Electron-transfer reactions of nitroxyl radicals with one-electron reduced quinones and viologens

Abstract

Second-order rate constants have been estimated in aqueous solution (pH 7) for reaction between one-electron reduced quinone and viologen (1,1′-disubstituted-4,4′- and 2,2′-bipyridinium dihalide) species and the persistent nitroxyl radicals TMPN, TAN and NPPN using the pulse radiolysis spectrophotometric method. The rate constants determined for the piperidinic nitroxyls, TMPN and TAN (10⁴–10⁶ dm³ mol^–1 s^–1) were found to decrease by about one order of magnitude for each 100 mV increase in the one-electron reduction potential (E¹₇) of the viologen. In contrast, NPPN reacts with these species at a rate (≈ 10⁹ dm³ mol^–1 s^–1) which is almost independent of the viologen E¹₇ in the –350-–640 mV range studied.

In the case of the reaction of NPPN with a series of quinones, the rate constants were found to be dependent on E¹₇ of the quinones when E¹₇ > –200 mV. The dependence of determined rate constants upon E¹₇ is discussed in terms of theoretical treatments of electron-transfer processes.