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
M. Barfield, 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, Journal of the American Chemical Society, 117(10), 1995, pp. 2862-2876
Ab initio IGLO (individual gauge for localized molecular orbital) meth
ods of SCF-MO theory are used to extend studies of the conformational
dependencies of isotropic C-13 chemical shifts to include delta-effect
s. All of the C-13 chemical shifts in butane and the 1-substituted but
anes CH(3)CH(2)CH(2)CH(2)X (X = CH3, CN, OH, Fl are obtained as functi
ons 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-e
ffects, which are almost independent of phi(2), alpha-, gamma-, and de
lta-effects depend on both dihedral angles. The similarities in the su
rface and contour plots for each of the effects suggest a positional d
ependence, while the surface elevations are primarily determined by th
e nature of the substituent. The calculated stereochemical dependencie
s of beta-, gamma-, and delta-effects compare favorably with the exper
imental results in substituted trans-decalins and bicyclo[2.2.1]heptan
es 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. A
nalyses of the IGLO local bond contributions show that the C-H bonds p
ointing 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 bo
nd.