Hyperpolarized Xe-129 NMR spectroscopic investigation of potentially porous shape-persistent macrocyclic materials

The application of continuous-flow hyperpolarized Xe-129 NMR spectroscopy to investigate the pores of three shape-persistent organic macrocycles is described. Peaks with xenon chemical shifts between 160 and 200 ppm are assigned to xenon atoms trapped in highly confined pores while NMR peaks with chemical shifts of 100 to 140 ppm are assigned to xenon present in channels that exist through the centre of the stacked macrocycles. With the aid of molecular dynamics simulations, connections between the Xe-129 NMR spectra and the X-ray diffraction structure of one of the solvated macrocycles, 1, have been made. The similarities in the Xe-129 NMR spectra of the three compounds containing hyperpolarized xenon suggest that all three hosts possess comparable pore structures. The Xe-129 NMR data provide information about the porous nature of the two compounds for which X- ray crystallographic analysis was not possible. Data obtained from two-dimensional Xe-129 NMR exchange spectroscopy experiments, variable-temperature Xe-129 NMR and molecular dynamics simulations suggest a mechanism whereby the xenon gains access to the highly confined sites via the channels.