Data analysis, electron transfer rate

The results of the theoretical analysis outlined above provide a deep and intuitively satisfying view of the electron transfer process and provide a quantitative basis for accounting for the magnitudes of electron transfer rate constants. Before applying the results to the experimental data in Table 1, it is necessary to remind ourselves of the limitations and approximations upon which the theory was based and to consider how comparisons between theory and experiment are to be made. 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

Experimental data on distance dependence continue to be gathered from studies of the nonexponential kinetics observed in rigid media and a new method has recently been claimed, based on the simultaneous analysis of kinetic and ESR data. The major development in recent years, however, has been the study of unimolecular electron transfer rates in specially synthesized binuclear complexes of known structure. Early work mostly involved systems with nonrigid, or not quite rigid, bridging groups, so that some doubt remained as to the operative electron transfer distance. In recent work this limitation has been removed in... 

The previous analysis has so far shown that photocurrent responses are directly linked to specifically adsorbed metalloporphyrin at the liquid/liquid boundary. The interfacial organisation of these species is affected by a series of parameters such as the bulk concentration and the Galvani potential difference. In this section, we shall concentrate on the dynamic and mechanistic aspects associated with the photocurrent signal. The information discussed in the previous sections concerning lifetimes of excited states and surface coverage will be relevant to the estimation of the heterogeneous electron-transfer rate constant from the photocurrent data. 

Me2-4,4 -bipy ) have been used to evaluate the rate constant for inter-molecular reaction transfer within the ion pair. Use of a thermochemical cycle has provided a value for the rate of the overall reaction [Fe(CN)6] + PQ " (k2 0.2 X 10" M s ), which compares well with Marcus theory predictions. It is suggested that data on electron transfer processes may be derived from analysis of charge transfer absorption band characteristics. 

References to a number of other kinetic studies of the decomposition of Ni(HC02)2 have been given [375]. Erofe evet al. [1026] observed that doping altered the rate of reaction of this solid and, from conductivity data, concluded that the initial step involves electron transfer (HCOO- - HCOO +e-). Fox et al. [118], using particles of homogeneous size, showed that both the reaction rate and the shape of a time curves were sensitive to the mean particle diameter. However, since the reported measurements refer to reactions at different temperatures, it is at least possible that some part of the effects described could be temperature effects. Decomposition of nickel formate in oxygen [60] yielded NiO and C02 only the shapes of the a—time curves were comparable in some respects with those for reaction in vacuum and E = 160 15 kJ mole-1. Criado et al. [1031] used the Prout—Tompkins equation [eqn. (9)] in a non-isothermal kinetic analysis of nickel formate decomposition and obtained E = 100 4 kJ mole-1. 

It follows from the above discussion that an electron transfer reaction that appears reversible at one (low) sweep rate may change to a quasi-reversible or even an irreversible process at higher sweep rates. This should be kept in mind since the application of LSV and CV in kinetics studies usually includes the recording of voltammograms at a number of different sweep rates (see below), and the analysis of the data is usually based on the assumption that the electron transfer is reversible. 

Though much research on the influence of the solvent on the rate of electrode reactions has been done in recent years the problem is still far from a profound understanding. The basic question is the role of the dynamic and energetic terms in the control of the kinetics of simple electron-transfer electrode reactions. To answer this question it is essential to have reliable kinetic data for analysis. Unfortunately some kinetic data are too low and should be redetermined, preferably using submicroelectrodes. 

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