Radical Recombination Research Articles

Enhanced Catalytic Probe Design for Mapping Radical Density in the Plasma Afterglow.

The electrification of chemical processes using plasma generates an increasing demand for sensors, monitoring concentrations of plasma-activated species such as radicals. Radical probes are a low-cost in situ method for spatially resolved quantification of the radical density in a plasma afterglow using the heat from the exothermal recombination of radicals on a catalytic surface. However, distinguishing recombination heating from other heat fluxes in the system is challenging. In this study, we present a heat flux analysis based on probe measurements inside the reactor, with simultaneous IR imaging monitoring of the temperature of the reactor wall. The impact of radiation heat on a single thermocouple as well as the advantage of a dual thermocouple setup (using a catalytic unit together with a reference thermocouple) is shown. We add a heat sink with a monitored temperature to the dual thermocouple setup, allowing the determination of conductive and radiative heat fluxes. The heat sink gives more information on the measurement and reduces ambiguities in the evaluation used by others. The probe was tested by mapping N atom densities throughout the plasma afterglow of our reactor, enabling evaluation of the recombination kinetics of the radicals in the gas phase. Three-body recombination was shown to be the main pathway of recombination, with a recombination rate of krec = (2.0 ± 0.9)·10-44 m6/s, which is in line with the known literature findings, demonstrating that the measured species are N radicals and the probe did not influence the plasma or recombination reactions in the afterglow.

Ether and ester formation from peroxy radical recombination: a qualitative reaction channel analysis

Abstract. The least volatile organic compounds participating in atmospheric new-particle formation are very likely accretion products from self- and cross-reactions of peroxy radicals (RO2). It has long been assumed that the only possible accretion product channel in this reaction is that forming a peroxide (RO2+RO2→ROOR+O2), but it has recently been discovered that a rapid alkoxy radical (RO) decomposition may precede the accretion step of the mechanism, forming slightly fragmented but more stable ether (ROR) or ester (RC′(O)OR) accretion products. In this work, the atmospheric implications of this new reaction channel have been explored further by using a modified version of the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) software to generate a large amount of representative RO2 + RO2 reactive pairs formed from the oxidation of typical primary hydrocarbons and by applying structure–activity relationships (SARs) to predict the potential accretion products. These data are analysed in terms of the formation of low-volatility products, and new discoveries are presented on what types of RO2 are especially efficient (and which are surprisingly inefficient) at forming accretion products. These findings are discussed in terms of the atmospheric relevance of these new RO2 + RO2 reaction channels. As the generation of these data rests on several simplifications and assumptions, many open questions worthy of later studies are also raised.

Piperazine pyrophosphate-functionalized Ni-MOF metal framework: Fabrication and synergistic explosion suppression mechanisms

Dust explosion suppression is crucial for ensuring safe industrial operations. To explore composite explosion suppression powders with higher performance, piperazine pyrophosphate (PAPP)-functionalized nickel-based metal–organic framework (PAPP@Ni-MOF) synergistic suppression materials were designed and synthesized. The effectiveness of this suppressant in inhibiting the explosion flame and intensity of polypropylene (PP) dust was investigated using explosion flame propagation and explosion pressure test apparatus. The results indicate that the combination of N/P elements and Ni-MOF exhibits excellent catalytic carbonization performance. Adding 50 wt% PAPP@Ni-MOF can completely inhibit the propagation of PP explosion flames and the rise in explosion pressure. The maximum propagation distance of the explosion flame was reduced by 78.07 %, the overall flame propagation speed decreased by 84.54 %, and the maximum explosion pressure was reduced by 76.87 %. Through characterization experiments, the products before and after the explosion were analyzed. Combined with molecular dynamics (MD) simulation results, the efficient synergistic explosion suppression mechanism of PAPP@Ni-MOF for PP dust was revealed from the perspectives of thermal conduction isolation, key radical recombination, and volatile gas adsorption.

Influence of blocking groups on photo-oxidation of tyrosine and derivatives

One-electron oxidation of tyrosine primarily results in the formation of di-tyrosine, which can induce crosslinks leading to protein damage. In this study, we investigated the 3-carboxybenzophenone-sensitized photo-oxidation of Tyr derivatives through time-resolved and steady-state photolysis under anaerobic conditions to analyze the effects of blocking groups.The mechanism for primary and secondary photoreactions in the sensitized photo-oxidation of Tyr derivatives in aqueous solution was presented based on time-resolved analysis and mass spectrometric characterization of photo-oxidation products. Identified di-Tyr products (in addition to those mentioned more often in the literature, such as 3,3′/3,O’) were in some samples presented together with Tyr-CBH adduct (resulting from radical recombination between the tyrosyl radical and CBH•). This publication discusses the possible coulombic effects of interacting ionic species (sensitizer and quencher) on quenching rate constants and the effect of amine groups and steric factors on the distribution of stable products. However, a crucial finding of this work is that the more blocked the Tyr is, the more di-Tyr isomers are formed, suggesting that Tyr residue in proteins may form several forms of di-Tyr cross-links.

Aggregation of Albumins under Reductive Radical Stress.

The reactions of radicals with human serum albumin (HSA) under reductive stress conditions were studied using pulse radiolysis and photochemical methods. It was proved that irradiation of HSA solutions under reductive stress conditions results in the formation of stable protein aggregates. HSA aggregates induced by ionizing radiation are characterized by unique emission, different from the UV emission of non-irradiated solutions. The comparison of transient absorption spectra and the reactivity of hydrated electrons (eaq-) with amino acids or HSA suggests that electron attachment to disulfide bonds is responsible for the transient spectrum recorded in the case of albumin solutions. The reactions of eaq- and CO2•- with HSA lead to the formation of the same products. Recombination of sulfur-centered radicals plays a crucial role in the generation of HSA nanoparticles, which are stabilized by intermolecular disulfide bonds. The process of creating disulfide bridges under the influence of ionizing radiation is a promising method for the synthesis of biocompatible protein nanostructures for medical applications. Our Raman spectroscopy studies indicate strong modification of disulfide bonds and confirm the aggregation of albumins as well. Low-temperature measurements indicate the possibility of electron tunneling through the HSA protein structure to specific CyS-SCy bridges. The current study showed that the efficiency of HSA aggregation depends on two main factors: dose rate (number of pulses per unit time in the case of pulse radiolysis) and the temperature of the irradiated solution.

Synthesis of biodegradable polydisulfides from renewable resources

In this paper, we report the synthesis and preliminary characterization of polydisulfides from a renewable resource. Di-linoleic diol was first converted into dithiol via transesterification of methyl-3-mercaptopropionate catalyzed by Candida antarctica Lipase B (CALB). The kinetics of the reaction was monitored using a newly developed Thin Layer Chromatography method. The dithiol was then polymerized by Reversible Radical Recombination Polymerization (R3P), a scalable “green” polymerization process using triethylamine (TEA) catalyst, aqueous H2O2, and air. 3 wt% H2O2 yielded low molecular weight polymer, while with 30 wt% concentration DL-SS with Mn = 66,400 g/mol and polydispersity of 1.26 was obtained. 800 MHz NMR and Raman spectroscopy did not detect thiol end group signals in this polymer, indicating that it had a cyclic structure. Dithiothreitol degraded the polymer to monomer in two weeks. Full characterization of this new polymer is in progress.

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.

Photoinduced Low‐Valent Zirconium Catalysis for Cross‐Electrophile Coupling of Ethers

AbstractAccessing versatile C(sp3)−C(sp3) bond through the cross‐electrophile coupling of two distinct etheric C−O bonds is crucial in organic synthesis but remains barely explored. Herein, we report an innovative photoinduced low‐valent zirconocene catalysis enabling the reductive coupling of ethers with high activity and cross‐selectivity. Mechanistic investigation suggests that photoexcitation of low‐valent zirconocene facilitates the C(sp3)−O bond scission of benzylic ethers, leading to the benzylic radicals intermediate via a single‐electron reduction pathway. The subsequent recombination of this benzylic radical with the Zr center followed by carbomagnesiation generates benzylic Grignard reagents for downstream coupling with aliphatic ethers through an SN2‐like mechanism. In application, a wide range of ethers readily in situ derived from aldehydes and ketones becomes feasible with high functional group compatibility as well as excellent cross‐selectivity.

Photoinduced Low-Valent Zirconium Catalysis for Cross-Electrophile Coupling of Ethers.

Accessing versatile C(sp3)-C(sp3) bond through the cross-electrophile coupling of two distinct etheric C-O bonds is crucial in organic synthesis but remains barely explored. Herein, we report an innovative photoinduced low-valent zirconocene catalysis enabling the reductive coupling of ethers with high activity and cross-selectivity. Mechanistic investigation suggests that photoexcitation of low-valent zirconocene facilitates the C(sp3)-O bond scission of benzylic ethers, leading to the benzylic radicals intermediate via a single-electron reduction pathway. The subsequent recombination of this benzylic radical with the Zr center followed by carbomagnesiation generates benzylic Grignard reagents for downstream coupling with aliphatic ethers through an SN2-like mechanism. In application, a wide range of ethers readily in situ derived from aldehydes and ketones becomes feasible with high functional group compatibility as well as excellent cross-selectivity.

Drastically Accelerated Radical Recombination Kinetics of a Hexaarylbiimidazole Derivative.

More than 60 years have passed since the discovery of hexaarylbiimidazole (HABI), which exhibits a characteristic photochromism that produces colored lophyl radicals through a radical dissociation reaction induced by light irradiation and reverts to its original state through a radical recombination reaction in the dark. Lophyl radicals are relatively stable among organic radicals, have low reactivity with oxygen, and have a very slow radical recombination reaction rate. HABI has been used industrially as a photoinitiator to date. However, the guidelines for molecular design to accelerate the thermal reverse reaction of HABI are still unknown and remain a challenge. We found that suppressing the rotation of the phenyl groups attached to the 4- and 5-positions of the imidazole ring of HABI is effective in accelerating the radical recombination reaction. The simple molecular design strategy to accelerate the thermal reverse reaction of HABI is expected to improve the performance of photoinitiators and photoresponsive materials that utilize HABI as a photoresponsive unit.

Kinetics in high-temperature free-radical reactive processing: Grafting maleic anhydride onto polypropylene in the presence of nanoclay-supported peroxide

The kinetics of grafting and chain scission reactions during maleic anhydride (MA) grafting onto polypropylene (PP) using nanoclay-supported peroxide (o-MMT/DCP) in the molten state were evaluated through the development of a new methodology. A combination of characterization techniques and mathematical data manipulation was employed for this assessment. To inhibit radical propagation and recombination, tetramethylpiperidine-1-oxyl (TEMPO) was used as a free radical scavenger. Rheological stability tests demonstrated that TEMPO was more effective as a reaction inhibitor. The kinetics dataindicated that the use of o-MMT/DCP resulted in products with higher MA grafted levels and molecular weight (MW) compared to grafting performed in the absence of o-MMT. Additionally, some changes in molecular architecture were observed. The chain scission distribution function (CSDF) revealed that during the MA grafting process, the molecular weight of PP was reduced through chain scission, with a preference for longer chains. Furthermore, the presence of o-MMT was found to restrain the level of PP chain scission, likely due to recombination on the clay surface.

The comparison of the efficacy of 185/254 nm and 172 nm photolysis for the removal of sulfonamides and trimethoprim from aqueous solutions

The VUV (λ < 200 nm) photolysis can be used to eliminate organic substances without the addition of oxidant due to the efficient generation of OH from water. Two VUV light sources are commercially available: the low-pressure mercury vapor (LPM) and the Xe-excimer lamp, which emit 185/254 nm and 172 nm photons, respectively. This study compares the efficiency of the UV/VUV185nm and VUV172nm photolysis in removing four sulfonamides (SAs) and trimethoprim (TRIM) antibiotics. For the LPM lamp, 254 nm UV radiation ((4.21 ± 0.08) × 10−6 molphoton s−1) can contribute to the transformation of target substances besides the OH generated by the 185 nm VUV light. The VUV photon flux (1.02 × 10−5 molphoton s−1) of the Xe-excimer lamp significantly exceeded that of the LPM lamp (3.97 × 10−7 molphoton s−1); as H2O2 formation in pure water reflected, however, the efficiency of the transformation rate of the target substances was just slightly faster. For the LPM lamp, in addition to the direct UV photolysis of SAs, the 254 nm light enhances the OH formation via H2O2 photolysis, thereby partially compensating for the low VUV185nm photon flux. The high VUV172nm photon flux was primarily manifested in mineralization efficiency and not in the transformation rate of target compounds. The short penetration depth of 172 nm light favors the recombination of primary radicals and decreases the efficiency. The effect of the matrix and matrix components is complex; they act as a radical scavenger or (V)UV filter. The change of radical set and the role of the formed secondary radicals (Cl, Cl2−, and CO3−) in transformation must be considered.

Fast Recombination of Free Radicals in Solution and Microviscosity

Fast Recombination of Free Radicals in Solution and Microviscosity

Preparation of Acetylenediol (HOCCOH) and Glyoxal (HCOCHO) in Interstellar Analog Ices of Carbon Monoxide and Water

Enols—tautomers of ketones or aldehydes—are considered key intermediates in the formation of prebiotic sugars and sugar acids. Although laboratory simulation experiments suggest that enols should be ubiquitous in the interstellar medium, the underlying formation mechanisms of enols in interstellar environments are largely elusive. Here, we present the laboratory experiments on the formation of glyoxal (HCOCHO) along with its ynol tautomer acetylenediol (HOCCOH) in interstellar ice analogs composed of carbon monoxide (CO) and water (H2O) upon exposure to energetic electrons as a proxy for secondary electrons generated from Galactic cosmic rays. Utilizing tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, glyoxal and acetylenediol were detected in the gas phase during temperature-programmed desorption. Our results reveal the formation pathways of glyoxal via radical–radical recombination of two formyl (HĊO) radicals, and that of acetylenediol via keto-enol-ynol tautomerization. Due to the abundance of carbon monoxide and water in interstellar ices, glyoxal and acetylenediol are suitable candidates for future astronomical searches. Furthermore, the detection of acetylenediol in astrophysically relevant ices advances our understanding for the formation pathways of high-energy tautomers such as enols in deep space.

Para-selective nitrobenzene amination lead by C(sp2)-H/N-H oxidative cross-coupling through aminyl radical

Arylamines, serving as crucial building blocks in natural products and finding applications in multifunctional materials, are synthesized on a large scale via an electrophilic nitration/reduction sequence. However, the current methods for aromatic C–H amination have not yet attained the same level of versatility as electrophilic nitration. Here we show an extensively investigated transition metal-free and regioselective strategy for the amination of nitrobenzenes, enabling the synthesis of 4-nitro-N-arylamines through C(sp2)-H/N-H cross-coupling between electron-deficient nitroarenes and amines. Mechanistic studies have elucidated that the crucial aspects of these reactions encompass the generation of nitrogen radicals and recombination of nitrobenzene complex radicals. The C(sp2)-N bond formation is demonstrated to be highly effective for primary and secondary arylamines as well as aliphatic amines under mild conditions, exhibiting exceptional tolerance towards diverse functional groups in both nitroarenes and amines (>100 examples with yields up to 96%). Notably, this C(sp2)-H/N-H cross-coupling exhibits exclusive para-selectivity.

Mechanisms of Action in FLASH Radiotherapy: A Comprehensive Review of Physicochemical and Biological Processes on Cancerous and Normal Cells.

The advent of FLASH radiotherapy (FLASH-RT) has brought forth a paradigm shift in cancer treatment, showcasing remarkable normal cell sparing effects with ultra-high dose rates (>40 Gy/s). This review delves into the multifaceted mechanisms underpinning the efficacy of FLASH effect, examining both physicochemical and biological hypotheses in cell biophysics. The physicochemical process encompasses oxygen depletion, reactive oxygen species, and free radical recombination. In parallel, the biological process explores the FLASH effect on the immune system and on blood vessels in treatment sites such as the brain, lung, gastrointestinal tract, skin, and subcutaneous tissue. This review investigated the selective targeting of cancer cells and the modulation of the tumor microenvironment through FLASH-RT. Examining these mechanisms, we explore the implications and challenges of integrating FLASH-RT into cancer treatment. The potential to spare normal cells, boost the immune response, and modify the tumor vasculature offers new therapeutic strategies. Despite progress in understanding FLASH-RT, this review highlights knowledge gaps, emphasizing the need for further research to optimize its clinical applications. The synthesis of physicochemical and biological insights serves as a comprehensive resource for cell biology, molecular biology, and biophysics researchers and clinicians navigating the evolution of FLASH-RT in cancer therapy.

Thermochemical conversion of guaiacol with supercritical CO2: Experimental insights

The excessive CO2 in the environment resulting from the combustion of fossil fuels has posed a significant challenge in terms of its conversion and utilization. In this work, the thermochemical conversion of guaiacol under supercritical CO2 (scCO2) was proposed, which not only enabled the consumption of CO2 but also facilitated the conversion of biomass into valuable products. It was concluded that the carbon gasification efficiency (CE) and hydrogen gasification efficiency (HE) were 37.10 % and 51.77 %, respectively, when the CO reached the maximum of 8.72 ± 0.28 mol/kg at 700 °C for 20 min. The addition of catalyst promoted the production of H2 and increased CE, the H2 yield of guaiacol gasification under different catalysts was ordered as Al2O3 > KOH > without catalyst > KCl. As the reaction temperature increased, the unreacted guaiacol and phenol decreased and were gradually converted to liquid products such as pyrene and benzo[c]phenanthrene. With the increase in reaction temperature, the more fully guaiacol was reacted, the easier it was to carbonize and form more carbon microspheres. In addition, reaction pathways for free radicals were proposed, including cleavage of free radicals, isomerization of free radicals, secondary cleavage of free radicals and recombination of free radicals. In short, degrading guaiacol using scCO2 is a feasible approach, which provides a new direction for the disposal and energy recovery of biomass in the future.

Universality of hydroxyl radical generation in irradiated nanoenhancer suspensions: Vital role of concentration

Concentration plays an essential role in generating hydroxyl radicals in irradiated nanoenhancer suspensions. In this paper, we used coumarin-3-carboxylic acid as a hydroxyl radical-specific probe to investigate the hydroxyl radical production of different concentration nanodiamonds (NDs) and CeO2 NPs in phosphate-buffered saline under x-ray irradiation. NDs significantly enhanced hydroxyl radical production, and the maximum enhancement of hydroxyl radical production was observed at a concentration of 10 µg/ml, with an enhanced factor of 1.398 ± 0.262. CeO2 NPs can increase and scavenge hydroxyl radicals at different concentration ranges, with the lowest and highest enhanced factors of 0.623 ± 0.069 and 1.738 ± 0.264, respectively. We tested the hydrodynamic diameter at various concentrations to explore the concentration effect further. We found that with increasing concentration, there might be factors, such as hydroxyl radical recombination and nanoparticle agglomeration, that lead to changes in the enhancement factor. Based on the data from previous and present studies, the experimental results indicate that the concentration factor is essential for hydroxyl radical generation in nanoenhancer suspensions under ionizing radiation. We also provide possible mechanisms for enhancing hydroxyl radical production by nanoenhancers in water under ionizing radiation and the decrease in enhancement factor at high concentrations of nanoenhancers.

A numerical study on premixed laminar ammonia/air flames enriched with hydrogen: An analysis on flame–wall interaction

Ammonia (NH3) has been recognized as a potential carbon-free synthetic fuel of the near future. To enhance its low reactivity, one practical option is to blend it with hydrogen (H2). In this study, the transient head-on quenching of premixed laminar ammonia/air flames enriched with hydrogen is explored based on numerical simulations using detailed chemistry and the mixture-averaged transport model. In this respect, nine different test cases are studied for blending ratios (the molar ratio of hydrogen to the ammonia/hydrogen mixture) of 0.0, 0.2, and 0.4, equivalence ratios of 0.8, 1.0, and 1.2, wall temperatures of 300, 500, and 750 K, and pressures of 1, 2, and 5 atm. The results reveal that the quenching distance (maximum absolute wall heat flux) decreases (increases) with increasing the blending ratio, the equivalence ratio, the wall temperature, and the pressure. For all the test cases, the quenching Peclet number changes between 1 and 3.5. In addition, the local heat release rate enhancement and the role of radical recombination reactions are highlighted at the time of quenching in the vicinity of the wall. This effect is augmented as the blending ratio, the equivalence ratio, and the wall temperature increase. Furthermore, the results show that the N2 pathway is the dominant pathway in consumption of NO near the wall at the time of quenching, in which R76 (NH2+NO⇔N2+H2O) poses the rate controlling role. In addition, the leading role of R85 (NH+NO⇔N2O+H) in consuming NO and converting it to N2O is highlighted at the time of quenching near the wall. Moreover, the significant roles of molecular diffusion and reaction source terms over convection are discussed for both NO and N2O species transport in the vicinity of the wall.Novelty and significance statement: The significance of this work is that using ammonia and its blends with hydrogen as promising carbon-free fuels for the future has several technical issues/uncertainties, which needs to be addressed fundamentally. One of the relatively unexplored issues in the combustion devices for ammonia/hydrogen/air flames is the head-on quenching phenomenon, which is investigated in detail in this study. The novelty of this work is that, for the first time, (1) the head-on quenching of various ammonia/hydrogen blends is systematically studied numerically, employing detailed chemistry, (2) effect of wall on the heat release rate chemistry is discussed wherein the role of radical recombination reactions is highlighted, and (3) the formation pathways of pollutant emissions (NO and N2O) for such flames are thoroughly investigated in the freely propagating flame scenario and in the vicinity of the wall at the time of quenching.

Flow effects on sonochemical oxidation activity in a 20 kHz ultrasonic horn system

Forced fluid flow, when properly aligned with the geometry of a 20 kHz horn-type sonoreactor, can increase its sonochemical efficiency (SE). We designed a flow-through reactor configuration to evaluate the effect of fluid dynamics on sonochemical activity. Using particle image velocimetry (PIV), we mapped the velocity profiles within the sonoreactor under flow rates up to 350 ml/min, and ultrasound power density from 142 to 498 W/L. KI dosimetry and sonochemiluminescence (SCL) are conducted to correlate fluid motion with chemical activity.At an ultrasound power density below 241 W/L, placing the probe near the water surface with a 288 ml/min flow rate maximized radical generation. Conversely, above 402 W/L, maximum SE is achieved with the probe near the vessel’s bottom at the same flow rate. At power densities below 241 W/L, external flow increases the liquid residence time in acoustically active zones from 0.16 to an average of 6.3 s, yielding enhanced chemical activity. Above 402 W/L, the introduction of forced flow broadens the acoustic jet, relocating bubbles from the active zone below the probe’s tip to the area around its neck. The expansion of this active zone leads to increased SE.A significant discrepancy is noted between Electron Paramagnetic Resonance (EPR) and KI dosimetry in measuring hydroxyl (OH.) radical concentrations. EPR suggests that most of these radicals are lost to recombination, highlighting the rapid dynamics of radical generation and recombination into hydrogen peroxide (H2O2) at interfacial regions.