The reduction of unsubstituted pquinones has been studied by measuring current-voltage curves at a dropping mercury electrode. Rate constants and transfer coef€icients for the reduction were derived by Koutecky's method. The energies of the various possible intermediate species are estimated and it is suggested, in agreement with Vetter,3 that two oneelectron transfers of almost equal activation energy occur. The value of the free energy of activation is consistent with the Hush 2o-Marcus 19 theory of electron transfer reactions. The effect of the molecular size of the quinone is expressed primarily through the energy of formation of the semiquinone.* DMF-dimethyl formamide : the results for this system were obtained from a cathodic wave analyzed by the procedure described elsewhere.10
Calculations are presented concerning two aspects of the electrode reactions of molecules M* in electronically excited states. Firstly, the current which would flow through an electrode is computed for the experimental arrangement in which M* is generated by irradiation of a solution of the ground-state molecules M through the electrode, which is assumed to be potentiostatically controlled in a region of potential where only M* is electroactive. Secondly, the rate of electrogeneration of M* is calculated, assuming the potential of the electrode to be switched instantaneously from one at which an anion M" is formed at a diffusion-controlled rate to one at which oxidation of Mto M* is feasible. In each calculation the quenching of M* to M by electron exchange with the electrode is allowed for by using for this purpose arecently derived for mulation of an electrochemical rate constant valid at high overpotentials.
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