Computational study on the mechanism and kinetics for the reaction between HO2 and n-propyl peroxy radical

The influence of (H2O)1-2 in the HOBr + HO2 gas-phase reaction

The HOBr + HO₂ reaction in the absence of water has three different channels for the abstraction of H to generate the corresponding products. The dominant channel is the generation of BrO + H₂O₂. The introduction of water molecules influences this dominant reaction via the way the reactants interact with the water molecules. The addition of water molecules decreases the energy barrier and increases the rate coefficient of the reaction. Interestingly, water works as a catalyst and we obtain BrO + H₂O₂, like in the reaction without water, or the water works as a reactant and we obtain products other than BrO + H₂O₂. The rate coefficients of the HOBr + HO₂ reaction in the presence of water are calculated to be faster than the reaction in the absence of water. However, other pathways in the presence of water are slower than the reaction in the absence of water. The water-assisted effective rate coefficients for the HOBr + HO₂ reaction are also larger than those for the reaction in the absence of water. The influence of a water dimer is not as important when compared with one water molecule. In summary, a single water molecule has a positive catalytic influence in enhancing the HOBr + HO₂ reaction.

Computational Study on the Reaction of β-Hydroxyethylperoxy Radical with HO2 and Effects of Water Vapor

The reaction of β-hydroxyethylperoxy radical (β-HEP) and HO₂ with and without water was studied using quantum chemistry and kinetic calculations. The main products are HOCH₂CH₂OOH and ³O₂ for the reaction with and without water, while all other reaction channels can be neglected. The rate coefficients of the reaction follow negative temperature dependence. The pseudo-second-order rate coefficients are 2-4 orders of magnitude smaller for the reaction with saturated water vapor, indicating the negligible contribution of water in this reaction. This is probably also true for other peroxy radicals (except for HO₂), indicating that a large part of previous results on the water enhancement of reaction rate coefficients might have overestimated the influence of water.