A pulsed gradient spin echo NMR study of guest encapsulation by hydrogen-bonded tetraurea calix[4]arene dimers

Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights

“…) was exactly the same as the one determined for the dimer [68] and illustrates that, as expected, the encapsulated benzene and the dimer diffuse as a single molecular entity. A titration experiment in which the diffusion coefficients of the dimer and the DMSO molecules added to the benzene solution of 26·26 were followed demonstrated that it was possible to determine by diffusion NMR spectroscopy that about four DMSO molecules per molecule of 26 are required to disrupt the dimer.…”

“…Moreover, the encapsulated molecules, which are in slow exchange with the bulk, should have the same diffusion coefficient as the capsule itself since the capsule and the encapsulated molecules diffuse as a single molecular entity. [68] Such a diffusion-based argument was used recently to probe benzene encapsulation in the tetraureacalix [4]arene dimer (26·26).…”

“…[67] Indeed, in recent years, such systems have been used to stabilize reactive intermediates [67a] and enhance the rate of some reactions. [67b,c] The diffusion coefficient should be a powerful tool for detecting and probing encapsulation, [68] since the encapsulated guests are generally much smaller molecular species than the capsule itself and, therefore, should have a much higher diffusion coefficient in its free state. Moreover, the encapsulated molecules, which are in slow exchange with the bulk, should have the same diffusion coefficient as the capsule itself since the capsule and the encapsulated molecules diffuse as a single molecular entity.…”

“…A titration experiment in which the diffusion coefficients of the dimer and the DMSO molecules added to the benzene solution of 26·26 were followed demonstrated that it was possible to determine by diffusion NMR spectroscopy that about four DMSO molecules per molecule of 26 are required to disrupt the dimer. [68] Frish et al subsequently used a similar approach to probe the encapsulation of a tropylium cation by 26·26 in an attempt to evaluate the relative importance of electronic effects, namely, the importance of p-cation interactions [69] , in determining the guest affinity for the cavity of the dimer. It was found that p-cation interactions indeed play an important role in determining the relative affinity toward the dimer cavity, as it was found that the tropylium cation affinity is about four orders of magnitude larger than that of benzene, which happens to have a very similar size.…”

Synthesis and capsule formation of upper rim substituted tetra-acrylamido calix[4]arenes

“…Similar changes in diffusion coefficient, along with chemical shift changes in the 1 H-NMR spectrum, are observed for the inclusion complex with triazine 5b·5b·7 accompanied by a change of diffusion coefficient from 2.2 × 10 −10 m 2 s −1 to 7.90 × 10 −11 m 2 s −1 on encapsulation. Change in a diffusion coefficient is in line with encapsulation, as discussed by Cohen et al 21, 22 Encapsulation appears to be irreversible. If inclusion complex 5b·5b·7 is treated with an excess of 6 no exchange of the included guest is apparent after 400 h by 1 H-NMR spectroscopy and the same was observed for the reverse experiment, in which inclusion complex 5b·5b·6 was treated with an excess of 7.…”