Double-bond fixation in tricyclopenta[cd,f,h]azulene and its higher homologues

Abstract

The C–C bond distortions in tricyclopenta[cd,f,h]azulene and its higher homologues have been examined by using the symmetry rule and the semiempirical Pariser–Parr–Pople (PPP)-type SCF MO method. In tricyclopenta[cd,f,h]azulene and tricyclohepta[a,c,ef]heptalene, it is predicted that a molecular-symmetry reduction (C_2v→C_s) accompanied by a marked double-bond fixation should occur in the carbon skeleton. In contrast, other homologues are expected to show a moderate bond fixation, retaining the full molecular symmetry group. This difference is explained in terms of a pseudo-Jahn–Teller effect.