Electron transfer rate constants measurement

Table 2 Bimolecular Electron-Transfer Rate Constants Measured When the Metal Complexes Were Intercalated in BAZrP (0.008% by Weight)...

Fig. 7.38 a) Single reduction wave, corrected for diffusion, for a 10 mM Mo(CN) solution at an Au electrode derivatized with a tetradecanethiol monolayer, b) Density of electronic states for Mo(CN)x calculated as the derivative of the heterogeneous electron transfer rate constant measured at the insulating layer on Au (Figs. 7,36 and 7.37). Solid curve, best fit to the experimental results dashed curve, the Gaussian distribution predicted by the Marcus theory. 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Figure 4 shows the temperature dependence of the decay kinetics measured in PVA films in the 665-nm band (triangles). Some measurements at lower temperatures in the 545-nm band are also shown (circles). The electron transfer rate constant increases by a factor of 2 as the temperature is lowered from 295 K to 100 K, at which point the rate becomes independent of temperature within experimental error. The solid curve through the experimental data in Figure 4 is a theoretical fit that will be discussed below. 

Electrochemical electron-transfer rate constants have been measured less frequently... 

The most striking application of electron transfer theory has been to the direct calculation of electron transfer rate constants for a series of metal complex couples.36 37 46 The results of several such calculations taken from ref. 37b are summarized in Table 2. The calculations were made based on intemuclear separations appropriate to the reactants in close contact except for the second entry for Fe(H20)j3+/2+, where at r = 5.25 A there is significant interpenetratidn of the inner coordination spheres. The Ke values are based on ab initio calculations of the extent of electronic coupling. k includes the total contributions to electron transfer from solvent and the trapping vibrations using the dielectric continuum result for A0. the quantum mechanical result for intramolecular vibrations, and known bond distance changes from measurements in the solid state or in solution. 

There has been keen interest in determination of activation parameters for electrode reactions. The enthalpy of activation for a heterogeneous electron transfer reaction, AH X, is the quantity usually sought [3,4]. It is determined by measuring the temperature dependence of the rate constant for electron transfer at the formal potential, that is, the standard heterogeneous electron transfer rate constant, ks. The activation enthalpy is then computed by Equation 16.7 ... 

It is much more popular to use nonaqueous solvents for low-temperature studies. There are two motivations, the more common of which is the desire to make measurements down to the lowest temperature possible using a solvent/ electrolyte system compatible with the chemical properties of the substances to be studied. In other instances, the purpose of the experiments is to study the effect of solvent on a temperature-sensitive parameter (e.g., a heterogeneous electron-transfer rate constant [5]), so a variety of solvents is sought in which low-temperature measurements can be made. 

Forster and Keyes prepared the tetrazine containing [Os(II)(BL)Os(III)] dimer (see below) and examined light induced intramolecular electron transfer in the system. They also measured heterogeneous electron transfer rate constants for oxidation of the complex at a Pt electrode [38]. The work is... 

Fig. 4. Photoinitiated electron transfer rate constant (natural logarithm) vs. edge-to-edge porphyrin-quinone separation for C-P-Q triads 4-8 and related P-Q molecules. The separations shown are derived from H-NMR measurements...

Thus, from fluorescence lifetime and transient absorption measurements we gathered the electron-transfer rate constants, i.e. both for charge-separation and for charge-recombination. Next, we plotted these rate constants as a function of donor-acceptor distance. From the resulting linear dependence (Fig. 9.26) it is possible to determine the attenuation factors p for the presented donor-acceptor... 

Electron transfer rate constants of outer sphere redox reactions can be measured relatively easily at n-type semiconductor electrodes. This is because electrons are withdrawn from the surface under depletion conditions, so that their concentration is lower than in the bulk. Under ideal... 

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