N^++H_2→NH^++H反应机理的热力学研究

A Study on the Mechanism and Thermodynamic Properties of the Reaction N^++H_2→NH^++H

应用量子化学耦合簇理论CCSD和MP2方法对文[1]提出的Titan大气中可能生成NH3的链式反应中第二个反应:N++H2→NH++H进行了热力学计算以及反应机理的分析研究.发现:(a)反应在Titan环境中不具有反应自发性,其转变温度为1797.6K,高于这个温度反应才有可能自发正向进行;(b)此反应的过渡态为线形,正反应活化能为118.833kJ.mol-1,在不同温度下,尤其是低温下反应的平衡常数很小;(c)由于此反应存在高的反应势垒,可以认为这6个链式反应不是在Titan大气的低温环境中自然合成NH3的一个通道;(d)耦合簇理论方法和MP2理论两种方法的计算结果相吻合,互相印证了结论的可靠性.

The thermodynamical properties ofthe followingreaction： N^＋ ＋ H2→NH^＋ ＋ H,whieh was one reaction among six reactions that theoreticallyproposed by Atreya in 1986 and was cited in 2003 by Bernard who assumed that this chain reaction would lead to ammoniaformation in Titan＇s atmosphere have been calculated by means of the coupled duster singles and doubles （CCSD）at the CCSD/cc-pvdz level. The geometries of the reactants and products of reaction have been optimized, the energies of this reaction have been computed. Andthe mechanism of the reaction was investigated atthe MP2/6-311g（d） level. The analysis of the results shows that： （ Ⅰ ） The Free Energies of this reaction are positive, It means it can not conduct spontaneouslyin Titan＇ slow temperature environmem. （ Ⅱ ） The reaction is an endothermic reaction, its converted temperatures is 1797.6K. When T 〈 1797.6K, Thereaction can not conduct forward spontaneonsly. And such a barrier of 118. 833kJ/molis probably too high to allow the reaction to occur in the atmosphere of Titan. （ Ⅲ ） The resultis verified by vibrational analysis and IRC calculations. So, we can draw a conclnsion that this chain reaction is not a pathway tolead to ammonia （gas phase）formation in Titan＇s atmosphere.