Yu.I. Kharkats

A theory of electrode reactions through bridge transition states; bridges with a discrete electronic spectrum

Summary A theory has been developed for outer-sphere non-adiabatic electron transfer reactions at electrodes, to or from which an electron is transferred via ionic or molecular bridge species adsorbed on the electrode surface. Assuming a discrete energy spectrum for the discharging species and for the bridge, and neglecting interaction between adsorbed bridges, current-voltage relationships have been calculated for all overvoltages, and it is shown that for suitable relative positions of the electronic terms, the rate of the bridge-assisted electron transfer is higher than that of the direct electron transfer. Experimental data for various systems have been considered, and a semiquantitative agreement with the theory is found for the reduction of [Fe(CN)6]3−.

Theory of homogeneous reactions involving proton transfer

Abstract A quantum-mechanical theory for proton transfer processes in solution is given. The Bronsted rule and isotope effect for these processes are also discussed.

Ion transfer through a phase boundary: a stochastic approach

Abstract The kinetics of the transfer of heavy particles (ions) through the interface of two ion-conducting condensed media have been investigated theoretically in the framework of a stochastic approach. Expressions have been derived for the exchange current and for the relation between the current and overpotential. The difference between these expressions and those of the theory of absolute reaction rates has been analysed. The dependence of the ion current on the overpotential has been found to differ from the traditional expression used in the case of electron transfer because the real potential barrier near the top is smoothed and its maximum therefore suffers an additional shift in an electric field. The deviation of the barrier shape near the top from a parabola has also been taken into consideration.

Asymmetry of inner-sphere reorganization energy for heterogeneous electron transfer

The approach to the calculation of the activation energy of electrochemical charge transfer is considered for systems characterized by asymmetry of inner-sphere reorganization (i.e. by the difference in inner-sphere reorganization energies for direct and reverse reactions). Model calculations are carried out and the region of parameters is evaluated for which the effect is pronounced. A quantum-chemical analysis of inner-sphere reorganization is presented for four cobalt and chromium chelate aminocarboxylate complexes and the corresponding contribution to activation energy is discussed in connection with the difference in rate constants.

A theory of electrode reactions through bridge transition states bridge with a quasicontinuous energy spectrum

Summary A theory is developed for outer-sphere non-adiabatic electron transfer reactions at electrodes to or from which an electron is transferred via ionic or molecular species adsorbed on the surface. Current-voltage curves have been calculated for the case where the coverage of the electrode necessitates taking account of interaction between the adsorbed species. This is done by assuming that the participating energy levels of the adsorbed ions form a two-dimensional surface energy band in which the electron is delocalized in the intermediate virtual state.

Theory of the exaltation effect and the effect of correlation exaltation of migration current

Abstract A theory has been developed for the effect of migration current exaltation (increase of the limiting current of cation reduction due to a conjugated reaction of reduction of a neutral substance). The effect of correlation exaltation of currents observed during parallel reactions of cation reduction in the absence of an indifferent electrolyte is also considered. The theory of these effects is based on the analysis of the corresponding sets of electrodiffusion equations. The obtained expression for exaltation current differs from the result obtained by the approximate method of Heyrovsky, in which a linear relationship is postulated between the migration current and total current and which does not account for the important role played by reduction products of the neutral substance in migration transfer.

Transitions in superionic crystals

Abstract A theory of phase transitions in solid electrolytes is developed. Using mean field approximation the expression for the free energy and the equation of state are analysed and shown to be determined by two dimensionless parameters which depend on the properties of the crystal. The parameter plane has eight characteristic regions. In two regions phase transitions are absent. Single transitions occur in four regions. The specificity of unsymmetrical systems is revealed in the remaining two regions in both of which two phase transitions take place. Expressions for the temperatures of the corresponding transitions are presented. It is shown that with the increase of temperature the degree of cation disorder may either decrease or increase. In symmetrical systems double transitions are absent. Comparison of the theory with experimental data is presented.

Estimation of the free energy of activation for electron transfer reactions involving dipolar reactants and products

Summary An expression for the free energy of activation for electron transfer reactions involving dipolar reactants and products is derived on the basis of the Marcus-Hush theory, the dipolar species being represented as two adjoining spheres. The estimated and experimental free energies of activation for electron transfer to representative substituted nitro-compounds are compared and discussed with respect to location of the reaction site, orientation of the reactant, and imaging effects.

On the temperature behaviour of an adsorption system described by a Frumkin-type isotherm

Abstract A theory is developed describing stepwise coverae changes with temperature in a system of adsorbed species interacting on the surface. The temperature patterns of coverage is shown to be significantly more interesting and variegated than the concentration pattern. The existence of double transitions in the system is predicted and the conditions for their realization are analyzed.

The effect of a low dielectric constant interlayer on the current—voltage relationship for simple electrode processes

Abstract We have investigated the effects of a thin, structureless dielectric layer between a metal electrode and a surrounding aqueous solution on the electrostatic work terms, the solvent reorganization energy and the rate of simple electrochemical processes. This layer represents the compact double-layer or a region enriched in the organic component in mixed aqueous—organic solvents. Close to the electrode the work terms increase due to less favourable solvation. This effect is, however, only significant for very small ions. The solvent reorganization energy first increases due to resolvation of an infinite manifold of image charges in addition to the real ion. As the ion crosses the boundary to the interlayer, the solvent reorganization energy drops drastically due to the low polarity of this region. As a result, electron transfer from the contact distance of the ion to the metal surface, i.e. inside the interlayer, is always more favourable than electron transfer across the layer.