Electron transfer kinetics study

Fiomogeneous cross-reaction electron-transfer kinetic studies suggest that many other Cu(II/I) systems obey Scheme 1. However, few Cu(II/I) systems have been subjected to sufficiently low temperature or rapid-scan CV measurements to demonstrate the presence of rate-limiting conformational changes. 

The material of this section will focus on results of homogeneous electron transfer kinetic studies involving reactions between electrochemically generated redox reactants and on direct heterogeneous electron transfer kinetic studies. An overview of the work in this area concludes this chapter. 

Homogeneous Electron Transfer Kinetic Studies of Biological Molecules by Electrochemical Techniques... 

Electron-transfer kinetics studied using SAMs self-assembled on a variety of noble metals in addition to gold Such studies will address issues associated with the density-of-states assumptions described in Sec. III.A [93] and will probe how the different orbitals of these metals contribute to the overall electronic coupling [116] as the coupling gets stronger (and the electron-transfer rate constant gets faster) with shorter or more effective (e.g., unsaturated) tethers. 

Substantial loss in sensitivity is expected for analytes with slow electron-transfer kinetics. This may be advantageous for measurements of species with fast electron-transfer kinetics in the presence of a species (e.g., dissolved oxygen) that is irreversible. (For the same reason, the technique is very useful for the study of electron processes.) Theoretical discussions on AC voltammetry are available in the literature (16-18). 

EPR studies on electron transfer systems where neighboring centers are coupled by spin-spin interactions can yield useful data for analyzing the electron transfer kinetics. In the framework of the Condon approximation, the electron transfer rate constant predicted by electron transfer theories can be expressed as the product of an electronic factor Tab by a nuclear factor that depends explicitly on temperature (258). On the one hand, since iron-sulfur clusters are spatially extended redox centers, the electronic factor strongly depends on how the various sites of the cluster are affected by the variation in the electronic structure between the oxidized and reduced forms. Theoret-... 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

In biological systems, electron transfer kinetics are determined by many factors of different physical origin. This is especially true in the case of a bimolecular reaction, since the rate expression then involves the formation constant Kf of the transient bimolecular complex as well as the rate of the intracomplex transfer [4]. The elucidation of the factors that influence the value of Kf in redox reactions between two proteins, or between a protein and organic or inorganic complexes, has been the subject of many experimental studies, and some of them are presented in this volume. The complexation step is essential in ensuring specific recognition between physiological partners. However, it is not considered in the present chapter, which deals with the intramolecular or intracomplex steps which are the direct concern of electron transfer theories. 

As part of a subsequent study concerning primarily second-site revertant yeast iso-l-cytochrome c variants, Hazzard et al. evaluated the effect of converting Lys-72 to an aspartyl residue by site-directed mutagenesis on the electron transfer kinetics of the cytochrome c-cytochrome c peroxidase complex [136]. Lys-72 was of interest for this purpose, because it is involved in the hypothetical model for the complex formed by these two proteins that was proposed by Poulos and Kraut on the basis of molecular graphics docking [106]. In these... 

In a recent upsurge of studies on electron transfer kinetics, importance was placed on the outer shell solvent continuum, and the solvent was replaced by an effective model potential or a continuum medium with an effective dielectric constant. Studies in which the electronic and molecular structure of the solvent molecules are explicitly considered are still very rare. No further modem quantum mechanical studies were made to advance the original molecular and quantum mechanical approach of Gurney on electron and proton (ion) transfer reactions at an electrode. 

A number of studies were done in order to determine which of these various factors contribute to the large peak separations observed here. First, it is well known that the effects of resistive elements can be obviated by applying positive feedback [132]. When positive feedback was applied to a thin-film confrol elecfrode similar fo fhaf described in Fig. 27, the peak separation decreased from 0.8 to —0.35 V (Fig. 28). These data show that resistance does, indeed, contribute to the large AEp values observed here. However, the fact that —0.35 V of this peak splitting cannot be removed by applying positive feedback clearly indicates that slow electron transfer kinetics also contribute to AEp. ... 

The STM has also been used to follow the evolution of surface-confined reactions such as the oxidation of adsorbed sulfide to form adsorbed Sg and iodide to polyiodide [275,288,289]. The substrate exerts a strong influence on the dimensions and ordering of the adsorbed molecules, particularly the formation of the first monolayer. In a similar manner, studies of the impact of different adlayer structures on the electron transfer kinetics of various soluble redox species have been initiated [290]. 

Recent structural studies and electron transfer kinetic experiments focus on structures in which a site-specific covalent crosslink between cytochrome c and cytochrome c peroxidase subunits exists. One of these used site-directed mutagenesis to form a disulfide bond between a V197C mutant CcP and an A81C... 

An alternative application of flash photolysis to study myoglobin electron transfer kinetics has been employed by Hofifinan and co-workers 156). In this approach, the photoactive zinc-substituted derivative of Mb is mixed with an equivalent amoimt of ferricytochrome bs to form an electrostatically stabilized binary complex. Upon transient irradiation, the strongly reducing Zn-Mb intermediate is formed, and the kinetics of ferricytochrome reduction within the preformed complex can be monitored spectrophotometrically. The resulting kinetics represents a mixed-order process consistent with electron transfer both within the electrostatically stabilized complex and between the dissociated components of the complex. 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

Morris studied the aqueous solution voltammetric behavior of some uranyl coordination complexes to learn how changes in the ligand environment influence the redox potentials and heterogeneous electron-transfer kinetic parameters for the single-electron transfer... 

Yamamoto also explored triphenylamine core dendrimers of the form Corei7-Rpti2-Periph15.125 In this system, the redox process studied was the oxidation of this core moiety. They showed that as the generation of the dendrimer increased from 1 to 4, the shape of the CV broadened, indicating slowing electron transfer kinetics. 

Recently, the electron-transfer kinetics in the DSSC, shown as a schematic diagram in Fig. 10, have been under intensive investigation. Time-resolved laser spectroscopy measurements are used to study one of the most important primary processes—electron injection from dye photosensitizers into the conduction band of semiconductors [30-47]. The electron-transfer rate from the dye photosensitizer into the semiconductor depends on the configuration of the adsorbed dye photosensitizers on the semiconductor surface and the energy gap between the LUMO level of the dye photosensitizers and the conduction-band level of the semiconductor. For example, the rate constant for electron injection, kini, is given by Fermi s golden rule expression ... 

From Eq. (9.6), mo D/d if L<1. This suggests the usefulness of SECM for the study of rapid heterogeneous electron transfer kinetics, the largest ks values that can be determined being of the order of D/d. The largest ks values measurable by voltammetry at UMEs are of the order of D/a. It is easier to get a small L value with SECM than to prepare a UME of very small a value. For example, ks for the Fc+/Fc couple in AN has been determined by SECM to be 3.7 cm s-1 [29]. 

The coordination chemist may be interested in the electrosynthesis of compounds, the generation and detection of unstable species in unusual oxidation states and the study of their mechanisms of decay or their spectroscopic properties, or in simply obtaining thermodynamic data. These, and related topics such as using electrogenerated metallo intermediates to catalyze the transformation of inert molecules, modifying the properties of an electrode surface by adsorbing or otherwise binding a coordination compound to it, or fundamental aspects of electron-transfer kinetics, are readily studied by the application of modem electrochemical techniques. 

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