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Comparison of Raman and IR Spectroscopy

By
Dr. Mehboob Peeran

Reviewed By
Dr. K.G. Srinivasamurthy

Mutual exclusion principle

In a molecule with a center of symmetry it is seen that vibrations that are Raman active are IR inactive and vice-versa, this is called the Principle of mutual exclusion (eg, as in CO2 see details in the end). In molecules with different elements of symmetry, certain bands may be active in IR, Raman, both or neither. For a complex molecule that has no symmetry except identity element, all of the normal modes are active in both IR and Raman. This does not; however mean that they can be observed. In both types the neighbouring strong bands may obscure weak bands, while others may be intrinsically too weak to be observed even if they are theoretically “allowed”.

In general the strong bands in the IR spectrum of a compound corresponds to weak bands in the Raman and vice versa. This complimentary nature is due to the electrical characteristic of the vibration. If a bond is strongly polarised, a small change in its length such as that occurs during a vibration, will have only a small additional effect on polarisation. Vibrations involving polar bonds ( C-O , N-O , O-H ) are therefore, comparatively weak Raman scatterers. Such polarised bonds, however, carry their charges during the vibrational motion, ( unless neutralised by symmetry factors), which results in a large net dipole moment change and produce strong IR absorption band. Conversely, relatively neutral bonds ( C-C , C-H , C=C ,) suffer large changes in polarisability during a vibration, though this is less easy to visualise. But the dipole moment is not similarly affected and vibrations that predominantly involve this type of bond are strong Raman scatterers but weak in the IR.

Mutual exclusion principle as seen in CO2

In molecules having inversion center, none of the normal modes of vibrations will be both Raman and IR active. This is known as “mutual exclusion principle”. A simple molecule which obeys this principle is CO2. Carbondioxide has an inversion center or center of symmetry. The following are its normal modes of vibrations. The IR and Raman active modes are indicated below each type of vibration.

Mutual exclusion Principle in Carbondioxide

Differences between IR and Raman methods

 

Raman IR

1

It is due to the scattering of light by the vibrating molecules.

It is the result of absorption of light by vibrating molecules.

2

The vibration is Raman active if it causes a change in polarisability.

The vibration is IR active if there is a change in dipole moment during the vibration.

3

The molecule need not possess a permanent dipole moment.

The vibration concerned should have a
change in dipole moment due to that vibration.

4

Water can be used as a solvent.

Water cannot be used due to its intense absorption.

5

Sample preparation is not very elaborate sample can be almost in any state.

Sample preparation is elaborate
Gaseous samples can rarely be used.

6

Gives an indication of covalent character in the molecule.

Gives an indication of ionic character in the molecule.

7

Cost of instrumentation is very high

Comparatively inexpensive.

References:

1. Physical methods of Chemistry : Drago
2. Instrumental methods of analysis : Willard
3. IR and Raman spectra of Inorganic and coordination compounds : Nakamoto
4. Instrumental analysis for Science and Technology : W. Ferren