The electronic structure of heteroaromatic molecules. Part 1. Six-membered rings

Abstract

Spin-coupled theory is applied to the π electrons of benzene and of the heteroaromatic molecules pyridine, pyridazine, pyrimidine, and pyrazine. A companion paper deals with five-membered rings. The spin-coupled wavefunction is very flexible and includes the familiar delocalized molecular orbital description as just one of the many special cases to which our calculations might converge. The actual solutions correspond to energies considerably lower than those afforded by molecular orbital (MO) theory and consist of localized orbitals which are clearly deformed C(2p) or N(2p) functions. This provides a clear picture of the behaviour of correlated electrons in systems of this kind. The special stability pf aromatic molecules arises from the mode of spin coupling and not from delocalized orbitals. Except for small but crucial distortions of the orbitals, there is a marked similarity to the classical valence bond (VB) descriptions of these systems. The spin-coupled representation of benzenoid heterocyclic systems incorporates a great deal of electron correlation while still retaining a clear-cut visuality. At the very least, it must now be regarded as a credible alternative to the MO-based view of these molecules and there are good reasons to reconsider our attitudes to the classical VB descriptions of these systems.