Electrochemical and spectroscopic measurements for stable nitroxyl radicals

A nitroxyl radical, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), is known to be oxidized electrochemically at 3.5 V versus lithium [4] and [5]. Since this reaction is reversible in the aprotic electrolyte, we can use it as a cathode reaction in lithium rechargeable battery. Some nitroxyl radical compounds which have different structures have been prepared and their electrochemical behavior and spectroscopic properties have been studied. The electrochemical measurements in aprotic electrolyte revealed that most nitroxyl radical compounds show reversible redox behavior similar to that of TEMPO independent of their structures in the range of −0.15-0.20 V versus Ag/Ag⁺ (3.69-4.04 V versus Li/Li⁺). The redox potentials for these materials were found to be predictable approximately by quantum calculations. Thus, various molecular designs tailored to desired redox potentials would be possible as active materials for lithium rechargeable batteries, and their specific capacities, mechanical properties and colors can be controlled within limits.     

Electrochemical chlorination of azulene derivatives: Insight into the mechanism of anodic oxidative chlorination

The cyclic voltammetry behavior of a series of five 1-substituted azulenes has been investigated as well as the electrochemical chlorination of these compounds. In the case of azulene compounds containing electron withdrawing groups which have higher oxidation potentials than that of the chloride ion, the electrochemical chlorination led usually to 3-chloro derivatives. The electrochemical chlorination fails for azulenes with lower oxidation potentials. Additionally some polyhalogenated compounds were obtained by controlled potential electrolyses. The paper also discusses the reaction mechanism of the electrochemical halogenation of 1-substituted azulenes.     

Electrochemical studies on doping of polyacetylene

The mechanism for electrochemical doping of polyacetylene was studied using cyclic voltammetry. The I–V curve of a thin (CH)_x filme (<1 μm) electrode exhibited a redox peak with a formal redox potential of +0.65 V vs. sodium calomel electrode. Approximately 30% of the total charge that oxidized (CH)_x was not reversible when held at the open circuit voltage of the cell. A more negative potential was needed to recover the remaining charge. This large charge-trapping phenomenon was the consequence of the (CH)_x film being doped. Using a thick film (?50 μm) electrode or freestanding film (～0.1 mm) as an electrode, the I–V curve gave only a broad re-reduction peak at +0.4 V. The disappearance of the well-defined redox peak implies that the redox process revealed by the thin film data may not be the predominate mechanism for the doping process.     

H-MCM-22 zeolitic catalysts modified by chemical liquid deposition for shape-selective disproportionation of toluene

Catalysts were prepared by dealumination of H-MCM-22 zeolite with oxalic acid leaching, followed by shaping and liquid phase modification with siloxane. The catalytic performance was measured with selective disproportionation of toluene in a fixed-bed reactor. The unmodified H-MCM-22 zeolite exhibited much higher initial activity than H-ZSM-5 with a thermodynamic equilibrium distribution of xylene products. The oxalic acid treatment of H-MCM-22 improved the selectivity for para-xylene by 10–18%. The modification by siloxane of the oxalic acid treated H-MCM-22 turned out to be highly selective for para-xylene, however, with the expense of the conversion. It is proposed that the high para-selectivity was closely associated with the contraction of micropore windows and the elimination of acid sites at the external surface by the deposited silica. Therefore, the modified H-MCM-22 is a potential catalyst for the selective disproportionation of toluene with a high para-selectivity.     

Electrochemical mechanism and kinetics studies of haloperidol and its assay in commercial formulations

The kinetics and mechanism for electrochemical reduction of haloperidol, a psychotherapeutic drug used in the treatment of schizophrenia, were studied using square wave and cyclic voltammetries allied to a hanging mercury drop electrode. The experimental and voltammetric parameters were optimized at 0.04 mol L⁻¹ Brinton–Robinson buffer (pH 10), with a pulse potential frequency of 100 s⁻¹, a pulse amplitude of 30 mV and scan increment of 2 mV. Two well-defined peaks were observed, which exhibited properties of fast electron transfer with a strong adsorption process of reactants and products on the electrode surface. The first peak was related to a fast and reversible anion-radical formation originating from the reduction of the carbonyl group, and the second was related to the irreversible reduction of the anion-radical previously formed. Analytical parameters such as: linearity range, equation of the analytical curves, correlation coefficients, detection and quantification limits, recovery efficiency, and relative standard deviation for intraday and interday were compared to similar results obtained by use of the UV–vis spectrophotometry technique, and the analytical results obtained in commercial formulations show that the voltammetric procedure using a hanging mercury drop electrode is suitable for analyzing haloperidol in complex commercial formulation samples.     

Electrochemical responsive arrays of sulfonatocalixarene groups prepared by free radical polymerization

A styrene based water soluble polymer containing pendant sulfonated calix[4]arene groups has been synthesized by using free radical polymerization combined with post-polymerization sulfonation chemistry. The monomer 25-(4-vinylbenzyl)-26, 27, 28-hydroxy-calix[4]arene (VBC4A) was prepared in 3 steps: (1) reduction of 4-vinyl benzoic acid to the respective alcohol (2) formation of the bromide by the Appel reaction and (3) synthesis of the respective ether by Williamson O-alkylation reaction with calix[4]arene. Polymerization was accomplished by azobisisobutyronitrile (AIBN) initiated free radical polymerization technique to afford P(VBC4A) with Mn value of 7090 g/mol and moderate polydispersity. The resulting P(VBC4A) was subsequently reacted with acetyl sulfate to afforded the sulfonated polymer. The reaction was followed by nuclear magnetic resonance and infrared spectroscopy, and the results suggest that the sulfonation reaction lead to nearly quantitative functionalization of the calixarene functional polymer. Elemental analysis by X-ray photoelectron spectroscopy confirmed these findings on the degree of sulfonated functionalization. Electro-responsive properties of the sulfonated polymer were studied by cyclic voltammetry and isothermal titration calorimetry in phosphate buffer saline solution. A response to electrochemical stimulus is observed where guest molecules of methyl viologen incorporate and dissociate with P(VBSC4A).     

On the σ-dimerization of N,N-dimethyl-p-toluidine during anodic oxidation—A reinvestigation

The anodic oxidation of N,N-dimethyl-p-toluidine (2) was studied using cyclic voltammetry and spectroelectrochemistry. In contrast to former studies, the experimental measurements showed that the radical cations of 2 reversibly dimerize at the methyl substituted p-position of the dimethylamino group and form σ-dimers. Theses results again clearly demonstrate that σ-dimerization takes place even when reactive sites are blocked by substituents such as alkyl or phenyl groups. The findings are also relevant for the interpretation of σ-dimerization during electropolymerization and charging of conducting polymers.     

Electrochemical synthesis of new catechol derivatives

Electrochemical oxidation of catechols (1a-e) has been studied in the presence of 2-acetylcyclopentanone (3) as a nucleophile in aqueous solutions, using cyclic voltammetry and controlled-potential coulometry. The results indicate that the quinones derived from catechols (1a-e), participate in Michael addition reactions with anion of 2-acetylcyclopentanone (3) and via EC mechanism pathway, converted to corresponding catechol derivatives (4a-e). In this work, we derived novel catechol derivatives with good yields based on electrochemical oxidation in aqueous solutions, at carbon electrode in an undivided cell, using environmentally friendly method.     

Electrochemical investigations of antioxidant interactions with radical anion and dianion of 1,3-dinitrobenzene

Interactions of five antioxidants (AO), quercetin (Q), morin (M), rutin (R), ascorbic acid (AA) and β-carotene (β-C) with anion radical and dianion of 1,3-dinitrobenzene (1,3-DNB) in two aprotic solvents – dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) – have been studied by cyclic voltammetry using glassy carbon electrode. Electrochemical parameters – peak potential (E_p), half-wave potential (E_1/2), and peak current (i_p) – for the reduction of 1,3-DNB before and after the addition of various concentrations of antioxidants, were evaluated. A gradual decrease in the oxidation peak current and finally irreversibility in 1,3-DNB radical anion and dianion systems upon the addition of antioxidant, reveals their interactions. The homogeneous bi-molecular rate constant (k₂) was determined from electrochemical data. In comparison to all other antioxidants used, enhanced homogeneous second order rate constant for the interaction of morin with 1,3-DNB anion radical and dianion, was observed. This aspect is attributable to protonation initiated by hydrogen bonding and greater acidic nature of morin.     

Electrochemical study of catechol-boric acid complexes

The electrochemical behavior of catechol-boric acid complexes in aqueous solutions has been studied using cyclic voltammetry and steady-state voltammetry with a rotating disk electrode. Different patterns of reactivity and various kinetic pathways have been examined. Based on a CE mechanism, the dissociation constant and homogeneous rate constants of complex formation and dissociation were estimated for each pathway by comparing the experimental cyclic voltammograms with the digitally simulated results. Also, the effects of pH and substituted groups on dissociation constants and reaction kinetics were studied.     

Electrochemical synthesis of polymers with alternate phenothiazine and bithiophene units

An electrochemical and spectroelectrochemical study of a new monomer—N-alkyl-bis(thiophene)phenotiazine have been presented. Electrochemical polymerization of the monomer leads to a polymer with alternate phenothiazine and bithiophene units. Polymerization proceeds after bielectronic oxidation of the molecule. Monoelectronic oxidation gives a stable radical cation with spin located on phenothiazine. Electrochemical properties of the polymer depend on the thickness of the film deposited on the electrode. For thin layers one can observe a characteristic redox couple of phenothiazine oxidation to a radical cation. Analysis of the polymer behaviour and the results of spectroelectrochemical measurements indicate a mixed-type of polymer conductivity.     

Transesterification of Jatropha curcas oil glycerides: Theoretical and experimental studies of biodiesel reaction

Vegetal oil, also known as triglycerides, is a mixture of fatty acid triesters of glycerol. In the triglycerides alkyl chains of Jatropha curcas oil, predominate the palmitic, oleic and linoleic fatty acids. The process usually used to convert these triglycerides to biodiesel is called transesterification. The overall process is a sequence of three equivalent, consecutive and reversible reactions, in which di- and monoglycerides are formed as intermediates. Semi-empirical AM1 molecular orbital calculations were used to investigate the reaction pathways of base-catalyzed transesterification of glycerides of palmitic, oleic and linoleic acid. The most probable pathway and the rate determining-step of the reactions were estimated from the molecular orbital calculations. Our results suggest the formation of only one tetrahedral intermediate, which in a subsequent step rearranges to form the products. The rate determining-step is the break of this tetrahedral intermediate.     

Electrochemical studies of the nickel catecholate complexes for detection of sulphur dioxide gas

Nickel catecholate complexes, bis(diphenylphosphino)ethanenickelcatecholate [(dppe)NiO₂C₆H₃4-R₁] R₁ = CH₃ (1), C(CH₃)₃ (2), H (3) and F (4)] were studied using CV and SWV techniques between −1.5 and +1.5 V at 50 mV s⁻¹ in cathodic and anodic sweeps. Glassy carbon electrode was used with 0.1 M tetrabutylammonium tetrafluroborate (TBATFB) in CH₂Cl₂. Complexes 1-4 before addition of SO₂, showed approximately reversible behaviour (i_pa/i_pc ≈ 1 and ΔE ≈ 77-88 mV); oxidation peaks observed between +313 and +524 mV, reduction peaks between +225 and +436 mV, depending on the substituent on the ligand. Detection limit 0.01 ppm, linear range 0.01-20 ppm, sensitivity 0.02 ppm/mV (with SWV) were obtained. The complexes 1 and 2 could be used as voltammetric sensors while complexes 3 and 4 would be suitable as amperometric sensors, for monitoring industrial SO₂ emissions.     

Electrochemical and corrosion studies of poly(nickel-tetraaminophthalocyanine) on carbon steel

Nickel-tetraaminophthalocyanine [TAPcNi] was electropolymerized from the complex monomeric solution, onto carbon steel substrates, yielding thin adherent films of poly[TAPcNi]. The investigation of such polymer-modified electrodes was carried out by means of cyclic voltammetry, UV-vis spectroscopy, FT-IR spectroscopy and spectroelectrochemistry. The preparation of TAPcNi modified electrode was also carried out by electropolymerization of a preformed molecular film of TAPcNi, after applying a drop of TAPcNi dimethylsulphoxide solution onto carbon steel, and allowing it to dry. The comparison of the corrosion behavior of the two types of polymer-coated electrodes was carried out by electrochemical impedance spectroscopy (EIS) in acid medium. It was found that the structure and morphology of each polymer greatly influence their redox behavior and corrosion inhibition performance for steel in hydrochloric acid. The film prepared by the drop-dry method offered a better corrosion protective efficiency while the electropolymerized film presented a more conductive behavior.     

One-electron redox reaction of di-tert-butyl nitroxide at platinum electrode in acetonitrile

The electrochemical oxidation of di-tert-butyl nitroxide (DTBN) at a platinum electrode in acetonitrile was examined. The cyclic voltammogram indicated an irreversible response during a normal time scale measurement, whereas chemically reversible voltammograms were obtained during a shorter time using a micro disk electrode with relatively fast sweep rates. The apparent formal redox potential and heterogeneous electron transfer rate constant were estimated to be 0.218 V (versus Fc⁺|Fc) and 0.035 ± 0.015 cm s⁻¹ from the digital simulation analysis.     

Electrochemical studies of magnetite coating on carbon steel in ascorbic and picolinic acid

Electrochemical impedance and cyclic voltammetric measurements of magnetite coating formed on carbon steel substrates were carried out in deaerated solutions of 1.0 × 10^–2 M ascorbic acid and 3.5 × 10^–2 M picolinic acid at 28 °C. Impedance data showed that the interaction between magnetite and ascorbic acid is associated with an incubation period, after which reductive dissolution takes place accompanied by oxidation of ascorbic acid. Once the magnetite film loses its thickness and protective properties on prolonged exposure to ascorbic acid, the charge transfer resistance and the capacitance is reduced. Initial dissolution of magnetite in picolinic acid is due to congruent chemical dissolution with little contribution from reduction by the electrons arising from the corrosive attack of acid on the carbon steel. With time the contribution of the latter process increases as more and more of the substrate opens up. Cyclic voltammetric studies showed that there is an adsorption phenomenon leading to the formation of a surface ferric complex that undergoes reduction to the ferrous species and transfers to the bulk solution. The accumulation of the ferrous ion accelerates the dissolution resulting in an increase in current. Dissolution of magnetite in picolinic acid is slow. The better complexing capability of picolinic acid compared to ascorbic acid does not permit the accumulation of the ferrous ion as indicated by the absence of a peak in the voltammogram.     

亚硝酰自由基及其选择性氧化多糖类物质研究进展

亚硝酰自由基能选择性氧化天然多糖类高分子,如纤维素,淀粉,甲壳质,壳聚糖等,从而为制备这些天然高分子的氧化产物或进一步制备氧化产物的衍生物,对环境友好高分子,可生物降解高分子,医用高分子的开发具有较大的应用前景。本文在简介亚硝酰自由的基础上,对其选择性氧化多糖类物质的研究进展作了综述。     

聚丙烯胺二茂铁电化学特性的研究

在水溶性高分子聚丙烯胺(PAA)的骨架上引入电化学活性二茂铁(Fc)基团,合成了具有电化学活性的水溶性高分子聚丙烯胺二茂铁(PAA-Fc).采用紫外和红外吸收光谱对PAA-Fc进行了表征,并利用循环伏安法研究了其电化学特性.实验结果表明:PAA-Fc具有明显的电化学活性,可在L-脯氨酸脱氢酶与电极表面之间实现电子传递.     

Electrochemical studies of octadecanethiol and octanethiol films on variable surface area mercury sessile drops

Octadecanethiol (ODT) and octanethiol (OT) films at the mercury-electrolyte interface are examined using cyclic voltammetry and differential capacitance measurements at a single frequency. A mercury flow-system is used to alter the volume, and therefore, the surface area and surface pressure of the mercury electrode. Manipulation of the mercury electrode's volume enables the introduction and removal of defects in the insulating thiol films. OT and ODT film behavior are contrasted under conditions of expansion and contraction. ODT forms extremely impermeable layers that allow 1000 time less redox probe current than seen on uncoated drops. Expansion of the mercury electrode to increase the electrode surface area produces defects and pinholes in the thiol film. These defects are almost completely removed when the drop is compressed back to its initial surface area. OT also forms insulating films on mercury sessile drops, however these films contain more defects than ODT films. While expansion of an OT-coated mercury drop increases redox probe current, recompression of the drop does not return the film to its initial condition. Pinholes and defects in the OT and ODT films can also be produced by cycling to negative potentials, which produce abrupt stripping peaks.     

Electrochemical study of azo-azulene compounds

An electrochemical study of several azo-azulene compounds (Az-NN-Y, where Y = substituted phenyl, pyridine, thiazole) was performed with cyclic and differential pulse voltammetry, as well as rotating-disk electrode methods. The objective of this work was to characterize their electrochemical properties and establish the influence of donor and acceptor substituents on azulene and azo group behaviour. Calculations were performed, using quantum mechanics-based methods, to correlate electrochemical reactivity with structure. Satisfactory correlations were found between experimental oxidation and reduction potentials as well as calculated ionization potentials and LUMOs.