Structure and Aromatic Character of Non-benzenoid Cyclically Conjugated Systems

The chemistry of aromatic compounds was originally confined to benzenoid hydrocarbons and their derivatives but has been enriched in the last decade by the discovery of numerous non-benzenoid, cyclically conjugated systems that are more or less like benzene in their properties. This has necessitated a new conception of “aromatic character”. Theoretical and experimental studies have been applied to this problem. In this connection, several selected representatives of this class of compound are described, mostly from studies by the author's own group, and relationships between structure and “aromatic character” are indicated.