Molecular exchange rates

When the data in this table are plotted, the graph shown in Fig. 33 is obtained. From this one can calculate a pKa of 2.85 for displacement of benzimidazole in D2O. In addition, since room temperature is above the coalescence temperature, it is possible to set a lower limit on the exchange rate between coordinated and uncoordinated benzimidazole of 3.1 X 102 sec-1. From Fig. 33 one can, by extrapolation, calculate the C(20)-methyl resonance of the base-on and "base-off forms to be 0.41 and 1.05 respectively. These numbers can be used, with the assumption of fast exchange, to determine the relative amounts of "base-on and base-off" species from the observed C(20)-chemical shift for any arbitrary sample. Such information would be useful, for instance, when investigating the displacement of benzimidazole by other Lewis bases. Thus for the simple case of benzimidazole displacement we have shown that NMR provides a method for studying the molecular conformation of vitamin B12. 

Table 1 Water-exchange rates and activation volumes determined on small molecular weight Gd111 chelates...

The 3H chemical shifts, coupling constants, temperature coefficients, exchange rates and inter-residual ROEs of the hydroxyl protons of various synthetic type II trisaccharides, analogues of (3-D-Galp-(l - 4)-p-D-GlcpNAc-(l - 2)-a-D-Man-(l - 0)(CH2)7CH3, were reported and interpreted,8 assisted with molecular dynamic simulations, to deduce key information on the conformational behavior of these important molecules in aqueous solution. 

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). 

Figure 20.9 Decreasing air-water exchange rate of 02 for a water surface covered with an increasingly thick film of oil. Experimental data by Downing and Truesdale (1955) (dots) are compared with curves calculated from Eqs. 20-39 and 20-40. K0l oM/w is the oil-water distribution coefficient of 02 D0l oi, is molecular diffusivity of 02 in oil. Note that the term containing the air-side exchange velocity, v,a, is neglected.

From the experimental point of view, significant variations in kobs can be induced by changes in solvent and/or molecular size. For example, there are relatively small contributions to A for the self-exchange reactions for the Ru(NH3)63+/2+and Ru(bipy)33+/2+couples in Table 1 and the effects of the differences in molecular radii on KA and Ao are sufficient to account for the difference in self-exchange rate constants of 106. 

The Free Volume and the Rate a of Particles Participating in Molecular Exchange Processes... 

Kinetic processes have been shown by NMR and other spectral data to involve both intra- and inter-molecular exchange for 1 1 complexes of alkaline earth metal ions and (52a) (Scheme 7).190 The internal exchange between the two monocycles occurs at a faster rate than the intermolecular process, however. 

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