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Approach to the Atmospheric Chemistry of Methyl Nitrate and Methylperoxy Nitrite. Chemical Mechanisms of Their Formation and Decomposition Reactions in the Gas Phase
journal contribution
posted on 2008-01-17, 00:00 authored by Juan F. Arenas, Francisco J. Avila, Juan C. Otero, Daniel Peláez, Juan SotoPotential energy surfaces, minimum energy reaction paths, minima, transition states, reaction barriers, and
conical intersections for the most important atmospheric reactions of methyl nitrate (CH3ONO2) and
methylperoxy nitrite (CH3OONO) on the electronic ground state have been studied (i) with the second-order
multiconfigurational perturbation theory (CASPT2) by computation of numerical energy gradients for stationary
points and (ii) with the density functional theory (DFT). The proposed mechanism explains the conversion of
unreactive alkyl peroxy radicals into alkoxy radicals: CH3O2 + NO ⇆ CH3OONO ⇆ CH3O + NO2 ⇆
CH3ONO2. Additionally, several discrepancies found in the comparison of the results obtained from the two
employed approaches are analyzed. CASPT2 predicts that all dissociation reactions into radicals occur without
an extra exit energy barrier. In contrast, DFT finds transition states for the dissociations of cis- and trans-methylperoxy nitrite into CH3O + NO2. Furthermore, multiconfigurational methods [CASPT2 and complete
active space SCF (CAS-SCF)] predict the isomerization of CH3ONO2 to CH3OONO to occur in a two-step
mechanism: (i) CH3ONO2 → CH3O + NO2; and (ii) CH3O + NO2 → CH3OONO. The reason for this has
to do with the coupling of the ground electronic state with the first excited state. Therefore, it is demonstrated
that DFT methods based on single determinantal wave functions give an incorrect picture of the aforementioned
reaction mechanisms.