Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

Despite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH⋅) at the air-water interface. Previous studies exhibited the recombination of OH⋅ into H₂ O₂ molecules and the capture of OH⋅ by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH⋅ in the form of (H₄ O₂ )⁺ that is essentially OH⋅-H₃ O⁺ , a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH⋅ scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H₄ O₂ )⁺ should be attributed to reactions with OH⋅-H₃ O⁺ rather than with the water dimer cation (H₂ O-OH₂ )⁺ .