Characterization of the HCN⋯CO complex and its radiation-induced transformation to HNC⋯CO in cold media: An experimental and theoretical investigation

The HCN⋯CO complex and its X-ray induced transformation to HNC⋯CO in solid noble gas (Ng) matrices (Ng = Ne, Ar, Kr, Xe) was first characterized by matrix isolation FTIR spectroscopy at 5 K. The HCN⋯CO complex was obtained by deposition of HCN/CO/Ng gaseous mixtures. The assignment was based on extensive quantum chemical calculations at the CCSD(T) level of theory. The calculations predicted two computationally stable structures for HCN⋯CO and three stable structures for HNC⋯CO. However, only the most energetically favorable linear structures corresponding to the co-ordination between the H atom of HCN (HNC) and the C atom of CO have been found experimentally. The HCN⋯CO complex demonstrates a considerable red shift of the H-C stretching vibrations (-24 to -38 cm, depending on the matrix) and a blue shift of the HCN bending vibrations (+29 to +32 cm) with respect to that of the HCN monomer, while the C=O stretching mode is blue-shifted by 15 to 20 cm as compared to the CO monomer. The HNC⋯CO complex reveals a strong red shift of the H-N bending (-77 to -118 cm) and a strong blue shift of the HNC bending mode (ca. +100 cm) as compared to the HNC monomer, whereas the C=O stretching is blue-shifted by 24 to 29 cm with respect to that of the CO monomer. The interaction energies were determined to be 1.01 and 1.87 kcal mol for HCN⋯CO and HNC⋯CO, respectively. It was found that the formation of complexes with CO had a remarkable effect on the radiation-induced transformations of HCN. While the dissociation of HCN to H and CN is suppressed in complexes, the isomerization of HCN to HNC is strongly catalyzed by the complexation with CO. The astrochemical implications of the results are discussed.