Calculations of the thermochemistry of six reactions leading to ammonia formation in Titan's atmosphere

Abstract: The thermochemical properties of the six reactions: (1) N2 (solar EUV)→N+ +N(4S)+e−, (2) N+ +H2 →NH+ +H, (3) NH+ +H2 →NH+ 2 +H, (4) NH+ 2 +H2 →NH+ 3 +H, (5) NH+ 3 +H2 →NH+ 4 +H, and (6) NH+ 4 +e− →NH3 +H, were theoretically proposed by Atreya in 1986 and were cited in 2003 by Bernard who assumed that this chain reaction would lead to ammonia formation in Titan''s atmosphere. The thermochemical properties of these six reactions have been calculated by means of the coupled cluster singles and doubles (CCSD) at the CCSD/cc-pvdz level, and the CCSD/6-311++g(3df,3pd) level, and G2 method. The geometries of the reactants and products of reactions have been optimized, the energies of reactions have been computed. The analysis of the results shows that: (I) The free energies of four reactions among these six reactions are negative. It means that these reactions, namely reactions (1)–(6) except reaction (2), can react spontaneously in Titan''s low temperature environment. The converted temperatures of reactions (3) and (5) are 11881.7 and 4596.9 K, respectively. (II) Reaction (2) is an endothermic reaction, its converted temperature is 1797.6 K. When , reaction (2) cannot react forward spontaneously. The barrier of reaction (2) is 26.154 kcal mol−1, which is probably too high to allow it to occur in the atmosphere of Titan. The rate for this reaction at 300 K has been calculated, and the value is . (III) The results of the three methods are more or less the same. So it is concluded that this chain reaction cannot be a pathway to lead to ammonia (gas phase) formation in Titan''s atmosphere. [Copyright &y& Elsevier]