Amide-pi interactions between formamide and benzene.

High-level ab initio calculations have been carried out using a formamide-benzene model system to evaluate amide-pi interactions. The interaction energies were estimated as a sum of the CCSD(T) correlation contribution and the HF energy at the complete basis set limit, for the geometries of the model structures at the energy minimum obtained by potential energy surface (PES) scans. NH/pi geometry in a face-on configuration was found to be the most attractive among the various geometries considered, with interaction energy of -3.75 kcal/mol. An interaction energy of -2.08 kcal/mol was calculated for the stacked N/Center type geometry, where the nitrogen atom of formamide points directly toward the center of the aromatic ring. The weakest C=O/pi geometry, where a carbonyl oxygen atom points toward the plane of the aromatic ring, was found to have energy minimum at an intermolecular distance of 3.67 A from the PES, with a repulsive interaction energy less than 1 kcal/mol. However, if there are simultaneous attractive interactions with other parts of the molecule besides the amide group, the weak repulsion could be easily overcome, to give a C=O/pi geometry interaction.