V. A. Kurmaz

Mechanism of electroreduction of intermediates with and without a proton donor

Electrochemistry of various categories of intermediates was comparatively studied over wide range of electrode potentials, concentrations of proton donors (H3O+, NH4+), and temperatures. The suggested kinetic model considered two parallel pathways of electron transfer: either to an adsorbed intermediate (radical) or to the respective metastable complex with a proton donor. The electroreduction of all studied intermediates was found to obey the model. The formation of metastable complexes was shown to be facilitated for radicals having one or more functional groups. The particular reduction pathway is determined predominantly by the difference between overvoltages of electron transfer to a radical and to its metastable complex with a proton donor under the same experimental conditions (i.e. nature and concentration of a proton donor, electrode potential).

Electrochemical Behavior of Electrodes Containing Nanostructured Carbon of Various Morphology in the Cathodic Region of Potentials

Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.

Electrochemical properties and reactivity of organonickel sigma-complex [NiBr(Mes)(bpy)] (Mes = 2,4,6-trimethylphenyl, bpy = 2,2′-bipyridine)

Electrochemical properties and reactivity of electrochemically activated forms of organonickel sigma-complex [NiBr(Mes)(bpy)] (where Mes = 2,4,6-trimethylphenyl, bpy = 2,2′-bipyridine) were studied. The activation of the organonickel sigma-complex was found to proceed under both electrochemical reduction and oxidation conditions to give coordinatively unsaturated forms of the complex: radical [Ni(Mes)(bpy)]• and cationic complex [Ni(Mes)(bpy)]+, respectively. It was shown experimentally that the active forms of organonickel complex [NiBr(Mes)(bpy)] can react with organic substrates (cyclohexene, octene-1, tetrahydrofuran) and convert nitriles (acetonitrile, acetonitrile-d3, chloroacetonitrile) into corresponding imines containing 2,4,6-trimethylphenyl fragment.

Thermodynamic and kinetic characteristics of intermediates of electrode reactions. Comparative laser photoemission study of the kinetics of electron transfer for certain alkylaryl and alkylhalide radicals

Using the laser photoemission technique, a comparative study of thermodynamic and energy characteristics of electron transfer (ET) is carried out for a number of alkylaryl intermediates (IM) (benzyl, benzhydryl radicals) and alkyl halides (chloromethyl and dichloromethyl radicals). It is found that the standard free energies of activation of radicals under study are 0.34–0.38 eV and the preexpoential factors are between 109 and 1010s−1 and weakly depend on the solution nature. The reorganization energies of the medium for these IM are estimated in terms of the classical Marcus theory. The results are compared with the literature data on the dissociative ET in monohalogenomethanes CH3Hal/CH3Hal−·, polychloromethanes CHnHal4 − n/CHnHal4−n−·, and some other systems. Possible reasons for the different probabilities of observing reversible ET for IM under study are discussed.

Study of the reactivity of organonickel sigma-complexes towards nitriles

The reactivity of organonickel sigma-complexes of type [NiBr(Ar)(bpy)], where Ar = 2,6-dimethylphenyl (Xyl), 2,4,6-trimethylphenyl (Mes), 2,4,6-triisopropylphenyl (Tipp), 2,4,6-tricyclohexylphenyl (Tchp), bpy = 2,2´-bipyridine, towards nitriles (acetonitrile, propionitrile, chloroacetonitrile, benzonitrile) has been investigated. This reaction leads to imines by formation of new carbon—carbon bond between aromatic fragment and nitrile group C≡N.

The elementary stages of electroreduction of alkyl(hetero)arylsulfones in water-organic media

Intermediates (IM) of methyl(2-pyridyl)sulfone (MPS) and tert-butyl(2-pyridyl)sulfone (TBPS) formed upon the transfer of the first electron are studied by methods of laser photoelectron emission (LPE). The capture constants of solvated electrons by the MPS and TBPS molecules were determined. The time-resolved voltammograms of each sulfone measured in water-organic mixtures (20–80% ethanol or 60% DMSO) are found to demonstrate a redox wave with half-wave potentials E1/2 = −1.34 and −1.37 V for MPS and TBPS, respectively. The dependence of rate constants for the one-electron IM reduction and oxidation on the potential is shown to obey the slow-discharge equation and the absolute magnitudes of rate constants are determined. The characteristic times of homogeneous transformations (decomposition, protonation) of MPS and TBPS radical anions do not exceed 3 × 10−7 s. The LPE data are compared with the results of preparative electrolysis and the mechanisms of both electrochemical and homogeneous reactions of IM are discussed.

Generation of phenylmethylketyl radicals in aqueous solutions and study of their transformation

Acetophenone and its intermediates formed upon the first electron transfer are studied by laser photoemission and traditional electrochemistry. It is shown that the intermediate reduction is affected by competition of the reactions of the formed radical-anions oxidation and their subsequent transformation to secondary products that are rapidly reduced at the electrode. From the comparison of the data obtained by the laser photoemission method and electrochemical measurements a conclusion was drawn that the product is a metastable complex (associate) of the radical-anion with water molecule; its formation rate constant is rather low (∼6 × 103 M−1 s−1). It was also concluded that bulk radical reactions dominate in aprotic media at moderate cathodic potentials; the acetophenone radical-anion is reduced at E ≤ −1.9 V (SCE). This conclusion agrees with the results of the acetophenone preparative electrolysis in DMFA, where marked yield of pinacon was observed at the potentials of limiting current of the 1st reduction wave, while the stage of 2nd electron transfer occurred at E ≤ −2.3 V (SCE).

Photoelectrochemical Behavior of Electrodes Containing One-Walled Carbon Nanotubes

Photoelectrochemistry of electrodes containing single-walled carbon nanotubes is studied for the first time ever. Photoemission of electrons from such electrodes illuminated by UV light is discovered. It is established that the photocurrent amplitude as a function of the imposed potential obeys the 5/2 law and that the work function into solution for nanotubes and a mercury electrode coincide to within 0.08 eV. Similarity of electrochemical behavior of intermediates on the nanocarbon and mercury electrodes is shown with the reduction of radicals ėCH2Cl as an example.

Effect of the EDL Structure on the Mechanism of Electroreduction of Adsorbed Intermediates

Effect of the Ψ1 potential on various kinetic modes of the β-hydroxyethyl radical reduction is studied using laser photoemission (LPE). A charged proton donor is found to make no impact on the electron transfer rate during the reduction of adsorbed radicals. Experimental results fit the model proposed earlier for the electroreduction of intermediates, which includes two parallel pathways for electron transfer—onto an adsorbed radical and onto a metastable complex radical–proton donor. Demonstrated is a complete analogy between time-resolved voltammograms for LPE-generated intermediates and classic polarograms.

Studying the dimerization reaction during the abstraction of halogen from organohalides by laser photoemission, controlled-potential electrolysis, and voltammetry

To continue earlier investigations into the dimerization reaction during the cathodic cleavage of a carbon-halogen bond and, in particular, to find an accessible way for synthesizing 1,4-butanediol, a comparative study of the dimerization of ethylene halohydrins and butyl and allyl halides is performed. On the basis of the data obtained by the laser photoemission (LPE), controlled-potential electrolysis, and voltammetry techniques, a general mechanism of the electrode reactions involving these compounds and their intermediates is proposed and recommendations on the optimization of the 1,4-butanediol synthesis are elaborated. According to LPE data, at pH < 8.1, the Β-hydroxyethyl radical reduction occurs with a preceding formation of a complex with a proton donor, whereas a direct electron transfer is characteristic of the butyl radical. This difference in mechanisms is offered as the main reason for the lesser capability of ethylene halohydrins to electrochemical dimerization as compared with butyl halides, where the octane yield reached up to 80–84%. The earlier assumption about a high electrocatalytic activity of the copper cathode in dimerization of ethylene halohydrins is confirmed, and possibilities of an iron cathode in this process are revealed. The dimer yield is found to increase in alkaline solutions and at lowered temperatures, specifically, at pH 11 and temperatures of 0–5°C., the 1,4-butanediol yield reached ∼17%.