On the Electron-transfer Reaction Rate between Different Molecular Species of Aromatic Hydrocarbons as Measured by ESR

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The rate constants of electron-transfer reactions between the anthracene anion radical and pyrene, and those between the former radical and 1,2-benzanthracene, were measured by the ESR method in tetrahydrofuran containing 20 mM of tetra-n-butylammonium perchlorate. In the analysis of the experimental results, correction of the ESR line width was made for the change in the radical concentration caused by the addition of neutral molecules. The rate constants thus obtained were (2.0±0.4)×107M−1sec−1 and (8.1±1.0)×10−7M−1sec−1 respectively at 25°C. Under the same experimental conditions, the rate constants of the electron exchange of anthracene and pyrene with their respective anions were about one-tenth of the rate constants of free anions. This fact led us to the supposition that the anion radicals had been combined with tetra-n-butylammonium ions into stable ion-pairs in our experiment. A semi-empirical method was proposed for the theoretical calculation of the rate constant according to the theory of R. A. Marcus. The agreement of the observed and the calculated rate constants was satisfactory. Further discussion of the dependence of the rate constant on the standard free energy of reaction was made in connection with the so-called "Linear Free Energy Relationship."
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