17O Magnetic Resonance of Aqueous Solutions of Vanadium(II) and Chromium(III)

Abstract

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‡ = 16.4 ± 0.6 kcal/mole, and ΔS‡ = 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.