The mobility and solvation structure of a hydroxyl radical in a water nanodroplet: a Born-Oppenheimer molecular dynamics study

Optical and Chemical Measurements of Solvated Electrons Produced in Plasma Electrolysis with a Water Cathode

It is known that glow discharges with a water anode inject and form solvated electrons at the plasma-liquid interface, driving a wide variety of reduction reactions. However, in systems with a water cathode, the production and role of solvated electrons are less clear. Here, we present evidence for the direct detection of solvated electrons produced at the interface of an argon plasma and a water cathode via absorption spectroscopy. We further quantify their yield using the dissociative electron attachment of chloroacetate, measuring a yield of 1.04 ± 0.59 electrons per incident ion, corresponding to approximately 100% faradaic efficiency. Additionally, we estimate a yield of 2.09 ± 0.93 hydroxyl radicals per incident ion. Comparison of this yield with other findings in the literature supports that these hydroxyl radicals are likely formed directly in the liquid phase rather than by diffusion from the vapor phase.

Valorization of Glycerol through Plasma-Induced Transformation into Formic Acid

To cope with climate change issues, a significant shift is required in worldwide energy sources. Hydrogen and bioenergy are being considered as alternatives toward a carbon neutral society, making formic acid - a hydrogen carrying product of glycerol - of interest for the valorization of glycerol. Here we investigate the plasma-induced transformation of glycerol in an aqueous nanosecond repetitively pulsed discharge reactor. We found that the water content in the aqueous mixture fulfilled a crucial role in both the gas phase (as a source of OH radicals) and the liquid phase (as a promotor of the dissolved OH radical's mobility and reactivity). The formic acid produced was linearly proportional to the specific input energy, and the most cost-effective production of formic acid was found with 10 % v/v glycerol in the aqueous mixture. A plausible reaction pathway was proposed, consisting of the OH radical-driven dehydrogenation and dehydration of glycerol. The results provide a fundamental understanding of plasma-induced transformation of glycerol to formic acid and insights for future practical applications.

The effects of the gas-liquid interface and gas phase on Cl/ClO radical interaction with water molecules

In the marine boundary layer (MBL), chlorine (Cl) and chlorine monoxide (ClO) are powerful oxidants with high concentrations. The gas-liquid interface is also ubiquitous in the MBL as a favorable site for atmospheric reactions. Understanding the role of water in Cl/ClO radical chemistry is essential for predicting their behavior in the atmosphere and developing effective strategies for mitigating their harmful effects. However, the research studies on the system of Cl/ClO radicals on the surface of water droplets are still insufficient. In previous studies, we have found unique results related to the hydroxyl radical at the interface using ab initio molecular dynamics (AIMD). In this work, we have used AIMD to investigate interactions between Cl/ClO radicals and water molecules at the gas-liquid interface. Radical mobility, radial distribution functions, coordination, and population analyses were conducted to investigate the surface preference, bonding pattern, and track Cl/ClO radicals in the water droplets. In addition, density functional theory (DFT) analysis was conducted to compare the results at the gas-liquid interface with those in the gas phase. We found that Cl/ClO radicals tend to remain near the gas-liquid interface in water droplet systems and outside of water clusters in gas phase systems. The ClO radical can form O*-H and Cl-O bonds with water molecules; however, neither the O*-O hemibond nor the Cl-H bond was detected in all systems. Different dominant structures were obtained for ClO in the interface and gas phase. The ClO radical can be bonded to one water molecule from its oxygen side, (H2O)0-Cl-O*-(H2O)1 at the interface, or to two water molecules from the chlorine and oxygen sides, (H2O)1-Cl-O*-(H2O)1 in the gas phase. Meanwhile, the Cl radical can only form a dominant structure like Cl*-(H2O)1 at the gas-liquid interface by making a Cl*-O hemibond. Providing a thorough explanation of the Cl/ClO radical behavior at the gas-liquid interface, this study will improve our understanding of the MBL's oxidizing capacity and pollution causes.

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

The surface-bulk partitioning of small saccharide and amide molecules in aqueous droplets was investigated using molecular dynamics. The air-particle interface was modeled using a 80 Å cubic water box containing a series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water bulk were examined at a molecular level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air-water interface as well as about the nonreactive interaction of the OH radicals with the organic solutes. It suggests that increased concentrations, preferred orientations, and decreased solvation near the air-water surface may lead to differences in reactivities between surface-active and surface-inactive molecules. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk.

Electrostatics and Chemical Reactivity at the Air-Water Interface

It has been recently discovered that chemical reactions at aqueous interfaces can be orders of magnitude faster compared to conventional bulk phase reactions, but despite its wide-ranging implications, which extend from atmospheric to synthetic chemistry or technological applications, the phenomenon is still incompletely understood. The role of strong electric fields due to space asymmetry and the accumulation of ions at the interface has been claimed as a possible cause from some experiments, but the reorganization of the solvent around the reactive system should provide even greater additional electrostatic contributions that have not yet been analyzed. In this study, with the help of first-principles molecular dynamics simulations, we go deeper into this issue by a careful assessment of solvation electrostatics at the air-water interface. Our simulations confirm that electrostatic forces can indeed be a key factor in rate acceleration compared to bulk solution. Remarkably, the study reveals that the effect cannot simply be attributed to the magnitude of the local electric field and that the fluctuations of the full electrostatic potential resulting from unique dynamical behavior of the solvation shells at the interface must be accounted for. This finding paves the way for future applications of the phenomenon in organic synthesis, especially for charge transfer or redox reactions in thin films and microdroplets.

High-throughput mapping of RNA solvent accessibility at the single-nucleotide resolution by RtcB ligation between a fixed 5'-OH-end linker and unique 3'-P-end fragments from hydroxyl radical cleavage

Given the challenges for the experimental determination of RNA tertiary structures, probing solvent accessibility has become increasingly important to gain functional insights. Among various chemical probes developed, backbone-cleaving hydroxyl radical is the only one that can provide unbiased detection of all accessible nucleotides. However, the readouts have been based on reverse transcription (RT) stop at the cleaving sites, which are prone to false positives due to PCR amplification bias, early drop-off of reverse transcriptase, and the use of random primers in RT reaction. Here, we introduced a fixed-primer method called RL-Seq by performing RtcB Ligation (RL) between a fixed 5'-OH-end linker and unique 3'-P-end fragments from hydroxyl radical cleavage prior to high-throughput sequencing. The application of this method to E. coli ribosomes confirmed its ability to accurately probe solvent accessibility with high sensitivity (low required sequencing depth) and accuracy (strong correlation to structure-derived values) at the single-nucleotide resolution. Moreover, a near-perfect correlation was found between the experiments with and without using unique molecular identifiers, indicating negligible PCR biases in RL-Seq. Further improvement of RL-Seq and its potential transcriptome-wide applications are discussed.