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) methods of SCF-MO theory are used to study and analyze the mathematical form of the conformational dependencies of the isotropic C-13 chemical shifts of cyclobutane and nitrile, methyl, and fluoro derivatives. Also included in this study are bicyclo[1.1.1]pentanes with the same substituents as these provide an upper limit for the pucker angle in four-membered rings. Calculated C-13 chemical shifts for all molecules in their energy optimized (HF/6-31G** level) equilibrium geometries compare favorably with the experimental data. Pucker angle averaging is a relatively small effect in which chemical shifts differ from the equilibrium geometry values by 1.0 ppm at most. Predictions that the effects of increasing strain will be to shift all the ring carbons to higher frequency except for the C1 carbon of 1-fluoro-compounds are confirmed by 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 contributions, and these are especially important for equatorial C-H or C-X bonds. Also examined are the angular dependencies of alpha-, beta-, and gamma-effects for the ring carbons for the three substituents. The prediction that these will be more shielding for dihedral angles in the range of about 110-160-degrees is qualitatively in conformity with experimental data in the bicyclo[n.1.1]alkanes series.