Fully benzenoid systems revisited

Abstract

Fully benzenoid hydrocarbons are benzenoid hydrocarbons in which benzene rings of a polycyclic benzenoid system are either assigned to isolated π sextets or are shown as “empty”. For a number of polycyclic benzenoid hydrocarbons we examined Kekule valence structures in order to characterize those that contribute to Clar's valence formulae depicting π sextets. We found that a particular weighting approach to Kekulé valence structures can result in a structural formula which in a number of benzenoids coincides with their Clar structure. The weights are determined by the smallest Pauling bond order that occurs for CC double bonds appearing in a Kekule valence structure. All valence structures which have maximal weights upon superposition produce a molecular structural formula which in some fully benzenoid systems coincides with Clar's representation but in others suggests a structure with fewer π sextets, suggesting, so to speak, less “perfect” fully benzenoid systems.