The Dependence of the Electron-transfer Rates of Aromatic Hydrocarbon Anion Radicals upon the Molecular Radii of the Reactants

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The applicability of the theory of R. A. Marcus is tested, the behavior of the rate constant being examined as a function of the molecular radii of the reactants. The rate constants of the electron-transfer reactions from the anion radicals of pyrene, perylene, and tetracene to their respective parent molecules are measured under experimental conditions in which the anion radicals are in a free ion or loose ion-pair state. The rate constants thus obtained and subsequently corrected for diffusion are 2.8×109, 3.0×109, and 2.6×109 M−1sec−1 respectively, in 1,2-dimethoxyethane at 25°C. These rate constants, as well as others obtained elsewhere for naphthalene and anthracene, are compared with those predicted theoretically, assuming a spherical and an oblate spheroidal molecular shape. The consistence between the theoretical and experimental results is not satisfactory; the variation in the observed rate constants with the molecular radii of the reactants is much smaller than that expected theoretically. The spin-exchange rate constants for anion radicals of pyrene, perylene, and tetracene are also determined to be 7.0×109, 5.1×109, and 4.8×109 M−1sec−1 respectively, in the same solvent at 25°C.
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