<P> The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near -, mid - and far - infrared, named for their relation to the visible spectrum . The higher - energy near - IR, approximately 14000--4000 cm (0.8--2.5 μm wavelength) can excite overtone or harmonic vibrations . The mid-infrared, approximately 4000--400 cm (2.5--25 μm) may be used to study the fundamental vibrations and associated rotational - vibrational structure . The far - infrared, approximately 400--10 cm (25--1000 μm), 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 . </P> <P> Infrared spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure . These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency . The energies are affected by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling . </P> <P> In particular, in the Born--Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are associated with the normal modes corresponding to the molecular electronic ground state potential energy surface . The resonant frequencies are also related to the strength of the bond and the mass of the atoms at either end of it . Thus, the frequency of the vibrations are associated with a particular normal mode of motion and a particular bond type . </P> <P> In order for a vibrational mode in a sample to be "IR active", it must be associated with changes in the dipole moment . A permanent dipole is not necessary, as the rule requires only a change in dipole moment . </P>

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