A Comprehensive Study of the Reaction NH2+ NO → Products: Reaction Rate Coefficients, Product Branching Fractions, andab InitioCalculations

Abstract

We have undertaken a comprehensive study of the reaction NH2(X2B1) + NO → N2 + H2O (1a) and NH2(X 2B1) + NO → N2H + OH → N2 + H + OH (1b). Experimental measurements of the reaction rate coefficient and product branching fraction are combined with accurate ab initio calculations to give a detailed picture of this important reaction. The rate constant of reaction 1 was investigated in the temperature range 203 K ≤ T ≤ 813 K using the laser photolysis/CW laser-induced fluorescence technique for production and detection of NH2. The rate coefficient was found to be pressure independent between 10 and 100 Torr and is well described by k1(T) = 1.65 × 10-7T -1.54 exp (−93 K/T) cm3/(molecule·s). The deuterium kinetic isotope effect for the reactions of NH2 and ND2 with NO was investigated at temperatures between 210 and 481 K. A small, temperature-independent isotope effect of kH/kD = 1.05 ± 0.03 was found. Additional experimental work focused on measuring the product branching fraction for production of OH, Φ1b, and its deuterium isotope effect at room temperature. Measurements were performed using the discharge-flow technique with mass spectrometric detection of products. OH from channel 1b was reacted with excess CO and measured as CO2. The room temperature branching fraction was measured as Φ1b = 9.0 ± 2.5% (NH2 + NO; T = 298 K) and Φ1b = 5.5 ± 0.7% (ND2 + NO; T = 298 K). Theoretical calculations have characterized the stationary points on the potential energy surface connecting reactants with products using G2 and G2Q levels of theory. These calculations support the experimentally observed temperature dependences and kinetic isotope effects.