Theoretical characterization of halogenated methylperoxy nitrites CX Y3−OONO (X, Y = H, F, Cl)

THEORETICAL STUDY OF MECHANISM FOR THE ATMOSPHERIC REACTION CF3CHFO2 + NO

The mechanism of the reaction CF3CHFO2 + NO was investigated using ab initio and density functional theory (DFT). The optimized geometries for all stationary points on the reaction energy surface were calculated using MP2 and B3LYP methods with the aug-cc-pVDZ basis set. Single-point energy calculations were performed using the coupled cluster method with single, double and perturbative triple configurations, CCSD(T). The most important energy minima on the potential energy surface (PES) were found corresponding to two conformers of the peroxynitrite association adducts, cis- CF3CHFOONO and trans- CF3CHFOONO , and the nitrate, CF3CHFONO2 . The radical pairs ( CF3CHFO + NO2 ) and the nitrate are formed through the breaking of the peroxy bond of trans- CF3CHFOONO and the rearrangement of cis- CF3CHFOONO , respectively. The nitrate can be decomposed to carbonylated species ( CF3CHO or CF3CFO ), nitryl fluoride (NO2F), nitrous acid (HONO), and radical pairs ( CF3CHFO + NO2 ), which are of potential atmospheric importance.

Effect of halogenation on the mechanism of the atmospheric reactions between methylperoxy radicals and NO. A computational study

The mechanism of the reactions between the halogenated methylperoxy radicals, CHX(2)O(2) (X = F, Cl), and NO is investigated by using ab initio and density functional quantum mechanical methods. Comparison is made with the mechanism of the CH(3)O(2) + NO reaction. The most important energy minima in the potential energy surface are found to be the two conformers of the halogenated methyl peroxynitrite association adducts, CHX(2)OONOcp and CHX(2)OONOtp, and the halogenated methyl nitrates, CHX(2)ONO(2). The latter are suggested to be formed through the one-step isomerization of the peroxynitrite adduct and may lead upon decomposition to carbonylated species, CX(2)O + HONO and CHXO + XNO(2). The ambiguous issue of the unimolecular peroxynitrite to nitrate isomerization is reconsidered, and the possibility of a triplet transition state involvement in the ROONOtp <--> RONO(2) rearrangement is examined. The overall calculations and the detailed correlation with the methyl system show the significant effect of the halogenation on the lowering of the entrance potential energy well which corresponds to the formation of the peroxynitrites. The increased attractive character of the potential energy surface found upon halogenation combined with the increased exothermicity of the CHX(2)O(2) + NO --> CHX(2)O + NO(2) reaction are suggested to be the important factors contributing to the enhanced reactivity of the halogenated reactions relative to CH(3)O(2) + NO. The calculated heat of formation values indicate the large stabilization of the fluorinated derivatives.