Probing the Guest-Mediated Structural Mobility in the UiO-66(Zr) Framework by 2H NMR Spectroscopy

The solid-state H-2 NMR technique (analysis of both the spectrum line shape and the spin lattice relaxation) was used to probe both slow and fast dynamical modes of the phenylene fragments of terephthalate linkers of the UiO-66(Zr) framework affected by the presence of benzene guest in the pores of the material. Such approach allowed us to probe different motions within a broad range of time scale, 10(-3)-10(-11) s. The internal dynamics in the UiO-66(Zr) framework is represented by torsional motions of the phenylene fragment of the linker induding 2-site 180 degrees flips (pi-flips) of the plane of the phenylene ring and its restricted librations. In the presence of benzene loaded in the MOF pores the rate of pi-flips decreases essentially and the activation barrier for this motion increases. The activation barrier has been found to increase almost in a linear fashion on benzene loading. Such observation is surprisingly unique among other MOFs with mobile linkers, like MIL-53(Al) or MOF-5. The fast librational motion occurs on a scale of similar to 10(10) Hz and shows no notable dependence on the guest loading. It has been established that anisotropy of T-1 relaxation of the H-2 NMR powder pattern of the phenylene fragments is especially sensitive to the librational motion when this motion is in a range of 10(7)-10(11) Hz. Within this range of libration frequencies, analysis of the anisotropic spin-lattice (T-1,) relaxation allows quantitative estimation of the rate of librational motion.