139La NMR Relaxation and Chemical Shift Studies in the Aqueous Nitrate and Chloride Solutions

Abstract

139La NMR relaxations and chemical shifts in the aqueous nitrate and chloride solutions at 274−343 K were studied. The solutions in this study were La(NO3)3−LiNO3 or −HNO3, and LaCl3−LiCl3 or −HCl systems. The dependencies of the slope for the plot of 1/T1 (s-1) vs η/T (cP 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 systems, the quadrupole coupling constants obtained by the temperature dependencies of 1/T1 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 increased 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 lanthanum were determined in all of the solutions by the T1 analyses using the Arrhenius equation. The activation energies in the nitrate concentration range from 0 to 0.1 mol/L were about 14 kJ/mol and in that above 1 mol/L about 18 kJ/mol . On the basis of the quantitative analysis for the activation energies, these values were regarded as the energies for a breaking the hydrogen bond of H2O−H2O. While in the chloride ion systems, the activation energies gradually decreased above 0.6 mol/L, indicating that hydrated lanthanum ions are gradually removed from a net structure of the aqueous solution by being packed by chloride ions. Chemical shifts obtained from a series of experiments also showed that chloride ions do not form inner-sphere complexes with lanthanum below about 4.3 mol/L. Furthermore, the chloride ions induce downfield shifts and nitrate ions induce upfield relative to the value of hydrated lanthanum. It is likely that the downfield shifts in the chloride systems are due to the result of an overlapping closed-shell repulsion.