Deep-space glycine formationvia Strecker-type reactions activated by ice water dust mantles. A computational approach

Abstract

A Strecker-type synthesis of glycine by reacting NH₃, H₂CO and HCN in presence of ice water (H₂O–ice) as a catalyst has been theoretically studied at B3LYP/6-31+G(d,p) level within a cluster approach in order to mimic reactions occurring in the interstellar and circumstellar medium (ICM). Results indicate that, despite the exoergonic character of the considered reactions occurring at the H₂O–ice surface, the kinetics are slow due to relatively high electronic energy barriers (ΔU^≠₀ = 15–45 kcal mol⁻¹). Reactions occurring within H₂O–ice cavities, in which ice bulk effects have been modeled by assuming a dielectric continuum (ε = 78), show energy barriers low enough to allow NH₂CH₂OH formation but not NHCH₂ (ΔU^≠₀ = 2 and 21 kcal mol⁻¹, respectively) thus hindering the NH₂CH₂CN formation, i.e. the precursor of glycine, through Strecker channels. Moreover, hydrolysis of NH₂CH₂CN to give glycine is characterized by high electronic energy barriers (ΔU^≠₀ = 27–34 kcal mol⁻¹) and cannot readily occur at cryogenic temperatures. Nevertheless, the facts that NHCH₂ formation can readily be achieved through the radical–radical HCN + 2H → NHCH₂ reaction [D. E. Woon, Astrophys. J., 2002, 571, L177–L180], and that present results indicate that the Strecker step of NHCH₂ + HCN → NH₂CH₂CN exhibits a relative low energy barrier (ΔU^≠₀ = 8–9 kcal mol⁻¹), suggest that a combination of these two mechanisms allows for the formation of NH₂CH₂CN in the ICM. These results strengthen the thesis that NH₂CH₂CN could have been formed and protected by icy dust particles, and then delivered through micro-bombardments onto the early Earth, leading to glycine formation upon contact with the primordial ocean.