Water exchange rate constants measured by oxygen-17 NMR

Water Exchange Rate Constants Measured by Oxygen-17 NMR 

Slow Exchange Studied by Isotopic Labeling Technique 

To study water exchange on aqua metal ions with very slow exchange of water molecules, an isotopic labeling technique using oxygen-17 can be used. A necessary condition for the applicability of this technique is that the life time, tm, of a water molecule in the first coordination shell of the ion is much longer that the time needed to acquire the 0-NMR spectrum. With modern NMR spectrometers and using enrichments up to 40% in the acquisition time can be as short as 1 s. 

If the aqua complex is very inert, as for example in the case of [Rh(H20)6] or [Ir(H20)6], the experiment is best performed by dissolving the complex enriched in 0-water, [M(H2 0)6] , in non-enriched water (2,26). Preparing the initial condition in this way leads to a relatively intense signal for bound water. The exchange rate constant can then be measured by observing the decrease in the bound water signal with time. The decrease in the mole fraction of labeled water coordinated to the metal, x, is described by Eq. (7)  

The mole fractions of labeled water at t = 0 and at equilibrium are noted as Xq and Xoo, respectively (Pig. 4). In the end, the signal of bound water becomes small and difficult to quantify. But, this does not influence the quality of the measured rate constant because the mole fraction at equilibrium, x, is known from the concentration of the metal ion and the coordination number. These experiments can be performed at variable temperature and at variable pressure to obtain activation enthalpies and entropies as well as activation volumes. 

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Water exchange rate constants measured by oxygen-17 NMR... 

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