Electron-transfer reaction reverse

Several of these features remain unexplained but it is clear that here we have an example of a relatively well-behaved reversible electron transfer reaction involving radical intermediates. 

As mentioned above, the distribution of the various species in the two adjacent phases changes during a potential sweep which induces the transfer of an ion I across the interface when the potential approaches its standard transfer potential. This flux of charges across the interface leads to a measurable current which is recorded as a function of the applied potential. Such curves are called voltammograms and a typical example for the transfer of pilocarpine [229] is shown in Fig. 6, illustrating that cyclic voltammograms produced by reversible ion transfer reactions are similar to those obtained for electron transfer reactions at a metal-electrolyte solution interface. 

Recently2 it has been asserted that the very existence of dissociative electron transfer reactions is ruled out by application of the principle of microscopic reversibility. The line of argument was as follows. In the reaction of the cleaving substrate RX, say, with an electron donor D (the same argument could be developed for an oxidative cleavage triggered by an electron acceptor),... 

Let s look at the little strip cartoon in Figure 7.7, which shows the surface of a copper electrode. For clarity, we have drawn only one of the trillion or so atoms on its surface. When the cell of which it is a part is permitted to discharge spontaneously, the copper electrode acquires a negative charge in consequence of an oxidative electron-transfer reaction (the reverse of Equation (7.7)). During the oxidation, the surface-bound atom loses the two electrons needed to bond the atom to the electrode surface, becomes a cation and diffuses into the bulk of the solution. 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

E1/2 being the reversible half-wave potential of the electron-transfer reaction with respect to ferrocene. The suggested offset value, however, differs somewhat from group to group. 

In this review, wherever electrochemistry is concerned, the reversibility of a reaction refers firstly to the chemical reversibility. It also requires that the electron transfer reaction occurs at such a rate that the rate of the whole electrodic process, which is measured by the output current of the electrode, is controlled by the diffusion of the redox species towards the electrode surface. Furthermore, the surface concentrations of O and R at a given potential should be governed by the Nemst equation. 

An Alternative Mechanism. Considering the facility of the electron transfer reactions to which a great deal of this symposium has been devoted, we have to worry whether our "proton transfer" reactions may not really be the result of electron transfer in the reverse direction followed by hydrogen transfer. As Bergman (26) has recently reported that another hydride anion may act as a one-electron reducing agent, and as we have evidence implicating 0s(C0) H as an intermediate in a number of... 

For a preceding chemical reaction two mechanisms are possible, depending on whether the electron transfer is reversible or irreversible. 

When K is small, the electron transfer reaction again appears as a simple reversible process, except that the peak current will be smaller than what one would expect on the basis of the quantity of the (erroneously assumed to be active) Y species placed in solution. This results because the concentration of the really active species C0x> being determined by the equilibrium of the preceding reaction, is equal to only a fraction of the species Y placed in solution ... 

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...

The electron-transfer reaction should be simple, fully reversible and preferably involve the transfer of only one electron... 

As a rule of thumb , reversibility (in the electrochemical sense) implies that the electron-transfer reaction is sufficiently swift for the current to obey equation (6.6) instantly and that no chemical processes accompany the electron-transfer reaction - see Section 6.3.4. 

Because the CV is stretched, the separation between the peaks AE, (anodic and cathodic) increases from its theoretical values of 59/a mV, which characterizes a fully-reversible electron-transfer reaction (.see Table 6.3). The magnitude of the overshoot depends on the time lag, and therefore as the scan rate increases, so the separation between the peaks increases, thus causing the CV to look even more stretched. 

Next, we need to decide on what we think is occurring in terms of the system actually before us. Let s suppose that we have a CV which looks as though it describes a simple single reversible electron-transfer reaction. From the experimental trace of current against potential, it should be easy to obtain the standard electrode potential E . In addition, before we start, we measure the area of the electrode. A, and the thermodynamic temperature, T. Next, knowing A, T and E , we estimate a value for the exchange current lo, run a simulation, and note how similar (or not) are... 

CNT randomly dispersed composites Many soft and rigid composites of carbon nanotubes have been reported [17]. The first carbon-nanotube-modified electrode was made from a carbon-nanotube paste using bromoform as an organic binder (though other binders are currently used for the paste formation, i.e. mineral oil) [105]. In this first application, the electrochemistry of dopamine was proved and a reversible behavior was found to occur at low potentials with rates of electron transfer much faster than those observed for graphite electrodes. Carbon-nanotube paste electrodes share the advantages of the classical carbon paste electrode (CPE) such as the feasibility to incorporate different substances, low background current, chemical inertness and an easy renewal nature [106,107]. The added value with CNTs comes from the enhancement of the electron-transfer reactions due to the already discussed mechanisms. 

Liquid-phase electron-transfer reactions that lead to the formation of ion-radicals can be reversible. The equilibria of these reactions can be managed to obtain the desired results. This chapter... 

No such confusion arises for decalins. In decalin mixtures, a reversible electron-transfer reaction takes place ... 

Let us consider molecular switches based on intramolecular electronic transition. Generally, transfer of energy or an electron within a molecule proceeds in femtoseconds. The aim is to produce molecular electronic devices that respond equally rapidly. Molecular switches that employ optically controlled, reversible electron-transfer reactions sometimes bring both speed and photostability advantages over molecular switches which are usually based on photochemical changes in their molecular structure. Important examples are the molecnlar switches depicted in Scheme 8.3 (Debreczeny et al. 1996). 

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