Instant Notes: Analytical Chemistry

groups. With long chains, a weak band around 750 cm-^1 is due to rocking of the

(–CH 2 – )ngroups. A similar spectrum is observed with polyethylene film.

If the compound has no methyl groups, for example cyclohexane, the

spectrum is simplified, as only the methylene absorptions are observed. If

hydrogens are substituted by halogens, peaks corresponding to the halogen

occur at about 700 cm-^1 for –C–Cl, 600 cm-^1 for –C–Br or 500 cm-^1 for –C–I.

Example 2. Aromatic compounds, especially those derived from benzene, show

bands characteristic of their structure and substitution. A thin film of poly-

styrene, (C 6 H 5 – CH–CH 2 )nshown in Figure 2 is often used for calibrating the

wavenumber scale of an IR spectrometer.

E11 – Infrared and Raman spectrometry: applications 243

4000 3000 2000

cm–1

1500 1000 500

% T

Fig. 2. Polystyrene film.

The aromatic –CH–stretch occurs just above 3000 cm-^1 and the aliphatic just

below.

The weak absorptions due to overtones in the 2000–1800 cm-^1 region are

typical of a monosubstituted benzene, as are the bands in the 800–600 cm-^1

region due to out-of-plane (OOP) bending of the C–H bonds. Ring vibrations

give bands between 1600 and 1500 cm-^1 whereas the rest of the spectrum can be

regarded as a “fingerprint” of the entire structure.

A particularly important peak in an IR spectrum is the carbonyl, >C=O

absorption around 1700 cm-^1. It is nearly always a very strong peak. Figure 3

4000 3000 2000

cm–1

1500 1000 500

% T

Fig. 3. Tribehenin: (triglyceride ester of C 22 carboxylic acid).