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Origins of the Exalted b 2u Frequency in the First Excited State of Benzene

1996; American Chemical Society; Volume: 118; Issue: 3 Linguagem: Inglês

10.1021/ja952376w

1943-2984

Sason Shaik, Avital Shurki, David Danovich, Philippe C. Hiberty,

Advanced Chemical Physics Studies

A mechanism for the frequency exaltation of the b2u mode in the 1B2u 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 b2u coordinate. The memory of the avoided crossing of the π-Kekulé forms dictates that the b2u mode of the ground state would undergo shortening of the double bonds and lengthening of the single bonds, while at the same time the b2u 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 b2u frequency in the ground state, its mode mixing (Duschinsky mixing) with the hydrogen b2u rocking vibration, its frequency exaltation in the 1B2u 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.