Methyl rotational barriers in amides and thioamides

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-C(Me)-N-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-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 CH3-N-CH3 bond angles in the transition states.