The infrared region of electromagnetic spectrum extents from the red-end of visible spectrum out to microwave region.  The region includes radiation at wavelength between 0.75 and 300 microns or in wave numbers between 13,000 and 33 cm-1

1. Infrared absorption
spectroscopy
(FTIR)
Prepared by
Prof.(Dr.) Dinesh Kumar Mehta
MMCP, (MMDU), Mullana

2. Introduction
 The infrared region of electromagnetic spectrum extents from
the red-end of visible spectrum out to microwave region.
 The region includes radiation at wavelength between 0.75 and
300 microns or in wave numbers between 13,000 and 33 cm-1
From application and instrumentation point of view, the
infrared region has been subdivided into three regions.
1. Near IR region (14290 – 4000 cm-1
)
2. Middle IR region (4000-666 cm-1
)
3. Far IR region (666-33 cm-1
)
The spectral region of greatest use in analytical applications for
studying organic compounds is the mid-infrared region.
The technique is based upon the simple fact that a chemical
substance shows marked selective absorption in the infrared
region.

3. After absorption of IR radiations, the molecules of a chemical
substance vibrate at many rates of vibration, giving rise to close
packed absorption bands, called an IR absorption spectrum.
Which extend over a wide wavelength range.
 Various bands will be present in IR spectrum which will
correspond to the characteristic functional groups and
bonds present in a chemical substance.
 Thus, an IR spectrum of a chemical substance is a
fingerprint for its identification.
 Infrared spectra are usually plotted as percent
transmittance rather than as absorbance as the ordinate.
This makes absorption bands appear as dips in the curve
rather than as maxima, as in case of ultra violet and visible
spectra.
 Each dip in the spectrum is called a band or peak and
represents absorption of infrared radiation at that
frequency by sample.

4.  The transmittance is 0%, if all the radiation is absorbed
and transmittance is 100% for no absorption.
Range of IR
Near IR: 0.8 to 2.5 µ m (12000 cm-1
– 4000 cm-1
) Analyzing
mixtures of aromatic amines
Determination of protein, fat, moisture, oil content.
Middle IR: 2.5 to 15 µ m (4000 cm-1
– 667 cm-1
)
Also known as vibration- rotation region.
This region is divided into:
1. Group frequency region: 4000 cm-1
– 1500 cm-1
2. Fingerprint region: 1500 cm-1
– 667 cm-1
Far IR: 15 to 1000 µ m (667 cm-1
–10 cm-1
)
Study of inorganic or organometallic compounds
Sensitive to changes in overall structure of the molecule

5. Principle of IR

6. Factors Influencing Vibrational Frequencies
The value of vibrational frequency of a bond calculated by
Hooke’s Law is not always equal to their observed value. The
force constant is changed with the electronic and steric effects
caused by other groups present in the surroundings.
Following are some important factors affecting the vibrational
frequency of a bond
Effect of Bond Order
Bond order affects the position of absorption bands. Higher the
bond order larger is the band frequency. A C-C triple bond is
stronger than a C=C bond, so a C-C triple bond has higher

7. stretching frequency than does a C=C bond. The C-C bonds
show stretching vibrations in the region from 1200-800 cm-1
but these vibrations are weak and of little value in identifying
compounds. Similarly, a C=O bond stretches at a higher
frequency than does a C-O bond and a C-N triple bond stretches
at a higher frequency than does a C=N bond which in turn
stretches at a higher frequency than does a C-N bond.
Resonance and Inductive Electronic Effects
Structural features of the molecule
 such as electron delocalization
 The electronic effect of neighbouring substituents
 Hydrogen bonding
Electron delocalization For example, the IR bands for the
carbonyl group in 2-pentanone, 2-cyclohexenone appears at
different frequencies. The 2-Cyclohexenone absorbs at a lower
frequency because the carbonyl group has less double-bond
character due to electron delocalization.

8. Inductive effect:
Electron releasing groups attached to the carbonyl group tend
to favour the polar contribution by mesomeric effect and thus
lower the bond order of the C=O bond (less double bond
character) and hence resulting in a decrease of the carbonyl
stretching frequency.
Electron withdrawing groups suppress the polar contribution
with an effective increase in the double bond character and
hence resulting in the increase of the frequency of absorption
Hydrogen Bonding
The presence of hydrogen bonding changes the position and
shape of an infrared absorption band.

9. Frequencies of both stretching as well as bending vibrations are
changed because of hydrogen bonding. The X-H stretching
bands move to lower frequency usually with increased intensity
and band widening. The X-H bending vibration usually shifts
to higher frequencies. Stronger the hydrogen bonding,
greater is the absorption shift from the normal values. The
two types of hydrogen bonding (intramolecular and
intermolecular) can be differentiated by the use of infrared
spectroscopy.
Fermi Resonance
Fermi resonance is a phenomenon which was first explained by
the Italian physicist Enrico Fermi to account for shifting of
the energies and intensities of absorption bands in an infrared
spectroscopy.
Fermi resonance was first discovered in carbon dioxide by
Fermi and it is also found in the vibrational spectra of
aldehydes,