The Applicability of the Theory of R. A. Marcus to the Electron-Transfer Reactions between Polycyclic Aromatic Hydrocarbons and Their Anion Radicals

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Abstract
The rates of electron-transfer from the anthracene-anion radical to anthracene, 1,2-benzanthracene, and pyrene are measured by the electron spin resonance method, with a line-broadening procedure, under experimental conditions in which anion radicals are in a state of free ions or of loose ion pairs. The dependence of the observed rate constants on the standard free energy of reaction, the dielectric constant of the solvent, and the temperature are discussed on the basis of the theory of R. A. Marcus. The free energy of nuclear reorganization, as estimated from the ΔG°-dependence of the rate constants, is much smaller than that predicted theoretically. The observed rate constants are almost independent of the dielectric constants of the solvents; consequently, the reorganization free energy of the solvent must be smaller than the theoretical value. The entropy term contributes to the activation energy to almost the same extent in the solvents with different dielectric constants; this is inconsistent with the theoretical prediction and requires a smaller value for the free energy of solvent reorganization than its theoretical value.

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