Geometries of three-membered rings in triangulanes and spirocyclopropanated bicyclopropylidenes: experimental studies and a general bond increment scheme

Abstract

Crystal structural studies of several linear and branched triangulanes, together with spiro-cyclopropanated bicyclopropylidenes, by X-ray diffraction reveal significant differences between proximal and distal bond lengths in the three-membered rings. Combined with previously published structural data, these results allowed us to elaborate a general additivity scheme describing the geometry of such hydrocarbons. The p donor spirocyclopropane and a double bond shorten the proximal bonds by 0.0234 and 0.0327 Å and lengthen the distal bonds by 0.0227 and 0.0327 Å with respect to the unperturbed value of 1.5008 Å. The results of these calculations were compared with those obtained by semiempirical and ab initio methods and explained qualitatively by using the Fragment Molecular Orbital (FMO) approach.