Bonding in benzodicyclobutadiene isomers: insights from modern valence bond theory

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 I is the most aromatic, followed by benzo[1,2:4,5]dicyclobutadiene isomer II , whereas benzo[1,2:3,4]dicyclobutadiene is close to being a non-aromatic conjugated system.