Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.
Rotational transitions are of little use to the spectroscopist.
Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collisions and other interactions. Vibrations fall into the two main catagories of stretching and bending . Change in angle between two bonds. You should now understand how certain molecules absorb infra red radiation, and the effects that this absorption has.