An ab initio study on the mechanism of the atmospheric reaction NH2+O3→H2NO+O2

The atmospheric reaction NH2+O-3 -->H2NO+O-2 has been investigated theoretically by using MP2, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. At the MP2 level of theory, the hypersurface of the potential energy (HPES) shows a two step reaction mechanism. Therefore, the mechanism proceeds along two transition states (TS1 and TS2), separated by an intermediate designated as Int. However, when the single-reference higher correlated QCISD and the multiconfigurational CASSCF methodologies have been employed, the minimum structure Int and TS2 are not found on the HPES, which thus confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of the reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of the reaction to be within the range of 3.9 to 6.6 kcalmol(-1), which is somewhat higher than the experimental value.[1] The calculated reaction enthalpy is -67.7 kcalmol(-1).