OXYGEN-17 MAGNETIC RESONANCE STUDIES OF ION SOLVATION . HYDRATION OF ALUMINUM (3) AND GALLIUM (3) IONS

The hydration of aluminum(III) and gallium(III) ions has been studied by oxygen-17 nuclear magnetic resonance. The oxygen-17 signal of the noncoordinated water molecules was shifted to a lower field than that of coordinated molecules by the addition of cobaltous perchlorate. The magnetic field was modulated at a frequency of 2000 cps and the first side band of the absorption mode recorded. Nuclear magnetic resonance relaxation times and the rate of water molecule exchange have been calculated from the data by a least-squares fit of 13 parameters to theoretical equations. The lifetimes of the water molecules in the hydration sphere at 25° were found to be: 7.5 sec for Al(III) (by extrapolation from the high-temperature data) and 5.5 × 10−4 sec for Ga(III) ions. The enthalpies of activation are 27 and 6.3 kcal/mole and the entropies of activation 28 and – 22 eu for Al(III) and Ga(III) ions, respectively. The transverse relaxation time of oxygen-17 bound to Al(III) ions at 25° was found to be 3.2 × 10−4 sec and ascribed to the time-dependent perturbation of the interaction between the quadrupole moment of the oxygen-17 nuclei and the intramolecular electric field gradients existing in the water molecule, due to the tumbling of the complex. The intermolecular contribution arising from the charge of the ion is negligible relative to that of intramolecular mechanism. The kinetic parameters are discussed in terms of the ionic charge, radius, and structure of the complex. Correlations with the charge, ionic size, and ligand field stabilization energies of other ions are shown to be consistent with the obtained data. © 1968, American Chemical Society. All rights reserved.