Hydrogen bonded complexes of acetylene and boric acid: A matrix isolation infrared and ab initio study

The infrared spectra of the hydrogen bonded complexes of acetylene-boric acid have been investigated in solid nitrogen matrix. We have observed the 1:1 acetylene-boric acid complex in the nitrogen matrix. Formation of the complex was evidenced from the shifts in the vibrational frequencies corresponding to the modes involving the acetylene and boric acid submolecules in the complex. The structure of the complexes and the energies were computed at HF, B3LYP and MP2 levels of theory using 6-31++G** basis sets. Only one minimum was obtained, which corresponded to a complex with both OH⋯π and CH⋯O interactions. In this complex boric acid acts as a proton donor to the acetylene π-cloud and proton acceptor to the acidic hydrogen in acetylene. The computed vibrational frequencies of acetylene–boric acid complexes at B3LYP/6-31++G** level corroborated well with the experimental frequencies. Calculations were also performed for the higher 2:1 and 3:1 acetylene–boric acid complexes.