Water exchange, limits

Now we can proceed to assemble the positive evidence for the path (I II -> IV, Fig. 7). Once the outer sphere complex, (II), is formed, all replacements of water should occur at the same rate, k - lO- If the ion pairing constant Ka is known, or a limiting rate of anion entry corresponding to saturation of the association is observable, the rates of conversion of (II) into (IV) may be compared for various X. All should be equal to / -h20 if the activation mode is d, but they will not equal the rate of water exchange which was identified with on the D path. The reason is that species (II) has a number of solvent molecules in its... 

Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

We should note that the water exchange rate remains relatively constant on introducing different substituents on the ligand, which do not directly interfere in the inner coordination shell. Similar rates have been reported for the different bisamide DTPA derivatives (57,59,60,68). Even bulky substituents on the carbon backbone of the DTPA had limited influence on the water... 

In the IPCM calculations, the molecule is contained inside a cavity within the polarizable continuum, the size of which is determined by a suitable computed isodensity surface. The size of this cavity corresponds to the molecular volume allowing a simple, yet effective evaluation of the molecular activation volume, which is not based on semi-empirical models, but also does not allow a direct comparison with experimental data as the second solvation sphere is almost completely absent. The volume difference between the precursor complex Be(H20)4(H20)]2+ and the transition structure [Be(H20)5]2+, viz., —4.5A3, represents the activation volume of the reaction. This value can be compared with the value of —6.1 A3 calculated for the corresponding water exchange reaction around Li+, for which we concluded the operation of a limiting associative mechanism. In the present case, both the nature of [Be(H20)5]2+ and the activation volume clearly indicate the operation of an associative interchange mechanism (156). 

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

This hypothesis was tested using DMSO (5) to limit both proton and water exchange whereupon the 170 signals of the four oxygen sites in the [Re0(H20)(CN)4] and [ReO(OH)(CN)4]2- ions were clearly identified (Fig. 10). 

Ad. 1. Theoretical considerations including calculations based on the knowledge of anatomical features. While considering whether or not it is reasonable to believe that the non-mono-exponential transverse relaxation reflects an intra- and extra-cellular compartmentalisation, it should be questioned if the slow-exchange assumption is reasonable taking into account the macroscopic features of a muscle cell from a theoretical point of view. The NMR-timescale criterion for exchange limit is comparison of the quantity (rr1 + t"1), where r, and t0 are the average lifetime of a water molecule inside and outside the cell, respectively, with Rt — R , which is the absolute difference in the intrinsic relaxation rates. If R —Rfl > (h 1 + ), the... 

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