Key Radical Research Articles

Mechanistic Interrogation of Photochemical Nickel-Catalyzed Tetrahydrofuran Arylation Leveraging Enantioinduction Data.

This manuscript details the development of an asymmetric variant for the Ni-photoredox α-arylation of tetrahydrofuran (THF), which was originally reported in a racemic fashion by Doyle and Molander. Leveraging the enantioselectivity data that we obtained, a complex mechanistic scenario different from those originally proposed is uncovered. Specifically, an unexpected dependence of the product enantiomeric ratio was observed on both the halide identity (aryl chloride vs bromide substrates) and the Ni source. Stoichiometric experiments and time course analyses of the evolution of product enantioselectivity with time revealed a different initial behavior for reactions carried out with Ni(II) and Ni(0) precatalysts that later converge into a common mechanism. For studying the predominant pathway, this paper describes a rare example of the syntheses of chiral bisoxazoline Ni(II) aryl halide complexes, which proved essential for probing enantioselectivity via stochiometric experiments. These experiments identify the Ni(II) aryl halide complex as the primary species involved in the key THF radical trapping event. A multivariate linear regression model is presented that further validates the dominant mechanism and delineates structure-selectivity relationships between ligand properties and enantioselectivity. EPR analysis of Ni(0)/aryl halide mixtures highlights the fast access to a variety of Ni complexes in 0, +1, and +2 oxidation states that are proposed to be responsible for the initial divergence in mechanism observed when using Ni(0) precatalysts. More broadly, beyond advancing the mechanistic understanding of this THF arylation protocol, this work underscores the potential of leveraging enantioselectivity data to unravel intricate mechanistic manifolds within Ni-photoredox catalysis.

A modified phosphorus-based inhibitor for dry water inhibitor and its chemical mechanisms in preventing benzoyl peroxide explosion

Dust explosions caused by benzoyl peroxide (BPO) pose a significant risk to the chemical industry, making the development of high-performance inhibitors essential. The effect of a novel inhibitor on the explosion behavior of BPO dust was studied through experiments and simulations. The chemical mechanism behind the suppressed BPO dust explosion was clarified. The primary explosion hazard of BPO stems from the instability of the oxygen–oxygen bond. To address this, a novel phosphorus-based dry water powder inhibitor, MAP-DW, was prepared which effectively captures key reactive radicals involved in the chain reaction. At the optimal explosion concentration of BPO, a concentration of 400 g/m3 of MAP-DW reduced the maximum explosion pressure and the maximum explosion pressure rise rate by 96.48 % and 99.58 %, respectively. The suppression effects of MAP-DW on the explosion of BPO dust include heat absorption, oxygen dilution, thermal insulation, and capture of key free radicals. The kinetic simulation results showed that the suppression cycle between phosphorus-containing substances captured key chain reaction radicals, and the coupled suppression among NH3, H2O, and phosphorus-containing substances dominated the suppression mechanism of MAP-DW on BPO dust explosion. This study provides new insights into the characterization and suppression of BPO dust explosion, while also supplying crucial data for the safe utilization of peroxides.

Mercury Reduction by Agricultural Organic Waste-Derived Dissolved Organic Matter: Kinetic Analysis and the Role of Light-Induced Free Radicals.

Agricultural organic wastes can leach dissolved organic matter (DOM) into surrounding water bodies, establishing them as significant sources of aquatic DOM. Given the importance of DOM in the biogeochemical cycling of mercury (Hg), this DOM may further mediate divalent Hg (Hg(II)) reduction, a process that remains poorly understood. This study investigated the Hg(II) reduction using DOM derived from six representative agricultural wastes, categorized into livestock manure (chicken, pig, cow) and crop straw (rice, corn, rapeseed), with systematic considerations of the kinetics of reduction processes and the involvement of key free radicals. Results revealed that photoreduction was the primary pathway for Hg(II) reduction, with pig manure DOM exhibiting the highest efficiency at 36%. Key DOM quality parameters, such as protein-like components, have been identified as critical determinants of Hg(II) photoreduction capacity. Furthermore, free radicals induced by DOM could either enhance or inhibit Hg(II) reduction capacities. Specifically, in livestock manure, the superoxide anion (O2•-)·was identified as the primary radical promoting Hg(II) photoreduction of pig manure DOM. In crop straw, hydroxyl radicals (·OH) were found to inhibit Hg(II) photoreduction, whereas O2•- promoted the Hg(II) photoreduction of rice straw DOM. These findings provide valuable insights into the role of agricultural organic wastes in the biogeochemical cycling of Hg within aquatic ecosystems.

Photocatalyst-free light-promoted carbohydrate synthesis and modification

Photoredox catalysis has recently emerged as a powerful approach for preparing oligosaccharides because it uses mild conditions, is compatible with partially or completely unprotected carbohydrate substrates, and exhibits impressive regio‐ and stereo‐selectivity and high functional group tolerance. However, most catalytic photoredox reactions require an external photocatalyst (organic dye or expensive transition-metal complex) to deliver key glycosyl radicals. Several photocatalyst-free photocatalytic reactions that avoid the use of expensive metal salts or organic-dye additives have received significant attention. In this review, we highlight the most recent developments in photocatalyst-free light-promoted carbohydrate synthesis and modification, which is expected to inspire broad interest in further innovations in the green synthesis of saccharides.

Experimental Investigation and Chemical Kinetics Analysis of Carbon Dioxide Inhibition on Hydrogen-Enriched Liquefied Petroleum Gas (LPG) Explosions

The utilization of hydrogen-enriched liquefied petroleum gas (LPG) is an effective means of reducing carbon emissions, but the special physical and chemical properties of hydrogen have raised concerns among the public. To delve into the intricate chemical kinetic mechanisms governing the inhibitory effect of CO2 on the explosion of hydrogen-enriched LPG, this study systematically investigated the influence of varying CO2 concentrations (3%, 6%, and 9%) on the explosion characteristics of hydrogen-enriched LPG (hydrogen ratio ranging from 0 to 0.5) within a 20 L spherical explosion chamber. Subsequently, a chemical kinetic analysis was conducted, focusing on the explosion reaction dynamics of the H2/LPG/CO2/Air mixture, encompassing temperature sensitivity assessments and the production rates of key free radicals. The findings reveal that although hydrogen incorporation does not significantly alter the maximum explosion pressure of LPG, it markedly accelerates the explosion reaction rate, posing a challenge for CO2 in effectively inhibiting the explosion of hydrogen-enriched LPG. CO2 functions as a stabilizing third body within the reaction system, diminishing the collision frequency among free radicals, hydrogen molecules, hydrocarbon molecules, and oxygen molecules, thereby slowing down the reaction rate. As the proportion of hydrogen increases, the concentration of ·H radicals, known for their high reactivity, escalates, rapidly completing the propagation phase of the chain reaction and intensifying the overall generation rates of critical free radicals, including ·H, ·O, and ·OH. Notably, the key reaction H+O2⇋O+OH, which governs the reaction temperature, undergoes significant enhancement, further accelerating the explosion reaction rate and ultimately diminishing the inhibitory efficacy of CO2 against the hydrogenated LPG explosion. Furthermore, as the amount of hydrogen added increases, hydrogen’s competitiveness for oxygen within the reaction system markedly improves, attenuating the oxidation of hydrocarbons. Concurrently, the alkane recombination reaction, exemplified by C3H6+CH3(+M)⇋sC4H9(+M), is strengthened. These insights provide valuable understanding of the complex interactions and mechanisms during the explosion of hydrogen-enriched LPG in the presence of CO2, with implications for the safe application of hydrogen-enriched LPG.

Effect of mesh aluminum alloy on hydrogen and propane explosion characteristics

In order to explore the effect of mesh aluminum alloy (MAA) on the explosion characteristics of different types of gases, experiments were conducted by filling MAA in hydrogen and propane explosion pipes. The explosion characteristic parameters of hydrogen and propane with different volume fractions under the action of MAA were obtained. The chain reaction process and thermodynamic reaction mechanism of hydrogen and propane explosions were studied using the numerical simulation software CHEMKIN-PRO and Materials Studio. The experimental results show that MAA significantly promotes hydrogen explosions but inhibits propane explosions. Simulation results show that ·H, ·O, and ·OH are key radicals in the hydrogen explosion process. As the hydrogen volume fraction increases, the number of radicals in the reaction system also increases. When MAA is filled in the hydrogen explosion test pipe, the turbulence of the flame structure inside the pipe increases the collision probability between key radicals such as ·H, ·O, and ·OH, sharply increasing the temperature of the explosion reaction system, resulting in a promoting effect of MAA on hydrogen explosions. When MAA is introduced into the propane explosion test pipe, it creates a barrier and wall effect. This reduces the number of key radicals in the reaction system and lowers the likelihood of radical collisions, which slows down the rates of multiple chain reactions and inhibits the propane explosion. Thermodynamically, it can be seen that propane requires more energy than hydrogen for key elementary reactions to explode, making it more difficult to react.

1D modeling of plasma streamers at ammonia-air flame conditions

Abstract This work is a self-consistent 1D modeling of streamers in ammonia-oxygen-nitrogen-water mixtures. A fluid model that includes species transport, electrostatic potential, and detailed chemistry was developed and verified. This model is then used to simulate the avalanche, streamer formation and propagation phases, driven by a nanosecond voltage pulse, at different thermochemical conditions derived from a 1D laminar premixed ammonia-air flame. The applicability of the Meek’s criterion in predicting the streamer inception location was successfully confirmed. Streamer formation and propagation duration were found to vary significantly with different thermochemical conditions, due to the difference in ionization rates. The thermochemical state also affected the breakdown characteristics which was tested by maintaining the background reduced electric field constant. Detailed kinetic analyses revealed the importance of O(1D) in the production of key radicals, such as O, OH, and NH2. Furthermore, the contributions of the dissociative electronic excitation of NH3 towards the production of H and NH2 radicals have also been reported. Spatial and temporal evolution of the electron energy loss fractions for various inelastic collision processes at different thermochemical states uncovered the input plasma energy spent of fuel dissociation and the large variability in the dominant processes during the avalanche and streamer propagation phases. The methodology and analyses reported in this work are key towards developing effective strategies for controlled nanosecond-pulsed non-equilibrium plasma sources used for ammonia ignition and flame stabilization.

Photoredox-Catalyzed Phosphine-Mediated Successive Deoxygenation of Sulfonyl Oxime Salts Enables Anti-Markovnikov Hydrothiolation of Alkenes.

Stable and easy-to-handle sodium salts of sulfonyl oximes were first identified to proceed via visible-light-driven phophine-mediated successive deoxygenation to realize the anti-Markovnikov hydrothiolation of alkenes, which could serve as an odorless sulfur source. Mechanistic studies revealed that the key thiyl radical intermediate could be generated in situ from the sulfonyl oxime anion via a phosphine-mediated fragmentation and a sequential deoxygenation process. Notably, a wide range of alkenes, including acrylamides, acrylates, vinyl ketones, vinyl sulfones, and acrylonitriles, are competent substrates for this protocol, which is highly beneficial for the construction of structurally diversified organosulfur compounds.

Sodium Hypophosphite as a Halogen Atom Transfer (XAT) Agent under Photocatalytic Conditions.

The ability of sodium hypophosphite to generate the phosphorus-centered radical, which can activate the carbon-halogen bond via the halogen atom transfer (XAT) is described. The hydroalkylation of nonactivated alkenes with methyl bromoacetate was performed using sodium hypophosphite as reducing agent under photocatalytic conditions. The key phosphorus centered radical is formed from the hypophosphite anion by hydrogen atom abstraction.

The effect of ethane on the chain reaction characteristics of methane–air mixtures explosions

In order to study the effect of ethane on gas explosions, the macroscopic properties, thermodynamics and kinetics of gas explosions were investigated using Chemkin numerical simulation software with the GRI-Mech 3.0 mechanism. Ethane was added to methane-air mixtures at seven different concentrations, such as 0%, 0.1%, 0.3% and so on, at constant initial temperature (1200 K) and pressure(0.9atm), respectively. The results show that the maximum explosion pressure, explosive power and ·H content was all maximized when the volume fraction of ethane was 3%, which was the most effective volume addition to promote gas explosion. Ethane reduced the time to peak energy of key radical reactions from 0.049s to 0.003s and reduced the sensitivity factors for reactions such as R155 and R158. When the added ethane was 8%, R158 and R98 shifted from inhibiting methane consumption to promoting methane consumption. Calculated by Arrhenius and Lindemann formulae found that ethane can act as the third molecule in unimolecular reactions, increasing the reaction rate constant and decreasing the activation energy, so accelerating the reaction. Due to the low addition of ethane, there was still a large amount of N2 acting as the third molecule, so the decrease in activation energy was small.

H-atom-assisted formation of key radical intermediates in interstellar sugar synthesis

Context. Despite the identification of the smallest sugar molecule, glycolaldehyde (GA), in the interstellar medium (ISM), its mechanism of formation in the ISM is still not fully understood. A more profound understanding of the interstellar chemistry of GA and related molecules could provide insights into whether larger sugar molecules can also form and survive under such conditions. Aims. The primary objectives of this research are to delve into the sugar formation mechanism in the ISM, especially in dark molecular clouds; unravel intricate details of H-atom-mediated reactions involving glyoxal (GO), GA, and ethylene glycol (EG); and identify intermediates playing potential roles in the formation of larger sugars or serving as intermediates in the destruction reaction paths of sugar molecules. Methods. The study utilizes the para-H2 matrix isolation method with infrared (IR) spectroscopic detection and quantum chemical computations to investigate H-atom reactions of GO, GA, and EG at a low temperature. Results. Several radical products were spectroscopically identified that might be key active species in the interstellar formation of larger sugar molecules.

Electrooxidative 1,3-Oxo/Carboamination of Arylcyclopropanes.

Herein, the work demonstrates an electrochemically paired electrolysis approach facilitating the efficient achievement of the electrooxidative 1,3-oxo/carboamination of arylcyclopropanes under mild conditions. The formation of 1,3-arylamination of arylcyclopropanes involves commercially available amine redox mediators through a radical-radical process. In addition, the successful execution of β-amino ketones also occurs under atmospheric conditions. The control experiments supported the existence of key benzylic radical intermediates in the reaction pathway.

Electron Donor-Acceptor Complex Enabled Cyclization/Sulfonylation Cascade of N-Heterocycles with Thianthrenium Salts.

We report a visible-light-promoted cyclization/sulfonylation cascade of N-heterocycles with thianthrenium salts using DABSO as the SO2 surrogate. This method features excellent functional group tolerance, wide substrate scope, and late-stage elaboration of bioactive relevant molecules. Mechanistic investigations reveal that the photoactive electron donor-acceptor (EDA) complexes between thianthrenium salts and DABCO are capable of the generation of aryl radicals, which induce the following SO2 insertion by attacking DABSO, thus triggering the key radical cyclization step.

N-Heterocyclic Carbene Enabled Functionalization of Inert C(Sp3)-H Bonds via Hydrogen Atom Transfer (HAT) Processes.

Developing methods to directly transform C(sp3) -H bonds is crucial in synthetic chemistry due to their prevalence in various organic compounds. While conventional protocols have largely relied on transition metal catalysis, recent advancements in organocatalysis, particularly with radical NHC catalysis have sparked interest in the direct functionalization of "inert" C(sp3) -H bonds for cross C-C coupling with carbonyl moieties. This strategy involves selective cleavage of C(sp3) -H bonds to generate key carbon radicals, often achieved via hydrogen atom transfer (HAT) processes. By leveraging the bond dissociation energy (BDE) and polarity effects, HAT enables the rapid functionalization of diverse C(sp3)-H substrates, such as ethers, amines, and alkanes. This mini-review summarizes the progress in carbene organocatalytic functionalization of inert C(sp3)-H bonds enabled by HAT processes, categorizing them into two sections: 1) C-H functionalization involving acyl azolium intermediates; and 2) functionalization of C-H bonds via reductive Breslow intermediates.

Flame propagation mechanism of methanol fuel spray explosion in a square closed vessel

Methanol, as an important chemical raw material and clean energy source, easily forms explosive droplet clouds during its production, processing, and usage. Therefore, understanding the evolution and propagation mechanisms of methanol spray explosion flames is crucial for its widespread use and the design of safety measures. This paper studies the flame propagation behavior of methanol spray explosions using a 16.2-L visualized enclosed vessel. The results indicate that the flame structure of methanol spray explosions is similar to that of premixed gas explosions but significantly differs from traditional fossil fuels. It demonstrates homogeneous combustion characteristics primarily governed by the kinetics-controlled regime. As the methanol spray concentration increases, both the maximum flame propagation speed and the maximum explosion pressure initially increase and then decrease, reaching their peak at a concentration of 224.68 g/m3, with values of 9.96 m/s and 0.72 MPa, respectively. The heat losses during combustion exhibit a trend opposite to that of explosion pressure. Numerical simulation results indicate that the concentrations of key radicals such as O, H, and OH vary significantly between lean and rich combustion states. The increase in H radical concentration enhances the elementary reactions that suppress flame temperature and promotes chain-terminating reactions.

Photoinduced Vicinal Sulfamoyloximation of Alkenes: Harnessing Bifunctional Nitrosamines via a Rapid Radical Trapping Strategy.

We developed a photoinduced method for vicinal sulfamoyloximation of alkenes using N-nitrosamines as bifunctional reagents, with DABSO serving as both a sulfonyl source and a rapid aminyl radical trap. This strategy prevents radical recombination, enabling bifunctional activation under neutral conditions to generate key sulfamoyl radicals. It accommodates broad substrate scope and functional group compatibility, enabling late-stage modifications of bioactive molecules and expanding sulfonamide diversity in organic synthesis.

Expanding the Chemical Space of Electrophilic β-Glycosyl β-Lactams through Photoinduced Diastereoselective Functionalization.

Herein, we present a photoinduced diastereoselective C-3 functionalization of electrophilic β-glycosyl β-lactams. The developed protocol is simple, mild, and scalable and explores the use of 3-exomethylene β-lactams as reaction partners in a Giese type reaction. The key nucleophilic alkyl radical is generated by a photoinduced electron transfer process in the EDA complex formed by NHPI and Hantzsch esters. The diastereoselective hydrogen atom transfer to the β-lactam radical intermediate enables the synthesis of various N-phenyl β-glycosyl β-lactams.

Experimental and chemical kinetic modeling study on the inhibition performance of pentauoroethane blended with carbon dioxide in methane-air mixture explosions

To explore the inhibition effectiveness and mechanisms of pentafluoroethane (C2HF5), carbon dioxide (CO2), and their blends, 9.5% methane (CH4)-air mixture explosions under different inhibitors were tested by employing a 20 L spherical explosion system. A quadratic regression model was developed to predict impacts of factors on maximum explosion pressure (pm) through response surface method. The reactions and mechanisms under blended inhibitors were numerically investigated. Results depicted that increasing volume fractions of inhibitor led to decrease in pm, maximum rate of pressure rise, and delay in explosion induction time. Blended inhibitors demonstrated superior inhibition effectiveness, optimal inhibition-environmental was achieved at 4.5% C2HF5 blended with 5.5% CO2. A quadratic polynomial relation existed among pm, volume fractions of C2HF5, CO2, and CH4 during the explosion. Key free radicals were weakened and the promoted explosive elementary reactions were primarily inhibited. CH4+OHCH3+H2O and CH4+HCH3+H2 were the most diminished reactions by blended inhibitors compared to CO2 alone. Carbon-containing reactions R82 and R84, and fluorinated reactions R1025 and R1099 reduced the key free radicals, interrupted the chain reactions and promoted the production of HF. Findings will provide new ideas for prevention of CH4-air mixture explosions and reduce hazardous effect.

Reaction mechanism of antioxidant inhibition of hydroxyl radical in coal oxidation.

To solve the problem of unclear targeted inhibition of key free radicals by antioxidants, this paper took the hydroxyl radical with the highest oxidation activity of coal as the inhibition target and selected five antioxidants such as ethylene diamine tetraacetic acid, tea polyphenol, citric acid, vitamin C, and proanthocyanidins. Based on the theory of quantum chemistry, the characteristics and oxidation pathway of antioxidants inhibiting coal oxidation of hydroxyl radical were analyzed. Analyze the influence characteristics of antioxidants on the evolution of free radicals in coal through an electron paramagnetic resonance experiment (ESR). The results showed that the electron density of antioxidants was mainly distributed in the functional groups of carboxyl and hydroxyl, which played a key inhibitory role, and the vicinity of carboxyl and hydroxyl and other functional groups was positive potential, which was the active site of inhibiting hydroxyl radical. The order of inhibitory capacity of the five antioxidants was determined as GTP > PC > EDTA > CA > VC. It is concluded that the energy barrier of hydroxyl radical inhibition by citric acid is much lower than that of EDTA. For the hydrogen extraction reaction, VC inhibited the hydroxyl radical pathway with a higher energy barrier than the other three antioxidants. The mechanism of five antioxidants inhibiting •OH reaction was comprehensively analyzed. It was found that tea polyphenols have more active sites that can react with •OH to quench it, so the inhibition of tea polyphenols should be the most significant. When antioxidants inhibit coal spontaneous combustion, the type, complexity, concentration, and linewidth of free radicals in coal molecules are lower than those in raw coal, with GTP antioxidants having the best inhibitory effect. The research results have important theoretical and practical significance for revealing the mechanism of coal spontaneous combustion inhibition and developing directional coal spontaneous combustion inhibition technology.

Study on auto-ignition characteristics of N-heptane/methanol/ammonia mixed fuel

An investigation was carried out on the auto-ignition properties of n-heptane/ammonia/methanol fuel mixtures, characterized by a small amount of n-heptane concentration (2 %). The experiment was conducted using a rapid compression machine (RCM) within a temperature bracket of 790–1112 K, under pressures of 1.5 and 3.05 MPa. The fuel incorporates a methanol content of 0 to 98 %, with mixture equivalence ratios set at 0.5, 1.0, and 2.0. Observations indicate that the fuel component and the equivalence ratio influence the ignition delay time (IDT). It was found that the reactivity of the mixtures increases upon the induction of n-heptane. For the 1 % CH3OH mixture, the IDT is the shortest with stoichiometry. When the methanol content increases, the IDT decreases with increased equivalence ratios. In addition, a detailed mechanism has been modified to improve the predictive performance of the experimental mixture. The chemical reaction kinetic analysis shows that n-heptane promotes the combustion reaction. The consumption of fuels in the combustion process follows a sequential pattern, with n-heptane being consumed first, followed by methanol and ammonia. Both n-heptane and methanol experience nearly complete consumption before the combustion. In contrast, the consumption of ammonia proceeds at a significantly slower rate and is primarily consumed in substantial quantities at the combustion stage. The dehydrogenation reactions compete for key radicals, which inhibit the oxidation process of the system. The extraction of H from methanol by HO2 to produce H2O2 is a sensitive reaction to promote combustion.