Ab initio SCF–M.O.–CI description of the lowest excited states of planar and twisted styrene by an extended π orbital basis

Abstract

The results of two different ab initio CI descriptions of planar and twisted styrene have been compared, the distinction lying in the basis used for π orbitals, which was minimal in one case and extended in the other, while minimal basis orbitals were used throughout for the σ skeleton. The need for a more accurate description seems to arise in the vicinity of the perpendicular geometry, where the zwitterionic states relevant in that region are strongly stabilized with respect to the diradical-type states by the presence of diffuse orbitals. In particular, the state corresponding to electron migration from methylene to benzyl becomes the lowest excited state at the twisted geometry in the extended basis description. The theoretical results are discussed in the light of current experimental knowledge of the system.