Origins of the Exalted b_2u Frequency in the First Excited State of Benzene

Abstract

A mechanism for the frequency exaltation of the b 2u mode in the 1 B 2u excited state of benzene is presented and supported by quantitative modeling (by means of both full π-CI as well as ab initio VB calculations of benzene). It is shown that the two Kekulé structures of benzene undergo avoided crossing along the b 2u coordinate. The memory of the avoided crossing of the π-Kekulé 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 Kekulé-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 1 B 2u excited state, and the state selectivity of this exaltation. The Kekulé-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.