General Consideration of the Electron Transfer Process in Solution

General Consideration of the Electron Transfer Process in Solution 

The mechanism of the electron transfer itself has been given considerable discussion, and attention has been directed to it as a barrier penetration phenomenon (84,104,1ST). This emphasis is justified only to the extent that other reasonable possibilities exist. Let us examine at the outset one such alternative process which has also come up for consideration, namely one in which the electron spends sufficient time in the solvent to be solvated. For a reaction with Fe+ + as reducing agent, the mechanism corresponding to this process would be formulated as follows  

If the model is modified by postulating that the electron is strongly affected by the presence of the oxidizing agent, the conditions for barrier penetration are also met, and it is difficult to see what experimental criteria would distinguish the two cases, whether the electron surmounts a 

The limited conclusions reached in the preceding paragraph should not be taken to imply that the description of these processes as barrier penetration phenomena is always apt. It is useful to consider the extreme mechanisms in one the geometries of the oxidized and reduced forms, at least as regards the first sphere of coordination, are the same, and substitutions in the first coordination sphere are not readily accepted in the other there are important changes in the first coordination sphere accompanying the electron transfer, and further, at least one of the species readily accepts substitution, and the coordination spheres interpenetrate in the activated complex. It will be noted that more than one condition has been specified for each case. The conditions are not necessarily combined as they appear here, and other cases can arise. The particular combinations have been chosen to provide the greatest contrast in behavior which can be expected. 

The systems of the first class afford the closest approach to a simple barrier penetration process, and perhaps they more readily respond to a theoretical analysis. It can reasonably be supposed that for these systems orbital overlap for the two ions is small, so that the frequency of the electronic transition is small, and there is no substantial binding between the two exchanging centers. A model of this kind presumably corresponds to the weak overlap cases as defined and discussed by Marcus (8 ). In attempting to calculate the rates of these reactions, besides the problem of the shape and height of the barrier for the electron transfer, electrostatic interaction of the reactants must be dealt with and the energy necessary to distort the solvent and ionic atmosphere about each ion to make the enei of the electron equal at the two sites. Different workers have emphasized different ones of these factors, and serious differences of opinion are recorded. 

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IV. General Consideration of the Electron Transfer Process in Solution. . 8... 

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