Exchange of water molecules

At the same time, as the concentration decreases the exchange of water molecules by cooperative processes becomes easier and so significant fluctuations in the coordination numbers are observed, which are estimated to be about 1. At even lower concentrations, ion-water correlation patterns will become obscured and then undetectable. As an extrapolation, dynamic processes might contribute more and more to the description of the solution. The strong interaction between Li+ and OH2 (dH. i = 34 kcal/mole in the vapor phase 130>) may cause the cation-water complexes to remain quite well-defined tetrahydrates, on the average, despite all dynamic effects. 

Fig. 1. Rate constants, for the exchange of water molecules from the first coordination sphere of a given metal ion and the corresponding mean life times, tm = l//zex, of a particular water molecule measured by " 0 NMR. The gray bars indicate values determined by NMR isotope exchange technique (except Cr + ).

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. 

Although OH reacts at near-diffusion-controlled rates with inorganic anions [59], there seems to bean upper limit of ca. 3 x 10 dm mol sec in the case of simple hydrated metal ions, irrespective of the reduction potential of M"". Also, there is no correlation between the measured values of 43 and the rates of exchange of water molecules in the first hydration shell of, which rules out direct substitution of OH for H2O as a general mechanism. Other mechanisms that have been proposed are (i) abstraction of H from a coordinated H2O [75,76], and (ii) OH entering the first hydration shell to increase the coordination number by one, followed by inner-sphere electron transfer [77,78]. Data reported [78] for M" = Cr, for which the half-life for water exchange is of the order of days, are consistent with mechanism (ii) ... 

There is a conceptual model of hydrated ions that includes the primary hydration shell as discussed above, secondary hydration sphere consists of water molecules that are hydrogen bonded to those in the primary shell and experience some electrostatic attraction from the central ion. This secondary shell merges with the bulk liquid water. A diagram of the model is shown in Figure 2.3. X-ray diffraction measurements and NMR spectroscopy have revealed only two different environments for water molecules in solution of ions. These are associated with the primary hydration shell and water molecules in the bulk solution. Both methods are subject to deficiencies, because of the generally very rapid exchange of water molecules between various positions around ions and in the bulk liquid. Evidence from studies of the electrical conductivities of ions shows that when ions move under the influence of an electrical gradient they tow with them as many as 40 water molecules, in dilute solutions. 

Table 15 Residence Times and Rate of Exchange of Water Molecules and Oxygen...

Dimethylgold(III) species with coordinated hydroxide, nitrate and perchlorate groups have been prepared (274), and the latter two found to exist in aqueous solution as ri,s-[AuMe2(OH2)2]+X-. The 170-NMR spectrum of [AuMe2(C104)] in aqueous solution exhibited (275) only one resonance down to 5°C, indicating that rapid exchange of water molecules occurred above this temperature. 

Exchange of Water Molecules between Enzyme Surface and Bulk Organic Solvent... 

The mechanism of a proton transfer in an allyl system involving the hydroxide ion has been investigated by the RHF/6-31+G and MP2/6-31+G methods with one, two, and four water molecules.153 A possible mechanism of intramolecular proton transfer involving easy exchange of water molecules between the first and second coordination spheres of the propenide ion was considered. 

Fig. 2.47. Hydration number n in relation to coordination number CN in the motion of an ion in water and in relation to exchange of water molecules. CW, coordinated HgO HW, primary hydrating HjO molecules. (Reprinted from J. O M. Bockris and P. P. 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]. 

Qualitative information about the rate of ligand substitution in octahedral complexes is summarized in Table 18.1. Quantitative data are obtained by application of ligand field theory. The rate constants may vary by many orders of magnitude. Thus, for the exchange of water molecules in the first coordination sphere of 3d transition elements the following values have been measured Cr (d" ) 7 10 s Cr (d ) 5-10- s-i Mn2+(d5) 3-lO s- Fe2+(d ) 3-10 - Co + d ) 1 lOS Ni2+(d ) 3 104s-i. 

Kobayashi et al. studied the catalytic activity of many metal salts in Mukaiyama-aldol reactions in aqueous THE They came to the conclusion that the catalytic activity of a metal in aqueous media should be related both to the hydrolysis constant, /Ch, and water exchange rate constant (WERC) of the metal [8]. All metals with good catalytic activity had p/Ch values ranging between 4.3 and 10.08 and WERC > 3.2 X 10 s This was because when for a metal is < 4.3, the metal cation is readily hydrolyzed to generate oxonium ion, which then helps the decomposition of the silyl enol ethers. When pMh > 10.08 the Lewis acidity of the metal is too low to promote the reaction. When the WERC is < 3.2 x 10 m s, exchange of water molecules seldom occurred and aldehydes had a very little chance to coordinate to the metal to be activated. The metals which fulfill these criteria are Sc(III), Fe(II), Cu(II), Zn(ll), Y(IIl), Cd(Il), Ln(Ill) and Pb(ll). 

In a computer simulation of fluoride ions in water, possible intermolecular scalar relaxation coupling mechanism was studied, modulated by water dynamics (exchange of water molecules in the first hydration sphere) due to modulation of the J coupling [81]. 

Table 1. A compilation of rate coefficients and activation parameters for exchange of water molecules from the inner-coordination sphere of Al(III) complexes to the bulk solution, as determined from 0-NMR. Included in these data are estimates for the substituted s-Keggin molecules.

Casey WH, Phillips BL (2000) The kinetics of oxygen exchange between sites in the Ga04Ali2(0H)24(H20)i2 (aq) molecule and aqueous solution. Geochim Cosmochim Acta 65 705-714 Casey WH, Phillips BL, Nordin JP, Sullivan DJ (1998) The rates of exchange of water molecules from Al(III)-methylmalonate complexes The effect of chelate ring size. Geochim Cosmochim Acta 62 2789-2797... 

When cells are exposed to hyper- or hypo-osmotic solutions, they immediately lose or gain water, respectively. Even in an isotonic medium a continuous exchange of water occurs between living cells and their surroundings. Most cells are so small and their membrane so leaky that the exchange of water molecules measured with isotopic water reaches equilibrium in a few milliseconds. 

The hydroxyl radical reacts with many inorganic anions at near-diffusion controlled rates (k 10 ° dm mol s [5]), but with equated metal ions (M"" = Tl Ag Cu Sn Fe Mn ) the rate constants (k) have an upper limit of 3 x 10 dm mol" s" There is no correlation between k and the rates of exchange of water molecules coordinated to which rules out ligand substitution as a general mechanism. Other possibilities are abstraction of H from a coordinated water molecule or OH entering and expanding the... 

Exchange of water molecules between different solvation shells... 

The exchange of water molecules in the hydration sphere for other dissolved species can be extended to include "mixed ligand complexes", i.e. those in which water molecules have been replaced by two or more different types of ligands, and "multi-dentate"... 

Several of the water moleculas of high occupancy are involved in as many as five hydrogen bonds, to 0 and N atoms of the protein and to other water molecules this reflects the fact that the structure is a dynamic one, and that the bonding shifts from instant to instant. The occupancy figures represent an average over time they indicate the fraction of the time that the site is occupied, but give no indication as to the rapidity of exchange of water molecules in and out of the site. 

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