Slow exchanges of water molecules

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. 

In the case of some ion-transfer reactions the chemical desolvation step controls the rate of the overall process and the currents observed are lower than those expected for the process limited solely by the mass transport rate. The formation of such less-hydrated species was attributed [210] in the case of the electroreduction of nick-el(II) in water to a slow exchange of water molecules from the first solvation sphere of Ni(II) under the influence of the crystal field stabilization. A similar mechanism was found for Ni(II) and Co(II) in methanol [211]. 

There is other evidence that the exchange of water molecules between the site in A W B and bulk solvent is slow compared to both proton transfer within the complex and the separation of A from B. Regarding the former, the fact that so many proton-transfer reactions in which AG° is negative are diffusion-controlled proves that the proton-transfer step is fast compared to the dissociation of the complex. Regarding the latter, if the departure of a water molecule from A W B were fast compared to the dissociation of the complex, one would expect to find more examples of rapid direct bimolecular proton transfer without solvent participation. 

The anions and cations in solution are normally hydrated and although hydration is a dynamic process, it is usually possible to identify a small number of water molecules in the hydration shell immediately surrounding the ion whose exchange rate is slow in comparison with other processes that might take place as the ions approach and recede from the interface. The picture of Figure 1.6 is, in any case, intended to represent a dynamic... 

Because distance and time can be coupled by motion, we could also view the timescales available to be probed with NMR and would find the same staggering range (Belton, 1995). Time constants for molecular processes can be quantified by magnetic resonance techniques ranging from extremely fast (picoseconds, such as for the tumbling of water molecules) to extremely slow (tens of seconds, such as for selected chemical reactions or exchange). 

Transport-related non-equilibrium behavior (e. g., physical non-equilibrium) is excluded, which plays an important role in non-ideal solute transport in the field and in some experimental column systems. Physical non-equilibrium is due to slow exchange of solute between mobile and less mobile water, such as may exist between particles or between zones of different hydraulic conductivities in the subsurface soil column, and occurs for sorbing and non-sorbing molecules alike. 

In macromolecules, slow exchange effects often quench the relaxivity (Pig. 30) (37) even in the presence of water molecules directly coordinated to iron(III) (91). For instance, in methemoglobin the relaxation rates are attributable to one water molecule coordinated to the paramagnetic center... 

The opposite signs for the neutron scattering power of hydrogen and deuterium (—0.38 and +0.65) offers the possibility for investigating (slow) self-diffusion between different water sites and/or localization of water molecules with different mobility if diffraction experiments are carried out for a sample where D20 is exchanged in steps vs. H20. 

In contrast to protonated formaldehyde itself, the proton bound dimer between unlabelled formaldehyde and 180 labelled water reacts with a second molecule of 18OH2by slow exchange of the carbonyl 160 [131]. 

The solvation of chromium(llI) ion in certain mixed-solvent systems has been studied in experiments which are relatively free of ambiguity. The exchange of solvent molecules between the mixed solvent and the solvated species Cr(OH2)w (So)n3+ (So = organic solvent component) is a very slow process. The species with solvation shells having different compositions can be separated from one another by column ion-exchange procedures. Analytical procedures based upon such separations allow evaluation of equilibrium constants for reactions involving replacement of coordinated water by the polar organic component. These equilibrium constants are reviewed in this chapter with attention focused upon the dependence of the equilibrium constants upon solvent composition, and the relationship of relative values of the equilibrium constants to the statistically expected values. 

A. Theories of Fast and Slow Exchange. In 1957i Zimmerman and Britten published a theoretical treatment of the relaxation of water protons absorbed on silica gel (l6). In this system, both uni- and multiphasic relaxation decays were observed. The authors were able to account for the change in the number of observed phases by taking into account the relaxation times of the water protons in bound (absorbed) or free (non-absorbed) states, and the lifetimes of water molecules in each state. Two asymptotic expressions were derived, which have frequently been used in subsequent studies of water relaxation. 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

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