8: Selected Modes in Vibrational Spectroscopy

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In the case of complex molecules, the vibrational spectra can appear as a "forest" of overlapping peaks. Sometimes it is impractical to interpret all of the vibrational peaks for a molecule. In some cases, vibrational signatures from particular functional groups are more useful because they are distinguished by their intensity and energy in a vibrational spectrum. One classic example are the vibrational stretches from carbonyl (C=O) functional groups, which give intense vibrational bands in the IR spectrum that are clearly resolved from other vibrations in organic molecules. In the case that interpretation of vibrational spectra can be limited to selected vibrational modes, like that of the C=O stretch, the application of symmetry and group theory can be focused on just these modes.

8.1: Application of symmetry to carbonyl vibrational modes