From experiment,the infrared portion of the electromagnetic spectrum is usually divided into three regions infrared named for their relation to the visible spectrum. The higher-energy near-IR can excite overtone or harmonic vibrations. The mid-infrared may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The names and classifications of these subregions are conventions, and are only loosely based on the relative molecular or electromagnetic properties.
Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms.