Concerted Proton-Transfer Mechanism and Solvation Effects in the HNC/HCN Isomerization on the Surface of Icy Grain Mantles in the Interstellar Medium
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- 作者：Denise M. Koch ; Cé ; line Toubin ; Sichuan Xu ; Gilles H. Peslherbe ; James T. Hynes
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：October 18, 2007
- 年：2007
- 卷：111
- 期：41
- 页码：15026 - 15033
- 全文大小：307K
- 年卷期：v.111,no.41(October 18, 2007)
- ISSN：1932-7455
文摘
In order to better understand the isomerization between HNC and HCN on icy grain (or comet nuclei) surfacesin the interstellar medium in connection with a Strecker synthesis route to glycine, B3LYP/6-31+G(d,p)calculations have been carried out on the mechanisms of direct proton transfer (PT), where water moleculesplay a purely solvating role, and indirect PT, where water molecules participate in a proton relay mechanism.In the direct PT mechanism, a rather high-energy barrier exists for isomerization of HNC to HCN. In themuch more important indirect mechanism, a concerted PT process is possible for isomerization in the presenceof several water molecules. The calculations show that three water molecules bound to HNC and HCN giverise to a ring reaction significantly favoring the isomerization, a mechanism previously found for this reactionby Gardebien and Sevin (J. Phys. Chem. A 2003, 107, 3925). Further quite important solvation effects areincluded in the present work by addition of explicit solvating water molecules, and by a comparison withPolarizable Continuum Model (PCM) solvation. The final calculated free-energy barrier at 50 K is 3.4 kcal/mol for the isomerization of HNC to HCN with three water molecules in a ring acting as a bridge for concertedPT and seven explicit solvating water molecules; PCM solvation of this entire system leads to a furtherfree-energy barrier reduction of 0.8 kcal/mol. The back isomerization of HCN to HNC, however, is unlikely,with an estimated free-energy barrier of 9.5 kcal/mol at 50 K. These results imply that, on icy surfaces in theinterstellar medium, the isomerization of HNC to HCN occurs relatively easily, and the implications for theStrecker synthesis of glycine are discussed.