Cyclic voltammetric study of the disproportionation reaction of the nitro radical anion from 4-nitroimidazole in protic media

We have chosen 4-nitroimidazole (4-NIm) as a prototype of nitroimidazolic compounds in order to carry out an exhaustive cyclic voltammetric study in protic media, 0.1 M Britton–Robinson buffer+0.3 M KCl with ethanol as a co-solvent. The one electron reduction of 4-NIm in protic media at alkaline pH produces a stable nitro radical anion on the time scale of the cyclic voltammetric technique. The nitro radical anion decays according to a coupled chemical reaction and we have focused the study to follow this reaction using cyclic voltammetric methodology. The one electron reduction of the 4-NIm to form the nitro radical anion and the subsequent reaction of the radical obey an EC₂ mechanism that follows very well the cyclic voltammetry theory for the disproportionation reactions previously described by Olmstead and Nicholson (Anal. Chem. 41 (1969) 862). We have obtained the disproportionation rate constant k₂ which is strongly dependent on both pH and ethanol content according to the following regression equations: log k₂=?0.932pH+12.771 and log(10^-3k₂)=?1.998log[% EtOH]+3.873. The results obtained from this study in protic media differ substantially from previous studies in aprotic media wherein the nitro radical anion was not stabilized.     

Reactions of the nitro radical anion of metronidazole in aqueous and mixed solvent: a cyclic voltammetric study

Cyclic voltammetry was used to investigate the electrochemical reduction of metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol) at glassy carbon and gold electrodes at different pHs in aqueous solution as well as in mixed solvent viz., aqueous dimethyl formamide. The electrogenerated nitro radical anion undergoes a disproportionation reaction, the rate constant of which is dependent on the pH, solvent composition and electrode material. The interactions of the nitro radical anion with thymine and cytosine were also investigated using a cyclic voltammetric technique. Both the bases were found to react with metronidazole nitro radical anion. The rate constants for such reactions in aqueous solutions were 3.5 × 10³ and 3.0 × 10³ dm³ mol^?1 s^?1 for thymine and cytosine, respectively.     

Cyclic voltammetric determination of free radical species from nitroimidazopyran: a new antituberculosis agent

PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine) is being tested as antituberculosis drug. Little is known on the action mechanism of PA-824; however the reduction of the nitro group seems to be a key step in the metabolic activation, as is observed for the well-known bactericidal metronidazole. Consequently, this paper is focused on the cyclic voltammetric behavior of PA-824 with the aim of revealing the formation and stability of the corresponding nitro radical anion and its comparison with the metronidazole behavior.Both compounds PA-824 and metronidazole reveal, in aprotic medium (DMSO + 0.1 tetrabutylammonium hexafluorophosphate), a similar reduction pattern showing a well-resolved couple due to nitro reduction to form the corresponding nitro radical anion. The electrode reaction obeys an EC₂ mechanism with a dimerization reaction as the chemical step in aprotic medium. Using cyclic voltammetry theory for a dimerization reaction we have calculated the second-order decay constants, k_2,dim, and the half-life time, t_1/2, for the nitro radical anions formed from PA-824 and metronidazole. We have obtained k_2,dim values of 2.22 × 10² and 2.58 × 10⁴ M^?1s^?1 for metronidazole and PA-824, respectively. Our voltammetric results show that the PA-824 nitro radical anion requires more energy for formation (about 200 mV) and it is approximately 100 times less stable than the metronidazole radical anion.     

Reversible dimerization of the anion radicals of some dicyanonaphthalenes

The electrochemical reduction of 7 of the 10 isomeric dicyanonaphthalenes has been studied in N,N-dimethylformamide. The studied compounds were 1,2-, 1,3-, 1,4-, 2,3-, 1,8-, 2,6- and 2,7-dicyanonaphthalene. For most of the isomers, the one-electron reduction to the anion radical occurred as a simple, reversible reduction without complications from coupled chemical reactions. However, for 1,3-, 2,3- and 2,7-dicyanonaphthalene, the reduction was affected by a reversible dimerization of the anion radicals. Dimerization equilibrium constants were determined at various temperatures. DFT calculations of the anion radicals of all 10 isomers provided spin densities at ring carbons 1–8. The highest spin density for carbons 1–8 not bearing a cyano group was largest for the three isomers with detectable dimerization. Not only do these calculations suggest the site of dimerization, it was found that the free energy change for dimerization varies in an approximately linear fashion with spin density.     

Investigation of anion adsorption on platinum electrodes in aqueous media by probe beam deflection

Probe beam deflection (PBD) was used to monitor concentration gradients adjacent to the platinum electrode surface in basic and acidic aqueous media. It was observed that the PBD technique can detect anion adsorption on platinum during a cyclic voltammetry experiment. Also, PBD can measure the potential-dependent extent of adsorption of various anions including ClO₄^?, H₂PO₄^? and Cl^? on the Pt electrode surface.     

Thermodynamic studies of anion adsorption at the Pt(111) electrode surface in sulfuric acid solutions

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(111) electrode in solutions of H₂SO₄ with an excess of an inert electrolyte (0.1 M HClO₄). A high quality Pt(111) single crystal electrode with a low defect density was employed in this study. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using the electrode potential and the charge as independent variables has been performed. The best results were obtained when the charge was the independent variable. These results show that the maximum Gibbs excess of (bi)sulfate attains a value of ～3×10¹⁴ ions cm^?2, which corresponds to the packing density of ～0.2 monolayer. This number agrees well with the results of a radioactive labeling method (Kolics and Wieckowski, J. Phys. Chem. Sect. B 105 (2001) 2588) and with recent STM studies (Itaya, Prog. Surf. Sci. 58 (1998) 121; Funtikov, Linke, Stimming and Vogel, Surf. Sci. 324 (1995) L343). The calculated Esin–Markov coefficient and electrosorption valencies exhibit an important dependence on the electrode potential (or charge).     

Investigation of potential inversion in the reduction of 9,10-dinitroanthracene and 3,6-dinitrodurene

In the case of two-electron reduction of a compound, potential inversion refers to the situation where introduction of the second electron occurs with greater ease than the first. That is E⁰₁?E⁰₂<0 where E⁰₁ and E⁰₂ are the standard potentials for the two steps of reduction. The extent of potential inversion in the reduction of 9,10-dinitroanthracene, 1, and 3,6-dinitrodurene, 2, has been assessed by steady-state microelectrode voltammetry, cyclic voltammetry under conditions of near-reversible behavior, cyclic voltammetry under conditions of quasireversible behavior and electrochemical impedance spectroscopy (EIS). The studies were conducted in acetonitrile at 298 K. Cyclic voltammetry under quasireversible conditions and EIS were most sensitive to small changes in E⁰₁?E⁰₂. The value of E⁰₁?E⁰₂ for 1 was found to be ?107 mV and that for 2 was ?280 mV.     

Glucose sensing based on an effective conversion of O2 and H2O2 into superoxide anion radical with clay minerals

An effective promotion for the generation of superoxide anion radical from H₂O₂ and O₂ was accomplished with montmorillonite K10 clay mineral (denoted Mont. K10). This catalytic event was characterized by a fluorescence assay using amplex red and superoxide dismutase (SOD) as probes. The iron species (Fe^II and Fe^III) contained in Mont. K10 are thought to contribute to this catalytic event. Due to this property, a glucose sensor was fabricated on the basis of Mont. K10, amplex red, and glucose oxidase (GO_x). When glucose is injected with amplex red over the clay|GO_x electrode, H₂O₂ is produced in the enzyme zone, transduced to superoxide anion radical by the clay, and quenched by amplex red. Since the resulting product, resorufin, is emission-active under the irradiation of visible light (λ_ex: 563 nm; λ_em: 583 nm, pH 8), the analyte can thus be recognized and reported as optical signals with a luminescence spectrometer via a Y-type optical fiber atop of the electrode. The optical signal showed a linear response to glucose up to 150 μM at pH 8. The lower and upper limits were estimated to be 1.0 and 150 μM, respectively, at this pH. These results suggest that Mont. K10 is a promising ${\mathrm{O}}_2^{-\text{<mglyph src="https://sdfestaticassets-us-east-1.sciencedirectassets.com/shared-assets/16/entities/rad"></mglyph>}}$ transducer.     

Electrochemical adsorption studies of urea on copper surface in alkaline medium

The inhibitive action of urea on the corrosion behavior of copper was investigated in borate buffer pH (8.4) by means of cyclic voltammetry and electrochemical impedance spectroscopy (EIS). A consistent trend of increase in inhibition efficiency was observed as a function of concentration of urea. The adsorption of urea on copper surface was found to obey the Langmuir adsorption isotherm with ΔG = −35.2 kJ/mole and adsorption constant K = 2.7 × 10⁴ dm³/mole. Impedance spectroscopic measurements confirmed that charge transfer resistance increases in presence of urea upon increasing its concentration.     

Voltammetric investigation of the anodic dimerization of p-halogenoanilines in DMF: Reactivity of their electrogenerated cation radicals

The electrodimerization mechanisms of p-chloro- and p-bromoaniline were studied in unbuffered DMF medium. By combined application of conventional and fast voltammetry (100 kV s^?1 range), the primary radical cation intermediates, formed by the one electron oxidation of each p-halogenoaniline were characterized. The overall reaction path involves a dimerization via a N–C bond formation and de-halogenation at the para position. A detailed mechanistic investigation demonstrates that this proceeds through a fast reversible deprotonation of the primary radical cation followed by the subsequent N–C bond formation between the resulting radical and its parent radical cation which is the rate-determining step of the sequence (e-p-RRC-p kinetic sequence). The effect of a relatively strong, but weakly nucleophilic base, 2,6-lutidine, has been also investigated and confirmed the involvement of the fast deprotonation pre-equilibria. The fast voltammetric experiments were simulated and the apparent rate constants for the overall deprotonation/dimerization sequence obtained on the basis of peak potential shift analysis were thus confirmed.     

An in situ UV-vis and IR spectroelectrochemical study of the deposition of a hydroquinone anion salt on platinum electrodes from dichloromethane solutions

Cyclic voltammetry with a platinum electrode of hydroquinone (BQH₂) solutions in dichloromethane, containing tetrabutylammonium perchlorate supporting electrolyte, shows a sharp asymmetric irreversible oxidation peak at about ?0.3 V (SCE). This feature is seen, in addition to the expected features in this system, when the cycle is extended to potentials more negative than ?0.6 V (SCE). Cyclic voltammetry, in situ UV-vis and infrared spectroelectrochemistry have shown that hydroquinone anion (BQH^?) is formed at negative potentials and this appears to arise via surface decomposition of hydroquinone to p-benzosemiquinone (BQH) followed by reaction of BQH with the p-benzoquinone radical anion (BQ^?). The sharp asymmetric peak in the cyclic voltammograms is due to oxidation of the hydroquinone anion in the insoluble tetrabutylammonium salt on the electrode surface. The oxidation of BQH^? appears to occur via disproportionation of (BQH) and leads to BQH₂ and p-benzoquinone (BQ) as the products.     

Electrochemical studies of single-wall carbon nanotubes in aqueous solutions

Studies on the voltammetric responses, capacitance, and charge storage capability of single-wall carbon nanotube sheets or papers are described. Broad redox responses have been observed probably due to the presence of oxygen-containing functional groups attached to the surface of the nanotubes or to the impurities produced during nanotube purification in nitric acid. Annealing the material at 900°C eliminates these responses. The voltammetry and capacitance of the nanotube paper varies little with either anion or cation molar mass, ion charge, or ion hydrophobicity. Acids as a group tend to produce higher capacitance but similarity between the electrochemical responses in various acids is also observed. The small variation of the capacitance within a wide range of scan rates suggests that in the time scale investigated the double layer charging process is controlled by the RC time constant and not by electrolyte ion diffusion into the pores of the nanotube paper.     

Electrochemical behavior of quercetin: Experimental and theoretical studies

Electrochemical oxidation of quercetin, as important biological molecule, has been studied in 0.1 M phosphate buffer solution, using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry as well as quantum mechanical calculations. The heterogeneous charge transfer rate constant, k′, transfer coefficient, α, and exchange current density, j₀, for oxidation of quercetin at the glassy carbon electrode are determined as 4.84 × 10^?2 cm s^?1, 0.65 ± 0.01 and (1.17 ± 0.39) × 10^?7 A cm^?2, respectively. The formal potential, E^0′, of quercetin is pH dependent with a slope of ?60.1 mV per unit of pH which is close to the anticipated Nernstian value of ?59 mV for a two electrons and two protons process. The standard formal potential, E⁰, of quercetin was found to be equal with 558 mV versus saturated calomel electrode (SCE). The mechanism of oxidation was deduced from voltammetric data in various pHs and also in different concentrations of quercetin. The diffusion coefficient of quercetin was calculated as 3.18 × 10^?6 cm² s^?1 for the experimental condition, using chronoamperometric results. The results of density functional theory (DFT) calculations for the oxidation of quercetin in aqueous solution, are also presented. The theoretical standard electrode potential of quercetin is obtained to be 568 mV versus SCE, which is in good agreement with the experimental value. The discrepancy between theoretical and experimental values is only 10 mV. The agreement verifies the accuracy of experimental method and the validity of mathematical model.     

Novel studies on the electrochemical oxidation of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) in acetonitrile at platinum electrodes

The electrochemical oxidation mechanism of 2-[4-(N,N-dimethylamino)phenyl]-6-methyl benzothiazole (DPMB) is studied in a 0.1 M N(C₄H₉)₄ClO₄ + acetonitrile (ACN) reaction medium by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential bulk electrolysis at platinum electrodes. The primary radical cation formed by the one electron oxidation of DPMB undergoes a deprotonation process, which is the rate-determining step, followed by a radical–radical coupling. On the other hand, an initial quasi-reversible monoelectronic charge transfer mechanism is inferred from cyclic and square wave voltammograms recorded at scan rates and frequencies higher than 0.4 V s^?1 and 40 Hz, respectively. Diffusion coefficients of DPMB at different temperatures were calculated from the quasi-reversible convoluted cyclic voltammograms. DigiSim® and COOL software were used to fit the quasi-reversible cyclic and square wave voltammetric responses, respectively. Formal potentials, formal rate constants and positive transfer coefficients at different temperatures were evaluated from the fitting of cyclic voltammograms. The experimental activation parameters were also determined. The effects of the analytical concentration of the reagent and the temperature, as well as the addition of trifluoracetic acid and a strong base such as lutidine on the electrochemical responses are discussed. A general reaction mechanism as well as probable structures for dimeric products are proposed.Besides, the presence of an acid–base equilibrium in DPMB solutions is also studied by employing UV–Vis spectroscopic measurements at different trifluoracetic acid concentrations. An apparent value of (1.5 ± 0.2) × 10³ M^?1 was estimated for the DPMB basic constant at 20.0 °C     

A comparative evaluation on the voltammetric behavior of boron-doped diamond (BDD) and glassy carbon (GC) electrodes in different electrolyte media

Voltammetric responses of boron-doped diamond (BDD) and glassy carbon (GC) electrodes on the anodic oxidation of two dissimilar compounds namely 2,6-dimethoxyphenol and 1,3,5-trimethylbenzene in acidic, neutral and basic media have been explored. Cyclic voltammetric (CV) analysis reveals that the BDD electrode shows wider cathodic potential window and lower background current in all the media than the GC. However, in the anodic side, the window is wide only in aqueous acidic medium and the background limit for both the electrodes is similar in both neutral and alkaline media containing solvents other than water. Further, the anodic oxidation of 2,6-dimethoxyphenol takes place at less potential on the GC when compared to the BDD and the oxidation peak current is also higher on the former electrode. CV results show that no anodic peak appears for the oxidation of 1,3,5-trimethylbenzene on the GC electrode in any of the above media, whereas in the acidic medium, a broad anodic wave appears on the BDD.     

Mechanistic investigation of the iron-mediated electrochemical formation of β-hydroxyesters from α-haloesters and carbonyl compounds

The electrochemical behavior of Fe^I in the presence of α-haloesters has been studied by means of cyclic voltammetry and preparative-scale electrolyses with the intent to investigate the electrochemical coupling reaction between α-haloesters and carbonyl compounds performed in the presence of FeBr₂ and a sacrificial iron anode. It is established that the reaction is initiated by the electrogeneration of an iron (I) species, which has to be stabilized by the presence of 2,2′-bipyridine (bpy). However, the reactivity of the electrogenerated Fe^I is governed by the bpy concentration. Indeed, for bpy/Fe ratios higher than or equal to 3, an outer-sphere electron transfer has been shown between Fe^I and α-haloesters, but no coupling product was obtained under these conditions. Conversely, for smaller bpy/Fe ratios the coupling reaction occurs, suggesting the existence of an oxidative addition between Fe^I and α-haloesters. The presence of a sacrificial iron anode in preparative-scale experiments is useful to decrease the bpy/Fe ratio by generation of Fe²⁺ cations.     

Uncovering the influence of antioxidants on polyphenol oxidation in wines using an electrochemical method: Cyclic voltammetry

The influence of sulfur dioxide, glutathione and ascorbic acid on the cyclic voltammograms of four representative wine polyphenols (catechin, caffeic acid, rutin and quercetin) and five different wines was investigated using a glassy carbon electrode in a model wine solution. Sulfur dioxide increased the anodic current and decreased the cathodic current for all four polyphenols and all wines, pointing to a rapid interaction of SO₂ with the oxidized polyphenol quinones. A similar trend was seen for glutathione, except that in the case of quercetin, addition of glutathione led to the formation of a second set of voltammetric peaks, corresponding to redox activity of a glutathione derivative. However, ascorbic acid produced no additional effect on the cyclic voltammograms of wine polyphenols and wines, beyond that expected for a simple sum of the polyphenol and ascorbic acid responses, with the exception that adsorption of quercetin and rutin on the carbon electrode caused a shift in the ascorbic acid oxidation peak to more positive potentials.     

Carbon paste electrodes of the mixed oxide SiO2/Nb2O5 prepared by the sol–gel method: dissolved dioxygen sensor

A carbon paste of SiO₂/Nb₂O₅ material was used as the electrode in the development of a dissolved dioxygen sensor in 1.0 mol l^?1 KCl solution at pH 6.2. The material was prepared by the sol–gel method. In the investigation of its electrochemical properties, linear and cyclic voltammetric and chronoamperometric techniques were employed. Dioxygen reduction, which was diffusion controlled, occurred at ?280 mV vs. SCE by a two electron mechanism, producing peroxide. A linear response between the cathodic peak current intensity and the dissolved O₂ concentration was obtained for the region between 1.0 and 13.6 mg l^?1. The stability proved to be very good over successive voltammetric cycles. The electrode response time was about 5 s. The electron transfer reactions were explained as being to an n-type semiconductor of niobia dispersed in the silica surface.     

Anion effect on the electrochemistry of TTF in polymer solvent

Cyclic voltammetry of a solid polymer solution of a tetrathiafulvalene (TTF) derivative showed different behaviours depending on the nature of the electrolyte anion. A lower potential and current peak for the second redox reaction in the presence of perchlorate revealed a specific interaction between this anion and the TTF dication. This effect also indicates a coupling between the physical diffusion and electron self-exchange during the voltammetric scan. No interaction between the TTF dication and triflimide or TFSI-bis (trifluoromethanesulfonyl) imide-anion was observed. This confirms the exceptional softness of this anion and underlines the role that the nature of the electrolyte anion could play on both the ionic and the electronic conductivities of the electrode polymer materials.