NMR approach of the electronic properties of the hydrated trivalent rare earth ions in solution

The electronic properties of the lanthanide Ln(3+) ions are systematically investigated through the NMR of the protons of the tetramethylammonium ion (CH3)(4)N+, used as a probe in D2O solution. The effective magnetic moments on the Ln(3+) ions are obtained by measuring the paramagnetic shift of the proton resonance line duc to the demagnetizing field that is proportional to the rare earth concentration. These results allow to estimate the sizes of the total splitting of the crystal field acting on the ground multiplet of the various lanthanides. The measured intermolecular longitudinal relaxa tion rates of the (CH3)(4)N+ protons are compared with the computed values, both of the usual Solomon theory and of the Curie mechanism. The measured values are intermediate between those predicted by the two theories. This a llows a rattler accurate determination of the lifetimes of the electronic l evels of the Ln(3+) ions, as they little depend on the details of the descr iption for the relative spatial microdynamics of the (CH3)(4)N+/Ln(3+) repu lsive ion pair.