Energetics and cooperativity in three-center hydrogen bonding interactions. I. Diacetamide-X dimers (X=HCN, CH3OH)

High level ab initio calculations are carried out on diacetamide–X (DA–X) dimers, X=HCN, CH3OH. The dimers are used as model systems to investigate the energetics and cooperative phenomomena in intermolecular three-center hydrogen-bond (H-bond) interactions relative to two-center H-bond interactions. The trans–trans conformer of diacetamide is chosen as a suitable model for intermolecular three-center H bonding where one H atom is interacting with two acceptor atoms. The proton–acceptor atoms are rigidly held in the same molecule. For both model systems, it is found that the calculated interaction energy per H bond is appreciably smaller in the three-center than in the two-center H-bond dimers, suggesting possibly a general characteristic of intermolecular three-center H bonds, namely, a negative cooperativity. More importantly, it is found that frequency shifts, intensity factors, bond lengths, and H1 nuclear magnetic resonance chemical shifts all support the energetic calculations in that the intermolecular three-center H-bond dimers exhibit marked negative cooperative effects. Despite the negative cooperativity, the three-center DA–HCN dimer is actually energetically favorable over the two-center counterpart, whereas the three-center DA–CH3OH dimer is energetically unfavorable over the two-center counterpart.