Modern valence-bond description of the electronic structure of benzocyclobutadiene

The ground state of benzocyclobutadiene, a bicyclic molecule with 8 pi electrons containing a benzene and a cyclobutadiene ring, is studied by means of modem valence bond (VB) theory in its spin-coupled (SC) form and the complete-active-space self-consistent field (GAS SCF) approach. The CAS SCF wave function is used to optimize the geometry, and SC theory-to obtain a well-correlated and easy to visualize and understand model of the active space hosting the pi electrons. It is shown that the pi system of the ground state of benzocyclobutadiene can be described with sufficient accuracy as a combination of the pi systems of a distorted benzene ring and an isolated double bond. Each of the eight SC orbitals is found to be well-localized about one carbon atom only, with small distortions toward its nearest neighbors. The analysis of the optimal active-space spin-coupling pattern within the SC wave function for benzocyclobutadiene shows unambiguously that this molecule inherits neither the aromatic nor the antiaromatic character of its cyclic components, and it is most appropriate to regard it as nonaromatic. (C) 1996 John Wiley & Sons, Inc.