Molecular orbital theory of the hydrogen bond. PI electrons as proton acceptors

Abstract

Ab initio SCF calculations with minimal STO-3G and slightly extended 4-31G basis sets have been performed on dimers having HF and H 2O as proton donors, and the π electrons of C 2H 2, C 2H 4, HCN, and H 2CO as proton acceptors. The equilibrium dimers with C 2H 2 and C 2H 4 have relatively weak hydrogen bonds which form preferentially at or near the midpoint of the C–C π bond. However, distortions of these dimers which move the hydrogen bonded proton away from the midpoint are associated with only a small amount of destabilization. With HCN and H 2CO as proton acceptors, pi hydrogen bonding is most favorable at the negative end of the π bonds. However, these pi dimers are not equilibrium structures on the intermolecular potential surfaces, but relax to structures in which hydrogen bond formation occurs through a lone pair of electrons.