Simple electron transfer

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

Mr 220-250 kDa. Figure 1 shows an overall electron transfer pathway for the nitrogenases where the Fe proteins act as very specific, essential electron donors to the larger proteins. This is not the only role for the Fe proteins (see Section IV,C) and their role in the mechanism is almost certainly more complex than that of a simple electron transfer agent (see below. Section V). Electron transfer from the Fe protein to... 

A simple electron transfer was proposed. Ir(VI) does not retard reaction and evidently the initial species ClOs must oxidise further Ir(ri[) very rapidly. 

Simple electron transfer followed by rapid protonation would give the ketyl radical which dimerises very rapidly. 

The reaction of eq. 16.9 will regenerate the antioxidant Arj-OH at the expense of the antioxidant At2-OH. Despite the fact that such regeneration reactions are not simple electron transfer reactions, the rate of reactions like that of eq. 16.9 has been correlated with the E values for the respective Ar-0. Thermodynamic and kinetic effects have not been clearly separated for such hierarchies, but for a number of flavonoids the following pecking order was established in dimethyl formamid (DMF) by a combination of electrolysis for generating the a-tocopherol and the flavonoid phenoxyl radicals and electron spin resonance (ESR) spectroscopy for detection of these radicals (Jorgensen et al, 1999) ... 

In a simple electron transfer reaction, the reactant is situated in front of the electrode, and the electron is transferred when there is a favorable solvent fluctuation. In contrast, during ion transfer, the reactant itself moves from the bulk of the solution to the double layer, and then becomes adsorbed on, or incorporated into, the electrode. Despite these differences, ion transfer can be described by essentially the same formalism [Schmickler, 1995], but the interactions both with the solvent and with the metal depend on the position of the ion. In addition, the electronic level on the reactant depends on the local electric potential in the double layer, which also varies with the distance. These complications make it difficult to perform quantitative calculations. 

As demonstrated in Section 2.2, the energy of activation of simple electron transfer reactions is determined by the energy of reorganization of the solvent, which is typically about 0.5-1 eV. Thus, these reactions are typically much faster than bondbreaking reactions, and do not require catalysis by a J-band. However, before considering the catalysis of bond breaking in detail, it is instructive to apply the ideas of the preceding section to simple electron transfer, and see what effects the abandomnent of the wide band approximation has. 

First, we shall discuss reaction (5.7.1), which is more involved than simple electron transfer. While the frequency of polarization vibration of the media where electron transfer occurs lies in the range 3 x 1010 to 3 x 1011 Hz, the frequency of the vibrations of proton-containing groups in proton donors (e.g. in the oxonium ion or in the molecules of weak acids) is of the order of 3 x 1012 to 3 x 1013 Hz. Then for the transfer proper of the proton from the proton donor to the electrode the classical approximation cannot be employed without modification. This step has indeed a quantum mechanical character, but, in simple cases, proton transfer can be described in terms of concepts of reorganization of the medium and thus of the exponential relationship in Eq. (5.3.14). The quantum character of proton transfer occurring through the tunnel mechanism is expressed in terms of the... 

Simple electron transfer at electrodes Proton transfer to cyanocarbon bases... 

Chemiluminescence is defined as the production of light by chemical reactions. This light is cold , which means that it is not caused by vibrations of atoms and/or molecules involved in the reaction but by direct transformation of chemical into electronic energy. For earlier discussions of this problem, see 7 9h Recent approaches towards a general theory of chemiluminescence are based on the relatively simple electron-transfer reactions occurring in aromatic radical-ion chemiluminescence reactions 10> and on considerations of molecular orbital symmetry as applied to 1.2-dioxetane derivatives, which very probably play a key role in a large number of organic chemiluminescence reactions 11>. 

R. A. Marcus for simple electron-transfer reactions 10> which, in appropriate modification, appears to be theoretically valid for more complex chemiluminescence reactions, too (e.g. 13>. 

According to the Butler-Volmer law, the rates of simple electron-transfer reactions follow a particularly simple law. Both the anodic... 

Figure 13.5 Cyclic voltammogram of a simple electron transfer reaction.

While N can be calculated for a given geometry, it is usually determined experimentally by using a simple electron-transfer reaction. A... 

At first sight, these strong effects might not seem to be predictable, given that the ferrocene reactant is uncharged and thus the formation of the precursor complex should be unaffected by the charge of the other reactant. The reaction of the ion-paired species, however, is not a simple electron-transfer reaction, because transfer of the anion must also occur. A detailed understanding of the dynamics of the process remains to be developed. 

FIGURE 2.31. Concentration profiles in steady-state (stirring or circulation electrolysis showing the various region of interest for a simple electron transfer reaction (top) and EC process with a fast follow-up reaction (bottom). 

The basic assumption of the Marcus theory of electron transfer is that only a weak interaction between the reactants is needed in order for a simple electron-transfer process to occur. Marcus theory considers... 

One expects the impact of the electronic matrix element, eqs 1 and 2, on electron-transfer reactions to be manifested in a variation in the reaction rate constant with (1) donor-acceptor separation (2) changes in spin multiplicity between reactants and products (3) differences in donor and acceptor orbital symmetry etc. However, simple electron-transfer reactions tend to be dominated by Franck-Condon factors over most of the normally accessible temperature range. Even for outer-... 

Studies of the simple electron transfer properties of coordinated dioxygen are in a preliminary stage. The definition of reactivity patterns is obscured by inconsistencies and the lack of key pieces of information, as indicated by the comments above. Nevertheless, there is hope that some general features will emerge ... 

Once again the voltammetric response will differ to a greater or lesser extent with respect to a simple electron transfer depending on the values of either the equilibrium constant, K, or the kinetics of the chemical complication (kf and kT). 

If the rate of the chemical complication is high, the system will always be in equilibrium and the voltammogram will apparently look like a non-complicated reversible electron transfer. However, as a consequence of the continual partial removal of the species Red from the electrode surface, the response (if, as in the present case, one is considering a reduction process) is found at potential values less negative than that of a simple electron transfer by an amount of ... 

In addition to the development of new methods, new applications of molecular dynamics computer simulation are also needed in order to make comparisons with experimental results. In particular, more complicated chemical reactions, beyond the relatively simple electron transfer reaction, could be studied. Examples include the study of chemical adsorption, hydrogen evolution reactions, and chemical modification of the electrode surface. All of the above directions and opportunities promise to keep this area of research very active ... 

We consider a simple electron transfer reaction (an outer-sphere electron transfer) between h3 rated redox particles OX /RED and a metal electrode M as shown in Eqn. 8-1 ... 

We compare a simple electron transfer reaction of hydrated redox particles, OX , + e(STD) = RED, with an electron transfer reaction of complex redox particles. 

The POs identified above can also be used for the analysis of other observables of our simple electron-transfer model. For example, it has been shown that a calculation employing two POs qualitatively reproduce the short-time evolution of the probability distribution P(x, t) = (f) /2) x) (x ( i2 l I (O)... 

Reinmuth notation. In the electrochemical world, the sequence of electrode and/or chemical reactions that occur are described by a simple shorthand code. Simple electron-transfer reactions are called E reactions. In the same shorthand system, a multiple electron-transfer reaction such as Fe " Fe " -> Fe is an EE reaction , i.e. the product of an electron-transfer process itself undergoes a second electron-transfer process. (Note that the two electron-transfer processes might occur at the same time, in which case it is merely an E reaction.) The vanadium pentoxide system illustrated in Figure 6.14 is another example of an EE system. 

If the rate constants for parallel reactions are to be resolved, then analysis of the products is essential (Sec. 1.4.2). This is vital for understanding, for example, the various modes of deactivation of the excited state (Sec. 1.4.2), Only careful analysis of the products of the reactions of Co(NH3)jH20 + with SCN, at various times after initiation, has allowed the full characterization of the reaction (1.95) and the detection of linkage isomers. Kinetic analysis by a number of groups failed to show other than a single second-order reaction.As a third instance, the oxidation of 8-Fe ferredoxin with Fe(CN)g produces a 3Fe-cluster, thus casting some doubt on the reaction being a simple electron transfer. 

A number of Fe(II) and Fe(III) chelates exist in low spin, high spin equilibrium in solution. They therefore afford an excellent opportunity to study the dynamics of a relatively simple electron-transfer and a number of very rapid reaction techniques have been applied to these systems (Chapters 3 and 7). Spin state interconversions are slightly more rapid in Fe(III) complexes. 

In addition to simple electron transfers in which no chemical bond is either broken or formed, numerous organic reactions, previously formulated by movements of electron pairs, are now understood as processes in which an initial electron transfer from a nucleophile (reductant) to an electrophile (oxidant) produces a radical ion pair, which leads to the final products via the follow-up steps involving cleavage and formation of chemical bonds [11-23], The follow-up steps are usually sufficiendy rapid to render the initial electron transfer the rate-determining step in an overall irreversible transformation [24], In such a case, the overall reactivity is determined by the initial electron-transfer step, which can also be well designed based on the redox potentials and the reorganization energies of a nucleophile (reductant) and an electrophile (oxidant). 

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