The Dominant Role of Electric Field and the Questionable Role of Anisotropy Effects of Single-Bond Substituents in 1H NMR Shifts. Shielding Mechanisms of and Steric Distortions in Some Monosubstituted Steroids

Article abstract of DOI:10.1021/jo01305a027

1H NMR shifts of 19-methyl protons in cholestanes bearing halo and hydroxy substituents in 3- and 5-positions are reported, compared with 2-, 4-, and 6-substituted steroids, and analyzed for the relevant shielding mechanisms.Linear electric-field effects correctly predict sign and magnitude of observed shift increments as well as the substantial differences between 3α- and 3β-substituted compounds; substituents situated closer to the observed nuclei show deviations ascribed to through-bond effects.Except for the oxo substituent, no correlation is found with anisotropy effects calculated from literature Δ_χ values.On the basis of MM1 force field minimized structures, steric distortions as van der Waals effects and sterically induced charge polarizations are investigated and are found to be negligible with the exception of the 3-oxo steroid.Shifts of the protons at C3 and those of some 17-substituted estr-4-ene derivatives are also reported.