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Issue 8, 2011
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Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

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Abstract

Superoxide ion (O2˙) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)33+], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙–Sc(HMPA)33+ complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)33+ to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)33+ complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)33+ depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)33+-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)33+-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)33+ to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)33+. The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)33+ occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)33+ depending the number of Sc(HMPA)33+ (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.

Graphical abstract: Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

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Publication details

The article was received on 18 Jun 2010, accepted on 11 Nov 2010 and first published on 06 Jan 2011


Article type: Paper
DOI: 10.1039/C0CP00916D
Phys. Chem. Chem. Phys., 2011,13, 3344-3352

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    Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

    T. Kawashima, K. Ohkubo and S. Fukuzumi, Phys. Chem. Chem. Phys., 2011, 13, 3344
    DOI: 10.1039/C0CP00916D

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