PROTON SPIN RELAXATION AND EXCHANGE PROPERTIES OF HYDRATED CHROMIC IONS IN H2O AND H2O-D2O MIXTURES

Experimental results are presented on proton spin relaxation in Cr(III) solutions of H2O and mixtures of H2O and D2O, in fields from 1.1 G to 14 kG, and at temperatures from 0 to 99°. Water solutions containing 0.10 M HClO4 give results which reflect the relaxation and exchange properties of Cr(H2O)63+ alone, uncomplicated by the effects of hydrolysis products or polymers which cause thermal hysteresis or by acid-catalyzed proton exchange which is present in solutions of high acidity. Measurements on weakly acidified solutions confirm that Cr(H2O)63+ is a more strongly relaxing species than Cr(H2O)5OH2+, but the latter undergoes more rapid proton exchange with the solvent water. A homogeneous (local) proton exchange mechanism involving the coordinated OH- ion is proposed to account for this result. Measurements on 0.10 M HClO4 solutions containing varying mole fractions of H2O and D2O indicate that the principal effect of deuteration at low temperatures is an increase in the lifetime of protons in the primary hydration sphere of Cr3+, resulting from the greater basicity of H2O as compared to D2O. At high temperatures T2 remains constant, whereas T1 decreases with deuteration due to slower rotational motion of the hydrated complex. The latter is correlated with the marked increase in viscosity which occurs in going from H2O to D2O.