Spontaneous Water Radical Cation Oxidation at Double Bonds in Microdroplets

Abstract

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]+·: a covalent adduct of water radical cation (H2O+·) 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.

Simultaneous and Spontaneous Oxidation and Reduction in Microdroplets by the Water Radical Cation/Anion Pair

“…The water radical cation adduct ( 2 a ) was observed at m/z 142. This observation is consistent with a high‐throughput DESI‐MS analysis of a library of 21 000 non‐proprietary chemicals, in which water radical cation adducts ([M+H 2 O] + ⋅) of significant intensity were found in 65 % of all the X=Y double bond‐containing compounds [27] . Replacing water by heavy water (D 2 O) gave the corresponding deuterated ions (Figure 1c) while [M+18] + ⋅ was still seen due to rapid H−D exchange.…”

“…These facts suggest that the substituents on the reactants greatly affect the outcome of the redox reactions. The proposed oxidation mechanism, which is analogous to the spontaneous oxidation of the carbon-heteroatom double bond in microdroplets, [26,27] involves the production of the water radical cation adduct, followed either by direct homolysis of the PÀ C bond or by 1,2-migration and then CÀ O cleavage (see Supporting Information, Figure S7). The reduction mechanism is still under investigation.…”

“…This observation is consistent with a high-throughput DESI-MS analysis of a library of 21 000 non-proprietary chemicals, in which water radical cation adducts ([M + H 2 O] + * ) of significant intensity were found in 65 % of all the X=Y double bond-containing compounds. [27] Replacing water by heavy water (D 2 O) gave the corresponding deuterated ions (Figure 1c) while [M + 18] + * was still seen due to rapid HÀ D exchange. Tandem mass spectrometry (see Figure S1 in Supporting Information) showed that the water adduct, RP(OR) 2 (OH) 2…”

Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets

“…The strong oxidizing power of H 4 O 2 + has been beautifully illustrated by the spontaneous oxidation in water microdroplets of benzene, [26] aromatic sulfones to sulfonic acids, [27] and the oxidation of molecules containing double bonds. [28] The H 4 O 2 + cation has commonly been referred to as the water dimer cation, but as we will argue this often might be a misnomer. The H 4 O 2 + cation can exist in two forms.…”

“…It seems that the H 4 O 2 + cation has not previously been observed in uncharged water microdroplets as reported here. The strong oxidizing power of H 4 O 2 + has been beautifully illustrated by the spontaneous oxidation in water microdroplets of benzene, [26] aromatic sulfones to sulfonic acids, [27] and the oxidation of molecules containing double bonds [28] …”