Nuclear Magnetic Resonance Studies of Solutions of the Rare-Earth Ions and Their Complexes. IV. Concentration and Temperature Dependence of the Oxygen-17 Transverse Relaxation in Aqueous Solutions

Abstract

The concentration and temperature dependence of oxygen-17 linewidths in solutions of rare-earth perchlorates have been investigated. Empirical models are proposed to account for the observed behavior. An equation relating the apparent relaxation rate to the concentration and to the NMR parameters of the nuclei under conditions of fast chemical exchange has been derived. It was used in the interpretation of the experimental results. Lower limits have been estimated for the rate constants of water exchange between the aquo complexes of Tb3+, Dy3+, Ho3+, Er3+, and Tm3+ and the solvent. These are found to be in the range 0.3–2.6 × 107 sec−1 and to follow the trend of the ionic radii. The upper limit for the activation enthalpy of the exchange process is estimated to be 5 kcal/mole. The upper limits of the electron relaxation times are in the range 2.8–5.0 × 10−13 sec. The temperature dependence of the oxygen-17 transverse relaxation rates in these systems is discussed in terms of the microdynamic behavior of the molecules in water and electrolyte solutions. The observed trends comply with a model according to which there are two kinds of water molecules differing in their activation enthalpies for rotation. The relative contribution of the “hydrogen-bonded” and coordinated molecules prevails at lower temperatures, whereas that of the “free” molecules becomes pronounced at higher temperatures. It is suggested that spherically shaped samples be preferred for studies of 17O line width in paramagnetic solutions in order to minimize inhomogeneous broadening. It is shown that this may be a source of some inconsistencies found in the literature.