Infrared Spectroscopy- Definition, Principle, Applications

Infrared spectroscopy is the measurement of the interaction of IR radiation with compounds. IR region involves the range between region 400-4000 cm-1. IR radiation causes the excitation of molecules from lower to higher vibrational levels. These vibrational levels are associated with closely spaced rotational levels. So IR spectra are also known as vibrational rotational spectra. The bonds accompanied by a change in dipole moment are IR active.

Infrared Spectroscopy
Infrared Spectroscopy

Requirements for Infrared Spectroscopy

  • Compounds should have a dipole moment.
  • Two compounds will never show similar IR spectra if they are not enantiomers.

Principle of Infrared Spectroscopy

When IR radiation is passed through the IR active compounds, they will get excited and show specific vibrational rotational spectra, which are characteristic to the functional group of compounds.

The frequency and wavelength of absorption relative to the mass of the atom, force constant of bonds and gemetry of atoms. The band position in IR spectra are represented as wavenumber i.e. .

= 1/λ(wavelength)

Transmittance (T) represents the bond intensities.

T= I/Io

where, I= intensity of transmitted light

Io= intensity of incident light

Types of Vibrations

There are generally two types of vibrations. They are:

Stretching vibrations

It involves the rhythmical movement along the bond axis. The interatomic distance is increasing or decreasing. It is further subdivided into symmetric and asymmetric stretching.

a. Symmetric stretching: Movement 0f atom with respect to the particular atom in same direction.

b. Asymmetric stretching: One atom approaches the central atom while the other departs from it.

Bending Vibration

In this type of vibration, the position of the atom changes with respect to the original bond axis. There are four types of bending vibrations.

a. Scissoring vibration: Two atoms approaches each other.

b. Rocking vibration: The movement of atoms takes place in the same direction.

c. wagging vibration: Two atoms move up and down the plane with respect to the central atom.

d. Twisting vibrations: In this type of vibration, one of the atoms moves up the plane while the other moves down to the plane with respect to the central atom.

The value of the stretching vibrational frequency can be calculated by the application of Hook”s law. Which is represented as:

⊽= 1/2πC √k/μ

μ= Reduced mass i.e., (m1.m2/m1+m2)

k= Force constant

Different regions of IR spectra

  1. Functional group region: It involves the region between 1500-4000 cm-1. The characteristic stretching frequency of different functional groups of compounds occurs in this region. The absence of absorption on the functional group’s specific assigned region indicates the absence of such a functional group.
  2. Fingerprint region: The region below 1500 cm-1 is called the fingerprint region. It is rich in many absorption bands and soulders caused by bending vibrations. Two compounds with the same functional group have a similar IR absorption band above 1500 cm-1, but their IR spectra differ in the fingerprint region. As a result, the fingerprint region is also used to distinguish between different compounds.

Factors affecting IR absorption

  • Inductive effect: The alkyl group (+I) effect results in bond weakening, and thus the force constant decreases. which causes a decrease in absorption wavenumber.

Formaldehyde (HCHO) = 1750 cm-1

Acetaldhyde (CH3CHO) = 1745 cm-1

Acetone (CH3COCH3) = 1715 cm-1

The effect of the electronegative group (-I) causes an increase in bond order, which raises the force constant, leading to an increase in the absorption wavenumber.

Acetone (CH3COCH3) = 1715 cm-1

Chloroacetone (CH3COCH2Cl) = 1725 cm-1

Dichloroacetone (CH3COCHCl2) = 1740 cm-1

  • Field effect: In ortho-substituted compounds, a lone pair of electrons on two atmos interact with each other in space, changing the vibrational frequencies of both groups.
  • Hydrogen bonding: Hydrogen bonding causes a remarkable downward frequency shift. The stronger the hydrogen bonding, the greater the absorption shits towards a lower wavenumber than the normal value. in dilute solution aliphatic alcohols, the sharp band appears at 3650 cm-1 due to the presence of the free -OH group, while broadband is noticed at 3350 cm-1 in concentrated solution due to the presence of hydrogen-bonded -OH group.

Applications of IR spectroscopy

  • Identification of organic compounds: The identity of organic compounds can be established from the fingerprint region. If the fingerprint region of an unknown compound exactly matches the fingerprint region of the known compounds, the identity of the compound is confirmed.
  • Analysis of functional groups: Different functional groups produce different IR absorption bands. As a result, IR spectra can be used to identify functional groups.
  • Conformational analysis: This technique is quite useful in determining the relative stability of various conformations of cyclic compounds.
  • Geometrical isomerism: It is also used for the distinction between cis and trans isomers.
  • This technique is also used to distinguish between intramolecular and intermolecular types of hydrogen bonding and to study chemical reactions.


  1. Silverstein, R. M., Webster, F. X., & Kiemle, D. J. (2005). Spectrometric identification of organic compounds. Hoboken, NJ: John Wiley & Sons.

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