Theoretical rate constants for the NH₃ + NO_x → NH₂ + HNO_x (x = 1, 2) reactions by ab initio MO/VTST calculations

Potential energy surfaces for the reactions of NH₃ with NO_x (x = 1, 2) have been studied by ab initio molecular orbital Gaussian 1 (G1) and Gaussian 2 (G2) methods. Both reactions have been shown to be endothermic and to proceed by the abstraction of a hydrogen atom from ammonia to produce NH₂ and HNO_x. The calculated heats of reaction are in close agreement with experimental measurements. Reaction 1, NH₃ + NO, does not have a reverse barrier at the G2 level of theory. Reaction 2, NH₃ + NO₂, can occur by three channels, leading to HNO₂ (2a), cis-HONO (2b), and trans-HONO (2c), and each mechanism involves the formation of NH₂·HNO₂ or NH₂·HONO intermediate complexes. Mechanism 2b has been found to be dominant. Theoretical rate constants for (1), (2b); and their reverse reactions have been computed by VTST in conjunction with detailed balancing for the temperature range of 300-5000 K. The following least-squares fitted expressions are recommended for practical applications: k₁ = 1.72 × 10^-17T^1.73 e^-28454/T, k_-1 = 6.02 × 10-^1.63T^1.63 e^630/T, k_2b = 3.92 × 10^-23T^3.41 e^-11301/T, k_-2b = 11.8 × 10^-23T^3.02 e^2487/T, in cm³/(molecule·s). The apparent activation energies calculated variationally for 300 ≤ T ≤ 1000 K, 58.3 and 25.6 kcal/mol for (1) and (2), respectively, agree well with experiments.