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

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.