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 "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.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.

The complete basis set (CBS) extrapolation model chemistry of Petersson and co-workers was used to explore the potential energy surface of the ammonia inversion mode. The CBS-QCI theoretical energies were calculated using 41 points along the inversion surface at the MP2/6-311++G** geometries. A variety of techniques were explored to model the potential surface. Subsequent numerical solution of the one-dimensional Schrödinger equation produced energy levels for ammonia isotopomers in good agreement with experimental transitions. Accounting for the variable nature of the reduced mass with inversion coordinate is shown to be of significance. This study is an important first step in producing reliable methods for making ab initio thermodynamic corrections from ΔE(0 K) to ΔG(298 K) in other nitrogen-containing systems. Because no experimental methods generate data at 0 K, these corrections provide a crucial link between experimental thermochemical energies and ab initio theory.

The methyl rotational barriers for a series of N-methyl-substituted amides and thioamides have been calculated at the MP2/6-311+G** level. A comparison of the N-methylformamide and methyl formate barriers indicates that the H [bond] C(Me) [bond] N [bond] H eclipsed torsional arrangement destabilizes an amide by about 0.8 kcal/mol. A comparison of thioamides and amides showed the importance of steric repulsion between the sulfur and a methyl hydrogen in the Z-forms of the thioamides. The C [bond] N bond rotation transition states of the N,N-dimethyl amides have much larger methyl rotational barriers than found in the ground states. They can be attributed to the smaller CH(3)(-)N [bond] CH(3) bond angles in the transition states.

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