Bonding in benzodicyclobutadiene isomers: insights from modern valence bond theory

Spin-coupled (SC) theory, which uses a compact and easy-to-interpret wavefunction that is comparable in quality to a complete active space self-consistent field construction, is used to obtain modern <i>ab initio</i> valence bond descriptions of the π‑electron systems of three benzodicyclobutadiene isomers. The geometries at the two energy minima for benzo[1,2:4,5]dicyclobutadiene (<b>I</b> and <b>II</b>) differ mostly in the lengths of the annulated CC bonds (<i>ca</i>. 1.41 and 1.55 Å, respectively). Analysis of the π-space SC wavefunctions indicate that the electronic structure of isomer <b>I</b> resembles a fairly standard benzene ring augmented with two exocyclic double bonds; its resonance pattern is dominated by two Kekulé-like and three less-important Dewar-like structures. Contrary to assumptions made in previous work, the resonance pattern for isomer <b>II</b> features two para-bonded structures which are at least as important as any of the Kekulé-like structures. In the case of benzo[1,2:3,4]dicyclobutadiene, analysis of the π-space SC wavefunction shows that the expected Kekulé-like structure is indeed the most important, but this system provides further evidence that reliance on chemical intuition can sometimes lead to erroneous predictions about the relative importance of resonance structures; certain para-bonded structures turn out to be more important than any of the other four Kekulé-like structures. The SC calculations suggest that benzo[1,2:4,5]dicyclobutadiene isomer <b>I</b> is the most aromatic, followed by benzo[1,2:4,5]dicyclobutadiene isomer <b>II</b>, whereas benzo[1,2:3,4]dicyclobutadiene is close to being a non-aromatic conjugated system.