Electrochemical and ESR studies of <Emphasis Type="Italic">tert</Emphasis>-butanol oxidation mechanism in the presence of radical cations pyrazine-di-N-oxide and its substituted derivatives as mediators

The methods of cyclic voltammetry, ESR electrolysis, and quantum chemical simulation were used to study the tert-butanol (tert-BuOH) oxidation mechanism in the presence of mediator cation radicals of pyrazine-di-N-oxide, 2,5-di-Me- and 2,3,5,6-tetra-Me-pyrazine-di-N-oxdides. This study was carried out on carbon glass (CG) and Pt electrodes in 0.1 M LiClO₄ solution in acetonitrile and on Au electrode in tert-butanol containing 0.05 M LiClO₄. The ESR spectra of cation and anion radicals of aromatic di-N-oxides were recorded in tert-BuOH. The quantum chemical simulation of the reaction between pyrazine-di-N-oxide radical cation and C-H bond in tert-BuOH was performed. The results were explained in the terms of the general two-electron oxidation mechanism of tert-BuOH in the complex with aromatic di-N-oxide cation radical as mediator.     

Electrochemical and ESR studies of the oxidation mechanism of pyrazine-di-N-oxides in the presence of methanol and its deuterated derivatives

The mechanism of oxidation of pyrazine-, 2,5-di-Me-, and 2,3,5,6-tetra-Me-pyrazine-di-N-oxides in the presence of methanol and its deuterated derivatives (CH₃OD, CD₃OD), i.e., compounds exhibiting the high energy of C-H bond dissociation, is studied by the methods of cyclic voltammetry, ESR electrolysis, and quantum chemical modeling. The study is carried out on a glassy carbon (GC) electrode in acetonitrile and on an Au electrode in solutions of different alcohols (methanol and its deuterated derivatives CH₃OD, CD₃OD). In alcohol solutions, the ESR spectra of radical cations and radical anions of the tested aromatic di-N-oxides are observed. The quantum chemical simulation of the reaction of the pyrazine-di-N-oxide radical cation with the MeOH C-H bond is carried out. The results obtained are explained within the framework of the E₁C₁E₂C₂ mechanism for a two-stage electrode process determined by the catalytic current of the second electrode stage. The overall two-electron catalytic oxidation of an alcohol within its complex with the pyrazine-di-N-oxide radical cation is proposed.     

Substituted pyrazine-di-N-oxides as the mediators of catalytic oxidation of organic compounds

The mechanism of oxidation of 2,5-dimethyl-, 2,3,5,6-tetramethyl-, 2,3-dimethyl-5,6-cyclohexa, and 3-phenyl-5,6-cyclohexapyrazine-di-N-oxides is studied by cyclic voltammetry, quantum chemical simulations, and ESR electrolysis. The studies are carried out on electrodes of glassy carbon and Pt in 0.1 M LiClO₄ solutions in acetonitrile. ESR spectra of radical cations of substituted pyrazine-di-N-oxides are recorded. The effects of the temperature, oxygen, and the additions of water, pyridine, and acid on the shape of cyclic voltamograms and the intensity of ESR signals of pyrazine-di-N-oxides are studied. A quantum-chemical simulation of the reaction of pyrazine-di-N-oxide radical cations with acetonitrile is carried out. The oxidation of substituted pyrazine-di-N-oxides is described by the E₁C₁E₂C₂ mechanism, which includes the stage of the formation of a complex between the di-N-oxide radical cation and acetonitrile.     

Metal- and chemical-oxidant-free C-H/C-H cross-coupling of aromatic compounds: the use of radical-cation pools

Pool and couple: A method for oxidative C-H/C-H cross-coupling has been developed using "radical-cation pools". Aromatic compounds react with aryl radical cations, which are generated and accumulated by low-temperature electrolysis (see scheme). This method avoids both the nonselective oxidation of substrates and oxidation of products and effects the C-H/C-H cross-coupling of aromatic compounds without metal complexes and chemical oxidants.     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

Computational studies on the cyclization of polycyclic aromatic hydrocarbons in the synthesis of curved aromatic derivatives.

Computational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six- or five-membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial C-C bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic C-H cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest-energy mechanism. The formation of five-membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic C-H dissociation to afford the corresponding cations is the most favourable process. These cations react with H* without barrier to give H2* Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors.     

Co3O4纳米晶催化氧化甲烷的理论研究：C-H键活化的晶面效应及活性中心

甲烷是一种在自然界中大量存在的原材料,在取代原油和合成重要化工产品等许多领域具有潜在的应用价值. 然而,由于CH₄中C-H键的键能特别大（约~4.5 eV）,如何实现甲烷的绿色有效转化在化学化工领域仍然是一个挑战. 本文采用密度泛函理论对Co₃O₄（001）和（011）晶面活化甲烷C-H键的机理进行了理论研究,得到了如下结论：（1） CH₄的C-H键在Co₃O₄晶面的解离具有很高的活性,只需要克服大约1 eV的能垒;（2）与Co^2＋相连的Co-O离子对是CH₄活化的活性位点,其中两个带正负电荷的离子对C-H解离起着协同作用,帮助产生Co-CH₃和O-H物种;（3）（011）面的反应活性明显大于（001）面,与实验的观察一致. 本文的计算结果表明,Co₃O₄纳米晶面对CH₄中C-H键的活化表现出明显的晶面效应和结构敏感效应,Co-O离子对活性中心对于活化惰性的C-H键发挥了关键作用.     

Electrochemical and ESR study of the mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol on glassy carbon and single-walled carbon nanotube electrodes

The mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol was studied by cyclic voltammetry on glassy carbon (GC) and single-walled carbon nanotube (SWCNT) electrodes in 0.1 M LiClO₄ solutions in acetonitrile. The effect of cyclohexanol on the shape of the cyclic voltammograms of phenazine-di-N-oxide and the intensity of the ESR signal of its radical cation was investigated. It was shown by ESR that the products of the one-electron oxidation and reduction of phenazine-di-N-oxide were radical cations and anions. The catalytic currents were recorded during the oxidation of phenazine-di-N-oxide on the SWCNT and GC electrodes in the presence of cyclohexanol. The results were explained in terms of the E₁C₁E₂C₂ mechanism of the two-stage electrode process characterized by the catalytic current recorded at the second electrode stage. The overall two-electron catalytic oxidation of cyclohexanol in the complex with the phenazine-di-N-oxide radical cation was assumed to occur. It was shown that SWCNT electrodes can be used in the electrocatalytic oxidation of organic compounds in the presence of the electrochemically generated phenazine-di-N-oxide radical cation.     

Aromatic C-H...O interactions in a series of bindone analogues. NMR and quantum mechanical study

The aromatic C-H...O hydrogen bonding within the series of the structurally relative indenone derivatives has been studied. The presence of the hydrogen bonds is corroborated by the large low-field chemical shifts of the protons involved in the hydrogen bond observed experimentally and reproduced by quantum mechanical calculations. Further confirmation is provided by analysis of the orbital overlap coefficients, (13)C NMR chemical shifts, and one-bond spin-spin coupling constants J((13)C-(1)H). The relationship between molecular geometry and (1)H NMR chemical shifts of involved protons has a complex nature, but the C-H...O distance is the principal factor.     

Macrocycles,derivatives of nitrogen-containing heterocycles 2. Synthesis and electrochemical properties of diindolizinadiquinoxalinacyclooxaalkaphanes

Phenylindolizinylquinoxalinomonopodands, obtained by the reaction of 1-(phenylindolizin-2-yl)quinoxalin-2(1H)-one with corresponding dibromotrioxa- and dibromopentaoxaalkanes, undergo oxidative dehydrocyclization assisted by molecular iodine to yield new redox-active diindolizinadiquinoxalinacyclooxaalkaphanes. Using CVA, the indolizine fragments of the heterocyclophanes in acetonitrile are found to undergo three-step oxidation with transfer of one electron in each step. The first and the third steps are reversible, whereas the second is irreversible. The oxidation at potentials of the first peak leads to the stable radical cations registered by ESR (g = 2.0024, a _2N = 0.26 mT).     

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)nCO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar.+(CH2)nCO2H) or radical zwitterions (Ar.+(CH2)nCO2-) depending on pH, and pKa values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with (-)OH (k-OH)) have been obtained. These values are similar to those obtained previously for the (-)OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.     

On a possible radical-cation mechanism of the biomimetic oxidation of the saturated hydrocarbon 1,3-dimethyladamantane in a Gif-type system containing a Fe2+ salt, picolinic acid, and pyridine

Based on an analysis of the ratio between the final products of the oxidation of 1,3-dimethyladamantane (1,3-DMA) with hydrogen peroxide under new catalytic conditions, an EPR-spectroscopic study of the 1,3-DMA radical cation under model radiation-chemical conditions, and the results of PM3 quantum-chemical calculations, a new mechanism was proposed for the biomimetic oxidation of the saturated hydrocarbon. This mechanism involves the intermediate formation of the 1,3-DMA radical cation, in which a tertiary C-H bond is selectively activated. Next, oxene (oxygen atom) is inserted into this C-H bond to form a tertiary alcohol. It was found that a comparison of the composition of final products in the oxidation of saturated hydrocarbons under conditions of a real chemical experiment with the structure and reactivity of their radical cations under model radiation-chemical conditions can be a methodologically new general technique for the analysis and prediction of the reactivity of saturated hydrocarbons under oxidative conditions.     

ENDOR AND ESR STUDIES OF RADICALS FORMED IN FRIEDEL-CRAFTS REACTIONS

The present study deals with the study of some Friedel-Crafts alkylating system and the accurate measurement of hyperfine coupling constants by ESR and ENDOR techniques. The results indicate that the observed ESR spectra are due to polycyclic aromatic radical cations formed from their parent hydrocarbons.It is suggested that benzyl halides produced in the Friedel-Crafts alkylating reaction undergo Scholl condensation reaction to give polycyclic aromatic hydrocarbons, which were converted into the corresponding polycyclic aromatic radical cations in the presence of aluminum chloride.     

Chemical and thermal stability of N‐heterocyclic ionic liquids in catalytic C–H activation reactions

¹H‐NMR spectrum analyses are applied to study the chemical and thermal stability of selected N‐heterocyclic ionic liquids within the reaction system that can highly efficiently activate a C–H bond of methane and convert it into the C–O bond in methanol. Our results indicate that under such reaction conditions involving using a powerful Pt‐based catalyst and strong acidic solvent, the aromatic ring of an imidazolium cation becomes unstable generating an ammonium ion (NH₄⁺). Our results also suggest that the instability of the imidazolium ring is more chemically (participation in reactions) than thermally based. Modifications of the aromatic ring structure such as pyrazolium and triazolium cations can increase the chemical/thermal stability of ionic liquids under these reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation

To investigate the initial chemical events associated with high-temperature gas-phase oxidation of hydrocarbons, we have expanded the ReaxFF reactive force field training set to include additional transition states and chemical reactivity of systems relevant to these reactions and optimized the force field parameters against a quantum mechanics (QM)-based training set. To validate the ReaxFF potential obtained after parameter optimization, we performed a range of NVT-MD simulations on various hydrocarbon/O2 systems. From simulations on methane/O2, o-xylene/O2, propene/O2, and benzene/O2 mixtures, we found that ReaxFF obtains the correct reactivity trend (propene > o-xylene > methane > benzene), following the trend in the C-H bond strength in these hydrocarbons. We also tracked in detail the reactions during a complete oxidation of isolated methane, propene, and o-xylene to a CO/CO2/H2O mixture and found that the pathways predicted by ReaxFF are in agreement with chemical intuition and our QM results. We observed that the predominant initiation reaction for oxidation of methane, propene, and o-xylene under fuel lean conditions involved hydrogen abstraction of the methyl hydrogen by molecular oxygen forming hydroperoxyl and hydrocarbon radical species. While under fuel rich conditions with a mixture of these hydrocarbons, we observed different chemistry compared with the oxidation of isolated hydrocarbons including a change in the type of initiation reactions, which involved both decomposition of the hydrocarbon or attack by other radicals in the system. Since ReaxFF is capable of simulating complicated reaction pathways without any preconditioning, we believe that atomistic modeling with ReaxFF provides a useful method for determining the initial events of oxidation of hydrocarbons under extreme conditions and can enhance existing combustion models.     

Structure-reactivity relationships in reactions of alkane radical cations: Study of the proton transfer from alkane radical cations to alkane molecules in γ-irradiated disordered CCl3F/alkane systems by ESR spectroscopy at cryogenic temperatures

A summary is presented of ESR results obtained in γ-irradiated disordered CCl₃F/alkane systems at cryogenic temperatures, with respect to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The nature of the alkyl radicals formed by proton transfer is indicative for the site of proton donation and is derived unambiguously from ESR results by comparison with powder spectra of authentic isomeric alkyl radicals, obtained by γ-irradiation of various chloro and bromoalkanes in perdeuterated cis-decalin. The experiments can be divided into two main classes. (i) Experiments on n-alkane radical cations in the extended all-trans conformation, i.e. ESR results on the system CCl₃F/heptane. The ESR spectrum of γ-irradiated CCl₃F/heptane consists of a triplet due to heptane radical cations in the extended all-trans conformation. In this conformation, the unpaired electron is delocalized over the carbon-carbon σ-bonds as well as the two chain-end carbon-hydrogen bonds that are in the plane of the C---C skeleton. Superimposed on the ESR triplet is a low-intensity spectrum due to heptyl radicals, which increases drastically with increasing heptane concentration. The formation of these heptyl radicals can be attributed unambiguously to proton transfer from heptane radical cations to heptane molecules, taking place in small heptane clusters to which positive-hole transfer still occurs efficiently. At the onset of proton transfer with increasing heptane concentration only primary heptyl radicals are present, clearly showing that the proton transfer takes place selectively from a chain-end position, in accordance with the electronic structure of the reacting radical cations. At higher heptane concentration secondary heptyl radicals also appear as a result of intermolecular radical-site transfer, i.e. the nature of the heptyl radicals becomes governed by their thermodynamic stability. (ii) Experiments on n-alkane radical cations in the gauche-at-C₂ conformation, i.e. ESR results on the system CCl₃F/octane. The ESR spectrum of γ-irradiated CCl₃F/octane indicates that octane radical cations are largely in the gauche-at-C₂ conformation in this matrix, with large unpaired-electron (and positive-hole) density on one planar chain-end C---H bond and one planar penultimate C---H bond at the other side of the radical cation. Careful investigation of ESR spectra with increasing octane concentration clearly reveals that in this case secondary octyl radicals are present from the very onset of proton transfer, in accordance with the electronic structure of the reacting radical cations. The results clearly point to proton-donor site selectivity in the proton transfer from alkane radical cations to alkane molecules and to a strict dependence of the site of proton donation on the electronic structure and conformation of the reacting radical cations.     

Radical Cation of Pyrazine-di-N-oxide as a Mediator in Electrocatalytic Oxidation of Organic Compounds

The mechanism of oxidation of cyclohexanol, methanol, diethyl ether, triethyl orthoformate, and cyclohexane in the presence of a mediator—electrochemically generated radical cation of pyrazine-di-N-oxide (PyrDNO)—is studied on glassy carbon and platinum in a 0.1 M LiClO₄ solution in acetonitrile employing cyclic voltammetry, ESR electrolysis, and gas chromatography. Effect of temperature, additives of acid and water, oxygen, and the nature of the substrate and solvent on the shape of cyclic voltammograms and intensity of ESR signal of PyrDNO is examined. ESR spectra for radical cations and anions of PyrDNO with g factors equal to, respectively, 2.0090 and 2.0031 are recorded. A mechanism for the overall two-electron catalytic oxidation of an organic substance, which involves a stage in which it complexes with the radical cation of PyrDNO, is suggested.     

A simple method for quantitative estimation of C-H bond lengths in hydrocarbon radical cations

A quantitative correlation between deviations of the C-H bond lengths of hydrocarbon molecules during their adiabatic single ionization and isotropic hyperfine coupling constants with protons of their primary radical cations (RC) was established. A simple method for estimation of the C-H bond lengths of hydrocarbon RC was proposed on the basis of the correlation found. Specific features of the structure of the RC of methane and severaln-alkanes of the general formula [H^*(CH_{2 n} H^*]⁺,n = 0 to 12, were analyzed. A widely used empirical rule, according to which deprotonation of hydrocarbon RC during the ion-molecular process is determined by the proton possessing the highest spin density, was refined and geometrically substantiated.     

The First Step of the Oxidation of Elemental Sulfur: Crystal Structure of the Homopolyatomic Sulfur Radical Cation [S8].+

The oxidation of elemental sulfur in superacidic solutions and melts is one of the oldest topics in inorganic main group chemistry. Thus far, only three homopolyatomic sulfur cations ([S₄]²⁺, [S₈]²⁺, and [S₁₉]²⁺) have been characterized crystallographically although ESR investigations have given evidence for the presence of at least two additional homopolyatomic sulfur radical cations in solution. Herein, the crystal structure of the hitherto unknown homopolyatomic sulfur radical cation [S₈]^.+ is presented. The radical cation [S₈]^.+ represents the first step of the oxidation of the S₈ molecule present in elemental sulfur. It has a structure similar to the known structure of [S₈]²⁺, but the transannular sulfur⋅⋅⋅sulfur contact is significantly elongated. Quantum-chemical calculations help in understanding its structure and support its presence in solution as a stable compound. The existence of [S₈]^.+ is also in accord with previous ESR investigations.     

Br＋活化甲烷C—H键反应机理的理论研究

化甲烷催化剂的可能性. 在B3LYP/6-311＋＋G(3df,3p)和MP2/6-311＋＋G(3df,3p)水平下优化了反应通道上各驻点(反应物、中间体、过渡态和产物)的几何构型. 在G2M(＋)水平下计算了各物种的能量. 研究结果表明: CH4与Br＋(3P)反应存在三条不同的吸热反应途径, 与Br＋(1D)反应存在二条不同的放热反应通道. 反应更易于通过单重态反应通道进行. 理论结果不仅较好地解释了实验事实, 还说明Br＋有可能成为一种活化甲烷的催化剂.