A theoretical study of the formation of the aminoacetonitrile precursor of glycine on icy grain mantles in the interstellar medium

摘要

Electronic structure calculations have been carried out for one of the key reactions in a Strecker synthesis route to the amino acid glycine, in connection with amino acid production in the interstellar medium (ISM). Density functional calculations at the B3LYP/6-31+G(d,p) level have been performed for the reaction between methanimine, CH2NH, and the two isomers HNC/HCN, leading to aminoacetonitrile-a known precursor of glycine-in both the gas phase and on a model icy grain surface. Three mechanisms are evidenced in the reference gas-phase calculations; for CH2NH reacting with HCN, there are two routes referred to as indirect and direct, and for CH2NH reacting with the isomer HNC, a one-step mechanism is found. All of these reaction paths have quite high barriers, but on a model interstellar grain icy surface, very considerable barrier reduction results due to a concerted proton relay mechanism. Explicit water molecules in a reaction ring are shown to participate in this relay mechanism in the reactions of CH2NH both with HCN and with the HNC isomer. The inclusion of two explicit H2O molecules leads to the strongest effect for the concerted proton transfer. With several further solvating waters included, this proton relay route to aminoacetonitrile for the HNC isomer via a direct mechanism is found to have a very low free-energy barrier at 50 K, Delta G(50K) < 1 kcal/mol, and thus appears to be feasible in the ISM. The corresponding reaction with HCN, however, has a much higher barrier, Delta G(50K) = 7 kcal/mol. The significance of these results for glycine production in the ISM is discussed.