A molecule with N atoms, has 3N degrees of freedom, of which 3N-6 are vibrations .
Some vibrations are skeletal , i.e. they involve almost the whole molecule.
Other vibrations are more or less localized, confined to a small number of atoms of a functional group. Below we use methylamine as an example, showing only the six vibrations involving the hydrogens of the amino group. Note how in some of them the methyl group participates as well, in particular the low frequency ones. Rocking IR . Because of the high symmetry, D6h, the picture is not necessarily more complicated.