Ab initio molecular orbital and infrared spectroscopic study of the conformation of secondary amides: derivatives of formanilide, acetanilide and benzylamides

Ab initio molecular orbital calculations at HF/4-31G level and infrared spe ctroscopic data for the frequencies are applied to analyse the grouping in a series model aromatic secondary amides: formanilide; acetanilide; o-methy lacetanilide; 2,6-dimethylformanilide, 2,6-dimethylacetanilide; N-benzylace tamide and N-benzylformamide. The theoretical and experimental data obtaine d show that the conformational state of the molecules studied is determined by the fine balance of several intramolecular factors: resonance effect be tween the amide group and the aromatic ring, steric interaction between var ious substituents around the -NH-CO- grouping in the aromatic ring, conjuga tion between the carbonyl bond and the nitrogen lone pair as well as direct field influences inside the amide group. (C) 1999 Elsevier Science B.V. Al l rights reserved.