Some studies of long range shielding effects on chemical shifts

Long range shielding parameters for the C-H, C=0, and thioketal groups were derived by least squares analysis of chemical shifts of C-18 and C-19 methyl groups in 5a,14a-androstane derivatives. The sensitivity of these parameters to structural approximations, concerning conformations of methyl groups and steroid framework, were studied. The C-H bond anisotropy is thought to be small, but no definitive value could be obtained. An electrostatic and a magnetic shielding mechanism were found to be operative in the cases of the carbonyl and thioketal groups. Shielding parameters for the C=0 bond agree reasonably well with previous results. The thioketal parameters are not considered physically meaningful quantitites due to the severity of approximations and the small number of equations, but they predict chemical shifts rather well. Conformational approximations, concerning methyl groups and steroid ring systems were found to have only a moderate effect on shielding parameters and calculated shifts.