Abstract
From the structural-chemical point of view, the ‘non-benzenoid aromatic compounds’ are reviewed. Hückel's (4n+2) rule has been proved by the X-ray analysis of several annulenes. It was pointed out, however, that resonance energies of non-alternant hydrocarbons are overestimated. Structural studies on the conjugated odd-membered ring systems have shown that these systems are very sensitive to various kinds of interactions, electronic, steric and intermolecular hydrogen bonding effects. The characteristic chemical and physical behaviours can be reasonably related to the structures.
