A hydrogen-1, chlorine-35, and lanthanum-139 NMR coordination study of the lanthanum (III) ion in aqueous solvent mixtures

A La(III) hydration study has been carried out for solutions of La(ClO₄)₃ and, in a preliminary way, La(NO₃)₃ in aqueous mixtures with acetone-d₆ and Freon-12, using hydrogen-1, chlorine-35, and lanthanum-139 NMR spectroscopy. Low temperature, proton magnetic resonance experiments allowed the direct observation and area evaluation of separate signals for water molecules in the primary solvation shell of La(III) and in bulk medium. Measurements over a wide range of salt and solvent concentration gave a maximum La(III) hydration number of 6 and no evidence for inner-shell ion-pairing in La(ClO₄)₃ solutions. Chlorine-35 chemical shift and linewidth data in these solutions confirmed the absence of contact ionpairing. Hydration numbers of 3–4 for La(III) in several La(NO₃)₃ solutions clearly indicated inner-shell complex formation. Lanthanum-139 chemical shift and linewidth measurements for these systems revealed the presence of some process, possibly hydrolysis, in the La(ClO₄)₃ solutions at extremely high acetone-d₆ concentrations.