Matrixinfrared spectra and ab initio calculations of the acetylene complexes with nitric and nitrous acids

The hydrogen bonded complexes formed by an acetylene molecule with nitric and nitrous acids have been observed and characterized in argon matrices. Eight perturbed HNO3 vibrations and three perturbed C2H2 modes were identified for the C2H2⋯HNO3 complex. Five perturbed trans-HONO and four cis-HONO vibrations, and three perturbed C2H2 vibrations were found for the C2H2 complexes with the trans- and cis-isomers of nitrous acid. The perturbation of the OH group vibrations proves that the complexes involve an O–H⋯π type of hydrogen bond with the OH group acting as a proton donor to the π electron density of the acetylene molecule. Theoretical studies of the structure and spectral characteristics of the studied systems were carried out at the electron correlation level with the 6-311++G(2d,2p) basis set. The calculated spectral parameters for the O–H⋯π type complexes are in good agreement with experimental data. The calculations also show the stability of the much more weakly bound C2H2⋯HNO3 structure that involves a C–H⋯O type of bond with the acetylene molecule acting as a proton donor and the nitric acid as a proton acceptor. Two observed bands are tentatively assigned to this type of complex.