Encapsulation and Molecular Capsules

Molecular capsules [29, 30] are molecules that can accommodate smaller molecules in their cavities. Therefore, diffusion coefficients which reflect the effective molecular size of the investigated molecular species should be a sensitive tool for 

The most plausible explanation for this observation is that the DMSO indeed disintegrates the hydrogen-bond aggregates [31a]. 

The power and simplicity of DOSY in probing intermolecular interactions in general and encapsulation in particular, is shown in Fig. 6.10. Upon addition of the cobaltocenium cation (13) to the C2D4CI2 solution of the dimer 12-12, an extra peak at about 2.8 ppm was observed beyond that of 13. When ferrocene (14) was added to the solution, no additional high-field peak was observed. The DOSY spectrum, in Fig. 6.10b, of the C2D4CI2 solution of dimer 12 12 in the presence of 13 and 14 clearly demonstrates how trivial it is to map the mutual interactions that prevail in the solution between the different molecular species [31c]. Only the peak 

13 and 14 and (b) the DOSY spectrum (400 MHz, 298 K) of a C2D4CI2 solution of 12 12 in the presence of 13 and 14, showing that the peak at 2.72 ppm, suspected to be the encapsulated cobaltocenium cation, has the same diffusion coefficient as all the peaks of 12 12. The peaks of the free ferrocene and cobaltocenium cation at 4.06 and 5.61 ppm, respectively, were found to have much higher diffusion coefficients, as expected (data from ref. [31c]). (Reproduced with permission from ref. [10]. Copyright 2005 Wiley-VCH.) 

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 

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