Atmospheric degradation mechanism of benzyl peroxy radical: A theoretical study

Abstract

The reaction mechanisms of the benzyl peroxy radical (PhCH2O2) with NO, NO2, HO2, and self-reaction in atmosphere, were studied by the quantum chemistry calculations at the cam-B3LYP/dgtzvp and CCSD(T)/CBS levels of theory. The predominant products of the PhCH2O2+NO reaction were predicted to be PhCHO, NO2 and PhCH2ONO2 with certain amount of HONO, where the proportion of HONO is probably pressure dependent. PhCH2O2NO2, with thermal stability close to that of general alkylperoxy nitrates, is formed by the recombination of PhCH2O2 and NO2, whose reverse reaction is most favorable for PhCH2O2NO2 dissociation. The PhCH2O2+HO2 reaction will mainly form PhCH2O2H and O2 by hydrogen abstraction via a hydrogen bonded complex on the triplet potential energy surface. Meanwhile, this reaction on the singlet potential energy surface is also feasible under atmospheric condition. A newfound compound, dibenzyl tetroxide (PhCH2O4CH2Ph) formed from the dimerization of PhCH2O2, was 11.11 kcal/mol more stable than two PhCH2O2 radicals. The pathway of PhCH2O4CH2Ph leading to two PhCH2O radicals and an 3O2 molecule is the most favorable exit channel, in which 3O2 can abstract a hydrogen atom from anyone of both PhCH2O radicals to produce PhCHO and HO2, and HO2 then transfers the hydrogen atom to the other PhCH2O to form PhCH2OH with certain probability. The reaction of PhCH2O radical with O2 in atmosphere is the most probable pathway, which produces PhCHO and HO2. However, as the concentration of PhCH2O is adequate, the self-bimolecular reaction of PhCH2O forming PhCH2O2CH2Ph, PhCHO and PhCH2OH may also be important. The reaction mechanisms of this study are in good accordance with the reported experimental observations.