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13.2 Infrared and Raman spectroscopy

13.2.1 Principles

Within the electromagnetic spectrum (Fig. 12.1), the energy range below the UV/Vis

is the infrared region, encompassing the wavelength range of about 700 nm to 25mm,

and thus reaching from the red end of the visible to the microwave region. The absorption

of infrared light by a molecule results in transition to higher levels ofvibration

(Fig. 12.3).

For the purpose of this discussion, the bonds between atoms can be considered

as flexible springs, illustrating the constant vibrational motion within a molecule

(Fig. 13.1). Bond vibrations can thus be eitherstretchingorbending(deformation)

actions. Theory predicts that a molecule with n atoms will have a total of 3n 6

fundamental vibrations (3n5, if the molecule is linear): 2n5 bending, andn 1

stretching modes (Fig. 13.2).

Infrared and Raman spectroscopy give similar information about a molecule, but

the criteria for the phenomena to occur are different for each type. For asymmetric

molecules, incident infrared light will give rise to an absorption band in the infrared

spectrum, as well as a peak in the Raman spectrum. However, as shown in Fig. 13.2,

symmetric molecules, such as for example CO 2 , that possess a centre of symmetry

show a selective behaviour: bands that appear in the infrared spectrum do not appear

in the Raman spectrum, and vice versa.

Aninfrared spectrumarises from the fact that a molecule absorbs incident light

of a certain wavelength which will then be ‘missing’ from the transmitted light.

The recorded spectrum will show an absorption band.

ARaman spectrumarises from the analysis of scattered light, and we have already

introduced the basics of inelastic light scattering in Section 12.6.3. The largest part

of an incident light beam passes through the sample (transmission). A small part is

scattered isotropically, i.e. uniformly in all directions (Rayleigh scatter), and possesses

the same wavelength as the incident beam. The Raman spectrum arises from the fact

C

H H

H C

H O

Fig. 13.1Possible stretching vibrations in acetaldehyde.

523 13.2 Infrared and Raman spectroscopy