Water exchange rates for

Fig. 2. Calculated relaxivities as a function of the water exchange rate for various proton Larmor frequencies and rotational correlation times, tr. The simulations have been performed by using the common Solomon-Bloembergen-Morgan theory of paramagnetic relaxation.

Water exchange rate = 560 s per coordination site recalculated to second-order units 4k, ,, /55 Water exchange rate for a particular coordination site. 

The mean residence lifetime of coordinated water protons represents one of the most important parameters that control the relaxivity of Gd(III) complexes 18). For several years it had been assumed that xm for the low molecular weight polyaminocarboxylate Gd(III) complexes was of the order of a few ns, as found for the octaaqua ion. In 1993, Merbach reported the first direct measurement of the water exchange rate for [GdDTPA(H20)] and [GdD0TA(H20)] and found that for these CAs the rate of water exchange was nearly three orders of magnitude lower than that for [Gd(H20)8] 49). 

The larger Mg " " ion is hexa-coordinated. The water exchange rate for [Mg(H20)6] " (40) lies between those of the d-transition metals [Co(H20)6] + and [Ni(H20)e] + (Tables I and Table II), and reflects the order of ionic radii of these three ions. The measured activation volume is... 

P 20.4 Experimental Determination of the Total Air-Water Exchange Rate for Two Chlorinated Hydrocarbons in a River... 

The nine-coordinate Gd(III) poly(amino carboxylates) all have positive activation volumes as indicative of dissociatively activated water exchange, and much lower exchange rates in comparison to the Gd(III) aqua ion (Table 1). Several factors should be considered to rationalize the decreased water exchange rates. For many of these complexes only the coordination number of nine exists... 

In the absence of data on water exchange rates for lighter lanthanides, it is difficult to say whether water exchange rates will be similar to sulphate complex formation rates for the lighter lanthanides. 

If we use this rate constant to calculate the water exchange rate following the derivation used for the T1(EDTA) -X system, we obtain = k(IK s = 1.3 10 s K a = 0.012 M ). This value is close to the water exchange rate for the Eu q ion (296). It is possible that Tl(EDTA)aq" behaves similarly to rare-earth aminopolycarboxylate complexes, in which some water molecules can also be coordinated to the metal ion. 

The kinetics of the reactions of lead complexes (Section IV) in aqueous solutions and mixed media is of equal importance to understanding the fundamental environmental (Section V) and biological chemistry (Section VI) of lead. It is important to note that although lead forms very thermodynamically stable complexes, ligands bound to lead tend to be extremely labile in aqueous solutions The water-exchange rate for lead is 7 x 10 s (49). 

The composition of the primary hydration sphere of the lanthanide ions continues to be a matter of intense interest from both experimental and theoretical points of view. Lincoln (1986) discussed both hydration numbers and water exchange rates for the lanthanides, ending with the conclusion that the most probable hydrated lanthanide ion is R(H20)9 for the entire series, with the qualifier that new data could alter this opinion. 

Water is a good ligand and has been used to determine water exchange rates for many metals, and metals with a variety of oxidation states. The values can serve as a general guide to gauge what you might expect to happen for a certain metal ion as per the oxidation state. It s particularly useful if you want to compare the relative reaction rates of two different metals, or to determine the relative reaction rates of the same metal but with different oxidation states. 

The comparatively large water-exchange rates for these trivalent ions have been attributed to their having co-ordination numbers greater than six. 

The predictions of this theory are qualitatively correct but it does not explain the wide range of reactivities, especially of the labile systems. For example, why is the water exchange rate for Ni(II) 10 times slower than that for Co(II) and >10 times slower than that for Cu(II) ... 

FIGURE10.13 Example model-predicted sediment-water exchange rates for solids and PCB s in the Lower Fox River. 

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