Chloroform encapsulated in p-tert-butylcalix[4]arene:: Structure and dynamics

A complex of p-tert-butylcalix[4]arene and chloroform has been prepared and characterized in the solid state using, for the first time for any calixarene complex, the Rietveld powder XRD method, as well as variable-temperature H-2 and C-13 solid-state NMR. The H-2 NMR spectra are composed of a central isotropic peak, attributed to a small fraction of CDCl3 in the free motion regime inside the crystal lattice, and a quadrupolar powder pattern. H-2 spin-lattice relaxation times lead to activation energies for the molecular motion of 4.9+/-0.5 and 6.0+/-0.6 kJ mol(-1) for the two spectral components, respectively. The temperature dependence of the static H-2 spectra shows a very unusual increase in the effective quadrupolar coupling constant with temperature, caused by the change in the geometry of the complex. We show that the complexation is caused by weak dispersion interactions. The C-13 MAS NMR spectrum of the guest molecule changes upon prolonged heating, while thermal analysis confirms the presence of an exothermic transition attributed to an intermediate state before the choroform is released.