The 1B2u excited state of benzene: Force field and normal coordinates

Abstract

A complete valence force field is presented for the ¹B_2u excited state of benzene. Perprotonated and perdeuterated isotopes were used in the refinement which matches known frequencies to within one half percent. New excited state vibrational frequencies from two‐photon spectroscopy enable reliable planar and out‐of‐plane force fields to be calculated. The frequencies of experimentally unobserved vibrational modes are given with the forms of the normal coordinates in the electronic excited state. These coordinates, for modes which strongly coupled the ground and excited states, are found to be mixtures of the ground state modes in accordance with the theory of vibronic coupling. The force field predicts an isotope shift upon electronic excitation which is in close agreement with experiment.