MOLECULAR CAPACITANCE - SEQUENTIAL ELECTRON-TRANSFER ENERGETICS FOR SOLUTION-PHASE METALLIC CLUSTERS IN RELATION TO GAS-PHASE CLUSTERS AND ANALOGOUS INTERFACES

A simple electrostatic treatment of the energetics of sequential solution-phase electron transfers involving metallic clusters and other large solutes is outlined and related to the conventional description for charging of gas-phase clusters with the objective of linking these phenomena and assessing phenomenologically the role of solvation in the former class of systems. The common occurrence of electron-transfer sequencies for solution-phase solutes suggests the concept of ''molecular capacitance'', denoting the dependence of the surface solute charge density upon the electrode potential; the present treatment is related to conventional descriptions of the capacitance of metal electrode-solution interfaces. The notion of the ''potential of zero charge'', E(pzc), for cluster solutes, which also emerges from the electrostatic treatment, is related to the corresponding better-known quantities for both metal-solution and metal-vacuum interfaces. The analysis therefore provides an instructive link between the charging energetics of spherical clusters and planar surfaces in solution- versus gas-phase environments. Illustrative applications of this treatment are outlined for the cathodic charging of buckminsterfullerenes and high-nuclearity platinum carbonyl clusters. The applicability of the dielectric-continuum description of solvent-dependent cluster charging is examined. While only of semiquantitative validity, the analysis can prove useful for separating classical electrostatic and quantum (molecular-orbital) contributions to the cluster capacitance. The approximate correspondence between E(pzc) for the platinum carbonyl clusters and for analogous platinum-solution and -vacuum interfaces is also noted.