Benzene is the most common example of an aromatic molecule. It has the features that represent the entire class of such molecules. The pi-pi* transisions in all aromatic molecules come in pairs, one strong and allowed and one weak and "forbidden". As we have said numerous times, no transition is really forbidden in quantum mechanics. But, the weak bands are qualitatively different than the strong bands and this is the main point. The strong bands are allowed by a Franck-Condon mechanism that involves coupling to totally symmetric modes. The weak bands are forbidden and involve state mixing promoted by non-totally symmetric modes.
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a href="pdf/vibronic_benzene.pdf">Vibronic Coupling in Benzene PDF
