Irreducible Representation and IR stretching bands

[remeday86]

After you find the irreducible representation of a molecule, how do you determine the number of IR active bands?

For example, in methane, the pointgroup is T_d , then you found that the irreducible representation is A₁ + T₂.

How do you find the number of IR active and Raman active bands?

Thanks

Btw, a friend of mine said there are 2 IR active bands but I am not sure how to figure that out. Also, I know that T is triple degenerate and A is nondegenerate. But not sure how that relates to the number of IR active bands.

 

[DrDu]

Methane is composed of N=5 atoms so there are 3N-6=9 different vibrations. A1+T2 makes up for only 4 of them.