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.

Differences between IR and Raman methods

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