Panajapo Pannipa, Suwannakham Parichart, Promma Phorntep, Sagarik Kritsana
School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
R Soc Open Sci. 2024 May 22;11(5):231957. doi: 10.1098/rsos.231957. eCollection 2024 May.
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 ( ) state, thermochemical processes after non-radiative → 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 HO…CO van der Waals and HO-OC hydrogen bond precursors (Ch (1)_Step (1)) possess considerably lower energy barriers than the state pathways. The Gibbs free energy barriers (∆ ) calculated after the non-radiative relaxations indicated higher spontaneous temperatures ( ) 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 relaxation energy leads to the separation of the weakly associated HO…CHNH…CO complex into single molecules. The NVE-MDSH results also confirmed that the molecular processes after the relaxations are thermally selective, and because the non-radiative relaxation temperatures are exceedingly higher than , 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.
利用量子化学方法、过渡态理论以及含表面跳跃动力学的微正则分子动力学模拟（NVE-MDSH），研究了在寒冷星际介质中由基本气体分子形成甘氨酸（Gly）的可能性。该理论研究着重探讨了最低单重激发态下的五条光化学途径、非辐射S₁→S₀弛豫后的热化学过程以及NVE系综中的光热能量转换。优化后的反应途径表明，为生成反应性单重态二羟基卡宾（HOCOH）中间体，涉及HO…CO范德华力和HO-OC氢键前体的光化学途径（Ch(1)_Step(1)）比S₁态途径具有低得多的能垒。非辐射S₁→S₀弛豫后计算得到的吉布斯自由能垒（∆G）表明，HOCOH中间体（Ch(1)_Step(1)）形成的自发温度（T）高于甘氨酸形成（Ch(1)_Step(2)和Ch(4)）的自发温度。尽管Ch(4)中的三分子反应具有低能垒且在热力学上有利，但高放热的S₁→S₀弛豫能量导致弱结合的HO…CHNH…CO络合物分离成单个分子。NVE-MDSH结果还证实，S₁→S₀弛豫后的分子过程具有热选择性，并且由于非辐射S₁→S₀弛豫温度远高于T₀，连续反应途径上甘氨酸的形成是非协同的，产率低且有几种副产物。基于理论结果，提出了促进所需光化学产物的光热控制策略。它们可作为未来光化学反应理论和实验研究的指导方针。
