Dimerization of HCN In Interstellar Icy Grain Mantles; A DFT Study

摘要

The dimerization of hydrogen cyanide (HCN) is the first step in potential mechanisms suggested for interstellar and prebiotic syntheses of nucleobases and amino acids. Adenine can be formed by the pentamerization of HCN, guanine can be formed from an HCN tetramer, 4-aminoimidazole-5-carboni-trile, by adding H2O and cyanogen (C2N2), and glycine can be formed by the trimerization of HCN, followed by hydrolysis. The widespread presence of HCN in the interstellar medium (ISM) suggests the possible syntheses of nucleobases or amino acids from HCN in interstellar space. However, previous studies have shown that the dimerization of HCN cannot occur in the interstellar gas phase by thermal reaction because its activation energy (E_a) is too high. For the thermal dimerization reaction to occur in the gas phase at low temperature and ultra-high vacuum such as in the ISM, the E_a should be less than a few kJ/mol, but the value estimated with CBS-QB3 calculations is 295 kJ/mol. When assisted by two H2O molecules, the E_a was lowered to 134 kJ/mol,5 still too high for the reaction to occur in the interstellar gas phase. However, dimerization in condensed phases, such as aqueous solution and ice, is more probable because the concentrations of the reactants are much higher than those in the gas phase. We showed that the E_a was lowered by catalytic water and/or water-solvation, using theoretical explicit and implicit methods. In this work, we examined the dimerization of HCN to form the Z-conformer of iminoacetonitrile ((Z)-HN=CHCN, 1), on or within ice mantles of ISM grains using quantum chemical calculation.