Rule of Mutual Exclusion:
The IR and Raman activities of different modes of vibration of a molecule depend on its symmetry characteristics. An analysis of the IR and Raman spectra of a large number of molecules has lead to an extremely important general rule known as the rule of mutual exclusion. It states in which, "for a molecule having a centre of symmetry the Raman active vibrations are IR inactive and vice versa". Therefore, if the molecule does not have a centre of symmetry then some vibration may be Raman as well as IR active.
The application of this rule can provide very useful structural information. For example, the fact that the Raman and IR spectra for N2O have signals at same wavenumber suggests that the molecule does not have a centre of symmetry. This information could be used to conclude in which the structure of the molecule is N - N - O and not N - O - N. Have a look at the Table again and check which of the two molecules given there has a centre of symmetry.