Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Spontaneous oxidation of compounds containing diverse X=Y moieties (e.g., sulfonamides, ketones, esters, sulfones) occurs readily in organic-solvent microdroplets. This surprising phenomenon is proposed to be driven by the generation of an intermediate species [M+H2O](+center dot): a covalent adduct of water radical cation (H2O+center dot) with the reactant molecule (M). The adduct is observed in the positive ion mass spectrum while its formation in the interfacial region of the microdroplet (i.e., at the air-droplet interface) is indicated by the strong dependence of the oxidation product formation on the spray distance (which reflects the droplet size and consequently the surface-to-volume ratio) and the solvent composition. Importantly, based on the screening of a ca. 21,000-compound library and the detailed consideration of six functional groups, the formation of a molecular adduct with the water radical cation is a significant route to ionization in positive ion mode electrospray, where it is favored in those compounds with X=Y moieties which lack basic groups. A set of model monofunctional systems was studied and in one case, benzyl benzoate, evidence was found for oxidation driven by hydroxyl radical adduct formation followed by protonation in addition to the dominant water radical cation addition process. Significant implications of molecular ionization by water radical cations for oxidation processes in atmospheric aerosols, analytical mass spectrometry and small-scale synthesis are noted.

Automated summary

Compounds containing diverse X=Y moieties (such as sulfonamides, ketones, esters, sulfones) readily undergo spontaneous oxidation in organic-solvent microdroplets, driven by the formation of an intermediate species [M+H2O](·+), which is a covalent adduct of water radical cation (H2O·+) with the reactant molecule (M). The presence of this adduct is observed in positive ion mass spectrometry, while its formation at the air-droplet interface is indicated by the dependence of oxidation product formation on spray distance and solvent composition. The formation of a molecular adduct with the water radical cation is a significant route to ionization in positive ion mode electrospray for compounds with X=Y moieties lacking basic groups.