Selection rules for an infrared or Raman active mode of vibration

Selection rules for an infrared or Raman active mode of vibration 

One of the important consequences of precisely denoting molecular symmetry is seen in infrared and Raman spectroscopy. For example, an IR spectrum records the frequency of a molecular vibration, i.e. bond stretching and molecular deformation (e.g. bending) modes. However, not all modes of vibration of a particular molecule give rise to observable 

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

A different selection rule apphes to Raman spectroscopy. For a vibrational mode to be Raman active, the polarizability of the molecule must change during the vibration. Polarizability is the ease with which the electron cloud associated with the molecule is distorted. 

A different selection rule applies to Raman spectroscopy  

---