Reaction rates electron transfer processes

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. 

For most metals (Hg, Au, Pt, Pb) and solvents (DMSO, CH3CN, propylene carbonate) investigated it is again found that the first electron transfer process is rate limiting and, in solution, the nature of the products from the reaction is a strong function of the conditions under which the reaction is performed. 

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

Electron and charge transfer reactions play an important role in many chemical and biochemical processes. Dynamic solvation effects, among other factors, can largely contribute to determine the reaction rate of these processes and can be studied either by quantum mechanical or simulation methods. 

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... 

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

A preliminary electrochemical overview of the redox aptitude of a species can easily be obtained by varying with time the potential applied to an electrode immersed in a solution of the species under study and recording the relevant current-potential curves. These curves first reveal the potential at which redox processes occur. In addition, the size of the currents generated by the relative faradaic processes is normally proportional to the concentration of the active species. Finally, the shape of the response as a function of the potential scan rate allows one to determine whether there are chemical complications (adsorption or homogeneous reactions) which accompany the electron transfer processes. 

It is not uncommon that in electron transfer processes one observes that at low scan rates the process behaves reversibly, whereas at high scan rates the process behaves irreversibly (such behaviour is more easily seen for processes that are not complicated by coupled reactions). Processes occurring in the transition zone between reversible and irreversible behaviour are called quasireversible. 

It is conceivable that the presence of such complications must affect the shape of the cyclic voltammograms, and hence perturb to some extent the diagnostic criteria for the above-mentioned fundamental electron transfer processes. As these reactions proceed at their own rates, cyclic voltammetry will be able to detect them only if their rates fall within the time scale of the voltammetric technique (which ranges from a few tens of seconds to a few milliseconds). 

In photosynthetic systems, some electron transfer processes exhibit nonexponential kinetics at low temperature, which are generally attributed to the existence of different conformations of the system. While the differences between the reaction rates corresponding to these conformations do not exceed a factor of four in some cases [157,158,159], they are sufficient to lead to different quantum yields in others [160, 161]. Sometimes, the heterogeneous character of the kinetics disappears at room temperature, which probably reflects a fast exchange between the conformations that are frozen at low temperature [157, 158]. A systematic study of all these effects, similar to that performed in Ref [159], could give useful information about the nature of the conformational differences. 

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