AB-INITIO IGLO MO STUDIES OF THE CONFORMATIONAL (PUCKER ANGLE) AND SUBSTITUENT DEPENDENCIES OF ISOTROPIC C-13 CHEMICAL-SHIFTS IN 1-SUBSTITUTED CYCLOBUTANES AND BICYCLO[1.1.1]PENTANES

Ab initio IGLO (individual gauge for localized molecular orbital) meth ods of SCF-MO theory are used to study and analyze the mathematical fo rm of the conformational dependencies of the isotropic C-13 chemical s hifts of cyclobutane and nitrile, methyl, and fluoro derivatives. Also included in this study are bicyclo[1.1.1]pentanes with the same subst ituents as these provide an upper limit for the pucker angle in four-m embered rings. Calculated C-13 chemical shifts for all molecules in th eir energy optimized (HF/6-31G* level) equilibrium geometries compare favorably with the experimental data. Pucker angle averaging is a rel atively small effect in which chemical shifts differ from the equilibr ium geometry values by 1.0 ppm at most. Predictions that the effects o f increasing strain will be to shift all the ring carbons to higher fr equency except for the C1 carbon of 1-fluoro-compounds are confirmed b y the experimental shifts for the 1-substituted bicyclo[1.1.1]pentanes . Conformational and substituent effects are analyzed in terms of the angular dependence of the IGLO C-H and C-C bond paramagnetic contribut ions, and these are especially important for equatorial C-H or C-X bon ds. Also examined are the angular dependencies of alpha-, beta-, and g amma-effects for the ring carbons for the three substituents. The pred iction that these will be more shielding for dihedral angles in the ra nge of about 110-160-degrees is qualitatively in conformity with exper imental data in the bicyclo[n.1.1]alkanes series.