STRUCTURAL ASPECTS OF CYCLOPROPYL CONJUGATION - EXPERIMENTAL STUDIES AND AB-INITIO CALCULATIONS

Low temperature crystal structural studies of small cyclopropyl-conjug ated unsaturated hydrocarbons reveal significant differences in bond l engths between carbon atoms mainly within the three-membered rings but also for adjacent C-C single bonds. The strongest influence of conjug ation was observed for spiro[2.4]hepta-4,6-diene (1), where the differ ence in the lengths of three-membered ring bonds is 0.050 Angstrom (X- ray). The same effect is also obvious from ab initio calculations at H F/6-31G(d) (to a minor extent) and MP2/6-31G(d) levels, as shown by a difference analysis of bond lengths which was performed in order to ca ncel out systematic errors and differences for both methods. Static di fference electron density maps were derived from the experimental data by multipole refinements. These maps showed shifts of electron densit y to exocyclic locations within the planes of the three-membered rings and significant bond ellipticities for the adjacent single bonds. For 1,1-diethenylcyclopropane (4) a nonsymmetric conformation was found i n the crystal lattice, very close to the global minimum structure calc ulated by ab initio methods.