AB-INITIO IGLO STUDIES OF THE CONFORMATIONAL AND SUBSTITUENT DEPENDENCIES OF ALPHA-EFFECTS, BETA-EFFECTS, GAMMA-EFFECTS, AND DELTA-EFFECTS IN THE C-13 NMR-SPECTRA OF 1-SUBSTITUTED BUTANES

Ab initio IGLO (individual gauge for localized molecular orbital) methods of SCF-MO theory are used to extend studies of the conformational dependencies of isotropic C-13 chemical shifts to include delta-effects. All of the C-13 chemical shifts in butane and the 1-substituted butanes CH(3)CH(2)CH(2)CH(2)X (X = CH3, CN, OH, Fl are obtained as functions of the torsion angles about the C1-C2 (phi(1)) and C2-C3 (phi(2)) bonds with structures optimized at the HF/6-31G* level. The calculated C-13 chemical shifts, averaged over the two dihedral angles, compare favorably with the experimental data. In contrast to calculated beta-effects, which are almost independent of phi(2), alpha-, gamma-, and delta-effects depend on both dihedral angles. The similarities in the surface and contour plots for each of the effects suggest a positional dependence, while the surface elevations are primarily determined by the nature of the substituent. The calculated stereochemical dependencies of beta-, gamma-, and delta-effects compare favorably with the experimental results in substituted trans-decalins and bicyclo[2.2.1]heptanes over a wide range of torsion angles. For all substituents the delta-effects assume their largest values for syn-axial (g(+)g(-)/g(-)g(+)) conformations, and gamma-gauche effects in these arrangements are 4-5 ppm deshielded in comparison with g(+)g(+) and g(+)t conformations. Analyses of the IGLO local bond contributions show that the C-H bonds pointing toward and away from the substituent are primarily responsible for the deshielding of the delta-carbon in syn-axial arrangements and appear to be independent of compression of the proximate C delta-H bond.