Parallel electron transfers

A similar approach may be used to evaluate kinetic parameters for parallel electron transfer of the kind 

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Branching mechanisms involve both consecutive and parallel electron transfers. The most important application of the RRDE in this context has been to the electrochemical reduction of oxygen [175], on which a large amount of research has been done. Different mechanistic models give rise to different expressions linking the rate constants, which can be compared with experimental data as in previous sections, the most important is the variation of (iD / h ) with rotation speed. A summary of different models has recently appeared [176] the conclusion of which is that, at platinum, the model of Damjanovic et al. [177] is correct diagnostic criteria to test the model have been developed. 

Achieving a high quantum yield in pentad 57 may be ascribed in large part to the use of parallel electron transfer to compete with charge recombination steps. The use of this strategy is also illustrated by tetrads 51 and 52 (see above). 

As discussed above, the experimental values of the excited-state yields s are connected to competition between the parallel electron transfer processes. To a first approximation, in the case of DPA the 0eci is given by the ratio of the rate constants of reactions (33), (34) and (35) ... 

Miller NJ, Sampson J, Candeias LP, Bramley PM and Rice-Evans CA (1996) Anti-oxidant activities of carotenes and xanthophylls. FEES Lett 384 240-242 Mortensen A and Skibsted LH (1996a) Kinetics of parallel electron transfer from j8-carotene to phenoxyl radical and adduct formation between phenoxyl radical and carotene. Free Rad Res 25 515-523... 

With the aim of establishing the relationship between the formal potentials for the parallel electron-transfer pathways given by Eqs. 14.3 and 14.4, a thermodynamic analysis of the voltammetry of Red(solid) adhered to an electrode surface can be undertaken ... 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

The presupposition is that parallel electrochemical reactions (i.e., ion or electron transfer) occur across the phase boundary, if the measured ions and interfering ions are both present in the solution. A redox process in which electrons pass the phase boundary is also considered an interfering electrochemical reaction. 

The last three columns contain the parameters which give the most reliable information on the nucleophilic character of the solvent for a wide range of solvents, namely Koppel and Paju s -values (1974), Schleyer s iVBS-values (Schadt et al., 1976), and Taft s / -values (Kamlet et al., 1983). The columns showing the type of product demonstrate roughly that increasing nucleophilicity favors the formation of products of homolytic intermediates. The fact that there is no exact correlation with nucleophilicity is likely to be due to the (one-) electron transfer capabilities of these solvents, which do not parallel the nucleophilicity (see Sec. 8.6). 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

From an analysis of data for polypyrrole, Albery and Mount concluded that the high-frequency semicircle was indeed due to the electron-transfer resistance.203 We have confirmed this using a polystyrene sulfonate-doped polypyrrole with known ion and electron-transport resistances.145 The charge-transfer resistance was found to decrease exponentially with increasing potential, in parallel with the decreasing electronic resistance. The slope of 60 mV/decade indicates a Nemstian response at low doping levels. 

A Mossbauer study of the protein reacted with benzaldehyde (in parallel with EPR detection of Mo(V) signals) shows partial reduction of the iron—sulfur centers, indicating the involvement of the clusters in the process of substrate oxidation and rapid intramolecular electron transfer from the molybdenum to the iron—sulfur sites. 

The electron transfer step is the only reaction step, which means that other parallel or consecutive steps are absent. 

FIG. 7 Simplified equivalent circuit for charge-transfer processes at externally biased ITIES. The parallel arrangement of double layer capacitance (Cdi), impedance of base electrolyte transfer (Zj,) and electron-transfer impedance (Zf) is coupled in series with the uncompensated resistance (R ) between the reference electrodes. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

Secondly, Fig. 5 shows that the polymeric rate constants parallel values of heterogeneous rate constants that have been observed for the electrochemical reactions of solutions of the corresponding dissolved porphyrin monomers. (The slope of the line is 0.5). This re-emphasizes what was said above, that measurements of electron hopping in polymers can give rate constants that are meaningful in the context of the metalloporphyrin s intrinsic electron transfer chemistry. 

Alkyl radicals share many of the desirable properties of or-ganometals described above, insofar as electron transfer reactions are concerned. Thus the steric properties of alkyl radicals with a- and (3-branches follow the trends in Figure 2. Moreover, the direct parallel in their donor properties is shown in Figure 3 by... 

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