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 [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.
Wiberg
Kenneth B
KB
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA.
Rush
Daniel J
DJ
D000577:Amides / Q000737:chemistry*; D008968:Molecular Conformation; D013438:Sulfhydryl Compounds / Q000737:chemistry*
D000577:Amides; D013438:Sulfhydryl Compounds