Kinetics rate-determining electron transfer

A general difficulty encountered in kinetic studies of outer-sphere electron-transfer processes concerns the separation of the precursor formation constant (K) and the electron-transfer rate constant (kKT) in the reactions outlined above. In the majority of cases, precursor formation is a diffusion controlled step, followed by rate-determining electron transfer. In the presence of an excess of Red, the rate expression is given by... 

Cyclic voltammetry, kinetic studies, and DFT calculations using a BP functional and the TZVP basis set showed that the major pathway of the non-regiospeciflc zinc-reduced titanocene-mediated ring opening of epoxides was initiated by a titanium dimer-epoxide compound that reacted in a rate-determining electron transfer mechanism 25 The calculations showed that the transition state is early so the stereoselectivity is determined by steric effects rather than by the stability of intermediate radicals. This was confirmed by studies with more sterically crowded catalysts. 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

The kinetics of SO oxidation by [PtClg] have been studied over a wide range of experimental conditions.On the basis of the experimental evidence, a 2e reduction mechanism was proposed. The reduction of Cu(II) by SOl proceeds via Cu"SOf and Cu2S030H intermediates. " The rate law is consistent with consecutive first-order reactions leading to a mechanism involving complexation followed by a rate-determining electron transfer. Inner-sphere electron transfer is also assumed for the electron transfer between Fe(III) and The reduction... 

In the absence of oxygen, at 80 °C, cobalt(III) acetate oxidizes cyclohexane to cyclohexyl acetate and 2-acetoxycyclohexanone as main products [35]. The lack of a deuterium kinetic isotope effect in this reaction is consistent with rate-determining electron transfer to... 

The trends of kinetic rates of electron transfer processes due to changes in electrode potential E are now obvious. Increasing E relative to FP decreases the value of AG and accelerates the anodic reaction decreasing E relative to FP accelerates the cathodic reaction. Using this formalism, it is straightforward to determine the current... 

The kinetic behaviour in 20 % aqueous methanol reveals a first-order dependence on both oxidant and reductant and the activation enthalpy associated with the second-order rate constant is too small (2 kcal mol" ) to indicate fission of a C—H bond in the rate-determining step. An isotope effect (/ch/Atd) of 1.68 observed with the 4,4 -dideuterio-compound suggests rate-determining electron transfer in which the C-4—H bond is weakened, followed by rapid H+ or H release. The amount of... 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

The passage from one control to the other is pictured in Figure 2.5 for the cathodic peak potential and the peak width as a function of the scan rate and of the intrinsic parameters of the system. We note that increasing the scan rate tends to move the kinetic control from the follow-up reaction to the electron transfer step. It thus appears that the overall reaction may well be under the kinetic control of electron transfer, even if this is intrinsically fast, provided that the follow-up reaction is irreversible and fast. The reason is that the follow-up reaction prevents the reverse electron transfer from operating, thus making the forward electron transfer the rate-determining step. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

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