Strength of the NH···OC and CH···OC Bonds in Formamide and<i>N</i>-Methylacetamide Dimers

Abstract

The structures of the dimers of formamide and N-methylacetamide have been calculated at the ab initio electronic structure theory level, second-order Møller−Plesset perturbation theory (MP2) with augmented correlation consistent basis sets. Five unique structures were optimized for the formamide dimers at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. At the optimized geometries obtained with the aug-cc-pVTZ basis set, MP2 energies were evaluated with the aug-cc-pVQZ basis set, allowing an extrapolation of the energies to the complete basis set limit. Four structures were found for the N-methylacetamide dimer at the MP2/aug-cc-pVDZ level, and single-point energies were calculated at the MP2/aug-cc-pVTZ level. In both systems, the basis set superposition error was estimated with the counterpoise method. The strength of the NH···OC bond has a mean value of 7.1 kcal/mol in the formamide dimers and a mean value of 8.6 kcal/mol in the N-methylacetamide dimers. The difference in hydrogen bond strengths is attributed to differences in basicity at the carbonyl oxygen receptor site. In several dimers CH···OC hydrogen bonds play an important role in stabilizing these intermolecular complexes, increasing the interaction energy by 1.1−2.6 kcal/mol per interaction.