EXPERIMENTAL VIBRATIONAL SPECTRA OF POLYATOMIC MOLECULES: REGISTRATION OF IR ABSORPTION AND RAMAN SPECTRA

3N-6 gives us the number of vibrational coordinates and tells us how mnay vibrational modes exists.
NOT how many spectral bands you will actually see.
Here are the reasons that would make spectral bands more or less than 3N-6:

Symmetry resticrtions

Number of spectral bands should be lower due to symmetry resticrtions. What is symmetry in molecular systems and what does symmetry restrictions refer to?
5 types of symmetry elements are sufficient to define symmetry of any molecule concerning vibrational movement.

DISCUSS: IR ACTIVE AND RAMAN ACTIVE

Investigate the symmetric and asymmetric movements; symmetry operations and symmetry group of the molecule. We know if if dipole moment doesn't change, we have IR inactive and as some vibrations doesn't change polarizibility, meaning Raman inactive. We have to be able to read the Character table.

CHARACTER TABLE