Harnessing the High Interfacial Electric Fields on Water Microdroplets to Accelerate Menshutkin Reactions.

Even though it is still an emerging field, the application of a high external electric field (EEF) as a green and efficient catalyst in synthetic chemistry has recently received significant attention for the ability to deliver remarkable control of reaction selectivity and acceleration of reaction rates. Here, we extend the application of the EEF to Menshutkin reactions by taking advantage of the spontaneous high electric field at the air-water interfaces of sprayed water microdroplets. Experimentally, a series of Menshutkin reactions were accelerated by 7 orders of magnitude. Theoretically, both density functional theory calculations and ab initio molecular dynamics simulations predict that the reaction barrier decreases significantly in the presence of oriented external electric fields, thereby supporting the notion that the electric fields in the water droplets are responsible for the catalysis. In addition, the ordered solvent and reactant molecules oriented by the electric field alleviate the steric effect of solvents and increase the successful collision rates, thus facilitating faster nucleophilic attack. The success of Menshutkin reactions in this study showcases the great potential of microdroplet chemistry for green synthesis.

Spontaneous Reduction of Transition Metal Ions by One Electron in Water Microdroplets and the Atmospheric Implications.

Freshman chemistry teaches that Fe3+ and Cu2+ ions are stable in water solutions, but their reduced forms, Fe2+ and Cu+, cannot exist in water as the major oxidation state due to the fast oxidation by O2 and/or disproportionation. Contrary to these well-known facts, significant fractions of dissolved Fe and Cu species exist in their reduced oxidation states in atmospheric water such as deliquesced aerosols, clouds, and fog droplets. Current knowledge attributes these phenomena to the stabilization of the lower oxidation states by the complexation of ligands and the various photochemical or thermal pathways that can reduce the higher oxidation states. In this study, by spraying the water solutions of transition metal ions into microdroplets, we show the results of the spontaneous reduction of ligated Fe(III) and Cu(II) species into Fe(II) and Cu(I) species, presenting a previously unknown source of reduced transition metal ions in atmospheric water. It is the spontaneously generated electrons in water microdroplets that are responsible for the reduction. Control experiments in the atmosphere and in a glove box filled with precisely controlled gaseous contents reveal that O2, CO2, and NO2 are the major competitors for the electrons, forming O2-, HCO2-, and NO2-, respectively. Taking these findings together, we opine that microdroplet chemistry might play significant but previously underestimated roles in atmospheric redox chemistry.

Spontaneous oxidation of I- in water microdroplets and its atmospheric implications.

Atomic and molecular iodine, I˙ and I2, play important roles in the atmosphere, such as the catalytic depletion of ozone and the oxidation of gaseous elemental mercury. It is known that the major source of I˙ and I2 in the atmosphere is the photodissociation of organoiodine molecules released by algae in the sea. In this study, we show the striking results of the spontaneous and ultrafast oxidation of I- into I˙, which further evolves into I2- and I3- in water microdroplets, presenting a previously unknown source of I˙ and I2 in atmospheric water, such as the sea spray or cloud microdroplets. Mass spectrometric evidence shows that spontaneously generated hydroxyl radicals in water microdroplets are responsible for the oxidation of I-. Taken together, we opine that microdroplet chemistry may adopt significant roles in atmospheric redox chemistry.

An 241Am Plasma Desorption Ionization (AmDI) Source Scavenged from Smoke Detectors for Ambient Mass Spectrometry Sampling.

The development of efficient, low-cost, easy-to-use ambient ionization methods has been a major goal of modern mass spectrometry. In this Letter, we present a gas-free, voltage-free, economic, and safe desorption ionization method using the plasma generated by a radioactive element, americium-241, scavenged from smoke detectors that equip almost every household. No other energy sources, such as laser, discharge, fast-moving carrier gas, solvent droplet, ultrasound, or heat are needed. We name this new method as americium-241 desorption ionization (AmDI). AmDI is tested for the detection of more than 20 volatile and nonvolatile chemicals under different sampling conditions, and the detection limit can be in the range of tens of picograms for some analytes. Mechanistically, we provide evidence that the α particles emitted from radioactive decay ionize ambient air, and the resulting plasma further energizes and ionizes the surface analytes for mass spectrometry detection. We anticipate wide applications of AmDI in mass spectrometric sampling in the near future because of the plethora of merits.