Probing molecular motions in metal-organic frameworks with solid-state NMR

Abstract

Molecular motions in metal-organic frameworks (MOFs) are important phenomena that involve the sorption of gases, the rotation of linkers, the diffusion of ions, the reaction of chemicals, and so on. Understanding these motional processes is essential for both fundamental science and the rational design of MOF materials with better functions. Solid-state nuclear magnetic resonance (SSNMR) is a unique tool that can capture dynamic pictures across a large range of time scale through a multitude of ways. It can determine the mode and rate of motion for selected molecular components either of the host framework or of the guest molecules. To characterize the molecular motions accurately and comprehensively, one needs to employ suitable SSNMR strategies according to different motion regimes. These strategies can be generally divided into four categories: exchange spectroscopy for relatively slow motion/reaction (100 ~ 103 Hz), lineshape analysis for intermediate motion (103 ~ 107 Hz), relaxation measurement for fast motion (107 ~ 1012 Hz), and pulsed-field gradient NMR for translational diffusion. This review provides brief introductions to relevant SSNMR strategies and presents recent progresses on the studies of molecular motions in MOFs.