A Theoretical Study of the Formation of the Aminoacetonitrile Precursor of Glycine on Icy Grain Mantles in the Interstellar Medium

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• 作者：Denise M. Koch ; C&eacute ; line Toubin ; Gilles H. Peslherbe ; James T. Hynes
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：February 28, 2008
• 年：2008
• 卷：112
• 期：8
• 页码：2972 - 2980
• 全文大小：359K
• 年卷期：v.112,no.8(February 28, 2008)
• ISSN：1932-7455

文摘

Electronic structure calculations have been carried out for one of the key reactions in a Strecker synthesisroute 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 betweenmethanimine, CH₂NH, and the two isomers HNC/HCN, leading to aminoacetonitrile-a known precursor ofglycine-in both the gas phase and on a model icy grain surface. Three mechanisms are evidenced in thereference gas-phase calculations; for CH₂NH reacting with HCN, there are two routes referred to as indirectand direct, and for CH₂NH reacting with the isomer HNC, a one-step mechanism is found. All of thesereaction paths have quite high barriers, but on a model interstellar grain icy surface, very considerable barrierreduction results due to a concerted proton relay mechanism. Explicit water molecules in a reaction ring areshown to participate in this relay mechanism in the reactions of CH₂NH both with HCN and with the HNCisomer. The inclusion of two explicit H₂O molecules leads to the strongest effect for the concerted protontransfer. With several further solvating waters included, this proton relay route to aminoacetonitrile for theHNC isomer via a direct mechanism is found to have a very low free-energy barrier at 50 K, mages/gifchars/Delta.gif" BORDER=0 >G_50K < 1kcal/mol, and thus appears to be feasible in the ISM. The corresponding reaction with HCN, however, has amuch higher barrier, mages/gifchars/Delta.gif" BORDER=0 >G_50K = 7 kcal/mol. The significance of these results for glycine production in the ISMis discussed.