Electron transfer quasi-reversible

They show that the electron transfer is reversible or quasi-reversible, the deviation from reversibility remaining quite modest for scan rates from 10 mV s to 100 mV s h Although the reversibility of the ferrocene-ferricinium in all the... 

It follows from the above discussion that an electron transfer reaction that appears reversible at one (low) sweep rate may change to a quasi-reversible or even an irreversible process at higher sweep rates. This should be kept in mind since the application of LSV and CV in kinetics studies usually includes the recording of voltammograms at a number of different sweep rates (see below), and the analysis of the data is usually based on the assumption that the electron transfer is reversible. 

The ec scheme, which is a very common mechanism in organic electrochemistry, is described by Equations (6.17) and (6.18). The cyclic voltammogram observed depends on the relative rates of the two steps. The simplest situation is where the electron transfer is totally irreversible the presence of the chemical reaction has no effect on the voltammogram obtained and no kinetic data related to the chemical reaction can be derived. This situation leads to the properties in Table 6.2. Similar properties can also arise when the rate of the electron transfer step is relatively fast if the rate constant for the chemical reaction is very large. The full range of other possibilities where the chemical reaction can be reversible or irreversible and the electron transfer either reversible or quasi-reversible has been considered in detail by Nadjo Saveant [7], and the various kinetic zones have been identified. In this chapter the only case to be discussed in detail is that where the electron transfer is reversible and the chemical reaction is irreversible. 

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

The present chapter will cover detailed studies of kinetic parameters of several reversible, quasi-reversible, and irreversible reactions accompanied by either single-electron charge transfer or multiple-electrons charge transfer. To evaluate the kinetic parameters for each step of electron charge transfer in any multistep reaction, the suitably developed and modified theory of faradaic rectification will be discussed. The results reported relate to the reactions at redox couple/metal, metal ion/metal, and metal ion/mercury interfaces in the audio and higher frequency ranges. The zero-point method has also been applied to some multiple-electron charge transfer reactions and, wheresoever possible, these results have been incorporated. Other related methods and applications will also be treated. 

Conducted in 10% CH2Cl2-90% acetonitrile for compounds [54] and [56] and in acetonitrile [55] upon addition of 2 equiv of the respective cation supporting electrolyte, 0.10 mol dm-3 TBABF4. The potential of the reduction current peak r, reversible q, quasi-reversible s, single reduction peak without corresponding reoxidation peak ec, electron transfer followed by a chemical reaction ec, ad, electron transfer followed by a chemical reaction with insoluble product which adsorbs on to the electrode surface. Prewaves are in parentheses. 

It is commonly assumed that an electron transfer behaves quasi-reversibly when the standard rate constant lies within the values expressed as a function of the highest and lowest scan rates v ... 

It is clear that the decrease of the rate of the electron transfer operated by the temperature makes the oxidation of ferrocene become quasi-reversible for both the electrode materials. Moreover, it is noted that for both types of electrode the faradaic current increases with temperature. For both the electrodes the oxidation process is governed by diffusion, since in both cases the plot of log(/p) vs. 1/T is linear. Furthermore, one should note in particular that, contrary to the naive expectation, for the superconducting electrode one does not observe any abrupt change in the response upon crossing the barrier from superconductor (that should exchange pairs of electrons) to simple conductor (that should exchange single electrons). 

Usually, however, electron transfers at the electrode are denoted by E , while chemical steps not involving the electrode are denoted by C . The ET may further be characterized as Er , Eqr , or Ej in the reversible, quasi-reversible, or irreversible case. It is usually not indicated how transport occurs. If the C-step is a dimerization, the symbol D is common, while an ET between two species in a (homogeneous) solution is denoted SET (for solution electron transfer) [18] or DISP (see, e.g. [19]). 

Fig. 8 Typical cyclic voltammograms of pure electron transfer reactions (a) effect of quasi-reversibility ks decreases from solid to dashed line) (b) effect of relative values of...

Rubredoxin is an electron-transfer protein with an Fe(IlI)/Fe(lI) redox couple at -0.31 V (SCE) in water (20). Our peptide model, [Fe( Cys-Pro-Leu-Cys-OMe)2] (Z = benzyloxycarbonyl) (21) exhibits its Fe(lll)/Fe(ll) redox couple at -0.50 V (SCE) in Mc2SO (9). This is similar to the value observed for the native protein when the difference of the solvent is taken into account. When the model complex is solubilized in water by formation of micelles with addition of the non-ionic detergent, Triton X-KX), we also observed a quasi-reversible redox couple at -0.37 V (SCE) (5). The Fe(lll) complexes of Cys-X-Y-Cys peptides also exhibit a characteristic MCD band at 350 nm due to ligand-to-metal charge transfer which has also been found in oxidized rubredoxin (4). 

The two-electron reduction of both 3 and 4+ resulted in the elimination of a dtc ligand [23]. For these compounds, the two-electron reduction could be resolved in two separate one-electron steps. The first electron transfer for both 3 and 4+ led to partial dissociation of a dtc ligand nevertheless this step was quasi-reversible, as evidenced by the scan rate dependence of the peak-to-peak separation (AEp) [24] (Sch. 4). This was assigned to the fact that the decoordinated sulfur atom... 

Many electrochemical studies on Cu(II/I) systems exhibit irreversible or quasi-reversible behavior or involve coupled chemical reactions. There are no known examples of Cu(II/I) systems where irreversibility can be definitively attributed to slow electron transfer. However, many... 

In a few instances, binuclear complexes have been reported in which the two copper atoms interact directly through a metal-metal bond. Two examples of such complexes are illustrated below [192, 193]. CV s run on the compound at left in both tetrahydrofuran and CH2CI2 indicated the presence of two separate electron-transfer steps that were quasi-reversible. 

The Pu +/Pu + couple for a series of Pu(IV)/(EDTA ) based complexes, where EDTA = ethylenediaminetetraacetate, has been studied as a function of pH and EDTA concentration [118]. The voltammetry was also studied with citrate and carbonate ions present in solution. At a relatively low pH of 2.3 and equimolar Pu +/EDTA concentrations a quasi-reversible one-electron reduction is observed for Pu(EDTA) at E /2 = 0.342 V versus SHE. The quasireversibility of this process remains as the pH is raised to 4.6. Additional voltammetry studies are discussed in the paper for the higher coordinate Pu + species, Pu(EDTA)-L (where L = EDTA, carbonate, citrato), all of which show irreversible electron-transfer behavior. 

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