An efficient electron transfer at the Fe/iron oxide interface for the photoassisted degradation of pollutants with H2O2

Fe-200 was synthesized through the calcination of iron powder at 200 °C for 30 min in air. On the basis of characterization by X-ray diffraction and X-ray photoelectron spectroscopy, Fe-200 had a core–shell structure, in which the surface layer was mainly composed of Fe₂O₃ with some FeOOH and FeO, and the core retained metallic iron. The kinetics and mechanism of the interfacial electron transfer on Fe-200 were investigated in detail for the photoassisted degradation of organic pollutants with H₂O₂. Under deoxygenated conditions in the dark, the generation of hydroxyl radicals in aqueous Fe-200 dispersion verified that galvanic cells existed at the interface of Fe⁰/iron oxide, indicating the electron transfer from Fe⁰ to Fe³⁺. Furthermore, the effects of hydrogen peroxide and different organic pollutants on the interfacial electron transfer were examined by the change rate of the Fe³⁺ concentration in the solution. The results indicated that hydrogen peroxide provided a driving force in the electron transfer from Fe²⁺ to Fe³⁺, while the degradation of organic pollutants increased the electron transfer at the interface of Fe⁰/iron oxide due to their reaction with OH.     

Radiation-Induced Hydrogen Transfer in Nucleic Acids and Proteins in the Solid State

A radiation-induced transfer of hydrogen from exchangeable to carbon-bound sites occurs in nucleic acids, proteins and their constituents. The G-values for this process are similar within a factor of 2–3 to the G-values for the initial production of stable radicals measured by ESR at room temperature. Unlike the production of radicals, hydrogen transfer in ribonuclease, DNA, and thymine is a linear function of dose in the 0–30 Megarad region. The tritium distributions obtained in proteins by hydrogen transfer are not a measure of the distribution of the “secondary” radicals. From the studies of the effect of temperature, of various scavengers (particularly nitric oxide) and of the dose dependence in the low dose region, it was inferred that for ribonuclease most of the hydrogen transfer takes place during the conversion of the radicals stable at 195°K to the radicals stable at room temperature. The same mechanism also appears to predominate in DNA.     

Electrochemical generation of triplet states VI—1. A mechanistic study of the luminescence processes at the anodic cleavage of 2,2′- and 4,4′-bipyranes in presence of aromatic hydrocarbons

During the anodic oxidation of mixtures of a fluorescent aromatic hydrocarbon and a 2,2′- or a 4,4′-bipyrane in acetonitrile/benzene the emission of the hydrocarbon is observed. it results from a systematic study using rotating disk and ring-disk electrode techniques that this luminescence is caused by the electron transfer between the pyranyl radicals generated in the anodic cleavage of the bipyrane and the hydrocarbon cation radicals. Measurements in a magnetic field and thermodynamical considerations prove a triplet ECL mechanism. As visible from the ECL intensity—potential curves the pyranyl radicals are generated by two different mechanisms. The heterogeneous one electron oxidation of the bipyrane and the following cleavage of a single bond is replaced with the indirect anodic oxidation by means of the reversible hydrocarbon redox system at more positive potentials.     

Photoinduzierte Polymerisation von Methylmethacrylat in Gegenwart von niederund hochmolekularen Chinoxalinderivaten

The photoinduced polymerization of methyl methacrylate in presence of 2,3-diphenylquinoxaline and 6-acrylamido-2,3-diphenylquinoxaline polymers is investigated. As photoexcited quinoxaline molecules are capable of hydrogen abstraction, which can be followed by hydrogen transfer, the formation of starting radicals in the considered system is referred to a hydrogen transfer from a H-donor to monomer molecules with monohydroquinoxaline radicals as transferring reagent. The polymerization mechanism based on this reaction is confirmed by endgroup analysis of the polymers and by the polymerization kinetics.     

Derivatives of 1,4‐naphthoquinone as visible‐light‐absorbing one‐component photoinitiators for radical polymerisation

Bifunctional visible‐light photoinitiators for free radical polymerisation based on a 1,4‐naphthoquinone skeleton were synthesised and characterised using proton nuclear magnetic resonance spectroscopy and chemical ionisation mass spectrometry. Their basic spectroscopic properties, such as absorption and low‐temperature phosphorescence spectra, and their red‐ox properties were also measured. These initiators contain an appropriate electron/hydrogen donor group in the skeleton and do not require an additional coinitiator for initiation. During irradiation they act both as a triplet photosensitiser and as a hydrogen/electron donor. The ability of each naphthoquinone to act as a photoinitiator strongly depends on its chemical structure. These studies suggest that initiator radicals are generated from the triplet state in an intermolecular electron/hydrogen transfer reaction.     

Elektrochemische erzeugung von triplettzustanden V.[1] elektrochemische lumineszenz des systems 2,4,6-triphenylpyryliumperchlorat-rubren

The electrochemical reduction of 2,4,6-triphenylpyrylium cations in acetonitrile and dimethyl-formamide yields triphenylpyranyl radicals existing in a reversible equilibrium with their dimers. The homegeneous electron transfer reaction between these radicals and the cation radicals of rubrene gives rise to electrochemical luminescence via the triplet state of rubrene. This leads to the conclusion that in addition to the well known Anion-Cation-Annihilation also electron transfer processes involving radicals can occur with formation of excited states.     

Camphorquinone-amines photoinitating systems for the initiation of free radical polymerization

Results of the camphorquinone/hindered piperidines, visible-light photoinduced polymerization of triethyleneglycol dimethacrylate are presented. The effectiveness of piperidines as a coinitiator is compared with a few aliphatic amines and aromatic amines. The main objective in this research was to study the mechanism of photoinitiation of polymerization. Reactive radicals that initiate the polymerization are formed by a mechanism of hydrogen atom abstraction by the triplet state of camphorquinone, mediated by photoinduced electron transfer. The different efficiencies of the aliphatic amines and of the aromatic amines affecting photopolymerization are explained on the basis of the different quenching reactivities of the excited states of camphorquinone.     

Polymerization of organic compounds in an electrodeless glow discharge. IV. Hydrocarbons in a closed system

The polymerization of hydrocarbons was investigated by measuring the hydrogen yield during the glow discharge polymerization in a closed system. It was found that the pressure change in the glow discharge polymerization of hydrocarbons was mainly due to the production of hydrogen and to the loss of vapor phase monomer by polymerization. The opening of triple or double bonds and cyclic structures plays an important role in the polymerization of hydrocarbons; however, these are not exclusive mechanisms. The major polymerization mechanism for saturated normal hydrocarbons seems to be by the formation of free radicals due to hydrogen abstraction and the recombination of these primary radicals. The polymerization due to this mechanism also seems to occur concurrently during the polymerization of hydrocarbons with multiple bond and/or cyclic structures. Aromatic hydrocarbons polymerize with very low hydrogen production, indicating that the utilization of an aromatic double bond is the major mechanism of polymerization.     

Understanding hydrogen atom transfer: from bond strengths to Marcus theory

Hydrogen atom transfer (HAT), a key step in many chemical, environmental, and biological processes, is one of the fundamental chemical reactions: A-H + B → A + H-B. Traditional HAT involves p-block radicals such as tert-BuO(?) abstracting H(?) from organic molecules. More recently, the recognition that transition metal species undergo HAT has led to a broader perspective, with HAT viewed as a type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H(?) (e(-) + H(+)), typically the electron transfers to the metal and the proton to a ligand. Examples with iron-imidazolinate, vanadium-oxo, and many other complexes are discussed. Although these complexes may not "look like" main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A-H bond strengths within a series of similar reactions. Like main group radicals, they abstract H(?) much faster from O-H bonds than from C-H bonds of the same strength, showing that driving force is not the only determinant of reactivity. This Account describes our development of a conceptual framework for HAT with a Marcus theory approach. In the simplest model, the cross relation uses the self-exchange rate constants (k(AH/A) for AH + A) and the equilibrium constant to predict the rate constant for AH + B: k(AH/B) = (k(AH/A)k(BH/B)K(eq)f)(1/2). For a variety of transition metal oxidants, k(AH/B) is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, k(AH/B) is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O-H or C-H bonds, occur in either water or organic solvents, and occur over a range of 10(28) in K(eq) and 10(13) in k(AH/B). The treatment of organic reactions includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. This Account includes a number of case studies to illustrate these and various other issues. The success of the cross relation, despite its simplicity, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with free energies, not with bond enthalpies. Moreover, the radical character or spin state of an oxidant is not a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for diverse cross reactions are accurately obtained from the HAT self-exchange rate constants, a remarkable and unprecedented result for any type of chemical reaction other than electron transfer. The Marcus cross relation thus provides a valuable new framework for understanding and predicting HAT reactivity.     

Hydrogen Isotope Effects in the Radiolysis of Water

The tritium fractionation between water and radicals formed in the radiolysis of dilute solutions of monomerous methyl methacrylate in H₂O-HTO and D₂O-DTO mixutres was studied. A parallel determination of the tritium content in molecular hydrogen was performed. Also, the isotopic composition of the initial molecular hydrogen was measured in the concentration range 10 to 90 Mol% D₂O of H₂O-HDO-D₂O mixtures. Hydrogen atoms, hydroxyl radicals and molecular hydrogen produced in the radiolysis of these solvents were found to be depleted in heavier hydrogen isotopes. The isotope effects on the composition of hydrogen atoms are discussed in terms of the rate isotope effects in proton transfer. The isotope effects on the composition of hydroxyl radicals are close to those on molecular hydrogen and their extent suggests ionic dissociation of water as a rate-determining step in the process of formation of both products.     

NIEDER- UND HOCHMOLEKULARE RADIKALKATIONEN VON CHINOXALINDERIVATEN

Quinoxaline derivatives are used as sensitizing agents in colour-photography and for photoinduced polymerization reactions. The reactivity of these molecules depends on their preference for hydrogen transfer. As models for investigation of these reactions quinoxaline radical cations were prepared, using both monomer derivatives and polymers with analoguous side chains. The radicals are generated by reduction or by UV-radiation and are characterized by their ESR-spectra and their decay during radiation.     

Hydrogen-Atom Transfer Reactions of Transition-Metal Hydrides

Many reactions of transition-metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second-order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition-metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon-centered radicals or by metal radicals. Reasonably accurate values are now available for the M-H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.     

Model studies on the mechanism of hals stabilization

Hindered Amine Light Stabilizers (HALS) are know to inhibit the photo-oxidation of polymers. A key reaction in their stabilization mechanism is believed to be the conversion of a hindered aminoether into a nitroxyl radical. Several different possible mechanisms for this conversion were explored. One, the elimination of the aminoether to form an olefin and hydroxylamine (an intermediate in the formation of a nitroxyl), while possible at high temperatures, cannot account for the inhibitory activity we observed for secondary and primary aminoethers. Direct radical displacement by peroxy radicals was also considered. However, the products predicted by this reaction pathway were not observed. Finally, oxidation of the nitrogen by a peroxy radical, by either electron transfer or a radical attack on the nitrogen, was investigated. While electron transfer was shown to be unlikely, direct oxidation of the aminoether nitrogen was supported by our results. A detailed mechanism for the reaction of both alkyl- and acyl-peroxy radicals with aminoethers is proposed.     

Electron-Induced Repair of 2′-Deoxyribose Sugar Radicals in DNA: A Density Functional Theory (DFT) Study

In this work, we used ωB97XD density functional and 6-31++G** basis set to study the structure, electron affinity, populations via Boltzmann distribution, and one-electron reduction potentials (E°) of 2′-deoxyribose sugar radicals in aqueous phase by considering 2′-deoxyguanosine and 2′-deoxythymidine as a model of DNA. The calculation predicted the relative stability of sugar radicals in the order C4′^• > C1′^• > C5′^• > C3′^• > C2′^•. The Boltzmann distribution populations based on the relative stability of the sugar radicals were not those found for ionizing radiation or OH-radical attack and are good evidence the kinetic mechanisms of the processes drive the products formed. The adiabatic electron affinities of these sugar radicals were in the range 2.6–3.3 eV which is higher than the canonical DNA bases. The sugar radicals reduction potentials (E°) without protonation (−1.8 to −1.2 V) were also significantly higher than the bases. Thus the sugar radicals will be far more readily reduced by solvated electrons than the DNA bases. In the aqueous phase, these one-electron reduced sugar radicals (anions) are protonated from solvent and thus are efficiently repaired via the “electron-induced proton transfer mechanism”. The calculation shows that, in comparison to efficient repair of sugar radicals by the electron-induced proton transfer mechanism, the repair of the cyclopurine lesion, 5′,8-cyclo-2′-dG, would involve a substantial barrier.     

Hydrogen tunneling and protein motion in enzyme reactions

Theoretical perspectives on hydrogen transfer reactions in enzymes are presented. The proton-coupled electron transfer reaction catalyzed by soybean lipoxygenase and the hydride transfer reaction catalyzed by dihydrofolate reductase are discussed. The first reaction is nonadiabatic and involves two distinct electronic states, while the second reaction is predominantly adiabatic and occurs on the electronic ground state. Theoretical studies indicate that hydrogen tunneling and protein motion play significant roles in both reactions. In both cases, the proton donor-acceptor distance decreases relative to its equilibrium value to enable efficient hydrogen tunneling. Equilibrium thermal motions of the protein lead to conformational changes that facilitate hydrogen transfer, but the nonequilibrium dynamical aspects of these motions have negligible impact.     

Roles of adsorbed OH and adsorbed H in the oxidation of hydrogen and the reduction of UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack> ions at Pt electrodes under non-conventional conditions

The roles of adsorbed hydroxyl radicals, OH, at a high temperature and adsorbed hydrogen atoms, H, in an acidic solution were investigated in the electrochemical reactions on Pt electrode by using potentiodynamic polarisation experiment, cyclic voltammetry and constant-potential electrolysis combined with UV/VIS analysis. From the analysis of the polarisation curves obtained from Pt electrode in a 0.185 M H₃BO₃ solution at 473 K, it was found that the reducing capability of dissolved hydrogen is significantly enhanced due to the increases of the mass transfer and the electron transfer rates. Especially, it is suggested that the stable Pt-OH_ad plays a significant role in the passivation reaction in the potential range from 0.60 to 0.75 V_SHE. From the analyses of the experimental results for the electrochemical reduction of UO₂²⁺ ions on Pt surface in a 1.0 M HClO₄ solution, it is recognised that the reduction reaction of UO₂²⁺ to U⁴⁺ ions is strongly dependent on the hydrogen atoms adsorbed on Pt electrode (indirect reduction of UO₂²⁺) as well as on the electrons transferred from Pt electrode (direct reduction of UO₂²⁺). In addition, the reduction mechanism of UO₂²⁺ ions involved in Pt-H_ad is also proposed.     

Electrochemical synthesis of peroxomonophosphate using boron-doped diamond anodes

A new method for the synthesis of peroxomonophosphate, based on the use of boron-doped diamond electrodes, is described. The amount of oxidant electrogenerated depends on the characteristics of the supporting media (pH and solute concentration) and on the operating conditions (temperature and current density). Results show that the pH, between values of 1 and 5, does not influence either the electrosynthesis of peroxomonophosphate or the chemical stability of the oxidant generated. Conversely, low temperatures are required during the electrosynthesis process to minimize the thermal decomposition of peroxomonophosphate and to guarantee significant oxidant concentration. In addition, a marked influence of both the current density and the initial substrate is observed. This observation can be explained in terms of the contribution of hydroxyl radicals in the oxidation mechanisms that occur on diamond surfaces. In the assays carried out below the water oxidation potential, the generation of hydroxyl radicals did not take place. In these cases, peroxomonophosphate generation occurs through a direct electron transfer and, therefore, at these low current densities lower concentrations are obtained. On the other hand, at higher potentials both direct and hydroxyl radical-mediated mechanisms contribute to the oxidant generation and the process is more efficient. In the same way, the contribution of hydroxyl radicals may also help to explain the significant influence of the substrate concentration. Thus, the coexistence of both phosphate and hydroxyl radicals is required to ensure the generation of significant amounts of peroxomonophosphoric acid.     

Effect of heat transfer on the distributions of OH and CH radicals in the boundary layer with ethanol combustion

The reasons for formation of superequilibrium concentrations of radicals are studied by means of joint consideration of experimental data on the distributions of CH and OH molecules formed during diffusion combustion of ethanol and data on heat transfer in the chemical reaction region. The air flow velocity near the stagnation point in experiments with combustion is 0.7 m/sec, and the flow velocity along a flat plate is 10 m/sec (the turbulence levels are 1 and 18%). Mutual locations of specific features in the distributions of the heat-release rate and temperature are analyzed and compared with the distributions of OH and CH radicals. For all turbulence levels and flow velocities considered, the maximum concentration of radicals is reached on the boundaries of the heat-release region, whose locations are determined by molecular transport mechanisms. It is demonstrated that this conclusion is applicable to experimental data on diffusion combustion of a submerged hydrogen jet in air. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 3–11, November–December, 2008.     

A New Mechanism of the Selective Photodegradation of Antibiotics in the Catalytic System Containing TiO2 and the Inorganic Cations

The mechanism of sulfisoxazole (SFF) selective removal by photocatalysis in the presence of titanium (IV) oxide (TiO₂) and iron (III) chloride (FeCl₃) was explained and the kinetics and degradation pathways of SFF and other antibiotics were compared. The effects of selected inorganic ions, oxygen conditions, pH, sorption processes and formation of coordination compounds on the photocatalytic process in the presence of TiO₂ were also determined. The Fe³⁺ compounds added to the irradiated sulfonamide (SN) solution underwent surface sorption on TiO₂ particles and act as acceptors of excited electrons. Most likely, the SFF degradation is also intensified by organic radicals or cation organic radicals. These radicals can be initially generated by reaction with electron holes, hydroxyl radicals and as a result of electron transfer mediated by iron ions and then participate in propagation processes. The high sensitivity of SFF to decomposition caused by organic radicals is associated with the steric effect and the high bond polarity of the amide substituent.     

Formation of Hydrazyl Radicals and Hydrazo Compounds by Photoreduction of Azo Dyes

The photoreduction of eighteen different monoazo dyes in aqueous and ethanolic solutions, containing d, l -mandelic acid or acetone as hydrogen donors, to the corresponding amines has been investigated by flash-photolysis and rapid-flow techniques. The photoreduction of azo dyes proceeds in stages involving hydrazyl radicals and hydrazo compounds. The former are formed in a rapid reaction of the azo dyes with the active reducing agent produced upon absorption of light by the hydrogen donor. These radicals are unstable and disproportionate with formation of hydrazo compounds and regeneration of the original azo dye. The hydrazo compounds are also unstable and decompose with formation of amines and regeneration of the dye. The reaction schemes presented are supported by kinetic evidence and electron spin resonance measurements.