GEOMETRICAL AND ELECTRONIC FEATURES OF THE BENZENE-RING IN BENZOCYCLOALKENES AND RELATED-COMPOUNDS

The fusion of a small, strained ring on a benzene molecule causes remarkable perturbations on the geometry of the latter molecule, perturbations which reduce with increasing number of atoms in the fused ring and become negligible as fusion involves rings larger than five-membered ones. The Cambridge structural database was searched for the experimental geometrical features of compounds containing from three- to ten-membered cycloalkene rings condensed with benzene. MO ab initio calculations at the 3-21 G level were employed for studying the calculated features, i.e. geometries, total energies, enthalpies of formation, strain energies and pi-electron distribution, in compounds containing cycloalkenes (from three- to five-membered rings) and related rings with further unsaturation, and in systems related to benzocyclopropene with a heteroatom (oxygen) or a positively charged carbon. The effect of the additional strain, generated in the molecule by the fusion of the two rings, on molecular geometry and pi-electron distribution is discussed. Strain, which is highest when a three-membered ring is annellated to the benzene molecule, seems to be the main factor responsible for the geometrical perturbations of these molecules, whereas the pi-electron distribution is mainly determined by interactions between the electrons of the atoms or groups at the functions of the annellated ring and those of the benzene pi-orbitals.