LA-139 NMR RELAXATION AND CHEMICAL-SHIFT STUDIES IN THE AQUEOUS NITRATE AND CHLORIDE SOLUTIONS

La-139 NMR relaxations and chemical shifts in the aqueous nitrate and chloride solutions at 274-343 K were studied. The solutions in this st udy were La(NO3)(3)-LiNO3 or -HNO3, and LaCl3-LiCl3 or -HCl systems. T he dependencies of the slope for the plot of 1/T-1 (s(-1)) vs eta/T (c P K-1) on the anion concentration showed that nitrate ions form inner- sphere complexes with lanthanum in a relatively low concentration but chloride ions scarcely form below about 4.3 mol/L. In the LaCl3-LiCl s ystems, the quadrupole coupling constants obtained by the temperature dependencies of 1/T-1 below about 4.3 mol/L ranged from 3.1 to 3.4 MHz , which agreed well with the value of hydrated lanthanum. In the LaCl3 -HCl systems, however, the quadrupole coupling constants slightly incr eased with an increase in chloride ion concentration even below about 4.3 mol/L, indicating that chloride ions possibly begin to form inner- sphere complexes. Activation energies for the rotational motion of lan thanum were determined in all of the solutions by the T-1 analyses usi ng the Arrhenius equation. The activation energies in the nitrate conc entration range from 0 to 0.1 mol/L were about 14 kJ/mol and in that a bove 1 mol/L about 18 kJ/mol. On the basis of the quantitative analysi s for the activation energies, these values were regarded as the energ ies for a breaking the hydrogen bond of H2O-H2O. While in the chloride ion systems, the activation energies gradually decreased above 0.6 mo l/L, indicating that hydrated lanthanum ions are gradually removed fro m a net structure of the aqueous solution by being packed by chloride ions. Chemical shifts obtained from a series of experiments also showe d that chloride ions do not form inner-sphere complexes with lanthanum below about 4.3 mol/L. Furthermore, the chloride ions induce downfiel d shifts and nitrate ions induce upfield relative to the value of hydr ated lanthanum. It is likely that the downfield shifts in the chloride systems are due to the result of an overlapping closed-shell repulsio n.