Electrochemical Study of 4‐Substituted Analogues of Megazol

The electrochemical behavior of three different megazol analogues substituted at position 4 and their comparison with the parent compound megazol in protic and aprotic media by cyclic voltammetry, Tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in both media with the reduction of the nitroimidazole group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized only in aprotic media for all these compounds. By applying cyclic voltammetric methodology we have calculated the dimerization reaction decay constants (k₂) of the corresponding nitro radical anions in aprotic media. The nitro radical anion obtained from the synthesized nitroimidazole compound having a bromine substituent in 4‐position (GC‐141) was significantly more stable than the corresponding radical formed from the compound lacking of the substituent in 4‐position, megazol.     

Voltammetric Behavior of Nitrofurazone at Highly Boron Doped Diamond Electrode

The electrochemical behavior of nitrofurazone (NFZ) at a highly boron doped diamond (BDD) electrode was studied in Britton‐Robinson (BR) buffer using cyclic voltammetry. NFZ was directly reduced to the amine derivative (RNH₂) in the pH range of 2.0 to 4.0 in a process involving six (6.0±0.4) electrons and six protons. In the range of pH 7.0 to 12 and, predominantly aqueous medium, the reduction step split into its two components: the reduction of NFZ to the radical anion (RNO₂^.?) and reduction of RNO₂^.? to hydroxylamine derivative (RNHOH) in processes involving one and three (3.1±0.1) electrons, respectively. On the anodic scan of the voltammograms and at pH 8.0, the oxidation of the hydroxylamine to the nitroso derivative (RNO), was observed in a process involving 2 (1.7±0.2) electrons and 2 protons. In addition and unreported in the literature on any electrode material, was the detection of a new oxidation peak at pH>8.0, which was observed regardless of whether NFZ had been previously reduced or not. The calculation of n, number of electrons, involved in each electrochemical step was satisfactorily accomplished using the Randles‐?evcik equation.     

Nitroradical Anion Formation from some Iodo‐Substituted Nitroimidazoles

The electrochemical behavior of iodo nitroimidazole derivatives such as 1‐methyl‐4‐iodo‐5‐nitroimidazole (M‐I‐NIm) and 1‐methyl‐2,4‐diiodo‐5‐nitroimidazole (M‐I₂‐NIm) and the parent compound 1‐methyl‐5‐nitroimidazole (M‐NIm), was studied in protic, mixed and non‐aqueous media. The electrochemical study was carried out using Differential Pulse Polarography (DPP), Cyclic Voltammetry (CV), Differential Pulse Voltammetry (DPV) and coulometry and as working electrodes mercury and glassy carbon were used. As can be expected, in all media, the effect of introduce iodo as substituent in the nitroimidazole ring produced a decrease of the energy requirements of the nitro group reduction. Certainly, this fact can be explained by the electron withdrawing character of the iodo substituent that acts diminishing the electronic density on the nitro group thus facilitating their reduction. In all the studied media the reduction of M‐NIm produced a detectable signal for a nitro radical anion derivative. In the case of M‐I‐NIm the nitro radical anion was only detectable in both mixed and non‐aqueous media. On the other hand the nitro radical anion for the M‐I₂‐NIm was detected only in non‐aqueous medium. When glassy carbon electrode was used as the working electrode in a mixed medium a detectable nitro radical anion derivative appeared for all compounds, thus permitting an adequate comparison between them. The obtained values of k₂ for M‐NIm, M‐I‐NIm and M‐I₂‐NIm in non‐aqueous medium were 5.81×10², 132×10² and 1100×10² M^?1 s^?1, respectively. From the obtained k₂ and t_1/2 values in this medium, it is concluded that there is a direct dependence between the presence of iodo substitution in the nitroimidazole ring with the stability of the nitro radical anion.     

Crystal Structures of the Potassium and Silver Salts of Nitroform

The solid state structures of three nitroformate (NF) salts were determined using single crystal X‐ray crystallography. The NF anion was found to be a non‐planar moiety which adopts either the commonly observed C_2v conformation or distorted propeller conformation (D₃) in the case of the silver salts, or, a C₂ conformation in the case of the potassium salt. This latter C₂ conformation has been uniquely observed for potassium nitroformate. All structures exhibit cation‐anion interactions that influence the structure of the anion. The ¹³C and ¹⁴N NMR spectra of the NF anion show broad singlets, which indicates the equivalence of the nitro groups in solution within the NMR time‐scale. In addition, the vibrational and mass spectra of potassium nitroformate and silver nitroformate monohydrate were recorded. Furthermore, the gaseous decomposition products of potassium nitroformate at 25 °C were detected using IR spectroscopy and mass spectrometry.     

Crystallographic characterization of the first reported crystalline form of the potent hallucinogen (R)‐2‐amino‐1‐(8‐bromobenzo[1,2‐b;5,4‐b′]difuran‐4‐yl)propane or `bromodragonfly': the 1:1 anhydrous proton‐transfer compound with 3,5‐dinitrosalicylic acid

The 1:1 proton‐transfer compound of the potent substituted amphetamine hallucinogen (R)‐2‐amino‐1‐(8‐bromobenzo[1,2‐b;5,4‐b′]difuran‐4‐yl)propane (common trivial name `bromodragonfly') with 3,5‐dinitrosalicylic acid, namely 1‐(8‐bromobenzo[1,2‐b;5,4‐b′]difuran‐4‐yl)propan‐2‐aminium 2‐carboxy‐4,6‐dinitrophenolate, C₁₃H₁₃BrNO₂⁺·C₇H₃N₂O₇⁻, forms hydrogen‐bonded cation–anion chain substructures comprising undulating head‐to‐tail anion chains formed through C(8) carboxyl–nitro O—H...O associations and incorporating the aminium groups of the cations. The intrachain cation–anion hydrogen‐bonding associations feature proximal cyclic R₃³(8) interactions involving both an N⁺—H...O_phenolate and the carboxyl–nitro O—H...O associations and aromatic π–π ring interactions [minimum ring centroid separation = 3.566 (2) Å]. A lateral hydrogen‐bonding interaction between the third aminium H atom and a carboxyl O‐atom acceptor links the chain substructures, giving a two‐dimensional sheet structure. This determination represents the first of any form of this compound and is in the (R) absolute configuration. The atypical crystal stability is attributed both to the hydrogen‐bonded chain substructures provided by the anions, which accommodate the aminium proton‐donor groups of the cations and give crosslinking, and to the presence of the cation–anion aromatic ring π–π interactions.     

Electrochemical reduction of CO2 in the presence of 1,3-butadiene using a hydrogen anode in a nonaqueous medium

The possibility of anodic generation of a solvated proton on a gas-diffusion electrode in an aprotic medium in the presence of carbon dioxide and 1,3-butadiene has been demonstrated. Formic acid was shown to be the only product of the reaction in the initially aprotic medium with the use of a hydrogen gas-diffusion anode. The influence of the counterion on the reactivity of the CO₂ ^.− radical anion in electrocarboxylation was shown experimentally. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 295–300, February, 1999.     

Solvation Dependences of Isotropic Hyperfine Interaction Constants and out-of-Plane Distortions of ortho-Substituted Nitrobenzene Radical Anions

For a number of ortho-substituted nitrobenzene radical anions (RAs) generated in DMF and its binary mixtures with water, it is shown that for radical anions with a substituent of minor effective volume in one ortho position to the nitro group, the dependences of the isotropic hyperfine interaction (ihfi) constants on the mole fraction of water are S-like and dictated by the medium composition and the concerted out-of-plane rotational and pyramidal distortions of the nitro group of the radical anion. The S-like shape of the solvation dependences of the ihfi constants is dictated by the dominant rotational distortions of the nitro group. For most radical anions with two ortho substituents or with one ortho substituent with a large effective volume, the S-like dependences are not observed, and the values of the nitrogen ihfi constants depend on the dominant pyramidal distortion of the nitro group. For the 2-tert-butylnitrobenzene radical anion in water, the nitrogen ihfi constant is a_N=25.62 G, which is typical of nitroaliphatic radical anions. This effect is explained based on the pyramidal structure of the nitro group in the case of its large rotation angles.     

Aerobic and Electrochemical Oxidative Cross‐Dehydrogenative‐Coupling (CDC) Reaction in an Imidazolium‐Based Ionic Liquid

The ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate [BMIm][BF₄] has demonstrated high efficiency when applied as a solvent in the oxidative nitro‐Mannich carbon? carbon bond formation. The copper‐catalyzed cross‐dehydrogenative coupling (CDC) between N‐phenyltetrahydroisoquinoline and nitromethane in [BMIm][BF₄] occurred with high yield under the described reaction conditions. Both the ionic liquid and copper catalyst were recycled nine times with almost no lost of activity. The electrochemical behavior of the tertiary amine substrate and β‐nitroamine product was investigated employing [BMIm][BF₄] as electrolyte solvent. The potentiostatic electrolysis in ionic liquid afforded the desired product with a high yield. This result and the cyclic voltammetric investigation provide a better understanding of the reaction mechanism, which involves radical and iminium cation intermediates.     

Voltammetric Determination of Nitronaphthalenes at a Silver Solid Amalgam Electrode

Abstract

Electrodes based on amalgam materials were re-introduced in electroanalytical chemistry in the year 2000, partially as reaction to unsubstantiated public fears of liquid mercury. In this publication, the voltammetric behavior of 1-nitronaphthalene and 2-nitronaphthalene was investigated at a mercury meniscus-modified silver solid amalgam electrode. The reduction mechanism in mixed neutral buffer-methanol medium includes the four-electron reduction to hydroxylaminoderivative followed by a two-electron reduction to the amine in acidic medium, similarly to mercury electrodes. In alkaline media, both compounds show the splitting of the main four-electron reduction peak typical for mercury electrodes in two new ones, the first one corresponding to a one electron reduction of the nitroderivative to the nitro radical anion, which was confirmed by microcoulometry. Using optimized conditions (differential pulse voltammetry, Britton-Robinson buffer pH 7.0 – methanol (9:1) medium) the calibration dependences are linear in the range of 2·10^?7 (4·10^?7) to 1·10^?4 mol L^?1 for 1-nitronaphthalene (2-nitronaphthalene). After preconcentration of the analytes from drinking and river water samples using solid phase extraction the limit of determination was lowered to ～3·10^?8 mol L^?1.     

Voltammetric Determination of BHT Antioxidant at Gold Electrode in Biodiesel

The voltammetric determination of synthetic antioxidant 2,6‐di‐tert‐butyl‐4‐methylphenole (BHT) was studied using linear‐sweep voltammetry (LSV) and cyclic voltammetry (CV) with a gold electrode and performed in isopropanol media containing either 0.1 mol L^?1 H₂SO₄ or 0.1 mol L^?1 LiClO₄ as supporting electrolyte. The results obtained have revealed that the most reliable detection was acquired in acidic media (isopropanol–H₂SO₄) whereas the use of isopropanol? LiClO₄ solution exhibited poorer reproducibility due to possible passivation of the electrode. Real samples of biodiesel mixture were analyzed without any special sample treatment or separation and results were compared with those obtained by FTIR‐spectroscopy.     

Investigation of the Electrochemical Reduction of Benzophenone in Aprotic Solvents Using the Method of Cyclic Voltammetry

The reduction of benzophenone (Bzph) in 3-pentanone (PEN), acetone (ACE), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (ACN) and dimethyl sulfoxide (DMSO) with n-tetrabutylammonium hexafluorophosphate (TBAPF₆) as background electrolyte was studied using the technique of cyclic voltammetry at the temperature of 263.15 K. The half-wave potentials (E _1/2) were extracted. The reduction of Bzph occurs in two successive one-electron steps to produce first the free radical anion Bzph⁻ and then the dianion Bzph²⁻. The results indicated that the radical anion Bzph⁻ is reoxidized to Bzph in all investigated solvent media whereas the dianion Bzph²⁻ is reoxidized to Bzph⁻ only in THF. The heterogeneous electron-transfer rate constants (k _s ) were evaluated by employing the electrochemical rate equation proposed by Nicholson. The rate of electron transfer for the Bzph/Bzph⁻ couple was found to be relatively slow in all investigated solvent media. Consequently, the electron-transfer processes can be recognized as quasi-reversible. The diffusion coefficients (D) of Bzph in the investigated solvent media have been calculated using the modified Randles-Sevcik equation. The effect of the physical and chemical properties of the solvent medium on the electrochemical behavior of Bzph has been examined.     

Photodynamic Action Mechanism Mediated by Zinc(II) 2,9,16,23‐Tetrakis[4‐(N‐methylpyridyloxy)]phthalocyanine in Candida albicans Cells

The photoreaction type I/type II pathways mediated by zinc(II) 2,9,16,23‐tetrakis[4‐(N‐methylpyridyloxy)]phthalocyanine (ZnPPc⁴⁺) was studied in Candida albicans cells. This photosensitizer was strongly bound to C. albicans cells at short times. After 30 min irradiation, 5 μM ZnPPc⁴⁺ produced ~5 log decrease in cell viability. Different probes were used to detect reactive oxygen species (ROS) in cell suspensions (~10⁶ CFU mL^?1). Singlet molecular oxygen, O₂(¹Δ_g), was observed by the reaction with 9,10‐dimethylanthracene (DMA) and tetrasodium 2,2‐(anthracene‐9,10‐diyl)bis(methylmalonate) (ABMM), whereas the nitro blue tetrazolium (NBT) method was used to sense superoxide anion radical (). Moreover, the effects produced by an anoxic atmosphere and cell suspensions in D₂O, as well as the addition of sodium azide and mannitol as ROS trapping were evaluated in the PDI of C. albicans. These investigation indicates that O₂(¹Δ_g) is generated in the cells, although a minor extension other radical species can also be involved in the PDI of C. albicans mediated by ZnPPc⁴⁺.     

Effects of temperature and anion type on the two-dimensional condensation of the 4,4′-bipyridine cation radical on mercury

The reduction of 4,4′-bipyridine (BPH²⁺₂) on mercury in an acid medium gives a very narrow sharp tail-less reversible voltammetric peak that can be ascribed to the formation of a two-dimensional (2D) phase of the cation radical BPH^·+₂ at the electrode according to the reaction

BPH²⁺₂ + e⁻ |BPH^·+₂|_2D

The corresponding oxidation peak possesses similar properties and arises from the destruction (fusion) of the 2D phase.In this work we studied the influence of some experimental variables, namely the type of anion present in the medium, the concentration of 4,4′-bipyridine and temperature on the 2D phase transition peaks. Also, we tested various analytical criteria to validate this assignation and fitted both voltammetric peaks numerically to the theoretical model developed for this purpose.     

Electrochemical Approach to the Radical Anion Formation from 2′‐Hydroxy Chalcone Derivatives

Three 2′‐hydroxy chalcone derivatives were electrochemically reduced to the radical anion by a reversible one‐electron transfer followed by a chemical dimerization reaction. Under suitable conditions of the medium, the one‐electron reduction produces very well resolved cyclic voltammograms due to the formation of the radical anion. By using appropriately the wide versatility of the cyclic voltammetric technique, was possible to study the generation of the radical anion and its stability.     

Carboxylate–phenolate tautomerism in 5‐[(nitrophenyl)diazenyl]salicylate anions

Aryldiazenyl derivatives of salicylic acid and their salts are used as dyes. In these structures, the carboxylate groups are engaged in short contacts with the cations and in hydrogen bonds with water molecules, if present. If both O atoms of the carboxylate group take part in such interactions, the negative charge is delocalized over the two atoms. In the absence of hydrogen bonds and contacts with cations, the negative charge is localized on one of the O atoms. In the crystal structures of tetramethylammonium 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoate and tetramethylammonium 2‐hydroxy‐5‐[(E)‐(2‐nitrophenyl)diazenyl]benzoate, both C₄H₁₂N⁺·C₁₃H₈N₃O₅⁻, all the interactions between the cations and anions are weak, and their effect on the geometry of the anions is negligible. Under these conditions, the 2‐nitro‐substituted anion is an almost pure phenol–carboxylate tautomer, whereas in the 4‐nitro‐substituted anion, the phenolic H atom is shifted towards the carboxylate group, and thus the structure of this anion is intermediate between the phenol–carboxylate and phenolate–carboxylic acid tautomeric forms. The probable formation of such an intermediate form is supported by quantum chemical calculations. Being the characteristic feature of this form, a short distance between the phenolic and carboxylate O atoms is observed in the 4‐nitro‐substituted anion, as well as in the structures of some 3,5‐dinitrosalicylates reported in the literature.     

Electrochemical reduction and quantification of menadione in sodium dodecyl sulfate micellar media

Menadione cyclic voltammograms show a pair of redox steps on glassy carbon electrode in 0.1 M H₃PO₄ with potential separation of 343 mV. Cationic, nonionic, and anionic surfactants micellar media significantly decrease the menadione peak potential separation. Statistically significant increase of menadione reduction current (3- and 4.4-fold) has been observed in Triton X 100 and sodium dodecyl sulfate (SDS) micellar media, respectively. Electrochemical reduction of menadione in 9 mM SDS micellar media is reversible diffusion-controlled one-electron process corresponding to formation of relatively stable semiquinone anion radical. The linear dynamic ranges of menadione determination are 7–560 and 600–2,550 μM with the limits of detection and quantification of 1.66 and 5.53 μM, respectively. The current concentration sensitivity is (8.6?±?0.2)?×?10³ μA μM^?1. The voltammetric method for the determination of menadione in pharmaceutical “Aekol” based on preliminary extraction with ethanol has been developed.     

Nitro radical anion formation from nitrofuryl substituted 1,4-dihydropyridine derivatives in mixed and non-aqueous media

Three new nitrofuryl substituted 1,4-dihydropyridine derivatives were electrochemically tested in the scope of newly found compounds useful as chemotherapeutic alternative to the Chagas' disease. All the compounds were capable to produce nitro radical anions sufficiently stabilized in the time window of the cyclic voltammetric experiment. In order to quantify the stability of the nitro radical anion we have calculated the decay constant, k2. Furthermore, from the voltammetric results, some parameters of biological significance as E7(1) (indicative of in vivo nitro radical anion formation) and KO2 (thermodynamic indicator of oxygen redox cycling) have been calculated. From the comparison of E7(1), KO2 and k2 values between the studied nitrofuryl 1,4-DHP derivatives and well-known current drugs an auspicious activity for one of the studied compounds i.e. FDHP2, can be expected.     

Electrocatalytic Reduction of Nitrite by Phosphotungstic Heteropolyanion. Application for Its Simple and Selective Determination

Three couples of reversible redox peaks of the PW₁₂O₄₀^3? (PW₁₂) anion, which are composed of two one‐electron and one two‐electron processes occur in the potential range from +0.25 to ?0.7 V in aqueous solutions. The electrocatalytic reduction of nitrite has been studied by the first redox couple of the PW₁₂ anion at the surface of a carbon paste electrode. Cyclic voltammetric and chronoamperometric techniques were used to investigate the suitability of PW₁₂ anion as a mediator for nitrite electrocatalytic reduction in aqueous solution with strongly acidic concentration of H₂SO₄. Results showed that H₂SO₄ 1.00 M is the best medium for this purpose. In the optimum concentration of H₂SO₄, the electrocatalytic ability about 500 mV can be seen and the homogeneous second‐order rate constant (k_s) for nitrite coupled catalytically to PW₁₂ anion was calculated as 2.52×10³ M^?1 s^?1 using the Nicholson–Shain method. According to our voltammetric experiments, the catalytic reduction peak current was linearly dependent on the nitrite concentration and the linearity range obtained was 3×10^?5 to 1.00×10^?3 M. The detection limit has been found to be 2.82×10^?5 M (2σ). This method has been applied as a selective, simple, and precise method for determination of nitrite in real samples.