Electron transfer rate initial correlations

Although there is a linear correlation between the reduction potentials of RX and the rate constants for their reaction with CrL(H20)2 , the pathway involves atom transfer and not electron transfer (S). An electron transfer pathway initially to give X" and a kinetically inert Cr(III) complex would not result in subsequent substitution to give the product CrXL(H20) . ... 

In order to understand the origin of the kinetic differences induced by the mutations at sites LI 81 and M208, we have attempted to correlate the electron transfer rate with the free energy of the initial chemistry. We have assumed that the electron transfer chemistry is dominated by a single, intermediate state of charge separation such as To... 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

As an example, we consider the oxidation of a series of monosubstituted anilines by Mn02 in batch systems. In this case, quite a good correlation between log kK (expressed relative to R of 4-chloroaniline) and E]/2(ArX ) is obtained (Fig. 14.20). The slope of -0.54 indicates that, similar to what we have postulated for the reduction of NACs by surface-bound Fe(II) (see Fig. 14.10 /), the overall reaction rate is determined not solely by the actual electron transfer but also by other steps such as precursor complex formation. Comparable results (slopes of between-0.5 and -0.6) were obtained for the reaction of Mn02 at pH 4 with a series of substituted anilines (Laha and Luthy, 1990), and with a series of substituted phenols at pH 4.4 (Stone, 1987). In all these cases, only initial pseudo-first-order rate constants determined with clean Mn02 were considered. In the presence of solutes such as Mn2+ that may adsorb to the oxide surface, much slower reaction rates and much... 

The artificial intelligence-superexchange method in which the details of the electronic structure of the protein medium are taken into account was used for estimating the electronic coupling in the metalloproteins (Siddarth and Marcus, 1993a,b,c). Fig.2.11 demonstrates a correlation of experimental and calculated ET rate constants for cytochrome c derivatives, modified by Ru complexes. The influence of the special mutual orientation of the donor and acceptor orbitals in Ru(bpy)2im HisX-cytochrome c on the rate of electron transfer was analyzed by the transition amplitude methods (Stuchebrukhov and Marcus, 1995). In this reaction the transferring electron in the initial and the final states occupies the 3d shell of the Fe atom and the 4d shell of Ru, respectively. It was shown that the electron is localized on t2g subshells of the metal ions. Due to the near-... 

For a CE mechanism, the electrode product of interest is formed via an initial chemical reaction. Consequently, the measured limiting current will directly correlate with the amount of electroactive product formed on the time-scale of the experiment. Thus, sufficiently slow rates of mass transport result in complete conversion of bulk material to electroactive product and under this condition the limiting current will be identical to that calculated from the expressions described in Table 5 for a simple electron transfer process (see Fig. 28a). As the electrode angular velocity (a>) or flow rate (Tf) increases, less of the material reaching the electrode will have converted into... 

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