Mechanistic aspects associated with the oxidation of -ascorbic acid at the 1,2-dichloroethane water interface

The biologically relevant oxidation of -ascorbic acid (AH₂) was studied at an externally polarised 1,2-dichloroethane water interface by a combination of electrochemical and spectroscopic techniques. In the presence of an electron acceptor (chloranil) in 1,2-dichloroethane, the basic features of the voltammetric signal are dependent on the concentration ratio of the redox species. In the presence of an excess of chloranil, a reversible signal is readily observed. A similar voltammetric response is developed when the semiquinone radical anion of chloranil (Q√⁻) is generated by homogeneous reduction in the presence of decamethylferrocene (DMFc) in the organic phase. These results suggest that Q√⁻ is formed homogeneously rather than by heterogeneous electron transfer from AH₂ to Q under these conditions. However, in the presence of an excess of ascorbate, an irreversible voltammetric response is the dominant feature. Replacing chloroanil by tetracyanoquinodimethane also provides irreversible features indicating that heterogeneous electron transfer can also take place. The occurrence of both homogeneous and heterogeneous electron transfer pathways is also suggested by potential modulated reflectance (PMR) spectroscopy.     

Nachweis von chloranil in der tüpfelanalyse

Mixing chloranil with tetrabase in ether solution gives no reaction. The redox reaction, with formation of a blue diphenylmethane dye, occurs only after evaporation of the ether. On this. effect can be based a selective identification of chloranil. It can be carried out as a spot test showing an identification limit of 0.25 γ of chloranil. The authors describe a preliminary test for aromatic compounds, which consists in heating the test material with potassium chlorate and conc. hydrochloric acid and identifying the hereby formed chloranil by means of tetrabase. The investigation of 71 compounds has shown that not all aromatic compounds can be converted to chloranil. but that there exist characteristic exceptions.     

Hydrogen transfer in the reaction of p-phenylenediamine with chloranil

Stage of electron transfer between p-phenylenediamines and chloranil in methanol was studied and a strictly reversible redox conversion of reagents was observed at low temperatures of ?50 to ?90°C. At positive temperatures, the proton transfer initiates a polycondensation yielding a polyaminoquinone of brown color. The nature of the absorption bands of charge-transfer complexes in the visible spectral range is discussed. Quantum-chemical calculations of excited reaction complexes and of absorption spectra of polyaminoquinone were performed.     

Tetrazine Assists Reduction of Water by Phosphines: Application in the Mitsunobu Reaction

Reaction of 3,6‐disubstituted‐1,2,4,5‐tetrazines with water and PEt₃ forms the corresponding 1,4‐dihydrotetrazine and OPEt₃. Thus PEt₃, as a stoichiometric reductant, reduces water, and the resulting two reducing equivalents serve to doubly hydrogenate the tetrazine. A variety of possible initial interactions between electron‐deficient tetrazine and electron‐rich PR₃, including a charge transfer complex, were evaluated by density functional calculations which revealed that the energy of all these make them spectroscopically undetectable at equilibrium, but one of these is nevertheless suggested as the intermediate in the observed redox reaction. The relationship of this to the Mitsunobu reaction, which absorbs the components of water evolved in the conversion of alcohol and carboxylic acid to ester, with desirable inversion at the alcohol carbon, is discussed. This enables a modified Mitsunobu reaction, with tetrazine replacing EtO₂CN=NCO₂Et (DEAD), which has the advantage that dihydrotetrazine can be recycled to tetrazine by oxidation with O₂, something impossible with the hydrogenated DEAD. For this tetrazine version, a betaine‐like intermediate is undetectable, but its protonated form is characterized, including by X‐ray structure and NMR spectroscopy.     

Studies on the photooxidation mechanism of polymers. II. The role of quinones as sensitizers in the photooxidative degradation of polystyrene

During the quinone-sensitized photooxidative degradation of polystyrene film and its solution in benzene, an initial rapid decrease of average molecular weight has been observed by GPC and viscosity measurements. The reaction rates are strongly increased by quinones such as p-quinone, duroquinone, anthraquinone, and chloranil. It has been suggested that this photosensitized degradation of polystyrene occurs by a singlet oxygen reaction which might be related to an energy transfer mechanism from excited triplet states of quinones to molecular oxygen. The photooxidative degradation of polystyrene in solution can be diminished by addition of typical singlet oxygen quenchers such as 1,3-cyclohexadiene or β-carotene.     

Oxidation of ascorbate and ascorbic acid at the aqueous|organic solution interface

An electron transfer reaction between ascorbate in an aqueous solution and oxidizing agents in an organic solution immiscible with water has been studied for the first time by polarography for charge transfer at the interface between two immiscible electrolyte solutions. A reversible electron transfer polarogram at the aqueous|organic solution interface could be observed when teterachlorobenzoquinone, dibromobenzoquinone and Meldola's Blue were used as oxidizing agents in the organic solution. The oxidation reaction of ascorbate at the aqueous|organic interface was discussed comparing with the reactions at the ordinary electrodes and in homogeneous solutions. The half-wave potentials of electron transfer polarograms at the aqueous|nitrobenzene interface were applied to evaluate the formal redox potential of ascorbate/ascorbate free radical.     

Toroidal hopping of a single hole through the circularly-arrayed naphthyl groups in hexanaphthylbenzene cation radical

Synthesis of a dendritic (soluble) hexanaphthylbenzene derivative is described in which the six naphthyl groups are connected to the central benzene ring in a propeller-shaped arrangement. Observation of multiple oxidation waves in its cyclic voltammogram as well as an intense charge-resonance transition (extending beyond 1600 nm) in its cation radical, generated by laser-flash photolysis using photoexcited chloranil as an oxidant, suggests that a single hole is mobilized via electron transfer (or hopping) over six identical (circularly arrayed) redox centers.     

Vibrational spectroscopic and density functional theory studies of chloranil-imidazole interaction

The present work reports vibrational spectra and density functional theory calculations for chloranil, imidazole and their complexes. The experimentally observed infrared and Raman bands have been assigned with the help of calculated vibrational frequencies and potential energy distribution analysis. Some bands of chloranil and imidazole have been found to shift on the complex formation due to partial electronic charge transfer from imidazole to chloranil. The charge transfer between these molecules is also corroborated by the electronic absorption spectroscopy and calculations. The theoretical values of the interaction energy of various possible chloranil-imidazole interactions suggest that the two molecules interact preferably via N and H atoms of imidazole and CO group of chloranil with their molecular planes almost perpendicular to each other.     

Theoretical study on the structure and UV–visible spectrum of pyridine–chloranil charge-transfer complex

The ground-state structure of the charge-transfer complex formed by pyridine (Py) as electron donor and chloranil (CA) as acceptor has been studied by full geometry optimization at the MP2 and DFT levels of theory. Binding energies were calculated and counterpoise corrections were used to correct the BSSE. Both MP2 and DFT indicate that the pyridine binds with chloranil to form an inclined T-shape structure, with the pyridine plane perpendicular to the chloranil. The CP and ZPE corrected binding energies were calculated to be 14.21 kJ/mol by PBEPBE/6-31G(d) and 23.21 kJ/mol by MP2/6-31G(d). The charge distribution of the ground state Py–CA complex was evaluated with the natural population analysis, showing a net charge transfer from Py to CA. Analysis of the frontier molecular orbitals reveals a σ–π interaction between CA and Py, and the binding is reinforced by the attraction of the O₇ atom of CA with the H₂₃ atom of Py. TD-DFT calculations have been performed to analyze the UV–visible spectrum of Py–CA complex, revealing both the charge transfer transitions and the weak symmetry-relieved chloranil π–π^* transition in the UV–visible region.     

Time-resolved EPR study of the photophysics and photochemistry of 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6.6]C61

Time-resolved (TR) EPR was used to study the photophysics and photochemistry of 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6.6]C61 (M1). The CW TREPR spectra of M1 in the photoexcited triplet state, frozen in a rigid matrix and in liquid solution at room temperature, were compared with those of 3C60. The introduction of the substituent on C60 has a striking effect on the spectra of the triplets, which is attributed to the lifting of the orbital degeneracy by the reduction in symmetry. Fourier transform (FT) EPR was used in an investigation of electron-transfer reactions in liquid solutions mediated by 3M1. Of particular interest was the system of M1/chloranil (CA)/perylene (Pe). Photoexcitation of M1 is found to lead to the formation of the chloranil anion radical and the perylene cation radical. From the chemically induced dynamic electron polarization (CIDEP) patterns in the FTEPR spectra and the dependence of the reaction kinetics on reactant concentrations, it was deduced that CA- is formed by two competing pathways following photoexcitation of M1: (1) direct electron transfer from 3M1 to CA followed by electron transfer from Pe to M1+ and (2) energy transfer from 3M1 to Pe followed by oxidative quenching of 3Pe by CA. In both pathways, M1 acts as a light-energy harvester and mediator of electron-transfer reactions from Pe to CA without itself being consumed in the process, that is, as a photocatalyst. It is found that the functionalization of C60 makes its triplet state a worse electron donor and acceptor, but it has no significant effect on the triplet energy transfer reaction.     

Electrochemically driven transfer of carboxyl-terminated PAMAM dendrimers at the water/dichloroethane interface

The transfer of PAMAM dendrimers bearing carboxylic acid peripheral groups between two immiscible liquids was studied by means of the three phase junction system, using a gold wire vertically crossing the interface and decamethyl ferrocene as the redox probe in the organic phase. While the voltammetric behavior indicates kinetic limitations of the overall ion–electron transfer process, thermodynamic data shows that the phase transfer process is entropically controlled. Four dendrimer generations were analyzed and it was found that the kinetics as well as the thermodynamics of the phase transfer reaction are size dependent.     

A New Redox Reaction of Ascorbic Acid at Platinum Electrodes

A pair of new redox peaks of ascorbic acid at a platinum electrode was found and studied in detailed by spectroelectrochemistry and electrochemistry technologies. This is a quasi‐reversible redox reaction with a one‐electron transfer process. The intermediate of tertiary carbon free radical exists in this process. The appearance reaction rate constant and the diffusion coefficient were investigated. A possible reaction mechanism has been proposed.     

缺电子芳烃侧链基的光氧化

研究了9,10-二氰蒽(DCA)和四氯对苯二醌(TCBQ)敏化的甲苯、对氯甲苯、对氰基甲苯和对硝基甲苯的电子转移光氧化反应。DCA和TCBQ均可敏化甲苯和对氯甲苯的光氧化。产物为相应的取代苯甲酸和取代苯甲醛。DCA和TCBQ均不能有效敏化对氰基甲苯和对硝基甲苯的光氧化, 但在反应体系中加入与反应物等摩尔的联苯为共敏化剂后, 两者即可顺利氧化为相应的取代苯甲酸和取代苯甲醛。通过荧光淬灭和共敏化剂联苯、无水盐高氯酸镁、O2捕获剂对苯二醌以及电子给体对二甲氧基苯等外加试剂对光氧化的影响讨论了反应历程。     

催化光度法测定水中痕量的α-萘胺

在Ｈ２ＳＯ４介质中,α－萘胺能减弱碘对亚砷酸与硫酸铈氧化还原反应的催化作用,研究了其动力学条件,建立了测定痕量α－萘胺的催化动力学光度分析方法。     

Spectrophotometric study of rate of formation and stability of the charge transfer complexes of aromatic aldoximes with chloranil

Rate constants for the reaction of p-substituted benzaldoximes with chloranil and the stability constants for the resulting charge transfer complexes have been determined spectrophotometrically in solution of pH 9 at 338 nm.     

A comparative study of the anion transfer kinetics across a water/nitrobenzene interface by means of electrochemical impedance spectroscopy and square-wave voltammetry at thin organic film-modified electrodes

The kinetics of the transfer of a series of hydrophilic monovalent anions across the water/nitrobenzene (W/NB) interface has been studied by means of thin organic film-modified electrodes in combination with electrochemical impedance spectroscopy and square-wave voltammetry. The studied ions are Cl-, Br-, I-, ClO4-, NO3-, SCN-, and CH3COO-. The electrode assembly comprises a graphite electrode (GE) covered with a thin NB film containing a neutral strongly hydrophobic redox probe (decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato)) and an organic supporting electrolyte. The modified electrode is immersed in an aqueous solution containing a supporting electrolyte and transferring ions, and used in a conventional three-electrode configuration. Upon oxidation of the redox probe, the overall electrochemical process proceeds as an electron-ion charge-transfer reaction coupling the electron transfer at the GE/NB interface and compensates ion transfer across the W/NB interface. The rate of the ion transfer across the W/NB interface is the limiting step in the kinetics of the overall coupled electron-ion transfer reaction. Moreover, the transferring ion that is initially present in the aqueous phase only at a concentration lower than the redox probe, controls the mass transfer regime in the overall reaction. A rate equation describing the kinetics of the ion transfer that is valid for the conditions at thin organic film-modified electrodes is derived. Kinetic data measured with two electrochemical techniques are in very good agreement.     

Electrochemical Behavior of Chloranil Chemically Modified Carbon Paste Electrode. Application to the Electrocatalytic Determination of Ascorbic Acid

A p‐chloranil modified carbon paste electrode was constructed and the electrochemical behavior of this electrode was studied in the aqueous solution with different pH. From the E_1/2–pH diagram for this compound the values of formal potential E^0' and pK_a of some different redox and acid‐base couples depending on the solution pH were estimated. The diffusion coefficient, D, value for p‐chloranil was estimated 1.5×10^?7 cm² s^?1. It has been shown by direct current cyclic voltammetry and double potential step chronoamperometry, that this p‐chloranil incorporated carbon paste electrode, can catalyze the oxidation of ascorbic acid in the aqueous buffered solution. Under the optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 325 mV less positive than that at an unmodified carbon past electrode. The catalytic oxidation peak currents was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 7×10^?5 M–4×10^?3 M of ascorbic acid with a correlation coefficient of 0.9998. The limit of detection (3σ) was determined as 3.5×10 ^?5 M. This method was used as simple, selective and precise voltammetric method for determination of ascorbic acid in pharmaceutical preparations.     

Spectroscopic study of chloranil charge-transfer complexes of amino acids and related compounds

The absorption maxima, molar absorptivities, infrared spectra, compositions, formation constants, and pH dependence of amino acid—chloranil complexes have been determined with purified chloranil The n-π charge-transfer interaction depends on the presence of an unprotonated amino group; pH 9 is optimal for complex formation, but once formed, the complex is stable in a highly acidic medium and may be quantitatively extracted by hexanol. The molar absorptivities of the chloranil complexes of glycine, iminodiacetic acid, NTA, EDTA, DTPA and TTHA were measured. There is a linear relationship between the logarithm of the molar absorptivities of their chloranil complexes and the number of carboxylic groups in the molecule. There is an inverse linear relationship between the molar absorptivities of chloranil—metal—EDTA complexes and the logarithm of the stability constants of the EDTA chelates. This leads to a new method of determining the stability constants of complexes involving a nitrogen-donating group.     

Photocatalytic Z-scheme Overall Water Splitting: Insight into Different Optimization Strategies for Powder Suspension and Particulate Sheet Systems

Achieving solar light-driven photocatalytic overall water splitting is the ideal and ultimate goal for solving energy and environment issues. Photocatalytic Z-scheme overall water splitting has undergone considerable development in recent years; specific approaches include a powder suspension Z-scheme system with a redox shuttle and a particulate sheet Z-scheme system. Of these, a particulate sheet has achieved a benchmark solar-to-hydrogen efficiency exceeding 1.1 %. Nevertheless, owing to intrinsic differences in the components, structure, operating environment, and charge transfer mechanism, there are several differences between the optimization strategies for a powder suspension and particulate sheet Z-scheme. Unlike a powder suspension Z-scheme with a redox shuttle, the particulate sheet Z-scheme system is more like a miniaturized and parallel p/n photoelectrochemical cell. In this review, we summarize the optimization strategies for a powder suspension Z-scheme with a redox shuttle and particulate sheet Z-scheme. In particular, attention has been focused on choosing appropriate redox shuttle and electron mediator, facilitating the redox shuttle cycle, avoiding redox mediator-induced side reactions, and constructing a particulate sheet. Challenges and prospects in the development of efficient Z-scheme overall water splitting are also briefly discussed.