Electron transfer, activation control homogeneous

Similar, although not exactly identical, analyses apply to the homogeneous case [(59), (47), 60)]. If the reaction proceeds in a stepwise fashion, there is again competition between the activation-controlled electron transfer (59),... 

In fact, the surface may mediate the requisite chemistry of the initially formed radical cation so that different products can be observed from the same intermediate when generated photoelectrochemically or by other means. The radical cation of diphenyl-ethylene, for example, gives completely different products upon photoelectrochemical activation 2 than upon electrochemical oxidation at a metal electrode or by single electron transfer in homogeneous solution, Eq. (31) . Surface control of... 

We see therefore that photoactive semiconductor particles provide ideal environments for control of interfacial electron transfer. Photoinduced electron-hole pairs formed on irradiated semiconductor suspensions, as in photoelectrochemical cells, allow for reactivity control not available in homogeneous solution. This altered activity derives from controlled adsorption on a chemically manipula-ble surface, controlled potential afforded by the valence band edge positions, controlled kinetics by virtue of band bending effects, and controlled current flow by judicious choice of incident light intensity. 

Experiments aimed at probing solvent dynamical effects in electrochemical kinetics, as in homogeneous electron transfer, are only of very recent origin, fueled in part by a renaissance of theoretical activity in condensed-phase reaction dynamics [47] (Sect. 3.3.1). It has been noted that solvent-dependent rate constants can sometimes be correlated with the medium viscosity, t] [101]. While such behavior may also signal the onset of diffusion-rather than electron-transfer control, if the latter circumstances prevail this finding suggests that the frequency factor is controlled by solvent dynamics since td and hence rL [eqn. (23), Sect. 3.3.1] is often roughly proportional to... 

Figure 36. The dependency of the homogeneous electron-transfer rate constant, A et on the potential difference, Erx - a- In the logarithmic plot, three asymptotes are noted, with slopes of 0, -1/118 mV, and -1/59 mV , representing diffusion-controlled, activation-controlled, and counter-diflfusion-controlled electron-transfer reactions, respectively [125]. The transfer coefficient...

Let (R)o and (P)o be the activities of R and P at Xq and (R)d> and (P)d, those at x = Xd> (the electron activity is not considered because it is implicitly included in the potentials). For all the electron transfer reactions investigated up to now, the rates of the diffusion-migration steps in Scheme 5 do not limit the overall process. Although this may be of importance for the prediction of the maximal rate of electron transfer at an electrode, we do not consider the possible kinetic limitation by either of these physical processes (compare the diffusion control of an homogeneous electron transfer discussed earlier). Thus, within this restriction, the existence of the diffusion-migration processes involves only thermodynamic contributions. Thus the overall rate constants in Scheme 5, defined in Eq. (98),... 

There are several examples of catenanes where ring movements can be induced by external stimulations like simple chemical reactions or homogeneous or heterogeneous electron transfer processes [91-93], but only very few cases are reported in which the stimulus employed is light. It has been shown that in azobenzene-containing [2]catenanes like 31 + (Fig. 29) it is possible to control the rate of thermally activated rotation of the macrocyclic components by photoisomerization of the azobenzene moiety [119, 120]. Such systems can be viewed as molecular-level brakes operated by light. 

The homogeneous outer sphere electron transfer reactions in solution occur at a rate that is noticeably Icj er than the diffusion rate. This peculiar behaviour has been explained through a three-step mechanism formation of a precursor complex from the separated reactants, actual electron transfer within this complex to form a successor complex and dissociation of the latter complex into separated products. The reaction rate is usually controlled by the electron transfer step, this step being governed by the Franck-Condon principle. This principle is embodied in classical electron transfer theories using an activated-complex formalism in which the electron transfer occurs at the intersection of two potential energy surfaces, one for the reactants and the other for the products. This implies that the second step necessarily involves the reorganization of the solvent before and after the electron transfer itself is produced. So, it is obvious that solvent must play an essential role in the rate of electron transfer reactions in solution. 

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