[Twickel]

Hi
How do I know if a molecule will be raman active. I am confused with the following.

A molecule is raman active if its polarizability changes. Now Atkins physical chemistry says " wheras the symmetric stretch changes the size of the molecule neither the asymmetric stretch nor the bending modes do"

My lectures notes also say bends are not active.
 But for one of the questions I have, it says that the asymmetric stretch of C2H2 is raman active but its symmetric bend does not.
Why is that?

[dipesh747]

The way I would use to see if a small molecule is raman active or not is to use group theory.

Work out what point group the molecule is in and then look in a table (which you will probably find in atkins, if not just google it) and see if on the table if it is raman or IR active.