Modern Valence-Bond Description of Homoaromaticity

Spin-coupled (SC) theory is used to obtain modern valence-bond descriptions of the electronic structures of local minimum and transition-state geometries of three species that have been considered to exhibit homoconjugation and homoaromaticity: the homotropenylium ion, C₈H₉⁺, the cycloheptatriene neutral ring, C₇H₈, and the 1,3-bishomotropenylium ion, C₉H₁₁⁺. The resulting compact SC wave functions are of comparable quality to complete-active-space self-consistent field constructions that are based on the same "N electrons in M orbitals" active spaces, but they are much easier to interpret directly. Analysis of the forms of the SC orbitals and of the overlaps between them, as well as an examination of the compositions of the associated resonance patterns, strongly suggest that both of the homotropenylium and 1,3-bishomotropenylium ions are homoaromatic at their local minimum geometries, with all of the other cases that were considered being nonaromatic. The SC results also show that the differences between "no-bond" and "bond" homoconjugated systems are very likely to be much smaller than previously thought.