Abstract
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 )+ .
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