Presence of H2SO4 in the atmosphere causes varieties of environmental problems. As known, H2SO4 can be yielded by the oxidation of H2SO3. In the atmosphere, the latter originates from abundant SO2 and moisture. Recently, a mechanism of H2SO3 formation on the ice surface in application to low temperatures has been described. Also, the rise of H2O2 concentration in polar core ice during the last decades has been observed. To the best of our knowledge, the detailed molecular mechanism of the H2SO3+H2O2 reaction resulting in the H2SO4 formation has been poorly investigated. Thereby, we suggest the possibility of oxidation of H2SO3 by H2O2 on the ice surface. Here, ab initio gas phase calculations have been done to understand the mechanism of H2SO4 formation by oxidation of H2SO3 with H2O2 in the gas phase as well as on the ice surface. In the gas phase, one stepwise and three concerted reaction pathways have been found. The stepwise mechanism proceeds via the formation of HOSO(OOH) intermediate which has been previously suggested in the experimental study. The concerted mechanisms proceed via the H2O2 O-O bond activation followed by the nucleophilic attack on the S atom. Two pathways with the addition of one H2O molecule have been found. First pathway proceeds via double proton transfer with the formation of (HO)3S(OOH) intermediate and the second one proceeds through H2O2 dissociation. It has been demonstrated that the addition of a water molecule can decrease the reaction barrier by as much as 16.0 kcal/mol. Additionally, one stepwise and one concerted mechanism of HSO3–+H2O2 reaction have been found possessing lower activation barriers than that for the H2SO3+H2O2 reaction. Further, ice surface calculations have been carried out. Two reaction mechanisms of H2SO3+H2O2 and one mechanism of the HSO3–+H2O2 reaction on ice surface have been revealed. In the case of H2SO3+H2O2 reaction, the first mechanism can be described as H2O2 O-O bond activation with subsequent H2O2 nucleophilic attack on the S atom with the energy barrier of 32.5 kcal/mol. Another mechanism proposed is the isomerization of H2O2 with the formation of H2OO followed by H2SO3+H2OO reaction. The energy barriers for these two steps are 31.4-34.7 and 9.5 kcal/mol, respectively. The mechanism of the HSO3–+H2O2 reaction is similar to the second one of the H2SO3+H2O2, however, has a lower energy barrier of 5.7 kcal/mol on the second reaction step.

Sulfuric acid formations by sulfurous acid and hydrogen peroxide in gas phase and on ice surface

2019 Fall National Meeting

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