By Sunil Bhardwaj

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If molecule has a center of symmetry then Raman active vibrations are IR Inactive and vice versa. e.g. CO2. For CO2 none of the vibrations are simultaneously active in both IR and Infra red. CO2 has centre of symmetry.

Polarisibility in a molecule

Centre of symmetry —> Present

Raman vibrations —> active

IR vibrations —> Inactive

Or

Raman vibrations —> Inactive

IR vibrations —> active

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. Carbon dioxide has an inversion center or center of symmetry

If molecule has no Center of symmetry then some but not necessarily all vibrations may be both IR and Raman active. e.g H2O.

Polarisibility in a molecule

The converse of the rule is also true i.e. if for a molecule both IR and Raman spectra do not have common lines, and then molecule has a center of symmetry.