Ab Initio MCSCF Study on Electronically Excited Singlet States of Fulvalene Systems: Energy Component Analysis of the Pseudo-Jahn−Teller Effect

Abstract

Geometry optimizations of the lowest (second) excited singlet states of the fulvalene systems were carried out by using the ab initio MCSCF method with the 6-31G(d) basis set. The electronically excited molecules are found to undergo a pseudo-Jahn−Teller (JT) distortion from D2h to C2v. On the basis of the results obtained, an energy component analysis of the total energy has been carried out to elucidate the physical picture of the pseudo-JT effect. Inspection of the energy components comprised in the total energy reveals that the stability of a less symmetric structure (C2v) is attributable commonly to the energy lowerings of the internuclear repulsion term and the interelectronic repulsive and kinetic terms due to σ electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT bond distortion. It is further found that in triafulvalene and heptafulvalene the nuclear−electron attractive and interelectronic repulsive terms due to π electrons contribut...