Electron transfer process driving force

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

As described in the introduction, certain cosurfactants appear able to drive percolation transitions. Variations in the cosurfactant chemical potential, RT n (where is cosurfactant concentration or activity), holding other compositional features constant, provide the driving force for these percolation transitions. A water, toluene, and AOT microemulsion system using acrylamide as cosurfactant exhibited percolation type behavior for a variety of redox electron-transfer processes. The corresponding low-frequency electrical conductivity data for such a system is illustrated in Fig. 8, where the water, toluene, and AOT mole ratio (11.2 19.2 1.00) is held approximately constant, and the acrylamide concentration, is varied from 0 to 6% (w/w). At about = 1.2%, the arrow labeled in Fig. 8 indicates the onset of percolation in electrical conductivity. 

This equation, based on the Marcus model, therefore gives us a relationship between the kinetics (kEr) and the thermodynamic driving force (AG°) of the electron-transfer process. Analysis of the equation predicts that one of three distinct kinetic regions will exist, as shown in Figure 6.24, depending on the driving force of the process. 

In the normal region, thermodynamic driving forces are small. The electron-transfer process is thermally activated, with its rate increasing as the driving force increases. 

The inverted region is so-called because the rate of the electron-transfer process decreases with increasing thermodynamic driving force. 

Using the dyad shown in Figure 6.26, it has been possible to investigate the driving-force dependence of the rate constants for the electron-transfer processes. 

These proton transfer processes increase the driving force of the electron transfer reactions, which can thus be considered in terms of a proton-coupled electron transfer process [57-60]. 

The exponential term of 4.7 in conjunction with 4.6 contain an important prediction, namely that three distinct kinetic regimes exist, depending on the driving force of the electron transfer process. The three kinetic regimes are also shown schematically in Fig. 4.2 (lower part) in terms of the classical Marcus parabolas ... 

The generalized mechanism for ion-pair annihilation as presented in Scheme 11 involves the rather circuitous route for radical-pair production [involving Eqs. (55) and (56), certainly in comparison with the direct electron-transfer pathway (Scheme 8)]. In other words, why do ion pairs first make a bond and then break it, when the simple electron transfer directly from anion to cation would achieve the same end The question thus arises as to whether electron transfer between Fe(CO)3L+ and CpMo(CO)3 is energetically disfavored. The evaluation of the driving force for the electron transfer process obtains from the separate redox couples, namely,... 

The driving force of the electron transfer process in the interface is the difference of energy between the levels of the semiconductor and the redox potential of the species close to the particle surface. The thermodynamically possible processes occurring in the interface are represented in Fig. 9 the photogenerated holes give rise to the D -> D + oxidative reaction while the electrons of the conduction band lead to the A -> A reductive process. The most common semiconductors present oxidative valence bands (redox potentials from +1 to + 3.5 V) and moderately reductive conduction bands (+ 0.5 to - 1.5 V) [115]. Thus, in the presence of redox species close or adsorbed to the semiconductor particle and under illumination, simultaneous oxidation and reduction reactions can take place in the semiconductor-solution interface. 

On the basis that the position of Etn at the semiconductor surface is dependent on the photon flux and that Etn has to lie above the HER redox level, a threshold in light intensity has been proposed217 for the sustained photoelectrolysis of water to occur. However, as discussed by other authors,218 no such threshold has been reported in the literature. It has been pointed out218 that the driving force for the photoinduced electron transfer process is related to the difference in standard potentials of the donor (say, an electron at the semiconductor CB edge) and the acceptor (say, protons in solution). This is independent of the carrier concentration and photon flux and thus a light intensity threshold for incipient product formation through photoelectrolysis should not occur.218... 

Light absorption modifies the driving force for electron transfer processes in all kinds of materials. As photoactivated species are always better oxidants and reductants than their ground state equivalents, an enhanced redox reactivity is usually observed in the excited state. Photoreactions are therefore ideally suited to trigger, study, and mimic bioinorganic electron transfer. 

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