Knowledge Bank

The relative intensities of vibronic transitions (Franck-Condon factors) can be used to determine changes in molecular geometry upon electronic excitation. Likewise, the presence or absence of the 0,0 transition and of progressions in non-totally symmetric vibrations can be used to the same end. While group theory allows the prediction of which transitions are allowed, it will be shown that cancellation of nearly equivalent terms in the vibrational overlaps greatly limits the formally allowed transitions which will be observed experimentally. The case of benzene phosphorescence is examined in detail both theoretically and experimentally. It is shown that contrary to previous $statements1$ the very weak non-totally symmetric progressions and 0,0 band for benzene are consistent with a geometrical distortion of the magnitude calculated on the basis of the dynamical Jahn-Teller $effect.2$ The strong $1600cm-1$ (non-totally) symmetric) progression seen in the phosphorescence of all methyl substituted benzenes, including 1,3,5-trimethylbenzene, is in sharp contrast to the $1000cm-1$ (symmetric) progression seen for benzene and all of its deutero-isomers. This would seem to imply a considerably larger distortion from hexagonal geometry for the methyl derivatives than for benzene.