be some differences, the major peaks are in similar positions, and some groups,
such as substituted alkynes R–CC–R¢and –S–S–, give stronger peaks in the
Raman than in the IR spectrum.
Further examples are given in Topic F2.
One technique which may be used for the identification of materials such as
drugs or polymers is a “decision tree”. For example, in considering the spec-
trum of a polymer shown in Figure7, the first decision might be based upon the
presence of a carbonyl band near 1730 cm-^1. In this spectrum, such a band is
observed, and the decisions are simplified by using the chart shown inFigure 8.
From the above chart, the sample is readily identified as polymethyl-
methacrylate as shown by the heavier lines inFigure 8. Similar charts are avail-
able for the polymers not possessing a carbonyl band and for other classes of
compounds.
Computerized analysishas enhanced the possibilities for identifying samples
by IR spectrometry. The spectrum is analysed to identify the peaks, which may
then be listed. This is much easier and more accurate than visual examination of
the hard copy spectrum.
Two procedures are then followed, in much the same way that the scheme in
Figure 8uses. Peaks, or groups of peaks associated with a particular structure
are assigned. Thus, a peak at 1750 cm-^1 would strongly indicate a carbonyl
compound, and peaks at 2950–2800 cm-^1 would probably belong to –CH
stretching of an aliphatic or alicyclic compound. This approach yields a list of
probable structural units (PSU).E11 – Infrared and Raman spectrometry: applications 245
4000 3000 2000
Raman shift (cm–1)1500 1000 500IntensityFig. 6. Raman spectrum of methyl benzoate (liquid).Fig. 7. Polymethylmethacrylate.4000 3000 2000
cm–11500 1000 500% T