the mass of the atoms/groups that are bonded

vibrations between light atoms have the highest frequency e.g. bonds to H

bond strength

double bonds (e.g. C=O)vibrate at a higher frequency than single bonds between the same pair of atoms

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

For phenylalanine N=23 therefore there are 72 vibrations