Acetylene as potential hydrogen-bond proton acceptor

Abstract

Ab initio calculations are used to assess the ability of the CC triple bond in acetylene to act as proton acceptor to donor molecules HF, HCl, HCN and HCCH. The strength of the interaction energy varies from 3 kcal/mol for the strongest of these complexes FH⋯HCCH, to 1 kcal/mol for the weakest (HCCH) 2. The X–H bond of all donors undergoes the elongation characteristic of H-bonds, along with the red shift of its stretching frequency. In addition, electron density is transferred from the proton-acceptor molecule to the donor, and the bridging H becomes more positive. However, the magnitudes of these density shifts are larger than might be expected from a conventional H-bond. Opposite to a H-bond, the proton-accepting C atom loses density. Decomposition of the total interaction energy indicates the dominant role played by Coulombic attraction, as in a H-bond, but the exchange repulsion, charge transfer, and polarization terms are disproportionately larger than is typically seen in true H-bonds.