Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

that only a few atoms in the molecule vibrate appreciably (i.e. suffer largecompres-

sions, extensions or bends). (Returning to the jelly analogy, although the whole jelly is

vibrating at a particular frequency, only parts of it move substantial distances during a

single vibration.)

For example, propanone molecules absorb infrared light at precisely 1715 cm–1.

As a result of this absorption, the C and O atoms of the C=O group undergo a

stretching motion at a higher amplitude. The motion of the other atoms in the

molecule is largely unaffected by photon absorption, i.e. the vibration of the molecule

is more or less localized around the carbonyl group. However, if propanone

molecules absorb infrared light at exactly 2950 cm–1, the C–H stretching motion

becomes more pronounced, and the rest of the molecule – including the C=O group –

moves very little.

An infrared spectrometer exposes the molecule under investigation to a range of

infrared light of differing wavenumbers during a ‘scan’. Peaks in the infrared spec-

trum occur at frequencies at which the different groups of atoms within the molecule

become ‘activated’ at their higher amplitudes. This is evident from the spectra in Figs

20.17–20.25, where the vibrations associated with key peaks are labelled.

A simple pattern is often observed in infrared spectra in that molecules containing

the same groups of atoms give rise to peaks which are roughly at the same wavenumber

382

Fig. 20.16(a) Stretching vibration and (b) bending vibration of the NH2 group.

Fig. 20.17Infrared spectrum of propanone CH 3 COCH 3.
(The peak marked is due to CO 2 in the spectrometer.)

Fig. 20.18Infrared spectrum of ethanal CH 3 CHO.