Atom Radical Research Articles

Critique and education under new climatic and digital conditions from a logic of division to a logic of gathering

In this article we argue that the critical paradigm can be understood in terms of a logic of never ending division. As such this paradigm is highly inadequate to help us dealing with two major societal challenges, viz. the digitization of our world and the ecological crisis. We first analyze in greater detail the today dominant critical way of looking, based on a brief historical overview. Then we develop our understanding of the growing impact of digitization in terms of an ontological shift in how we relate to ourselves, others and the world and, hence, in terms of a profound change regarding what it means to inhabit the world. This is because we no longer relate to things, but to information or what Han calls ‘non-things’. This situation, we go on arguing, comes with a radical fragmentation of the world into many personalized private ‘worlds’. This implies the impossibility of experiencing to belong to a shared world. It becomes, then, clear that if we want to address this issue, the critical paradigm won’t suffice. Instead we need a response that is postcritical and that focuses on how we can gather again people around some-thing they have in common. In essence this is an educational gesture. Therefore, we advocate for the importance of practices of collective study, and we conclude this article by arguing that this is also what we are in need of to give an appropriate response to the ecological challenges we are faced with.

Computational Study of SmI2-Catalyzed Intermolecular Couplings of Cyclopropyl Ketones: Links between the Structure and Reactivity.

SmI2-catalyzed intermolecular coupling reactions of cyclopropyl ketones with alkenes or alkynes offer an efficient strategy for furnishing diverse five-membered ring-containing molecular architectures. This study presents a systematic computational investigation to reveal the structure-reactivity relationships in these reactions. The reactivity of aryl cyclopropyl ketones is enhanced by the stabilized ketyl radical and cyclopropyl fragmentation, arising from the conjugation effect of the aryl ring, despite an obstacle emerging from the gauche styrene intermediate that elevates the energy barrier for radical trapping. By contrast, alkyl cyclopropyl ketones lack conjugation and exhibit high barriers for reduction and fragmentation but undergo facile radical trapping due to the minimal steric hindrance. Interestingly, ortho-substituted phenyl cyclopropyl ketones exhibit superior reactivity due to a balance between the moderate conjugation, promoting cyclopropyl fragmentation, and the pretwisted nature of the ortho-substituted phenyl that circumvents the hindrance posed by the gauche intermediate and facilitates the radical trapping. The markedly enhanced reactivity of bicyclo[1.1.0]butyl (BCB) ketones arises from facile fragmentation of the strained BCB motif. Bicyclo[2.1.0]pentyl (BCP) ketones, less strained than BCB ketones, are computationally verified to undergo efficient couplings with various partners, and this can be attributed to their stable fragmentation intermediates that facilitate radical trapping. Our findings provide insights that can aid in designing related reactions.

Reactivity of Atomic Oxygen Radical Anions in Metal Oxide Clusters.

Atomic oxygen radical anion (O⋅-) represents an important type of reactive centre that exists in both chemical and biological systems. Gas-phase atomic clusters can be studied under isolated and well controlled conditions. Studies of O⋅--containing clusters in the gas-phase provide a unique strategy to interpret the chemistry of O⋅- radicals at a strictly molecular level. This review summarizes the research progresses made since 2013 for the reactivity of O⋅- radicals in the atomically precise metal oxide clusters including negatively charged, nanosized, and neutral heteronuclear metal clusters benefitting from the development of advanced experimental techniques. New electronic and geometric factors to control the reactivity and product selectivity of O⋅- radicals under dark and photo-irradiation conditions have been revealed. The detailed mechanisms of O⋅- generation have been discussed for the reaction systems of nanosized and heteroatom-doped metal oxide clusters. The catalytic reactions mediated by the O⋅- radicals in metal clusters have also been successfully established and the microscopic mechanisms about the dynamic generation and depletion of O⋅- radicals have been clearly understood. The studies of O⋅- containing metal oxide clusters in the gas-phase provided new insights into the chemistry of reactive oxygen species in related condensed-phase systems.

Understanding Selectivity in Product Distributions from Laser Ablation of Organic Liquids.

Pulsed laser ablation in organic solvents is widely used to produce oxide-free metal and metal carbide nanoparticles, often with carbon coatings resulting from laser-induced reactions in the organic solvent. To gain insight into how the molecular structure of the solvent affects these reaction pathways, this work investigates ablation of the C6H14 isomers n-hexane, 2-methylpentane, and 3-methylpentane through characterization of the gas and liquid products with mass spectrometry. Ablation of each C6H14 isomer produces a distinct distribution of product molecular weights and isomers. 2-methylpentane preferentially produces C3 and C9, whereas 3-methylpentane produces C2, C4, C8, and C10 products. These preferential product distributions, along with the lack of such selectivity in n-hexane, arise from differences in the most favorable C-C bond scission pathways in each C6H14 isomer. Moreover, the particular isomers of C8H18, C9H20, C10H22, and C12H26 produced by ablation of each C6H14 isomer indicate that the vast majority of reaction pathways involve addition reactions between a fragment radical and parent C6H14 or between two C6H14 molecules, without molecular rearrangement. This propensity toward direct addition suggests that the chemical reactions induced by ultrashort pulsed laser ablation proceed on faster time scales than those of radical rearrangements.

Fe-Catalyzed α-C(sp3)-H Amination of N-Heterocycles.

Nitrogen-heterocycles are privileged structures in both marketed drugs and natural products. On the other hand, C-H amination reactions furnish unconventional and straightforward approaches for the construction of C-N bonds. Yet, most of the known methods rely on precious metal catalysts. Herein we report a site-selective intermolecular C(sp3)-H amination of N-heterocycles, catalyzed by inexpensive FeCl2,which allows the functionalization of a wide range of pharmaceutically relevant cyclic amines. The C-H amination occurs site-selectively in α-position to the nitrogen atom, even when weaker C-H bonds are present, and furnishes Troc-protected aminals or amidines. The method employs the N-heterocycle as limiting reagent and is applicable to the late-stage functionalization of complex molecules. Its synthetic potential was further illustrated through the derivatization of α-aminated products and the application to a concise total synthesis of the reported structure for senobtusin. Mechanistic studies allowed to propose a plausible reaction mechanism involving a turnover-limiting Fe-nitrene generation followed by fast H atom transfer and radical rebound.

Fe‐Catalyzed α‐C(sp3)–H Amination of N‐Heterocycles

Nitrogen‐heterocycles are privileged structures in both marketed drugs and natural products. On the other hand, C–H amination reactions furnish unconventional and straightforward approaches for the construction of C–N bonds. Yet, most of the known methods rely on precious metal catalysts. Herein we report a site‐selective intermolecular C(sp3)–H amination of N‐heterocycles, catalyzed by inexpensive FeCl2, which allows the functionalization of a wide range of pharmaceutically relevant cyclic amines. The C–H amination occurs site‐selectively in α‐position to the nitrogen atom, even when weaker C–H bonds are present, and furnishes Troc‐protected aminals or amidines. The method employs the N‐heterocycle as limiting reagent and is applicable to the late‐stage functionalization of complex molecules. Its synthetic potential was further illustrated through the derivatization of α‐aminated products and the application to a concise total synthesis of the reported structure for senobtusin. Mechanistic studies allowed to propose a plausible reaction mechanism involving a turnover‐limiting Fe‐nitrene generation followed by fast H atom transfer and radical rebound.

Electrochemical generation of H*-•OH redox pair for rapid removal of refractory nitro-organic micropollutants

This study developed an innovative reduction-oxidation coupling process for treatment of refractory organic wastewater. We constructed a series of N5-coordinated transition metals (TMs) (TMs = Mn, Fe, Co, Ni, Cu) single-atom sites supported on nitrogen-rich carbon nitride (SA-TM-C3N5) for electrochemical generation of the surface-adsorbed hydrogen atom (H*) and hydroxyl radical (•OH). The SA-Co-C3N5 exhibited the highest activity for production of the H*-•OH redox pair, which effectively degraded refractory nitro-organic micropollutants via the parallel reduction-oxidation pathways. The SA-Co-C3N5-mediated electro-Fenton-like system showed high removal efficiency (99 %) and mineralization capacity (78 %) toward metronidazole (MNZ) with low energy consumption (0.019 kWh (g TOC)–1). The trifunctional properties of the H* have contributed to the reduction of nitro compounds, the 2e– ORR for H2O2 production and the Fenton-like catalysis for •OH generation. This work sheds lights on the development of advanced reduction-oxidation coupling process for green and sustainable water purification.

Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics.

Hydroxyl radical (HO•) and chlorine atom (Cl•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO• and Cl• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO• and Cl• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl• were more thermodynamically favorable than HO•, but contrary results were obtained for HAT reactions. According to the free energy of activation (ΔGaq‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO• and SET by Cl•. The important role of SET in the HO• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO•/HO- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the ΔGaq‡ value of each reaction pathway of Cl• was lower than that of HO•, resulting in higher rate constants of Cl• with aromatic compounds than HO•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO• and Cl•. This study proposed general rules for the reaction mechanisms of HO• and Cl• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO• and Cl•.

Building Catalytic Reactions One Electron at a Time.

ConspectusClassical education in organic chemistry and catalysis, not the least my own, has centered on two-electron transformations, from nucleophilic attack to oxidative addition. The focus on two-electron chemistry is well-founded, as this brand of chemistry has enabled incredible feats of synthesis, from the development of life-saving pharmaceuticals to the production of ubiquitous commodity chemicals. With that said, this approach is in many ways complementary to the approach of nature, where enzymes frequently make use of single-electron "radical" steps to achieve challenging reactions with exceptional selectivity, including light detection and C-H hydroxylation. While the power of radical elementary steps is undeniable, the fundamental understanding of─and ability to apply─these in catalysis remains underdeveloped, constraining the palette with which chemists can make new reactions.Motivation to remedy this traditional underemphasis on radical catalysis has been intensified by the runaway success of outer-sphere photoredox catalysis, not only confirming the versatility of radicals in anthropogenic catalysis but also teaching the value of robust and well-understood catalytic cycles for reaction design. Indeed, I would argue the success of outer-sphere photoredox catalysis has been fueled by strong fundamental understanding of its underlying radical elementary steps, with consideration of single-electron transfer (SET) energetics allowing new reactions to be designed de novo with enviable confidence. However, outer-sphere photoredox catalysis is an outlier in this regard, with other mechanistic approaches remaining underexplored.Our research group is part of a growing movement to expand the vocabulary of synthetic radical catalysis beyond the traditional outer-sphere photoredox SET manifold, assembling new cycles comprised of hydrogen atom transfer (HAT), light-induced homolysis (LIH), and radical ligand transfer (RLT) steps in new combinations to achieve challenging transformations. These efforts have been made possible by the ever-growing understanding of these radical elementary steps and discovery of catalyst systems with significant mechanistic flexibility, most recently iron/thiol (Fe/S) cocatalysis.In this Account, I will focus on our efforts applying HAT and LIH steps in Fe/S cocatalysis, sharing broad guidelines we have found helpful for using these steps and demonstrating how they can be combined to make new reactions using three case studies: radical hydrogenation (HAT + HAT), decarboxylative protonation (LIH + HAT), and alkene hydrofluoroalkylation (LIH + HAT, with an intervening radical alkene addition). These efforts have highlighted the importance of several key parameters, including bond dissociation energy (BDE) and radical polarity, and I hope our findings similarly provide a valuable framework to others designing new radical catalytic reactions.

Enantioselective Synthesis of α‐Aryl Ketones by a Cobalt‐Catalyzed Semipinacol Rearrangement

A highly enantioselective cobalt‐catalyzed semipinacol rearrangement of symmetric α,α‐diarylallylic alcohols is disclosed. A chiral cobalt‐salen catalyst generates a highly electrophilic carbocation surrogate following hydrogen atom transfer and radical–polar crossover steps. This methodology provides access to enantioenriched α‐aryl ketones through invertive displacement of a cobalt(IV) complex during 1,2‐aryl migration. A combination of readily available reagents, silane and N‐fluoropyridinium oxidant, are used to confer this type of reactivity. An exploration into the effect of aryl substitution revealed the reaction tolerates para‐ and meta‐halogenated, mildly electron‐rich and electron‐poor aromatic rings with excellent enantioselectivities and yields. The yield of the rearrangement diminished with highly electron‐rich aryl rings whereas very electron‐deficient and ortho‐substituted arenes led to poor enantiocontrol. A Hammett analysis demonstrated the migratory preference for electron‐rich aromatic rings, which is consistent with the intermediacy of a phenonium cation.

Exploring antioxidative properties of xanthohumol and isoxanthohumol: An integrated experimental and computational approach with isoxanthohumol pKa determination

This study explores the antioxidative activities of xanthohumol (XN) and isoxanthohumol (IXN), prenylated flavonoids from Humulus lupulus (family Cannabaceae), utilizing the oxygen radical absorption capacity (ORAC) and ferric reducing antioxidant power (FRAP) assays along with computational Density Functional Theory methods. Experimentally, XN demonstrated significantly higher antioxidative capacities than IXN. Moreover, we determined IXN pKa values using the UV/Vis spectrophotometric method for the first time, facilitating its accurate computational modeling under physiological conditions. Through a thermodynamic approach, XN was found to efficiently scavenge HOO• and CH3O• radicals via Hydrogen Atom Transfer (HAT) and Radical Adduct Formation (RAF) mechanisms, while CH3OO• scavenging was feasible only through the HAT pathway. IXN exhibited its best antioxidative activity against CH3O• via both HAT and RAF mechanisms and could also scavenge HOO• through RAF. Both Single Electron Transfer (SET) and Sequential Proton Loss-Electron Transfer (SPLET) mechanisms were thermodynamically unfavorable for all radicals and both compounds.

Double Spin with a Twist: Synthesis and Characterization of a Neutral Mixed-Valence Organic Stable Diradical.

A convenient design strategy opens access to neutral open-shell mixed-valence species via the redox transformation of charged stable precursors, i.e., the spiro-fused borate anions. We have implemented this strategy for the synthesis of the first neutral mixed-valence diradical: two neutral mixed-valence radical fragments were assembled via a twisted biphenyl bridge. The diradical is a crystalline solid obtained in almost quantitative yield by using a facile synthetic procedure. It is stable at room temperature in the triplet ground state with a very small singlet/triplet gap. This metal-free diradical can reversibly form five redox states. The diradical exhibits an intense IVCT band in the NIR region and can be assigned as a Class 2 Robin-Day MV (mixed valence) system with weakly interacting redox centers. Computations suggest that this diradical finds itself in a unique tug-of-war between two electron delocalization patterns, Kekulé and non-Kekulé, which gives rise to two geometric isomers that are close in energy but drastically different in spin distribution and polarity. Such bistable spin-systems should be intrinsically switchable and promising for the design of functional spin devices. The scope and limitations of the new redox-strategy for the neutral MV radicals were also tested on other types of spiro-fused borates, revealing structural factors responsible for the evolution from transient to persistent and then to stable radicals.

An investigation of CO generation path upon loading coal from perspective of macromolecular simulation.

CO is a hazardous and pollutant gas that can be produced in many scenarios of coal-related operations. The study mainly investigated CO production process and mechanism when coal is subject to external forces. The effects of coal type, particle size, temperature, and inlet atmosphere on CO production from coal body fragmentation were investigated through coal loading experiments. Materials Studio software was used to carry out coal macromolecular mechanics simulation and molecular dynamics simulation, and the gas production mechanism of coal under loading was explored at the molecular level. It was found that under air atmosphere, the low degree of deterioration, small particle size, and elevated temperature are all more likely to cause coal samples to fragment and decompose to produce CO. The carbonyl group in the molecular structure of coal is shed or broken free radical fragments react with oxygen which may lead to CO formation.

Role of H2O Adsorption in CO Oxidation over Cerium-Oxide Cluster Anions (CeO2)nO- (n = 1-4).

Water (H2O) is ubiquitous in the environment and inevitably participates in many surface reactions, including CO oxidation. Acquiring a fundamental understanding of the roles of H2O molecules in CO oxidation poses a challenging but pivotal task in real-life catalysis. Herein, benefiting from state-of-the-art mass-spectrometric experiments and quantum chemical calculations, we identified that the dissociation of a H2O molecule on each of the cerium oxide cluster anions (CeO2)nO- (n = 1-4) at room temperature can create a new atomic oxygen radical (O•-) that then oxidizes a CO molecule. The size-dependent reactivity of H2O-mediated CO oxidation on (CeO2)nO- clusters was rationalized by the orbital compositions (O2p) and energies of the lowest unoccupied molecular orbitals of active O•- radicals modified by H2O dissociation. Our findings not only provide new insights into H2O-mediated CO oxidation but also demonstrate the importance of H2O in modulating the reactivity of the O•- radicals.

Study on pyrolysis characteristics of Gonghe oil shale using Py-GC/MS under different atmospheres

ABSTRACT Taking Gonghe oil shale as raw material, the rapid pyrolysis experiment was carried out using the Py-GC/MS device under different atmospheres. The effects of different pyrolysis atmospheres on the rapid pyrolysis products of Gonghe oil shale were studied. The results show that the H2 atmosphere can not only increase the content of saturated hydrocarbons and reduce the content of unsaturated hydrocarbons but also reduce the content of oxygen-containing heterocyclic compounds. This may be because more H free radicals under the H2 atmosphere are produced to react with the free radical fragments formed by kerogen decomposition. The coupling between H free radicals and free radical fragments can prevent the polymerization among macromolecular free radicals and inhibit the generation of heavy byproducts, which can promote the kerogen pyrolysis and increase the release of volatile matter. In addition, the pyrolysis atmosphere is conducive to the deoxygenation of oxygen-containing heterocyclic compounds to a certain extent.

Free radical fragmentation and oxidation in the polar part of lysophospholipids: Results of the study of blood serum of healthy donors and patients with acute surgical pathology

The interaction of reactive oxygen species with cell membrane lipids is usually considered in the context of lipid peroxidation in the nonpolar component of the membrane. In this work, for the first time, data were obtained indicating that damage to human cell membranes can occur in the polar part of lysophospholipids at the interface with the aqueous environment due to free radical fragmentation (FRF) processes. FRF products, namely 1-hexadecanoyloxyacetone (PAc) and 1-octadecanoyloxyacetone (SAc), were identified in human serum, and a GC-MS method was developed to quantify PAc and SAc.The content of FRF products in serum samples of 52 healthy donors was found to be in the range of 1.98–4.75 μmol/L. A linear regression equation, CPAc&SAc (μmol/L) = 0.51 + 0.064 × years, was derived to describe the relationship between age and content of FRF products. In 70 patients with acute surgical pathology in comparison with the control group of healthy donors, two distinct clusters with different concentration levels of FRF products were revealed. The first cluster: groups of 43 patients with various localized inflammatory-destructive lesions of hollow organ walls and bacterial translocation (septic inflammation) of abdominal cavity organs. These patients showed a 1.5–1.9-fold (p = 0.012) decrease in the total concentration of PAc and SAc in serum. In the second cluster: groups of 27 patients with ischemia-reperfusion tissue damage (aseptic inflammation), – a statistically significant increase in the concentration of FRF products was observed: in 2.2–4.0 times (p = 0.0001).The obtained data allow us to further understand the role of free-radical processes in the damage of lipid molecules. FRF products can potentially be used as markers of the degree of free-radical damage of hydroxyl containing phospholipids.

Experimental Reactivity of (MoO3)NO- (N = 1-21) Cluster Anions with C1-C4 Alkanes: A Simple Model to Predict the Reactivity with Methane.

Metal oxide clusters with atomic oxygen radical anions are important model systems to study the mechanisms of activating and transforming very stable alkane molecules under ambient conditions. It is extremely challenging to characterize the activation and conversion of methane, the most stable alkane molecule, by metal oxide cluster anions due to the low reactivity of the anionic species. In this study, using a ship-lock type reactor that could be run at relatively high pressure conditions to provide a high number of collisions in ion-molecule reactions, the rate constants of the reactions between (MoO3)NO- (N = 1-21) cluster anions and the light alkanes (C1-C4) were measured under thermal collision conditions. The relationships among the reaction rates of different alkanes were obtained to establish a model to predict the low rate constants with methane from the high rate constants with C2-C4 alkanes. The model was tested by using available experimental results in literature. This study provides a new method to estimate the relatively low reactivity of atomic oxygen radical anions with methane on metal oxide clusters.

Reduction of Ketones and Coupling of Ketyls by a Zn-Zn-Bonded Compound.

The α-diimine-ligated Zn-Zn-bonded compound [K(THF)2]2[LZn-ZnL] (1, L = [(2,6-iPr2C6H3)NC(Me)]22-) displays diverse reactivities toward a variety of ketones. In the reaction of 1 with benzophenone or 4,4'-di-tert-butylbenzophenone, a multielectron transfer process was observed to give bimetallic (Zn/K) complexes with both ketyl radical fragments and C-C coupled pinacolate moieties (products 2 and 3). In contrast, treating 1 with 9-fluorenone only afforded pinacolate complex 5. Moreover, the reactions of 1 with N- or O-heterocycle-functionalized ketones, i.e., di(2-pyridyl)ketone, 2,2-pyrrolidinone, 9-xanthenone, or 10-methyl-9(10H)-acridone, were also carried out. Besides different transformations of the ketone moiety, the heteroatoms (nitrogen or oxygen) are also involved in coordination with zinc or potassium ions, yielding discrete aggregates or polymeric structures of products 6-9.

Studying the Intrinsic Reactivity of Chromanes by Gas-Phase Infrared Spectroscopy.

Tandem mass spectrometry is routinely used for the structural analysis of organic molecules, but many fragmentation reactions are not well understood. Because several potential structures can correspond to a measured mass, the assignment of product ions is ambiguous using mass spectrometry alone. Here, we combine mass spectrometry with high-resolution gas-phase infrared spectroscopy and computational chemistry tools to identify product ion structures and derive collision-induced fragmentation mechanisms of the chromane derivatives Trolox and Methyltrolox. We find that protonated Trolox and Methyltrolox fragment identically via dehydration and decarbonylation, while deprotonated ions display substantially diverging reactivities. For deprotonated Methyltrolox, we observe unusual radical fragmentation reactions and suggest a [1,2]-Wittig rearrangement involving aryl migration in the gas phase. Overall, the combined experimental and theoretical approach presented here revealed complex proton dynamics and intramolecular rearrangement reactions, which expand our understanding on structure-reactivity relationships of isolated molecules in different protonation states.

Simulations on coal water slurry gasification by molecular dynamics method with ReaxFF.

Coal water slurry (CWS) is a new type of liquid coal product with low pollution, which is mainly used in the chemical industry to produce syngas (CO + H2). It is of great significance to study the microscopic mechanism of CWS gasification reaction for improving the efficiency of coal gasification. In this paper, the method of molecular dynamics based on reaction force fields (ReaxFF-MD) is used to study the gasification process of CWS/O2 system at different temperatures. The results show that, in the range of 1600-2400 K, the macromolecular network structure of lignite is decomposed into a large number of small molecular structures and a small number of light tar free radical fragments, and the types and quantities of reaction products increased rapidly. At 2400-4000 K, the free radical fragments of light tar are further decomposed and reacted with gasification agents, but the types and quantities of reaction products have little change. At 3600 K, a full gasification reaction occurred in the system, and the content of syngas is the highest. The model was established and optimized by Materials Studio (MS) software. Based on ReaxFF-MD method, Lammps software was used to simulate the gasification process of CWS/O2 system, and the reaction force field files containing C, H, O, N, and S element were used. By calculating the activation energy of gasification reaction, the rationality of the model and calculation method was illustrated. The post-processing of the results was implemented using OVITO, ChemDraw software, and self-programmed Python scripts.