Maximum Overlap Study of Some Bridged Annulenes

Abstract

The electronic and geometrical structures of 9,10-methano[10]annulene, 11,11-dimethyl-9,10 methano[10]annulene and 9,10-ethano-9,10 dihydronaphthalene are studied by the maximum overlap method. It is found that the first two molecules have bisnorcaradiene structures possessing very weak central bonds described by sp7 hybridisation states. Bending of these hybrid orbitals is highly pronounced indicating that the most probable place for electrophilic attack is the central bond on the side opposite from the bridge. Their overlapping is, however, by no means negligible concomitant with the experimental NMR evidence which suggests the presence of a direct bond between the bridgehead atoms. The calculated bond angles are in good accordance with available experimental data. The present results provide rationalization of the unusual dihedral angle of 136° found in 9,10-methano[10]annulene by X-ray measurements. Thus the current opinion that bridged annulenes tend to achieve a planar configuration of the olefinic carbon atoms in order to maximize conjugation between the rings should be revised.