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, ΔG 50K < 1 kcal/mol, and thus appears to be feasible in the ISM. The corresponding reaction with HCN, however, has a much higher barrier, ΔG 50K = 7 kcal/mol. The significance of these results for glycine production in the ISM is discussed.

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.