Ab initio studies on amides: acetamide, N-methylformamide and N-methylacetamide

Ab initio molecular orbital theory with the STO-3G and 4-31G basis sets has been used to investigate the geometries, preferred conformations, and barriers to internal rotation for acetamide, N-methylformamide and N-methylacetamide. Results are compared with corresponding previously reported data for formamide. For acetamide, the preferred conformation has the methyl group staggered with respect to the N-C bond whereas for N-methylformamide the methyl group is eclipsed with respect to this bond. Both N-methylformamide and N-methylacetamide prefer a Z-arrangement, i.e. methyl cis to C=O about the N-C bond. Experimentally determined barriers to internal rotation about the N-C bond generally lie within the range spanned by the STO-3G and 4-31G estimates.