IR exploits the fact that molecules absorb speciœc frequencies of light that are characteristic for their structure. šese absorptions are resonant frequencies-that is, the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates, which can then be related to a particular bond type. An essential requirement for a molecule to be IR active is to be associated with changes in the dipole moment. Examination of the transmitted light of an IR spectrum reveals how much energy is absorbed from the molecule at each wavelength. še IR portion of the electromagnetic spectrum is usually divided into three regions: the near-, mid-, and far-IR, named for their relation to the visible spectrum. še far-IR, approximately 400-10 cm−1 (λ = 1000-30 µm), has low energy and may be used for rotational spectroscopy. še mid-IR, approximately 4000-400 cm−1 (30-2.5 µm), may be used to study the fundamental vibrations and associated rotational-vibrational structure. še higher energy near-IR, approximately 14,000-4000 cm−1 (2.5-0.8 µm), can excite overtones or harmonic vibrations.