J. Electroanal. Chem., 204 (1986) 31-43 Elsevter Sequoia S.A., Lausanne - Prmted m The Netherlands 31 zyxwvutsrqp A THEORY OF ADIABATIC ELECTRON-TRANSFER REACTIONS l WOLFGANG SCHMICKLER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB Instrtut fiir Phpkahsche Chemre der Unrvemtiit Bonn, Wegelerstr 12. D-5300 Bonn (F. R G.) (Received 16th December 1985) ABSTRACT A theory for adiabatic electron-transfer reactions at metal electrodes IS presented The model Hamiltonian is similar to that of the Levich and Dogonadze theory for non-adiabatic reactions, but the rate is not calculated from perturbation theory but by techmques fanuhar from the Anderson-News model for adsorption. In the hmtt of comparatively weak mteractions, this model reduces to the Marcus theory; for stronger Interactions there are significant deviations. A possible classification of electrochem- ical electron-transfer reactions ts discussed. (I) INTRODUCTION For certain outer-sphere redox reactions occurring on metal electrodes, the rate constant seems to be essentially independent of the nature of the metal. Thus, Iwasita et al. [l] showed that the exchange current density of the [Ru(NH,),]*+/~+ couple is essentially constant on a series of metals comprising sd-, sp- and transition metals; this is true even when the metal surface is modified by metal adatoms deposited at underpotential [2]. This result is qualitatively in line with previous studies by Capon and Parsons [3], and Samuelson and Sharp [4], who showed that the rate of the system benzoquinone-benzoquinone anion radical is practically the same on several substrates. Whether there are any outer-sphere reactions whose rate does vary with the nature of the metal is still an open question; early studies which did show such a dependence [5-71 were performed with redox couples which later proved not to be of the outer-sphere type. These experimental observations cannot be explained within the Levich and Dogonadze theory [8] for electron-transfer reactions. According to this theory, the current density is proportional to the electronic overlap integral between the metal surface and the redox centre, and to the electronic density of states near the Fermi level of the metal. It is highly unlikely that the product of these two factors should be constant over a series of metals of different chemical character. A natural explanation is that reactions whose rate is independent of the metal proceed adiabatically. Classical theories for adiabatic electron-transfer reactions have been l Dedicated to the memory of R.R. Dogonadze. 0022-0728/86/$03.50 8 1986 Elsevier Sequoia S.A.