Compute the number of normal modes of vibration:

Problem:

Compute the number of normal modes of vibration for benzene.

 Draw the vibrational modes  of CO₂   molecule  using their given figureand  comment  on their IR activity.

Answer:

Since benzene is not a linear molecule and has twelve atoms in it, the number of vibrational modes will be [ (3 × 12)  - 5 = 31].

The vibration modes of carbon dioxide can be drawn as follows:

Out of these all the modes other than the symmetric stretching mode will be IR active.