Electron transfer influences

The fact that ET and conformational reactions thus are sequential (Scheme III), and not concerted, is an important factor in efforts to disentangle eonforma-tional and electron-transfer influences, because standard detection methods monitor only the ET event, and not conformational changes within one electronic state. In many, if not most, instances the measured time course of a single gated ET reaction is likely to be indistinguishable from a reaction without gating. 

Redox potentials of the halide ions explain that direct electron release to the benzenediazonium ion takes place only with iodide (and astatide, At ). This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light or other special procedures. For bromo- and chloro-de-diazoniations, catalysis by cuprous salts (Sandmeyer reaction) is necessary. For fluorination, the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen, i.e. a heterolytic dediazoniation without electron transfer. GaUi demonstrated that in chloro-de-diazoniations the yield is strongly dependent on the redox potential of electron transfer catalysts (highest yields with Cu" and Sn +), but that the rate of electron transfer influences the yield also. Electron transfer is likely to be the rate-limiting step of aryl radical formation in dediazoniations catalyzed by transition metal salts. 

As tire reaction leading to tire complex involves electron transfer it is clear that tire activation energy AG" for complex fonnation can be lowered or raised by an applied potential (A). Of course, botlr tire forward (oxidation) and well as tire reverse (reduction) reaction are influenced by A4>. If one expresses tire reaction rate as a current flow (/ ), tire above equation C2.8.11 can be expressed in tenns of tire Butler-Volmer equation (for a more detailed... 

Influence of the Kinetics of Electron Transfer on the Faradaic Current The rate of mass transport is one factor influencing the current in a voltammetric experiment. The ease with which electrons are transferred between the electrode and the reactants and products in solution also affects the current. When electron transfer kinetics are fast, the redox reaction is at equilibrium, and the concentrations of reactants and products at the electrode are those specified by the Nernst equation. Such systems are considered electrochemically reversible. In other systems, when electron transfer kinetics are sufficiently slow, the concentration of reactants and products at the electrode surface, and thus the current, differ from that predicted by the Nernst equation. In this case the system is electrochemically irreversible. 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

In addition to the above mechanism, the metal chelates can influence the process of quenching through electron-transfer ... 

Various light-induced reactions including hydrogen atom abstraction, electron transfer and (3-scission occur under the influence of UV light. Certain... 

Let us see now what happens in a similar linear scan voltammetric experiment, but utilizing a stirred solution. Under these conditions, the bulk concentration (C0(b, t)) is maintained at a distance S by the stilling. It is not influenced by the surface electron transfer reaction (as long as the ratio of electrode area to solution volume is small). The slope of the concentration-distance profile [(CQ(b, t) — Co(0, /))/r)] is thus determined solely by the change in the surface concentration (Co(0, /)). Hence, the decrease in Co(0, t) duiing the potential scan (around E°) results in a sharp rise in the current. When a potential more negative than E by 118 mV is reached, Co(0, t) approaches zero, and a limiting current (if) is achieved ... 

Samec Z, Weber J (1973) The influence of chemisorbed sulfur on the kinetic parameters of the reduction of Fe " ions on a platinum electrode on the basis of the Marcus theory of electron transfer. J Electroanal Chem Interfacial Electrochem 44 229-238... 

Yet the view that the rates of electron transfer in simple reactions are principally independent of the electrode metal (which for some time had been current in the electrochemical literature) cannot be maintained in this strict form. Many experimental data relating to the exchange current densities of reactions involving simple cations (such as Fe and Fe ) provide evidence that the electrode metal does exert a rather strong influence on the reaction rates. 

Influence of surface oxidation, 12-28 Potential energy surface, 66-73 Reaction order, 21 -22 Tafel slope, 18-20, 276-277, 297 Oxygen tolerance, 618-620 Outer sphere electron transfer, 33-38... 

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