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 $H1$-decoupled $C13$ cross-polarization magic-angle-spinning (MAS) NMR and two-dimensional $H1–C13$ correlation spectroscopy, a full assignment of the $C13$ and $H1$ chemical shifts has been made. Using $H1$ NMR relaxometry, the effects of thermal history on the structure of the system have been investigated. Rapidly cooled samples have $H1$ spin-lattice relaxation times $T1$⁠, which at low temperature $(T<60K)$ 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 $C13$ MAS spectra of the two forms provides structural insight into the nature of the disorder. Using $H1$ 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 $H1$ $T1$ 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.