Strength of the N-H center dot center dot center dot O=C and C-H center dot center dot center dot O=C bonds in formamide and N-methylacetamide dimers

The structures of the dimers of formamide and N-methylacetamide have been c alculated at the ab initio electronic structure theory level, second-order Moller-Plesset perturbation theory (MP2) with augmented correlation consist ent basis sets. Five unique structures were optimized for the formamide dim ers at the MP2/ aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. At the optimized ge ometries obtained with the aug-cc-pVTZ basis set, MP2 energies were evaluat ed with the aug-cc-pVQZ basis set, allowing an extrapolation of the energie s to the complete basis set limit. Four structures were found for the N-met hylacetamide 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 se t superposition error was estimated with the counterpoise method. The stren gth of the N-H O-...=C bond has a mean value of 7.1 kcal/mol in the formami de 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 C-(HO)-O-. ..=C hydrogen bonds play an important role in stabilizing these intermolecu lar complexes, increasing the interaction energy by 1.1-2.6 kcal/mol per in teraction.