New insights about the hydrogen bonds formed between acetylene and hydrogen fluoride: π⋯H, C⋯H and F⋯H.

A theoretical study of hydrogen bond strength and bond properties in the C2H2⋯(HF)-T, C2H2⋯2(HF)-T, C2H2⋯2(HF), C2H2⋯3(HF) and C2H2⋯4(HF) complexes was carried out at the B3LYP/6-311++G(d,p) theory level. In these systems, a strength competition between the π⋯H and C⋯H interactions was examined. Specifically the F⋯H hydrogen bond, its properties were studied through a comparison between the hydrogen fluoride and the higher-order complexes (trimer, tetramer and pentamer). Regarding the electronic properties, the hydrogen bond strength could not be determined by the supermolecule approach. Thus, the hydrogen bond energies were computed via NBO calculations. Additionally to NBO, the ChelpG charge calculations were used to interpret the intermolecular charge transfer. The QTAIM integrations were useful to predict the covalent character of the π⋯H, C⋯H and F⋯H hydrogen bonds. Moreover, values of hybrid orbitals (s and p) and atomic radii were also determined in order to justify the red shifts in the stretch frequencies of the HF bonds.