Atmospheric degradation mechanism of benzyl peroxy radical: A theoretical study

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.