Electron Correlation Effects for Adiabatic Electrochemical Reactions of Electron Transfer in a Model for an Electrode with an Infinitely Wide Conduction Band: Computing Adiabatic Free Energy Surfaces

The adiabatic free energy surfaces for adiabatic electrochemical reactions of electron transfer are calculated in a model for an electrode with an infinitely wide conduction band with exact allowance for electron–electron correlations. The surfaces are analyzed on the basis of a diagram of kinetic modes obtained earlier. It is shown that, as in a surface-molecule model for these reactions, the correlation effects play an essential role and lead to a considerable decrease in the activation free energies and to qualitatively different forms of adiabatic free energy surfaces in certain ranges of model parameters.     

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: General Relationships

Expressions for the calculation of the adiabatic free energy surfaces (AFES) and average number of electrons in the valence orbital of the reagent for adiabatic electrochemical reactions of electron transfer are obtained in terms of exactly solved models for a metal electrode with nearly empty and nearly filled conduction bands. The models are extreme cases of the Anderson model, which account exactly for the electron–electron correlation effects. In particular, the electrode model with a nearly filled conduction band can be applied to transition metals of Group VIII in the periodic table. Exact relationships connecting AFES and diagrams of kinetic modes (DKM) for electrodes with symmetric position of Fermi levels relative to the conduction band center are obtained. Two characteristic functions for analyzing the role of electron–electron correlation effects in the system under consideration are proposed and calculated. The results form a basis for calculating AFES and studying correlation effects in adiabatic electrochemical reactions of electron transfer and constructing DKM that would correspond to different electron transfer modes.     

Effect of electron-electron correlations on the activation free energy of adiabatic electrochemical reactions of dissociative electron transfer

Adiabatic free energy surfaces for adiabatic electrochemical reactions of dissociative electron transfer are calculated with exact allowance for the effects of electron-electron correlations in a model of an electrode with an infinitely broad conduction band. The role of correlation effects in these reactions is discussed. It is shown that, as in common adiabatic electrochemical reactions of electron transfer, correlation effects play a substantial role and lead to a considerable decrease in the activation free energies.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 412–418.Original Russian Text Copyright © 2005 by Kuznetsov, Medvedev, Sokolov.     

Interfacial electrochemistry of cytochrome c at tin oxide,indium oxide,gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10^?2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.     

Reversibility of electrode reactions involving organic compounds

A general empirical approach allowing one to describe the kinetics and evaluate the mechanism of the electrode electron transfer reactions is offered. The approach is based on the electrode potentials, the vertical ionization potentials (oxidation), and the affinity to electron (reduction). An equation linking kinetic and thermodynamic parameters is derived. Electrode reactions involving organic compounds are discussed in polarographic terms. The conclusion is drawn that most electron transfer reactions involving organic compounds are reversible, and that the irreversibility of the net electrode reaction is due to the irreversibility of subsequent chemical and electrochemical stages. An experimental observation of the slow electron transfer is possible in the cases of a substantial reorganization of molecules in the presence of fast subsequent chemical and electrochemical reactions.     

Hydrogen Evolution on a Mercury Electrode: The Effect of a Condensed Adsorption Layer

The electroreduction of hydrogen on a dropping mercury electrode with and without condensed adsorption layers (CAL) of organic compounds is studied by methods of classical polarography and laser photoemission. An analysis of effects CAL has on some reactions reveals that the CAL influence on the first-electron transfer and on reactions involving intermediates can retard or even completely block the process. The most profound changes occur in the processes that involve a proton donor. Possible reasons for different effects of CAL on the electron transfer processes are discussed.     

Electron Correlation Effects for Adiabatic Electrochemical Reactions of Electron Transfer in a Model for an Electrode with an Infinitely Wide Conduction Band: Diagrams of Kinetic Modes

For adiabatic electrochemical reactions, within the framework of a model for an electrode with an infinitely wide conduction band, exact results are obtained for critical regions that correspond to different possible types of electron transfer processes (transfer of a single electron with and without an intermediate state, simultaneous transfer of two electrons) and regions that correspond to electroadsorption of the reactant in certain charge states. These regions form a diagram of kinetic modes (DKM) in the space of model parameters. Analytical expressions for outermost curves of DKM are obtained for some extreme cases. For the general case of a model for an electrode with an infinitely wide conduction band, a DKM is constructed and investigated with exact allowance for the effects of electron–electron correlations.     

Effects of Electron Correlations in a Simple Model for Adiabatic Electrochemical Reactions: The Application to Particular Reactions

Adiabatic free-energy surfaces (AFES) for some typical electrode processes are calculated in the framework of the surface-molecule model for adiabatic electrochemical reactions of electron transfer previously suggested by the authors. The surfaces are analyzed using the proposed diagrams of kinetic modes. It is shown that correlation effects play a substantial role in the reactions and not only considerably diminish the free energy of activation but also lead to qualitatively different shapes of AFES in some regions of modeling parameters.     

Effects of Electronic Correlations for Adiabatic Electrochemical Reactions of Electron Transfer in Electrode Models with Nearly Empty and Nearly Filled Conduction Bands: Adiabatic Free Energy Surfaces

Adiabatic free energy surfaces (AFES) for adiabatic electrochemical reactions of electron transfer (ARET) are computed with exact allowance for electron–electron correlation effects (EECE) in models of electrode with nearly empty and almost filled conduction bands and analyzed on the basis of a diagram of kinetic modes obtained earlier. The EECE role in ARET for an electrode with an arbitrary Fermi level in a conduction band of an arbitrary width is discussed. In the general case, allowing for EECE gives at some model parameters results other than for the Fermi level coinciding with the conduction band center (model of a surface molecule, MSM). As in the case of MSM considered previously, EECE considerably reduce activation free energies and at some model parameters give qualitatively different AFES.     

Electron Correlations for Adiabatic Electrochemical Electron Transfer Reactions in a Model for an Electrode with an Infinitely Wide Conduction Band

Fresh general relationships for adiabatic free-energy surfaces (AFES) and corresponding diagrams of kinetic modes for adiabatic electrochemical electron transfer reactions are derived in the framework of an exactly solvable model for a metallic electrode with an infinitely wide conduction band. The model is a limiting case of the Anderson model applicable to the sp metals. In contrast to earlier studies of adiabatic reactions in a model for an electrode with an infinitely wide conduction band, this work accounts for the electron–electron correlation effects exactly. As an illustration, an AFES is calculated and a diagram of kinetic modes is constructed for a special case corresponding to the equilibrium electrode potential of a two-electron reaction. The exact AFES is compared with the AFES computed in the Hartree–Fock approximation and a spinless model. The correlation effects are shown to play a substantial role and lead to a considerable decrease in the activation free energy.     

Electron transfer at different electrode materials: Metals,semiconductors, and graphene

Electron transfer reactions are the most important processes at electrochemical interfaces. They are determined by the interplay between the interaction of the reactant with the solvent and the electronic levels of the electrode surface. Theoretical treatments only based on Density Functional Theory calculations are not sufficient. This review emphasizes mainly the effect of the electronic structure of the electrode material on electron transfer under different kinetic regimes. Our goal is to understand experimental results in the framework of a theory valid for arbitrary strengths of electronic coupling.     

Electrochemical approach to the development of a photoelectrode on the basis of photosynthetic reaction centers

Experimental approaches to the coupling of photoinduced charge separation in reaction centers (RCs) of the photosynthetic bacterium Rhodobacter sphaeroides R-26 with electron transfer to an electrode are discussed. Exogenous quinones are used as an electron transfer mediator. With the use of photopolarography it is shown that water-soluble ubiquinone can serve as a diffusionally mobile mediator of electron transfer. Some methods of quinone immobilization at an electrode have been developed to obtain a non-diffusional mediator of electron transfer. Quasi-reversible electrochemical kinetics were observed for aminonaphthoquinone immobilized as a monolayer at a Pt electrode. The ubiquinone-depleted RCs were subjected to affinity immobilization at these chemically modified electrodes probably due to the insertion of the immobilized quinone into the primary quinone Q_A binding site. The quantum efficiency of photocurrent formation was ca. 5% for the photoelectrode obtained. The electrochemical process of the immobilized quinone is shown to be the stage that limits electron transfer from RCs to the electrode.     

Direct electrochemical redox of tyrosinase at silver electrodes

The direct electron transfer reactions between tyrosinase and silver electrode were investigated by using cyclic voltammetry and potential-step chronoamperometry as well as current-step chronopotentiometry techniques. The kinetics of these reactions is quasi-reversible with two electron transfer reactions and 0.030 s(-1) apparent electrode reaction rate constant. The results demonstrate that neither electrode surface modification nor the inclusion of mediators is necessary to study the electron transfer reactions of tyrosinase at silver electrodes. Moreover, both the anodic and the cathodic currents are linear relationship with the tyrosinase concentration in the range of 1 x 10(-9) approximately 5 x 10(-8)moll(-1). It is possible to be used as a method of analyzing tyrosinase concentration.     

Synthesis,spectrophotometric, voltammetric,and density functional theory studies of tetrahydro[3,2-b]indolocarbazoles for sensing small molecules

Absorption spectra of tetrahydro[3,2-b]indolocarbazoles (THICZs) with respective molecular property and alkyl substituents have been prerecorded in diverse solvents in the scope between 200 and 600 nm for very fewer times in the literary study. The photo-physical behavior of liquified THICZs belonging to the existence surroundings. The solvatochromic behaviors of THICZs and solvent substance physical phenomenon can be analyzed by implementation of linear solvating energy state conception. Compound 4 shows excellent properties for sensing small molecules. The electrochemical behavior of some THICZs was investigated at carbon paste electrode where two electrode reactions were involved, irreversible oxidation-one electron transfer and quasi-reversible redox reactions forming phenolic followed by quinolone moiety electro active species. The density functional theory which means calculated molecular orbital energies (B3LYP/6-31G) and HOMO-LUMO gap/space for some presented indolocarbazoles have been performed.     

Quantum effect of intramolecular high-frequency vibrational modes on diffusion-controlled electron transfer rate: from the weak to the strong electronic coupling regions

The Sumi-Marcus theory is extended by introducing two approaches to investigate electron transfer reactions from weak-to-strong electronic coupling regime. One of these approaches is the quantum R-matrix theory, useful for dealing with the intramolecular vibrational motions in the whole electronic coupling domain. The other is the split operator approach that is employed to solve the reaction-diffusion equation. The approaches are then applied to electron transfer in the Marcus inverted regime to investigate the nuclear tunneling effect on the long time rate and the survival probabilities. The numerical results illustrate that the adiabatic suppression obtained from the R-matrix approach is much smaller than that from the Landau-Zener theory whereas it cannot be predicted by the perturbation theory. The jointed effects of the electronic coupling and solvent relaxation time on the rates are also explored.     

Initiation of cationic polymerization by photoinduced electron transfer

The objective of this paper is to discuss: (i) the general approaches to the initiation of cationic polymerization by photinduced electron transfer reactions (ii) the use of photoinduced electron transfer reactions for block copolymer synthesis. For the first, it is concluded that three general methods are currently available which involve reduction of onium salts by (a) photogenerated radicals, (b) photoexcited sensitizers or (c) electron donor compounds in charge transfer complexes. According to this view, a variety of initiating systems are discussed. For the second, recent developments on the application of photoinduced electron transfer reactions to the synthesis of block copolymer of monomers polymerizable with different mechanisms are presented.