The shielding of H-α in the stabilized rotameric forms of α,α-di-tert-butylthioacetic esters. The electric and magnetic anisotropic effects of the thione group

The currently accepted geometry of carbonyl magnetic anisotropic effects, if regarded as extendable to the thione group, imply that the more deshielded of the two H-α lines in the ¹H NMR spectra of α,α-di-tert-butylthioacetic esters should be assigned to the ‘180° form’ in which C?S is antiperiplanar to C-α? H. CNDO results, however, indicate the opposite; the line should be assigned to the ‘O° form,’ in which C? S eclipses C-α? H, if the predicted considerable increase in 1s density at H-α on going from the 0° form to the 180° form is taken into account. A change in 1s density at H-α as a result of increased branching of alkylation at C-α is also found. These specific effects must be taken into account when discussing the anisotropic effects of C?O or C?S on H-α shifts.