The atmospheric reaction (1) OH + O3-->HO2 + O2 was investigated theoretically by using MP2, QCISD, QCISD(T), and CCSD(T) methods with various basis sets. At the highest level of theory, namely, QCISD, the reaction is direct, with only one transition state between reactants and products. However, at the MP2 level, the reaction proceeds through a two-step mechanism and shows two transition states, TS1 and TS2, separated by an intermediate, Int. The different methodologies employed in this paper consistently predict the barrier height of reaction (1) to be within the range 2.16-5.11 kcal mol-1, somewhat higher than the experimental value of 2.0 kcal mol-1.

The atmospheric reaction NO2 + O3 --> NO3 + O2 (1) has been investigated theoretically by using the MP2, G2, G2Q, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. The results show that the reaction pathway can be divided in two different parts at the MP2 level of theory. At this level, the mechanism proceeds along two transition states (TS1 and TS2) separated by an intermediate, designated as A. However, when the single-reference higher correlated QCISD methodology has been employed, the minimum A and the transition state TS2 are not found on the hypersurface of potential energy, which confirms a direct reaction mechanism. Single-reference high correlated and multiconfigurational methods consistently predict the barrier height of reaction (1) to be within the range 2.5-6.1 kcal mol(-1), in reasonable agreement with experimental data. The calculated reaction enthalpy is -24.6 kcal mol(-1) and the reaction rate calculated at the highest CASPT2 level, of k = 6.9 x 10(-18) cm(3) molecule(-1) s(-1). Both results can be regarded also as accurate predictions of the methodology employed in this article.

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The atmospheric reaction NH2 + O3-->H2NO + O2 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 to be within the range of 3.9 to 6.6 kcal mol-1, which is somewhat higher than the experimental value. The calculated reaction enthalpy is -67.7 kcal mol-1.

The deviation of the NH(2) pseudo-first-order decay Arrhenius plots of the NH(2) + O(3) reaction at high ozone pressures measured by experimentalists, has been attributed to the regeneration of NH(2) radicals due to the subsequent reactions of the products of this reaction with ozone. Although these products have not yet been characterized experimentally, the radical H(2)NO has been postulated, because it can regenerate NH(2) radicals through the reactions: H(2)NO + O(3) --> NH(2) + O(2) and H(2)NO + O(3) --> HNO + OH + O(2). With the purpose of providing a reasonable explanation from a theoretical point of view to the kinetic observed behaviour of the NH(2) + O(3) system, we have carried ab initio electronic structure calculations on both H(2)NO + O(3) possible reactions. The results obtained in this article, however, predict that of both reactions proposed, only the H(2)NO + O(3) --> NH(2) + O(2) reaction would regenerate indeed NH(2) radicals, explaining thus the deviation of the NH(2) pseudo-first-order decay observed experimentally.