Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck-Condon region have been theoretically examined by means of ab initio complete active space self-consistent field (CASSCF) and the second order multireference Møller-Plesset calculations. The optimized structure of styrene in S(0) is planar but the torsional motion of the phenyl group is very floppy. The S(1) state is assigned to the local pi-pi* excitation within the benzene ring. On the other hand, S(2), above S(1) by 0.561 eV, is assigned to a state that resembles the so-called V-state of ethylene. The transition intensity of S(0)-S(1) is weak, while that of S(0)-S(2) is strong. This is in good agreement with the experimental absorption spectrum where the S(0)-S(1) and S(0)-S(2) transitions are in the energy range of 290-220 nm. The optimized geometry of S(1), characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S(1) state.