Quantum mechanical analysis of reaction mechanisms between atomic oxygen species O(³P) and O(¹D) with water molecules

Electric gas discharge plasmas generated in oxygen-containing gas mixtures produce atomic oxygen in both the ground triplet state O(³P) and the excited singlet state O(¹D). Upon contact with humidity or aqueous solutions, these oxygen atoms undergo reactions with water molecules. Despite extensive experimental and computational investigations, the underlying reaction mechanisms between atomic oxygen species and water molecules remain a topic of debate. We explored these interactions using high-level quantum mechanical calculations (i.e., CASSCF and CCSD(T)). Our calculations predict the formation of a relatively stable singlet oxywater species, ¹O.OH₂, as the product of O(¹D) and H₂O. We examined the transition of the singlet oxywater species to hydrogen peroxide through a unimolecular (1,2)-hydrogen shift. We calculated the energy barrier of the reaction and estimated the associated reaction rate constant. Moreover, we investigated the impact of solvents on the reaction pathway using an implicit solvation model of water, which indicated that the singlet oxywater species have a longer lifetime in water environment than in the gas phase. Although the existence of oxywater species has yet to be experimentally confirmed, considering a reaction channel for the production of the oxywater species and thereby hydrogen peroxide in plasma simulations may be crucial in predicting the final concentration of hydrogen peroxide.