Structure of hydrogen tetroxide in gas phase and in aqueous environments: relationship to the hydroperoxyl radical self-reaction

Hydrogen polyoxides are important species in atmospheric chemistry, advanced oxidation processes for wastewater treatment, and biological processes, among other fields. However, the electronic structure and chemical properties of the largest synthesized members of this chemical family remain poorly understood. In the present work, we have carried out a detailed theoretical study of hydrogen tetroxide (HO4H), which is a reaction intermediate of the hydroperoxyl radical (HO2) self-reaction. We have considered the molecule in gas phase, in microhydrated environments, in bulk water solution, and at the air-water interface. Very high level ab initio calculations have been carried out to describe the isolated molecule and the water complexes. Combined QM/MM molecular dynamics simulations have been performed to describe the system in liquid water and at the water surface. We show that the interactions with water strongly stabilize the tetraoxide adduct with respect to the (HO2)(2) dimer. The chemical process leading to hydrogen tetroxide from two separated hydroperoxyl radicals is predicted to be an exothermic and exergonic reaction at 298 K in all the studied media, with the reaction free energy being slightly smaller (in absolute value) in the condensed phase with respect to the gas phase. An estimation of the pK(a) of hydrogen tetroxide has been reported (7.3), which suggests that this species is less acidic than previously thought.