Shielding contributions to 1H and 13C nuclear magnetic resonance shifts in cyclohexane, methylcyclohexane, and n-butane

Abstract

Anisotropy effects (σan), linear and square electric field contributions (σE, σE2), and sterically induced polarizations (σst) are evaluated on the basis of MM2-generated molecular structures using a computer program (SHIFT) which allows the point of action of inducing and polarized bonds to be varied, and the number of interacting atoms or bonds to be selected. The linear electric field effect (σE) can be excluded as the source of the equatorial–axial1H shift difference, and ofthe methyl substituent effects, similarly σan for the methyl effect, and very likely for the equatorial–axial difference. Steric forces F(σst) describe distant shielding effects for 13C with a sensitivity of kc≅ 1 p.p.m. µdyn–1, and for 1H with kH; ≅ 0.05 p.p.m. µdyn–1 with the correct order of magnitude, including the expected 13C shielding in n-butane rotamers. In conclusion, the application of all now available computational options without arbitrary limitations shows that only a few limiting shielding mechanisms can be excluded, and that a parametrization of screening factors on the basis of hydrocarbon n.m.r. data alone is not feasible.