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

Abstract

Anisotropy effects (σ_an), linear and square electric field contributions (σ_E, σ_E²), and sterically induced polarizations (σ_st) 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 (σ_E) can be excluded as the source of the equatorial–axial¹H shift difference, and ofthe methyl substituent effects, similarly σ_an for the methyl effect, and very likely for the equatorial–axial difference. Steric forces F(σ_st) describe distant shielding effects for ¹³C with a sensitivity of k_c≅ 1 p.p.m. µdyn^–1, and for ¹H with k_H; ≅ 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.