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 H2O2 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 (H4O2)+ that is essentially OH⋅−H3O+, 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 (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+.

We report the use of 1,2,3-triazole (Tz)-containing water microdroplets for gas-phase carbon dioxide (CO 2 ) reduction at room temperature. Using a coaxial sonic spraying setup, the CO 2 can be efficiently captured by Tz and converted to formic acid (HCOOH; FA) at the gas−liquid interface (GLI). A mass spectrometer operated in negative ion mode monitors the capture of CO 2 to form the bicarbonate anion (HCO 3 − ) and conversion to form the formate anion (HCOO − ). Varied FA species were successfully identified by MS/MS experiments including the formate monomer ([FA − H] − , m/z 45), the dimer ([2FA − H] − , m/z 91; [2FA + Na − 2H] − , m/z 113), the trimer ([3FA − H] − , m/z 137), and some other adducts (such as [FA − H + H 2 CO 3 ] − , m/z 107; [2FA + Na − 2H + Tz] − , m/z 182).The reaction conditions were systematically optimized to make the maximum conversion yield reach over 80% with an FA concentration of approximately 71 ± 3.1 μM. The mechanism for the reaction is speculated to be that Tz donates the proton and the hydroxide (OH − ) at the GLI, resulting in a stepwise yield of electrons to reduce gas-phase CO 2 to FA.

Previous studies have shown that hydroxyl radicals can be formed at the water−gas surface of water microdroplets. We report the use of in situ generated hydroxyl radicals to carry out an organic transformation in one step, namely, the formation of anilines from aryl acids as well as both ammonia and primary/secondary amines via decarboxylation. Benzoic acids and amines are dissolved in water, and the solution is sprayed to form microdroplets whose chemical contents are analyzed mass spectrometrically. All intermediates and products are determined using mass spectrometry (MS) as well as in some cases tandem mass spectrometry (MS 2 ). These results support the following reaction mechanism: NR 2 OH, formed via reaction of the amine with •OH, reacts with benzoic acid to form an isocyanate via a Lossen rearrangement. Hydrolysis followed by liberation of CO 2 then delivers the aniline product. Notably, the scope of this transformation includes a variety of amines and aromatic acids and enables their conversion into aniline and N-substituted anilines, all in a single step. Additionally, this reaction occurs at room temperature and does not require metal catalysts or organic solvents.

Figure 1. Schematic diagram of the experimental setup. Communication pubs.acs.org/JACS

The properties of water microdroplets strikingly differ from bulk water. Using room-temperature water microdroplets, we find that toluene can react with CO2 to form phenylacetic acid in one step without any catalyst with negative high voltage applied at the sprayer source. The chemical components of these microdroplets are identified by mass spectrometry, and product structures are confirmed by tandem mass spectrometry. In this manner, we generate three drug molecules in a single step: 4-aminophenylacetic acid (epithelial peptide transporter PepT1 inhibitor), 3,4-dihydroxyphenylacetic acid (dopamine metabolite neurotransmitter), and phenylacetic acid (sodium salt form; treatment of urea cycle disorder). Mechanistic studies show that benzyl radicals formed from hydroxyl radicals at the water microdroplet interface drive these carboxylation reactions. This water microdroplet chemistry is general, allowing activation and subsequent carboxylation of aryl α-C–H groups.

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 H2O2 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 (H4O2)+ that is essentially OH⋅−H3O+, 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 (H4O2)+ should be attributed to reactions with OH⋅−H3O+ rather than with the water dimer cation (H2O−OH2)+.

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