Theoretical Rate Constants for the NH3 + NOx → NH2 + HNOx (x = 1, 2) Reactions by ab Initio MO/VTST Calculations

Potential energy surfaces for the reactions of NH 3 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 NH 2 and HNO x . The calculated heats of reaction are in close agreement with experimental measurements. Reaction 1, NH 3 + NO, does not have a reverse barrier at the G2 level of theory. Reaction 2, NH 3 + NO 2, can occur by three channels, leading to HNO 2 (2a), cis -HONO (2b), and trans -HONO (2c), and each mechanism involves the formation of NH 2 ·HNO 2 or NH 2 ·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: k 1 = 1.72 × 10 -17 T 1.73 e -28454/ T, k -1 = 6.02 × 10 -17 T 1.63 e 630/ T, k 2b = 3.92 × 10 -23 T 3.41 e -11301/ T, k -2b = 11.8 × 10 -23 T 3.02 e 2487/ T, in cm 3 /(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.