A 139La and 17O nuclear magnetic resonance study of the nature of La(III) first coordination sphere in acetonitrile solutions of hydrated lanthanum nitrate

Abstract

The nature of the first coordination sphere of the La(III) cation in acetonitrile solutions of La(NO3)3.6H2O was investigated by 139La and 17O 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 139La and 17O NMR results, obtained for various concentrations of La(NO3)3.6H2O, 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 139La and 17O NMR parameters (chemical shifts and relaxation rates) with the concentration of La(NO3)3.6H2O 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 17O NMR time scale. The 17O 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 17O and 139La. The direct interaction comes from an increase of the electric field gradient at the 17O nucleus when a water molecule coordinates to the La(III) cation. The indirect interaction results from an 17O–139La scalar coupling. The detailed analysis of the 17O NMR spectra also allowed the determination of the indirect coupling constant between coordinated 17OH2 and 139La(III), [Formula: see text]88 ± 13 Hz.