A radiation-chemical and quantum-chemical study of ascorbic acid interaction with α-hydroxyethyl radicals

The effect of ascorbic acid and its analogue 5,6-O-isopropylidenyl-2,3-O-dimethylascorbic acid (I), which has been synthesized for the purpose and does not contain mobile hydrogen atoms, on the formation of the products of continuous radiolysis of deaerated ethanol and its aqueous solutions has been studied. The ionization potentials, the molecular orbital energies, the enthalpies of homolytic dissociation of C-H and O-H bonds, and the enthalpies of H atom addition to the C=O group of the test compounds have been calculated by ab initio methods. The array of the experimental and calculated theoretical data suggests that both ascorbic acid in the undissociated form and compound I can oxidize α-hydroxyethyl radicals, whereas the monoanion of ascorbic acid acts as a reducing agent in the reactions with these transient radicals. The reduction of α-hydroxyethyl radicals in aqueous solutions by the ascorbic acid monoanion can follow both the hydrogen transfer and electron transfer mechanisms.     

Photoelectron emission spectroscopy of inorganic cations in aqueous solution

Threshold energies (6.1 <E_t ? 8.6 eV) are determined for photoelectron emission by 16 inorganic cations in aqueous solution E_t values are correlated with gas-phase ionization potentials, solvation and reorganization free energies, standard reduction potentials and ligand field stabilization energies (five transition metals). Dielectric saturation is shown to drastically lower threshold energies.     

Polarographic properties of ubiquinones with isoprenoid chains of different lengths

It has been shown that in aqueous ethanolic solutions the polarographic potentials E₁/₂ for ubiquinones with isoprenoid chains with different lengths are the same, –0.235 V (relative to a normal calomel electrode) at pH 7.9, and correspond to quasi-irreversible reduction with the addition of 2e^–/2H⁺. It has been found that the potential E₁/₂ for the ubiquinone UQ-O having no isoprenoid chain is more positive by 57 mV relative to the potentials of the other ubiquinones.Institute of Soil Science and Photosynthesis, USSR Academy of Sciences, Pushchino. Moscow Oblast. Translated from Khimiya Prirodnykh Soedinemii, No. 5, pp. 703–707, September–October, 1988.     

Kinetics of americium(VI) mass transfer through solid supported liquid membrane with HDEHP

The main regularities of membrane extraction of americium under conditions of different redox potentials in aqueous phases have been studied. The physico-chemical model of the process including steps of americium oxidation in feed solution, extraction by membrane, partial reduction on membrane surface, trans-membrane diffusion and reextraction to strip solution has been developed. The calculation of reduction rate constant on membrane surface has been carried out.     

Voltammetric analysis of hydroquinone, ascorbic acid, nitrobenzene and benzyl chloride in aqueous, non-aqueous, micellar and microemulsion media

Voltammetric behaviour of hydroquinone, ascorbic acid, nitrobenzene and benzyl chloride in aqueous, micellar, microemulsion and aprotic media on glassy carbon electrode under identical experimental conditions was compared. A general trend of decreasing peak currents in the order was noticed. The only exception was hydroquinone, which exhibited a slightly lower current in micellar medium. The peak potentials for each of these compounds varied in the order with due consideration for the two oxidation and reduction processes (negative E _P values for reduction). In micellar systems, which contain a predominantly aqueous phase where the cationic surfactant exhibits a catalytic effect, lower peak potentials and higher current are obtained. The peak potential and peak current is lower in microemulsion when compared to aprotic medium. The causes for such systematic variations and their analytical implications are discussed.     

In situ infrared spectroscopic investigations on the electrochemical properties of prussian blue-polyaniline-modified electrodes with various anionic Fe(II) complexes working as a mediator for the electroreduction of CO2

Various Fe(II) complexes have been incorporated into Prussian blue (PB)|polyaniline (PAn)-modified electrodes, and their spectroelectrochemical properties been investigated using in situ and ex situ FTIR methods. It is shown that large anionic complexes once incorporated in the PAn matrix are not dedoped during the potential cycling and the charge balance is maintained by dedoping or incorporating electrolyte cations. This electrode system was applied to the electrocatalytic reduction of CO₂ in aqueous solution, and the reduction products were identified by taking in situ FTIR spectra during the anodic stripping. At potentials higher than 0 V, the IR bands associated with the loss of carboxylic acid at 1362 cm⁻¹ and the gain of CO₂ at 2343 cm⁻¹ were simultaneously observed, indicating that the CO₂ was derived from the reoxidation of carboxylic acid. It is therefore confirmed that CO₂ can be reduced to organic species including carboxylic acid on the PB|PAn-modified electrode with anionic Fe(II) complexes in aqueous solution, with an indication that the existence of the anionic metal complex is essential to such mediated reduction of CO₂.     

Electrochemical properties of modified carbon paste electrodes containing some amino derivatives of 9,10-anthraquinone

The preparation and aqueous electrochemistry of carbon paste electrodes modified by some amino-substituted 9,10-anthraquinones are reported. In all cases, the reduction processes studied by cyclic voltammetry reveal a quasi-reversible behavior. The half-wave potentials were calculated as a function of the solution pH and from the resulting potential-pH plots the formal potentials and pK _a ^′ values of different redox and acid-base couples involved at various pH ranges were evaluated. The diffusion coefficients of different anthraquinone derivatives used in paraffin oil were calculated by chronoamperometry. Received: 3 May 1999 / Accepted: 15 November 1999     

The electrocatalytic reduction of carbon dioxide using cobalt tetrakis (4-trimethylammoniophenyl) porphyrin under high pressure

The electrocatalytic reduction of carbon dioxide in aqueous solution using Co [p-N(CH₃)₃) TPPI under pressure (4～22 kg · cm^?2) was carried out on solid-state metal electrodes such as In, Sn, Pb and Pb-Hg. The reduction products were mainly carbon monoxide and a small amounts of formic acid. The effects of the pressure of carbon dioxide, electrolytes, catalyst concentration, electrode materials and potentials on the reduction of carbon dioxide were studied. The optimum voltage for maximum current efficiency of reduction products was at ?1.2 Vvs. SCE for all four kinds of metal electrodes. High current efficiencies (>90%) were obtained with lead and amalgamated lead electrodes in an aqueous solution of NaHCO₃ containing 7.4 × 10×⁵ mol/L of catalyst.     

Mechanistic study of colchicine's reduction behavior

Electrochemistry/mass spectrometry (EC/MS) using two different types of electrolytic cells was employed for the systematic mechanistic study of colchicine's reduction, both in aqueous and non-aqueous media. In aqueous media, at around − 1 V vs. Ag/AgCl, colchicine suffers a single-electron reduction to a transient anion radical, which after a follow-up protonation leads to a neutral free radical (E_rC_i mechanism). Depending on the experimental conditions, the latter undergoes some dimerization. At more negative potentials (− 1.4 V vs. Ag/AgCl) and pH < 7, the free radical is undergoing another single-electron reduction and a subsequent protonation. In the absence of protons (aprotic media), the one-electron reduction gives the anion radical. This process becomes fully reversible at high scan rates (≥ 10 V/s).     

Characterization and Electrocatalytic Properties of Composite Poly(new fuchsin) and Phosphomolybdate Films

Organic/inorganic hybrid films of poly(new fuchsin) and phosphomolybdate (PMo₁₂O ) have been prepared in acidic aqueous solutions. These new combination films are stable, electrochemically active, and can be produced on glassy carbon, platinum, gold, and transparent semiconductor tin oxide electrodes. An electrochemical quartz crystal microbalance along with cyclic voltammetry and UV‐visible absorption spectroscopy were used to study the in situ growth of the hybrid films. The hybrid poly(new fuchsin) and PMo₁₂O films showed four obvious redox couples, and when transferred to various acidic aqueous solutions, the formal potentials of the four redox couples were found to be pH dependent. The electrocatalytic reduction of ClO , BrO , IO , SO , S₂O , H₂O₂, and NO by the hybrid poly(new fuchsin) and PMo₁₂O films was achieved in acidic aqueous solutions. In an aqueous solution at pH 1.5, a hybrid poly(new fuchsin) and PMo₁₂O film showed a higher electrocatalytic reduction activity of IO than BrO or ClO , and the order of electrocatalytic activity was IO >BrO >ClO . The order of electrocatalytic reduction of SO , S₂O , H₂O₂, and NO by hybrid poly(new fuchsin) and PMo₁₂O films in an aqueous solution at pH 1.5 was NO >H₂O₂>S₂O and SO . The electrocatalytic reactions of the poly(new fuchsin) and PMo₁₂O films were investigated using the rotating ring‐disk electrode method.     

Influence of micelles on the electrochemical behaviors of catechol and hydroquinone and their simultaneous determination

A simple and highly selective electrochemical method for the simultaneous determination of CAT and HQ at a glassy carbon electrode in micellar solutions has been developed. The electrochemical behaviors of CAT and HQ in aqueous CPB and SDBS micellar solutions have been studied by cyclic voltammetry. The oxidation peak potentials shift negatively, the reduction peak potentials shift positively, and the peak currents increase in the presence of CPB for both CAT and HQ. However, the oxidation peak potentials shift positively, the reduction peak potentials shift negatively, and the peak currents decrease in the presence of SDBS for both CAT and HQ. The electrochemical kinetic parameters for CAT and HQ in aqueous CPB and SDBS micellar solutions were also determined by chronocoulometry (CC) and chronoamperometry (CA). The cyclic and pulse differential voltammetric behaviors of the system consisting of CAT coexisting with HQ were also investigated in this work. It was found that the oxidation peak potential waves of CAT and HQ were separated by 100 mV in the presence of CPB in 0.10 M PBS (pH 6.8). Therefore, CAT and HQ can be determined simultaneously in such a system. This simple method was applied to the simultaneous determination of HQ and CAT in a household tap water sample and it exhibited high selectivity.     

Reduction of dicarbonyl compounds on the dme: Part VII. Electrochemical behaviour of glyoxal

Polagraphic and kinetic studies of both the first and the second reduction waves of glyoxal have been carried out in aqueous solutions in the pH range 0 – 11.Tafel slopes and reaction orders with respect to glyoxal, H⁺ ion and proton donors present in solutions have been obtained at potentials corresponding to the foot of the waves. Reactions mechanisms are proposed for this potential zone.The proposed mechanism accounts both for our experimental data and for those previously reported in the literature.     

Solvatochromism and solvation of ternary iron-diimine-cyanide complexes

Summary The solvatochromic behaviour of several complexes [Fe(LL)₂(CN)₂] with LL=Schiff base diimine has been established in a series of non-aqueous solvents, as has that of two analogues containing diazabutadiene ligands. Transfer chemical potentials have been derived from appropriate solubility measurements for several iron(II)-and iron(III)-diimine-cyanide complexes into aqueous methanol, and for [Fe(bipy)₂(CN)₂] into several binary aqueous solvent series. The usefulness of solvatochromic shifts and transfer chemical potentials as indicators of selective solvation is discussed. Kinetics of oxidation of catechol and of 4-t-butyl catechol by [Fe(bipy)(CN)₄]^– in aqueous solution are described.     

Polarographic reduction of nonquaternized and quaternized pyrophthalones in protogenic media

The polarographic reduction potentials were determined by classical polarography, millicoulometry, preparative electrolysis, and cyclical voltammetry, and the course of the electrochemical process for nonquaternized and quaternized pyrophthalones in 20% buffer solutions in aqueous dimethylformamide was studied. In the case of α and γ isomers of pyrophthalones, two-electron irreversible electrical reduction leads to the formation of an indan-1-on-3-yl-1,4-dihydropyridine, which exists in two protolytic forms. The compound was isolated and its structure was proved by means of mass spectrometry. The polarographic reduction of β-pyrophthalones is similar to the electrical reduction of 2-phenylindane-1,3-dione. The tautormeric-protolytic equilibria of quaternized and nonquaternized α-, β-, and γ-pyrophthalones were studied, and it was ascertained that individual tautomeric-protolytic forms participate in the electrochemical process at various pH values. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 218–227, February, 1980. We thank I. B. Mazheika and A. P. Gaukhman for determination of the mass spectra.     

Synthesis,polarography and herbicidal activity of quaternary salts of 2-(4-pyridyl)-1,3,5-triazines, 5-(4-pyridyl)pyrimidine, 2-(4-pyridyl)pyrimidine and related compounds

2-(4-Pyridyl)-1,3,5-triazine, 2-(4-pyridyl)-4-methyl-1,3,5-triazine, 2-(4-pyridyl)-4,6-dimethyl-1,3,5-triazine and 2-(4-pyridyl)pyrimidine have been prepared by modification of established triazine and pyrimidine syntheses. These compounds and some of their relatives have been converted to quaternary pyridinium salts. The polarographic reduction potentials of the salts in aqueous solution are pH dependent. The activity of the salts as post-emergent herbicides is reported.     

Transmembrane potentials in systems containing 18-crown-6

Electric potentials of MK-40 cation-exchange membrane, which is immersed in aqueous solutions of KCl, RbCl, and macrocyclic polyether 18-crown-6, have been measured. The possibility of preliminary calculations of biionic and multiionic potentials from equilibrium constants of ionic exchange and electrochemical mobilities of cations in the sulfo cationite is demonstrated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1489–1490, August, 1995.     

Electroprecipitation of Ag(II)/Ag(III) tetraphenylsulfonate porphyrin and electrocatalytic behavior of the films

The electrochemical precipitation on glassy carbon and gold electrodes of Ag(II) tetraphenylsulfonate porphyrin (Ag(II)TPPS) from aqueous HClO₄ solutions, is reported. Electrochemical quartz crystal microbalance (EQCM) results indicate the possible formation of an Ag(II)–Ag(III) porphyrin dimer species. This species is oxidized and reduced in two consecutive steps: oxidation at +0.31 and +0.36 V (vs. SCE) and reduction at +0.11 and +0.07 V. The films show catalytic behavior toward O₂ reduction in 10⁻² M HClO₄ at relatively low potentials (E<−0.1 V) but catalyze NO reduction at relatively high-reduction potentials (E<0.4 V). The electrochemical results seem to indicate that the catalytic cycle in the case of NO involves formation of Ag(II)TPPS–Ag(II)TPPS(NO)⁺ and its electroreduction to regenerate Ag(II)TPPS–Ag(III)TPPS and NO-reduction products.     

Hydration forces near charged interfaces in terms of effective ion potentials

Experiments and simulations provide detailed information on the structure and interaction of charged interfaces in aqueous solutions. In order to understand this information in terms of general principles it is favourable to use effective potentials, which describe the interaction of ions and surfaces in the presence of solvent. Effective potentials are derived from molecular simulations and are capable of discerning ion-specific behaviour. We argue that short-range interactions in aqueous solutions involve two characteristic lengths, the range of the interaction and the spatial periodicity, both about 2.5 ± 0.5 Å.     

Structure,solvatochromism, and solvation of trans-[CoIII(cyclam)(NCS)2](NCS) and the structure of [CoII(Me4cyclam) (NCS)]2[Co(NCS)4]MeOH

The structures of trans-[CoIII(cyclam)(NCS)2](NCS) and of [CoII(Me4cyclam)(NCS)]2[Co(NCS)4]·MeOH have been established by X-ray diffraction methods. The solvatochromic behavior of the trans-[Co(cyclam)(NCS)2]+ cation in several binary aqueous solvent mixtures is reported. Transfer chemical potentials for this complex from H2O into MeOH-H2O mixtures have been established from solubility measurements on its thiocyanate salt. The solvatochromic behavior of this cation is discussed in the context of other solvatochromic inorganic complexes; its transfer chemical potentials are discussed in relation to those of other cobalt(III) complexes.     

One- versus two-electron reaction pathways in the electrocatalytic reduction of benzyl bromide at silver cathodes

The electrocatalytic reduction of benzyl bromide at a silver cathode has been investigated in acetonitrile in the absence and presence of acids, using cyclic voltammetry (CV) and controlled-potential electrolysis (CPE). CV gives rise to two reduction waves, which represent the dissociative 1e⁻ reduction of PhCH₂Br to and Br⁻ followed by a further reduction of the benzyl radical to at more negative potentials. The charge stoichiometry (1e⁻ vs 2e⁻/molecule) and product distribution depend on the applied potential and reaction medium. In the absence of added acids, the reduction of PhCH₂Br at potentials of the first wave is a 1e⁻ process mainly yielding bibenzyl, whereas toluene becomes the principal product at potentials beyond the second wave. The addition of acids strongly modifies the dependence of selectivity on the applied potential. The presence of a strong acid changes the mechanism of the process, which now becomes a 2e⁻ reduction to toluene, even at potentials corresponding to the first reduction wave.