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For example, it is possible to distinguish between C–H vibrations in methylene (–CH 2 –)

and methyl groups (–CH 3 ).

Raman spectroscopy

The assignment of peaks in Raman spectra usually requires consideration of peak

position, intensity and form, as well asdepolarisation. This allows identification of

the type ofsymmetry of individual vibrations, but not the determination of structural

elements of a molecule. The depolarisation is calculated as the ratio of two intensities

with perpendicular and parallel polarisation with respect to the incident beam. The use

of lasers as light source for Raman spectroscopy easily facilitates the use of linearly

polarised light. Practically, the Raman spectrum is measured twice. In the second

measurement, the polarisation plane of the incident beam is rotated by 90.

Phenacetin

1610 s

H 3 C CN

OH

1660 s 3280 s 3050 s ( C H) 1240 s

O CH 3

2980 s ( C H)

CH 2

740 b (ring)

4000

70

60

50

40

30

20

10

0

3500 3000 2500 2000 1500 1000

Wavenumber (cm–1)

Transmittance (%)

500

Fig. 13.3FT–IR spectrum of phenacetin, the historically first synthetic fever reducer to go on the market.

Bands at the appropriate wavenumbers (cm^1 ) are shown, indicating the bonds with which they are associated,

and the type (s, stretching; b, bending).

525 13.2 Infrared and Raman spectroscopy