INTERACTIONS OF LA(III) WITH ANIONS IN AQUEOUS-SOLUTIONS - A LA-139 NMR-STUDY

The interactions of the La(III) cations with three anions (X), nitrate, chloride and perchlorate, in aqueous solutions in the pH range 4.0-6.5, were studied by La-139 NMR spectroscopy. A single model, involving the formation of the contact ion-pair (inner-sphere complex) (LaX)2+ was successfully and quantitatively applied to the chemical shift and the transverse relaxation rate data. Both measurements gave values for the thermodynamic equilibrium constants of formation of (LaX)2+ (K(th)) in good agreement (average K(th) = 0.45 +/- 0.05; 0.15 +/- 0.09; 0.03 +/- 0.01, respectively for nitrate, chloride and perchlorate). The complexes are characterized by chemical shifts of -25, 22 and -3.1 ppm and by transverse relaxation rates of 11.2, 5 and 1.65 kHz respectively for nitrate, chloride and perchlorate. The La-139 quadrupolar relaxation rate is not controlled by the reorientational correlation time. This finding is discussed, and it is suggested that the very fast exchange of water molecules in the first coordination sphere of La(III) is responsible for the time fluctuation of the electric field gradient at the La-139 nucleus site.