Origins of the exalted b(2u) frequency in the first excited state of benzene

A mechanism for the frequency exaltation of the b(2u) mode in the B-1(2u) excited state of benzene is presented and supported by quantitative modeling (by means of both full pi-CI as well as ab initio VB calculations of benzene). It is shown that the two Kekule structures of benzene undergo avoided crossing along the b(2u) coordinate. The memory of the avoided crossing of the pi-Kekule forms dictates that the b(2u) mode of the ground state would undergo shortening of the double bonds and lengthening of the single bonds, while at the same time the b(2u) mode of the excited state would stretch the double bonds and shorten the single bonds. As such, the Kekule-avoided crossing model provides a single mechanism that accounts for the origins of the low skeletal b(2u) frequency in the ground state, its mode mixing (Duschinsky mixing) with the hydrogen b(2u) rocking vibration, its frequency exaltation in the B-1(2u) excited state, and the state selectivity of this exaltation. The Kekule-avoided crossing model is suggested as a general mechanism for frequency exaltation of the localizing modes in those excited states of delocalized molecules that are the out-of-phase counterparts of the corresponding ground states.