Salt Effects on Nearly-Diffusion-Controlled Electron Transfer Reactions. Bimolecular Rate Constant and Cage Escape Yields in Oxidative Quenching of Tris(2,2'-bipyridine)ruthenium(II)

The rate constants for the quenching of excited Ru(bpy)32+ by methylviologen (MV2+) and Ru(NH3)5py3+ have been studied in aqueous solution as a function of the concentration (0.01-1 M) and type (NaCl, NaClO4, CaCl2) of added electrolyte. With MV2+ as quencher, the yield of products escaping cage recombination and the rate constant of their back electron transfer reaction have also been studied as a function of the concentration of added NaCl. The results have been compared with predictions based on expressions available in the literature for the ionic strength dependence of diffusional parameters kd and k-d. With uni-univalent electrolytes, the Debye and Eigen equations appear to be adequate for the calculation of kd and k-d, respectively, provided that the appropriate numerical integration over the interreactant distance is performed. Approximations leading to more tractable expressions (such as, e.g., those leading to a Brønsted-Bjerrum ionic strength dependence of kd) give rise to s...