PROTON AND DEUTERON RELAXATION IN AQUEOUS SOLUTIONS OF VANADYL(4) . EFFECTS OF ELECTRON SPIN RELAXATION AND CHEMICAL EXCHANGE

Proton and deuteron relaxation times in aqueous solutions of vanadyl(IV) sulfate have been studied as a function of temperature. The “excess” nuclear relaxation in this system is governed by both the relaxation time of a nucleus in the first hydration shell, T2m, and its mean residence time, Tm, These quantities have been separated by comparing the proton and deuteron transverse relaxation times in the same solution, taking advantage of the fact that T2mhT2md = (γd/γh)2. The temperature dependence of T2m governed by the variation of the electron relaxation rate, has thus been obtained. The observed behavior closely follows that predicted by the Kivelson theory of electron spin relaxation. It is, thus, demonstrated that reliable results for the electron relaxation and its temperature dependence may be obtained from nuclear relaxation studies. This approach has also yielded more accurate values for the kinetic parameters of the acid-catalyzed hydrogen ion exchange between the vanadyl-(IV) aquo complex and the uncoordinated water molecules. A deuterium isotope effect on the rate constants has been observed, and its relation to the reaction mechanism is discussed. © 1969, American Chemical Society. All rights reserved.