17O MAGNETIC RESONANCE OF AQUEOUS SOLUTIONS OF VANADIUM(2) AND CHROMIUM(3)

Oxygen 17 nuclei in aqueous V2+ and Cr3+ solutions give rise to a single NMR line emanating from the bulk solvent oxygens. In the V2+ case, the linewidth increases with temperature above 30° C due to chemical exchange between bulk and bound waters: k298 = 90±20 sec-1, ΔH 2 = 16.4±0.6 kcal/mole, and ΔS 2 = 5.5±1.5 e.u. The linewidths narrow with increasing temperature up to 30°C for a V2+ solution, and up to 120°C in the Cr3+ case. Relaxation in these regions is attributed to time-dependent interactions between the unpaired electrons of the paramagnetic ions and outer-sphere solvent molecules. A detailed treatment shows that it may be important to consider scalar as well as dipole-dipole interactions in interpreting the observed outer-sphere relaxation.