Theoretical study of reactions of N₂O with NO and OH radicals

The reactions of N₂O with NO and OH radicals have been studied using ab initio molecular orbital theory. The energetics and molecular parameters, calculated by the modified Gaussian-2 method (G2M), have been used to compute the reaction rate constants on the basis of the TST and RRKM theories. The reaction N₂O + NO → N₂ + NO₂ (1) was found to proceed by direct oxygen abstraction and to have a barrier of 47 kcal/mol. The theoretical rate constant, k₁ = 8.74 × 10^-19 × T^2.23 exp (-23,292/T) cm³3 molecule^-1 s^-1, is in close agreement with earlier estimates. The reaction of N₂O with OH at low temperatures and atmospheric pressure is slow and dominated by association, resulting in the HONNO intermediate. The calculated rate constant for 300 K ≤ T ≤ 500 K is lower by a few orders than the upper limits previously reported in the literature. At temperatures higher than 1000 K, the N₂ O + OH reaction is dominated by the N₂ + O₂ H channel, while the HNO + NO channel is slower by 2-3 orders of magnitude. The calculated rate constants at the temperature range of 1000-5000 K for N₂ O + OH → N₂ + O₂ H (2A) and N₂ O + OH → HNO + NO (2B) are fitted by the following expressions: k_2A = 2.15 × 10^-26 X T^-4.72 × exp(-18,400/T), k_2B = 1.96 × 10^-28 X T^4.33 × exp(-12,623/T), in units of cm³ molecule^-1 s^-1. Both N₂O + NO and N₂O + OH reactions are confirmed to enhance, albeit inefficiently, the N₂O decomposition by reducing its activation energy.