GIAO-CHF CALCULATIONS OF NMR CHEMICAL-SHIFTS FOR SOME N,N-DIMETHYLACETAMIDE DERIVATIVES USING OPTIMIZED GEOMETRIES

NMR chemical shifts and shift anisotropies for hydrogen, carbon, nitrogen, oxygen, fluorine and chlorine atoms in four C-substituted N,N-dimethylamides (XCON(CH3)(2) where X = H, CH3, F or Cl) have been calculated using ab initio GIAO-CHF method and optimized molecular structures. The molecular geometries for the studied molecules were obtained in the Berny optimization process implemented in the GAUSSIAN 92 code. In all the amides studied, two nonequivalent methyl groups were present due to hindered rotation about the C-N bond. An activation energy of 18.2 kcal mol(-1) was theoretically predicted for the internal rotation about the C-N bond in N,N-dimethylformamide (DMF, HCON(CH3)2). It was found that the difference in hydrogen and carbon shielding constants of the methyl groups can be calculated with satisfactory precision. For dimethylcarbamyl fluoride (FCON(CH3)(2)) the ab initio calculations suggest that the C-13 Shielding of the cis-methyl group is lower than for the trans-methyl group, in contrast to the other derivatives (X = H, CH3 and C1).