The driving force for Π-bond localization and bond alternation in trisannelated benzenes

To investigate the factors that may cause bond length alternation and π-bond localization in annelated benzenes, ab initio valence bond calculations were performed. The results reveal that the bond length alternation of annelated benzene is determined by the strain-induced hybridization change from the partially optimized geometries, in which the central benzene ring is constrained to a regular hexagon, to the equilibrium geometries rather than the previously recognized re-hybridization effect that the carbon atoms in the central ring are deviated from sp² hybridization. Meanwhile, the π-π interaction also provides a sort of driving force, which facilitates bond length alternation, which in turn magnifies π-bond localization. A subsequent potential energy curve study shows that σ-strain and π-π interactions have different mechanisms for the effect.