that a very small proportion of light scattered by the sample will have a different
frequency than the incident light. As different vibrational states are excited, energy
portions will be missing, thus giving rise to peaks at lower frequencies than the
incident light (Stokes lines). Notably, higher frequencies are also observed (anti-
Stokes lines); these arise from excited molecules returning to ground state. The
emitted energy is dumped onto the incident light which results in scattered light of
higher energy than the incident light.
The criterion for a band to appear in the infrared spectrum is that the transition
to the excited state is accompanied by a change indipole moment, i.e. a change in
charge displacement. Conversely, the criterion for a peak to appear in the Raman
spectrum is a change inpolarisabilityof the molecule during the transition.Infrared spectroscopy
The fundamental frequencies observed are characteristic of thefunctional groups
concerned, hence the termfingerprint. Figure 13.3 shows the major bands of an FT–IR
spectrum of the drug phenacetin. As the number of functional groups increases in
more complex molecules, the absorption bands become more difficult to assign.
However, groups of certain bands regularly appear near the same wavelength and
may be assigned to specific functional groups. Such group frequencies are thus
extremely helpful in structural diagnosis. A more detailed analysis of the structure
of a molecule is possible, because the wavenumber associated with a particular
functional group varies slightly, owing to the influence of the molecular environment.OCOModeStretching, symmetricStretching, asymmetricDeformationODeformationCOOCOOCOWavenumber1340 cm–12349 cm–1667 cm–1667 cm–1IR–
+
+
+
Raman+
–
–
–
Fig. 13.2Normal vibrational modes for CO 2. For symmetric molecules that possess a centre of symmetry,
bands that appear in the IR do not appear in the Raman spectrum.524 Spectroscopic techniques: II Structure and interactions