Mobility of Aromatic Guests and Isobutane in ZIF-8 Metal–Organic Framework Studied by ²H Solid State NMR Spectroscopy

Metal–organic framework ZIF-8 is renowned for its adsorption and separation properties influenced by the framework flexibility. Although much effort was put into understanding the molecular mechanism of such gate-opening (dynamical and static), the mobility of large guest themselves remained unclear. In this work, we present a detailed 2 H solid-state NMR investigation of large guests (xylene isomers, benzene, toluene, and isobutane) confined inside the ZIF-8. Evolution of line shape and T 1, T 2 relaxation times with temperature were interpreted in terms of existence of three different motional states for diffusing guest molecules: (i) the highly mobile guest molecules located in the central part of the cage; (ii) the bound state for a guest located on the cage wall or near the window; and (iii) the guest squeezing through the window with a subsequent release to the next cage. We report detailed quantitative analysis of molecular dynamics in each of the states and the kinetics of exchange between the bound and mobile states.