Radical-mediated proton transfer enables hydroxyl radical formation in charge-delocalized water

The Grotthuss mechanism has long defined our understanding of proton transfer in water, where protons migrate through hydrogen bond networks <i>via</i> structural diffusion. However, whether radical intermediates participate in this process remains unresolved. Herein, we demonstrate a radical-mediated proton transfer mechanism in acidic aqueous solutions. Using electron paramagnetic resonance spectroscopy, we detect hydroxyl radicals (˙OH) and water radical cations (H₂O˙⁺), while isotope-labeled high-resolution mass spectrometry confirms the presence of crown ether H₂O˙⁺ complexes in HCl solutions. Quantum simulations reveal that excess charge is delocalized over neighboring water molecules rather than localized on the hydronium ion (H₃O⁺), forming positively charged water clusters. We thus reveal that the proton transfer proceeds through ˙OH⋯H⁺···H₂O intermediates under ambient conditions. Our findings extend beyond the Grotthuss framework, proposing a mechanism driven by hydrogen bond imbalance and charge delocalization that spontaneously generates ˙OH in acidic environments. This work advances the understanding of proton transfer in water and has implications for acid-induced reactions, electrochemical processes, and degradation mechanisms in energy technologies.