53rd Meeting of the International Society of Electrochemistry

Russian Journal of Electrochemistry -     

Electron Transfer through Monolayers of Mixed Long-chain Aliphatic Acids and Alcohols

Properties of monolayers of cetyl, stearyl, and eicosyl alcohols and stearic and behenic acids and their mixtures are studied, along with the kinetics of a redox reaction of hemin adsorbed on these monolayers. Three-dimensional computer models of corresponding monolayers are constructed and compared with their electrochemical behavior. Properties of monolayers of cetyl alcohol in acid and alkaline solutions and stearyl alcohol in acid solutions correspond to package of molecules in a stretched conformation oriented normally to the electrode surface; for the other one-component systems, the monolayer structure is less regular. The hemin redox reaction rate has no correlation whatsoever with the monolayer thickness and is defined by the length of the electron transfer path (over a chain of covalent bonds and through van der Waals contacts between molecules). The stronger the deviation of the conformation of molecules from the stretched one, the shorter the effective path of the electron transfer. In mixed films of stearic acid and cetyl alcohol (1 : 6), it is sometimes possible to keep molecules of stearic acid in a stretched conformation, thus inhibiting the electron transfer along the hydrocarbon chain of this molecule to a maximum extent. The other mixed systems, while making the conformation more stretched, fail to provide for a completely regular structure.     

The surface potential of solvent and the intraphase pre-existing potential

Using own and literature data, the differences of real solvation energy for ferricenium and ferrocene in six solvents are found. These quantities are confronted with the calculated difference of the dielectric response energies plus nonelectrostatic energies for the redox couple. Such a comparison allows determining the sum of the surface and intraphase potentials. The comparison of these sums with the experimental values of the surface potential differences obtained by the measuring of Volta potentials allowed determining the differences of pre-existing intraphase potentials formed by solvent molecules on the ferrocene molecule. Thus, the intraphase potentials are evaluated for the first time, using an approach not based on the molecular-dynamic modeling. Using some approximations, the surface potentials of the studied solvents are found.     

Quantum chemical model of solvation for calculation of electrode potentials of redox processes involving ferrocene,cobaltocene, and their ions

Quantum chemical calculations of solvation energy for ferrocene and cobaltocene molecules and their ionic forms in water, acetonitrile, methanol, and acetone are performed in terms of the B3LYP density functional method by taking into account solvation effects and using the polarized continuum model (PCM). Standard electrode potentials of the corresponding redox pairs, the effect of solvent on them, and the overall energy of the transfer of cobaltocene cation and anion between two solvents are calculated. The calculation results well agree with the available experimental data. The present study provides sufficiently reliable grounds for the application of an ion—metallocene molecule redox pair as a pilot system for the comparison of electrode potentials and solvation energies in different solvents.     

Effect of the energy of zero vibrations and the entropy of reagents on the kinetic isotopic effect of the hydrogen evolution reaction

The effect the difference in zero energies and entropies of isotopic forms of reagents has on the difference in free energies of the elementary act of discharge of a hydronium ion for an initial equilibrium configuration of a reaction complex and equilibrium (ground) energy levels is considered. The analysis is performed for conditions of constancy of potential and constancy of overvoltage: only in the latter case one does not have to resort to extrathermodynamic assumptions. It is shown that differences in entropies of isotopic forms play a substantial role     

Free energy of charge transfer and intraprotein electric field: method of calculation depends on the charge state of protein at a given structure.

Free energy of charge transfer presents a basic characteristic of reactions such as protonation, oxido-reduction and similar. Evaluation of this quantity requires calculation of charging energy. Proteins are structured dielectrics, and a consistent incorporation of their structure into calculation of intraprotein electric field results in expression for charging energy of an active group in protein, which is essentially different from that for a simple dielectric. An algorithm for semi-continuum calculation of relevant free energies is described. First of the two components of charging energy in protein, energy of the medium response to charge redistribution in reactants, should be always calculated as the charging energy by the charge redistribution using the static dielectric constant of protein. The second term is interaction energy of the charge redistribution with the 'frozen' electric field of the system before reaction. Charges of protein groups, at which the protein structure has been determined, are often different from those before reaction of charge transfer, so is the corresponding intraprotein field. The field is expressed through either both the optical and static dielectric constants of protein or only optical one depending on whether the charges of protein groups before reaction and upon structural analysis are the same or not. Proper allowance for difference in charges of reacting groups before reaction and upon structural analysis of protein is thermodynamically necessary and quantitatively important. The expression for activation free energy for charge transfer in proteins is derived in the form presenting explicitly an invariant contribution of protein structure.     

From Editors of Special Issue

Russian Journal of Electrochemistry -