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

Abstract

A complex of p-tert-butylcalix [4]arene and chloroform has been prepared and characterized in the solid state using, for the Ðrst time for any calixarene complex, the Rietveld powder XRD method, as well as variable-temperature 2H and 13C solid-state NMR. The 2H NMR spectra are composed of a central isotropic peak, attributed to a small fraction of in the free motion regime inside the crystal lattice, and a CDCl 3 quadrupolar powder pattern. 2H spinÈlattice 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 2H spectra shows a very unusual increase in the e †ective 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 13C MAS NMR spectrum of the guest molecule changes upon prolonged heating, while thermal analysis conÐrms the presence of an exothermic transition attributed to an intermediate state before the choroform is released.

Binding Mechanisms in Supramolecular Complexes

“…This is an indication that an electrostatic mechanism predominates here. [518] Fullerene-calixarene interactions are characterized above all by gains in enthalpy DH; the corresponding losses in entropy TDS represent only about one-fourth of the DH gain, which can be explained by the round, largely dispersively bound and mobile "buckyballs". [517] Deuterium NMR relaxation studies of solid calixarene complexes with chloroform as the guest show an unusually high mobility of the guest molecule, which is regarded as an indication of dispersive interactions.…”

Characterization of dynamic processes using deuterium in uniformly 2H,13C,15N enriched peptides by MAS solid-state NMR

“…In the fast motion limit (10 À7 > s C > 10 À12 s), the deuterium quadrupolar tensor is reduced compared to the static case. The anisotropy of the spin-lattice relaxation time T 1 allows a quantification of the timescale of the motional process and to assign a motional model [11,[21][22][23][24].…”

Inhibitor radicals in styrene polymerization

“…The observed effects were qualitatively analyzed by the increase in the internal rotation barrier height of the guest methyl groups, caused by a CH⋅⋅⋅π interaction deep inside the cavity. To the best of our knowledge,10 this report is the first example in which the isotope effects are ascribed to the increase of the barrier height of the methyl internal rotation by the host–guest noncovalent interaction.…”