubstitutions at ortho, meta, para
VIBRATIONS
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Two types of vibrations are;
1. Stretching
i. Symmetric
ii. Asymmetric
2. Bending
i.
ii.
iii.
iv.
Scissoring
Rocking
Wagging
Twisting
FACTORS INFLUENCING
ABSORPTION
1.
Symmetry
◦ Symmetric compounds do not possess dipole moment
and are IR inactive.
◦ E.g. symmetric acetylene
2.
Coupling
◦ There are so many factors which cause coupled
vibration in IR and it will influence the intensity and
shape of the absorption bands.
◦ E.g. normally the band due to C C bond is around at
1650 cm-1 but due to mechanical coupling of two C
C systems in allene give two bands at 1960 and 1970
cm-1
3.
Fermi resonance
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Fermi resonance results in an unexpected shift
in energy and intensity of the bands.
E.g. the overtone of C--H deformation mode at
1400 cm-1 is always in Fermi resonance with
the stretch of the same bond at 2800 cm-1.
4. Hydrogen bonding
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It can change the shape and position of IR
bands.
Stronger the H-bonding greater the absorption
shift.
Intermolecular- broad bands
Intra-molecular-sharp bands
5.
Electronic effect
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6.
Electronic effects such as inductive, mesomeric
and field effect may cause shift in absorption
bands due to the change in absorption frequency.
E.g. inductive – acetone(1715 cm-1) and
chloroacetone(1725 cm-1)
mesomeric-acetophenone(1693 cm-1) and Paminoacetophenone(1677 cm-1)
Bond angles
◦
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Difference in bond angles can also leads to the
changes is absorption bands.
E.g.
APPLICATIONS OF IR
SPECTROSCOPY
1.
Identification of an organic compound
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The identity of an organic compound can be
established from its fingerprint region by
comparing the sample spectrum with the known
spectrum of the compound.
E.g. spectrum of n-hexanal
2. Qualitative determination of functional
groups
◦ The presence or absence of absorption bands
help in predicting the presence of certain
functional group in the compound.
Infrared Spectroscopy of Hydrocarbons
•Hydrocarbons compounds contain only C-H and C-C bonds, but there
is plenty of information to be obtained from the infrared spectra arising
from C-H stretching and C-H bending.
Carbon-Carbon Bond Stretching
•Stronger bonds generally absorb at higher frequencies because of their
greater stiffness.
•Carbon-carbon single bonds absorb around 1200 cm-1,
•C = C double bonds absorb around 1660 cm-1 , and C - C triple bonds
absorb around 2200 cm -1 .
In alkanes, which has very few bands, each band in
the spectrum can be assigned:
C–H stretch from 3000–2850 cm-1
C–H bend or scissoring from 1470-1450 cm-1
C–H rock, methyl from 1370-1350 cm-1
C–H rock, methyl, seen only in long chain alkanes,
from 725-720 cm-1.
Note that the change in dipole moment with respect to
distance for the C-H stretching is greater than that for
others shown, which is why the C-H stretch band is the
more intense.
•The absorptions of (alkenes) C = C double bonds, however, are useful for
structure determination.
•Most unsymmetrically substituted double bonds produce observable stretching
absorptions in the region of 1600 to 1680 cm-1.
•The specific frequency of the double bond stretching vibration depends on
whether there is another double bond nearby.
•When two double bonds are one bond apart (as in 1,3-cyclohexadiene) they are
said to be conjugated.
•conjugated double bonds are slightly more stable than isolated double bonds
because there is a small amount of pi bonding between them.
•This overlap between the pi bonds leaves a little less electron density in the
double bonds themselves.
•As a result, they are a little less stiff and vibrate a little more slowly than an
isolated double bond.
•Isolated double bonds absorb around 1640 to1680 cm-1, and conjugated double
bonds absorb around 1620 to 1640 cm-1 .
•The effect of conjugation is even more pronounced in aromatic compounds,
which have three conjugated double bonds in a six-membered ring.
• Aromatic C = C bonds are more like 1½ bonds than true double bonds, and
their reduced pi bonding results in less stiff bonds with lower stretching
frequencies, around 1600 cm-1.
•Carbon-carbon triple bonds in alkynes are stronger and stiffer than
carbon-carbon single or double bonds, and they absorb infrared light at
higher frequencies.
• Most alkyne C-C triple bonds have stretching frequencies between
2100 and 2260 cm-1.
•Terminal alkynes usually give sharp C-C stretching signals of moderate
intensity.
•The C –C stretching absorption of an internal alkyne may be weak or
absent, however, due to the symmetry of the disubstituted triple bond
with a very small or zero dipole moment.
The n-octane (Alkane) spectrum, C – C- absorptions
•The different vibrations of the different functional groups in the molecule give rise
to bands of differing intensity.
•The most intense band in the spectrum of n-octane shown at 2971, 2863 cm-1 and
is due to stretching of the C-H bond.
•One of the weaker bands in the spectrum of octane is at 726cm-1, and it is due to
long-chain methyl rock of the carbon-carbon bonds in octane.
•The change in dipole moment with respect to distance for the C-H stretching is
greater than that for the C-C rock vibration, which is why the C-H stretching band is
the more intense than C-C rock vibration.
The 1- hexene (Alkene) spectrum, C = C absorptions
In alkene compounds, each band in the spectrum can be assigned:
C=C stretch from 1680-1640 cm-1
=C–H stretch from 3100-3000 cm-1
=C–H bend from 1000-650 cm-1
The spectrum of 1-hexene shows additional absorptions characteristic of a
double bond.
The C -H stretch at 3080 cm-1corresponds to the alkene = C -H bonds involving
sp2 hybrid carbons.
The absorption at 1642 cm-1 results from stretching of the C = C double bond.
(The small peak at 1820 cm-1is likely an overtone at double the frequency of the
intense peak at 910 cm-1)
The 1-hexyne (Alkyne) spectrum, C ≡ C absorptions
In alkynes, each band in the spectrum can be assigned:
–C≡C– stretch from 2260-2100 cm-1
–C≡C–H: C–H stretch from 3330-3270 cm-1
–C≡C–H: C–H bend from 700-610 cm-1
The spectrum of 1-hexyne, a terminal alkyne, is shown below.
Aromatic compounds
•In aromatic compounds, each band in the spectrum can be assigned:
C–H stretch from 3100-3000 cm-1
overtones, weak, from 2000-1665 cm-1
C–C stretch (in-ring) from 1600-1585 cm-1
C–C stretch (in-ring) from 1500-1400 cm-1
C–H "oop" from 900-675 cm-1
•Note that this is at slightly higher frequency than is the –C–H stretch in
alkanes.
•This is a very useful tool for interpreting IR spectra.
•Only alkenes and aromatics show a C–H stretch slightly higher than 3000
cm-1.
Infrared Spectrum of Toluene
Characteristic Absorptions of Alcohols and Amines
•The O- H bonds of alcohols and the N - H bonds of amines are strong and
stiff.
•The vibration frequencies of O-H and N - H bonds therefore occur at higher
frequencies than those of most C -H bonds (except for alkynyl ≡C- H bonds).
•Alcohol O-H bonds absorb over a wide range of frequencies, centered
around 3300 cm-1.
•Alcohol molecules are involved in hydrogen bonding, with different
molecules having different instantaneous arrangements.
•The O-H stretching frequencies reflect this diversity of hydrogen-bonding
arrangements, resulting in very broad absorptions.
•Notice the broad O-H absorption centered around 3300 cm-1 in the IR
spectrum of 1-butanol.
•Amine N - H bonds also have stretching frequencies in
the 3300 cm-1 region,
or even slightly higher.
•Like alcohols, amines participate in hydrogen bonding
that can broaden the N - H absorptions.
•With amines, however, the absorption is somewhat
weaker, and there may be one or more sharp spikes
superimposed on the broad N-H stretching absorption:
often one N-H spike for a secondary amine ( R2NH) and
two
N-H spikes for a primary amine (RNH2 ) .
•These sharp spikes, combined with the presence of
nitrogen in the molecular formula, help to distinguish
amines from alcohols.
•Tertiary amines ( R3 N ) have no N - H bonds, and they
do not give rise to N-H stretching absorptions in the IR
spectrum.
Characteristic Absorptions of Carbonyl Compounds
•The C = O double bond produces intense IR stretching absorptions bcz of it
has a large dipole moment.
•Carbonyl groups absorb at frequencies around 1700 cm-1, but the exact
frequency depends on the specific functional group and the
rest of the molecule.
• For these reasons, IR spectroscopy is often the best method for detecting
and identifying the type of carbonyl group in an unknown compound.
Simple Ketones, Aldehydes
• The C = O stretching vibrations of simple ketones, aldehydes, and
carboxylic acids occur at frequencies around 1710 cm-1.
•These frequencies are higher than those for C = C double bonds because
the C = O double bond is stronger and stiffer.
Continue…
•In addition to the strong C = O stretching absorption, an aldehyde
shows a characteristic set of two low-frequency C - H stretching
frequencies around 2700 and 2800 cm-1.
•Neither a ketone nor an acid produces these absorptions.
•Notice the characteristic carbonyl
stretching absorptions in both spectra, as well as the aldehyde C - H
absorptions at 2720 and 2820 cm-1.
Carboxylic acids
•A carboxylic acid produces a characteristic broad O-H absorption in addition
to the intense carbonyl stretching absorption.
•Because of the unusually strong hydrogen bonding in carboxylic acids, the broad O-H
stretching frequency is shifted to about 3000 cm-1.
•This broad O-H absorption gives a characteristic overinflated shape to the peaks in the C- H
stretching region.
•Participation of the acid carbonyl group in hydrogen bonding frequently results in
broadening of the strong carbonyl absorption as well.
Characteristic Absorption of Organic Nitrogen
Compounds
N–O asymmetric stretch from 1550-1475 cm-1
N–O symmetric stretch from 1360-1290 cm-1
Absorption Spectrum of Organic Compounds Containing
Halogens
•Alkyl halides are compounds that have a C–X bond, where X is a halogen:
bromine, chlorine, fluorene, or iodine.
•C–H wag (-CH2X) from 1300-1150 cm-1
•C–X stretches (general) from 850-515 cm-1
•C–Cl stretch 850-550 cm-1
•C–Br stretch 690-515 cm-1
3. Distinction between 2 types of H-bonding
◦ If a compound having intra-molecular H-bonding it
will show broad bands in IR spectrum and if a
compound having intermolecular H-bonding then it
will show sharp well defined bands.
◦ E.g. o-nitrophenol shows broad bands due to intramolecular H-bond whereas p-nitrophenol shows
sharp bands due to intermolecular H-bonding.
4. Quantitative analysis
◦
◦
It can be done by measuring the intensity of the
absorption bands.
E.g. xylene exists as mixture of 3 compounds
which shows absorption bands at
ortho-740cm-1
meta-880cm-1
para-830cm-1
5. Study of chemical reactions
◦ It is useful for studying the chemical reactions.
◦ E.g.: reduction of butan-2-one to form butan-2-ol
Butan-2-one
(1710 cm-1)
Butan-2-ol
(3300 cm-1)
6. Study of keto-enol tautomerism
◦ Diketones and ketoesters exhibit keto-enol
tautomerism and this can be studied using IR
spectrum of the compound.
◦ E.g.: Ethyl acetoacetic etser.
C==O – 1733 cm-1
- 1710 cm-1
O---H – 3300 cm-1
C==O – 1645 cm-1
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