星际媒介H2NCH2CN与H2O反应中的H2O分子偶合质子转移机理和氢隧道效应

H2NCH2CN+H2O→H2NCH2C(OH)NH是一个重要的反应, 涉及到星际媒介中甘氨酸的形成, 与早期地球上的氨基酸起源有关. 如果没有考虑氢隧道效应, 在MP2/6-311+G(d,p)级别上计算反应能垒是254.7 kJ·mol-1, 在星际媒介中该气相反应很难进行. 在星际媒介冰颗粒表面上, 水分子催化反应增强了该化学反应的活性. H2NCH2CN与(H2O)3反应中的两个水分子作为催化剂降低活化能77.5 kJ·mol-1和活化自由能70.9 kJ·mol-1, 并且通过氢键桥协同传递质子. 量子氢隧道对于该反应进行至关紧要,采用小弯曲隧道(SCT)近似和正则变分过渡态理论(CVT)方法研究. 温度50 K时, 速率常数kSCT/CVT为1.86×10-23 cm3·molecule-1·s-1, 表明在星际媒介中通过质子隧道机理该反应容易进行. 研究结果与地球上的氨基酸起源于地球本身物质的观点相一致.

H2O-Coupled Proton Transfer Mechanism and HydrogenTunneling Effects in the Reaction of H2NCH2CN with H2O in the Interstellar Medium

The reaction H_2NCH_2N + H_2O→H_2NCH_2C(OH)NH is an important reaction related to the formation of glycine in the interstellar medium (ISM) and the origin of amino acids in early Earth. This reaction in the gas phase proceeds slowly and this is confmned by its energy barrier of 254.7 kJ·mol~(-1) as determined at the MP2/6-311+(d,p) level without considering hydrogen tunneling in the ISM. On icy grain surfaces in the ISM, however, its reactivity may be enhanced by H_2O catalytic reactions. The reaction of H_2NCH_2N with (H_2O)_3 is optimal as two H_2O molecules act as a catalyst with reducing the activation energy by 77.5 kJ·mol~(-1) and the activation free energy by 70.9 kJ·mol~(-1) A proton moves in a concerted way through a hydrogen bonded bridge. Quantum hydrogen tunneling is a crucial pathway for this reaction to proceed and was investigated using the small curvature tunneling (SCT) approximation and canonical variational lransition state theory (CVT). At 50 K, rate constant k~(SCT/CVT) is 1.86×10~(-23) cm~3·molecule~(-1)·s~(-1), which demonstrates that the reaction is feasible by a hydrogen tunneling mechanism in the ISM. These results may support the theory that the origin of amino acids is from the materials of the Earth itself.