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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.
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.
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.
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