Yurij I. Kharkats

Quantum chemical modelling of the heterogeneous electron transfer: from qualitative analysis to a polarization curve ☆

State-of-the-art in the field of quantum chemical modelling of the heterogeneous electron transfer processes is reviewed. Novel approaches originating from interplay between quantum chemistry and modern theory of charge transfer are discussed and illustrated by recent results on the calculation of relevant kinetic parameters for various electrochemical systems. Emphasis is made on the modelling of the inner-sphere reorganization and the works of approach, as well as on the consideration of reaction layer as an orientational ensemble of reagents. Recent approaches to the estimation of electronic transmission coefficient are analyzed. A possibility to employ traditional phenomenological theory to the analysis of experimental data is re-examined in the framework of microscopic treatment.

Effect of magnetic fields on convection in solutions containing paramagnetic ions

The objective of this study is to develop a mathematical model for the effect of magnetic fields on the corrosion process at an iron electrode in a solution containing paramagnetic and diamagnetic cations, Co2+ and Fe2+, respectively, and diamagnetic anions SO42−, and to verify the mathematical model experimentally. The hypothesis is that, in the presence of a magnetic field in solution, where a gradient of concentration of paramagnetic ions exists due to some electrode reaction in which they participate, an additional driving force acting on the solution will arise. This force, which has the same direction as the gradient of the paramagnetic ions, will cause a redistribution of velocities in the diffusion layer. As a result an additional convective transport of all the components of the solution will be generated. Correspondingly, limiting currents of reactions proceeding in an electrochemical system will become a function of the applied magnetic flux density. Experimentally, the study is carried out by potentiostatic and open-circuit potential methods, and the results obtained show an anodic polarization due to the magnetic field. Both the mathematical model developed and the results obtained experimentally, show that the magnetic field effect increased with increasing magnetic flux density and electrolyte concentration, and decreased with increasing stirring rate. The results show no effect due to the rate of change of the applied magnetic field, which indicates that the effect obtained cannot, apparently, be explained in terms of the Lorentz force.

Simple dielectric image charge models for electrostatic interactions in metalloproteins

An investigation has been made of the electrostatic potential from an excess point charge near dielectric interfaces by the method of image charges. Two configurations were considered: one is a planar slab of low constant dielectric permittivity enclosed between two infinite regions of larger constant dielectric permittivity, and the other is a dielectric sphere of low constant dielectric permittivity embedded in an infinite region of larger constant dielectric permittivity. The systems are representative of charges near bilayer membranes and globular proteins, respectively. The potentials reduce to simple analytical forms in these configurations. The notion of an ‘effective dielectric permittivity’ eeff is useful, with characteristic distance and orientation dependence caused by the image charge distributions. eeff extends from values between the two constant intrinsic dielectric permittivities, but always larger than the lower of the two, to values which exceed significantly the value in the embedding r...

The electrostatic Gibbs energy of finite-size ions near a planar boundary between two dielectric media

Abstract We have calculated analytically the electrostatic Gibbs energy profile for transfer of an ion between two bulk dielectric phases separated by a planar boundary. The profile incorporates the finite ionic size and dielectric image interactions. There is no discontinuity as the ion traverses the boundary and the model is well represented by a simple point charge when the ionic centre is beyond about half an ionic radius from the boundary. The profile is in line with observations based on interfacial inner-layer capacitances that ions can partly penetrate the boundary between two immiscible liquids at equilibrium. It represents an inner-layer capacitance contribution when the distance of closest approach is different for cations and anions.

Dielectric image effects in environmental reorganization free energies and inter-reactant work terms of metalloprotein electron transfer reactions

Abstract Kinetics of electron transfer between redox metalloproteins and small inorganic reaction partners has become a powerful tool for investigations of protein electron transport. We introduce here a model for metalloprotein electron transfer which incorporates essential features omitted in previous approaches to metalloprotein electron transfer data analysis. The protein is represented by a spherical region of low dielectric constant, with a conducting sphere excentrically located inside the protein simulating the metal centre. A conducting sphere outside the protein represents the small reaction partners, and the whole system is embedded in a dielectric solvent. The inter-reactant work terms and overall protein and solvent reorganization free energy for this model have been calculated. It appears that dielectric image interactions for multiply charged small reactants are important and comparable to interactions with both the protein surface charges and the solvent. The character of work terms and reorganization free energies for proteins is thus different from those of small ionic reactants. Cross relations and other frames where these features are disregarded should therefore be used with care for protein electron transfer.

Resonance effects in three-centre long-range electron transfer

Abstract Long-range electron transfer is usually treated by coupling donor and acceptor states perturbatively to high-energy intermediate states (superexchange). This needs modification when intermediate-state energies approach the initial—final state crossing. We present a formalism for three-level systems with strong electronic—vibrational coupling in the three states and covering all intermediate-state energies. The “normal” superexchange form is recovered at high energies. Finite resonance appear as the intermediate state traverses the initial—final state crossing. Still lower energies cause intermediate-state population but the process differs from sequential electron transfer due to intermediate-state vibrational nonequilibrium and electronic coupling to both donor and acceptor.

Sequential and coherent long-range electron transfer close to resonance with intermediate bridge groups, and new perspectives for in situ scanning tunnelling microscopy of adsorbed metalloproteins

Abstract Three-level electron transfer follows superexchange patterns when the intermediate electronic level is off-resonance with the donor and acceptor levels. Close to resonance, new patterns emerge where the intermediate level is temporarily populated in vibrationally coherent or incoherent modes. We discuss energy and distance relations associated with such electron transfer modes. These appear to accord with fast electron transfer in several chemical and biological systems. We also discuss some recent observations on in situ scanning tunnelling microscopy of metalloproteins and large transition metal complexes which enable, in principle, a distinction between superexchange, coherent, and sequential three-level electron transfer.

PROPERTIES OF POLARIZATION CURVES FOR ELECTROCHEMICAL CELLS DESCRIBED BY BUTLER-VOLMER KINETICS AND ARBITRARY VALUES OF THE TRANSFER COEFFICIENT

Theoretical investigation of potentiostatic electrolysis of a metallic salt in three component electrolyte solution was carried out for a cell consisting of two identical parallel electrodes. Analytical and numerical results are given for polarization curves for electrochemical cells with arbitrary values of trans- fer coefficient LY and exchange current density. Theoretical analysis of the electrodiffusion problem, based on an exact solution of the Nernst-Planck equations with boundary conditions of Butler-Volmer type, led to a formula for the polarization curve that is similar to the Tafel equation but with an effective transfer coefficient αeff = a(l - α). It was shown that, under certain conditions, the polarization curve can have two inflection points.

A view of fast charge separation in the bacterial photosynthetic reaction centre involving weak environmental damping of a populated auxiliary chlorophyll state

Abstract The initial fast electrical charge separation in the bacterial photosynthetic reaction centre involves electron transfer (ET) from the photo-excited special pair chlorophyll to pheophytin. The ET is assisted by the auxiliary chlorophyll monomer which may be populated in the sub-picosecond time range. We provide a new theoretical description of this process which rests on three-level coherent ET theory, incorporating environmental damping and partial vibrational relaxation in the intermediate state of auxiliary chlorophyll population. If damping is not too strong, this view offers a clue to the ultrafast, activationless and possibly coherent character of both steps, also in the limit when the electronic coupling between the three molecular entities is weak.

A simple approach to the solvent reorganization Gibbs free energy in electron transfer reactions of redox metalloproteins

Abstract We discuss a simple model for the environmental reorganisation Gibbs free energy, E r , in electron transfer between a metalloprotein and a small reaction partner. The protein is represented as a dielectric globule with low dielectric constant, the metal centres as conducting spheres, all embedded in a continuous solvent with high dielectric constant. This model has previously reproduced thermodynamic metalloprotein properties. E r takes an analytic form expressed by a continuous image charge distribution, and depends weakly on the globular radius, indicating that repolarisation at the globule/solvent interface contributes less to these effects than to the work terms and driving force.