中文摘要：

运用耦合簇理论OCSD方法对文献[1]提出的Titan大气中可能生成NH3的链式反应中第三个反应：NH^＋＋H2→NH2^＋＋H进行了热化学计算和分析．发现：①反应（3）在Titan环境中具有良好的反应自发性，在低温下自发反应可能性更大．反应（3）的转变温度为11881．7K，高于这个温度反应才不可能自发正向进行．运用量子化学MP2方法在6—311g（d）基组下计算研究了反应的机理，发现：②此反应分为两步反应．第一步为放热反应，无能垒．第二步反应的过渡态为平面构型，正反应能垒仅为38．991Kcal·mol^-1，完全可以由第一步反应放出能量来补偿．结果均经过振动分析和IRC计算验证．在不同温度下，尤其是低温下第一步反应平衡常数很大．③由于此反应有较高的反应自发性，可以认为谊反应在这六个在Titan大气的低温环境中自然合成NH3的链式反应中起着积极的作用．④辐合簇理论方法和MP2理论两种方法的计算结果是一致的，互相印证了结论的可靠性．

英文摘要：

The thermodynamica! properties of the following reaction： NH^＋ ＋ H2→NH^＋ ＋ H, which was one reaction among six reactions that theoretically proposed by Atreya in 1986 and was cited in 2003 by Bernard who assumed that this chain reaction would lead to ammonia formation in Titan＇ s atmosphere have been calculated by means of the coupled cluster 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, And the mechanism of the reaction was investigated at the MP2/6 - 311g（d） level. The analysis of the results shows that; （1）The Free Energies of this reaction are negative. It means it can conduct spontaneously in Titan＇ s low temperature environment. （2）The reaction proceeds in two steps. The first step involves the formation of molecular complex （MC）. In this step, the energy of the reaction system descends and there is no potential barrier. The second step is the rearrangement of MC to products with potential barrier of 38. 991 Kcal·mol^-1; this step is the rate control step for the overall reaction. The result is verified by vibrational analysis and IRC calculations. （3）Because of the energy which produced in the first step can compensate the barrier of 38. 991 Kcal· mol^-1 so, the reaction can conduct forward spontaneously in Titan＇s atmosphere with low temperature.