Chemical reactions in micron-sized droplets are often orders of magnitude faster than the equivalent bulk reactions due to the large interfacial effects. Spontaneous oxidation or reduction without external oxidants or reductants has been reported in aqueous microdroplets. One explanation is the radical cation/anion pair (H2O+./ H2O-.), recently argued to exist in pure bulk water, to provide the redox species. In this work, we reported spontaneous oxidation of heteroatom double bonds (e.g. sulfone to sulfonic acid) in non-aqueous microdroplets containing traces of water (&lt;1%). Meanwhile, the simultaneous oxidation and reduction of several phosphonates was discovered, providing supporting evidence that the radical pair is the source of reactive species in water microdroplets. Aromatic sulfone solutions were sprayed using nanoelectrospray ionization and the microdroplet reactions were directly measured by mass spectrometry. The oxidation of sulfones to corresponding sulfonic acids were detected. The significant influence of water on the reaction extent, as well as the observation of a key intermediate, water radical cation adduct (M+H2O)+., suggest that the water radical cation is the oxidant. To further explore the generality of the spontaneous oxidation in microdroplets, a high throughput screening experiment was conducted based on a 21,000-compound library. The formation of water radical cation adduct was found with 18% occurrence over all molecules and more than 65% in the set containing heteroatom double-bond compounds. This result was further confirmed by model monofunctional molecules, including sulfone, sulfonamide, ketone, ester, amide, and carbamate, where the corresponding oxidation products were seen. The investigation on the phosphonates gave the striking result that concurrent conversions of several phosphonates to phosphonic acids by spontaneous reduction and to pentavalent phosphoric acids by spontaneous oxidation were seen in microdroplets. This again suggests that the active reagent is the water radical cation/anion pair. To confirm the source of those active species, pure water was sprayed, and the water radical cation was observed directly as the ionized water dimer while the water radical anion was observed indirectly though the spontaneous reduction of carbon dioxide to formate. The peculiar redox properties of water in microdroplets provide new insights into its reactivity and its potential as a green oxidant and/or reductant.

Spontaneous oxidation and reduction by endogenous radical cation and anion pairs in microdroplets

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