Resonance in amides. Comparison of ab initio and semiempirical methodologies in the description of acetamide

The electronic description of acetamide is analyzed at different levels of theory, comprising both semiempirical and ab initio methodologies. Regardless of the basis set used, the level of correlation included and the population analysis considered, ab initio calculations always result in a density charge on the nitrogen atom larger than on the oxygen, a fact that seems to be at variance with the predictions of resonance. AMI and MNDO calculations agree with the ab initio results. PM3 and ZINDO, on the other hand, render the resonance-based description. The study, oriented to select the best semiempirical methodology for the treatment of large aliphatic amides, points to AM1 as the method of choice. (C) 1997 Elsevier Science B.V.