Buckminsterfullerene as a model reactant for testing electron transfer theory

Abstract

The importance of molecular structure and quantum mechanical properties in determining the kinetics of electron transfer has been discussed at length in the recent literature [l]. According to Marcus’ theory [2], the reactant is represented as a simple sphere, any changes in molecular configuration accompanying electron transfer being associated with an internal reorganization energy. In order to estimate the outer sphere reorganization energy one must assume a size and shape for the reacting molecule. Attempts to improve on the simple spherical model have included descriptions based on ellipsoids [3] and on collections of spheres [4]. Even when the molecule is approximately spherical, selection of its radius requires careful consideration of a variety of experimental data [5,6]. Considerable attention has been directed in recent months to the electrochemistry of buckminsterfullerene (C,), the new allotropic form of carbon 17-161. This molecule can be reduced in polar solvents in several one electron transfer steps [9-161. From the point of view of testing electron transfer theory, it would seem to be an ideal system. It is close to spherical in shape, and is chemically uniform over its surface. Because of its large size, the rate constant associated with electron transfer is expected to be large. We have recently studied the thermodynamics and kinetics of electroreduction of both C, and C,, in several polar solvents at platinum microelectrodes. In the present communication we present kinetic data for the first electron transfer to these molecules and compare the results obtained with those for other fast heterogeneous reactions studied under similar conditions. It should be noted that the C,, molecule is spheroidal in shape with the ratio of the shorter major axis to the longer equal to 0.86 1171. Data are also presented for some activation