Radical Recombination Research Articles

Evaluation and Modeling of Polylactide Photodegradation under Ultraviolet Irradiation: Bio-Based Polyester Photolysis Mechanism.

In our study, we investigated the accelerated aging process of PLA under 253.7 nm UV-C irradiation with the use of the GPC, NMR, FTIR, and DSC methods and formal kinetic analysis. The results of GPC and DSC indicated a significant degree of destructive changes in the PLA macromolecules, while spectroscopic methods NMR and FTIR showed maintenance of the PLA main structural elements even after a long time of UV exposure. In addition to that, the GPC method displayed the formation of a high molecular weight fraction starting from 24 h of irradiation, and an increase in its content after 144 h of irradiation. It has been shown for the first time that a distinctive feature of prolonged UV exposure is the occurrence of intra- and intermolecular radical recombination reactions, leading to the formation of a high molecular weight fraction of PLA decomposition products. This causes the observed slowdown of the photolysis process. It was concluded that photolysis of PLA is a complex physicochemical process, the mechanism of which depends on morphological changes in the solid phase of the polymer under UV radiation.

Rotational Spectrum and First Interstellar Detection of 2-methoxyethanol Using ALMA Observations of NGC 6334I

We use both chirped-pulse Fourier transform and frequency-modulated absorption spectroscopy to study the rotational spectrum of 2-methoxyethanol (CH3OCH2CH2OH) in several frequency regions ranging from 8.7 to 500 GHz. The resulting rotational parameters permitted a search for this molecule in Atacama Large Millimeter/submillimeter Array (ALMA) observations toward the massive protocluster NGC 6334I, as well as source B of the low-mass protostellar system IRAS 16293−2422. A total of 25 rotational transitions are observed in the ALMA Band 4 data toward NGC 6334I, resulting in the first interstellar detection of 2-methoxyethanol. A column density of 1.3−0.9+1.4×1017 cm−2 is derived at an excitation temperature of 143−39+31 K. However, molecular signal is not observed in the Band 7 data toward IRAS 16293−2422B, and an upper-limit column density of 2.5 × 1015 cm−2 is determined. Various possible formation pathways—including radical recombination and insertion reactions—are discussed. We also investigate physical differences between the two interstellar sources that could result in the observed abundance variations.

Cometary outbursts in the Oort cloud

Comet nuclei in the outer Solar system are constantly irradiated by cosmic rays at low temperatures. Accumulated high concentrations of radicals can undergo fast recombination with significant heating of cometary surface layers. We present the model of comet activity at large heliocentric distances caused by the recombination of radicals. We found that the considered mechanism can cause activity of comets in distant regions of the Solar system, even at the Oort cloud distances. Outbursts in distant comet reservoirs can be a new source of dust and ice particles contributing to the recently discovered anomalous diffuse light in the cosmic extragalactic background optic light and the unexpectedly high flux of dust particles detected by the New Horizons dust counter at the edge of the Kuiper belt. The orbits of small-radii comets in the Oort cloud are highly influenced by cometary outbursts. This effect may account for the observed decrease in the number of small-radius long-period comets.

Stable and selective multi-porous nickel nanoparticles catalysts for hydrocracking of dibenzyl ether supported by DFT studies

Preparation of clean fuel from biomass has a crucial significance on the development and utilization of renewable clean energy. In this study, a multi-stage porous mordenite supported nickel nanoparticles catalyst (Nix%/HMOR) was prepared, which showed that its acid content, acidity, porosity, specific surface area and metal dispersion were significantly improved while the crystallization strength was basically the same compared with microporous mordenite. In order to study the conversion mechanism of biomass energy, dibenzyl ether was selected as a typical lignin model compound. Under the condition of Ni10%/HMOR in n-hexane at 160 °C for 2 h under 5 MPa of initial hydrogen pressure, the conversion of dibenzyl ether was 100 % and the selectivity of methyl cyclohexane was 84.4 %, which manifested that Ni10%/HMOR had good activity in the selective hydrocracking process under mild conditions. Furthermore, density functional theory (DFT) was selected to study the activation energies of different reaction sites on dibenzyl ether bridge bonds and the recombination process of free radicals in the reaction process. It comes to a conclusion that the O atom on the C-O bridge bond is easier to be attacked by H*, and the C-O bond of the unsaturated aromatic ring is easier to break. During the recombination of free radicals, the two C-O bond breaking energy barriers of OMD are different, and H free radicals are more likely to attack the O atom in cyclohexane-methanol. It is conducive to the formation of stable compound 1 - (cyclohexylmethyl) − 1-methylcyclohexane and its isomers.

Visible Light‐Induced Enantioselective Radical Reactions Catalyzed by Redox‐Active Chiral Lewis Acids

AbstractThis Concept outlines cutting‐edge progress in catalytic and enantioselective radical reactions facilitated by redox‐active chiral Lewis acids upon visible light excitation. These catalytic systems exploit the high reactivity of radical species to enable efficient stereocontrolled radical recombination. We delve into the design, underlying mechanisms, and practical applications of these catalysts, aiming for precise stereocontrolled bond formation. The synergistic blend of chirality and reactivity has brought a new level of precision to enantioselective radical additions, reshaping the approach to asymmetric synthesis. This Concept focuses on the recent progress and effective application of redox‐active chiral Lewis acids, including Rh, Ir, Ga and Ni, in catalyzing enantioselective radical reactions, which offers a comprehensive perspective on modern methods and projecting future directions in enantioselective radical chemistry.

Sensitivity enhancement of radical-pair magnetoreceptors as a result of spin decoherence.

Electron spin relaxation is, on many occasions, considered an elephant in the room that challenges the idea of a radical-pair compass, a leading hypothesis for the navigation of migratory avian species. It has been widely recognized that an effective radical-pair magnetoreceptor requires a relaxation time that is long enough for an external magnetic field as weak as the geomagnetic field to significantly modify the coherent spin dynamics. However, previous studies proposed that certain spin relaxation, far quicker than the radical recombination reactions, could enhance, rather than degrade, the directional sensitivity of a radical-pair magnetoreceptor. Here, I investigate relaxation effects on the singlet-triplet interconversion of a model radical pair and find that the enhancement effect originates from population relaxation over a period of several microseconds as a result of efficient spin decoherence. Insights into the truncated spin systems shed light on the physics behind them. I further investigate the possibilities of such enhancement in cryptochrome-based magnetoreception, in which electron hopping takes place between tryptophan residues.

Alkene dialkylation by triple radical sorting.

The development of bimolecular homolytic substitution (SH2) catalysis has expanded cross-coupling chemistries by enabling the selective combination of any primary radical with any secondary or tertiary radical through a radical sorting mechanism1-8. Biomimetic9,10 SH2 catalysis can be used to merge common feedstock chemicals-such as alcohols, acids and halides-in various permutations for the construction of a single C(sp3)-C(sp3) bond. The ability to sort these two distinct radicals across commercially available alkenes in a three-component manner would enable the simultaneous construction of two C(sp3)-C(sp3) bonds, greatly accelerating access to complex molecules and drug-like chemical space11. However, the simultaneous in situ formation of electrophilic and primary nucleophilic radicals in the presence of unactivated alkenes is problematic, typically leading to statistical radical recombination, hydrogen atom transfer, disproportionation and other deleterious pathways12,13. Here we report the use of bimolecular homolytic substitution catalysis to sort an electrophilic radical and a nucleophilic radical across an unactivated alkene. This reaction involves the in situ formation of three distinct radical species, which are then differentiated by size and electronics, allowing for regioselective formation of the desired dialkylated products. This work accelerates access to pharmaceutically relevant C(sp3)-rich molecules and defines a distinct mechanistic approach for alkene dialkylation.

Growth kinetics and morphology characterization of binary polymeric fluid under random photo-illumination.

We present a comprehensive study using dissipative particle dynamics simulations to investigate phase separation kinetics (PSK) in three-dimensional (3d) polymeric fluids under random photo-illumination. We consider two scenarios: polymer blends with active radicals at one end of each immiscible chain and block copolymer (BCP) melts with photosensitive bonds linking incompatible blocks. The phase separation (PS) is induced by temperature quench of the initial homogeneously mixed system. Simultaneously, the system experiences random photo-illumination, simulated by two concurrent random events: (a) the recombination of active radicals in polymer blends and (b) the breaking of photosensitive bonds in BCP chains. Variations in the bond-breaking probability, Pb, mimic the change in light intensity. The length scale follows power law growth, R(t) ∼ tϕ, where ϕ represents the growth exponent. Increasing Pb results in a gradual transition in growth kinetics from micro-PS to macro-PS, accompanied by corresponding transition probabilities for both systems. Micro-PSK dominates the evolution process at low Pb values. The scaling functions exhibit data overlap for most scaled distances, indicating the statistical self-similarity of evolving patterns. Our study enhances the understanding of PSK in polymeric fluids, revealing the impact of photosensitive bonds and active radicals. Furthermore, it suggests the potential for designing novel polymeric materials with desired properties.

Effect of differential diffusion on head-on quenching of premixed NH[formula omitted]/H[formula omitted]/air flames within turbulent boundary layers

Direct numerical simulations (DNS) have been done for premixed laminar and turbulent NH3/H2/air flames propagating towards a cold wall. Two turbulent channel flows (with well-characterized turbulent boundary layers) at Reτ = 280 and 313 have been simulated for the turbulent flame cases. Near-wall low temperature chemistry has been considered. Effect of differential diffusion on the flame head-on quenching characteristics (quenching distance and maximum wall heat flux) has been checked in detail. Finally, two underlying mechanisms determining differential diffusion effects have been identified: (a) preferential diffusion of H radical increases the reaction rates of the near-wall low temperature (radical recombination) reactions, resulting in larger quenching distance and wall heat flux; (b) differential diffusion of fuels results in smaller flame heat release, thus, larger quenching distance and smaller wall heat flux. The effect of H radical preferential diffusion is suppressed in rich H2/air turbulent flames and strengthened in lean H2/air turbulent flames, due to the differential diffusion focusing/defocusing effects. The effect of differential diffusion of fuels is suppressed in turbulent flames, due to the enhanced mixing process, especially at low Damköhler numbers typical for NH3/H2/air flames interacting with highly turbulent flows.

Accelerated Recombination of Lophyl Radicals in Micelles: Rapid Controlled Self-Assembly of Micelles Formed by Amphiphilic Lophine Dimers and Release of Solubilized Substance by Photoirradiation.

Controlling the morphology of molecular assemblies formed by surfactants by photoirradiation enables the controlled release of incorporated substances, which can be applied to delivery systems for drugs and active ingredients. On the other hand, conventional photoresponsive surfactants and molecular assemblies have a slow response speed, making it difficult to control their functions at the desired time. In this review, I discuss our recent progress in the accelerated control of functions of photoresponsive molecular assemblies by using lophine dimer as a photochromic compound. The lophine dimer derivative dissociates into a pair of lophyl radicals that upon ultraviolet (UV) light irradiation, and these radical species thermally recombine although the recombination reaction is extremely slow due to the diffusion of lophyl radicals. By using the confined inner space of micelles formed by surfactants, the recombination reaction was extremely accelerated. With UV light irradiation, rapid morphological changes in micelles, formed by amphiphilic lophine dimers were observed by using in situ small-angle neutron scattering (in situ SANS) system. Moreover, the rapid controlled release of calcein as a model drug was achieved by UV light irradiation using the photoresponsive micelles. This rapid system can realize the controlled release of drugs truly at the desired time, developing an efficient and precise drug delivery system (DDS). Furthermore, it can be applied in a wide range of fields such as release control of active ingredients, efficient heat exchange control, and actuating systems.

Quantum effects in CH activation with [Cu2O2]2+ complexes.

We investigate the mechanism of primary alkane CH bond activation with dioxo-dicopper ([Cu2O2]2+) complexes, which serve as model catalysts for enzymes capable of activating CH bonds under mild conditions. As large H/D kinetic isotope effects (KIEs) are observed in enzymes and their synthetic mimics, we employ density functional theory along with variational transition-state theory with multidimensional tunneling to estimate reaction rate coefficients. By systematically varying ligand electrophilicity and substrate chain length, we examine trends in rate coefficients and kinetic isotope effects for the two proposed CH activation pathways - one-step oxo-insertion and two-step radical recombination. Although larger tunneling transmission coefficients are obtained for the radical pathway, the oxo-insertion mechanism yields higher rate coefficients on account of lower activation barriers. The question of the preferred CH activation mechanism, however, remains open: excellent agreement is observed between the predicted and known experimental KIE results for the radical pathway, while calculated Hammett slopes for the oxo-insertion pathway closely mirror experiments.

The role of chemistry in the retardant effect of dimethyl methylphosphonate in flame–wall interaction

Organophosphorous compounds can act as chemical inhibitors for fire suppression and have recently received significant attention in the combustion community due to their potential to induce flame extinction through a radical recombination process. To optimize their use, a comprehensive understanding of the extinction dynamics is essential. In this study, mixtures of methane with dimethyl methylphosphonate (DMMP) are investigated in laminar conditions, with a specific focus on flame–wall interaction. The ultimate objective is obtaining a deeper insight into the flame quenching process, which is synergistically enhanced by heat transfer to the wall and the chemical retardant effect of DMMP. Detailed chemistry information is implemented by setting up an ad-hoc skeletal kinetic mechanism. This is developed and validated for flame configurations with increasing complexity, including freely propagating premixed flames, head-on-quenching flames, and side-wall quenching flames. The results indicate that the skeletal mechanism is able to reproduce available experimental data for flame speed and ignition delay time, as well as the main flame features and quenching characteristics in flame–wall interactions for an increasing level of DMMP. Simulations of a side-wall quenching burner, incorporating a secondary fuel injection through a porous insert at the wall, are carried out. Chemical analyses provide detailed insight into the role of flame retardant during the flame quenching process. The results show that the HOPO/PO2 catalytic cycle is of major importance for the suppressant effect, further supported by the formation of CH3PO2 as an intermediate, increasing the formation of HOPO itself. Moreover, the CO mass fraction is observed to increase also because of the radical scavenging effect, inhibiting the conversion to CO2 via CO ＋ OH → CO2 ＋ H. Overall, this study advances the understanding of the chemical features of flame quenching in the presence of flame retardants containing implications for fire safety applications.

Evolution of spin coherence of radical pairs due to spin-selective recombination: Comparison of three models.

This study looked for a way to evaluate the validity of previously suggested models for describing the spin-selective recombination of radical pairs. As an example, for analysis, we used the conventional model, the model by Jones and Hore [Chem. Phys. Lett. 488, 90 (2010)], and the model by Salikhov [Am. J. Phys. Chem. 11, 67 (2022)]. To do this, analytical solutions to the evolution of the radical pair density matrix due to a radical pair's spin-selective recombination and singlet-triplet transitions in a strong magnetic field were obtained for the conventional model and the Jones and Hore model. Spin interactions included in the Hamiltonian were time-independent exchange interactions as well as Zeeman and hyperfine interactions. The most striking difference between the models' predictions appeared when considering the fraction of singlet pairs among all currently existing ones. In the Jones and Hore model, this ratio has the form of damped oscillations for any values of the spin-hamiltonian parameters. The conventional model and the Salikhov model both predicted that this ratio had the form of undamped oscillations in the absence of exchange interaction and at a sufficiently low recombination rate. Besides, the conventional model predicts the possibility of a resonance-like behavior of this ratio when singlet-triplet transitions in a part of the radical pair ensemble are completely suppressed by tuning the magnetic field strength. Possible experimental conditions in which distinguishing between the models seems to be most straightforward were suggested.

Reaction with Water Vapor Defines Surface Reconstruction and Membranolytic Activity of Quartz Milled in Different Molecular Environments.

Industrial processing of quartz (SiO2) and quartz-containing materials produces toxic dust. Fracturing quartz crystals opens the Si‒O bond and produces highly reactive surface species which mainly react with molecules like water and oxygen. This surface-reconstruction process forms silanol (Si‒OH) on the quartz surface, which can damage biological membranes under specific configurations. To comprehend the impact of the quartz surface restructuring on membranolytic activity, the formation and reactivity of quartz radicals produced in four distinct molecular environments with electron paramagnetic resonance (EPR) spectroscopy are evaluated and their membranolytic activity is measured through in vitro hemolysis test. The four molecular environments are formulated with and without molecular water vapor and oxygen (±H2O/±O2). The absence of water favored the formation of surface radical species. In water-rich environments, diamagnetic species prevailed due to radical recombination. Quartz milled in -H2O/±O2 acquired membranolytic activity when exposed to water vapor, unlike quartz milled in +H2O/±O2. After being stabilized by reaction with water vapor, the membranolytic activity of quartz is maintained over time. It is demonstrated that the type and the reactivity of radical sites on quartz are modulated by the outer molecular environment, ultimately determining the biological activity of milled quartz dust.

General relationships between the kinetic characteristics of bulk and geminate recombination of radicals in solutions

General relationships between the kinetic characteristics of bulk and geminate recombination of radicals in solutions

Across the stages: a multiscale extension of the generalized stochastic microdosimetric model (MS-GSM2) to include the ultra-high dose rate

Ultra-high dose rate (UHDR) irradiations with different types of radiation have shown a larger sparing of normal tissue and unchanged tumor control with respect to conventional delivery. In recent years, there has been an accumulation of experimental evidence related to the so-called FLASH effect. However, the underpinning mechanism remains, to date, extremely debated and largely unexplained, while the involvement of multiple scales of radiation damage has been suggested. Since it is believed that the chemical environment plays a crucial role in the FLASH effect, this work aims to develop a multi-stage tool, the multiscale generalized stochastic microdosimetric model (MS-GSM2), that can capture several possible effects on DNA damage at the UHDR regime, such as reduction of DNA damage yield due to organic radical recombination, damage fixation due to oxygenation, and spatial and temporal dose deposition effects, allowing us to explore most of the candidate mechanisms for explaining the FLASH effect. The generalized stochastic microdosimetric model (GSM2) is a probabilistic model that describes the time evolution of DNA damage in a cell nucleus using microdosimetric principles, accounting for different levels of spatio-temporal stochasticity. In particular, the GSM2 describes radiation-induced DNA damage formation and kinetic repair in the case of protracted irradiation without considering the Poissonian assumption to treat the number of radiation-induced DNA damage. In this work, we extend the GSM2, coupling the evolution of DNA damage to fast chemical reaction kinetics, described by a system of ordinary differential equations, accounting for an additional level of stochasticity, i.e., in chemistry. We simulate energy deposition by particles in a microscopic volume, which mimics the cell nucleus, in order to examine the combined effects of several chemical species and the time evolution of DNA damage. We assume that UHDR modifies the time evolution of the peroxyl radical concentration, with a consequent reduction in the yield of the indirect DNA damage. This damage reduction emerges only at UHDR and is more pronounced at high doses. Moreover, the indirect damage yield reduction depends on the radiation quality. We show that the MS-GSM2 can describe the empirical trend of dose- and dose rate-dependent cell sensitivity over a broad range, particularly the larger sparing of healthy tissue occurring at the FLASH regime. The complete generality of the MS-GSM2 also allows us to study the impact of different dose delivery time structures and radiation qualities, including high LET beams.

Non-gravitational Mechanism of Comets’ Ejection from the Oort Cloud Due to Cometary Outbursts

Cometary nuclei located in the Oort cloud accumulate high concentration of radicals in surface layers under cosmic ray irradiation at low temperatures. Recombination of radicals induced by an increase in the surface temperature of a comet by a close passing star, O/B stars, or nearby supernovae leads to the heating of the ice layer with the releasing of volatiles from the amorphous ice. When high gas pressure builds up beneath the cometary surface, dust and gas are ejected. The resulting jet of gas and dust can change the comet’s orbit in the Oort cloud. The studied non-gravitational mechanism can effectively expel comets with a radius of ≤1 km from the Oort cloud into the inner part of the Solar system. The total effect of cometary outbursts on the stability of cometary orbits during the evolution of Solar system can result in a decrease in the number of long-period small-radius comets.

Selenium‐Catalyzed Electrophilic Cascade Reactions of Vinylarenes with N‐Fluorobenzenesulfonimide: A Novel Strategy for the Synthesis of β‐Keto Sulfones

AbstractA selenium‐catalyzed electrophilic cascade reaction of vinylarenes with N‐fluorobenzenesulfonimide for the efficient synthesis of important β‐keto sulfones via a single operation is developed. The reactions of vinylarenes bearing electron‐withdrawing groups and electron‐donating groups proceeded smoothly in the air and gave products in up to 94 % yield. Mechanistic studies suggest that vinyl amines generated by oxidative amination of vinylarenes are the intermediates, the cleavage of N−S bond and the recombination of sulfonyl radical lead to 1,3‐migration of sulfonyl, and hydrolysis affords the products β‐keto sulfones.

Excellent conformality of atmospheric-pressure plasma-enhanced spatial atomic layer deposition with subsecond plasma exposure times

Atmospheric-pressure spatial atomic layer deposition (s-ALD) has emerged as a scalable deposition technique combining the advantages of ALD with high deposition rates, suitable for low-cost and high-volume applications. There is a growing interest in atmospheric-pressure plasma-enhanced spatial ALD (PE-s-ALD), e.g., to allow for deposition at reduced temperatures or for materials that are otherwise difficult to prepare by thermal ALD. For low-pressure PE-ALD, conformal films on high aspect ratio features have been achieved despite plasma radical recombination, and the aspects influencing conformality are fairly well understood. This work addresses surface recombination and conformality for atmospheric-pressure PE-s-ALD films. We demonstrate that conformality can be achieved for SiO2 and TiO2 films deposited by atmospheric-pressure PE-s-ALD inside high-aspect-ratio trenches with short plasma exposure times. Using plasma exposure of 0.73 s results in conformal SiO2 and TiO2 films in structures with aspect ratios of 74 and 219, respectively. Additionally, the recombination probabilities of oxygen radicals at atmospheric pressure are extracted to be 4×10−4 for SiO2 and 6×10−5 for TiO2. These results demonstrate that atmospheric-pressure PE-s-ALD can be used for conformal and high-speed depositions on 3D substrates.

Understanding the Reaction Chemistry of 1,1,3,3-Tetramethyldisilazane as a Precursor Gas in a Catalytic Chemical Vapor Deposition Process.

The reaction chemistry of 1,1,3,3-tetramethyldisilazane (TMDSZ) in catalytic chemical vapor deposition (Cat-CVD), including its primary decomposition on a heated W filament and secondary gas-phase reactions in a Cat-CVD reactor, was studied using 10.5 eV vacuum ultraviolet single-photon ionization and/or laser-induced electron ionization in tandem with time-of-flight mass spectrometry. It has been demonstrated that TMDSZ initially breaks down to form various species, including methyl radical (•CH3), ammonia (NH3), and 1,1-dimethylsilanimine (DMSA). The activation energies (Ea) for the formation of •CH3 and NH3 were determined to be 61.2 ± 1.0 and 42.1 ± 0.9 kJ mol-1, respectively, in the temperature range of 1400-2000 and 900-2400 °C. It was found that the formation of DMSA may have two different contributing routes, i.e., a concerted one (Ea = 33.6 ± 2.3 kJ mol-1) at lower temperatures of 900-1500 °C and a stepwise one (Ea = 155.0 ± 7.8 kJ mol-1) at higher temperatures of 2100-2400 °C. The secondary gas-phase reactions occurring in the Cat-CVD reactor environment were found to stem from two competing processes. The first one, free-radical short-chain reactions initiated by •CH3 formation and propagated by H abstraction reactions, is the dominating chemical process, producing many high-mass stable alkyl-substituted or silyl-substituted disilazane or trisilazane products via radical recombination reactions. Head-to-tail cycloaddition of unstable DMSA is the second contributing chemical process, which forms cyclodisilazane species. In addition, evidence was found for the conversion of NH3 into H2 and N2 in the Cat-CVD reactor.