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 |
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Authors: | S. N. Balasubrahmanyam S. Narasimha Bharathi G. Usha |
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Abstract: | 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. |
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