Thermal history effects and methyl tunneling dynamics in a supramolecular complex of calixarene and para-xylene

The low-temperature structure and dynamics of guest molecules of p-xylene incorporated in the isopropyl-calix[4] arene(2:1) p-xylene complex have been investigated by solid state nuclear magnetic resonance (NMR). Using one-dimensional 1H-decoupled 13C cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional 1H-13C correlation spectroscopy, a full assignment of the 13C and 1H chemical shifts has been made. Using 1H NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have 1H spin-lattice relaxation times T1, which at low temperature (T<60 K) are typically two orders of magnitude faster than those observed in annealed samples which have been cooled slowly over many hours. In both forms, the low-temperature relaxation is driven by the dynamics of the weakly hindered methyl rotors of the p-xylene guest. The substantial difference in T1 is attributed in the rapidly cooled sample to disorder in the structure of the complex leading to a wide distribution of correlation times and methyl barrier heights. A comparison of the linewidths and splittings in the high resolution 13C MAS spectra of the two forms provides structural insight into the nature of the disorder. Using 1H field-cycling NMR relaxometry, the methyl dynamics of the p-xylene guest in the annealed sample have been fully characterized. The B-field dependence of the 1H T1 maps out the spectral density from which the correlation times are directly measured. The methyl barrier heights are determined from an analysis of the temperature dependence.