Initial stages of anodic oxidation of silver in sodium hydroxide solution studied by potential sweep voltammetry and ellipsometry

The first stages of the oxidation of polycrystalline silver electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Ag₂O was found to be preceded by dissolution of silver species and deposition of a surface oxide. The equilibrium oxide coverage depended on the electrode potential and occurred within a few seconds. Surface oxide formation probably took place via a process of random electro-deposition. No ageing effect was observed in the chemisorbed layer.     

Convolution potential sweep voltammetry: Part IV. Homogeneous follow-up chemical reactions

The application of Convolution Potential Sweep Voltammetry to mechanism analysis and rate determination in electrochemical processes involving homogeneous chemical reaction is discussed. The formal analysis of the transition between pure diffusion control and pure kinetic condition is treated in the case of a first order follow-up reaction. The practical applicability of the method is then tested on the reductive pinacolization of acetophenone in acetonitrile using as operational parameters the sweep rate, the initial concentration and the water content of the medium.     

Electrochemical dissolution of chalcopyrite studied by voltammetry of immobilized microparticles

The characteristics of anodic electrochemical dissolution of chalcopyrite (CuFeS₂) powder in hydrochloric acid medium with sodium chloride have been studied. Cyclic voltammetry and chronopotentiometry of immobilized microparticles using paraffin-impregnated graphite electrode was employed. Present work is focused on electrochemical identification of chalcopyrite cathodic and anodic reaction products within the potential range of −0.7 to +0.8 V (vs. SCE) in hydrochloric acid solution containing sodium chloride and/or copper(II) chloride.     

Mechanism and exo-regioselectivity of organolanthanide-mediated intramolecular hydroamination/cyclization of 1,3-disubstituted aminoallenes: a computational study

The complete catalytic reaction course for the organolanthanide-assisted intramolecular hydroamination/cyclization (IHC) of 4,5-heptadien-1-ylamine by a prototypical [(eta(5)-Me5C5)2LuCH(SiMe3)2] precatalyst has been critically scrutinized by employing a reliable DFT method. A computationally verified mechanistic scenario for the IHC of 1,3-disubstituted aminoallene substrates has been proposed that is consistent with the empirical rate law determined by experiment and accounts for crucial experimental observations. It involves kinetically rapid substrate association and dissociation equilibria, facile and reversible intramolecular allenic C=C insertion into the Ln-N bond, and turnover-limiting protonation of the azacycle's tether functionality, with the amine-amidoallene-Ln adduct complex representing the catalyst's resting state. This mechanistic scenario bears resemblance to the mechanism that has been recently proposed in a computational exploration of aminodiene IHC. The unique features of the IHC of the two substrate classes are discussed. Furthermore, the thermodynamic and kinetic factors that control the regio- and stereoselectivity of aminoallene IHC have been elucidated. These achievements have provided a deeper insight into the catalytic structure-reactivity relationships in organolanthanide-assisted cyclohydroamination of unsaturated C-C functionalities.     

Voltammetry at partially covered electrodes: Part II. Linear potential sweep and cyclic voltammetry

Effect of inhomogeneity of the electrode surface on the linear potential sweep and cyclic voltammograms is investigated theoretically and experimentally using model electrodes partially covered with photoresist layer. Good agreement between the theoretical and experimental results is obtained.     

Normalized potential sweep voltammetry: Part III. Elimination of the effect of uncompensated Resistance and the application to heterogeneous kinetics of rapid charge-transfer processes

Uncompensated resistance (R_u) has a distoring effect on normalized potential sweep voltammetry (NPSV) slopes. This provides a simple and effective method to determine R_f^o, the value of the potentiostat feedback resistance necessary for full compensation. If the NPSV range is divided into overlapping segment, 1 and 2, corresponding to I_N of 0.20–0.50 and 0.50–0.80 respectively, the slopes m₁ and m₂ differ significantly when R_f differs form R_f^o. The difference, m₁-m₂, is negative for R_f<R_f^o and positive for R_f>R_f^o. Fine tuning of the potentiostat R_f setting so that the average value of the difference is the theoretical value can be accomplished in a minimum of time. Under these conditions, m₁ and m₂, as well as m_T, the slope of the entire correlation have very nearly the same values. Linear equations were derived from theoretical data which allow heterogeneous rate constants to be obtained directly from NPSV slopes. The precision in the NPSV slopes was observed to be of the order of ±0.002 which implies that the method should give reliable rate constants as great as 10cm s^? at a voltage sweep rate of 100V s^?1. The method is demonstrated using the reduction of benzonitrile and perylene in N,N-dimethylformamide and acetonitrile as examples.     

Microdetermination of double-stranded DNA by linear sweep voltammetry with phenosafranine.

A novel voltammetric method for the determination of microamounts of fish sperm double-stranded (ds) DNA based on its interaction with phenosafranine (PSF) is proposed in this paper. In a pH 3.5 Britton-Robinson (B-R) buffer solution, PSF had a well-defined second-order derivative linear-sweep voltammetric reductive peak at -0.32 V (vs. SCE) on a mercury electrode. After the addition of dsDNA into the PSF solution, the reductive peak current decreased significantly without a shift of the peak potential, and no new peak appeared. The experiment results showed that a new supramolecular complex was formed after the interaction of dsDNA with PSF, which resulted in a decrease of the diffusion coefficient, and then a decrease of the reductive peak current. The interaction conditions and the electrochemical detection conditions were carefully investigated. Under the optimal conditions, the decrease of the peak current was proportional to the dsDNA concentration in the range 1.0 - 40.0 microg/mL with the linear regression equation DeltaI(p)'(nA) = 32.59C(microg/mL) - 4.03 (n = 13, gamma = 0.998) and a detection limit of 0.25 microg/mL (3 sigma). The interaction mechanism was considered based on the aggregation of the dsDNA-PSF supramolecular complex; the stoichiometry of this supramolecular complex was calculated based on voltammetric data with a binding number of 3 and a binding constant of 2.76 x 10(12). This method was successfully applied to the determination of synthetic samples and the polymerase chain reaction (PCR) product of the nopaline synthase gene (NOS) DNA from genetically modified organisms (GMOs) with satisfactory results.     

Convolution potential sweep voltammetry V. Determination of charge transfer kinetics deviating from the Butler-Volmer behaviour

Convolution potential sweep voltammetry is applied to the analysis of charge transfer kinetics. The possibility of using the method without assuming a priori the validity of a given kinetic law, e.g. the Butler-Volmer law as usually done, is illustrated on the example of the reduction of tert-nitrobutane in acetonitrile and dimethylformamide. It is found that the post-factum experimental kinetics indeed deviate significantly from the Butler-Volmer behavior, involving a dependence of the transfer coefficient α upon the electrode potential. It is shown that the experimental variations of α match the orders of magnitude predicted by the Marcus theory. A procedure is proposed and applied for determining the reversible half-wave potential for redox couples in which one species is not quite stable chemically by combining the cyclic voltammetry and CPSV data.     

Electrochemical behavior of DNA at a silver electrode studied by cyclic and elimination voltammetry

Electrochemical characteristics of native and denatured calf thymus DNA have been studied by voltammetry on a silver electrode (AgE). Experimental results obtained from linear sweep or cyclic voltammetry (LSV or CV) have been employed in elimination voltammetry. The elimination voltammetry with linear scan (EVLS), using the linear combination of the total currents measured at different scan rates, enables one or two selected particular currents to be eliminated. The best results have been obtained by using a function eliminating the kinetic and charging currents (I(k),I(c)), and conserving the diffusion current (I(d)). This function makes it possible to increase significantly voltammetric signals of native and denatured DNAs, and to reveal processes not detectable by conventional electrochemical methods. The influence of electrochemical pretreatment of silver electrode surfaces and of starting and switching potentials on DNA voltammetric signals have been discussed. Silver electrodes coupled with elimination voltammetry represent promising tools for developing new nucleic acids biosensors.     

Electrochemical behavior of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives studied by cyclic voltammetry

This study demonstrates the electrochemical activity of selected 2-alkylidene-4-oxothiazolidine compounds containing different substituents at the C5-position on the Pt-electrode in 0.1 M TBAHFP + MeCN in the potential range from −2 V to 2 V. The mechanism of electrode processes is discussed. It is shown that a chemical step is involved in irreversible electrochemical processes. The effect of the substituent at the C5-position on reduction and oxidation potentials is also noted. The regions of a linear dependence of the current peak on the concentration of the compounds examined are determined. The text was submitted by the authors in English.     

Lewis acid-induced intramolecular Friedel-Crafts cyclization of 1,3-bis-exocyclic dienes. a new route to 4a-methyltetrahydrofluorenes

[reaction: see text] An efficient new method for constructing the 4a-methyltetrahydrofluorene skeleton was achieved via Friedel-Crafts intramolecular cyclization of 1,3-bis-exocyclic dienes. This strategy offers a simple and promising method for accessing complex structures.     

Mechanism of palladium-catalyzed diene cyclization/hydrosilylation: direct observation of intramolecular carbometalation

The results of kinetic, deuterium-labeling, and low-temperature NMR studies have established a mechanism for the palladium-catalyzed cyclization/hydrosilylation of dimethyl diallylmalonate (1) with triethylsilane involving rapid, irreversible conversion of the palladium silyl complex [(phen)Pd(SiEt(3))(NCAr)](+) [BAr(4)](-) [Ar = 3,5-C(6)H(3)(CF(3))(2)] (4b) and 1 to the palladium 5-hexenyl chelate complex [(phen)Pd[eta(1),eta(2)-CH(CH(2)SiEt(3))CH(2)C(CO(2)Me)(2)CH(2)CH=CH(2)]](+) [BAr(4)](-) (5), followed by intramolecular carbometalation of 5 to form the palladium cyclopentylmethyl complex trans-[(phen)Pd[CH2CHCH2C(CO2Me)2CH2CHCH2SiEt3](NCAr)]+ [BAr4]- (6), and associative silylation of 6 to release 3 and regenerate 4b.     

Heterogeneous electron-transfer kinetic parameters of metalloproteins as studied by channel-flow hydrodynamic voltammetry

Channel-flow hydrodynamic voltammetry was used to study the direct electron-transfer reactions of two metalloproteins, myoglobin and cytochrome c, under steady-state conditions at methyl viologen modified (MVM) gold foil electrodes. Utilization of a dual working electrode cell with this technique permitted determination of the heterogeneous electron-transfer kinetics for both the reduction and oxidation of myoglobin. The formal heterogeneous electron-transfer rate constants in pH 7.00 phosphate buffered solutions were found to be 8.9 (±1.5)×10⁻⁵ cm s⁻¹ for the reduction, and 7.7 (±1.2)×10⁻⁵ cm s⁻¹ for the reoxidation of myoglobin. The transfer coefficient values obtained were 0.21 (±0.01) for the reductive (α) and 0.82 (±0.01) for the oxidative (1−α) electrode reactions. Ionic strength and pH dependences were observed in these direct electron-transfer reactions. Collective current efficiency measurements in the myoglobin experiments indicated that an overall simple charge-transfer process occurred at the respective electrode interfaces. A formal rate constant of 3.4 (±0.2)×10⁻⁵ cm s⁻¹ with a transfer coefficient of 0.25 (±0.01) for the reduction (α) of cytochrome c was obtained by this hydrodynamic technique. The use of channel-flow hydrodynamic voltammetry in characterizing an electrode reaction as well as an interpretation of the data presented are discussed.     

Mass-spectrometric study of the products of intramolecular cyclization of 1,3-amido alcohols. 5-6-Dihydro-4H-1,3-oxazines and 2-oxazolines

The mass spectra of substituted 5,6-dihydro-4H-1,3-oxazines and 2-oxazolines were studied for the identification of the products of intramolecular cyclization of 1,3-amido alcohols. The fragmentation of the molecular ions of 1,3-oxazines under the influence of electron impact proceeds via both fragmentation of the retrodiene type and with the formation of rearrangement ions, the relative intensities of the peaks of which are determined by the nature and position of the substituents in the heteroring. The molecular ions of 2-oxazolines undergo fragmentation chiefly with the loss of a molecule of a ketone.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1539–1543, November, 1988.     

Hydrodynamic voltammetry at channel electrodes: Part VIII. Theory of reversible voltammograms for chronoamperometry and linear sweep voltammetry

An expression for the reversible transient current at a channel flow electrode was derived when a potential with any time variation was applied to the electrode. First, the expression was adapted to chronoamperometry. As the electrolysis time elapses, the current density distribution varies from a Cottrellian uniform distribution to a non-uniformly steady-state distribution. Second, the expression for the reversible linear sweep voltammogram was derived. For small values of a dimensionless potential sweep rate, the voltammograms are in a sigmoidal form similar to the steady-state curve. As the values become large, the voltammograms have a peak which is often observed in quiescent solution. The dependence of the peak current and the potential at the half peak current on the dimensionless potential sweep rate was examined.