DFT study of HCN and NC-CN reactions with hydrogen species

Energetic feasibility of the formation of aminoacetonitrile (or glycine nitrile) through a sequence of reactions of cyanogen (NdropC-Cdrop N) with hydrogen species was evaluated, using density functional theory (DFT) calculations at the B3LYP/6-31 + +G(d,p) theoretical level. A critical step in the cyanogen hydrogenation is the addition of first H atom. As a result of a positive activation barrier of similar to3-6 kcal mol(-1) (depending on the attachment site), this process is hardly feasible under cold gas-phase interstellar conditions, but might possibly be facilitated in the solid phase by proton tunneling effects. The addition of a third hydrogen atom requires much lower activation energy, especially if H adds on the NH group (0.2-0.8 kcal mol(-1)). The second and fourth steps (combination of C2HN2 and C2H3N2 radical species with hydrogen atoms) are highly exothermic; the need to get rid of excess energy makes them possible through grain-surface processes only. As a whole, most of the hydrogenation reactions under study can be expected to take place in the solid phase (icy and silicate grains), and not in the gas phase under cold interstellar conditions. (C) 2004 Wiley Periodicals, Inc.