C-13 nuclear magnetic resonance studies of the oriented molecules in the liquid crystal solvent. anisotropy of C-13 shielding constants in (CH3I)-C-13 and (CH3CN)-C-13

Anisotropies of the C-13 shielding constant (Delta sigma(C)) for CH3I and CH3CN are obtained from C-13 NMR studies of these molecules in the nematic solvents. The results are compared with the theoretical values obtained by the molecular orbital calculations using the current theory. The agreement is fairly satisfactory for CH3CN. but not for CH3I. The observed large negative value of Delta sigma(C) for CH3I (the theoretical prediction is positive) is discussed in terms of the possible sources of the heavy atom effect on the C-13 chemical shift.