Generation of Phenol and Molecular Hydrogen through Catalyst-Free C-H Activation of Benzene by Water Radical Cations

Rapid evaluation of vegetable oil varieties and geographical origins by ambient corona discharge ionization mass spectrometry

The composition and ratio of unsaturated fatty acids in vegetable oils play a crucial role in determining their overall quality. In this study, we present a corona discharge ionization mass spectrometry (MS) method for the rapid differentiation of vegetable oil varieties and their geographical origins under environmental conditions. Abundant water dimer radical cations, (H2O)2+•, were generated by the ionization setup, which effectively activated carbon‑carbon double bonds (C=C) to form epoxidized products. These epoxidation products were analyzed using tandem MS, generating diagnostic fragment ions that precisely identified CC bond positions. Statistical analysis models were subsequently developed using the resulting MS fingerprint data, revealing significant differences between various vegetable oils and olive oils from different origins. Key advantages of this method include minimal sample preparation, rapid analysis, and easily interpretable spectra. This study provides a new MS-based strategy for food quality assessment and offers a promising tool for identifying CC positional isomers in complex systems.

Ultrafast Dual Activation of C(sp3)-H and C(sp2)-H Bonds in an Arc Plasma-Initiated Microdroplet

This study demonstrates a method that utilizes arc plasma-induced microdroplet reactions to synthesize dual-activated products with C(sp3)-N and C(sp2)-O bonds starting from C-H bonds. This innovative process utilizes arc- and microdroplet-generated hydroxyl radicals and water dimer radical cations, opening new possibilities for the multisite derivatization of small molecules.

Ambient catalyst-free oxidation reactions of aromatic amines using water radical cations

Water radical cations play a pivotal role in various scientific and industrial fields due to their unique reactivity and capacity to drive complex chemical transformations. Here we explored the formation of quaternary ammonium cations through the direct oxidation reaction of aromatic amines, facilitated by water radical cations within water microdroplets. This process was monitored via in situ mass spectrometry and occurs under ambient conditions, negating the need for traditional chemical catalysts or oxidants and achieving an impressive yield of approximately 80%. Additionally, we employed a multi-channel spray system and enhanced both the reactant concentration and flow rate, thereby enabling gram-scale synthesis. These findings not only demonstrate the effectiveness and eco-friendliness of microdroplet chemistry but also provide a new understanding of heterogeneous ˙OH generation channels, thereby boosting the synthetic efficiency and sustainability of chemical processes.

Observing C-N bond formation in plasma: a case study of benzene and dinitrogen coupling via an arylnitrenium ion intermediate

Directly fixing dinitrogen into value-added organics is one of the core issues, and yet a long-standing challenge, in chemical synthesis. In earlier discrete studies, direct amination of benzene with N2 has been achieved via non-thermal plasma-liquid reaction. Nonetheless, the reaction mechanism thereof remains elusive and the amination product was non-selective primarily including aniline and diphenylamine. Herein, non-thermal plasma reaction in combination with on-line mass spectrometry was employed to probe the reaction pathway by on-line detection of the transient intermediate and the stable amination product. The long-lived atomic nitrogen ions N+(3P) as well as the arylnitrenium ions' intermediacy were shown to play a pivotal role in the amination process, and the product distribution was affected by an external hydrogen source and likely dependent on the competing hydrogen abstraction reaction and intersystem crossing of the initially generated triplet state arylnitrenium ions. The mechanistic investigation in this work has implications for plasma-based nitrogen conversion into organics, but also has broader relevance for understanding the C-N coupling by other means directly with N2.

Spontaneous Oxidation in Aqueous Microdroplets: Water Radical Cation as Primary Oxidizing Agent

Exploration of the unique chemical properties of interfaces can unlock new understanding. A striking example is the finding of accelerated reactions, particularly spontaneous oxidation reactions, that occur without assistance of catalysts or external oxidants at the air interface of both aqueous and organic solutions (provided they contain some water). This finding opened a new area of interfacial chemistry but also caused heated debate regarding the primary chemical species responsible for the observed oxidation. An overview of the literature covering oxidation in microdroplets with air interfaces is provided, together with a critical examination of previous findings and hypotheses. The water radical cation/radical anion pair, formed spontaneously and responsible for the electric field at or near the droplet/air interface, is suggested to constitute the primary redox species. Mechanisms of accelerated microdroplet reactions are critically discussed and it is shown that hydroxyl radical/hydrogen peroxide formation in microdroplets does not require that these species be the primary oxidant. Instead, we suggest that hydroxyl radical and hydrogen peroxide are the products of water radical cation decay in water. The importance of microdroplet chemistry in the prebiotic environment is sketched briefly and the role of partial solvation in reaction acceleration is noted.

Efficient catalyst-free N2 fixation by water radical cations under ambient conditions

The growth and sustainable development of humanity is heavily dependent upon molecular nitrogen (N2) fixation. Herein we discover ambient catalyst-free disproportionation of N2 by water plasma which occurs via the distinctive HONH-HNOH+• intermediate to yield economically valuable nitroxyl (HNO) and hydroxylamine (NH2OH) products. Calculations suggest that the reaction is prompted by the coordination of electronically excited N2 with water dimer radical cation, (H2O)2+•, in its two-center-three-electron configuration. The reaction products are collected in a 76-needle array discharge reactor with product yields of 1.14 μg cm-2 h-1 for NH2OH and 0.37 μg cm-2 h-1 for HNO. Potential applications of these compounds are demonstrated to make ammonia (for NH2OH), as well as to chemically react and convert cysteine, and serve as a neuroprotective agent (for HNO). The conversion of N2 into HNO and NH2OH by water plasma could offer great profitability and reduction of polluting emissions, thus giving an entirely look and perspectives to the problem of green N2 fixation.

Determination of C═C Positions of Unsaturated Fatty Acids in Foods via Ambient Reactive Desorption Ionization with Water Dimer Radical Cations

The positions of C═C bonds in unsaturated fatty acids (FAs) are one of the main factors determining the quality of food flavor. Herein, we developed an approach for the determination of C═C bonds of FAs by online epoxidation reaction with water dimer radical cations. The limit of detection for octenoic acid isomers was ∼9 μg/L. The positions of C═C bonds in trans-2/3-hexenoic acid, trans-2/3-octenoic acid, oleic acid, linoleic acid, and linolenic acid in black tea or olive oil samples were directly determined by the established method. These results indicate that the established method allows the rapid determination of unsaturated FAs in black tea and olive oil. The advantages of this approach include the analysis speed (∼1 min per sample), simple device, and no need for complex pretreatment. This study not only provides a strategy for the determination of C═C positions but also offers new possibilities for applications in the field of food chemistry.

Nitrogen Fixation by Benzene into Pyridine and Aniline in Water/Nitrogen Plasma

We demonstrated direct conversion of benzene into pyridine and aniline, assisted through exact mass measurements (m/z 80.0494, 93.0574, and 94.0651, respectively), through the interaction of benzene with water/nitrogen vapor plasma produced by corona discharge. Systematic analysis using a series of isotopic standards indicated that formation of pyridine and aniline occurred through the reaction between neutral benzene and reactive N+(OH2)2 in water/nitrogen plasma; exact mass measurements of products and intermediates supported this hypothesis. As the proportion of water vapor in plasma increased over time, the reaction proceeded from exclusive formation of protonated pyridine to formation of protonated aniline as the main product; theoretical simulations indicated that the presence of water vapor promoted proton migration to elicit formation of protonated aniline. The reactions we discovered suggest a new mechanism for direct nitrogen fixation.

Evidence for the co-existence of isomers of water dimer radical cations and their inter-conversion in a linear ion trap

Water dimer radical cations are regarded as key intermediates in many aqueous reactions and biochemical processes. However, the structure of the water dimer radical cations, and particularly the inter-conversion between their isomers, remain difficult to investigate experimentally due to their short lifetime and low abundance under ambient conditions. Furthermore, the isomers cannot be distinguished in a full mass spectra. In this study, we report the experimental evidence for the hemi-bonded and proton-transferred isomers of gas-phase water dimer radical cations, and the inter-conversion process between them in a linear ion trap at low pressure and near room temperature. Multiple collisions of isolated water dimer radical cations with He inside the ion trap were systematically investigated; first, under different trapping times (i.e., reaction times) ranging from 0.03 to 800 ms, and then at a very low collision energies ranging from 0.1% to 10% normalized collision energy. The proton-transferred isomers were dominant at shorter trapping times (≤250 ms), while the hemi-bonded isomers were dominant at longer trapping times (250-800 ms). Moreover, the difference in symmetry of the shapes of the H₂O^•+ signal profiles and the H₃O⁺ signal profiles implied the existence of two kinds of isomers and there were small potential differences between them. Our results also suggested that by tuning the experimental parameters the hemi-bonded isomers would become dominant, which could allow the study of novel chemical reactions involving the hemi-bonded two-center-three-electron (2c-3e) structure in a linear ion trap.

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]^+·: 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.