NMR parameters in the presence of exchange

NMR PARAMETERS IN THE PRESENCE OF EXCHANGE 4.3.1 Exact solutions for two-site exchange... 

Prompted by its crystal structure [34a] and the fact that some of these aggregates were formed in water-saturated chloroform solutions, the role of the water molecules in the formation of the hexameric capsules of 15c and 16c was also studied. Diffusion NMR was selected since the chemical shift of water protons in the presence of acidic compounds is very sensitive to the amount of water, the concentration of the acidic material, the exchange rate and solvent pH and is therefore not a predictable parameter. In addition, since water molecules are very small relative to the resorcinarene and pyrogallolarene hexamers, the water diffusion coefficient was anticipated to be a rather sensitive parameter to follow water interaction with these supramolecular structures. Indeed, when the water diffusion coefficient was followed as a function ofthe I5C/H2O ratio, a decrease in the diffusion coefficient of the water peak was observed with the increase of the 15c/H2O ratio, as shown in Fig. 6.12. Fig. 6.12a-d shows the signal decay of the water (Fig. 6.12a, c) and the... 

Because each chemical shift difference between exchanging sites as well as each spin coupling constant gives rise to a coalescence of its own when Av or J k (where k is the rate of site exchange), a system having many such parameters will, because of the presence of a multitude of internal clocks be more sensitive in the response of its NMR spectrum to temperature changes 87). Thus, within the limits of feasibility of computer treatment, the more shifts and coupled spins, the better. 

The activation parameters for the exchange reactions of 17 and 18 were determined by a combination of variable-temperature ll NMR lineshape analysis16 and spin saturation transfer experiments.17 Rate data for 17 were measured over a temperature range of 100 "C. Rates for compound 18 were measured over a 65 °C range. The enthalpy of activation was found to be considerably smaller in the case of 17 (12.2(2) kcal/mol) relative to 18 (17.6(3) kcal/mol). Ion pair dissociation is therefore facilitated by the presence of a lone pair of electrons on the boron substituent. The entropy of activation for 17 is -2.3(6) eu, while that of 18 is 8(1) eu. The more positive entropy of activation measured for 18 may be interpreted as the creation of two independent particles from a closely associated ion pair. 

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