Mechanisms of glycine formation in cold interstellar media: a theoretical study

The possibility of the formation of glycine (Gly) from fundamental gas molecules in cold interstellar media was studied using quantum chemical methods, transition state theory and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This theoretical study emphasized five photochemical pathways in the lowest singlet-excited (S ₁) state, thermochemical processes after non-radiative S ₁→S ₀ relaxations, and photo-to-thermal energy conversion in the NVE ensemble. The optimized reaction pathways suggested that to generate a reactive singlet dihydroxy carbene (HOCOH) intermediate, photochemical pathways involving the H₂O…CO van der Waals and H₂O-OC hydrogen bond precursors (Ch (1)_Step (1)) possess considerably lower energy barriers than the S ₀ state pathways. The Gibbs free energy barriers (∆G ) calculated after the non-radiative S ₁ →S ₀ relaxations indicated higher spontaneous temperatures (T _s) for the formation of the HOCOH intermediate (Ch (1)_Step (1)) than for Gly formation (Ch (1)_Step (2) and Ch (4)). Although the termolecular reaction in Ch (4) possesses a low energy barrier, and is thermodynamically favourable, the high exothermic S ₁ →S ₀ relaxation energy leads to the separation of the weakly associated H₂O…CH₂NH…CO complex into single molecules. The NVE-MDSH results also confirmed that the molecular processes after the S ₁ →S ₀ relaxations are thermally selective, and because the non-radiative S ₁ →S ₀ relaxation temperatures are exceedingly higher than T _s, the formation of Gly on consecutive reaction pathways is non-synergistic with low yields and several side products. Based on the theoretical results, photo-to-thermal control strategies to promote desirable photochemical products are proposed. They could be used as guidelines for future theoretical and experimental research on photochemical reactions.