Step Preceding Electron Transfer

In many cases the substrate, as added from the reagent bottle, is not the electroactive species in itself, but is transformed into one by reactions taking place upon 

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The reduction of arylalkyl halides of the triphenylmethyl type by electro-chemically generated outer sphere single electron donors offers an example of a sequencing of the bond-breaking and electron-transfer steps different from what has been described before. The cleavage of the halide ion then precedes electron transfer which thus involves the carbocation, a mechanism reminiscent of 8, 1 reactions (Andrieux et ai, 1984b) as shown in (102). The... 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

In a coupled electron transfer reaction, the preceding adiabatic reaction step influences the experimentally-determined rate constant even though the electron transfer step is the slowest for the overall redox reaction. This occurs when the relatively fast reaction step which precedes electron transfer is very unfavorable (i.e. Kx (kx/kfix) l)- In Hii case, ks will be influenced by the equilibrium constant for that non-electron transfer process such that ks = kgT Kx (Harris et ah, 1994 Davidson, 1996). It follows that the experimentally-derived X ( lobs) may contain contributions from both the electron transfer event and the preceding reaction step (i.e. obs [ ET. x])- For example, lo sfor interprotein electron transfer reactions may reflect contributions from an intracomplex rearrangement of proteins after binding to achieve an optimum orientation for electron transfer. As with a true electron transfer reaction, k3 will vary with AG° since ks is proportional to ksT, although H b may also be affected by the coupling. 

Although first-order kinetics in both and are commonly observed, the majority of electrochemical reactions occur in more than one step -. Thus, e.g., for metal electrodeposition, electron and phase transfer occur in two distinct steps. As for homogeneous redox processes, various chemical steps (e.g., ligand substitution) either preceding or following the electron-transfer step often are encountered. Also, there is evidence that multielectron transfer occurs in microscopically separable one-electron steps. Even for one-electron transfer where both Yq and Y, are solution species, a separate adsorption step often precedes electron transfer. 

Examples of the lader include the adsorption or desorption of species participating in the reaction or the participation of chemical reactions before or after the electron transfer step itself One such process occurs in the evolution of hydrogen from a solution of a weak acid, HA in this case, the electron transfer from the electrode to die proton in solution must be preceded by the acid dissociation reaction taking place in solution. 

These equations may be used directly to predict the effect of pressure on the chemical reactions preceding or following the electron transfer step and, by use of standard thermodynamic formulae, they may be modified to allow a consideration of the electron transfer step itself. For example, the electrode reaction... 

The importance of radical ions and electron-transfer reactions has been pointed out in the preceding sections (see also p. 128). Thus, in linear hydrazide chemiluminescence (p. 103) or acridine aldehyde or ketone chemiluminescence, the excitation steps consist in an electron transfer from a donor of appropriate reduction potential to an acceptor in such a way that the electron first occupies the lowest antibonding orbital, as in the reaction of 9-anthranoyl peroxide 96 with naphthalene radical anion 97 142> ... 

In the CE mechanism (Scheme 2.2), a first-order (or pseudo-first-order) homogeneous reaction precedes the electron transfer step. In the case where the initial electron transfer is fast enough not to interfere kinetically, the electrochemical response is a function of two parameters the first-order (or pseudo-first-order) equilibrium constant, K, and a dimensionless kinetic... 

When pc —> oo, the catalytic loop is complete. The reaction sequence and the current-potential responses are the same as in the two-electron ECE homogeneous catalytic mechanism analyzed in the preceding subsection. When pc —> 0, deactivation prevails, and if the first electron transfer and the deactivation steps are fast, the same irreversible current-potential responses are obtained as in a standard EC mechanism. 

The preceding approach applies to all linear systems that is, those involving mechanisms in which only first-order or pseudo-first-order homogeneous reactions are coupled with the heterogeneous electron transfer steps. As seen, for example, in Section 2.2.5, it also applies to higher-order systems, involving second-order reactions, when they obey pure kinetic conditions (i.e., when the kinetic dimensionless parameters are large). If this is not the case, nonlinear partial derivative equations of the type... 

The opposite situation (y/D/k -C <5), where the reaction layer is much thinner than the diffusion layer (as represented in the lower diagram of Figure 2.31) is more specific of electrochemistry, in the sense that the homogeneous follow-up reactions are more intimately connected with the electrode electron transfer step. The same pure kinetic conditions discussed earlier for cyclic voltammetry (Section 2.2.1) apply. In the case of a simple EC reaction scheme, as shown in the figure, the production of C in the bulk solution obeys exactly the same equations (2.32) to (2.34) as for B in the preceding case, as established in Section 6.2.8. 

The preceding analysis is made simpler if, as often the case, the first as well as the electron transfer step may be regarded as totally irreversible, and dimerization is so fast that pure kinetic conditions are fulfilled. The last simplification implies that Qb/Qr = 0 in equation (6.58). Integration of this equation, taking into account initial and boundary conditions (6.59) and equations (6.63), leads to... 

Two one-electron transfers with different extents of reversibility. In the case where not all the processes of a consecutive electron transfer sequence are reversible, the irreversibility of a particular step becomes evident by the absence of the reverse peak in its pertinent response. For all other aspects the preceding considerations remain valid. 

The reaction mechanisms of organic electrode reactions are thus composed of at least one ET step at the electrode as well as preceding and follow-up bond-breaking, bond-forming, or structural rearrangement steps. These chemical steps may be concerted with the electron transfer [15, 16]. The instrumental techniques described in this chapter allow the investigation of the course of the reaction accompanying the overall electrolysis. 

The electrode reaction typically includes a lot of steps, such as adsorption and desorption, one or several electron transfer steps, preceding, and/or... 

An explicit assumption in the preceding discussion was that, in the case of a stepwise process, the follow-up reaction is so fast that the electron-transfer step (59) is rate determining. A tacit additional assumption was that it is, however, not so fast that the decay of RX does not take place solely outside the diffusion layer surrounding the electron donor molecule. In other words, the reaction scheme (68) was assumed, where the parentheses represent a solvent cage. 

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

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