by Furong Wang, Uli Schmidhammer, Aurélien de La Landea and Mehran Mostafavi

Oxidation by the ultra-short lived radical cation of water, H₂O˙⁺, can potentially take place at the interface of water and numerous heterogeneous systems involved in radiation therapy, energy and environmental industries. The oxidation processes induced by H₂O˙⁺ can be mimicked in highly concentrated solutions where the nearest neighbors of H₂O˙⁺ may be molecules other than water. The reactivity of H₂O˙⁺ and D₂O˙⁺ is probed in hydrogenated and deuterated sulfuric acid solutions of various concentrations. The oxidized solute, sulfate radical, is observed at 7 ps and remarkably higher yields are found in deuterated solutions. The isotopic effects reveal the competition between two ultrafast reactions: proton transfer toward H₂O (D₂O) and electron transfer from HSO₄⁻ to H₂O˙⁺ (D₂O˙⁺). Density functional theory simulations decipher the electron transfer mechanism: it proceeds via sub-femtosecond charge migration and is not affected by isotopic substitution. This work definitively demonstrates why direct oxidation triggered by H₂O˙⁺ can be competitive with proton transfer.