Theoretical investigations on the reactions NH+HO2 and NH2+O2: Electronic structure calculations and kinetic analysis

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Electronic structure calculations at the MP2, B3LYP-DFT, and quadratic configuration interaction singles and doubles levels of theory, with 6-311++G** and 6-311++G(2df,2pd) basis sets, are reported for the stationary points on the NH+HO2 doublet potential energy surface. Also the transition state on the quartet surface for the direct hydrogen abstraction reaction has been identified. Two minima viz., HNOOH and NH2O2, of almost equal stability and with a very high interconversion barrier have been found. Preferential dissociation of HNOOH to HNO and OH is reported due to its high isomerization barrier. The favorable dissociation channels of the NH2O2 adduct are those leading to NH2+O2 and NH2O+O products. Detailed kinetic analyses have been performed on the calculated DFT-B3LYP potential energy surfaces using quantum statistical Rice–Ramsperger–Kassel theory. The calculated total rate constant for NH+HO2 reaction at 300 K and 1 atm is 1.52×1010 cm3 mol−1 s−1 and the predominant contribution to the disappearance of the HNOOH adduct is the HNO+OH dissociation channel, K31. The NH2+O2 reaction is found to be a slow reaction and the calculated rate coefficient is in good agreement with the upper limit predicted by the experimentalists.