Alternative prohibition rule

In the case of simple molecules, the question of whether a particular vibration is active in the IR spectrum can be answered by considering the forms of the normal modes [49-60, 66]. It can be seen in Fig. 1.4 that the dipole moment changes under all the active vibrations of the H2O molecule. In contrast, the vibrations of homopolar molecules such as H2 and N2 do not produce a dipole moment and thus are inactive in the IR spectrum. When a polyatomic molecule contains a center of symmetry, the vibrations synunetrical about this center are active in the Raman spectrum but inactive in the IR spectrum of this molecule, and vice versa. This result is known as the alternative prohibition rule. In general, the activity of the excitation in the IR spectrum cannot be predicted from such a qualitative analysis, but rather must be determined using group theory [51-54, 62]. 

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