Selection rule for an infrared active mode of vibration

Selection rule for an infrared active mode of vibration 

One of the important consequences of precisely denoting molecular symmetry is seen in infrared spectroscopy. An IR spectrum records the frequency of a molecular vibration, but not all modes of vibration of a particular molecule give rise to observable absorption bands in the IR spectrum. This is because the following selection rule must be obeyed for a vibrational mode to be IR active, it must give rise to a change in the molecular dipole moment (see Section 1.16). 

For a mode of vibration to be infrared (IR) active, it must give rise to a change in the molecular electric dipole moment. 

In the discussions of IR spectroscopy that follow, we are concerned only with fundamental absorptions, these being the dominant features of IR spectra. 

The transition from the vibrational ground state to the first excited state is the fundamental transition. 

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