Hyperpolarized ¹²⁹Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks

The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable-temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19 °C. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with X-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D exchange spectroscopy (EXSY) 129Xe NMR. It was found the exchange of xenon from adsorption sites within the IRMOF to the free gas space is much slower than that between the adsorption sites within the lattice. Cross-polarization experiments showed that the preferred adsorption sites were spatially removed from the phenylene rings of the network. This agrees with previous spectroscopic, structural, and computational studies of gas adsorption (H2, N2, Ar) in IRMOFs that indicate the preferred binding sites reside near the carboxylate groups of the inorganic clusters.