Mechanism classification electron transfer

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

It should be remembered that the classification criterion used above has nothing to do with the fact that the composition of the products may indicate that an atom or group has been transferred between the two metal centers. An inner-sphere mechanism may or may not be accompanied by atom or group transfer, and this may either occur as a consequence of the transition state structure or trivially be due to the fact that the complexes involved are substitution labile, i.e. exchange of ligands between themselves or the environment takes place at a rate faster than that of electron transfer. 

Following our initial, loose classification of redox reagents as belonging to either the one- or two-electron variety, we now proceed to examine a wide range of suspected cases of nonbonded electron transfer mechanisms in the light of the Marcus theory. We shall use an approach similar to the one developed in Section 5, namely, to see if Marcus theory can predict rate constants or other rate parameters that are in reasonable agreement with experimentally determined ones or otherwise are compatible with known facts. 

Fundamental work by Luche resulted in the hypothesis that ultrasound can influence and change reaction pathways in reaction types with single electron transfer [186, 187]. Ultrasound is also believed to influence reaction systems by mechanical effects [187]. An empirical classification of sonochemical reactions is divided into three types of effects purely chemical effects induced by sonochemical cavitation, hydrodynamic effects (mechanically induced cavitation), and by-passing mass-transport limitation. The latter effects are based on physical rather than chemical phenomena and judged to be false sonochemistry [188]. Nevertheless, these false effects (e.g. emulsification) are often important. The three types of effect are ... 

A second mode of classification of electrode reactions is based entirely on the electrode mechanism. Here it is necessary to know the number of chemical and electrochemical steps involved and the order between the different steps. By denoting an electrochemical step by an if and a chemical one by a C and postulating that every electrochemical step involves the transfer of one electron, it is immediately evident that, e.g., an ECEC process consists of ... 

Our conclusion can be extended to include all other substrates for the E—X bond and Y reagents. We emphasize that all the mechanisms are bimolecular, and presuppose the transfer of the corresponding ligands (X or from the substrate to the reagent Y together with its electrons. Such generation of radicals in ionic processes (Fig. 4.25a, b), according to Pryor s classification (53],... 

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