Ab initio chemical kinetics for the NH₂ + HNO_x reactions, part I: Kinetics and mechanism for NH₂ + HNO

The kinetics and mechanism for the reaction of NH₂ with HNO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single-point calculations at the CCSD(T)/6-311+G(3df, 2p) level based on geometries optimized at the CCSD/6-311++G(d, p) level. The major products of this reaction were found to be NH₃ + NO formed by H-abstraction via a long-lived H ₂N···HNO complex and the H₂NN(H)O radical intermediate formed by association with 26.9 kcal/mol binding energy. The rate constants for formation of primary products in the temperature range of 300-3000 K were predicted by variational transition state or RRKM theories. The predicted total rate constants at the 760 Torr Ar pressure can be represented by k_total = 3.83 × 10^-20 × T ^+2.47exp(1450/T )at T = 300-600 K; 2.58 x 10^-22 × T^+3.15 exp(1831/T )cm³ molecule^-1 s ^-1 at T = 600-3000 K. The branching ratios of major channels at 760 Torr Ar pressure are predicted: k₁ + k₃ + k₄ producing NH₃ + NO accounts for 0.59-0.90 at T = 300-3000 K peaking around 1000 K, k₂ accounts for 0.41-0.03 at T = 300-600 K decreasing with temperature, and k₅ accounts for 0.07-0.27 at T > 600 K increasing gradually with temperature. The NH₃ + NO formation rate constant was found to be a factor of 3-10 smaller than that of the isoelectronic reaction CH₃ + HNO producing CH₄ + NO, which has been shown to take place by barrierless H-abstraction without involving a hydrogen-bonding complex as in the NH2 case.