Bulk-water relaxation rate

The longitudinal inner-sphere relaxation rate, l/Ti, of bulk water protons is given by Equation (2) 13... 

The linear relationship between H NMR transverse relaxation rate and (1 av) is shown in Figure 30 for pregelled potato starch (Hills et al., 1999). The change in slope at about 0.90 c/w corresponds to the bulk water break (i.e., the removal of bulk water) in a corresponding adsorption isotherm. Equation... 

Several other nmr procedures have been used for the determination of fractionation factors. These have advantages in some systems. Instead of determining the effect of the concentration of an exchanging site on the averaged chemical shift, the effect on the averaged relaxation rate of water protons can be used in a very similar way (Silverman, 1981 Kassebaum and Silverman, 1989), For example, addition of the enzyme Co(ii)-carbonic anhydrase to an aqueous solution increases the observed value of XjT because the proton-relaxation rate is the average of that for the bulk solvent (cfl. 0.3 s ) and that for water bound to the cobalt ca. 6x 10 s ). The average is different in an H2O/D2O mixture if the bulk solvent and the Cobound solvent have different deuterium contents, and it has been used to determine a value for the fractionation factor of Co-bound water molecules in the enzyme. 

In Eq. (1) Riu) is the longitudinal i = 1) or transverse i = 2) relaxation rate of the bulk water protons, corresponding to that measured for an analogous diamagnetic solution. In practice, Ri, coincides with the value determined for pure water under identical conditions of pH, temperature, and observation frequency. Clearly, the above relation strictly holds only for dilute solutions, in the absence of solute-solute interactions and of variations in viscosity. 

This situation can be compared with the first one where the water core contains a hydrophilic paramagnetic complex. The vesicular water molecules relax faster than the bulk ones and the overall paramagnetic relaxation rate depends on the exchange rate of the water through the bilayer. The synthesis of such systems is cumbersome and, to the best of our knowledge, has not been reported. 

I/T2A is the relaxation rate of bulk water in absence of exchange, AmM is the chemical shift difference (in rad s ) between bound and bulk water in absence of exchange, and tm is the life time of a water molecule in the first coordination shell of the metal ion M. The reciprocal of tm is the exchange rate constant for the exchange of a water molecule. The direct influence of the paramagnetic center on outer sphere molecules is included in a term I/T20S-... 

Relaxation times Tt and T2 have been determined as a function of temperature and surface coverage in various zeolites, particularly of the faujasite type. The early experiments have been troubled by the very strong dependence of relaxation rates on the concentration of paramagnetic impurities. In order for the relaxation values to be meaningful, such impurities expressed as Fe content must be below ca. 6 ppm. Figure 38 shows the variation of Tt and T2 for water adsorbed in a particularly pure sample of zeolite Na-X (248). The authors (248) account for the experimental results using a model of the intracrystalline fluid, which is about 30 times as viscous as bulk water at room temperature. It shows a broad distribution of molecular mobilities (the ratio T,/T2 at the minimum in Tt is much larger... 

---