A LA-139 AND O-17 NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE NATURE OF LA(III) FIRST COORDINATION SPHERE IN ACETONITRILE SOLUTIONS OF HYDRATED LANTHANUM NITRATE

The nature of the first coordination sphere of the La(III) cation in acetonitrile solutions of La(NO3)(3).6H(2)O was investigated by La-139 and O-17 NMR. All three nitrate anions are coordinated to La(III). There is a competition between water and acetonitrile molecules for the occupancy of the La(III) first coordination sphere. The La-139 and O-17 NMR results, obtained for various concentrations of La(NO3)(3).6H(2)O, were quantitatively interpreted through a chemical equilibrium model. It was shown that the presence of only two La(III) species, {La(III) (NO3-)(3)} and {La(III) (NO3-)(3).H2O}, in rapid exchange in solution could account for all the NMR results. The variation of the La-139 and O-17 NMR parameters (chemical shifts and relaxation rates) with the concentration of La(NO3)3.6H(2)O results from the population increase of the species {La(III) (NO3-)(3).H2O}. The chemical exchange between free and coordinated water molecules is also fast on the O-17 NMR time scale. The O-17 NMR signal of water is considerably broadened when it coordinates to the La(III) cation. This broadening was attributed to both direct and indirect interactions between the two quadrupolar nuclei O-17 and La-139. The direct interaction comes from an increase of the electric field gradient at the O-17 nucleus when a water molecule coordinates to the La(III) cation. The indirect interaction results from an O-17-La-139 scalar coupling. The detailed analysis of the O-17 NMR spectra also allowed the determination of the indirect coupling constant between coordinated (OH2)-O-17 and La-139(III), \(1)J(O-17-La-139)\: 88 +/- 13 Hz.