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) d imers, X=HCN, CH3OH. The dimers are used as model systems to investigate th e energetics and cooperative phenomomena in intermolecular three-center hyd rogen-bond (H-bond) interactions relative to two-center H-bond interactions . The trans-trans conformer of diacetamide is chosen as a suitable model fo r intermolecular three-center H bonding where one H atom is interacting wit h two acceptor atoms. The proton-acceptor atoms are rigidly held in the sam e molecule. For both model systems, it is found that the calculated interac tion energy per H bond is appreciably smaller in the three-center than in t he two-center H-bond dimers, suggesting possibly a general characteristic o f intermolecular three-center H bonds, namely, a negative cooperativity. Mo re importantly, it is found that frequency shifts, intensity factors, bond lengths, and H-1 nuclear magnetic resonance chemical shifts all support the energetic calculations in that the intermolecular three-center H-bond dime rs exhibit marked negative cooperative effects. Despite the negative cooper ativity, 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. (C) 2001 Americ an Institute of Physics.