E.s.r. studies of intermolecular cation exchange in ether solutions of semiquinone ion pairs

Abstract

We have measured the rate, from line broadening in the slow exchange region, of the reaction between Na-2,5-ditertiarybutylparabenzosemiquinone ion pairs and NaI in THF. Over a 40-fold increase in NaI concentration, a plot of the rate against total salt concentration is a good straight line, whereas the plot against the square root of the salt concentration is markedly curved. This shows that the NaI ion pairs are the predominant species which attack the vacant site in the semiquinone ion pair. Experiments involving the suppression of the dissociation of NaI by the addition of tetraheptylammonium iodide also support this conclusion. The NaI experiments give a value for the velocity constant of 7.6 × 10⁸ 1 mol^–1 s^–1 at 20°C and measurements over a range of temperature give values for the pre-exponential factor and activation energy of ≈5.6 × 10¹⁰ l mol^–1 s^–1 and ≈ 10.0 kJ mol^–1, respectively. Evidence is presented, chiefly from similar experiments using NaBPh₄ instead of NaI, which indicates that the detailed mechanism of intermolecular cation exchange involves the formation of a triple ion species (i.e., a free radical anion with a sodium ion in each of the two sites) from the semiquinone ion pair by attack of the salt ion pair. We consider that the above velocity constant applies to this process.