Rate Saturation in Electron Transfer

This apparent low efficiency for reducing N2 was observed under conditions of saturating N2 and is not due to restrictions in electron transfer rate. Furthermore, the low efficiency is mirrored in bacterial cultures at 30°C (185). 

Bimolecular Rate constants for Electron Transfer between Carotenoid Pairs in Argon Saturated Hexane (CAR/- + CAR2 CAR, + CAR/-)... 

Bimolecular Rate Constants for Electron Transfer between Carotenoid Pairs in Argon Saturated Benzene... 

Scherer and Willig (65) have studied the rate enhancement, due to cations and protons, of electron transfer from the surface of an organic insulator crystal, such as perylene, to oxidized ions, such as [Fe(CN)g] and fMo(CN)g] ", in solution. In an electrochemical method such as this, the saturation current directly renders the rate constant for electron transfer at the crystal surface. Furthermore, electron transfer on [Fe(CN)6l or [Mo(CN)g] can be studied in the absence of reduced forms, whereas the salt effect can be measured up to the solubility limit. They found that for the same concentration of added electrolyte, rate constants increased with the increased charge of the cation. Up to s 1M rate enhancement was of the order Li < Na < Cs but at salt concentrations >3.5 M a reversal that could be explained by different hydrations of the cations took place. They also found a good linear correlation in the shift to higher redox potentials (simultaneously increasing rate constants) with higher salt concentrations. 

Other factors influence the rate of direct electron transfer processes. For example, the greater the conductivity of their ligands the more readily should electron transfer proceed between two complexes. Multiple bonds in cyanide ions are expected to provide a good path for electrons, and, indeed, electron transfer between a variety of similar cyanide complexes has been found to be rapid. The same is true of the electron conducting aromatic ligands in [M(phen)3] and [M(bipy)3] relative to the saturated ligands in [M(en)3]" and [M(NH3)6]. ... 

FIgurt 12 (a) Potential energy curve for electron transfer from P to Q. (b) Bell-shaped curve of rate constant of electron transfer vs. free energy change (AG) predicted by Marcus theory, (c) Saturation curve of rate constant of bimolecular electron transfer observed in solution. 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

The half-wave potentials of (FTF4)Co2-mediated O2 reduction at pH 0-3 shifts by — 60 mV/pH [Durand et ah, 1983], which indicates that the turnover-determining part of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or two reversible ETs and two protonation steps. In contrast, if an irreversible ET step were present, the pH gradient would be 60/( + a) mV/pH, where n is the number of electrons transferred in redox equilibria prior to the irreversible ET and a is the transfer coefficient of the irreversible ET. The —60 mV/pH slope is identical to that manifested by simple Ee porphyrins (see Section 18.4.1). The turnover rate of ORR catalysis by (ETE4)Co2 was reported to be proportional to the bulk O2 concentration [Collman et ah, 1994], suggesting that the catalyst is not saturated with O2. 

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

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