Molecular vibrations and IR spectroscopy of ethylene

For a non-linear molecule with N atoms, there are 3N-6 molecular vibrations

For ethylene N=6 therefore there are 12 vibrations

 

There are 4 C-H stretching modes (this is because there are 4 C-H bonds)

These modes are the highest frequency vibrations for ethylene

There are 4 C-H stretching modes. Use the D2h character table to determine IR and Raman activity

 

symmetry label of C-H mode   IR active   Raman active
Ag   No x2, y2, z2 Yes
B3g   No yz Yes
B1u z Yes   No
B2u x Yes   No

This is an illustration of the Exclusion Rule:

If a molecule has a centre of symmetry (inversion centre) then no normal mode can be active in both IR and Raman.