A direct nitrogen-15 NMR study of neodymium(III)-nitrate complex formation in aqueous solvent mixtures

A study of contact ion-pair formation between the neodymium (III) and nitrate ions in aqueous solvent mixtures has been carried out by a direct, low temperature, nitrogen-15 (¹⁵N) nuclear magnetic resonance (NMR) technique. At low temperatures, –90 to –120°C ligand exchange is slow enough to permit the observation of¹⁵N NMR signals for uncomplexed nitrate ion, and this anion in the primary solvation shell of Nd(III). In aqueous mixtures with inert acetone and Freon-12, resonance signals for Nd(NO₃)²⁺, Nd(NO₃) ₂ ¹⁺ , and two higher complexes are observed. Signal areas indicate these additional species are possibly a combination of the tetra-, penta-, and hexanitrato complexes, but not the trinitrato. In water-methanol, a medium of higher dielectric constant, complexation is much less and signals only for the mono-and dinitrato complexes are observed. The effect of solvent on complexation is demonstrated more clearly by a series of measurements in water-methanol-acetone mixtures.