Chemistry, Third edition

INFRARED SPECTROSCOPY

light, it increases its vibrational energy from the ground vibrational energy level to the

second vibrational energy level (v1). The transition involved is

HCl(E 0 ,v 0 ) + infrared photon →HCl(E 0 ,v 1 )

The v 0 →v 1 transition is the only important transition in the infrared spectra of most

diatomic molecules at room temperature. This, and the fact that diatomic molecules

have only one way of vibrating – a simple stretching of the bond between the two

atoms – explains why diatomics have only one major absorption peak in their infrared

spectrum (Fig. 20.15).

Polyatomic molecules

Molecules containing more than two atoms (polyatomic molecules) may also be

thought of as consisting of atoms connected by springs. For example, the propanone

molecule

may be represented as

wherevvvvare the ‘springs’. The atoms on either side of bonds in polyatomic

molecules may undergo stretching vibrations(Fig. 20.16(a)), but they may also

bend against the skeleton of the molecule. These are bending vibrations(Fig.

20.16(b)).

If a polyatomic molecule absorbs an infrared photon, the whole molecule (like

some microscopic jelly) vibrates with a higher amplitude. However, it is often found

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Fig. 20.15The infrared absorption peak of hydrogen chloride (HCl) at low resolution. (The gas is
dissolved in CCl 4 , but the solvent does not absorb in this wavenumber range).