Oxygen-17 nuclear magnetic resonance kinetic study of water exchange on the lanthanide(III) aqua ions

A variable-temperature 17O NMR study of aqueous lanthanide solutions is reported. Chemical shift measurements on 0.3 m Ln(ClO4)3/2 m HClO4 solutions of 12 paramagnetic lanthanide ions are consistent with a constant hydration number along the series. This observation is reinforced by spectrophotometric measurements in acidified Nd(ClO4)3 solutions. A critical review of the previously published results on this topic suggests that the hydration number is nine for all lanthanide ions. The study was made on high-field NMR spectrometers, extending the accessible kinetic NMR window toward larger rate constants. By a combination of the chemical shifts and the longitudinal and the transverse relaxation rates, the water exchange activation parameters were obtained for six heavy lanthanide ions. The exchange rate constants calculated at 298.15 K are 5.0 x 108 s-1 for Tb3+, 3.9 x 108 s-1 for Dy3+, 1.9 x 108 s-1 for Ho3+, 1.2 x 108 s-1 for Er3+, 8.1 x 107 s-1 for Tm3+, and 4.1 x 107 s-1 for Yb3+. Unsuccessful attempts were made to study the water exchange on Nd3+ and Eu3+. The regular decrease of the k298 value with ionic radius and the fairly constant negative activation entropies are interpreted as grounds for a similar exchange mechanism for the six ions successfully kinetically characterized. 45 references, 5 figures, 4 tables