A DIRECT N-15 NMR-STUDY OF ERBIUM(III)-NITRATE COMPLEXATION IN AQUEOUS SOLVENT MIXTURES

The extent of inner-shell ion-pair formation of Er3+ with nitrate ion in aqueous mixtures has been studied by nitrogen-15 (N-15) NMR spectroscopy. At low temperature, exchange is slow enough to permit the direct observation of N-15 signals for nitrate ions in the Er3+ solvation shell and in bulk medium. In water-acetone mixtures, N-15 NMR signals for the mono- and bis complexes are observed at low nitrate to Er3+ mole ratios, but only the bis complex is evident at higher anion concentrations. No spectral evidence for the tris complex was seen at any nitrate concentration. In water-methanol-acetone mixtures, signals for the mono and bis complexes persist even at higher nitrate concentrations, indicating a reduced tendency to ion-pair with increasing dielectric constant. Preliminary N-15 NMR results are presented for the nitrate complexes of other paramagnetic lanthanide ions.