An Ab Initio Study on the Mechanism of the Atmospheric Reaction NH₂+O₃→H₂NO+O₂

Abstract

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.

Ab Initio Study on the Mechanism of the Atmospheric Reaction OH+O3→HO2+O2

“…Therefore, this work gives us further indication that the experimental results obtained on the kinetic study of the NH 2 O 3 reaction [31] should be revised, as both our ab initio results and the experimental works of Back and Yokota [32] and Hack et al [33] predict that the reactivity of OH with ozone is higher than with NH 2 . A more rigorous critical evaluation of the results obtained here, however, indicates that although CCSD(T)//MP2 results agree fairly well with experimental data, the error introduced by the UMP2 ZPE of the ozone molecule makes CCSD(T)//QCISD results more reliable, especially when large basis sets are employed in both the single-point CCSD(T) energy calculations and the QCISD optimizations.…”

“…In our previous ab initio work on the mechanism of the NH 2 O 3 reaction [31] we pointed out this discrepancy and suggested that the experimental data should be revised. The calculations carried out in the present work on the isoelectronic OH O 3 system support this conclusion, because the barriers obtained at the same level of theory are always significantly lower for the OH O 3 reaction.…”

“…In fact, values of 0.045 were accepted as reliable in the work of Rienstra-Kiracofe et al, [35] so our highest value of 0.0481, obtained for the transition state TS1 at the CCSD(T)/6-311G-(3df,2p)//MP2/6-311 G(d,p) level, could be considered reliable enough. Moreover, the reliability of the single-reference methods employed to calculate the stationary points of the PES in this type of reactions was previously verified by us in a study on the isoelectronic NH 2 O 3 system [31] with the multiconfigurational CASSCF and CASPT2 methods. The fact that both QCISD(T) and CCSD(T) results fit reasonably well to experimental data can be considered to be one more indication of the successful performance of these kinds of methods in the study of reactions of ozone with small radicals such as OH, NH 2 , NO 2 , and NO.…”

“…It is in agreement with the reaction mechanism proposed for the Cl O 3 , NH 2 O 3 , and NO O 3 reactions, [29±31] which proceed through several steps at UMP2 level of theory. Hence, at the UMP2 level, reaction (1) [30,31] Although reaction (1) is a two-step reaction at the UMP2 level of theory, when the single-reference, higher correlated QCISD methodology was employed, the optimizations of the minimum Int and the transition state TS2 always converged towards the products, which confirms a direct reaction mechanism and that Int and TS2 could be interpreted as a mathematical artifacts of the UMP2 methodology, rather than actual stationary points on the PES of reaction (1). This is in agreement with the previously studied isoelectronic system NH 2 O 3 , [31] for which both singlereference QCISD and multiconfigurational CASSCF results confirm a single-step reaction mechanism.…”

Aspects of the Atmospheric Chemistry of Amides

“…In the atmosphere the radicals can in principle react with O 2 , NO, NO 2 or O 3 . The amino radical NH 2 [2,59] and also the N(CH 3 ) 2 radical [60] are known to react only slowly with O 2 , therefore, the atmospheric fate of the amino radicals in moderately polluted atmospheres is potentially reactions with NO, NO 2 and O 3 , as discussed in the literature, [2,59,60] with formation of nitrosamine and nitramine.…”