GEOMETRIES OF 3-MEMBERED RINGS IN TRIANGULANES AND SPIROCYCLOPROPANATED BICYCLOPROPYLIDENES - EXPERIMENTAL STUDIES AND A GENERAL BOND INCREMENT SCHEME

Crystal structural studies of several linear and branched triangulanes , together with spiro-cyclopropanated bicyclopropylidenes, by X-ray di ffraction reveal significant differences between proximal and distal b ond lengths in the three-membered rings. Combined with previously publ ished structural data, these results allowed us to elaborate a general addivity scheme describing the geometry of such hydrocarbons. The pi donor spirocyclopropane and a double bond shorten the proximal bonds b y 0.0234 and 0.0327 Angstrom and lengthen the distal bonds by 0.0227 a nd 0.0327 Angstrom with respect to the unperturbed value of 1.5008 Ang strom. The results of these calculations were compared with those obta ined by semiempirical and ab initio methods and explained qualitativel y by using the Fragment Molecular Orbital (FMO) approach.