ИСТИНА

The impact of non-covalent interactions on the processes induced by high-energy radiation is an important issue for basic chemical physics and biophysics. This effect should be especially visible at low temperatures, when even weak intermolecular interactions can stabilize specific frozen structure for an infinite time. Such situation is relevant to multicomponent astrochemical ices. As shown in our laboratory, matrix isolation technique is a useful tool for model studies of astrochemically important complexes [1]. HCN and relative species (HNC, H2CN, HCNH, CN etс.) are considered as possible precursors for prebiotic synthesis in interstellar medium [2], whereas CO and CO2 are the basic components of various astrochemical ices [3]. Recently, we have studied the radiation chemistry of HCN under matrix isolation conditions [4] and first reported on the radiation-induced transformations of its complexes of HCN and CO2 [5]. This work is focusing on the effect of weak intermolecular interactions with small molecules on the radiation chemistry of HCN and very low temperatures and their astrochemical implications. The experimental FTIR spectroscopic studies are complemented by the extensive quantum-chemical calculations at the CCSD(T) level. The HCN...CO2 and HCN...CO complexes in solid rare gas environment were obtained by deposition of HCN/CO2/Ng and HCN/CO/Ng gaseous mixtures (where Ng - Ne, Ar, Kr or Xe). The calculations predict two stable structures for both complexes, but only linear OC...HCN complex and OCO...HCN complexes were detected in our matrix experiments. The matrix samples were irradiated with X-rays at 6 K to different doses and then annealed carefully in the temperature range from 6 to 50 K. The IR-spectra were recorded at 6 K. Decomposition of CO2 under X-irradiation of HCN/CO2/Ng is negligible and decomposition of HCN is significantly less effective than that is in the HCN/Ng systems. The principal products of the radiolysis of triple systems are H, CN, HNC and CO. The thermally induced reactions of H atoms were studied in an argon and krypton matrices. These reactions result in appearance of the known absorptions of H2CN, trans-HCNH, HCO and HKrCN. In addition, we observed a CO2-induced feature, which was assigned to the N-coordinated complex of trans-HCNH radical with CO2 on the basis of comparison with calculations. To our knowledge, it is the first experimentally observed complex of this radical. The OC...HNC complex was detected in all the matrices. Somewhat surprisingly, this complex decays more efficiently than the isolated HCN in the HCN/CO/Ng systems under X-irradiation. The major products are H, CN, HNC and HCO. 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. This may be an important clue for solving an old astrochemical puzzle of high HNC abundance. Other astrochemical implications and mechanistic aspects are discussed. This work was supported by the Russian Science Foundation (project no. 14-13-01266). REFERENCES 1. S.V. Ryazantsev et al. J. Phys.Chem.A 2015, 119 2578-2586. DOI:10.1021/jp509313n 2. R. Glaser, et al. Astrobiol. 2007, 7 (3) 465-475. DOI:10.1089/ast.2006.0112 3. M.J. Mumma, et al. Annu. Rev. Astron. Astrophys. 2011, 49 471-524. DOI:10.1146/annurev-astro-081309-130811 4. S.V. Kameneva et al. Radiat. Phys.Chem.2016, 124 30-37. DOI:10.1016/j.radphyschem.2015.12.001 5. S.V. Kameneva et al. J. Chem. Phys. 2016, 145 214309. DOI:10.1063/1.4969075