Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

If a watch glass of volatile solvent (e.g. ethoxyethane or methanol) is placed below

the path of the infrared light, the intensity of detected infrared light falls (Fig.

20.13(b)). This reduction is caused by invisible solvent molecules in the vapour

absorbing some of the infrared light. It is customary to use wavenumber instead of

wavelength in infrared spectroscopy (Box 20.1).

‘Spring model’ of molecules

A diatomic molecule, such as HCl, can be thought of as two tiny masses (the H and

Cl atoms) connected by a microscopic spring (the covalent bond) (Fig. 20.14).

The vibration of a diatomic molecule involves the compression and extension of its

covalent bond. The number of ‘to and fro’ movements per second is called the vibra-

tional frequencyof the molecule. The greater is the vibrational energy of a

molecule, the higher is the amplitude of its vibration i.e. the greater is the distance

travelled by the atoms during the vibration.

All molecules consist of atoms in continuous vibration – even at 0 K. Infrared

absorption spectra are produced when molecules increase their vibrational energy by

absorbing infrared light. The electronicenergy of molecules does not change in these

transitions (see Fig. 20.4). When a hydrogen chloride molecule absorbs infrared

380

Fig. 20.13Demonstration of infrared absorption. Vapour from the solvent absorbs some of the

infrared light so that the intensity of light recorded by the detector falls.

BOX 20.1

Wavenumbers

In the infrared region, chemists depart from

the practice of plotting absorbance against

frequency or wavelength. Instead, spectra

usually consist of absorbance values

plotted against wavenumber. This gives

more conveniently sized values. The

wavenumber, symbolized  ̄, is the number

of complete wavelengths per centimetre.

This means that it is connected to

wavelength by the relationship

1

 ̄ = —



When is expressed in centimetres,  ̄ is in

cm–1– its usual units.

Wavenumbers are directly proportional to

the frequency (in Hz):

 ̄ (cm–1) = 3.333 × 10 –11×

Rearrangement of this equation shows that

1cm–1is equivalent to light of frequency

3.000× 1010 Hz. A high frequency means

a high wavenumber. In fact, some chemists

in conversation speak of frequency when

they mean wavenumber! Although incorrect,

this serves to remind us that only a simple

conversion factor separates the two

quantities.

Fig. 20.14The vibration of a
hydrogen chloride molecule is
caused by the stretching of its
chemical bond. The molecule
vibrates about 10^14 times a
second. Absorption of a
photon increases the
amplitude of the vibration, but
does not alter its frequency.