AMIDES .3. EXPERIMENTAL AND THEORETICAL-STUDIES OF THE EFFECT OF THE MEDIUM ON THE ROTATIONAL BARRIERS FOR N,N-DIMETHYLFORMAMIDE AND N,N-DIMETHYLACETAMIDE

The rotational barriers for N,N-dimethylformamide and N,N-dimethylacetamide have been investigated theoretically and experimentally. Calculations at the G2(MP2) theoretical level followed by correction to 25 degrees C reproduced the experimental gas-phase barriers. An examination of the geometries of these amides showed that the lower barrier for the acetamide resulted mainly from a ground state methyl-methyl repulsive interaction. The rotational barriers for the amides were measured in several solvents using NMR selective inversion-recovery experiments. The effect of solvent on the C-N rotational barriers was examined computationally using reaction field theory. This approach was found to give barriers that are in good agreement with experiment for aprotic, non-aromatic solvents which do not engage in specific interactions with the amides. The effect of a hydrogen bonding solvent, water, was studied via incorporating a water molecule hydrogen bonded to the oxygen and examining this ensemble using reaction field theory.