Concerted Proton-Transfer Mechanism and Solvation Effects in the HNC/HCN Isomerization on the Surface of Icy Grain Mantles in the Interstellar Medium

Abstract

In order to better understand the isomerization between HNC and HCN on icy grain (or comet nuclei) surfaces in 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 molecules play 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 the much more important indirect mechanism, a concerted PT process is possible for isomerization in the presence of several water molecules. The calculations show that three water molecules bound to HNC and HCN give rise to a ring reaction significantly favoring the isomerization, a mechanism previously found for this reaction by Gardebien and Sevin (J. Phys. Chem. A 2003, 107, 3925). Further quite important solvation effects are included in the present work by addition of explicit solvating water molecules, and by a comparison with Polarizable 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 concerted PT and seven explicit solvating water molecules; PCM solvation of this entire system leads to a further free-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 the interstellar medium, the isomerization of HNC to HCN occurs relatively easily, and the implications for the Strecker synthesis of glycine are discussed.

Gas phase basicities of polyfunctional molecules. Part 6: Cyanides and isocyanides

“…An indirect mechanism involving a concerted proton transfer through three molecules of water has been shown to reduce drastically the barrier for HCN ↔ HNC isomerization. 36 For more complex systems, the Dissociations of protonated n-propyl and i-propyl cyanides were proposed to follow similar reaction paths. 41 Indeed, the passage trough…”

“…A high critical energy barrier of 200‐220 kJ/mol has been calculated for this reaction. An indirect mechanism involving a concerted proton transfer through three molecules of water has been shown to reduce drastically the barrier for HCN ↔ HNC isomerization . For more complex systems, the cyanide/isocyanide isomerization, RCN ↔ RNC, may also involve 1,2‐R migration, an elementary step which can be seen as an internal rotation of the CN group inside the molecular system.…”

ETS‐NOCV Decomposition of the Reaction Force: The HCN/CNH Isomerization Reaction Assisted by Water

“…[30][31][32][33][34][35][36] This reaction in gas phase is characterized by a high energetic barrier (DE 6 ¼ 5 $53 kcal/mol). For example, the proton relay mechanism with four-water molecules [37][38][39] exhibits a barrier of ca. For example, the proton relay mechanism with four-water molecules [37][38][39] exhibits a barrier of ca.…”

“…[36] The reaction in solvent becomes more facile: many authors have demonstrated the catalytic effect of the explicit water molecules, [37,38] addition of water molecules substantially reduce the height of the barrier compared to the nonassisted gas-phase isomerization reaction. For example, the proton relay mechanism with four-water molecules [37][38][39] exhibits a barrier of ca. 19 kcal/mol.…”

Revisiting the seemingly straightforward hydrogen cyanide/hydrogen isocyanide isomerisation

“…This process has sometimes been viewed as a prototypical unimolecular reaction [11,12]. Lee and Rendell described it as '.…”