Kekule benzene vibration

One might have thought that the state- and mode-specificity of the frequency exaltation is dictated by the symmetry of the states and vibrational mode. Scheme 35 shows, however, that benzene derivatives where the DSh symmetry is clearly absent exhibit this frequency upshift of Kekule mode.3 234 235 Moreover, in all these systems, no other state and no other mode... 

For benzene several isomers are possible, most of them suggested in the last century as possible structures for benzene before the Kekule model was proposed and accepted. Prismane, Dewar benzene, and benzvalene (see Fig. 5) all might be formed from excited benzene molecules provided proper modes of vibration were... 

Figure 7.6c shows that the vibrational mode b2u interchanges the Kekule structures K and Kr. Since the excited state is made only from these Kekule structures, then by analogy to benzene, one will expect that the frequency of this mode will be larger in the B2u excited state than in the Ag ground state, which is indeed what is observed by two-photon spectroscopy (16,24). Another analogy... 

---