Clarification of the mechanism of interfacial electron-transfer reaction between ferrocene and hexacyanoferrate(III) by digital simulation of cyclic voltammograms

The reaction mechanism of electron transfer (ET) between ferrocene (Fc) in nitrobenzene (NB) and Fe(CN)(6)(3-) in water (W) was clarified by digital simulation of cyclic voltammograms. The voltammograms observed under various concentration conditions could not be elucidated by assuming a heterogeneous ET at the NB/W interface, whereas they were successfully elucidated in terms of the ion-transfer (IT) mechanism, in which a homogeneous ET between Fe(CN)(6)(3-) and Fc (partially distributed from NB) occurs in the W phase and the interfacial transfer of the resultant ferricenium ion (Fc(+)) is responsible for the current passage across the interface. The validity of the IT mechanism was further supported by spectroscopic detection of Fc(+) produced in the W phase without electrochemical control. These results show that the homogeneous ET proceeds more advantageously than the heterogeneous ET due to a pinpoint collision of redox species at the interface. This may be ascribed to the difference in the volume of reaction field between the homogeneous and heterogeneous ETs. The reaction field for the former has a thickness of ca. 200 mum, whereas that for the latter is restricted to an interfacial layer as thin as several Angstroms. Such a large difference in the volume of the reaction field would overcome the disadvantage of the IT mechanism, i.e., the small partition of Fc into the W phase. The low possibility of the ET mechanism has also been deduced from theoretical estimation of the standard rate constant and transfer coefficient for the hypothetical heterogeneous ET.