Deuterium NMR Studies of Framework and Guest Mobility in the metal-organic framework compound MOF-5, Zn₄O(O₂CC₆H₄CO₂)₃

Variable temperature wideline ²H NMR investigations of the mobility of aromatic rings in the framework of desolvated samples of the metal organic framework compound MOF-5 (Zn₄O(O₂CC₆H₄CO₂)₃) and the closely related solid MOCP-L have been performed. The aromatic rings in MOF-5 are stationary at temperatures lower than room temperature but perform 180° (π) flips at higher temperatures until all framework aromatic groups execute this motion at 373 K. The aromatic groups in MOCP-L demonstrate a higher degree of mobility with some motion even below 170 K. ²H NMR of perdeuterobenzene adsorbed onto MOF-5 at a loading of two molecules per Zn₄O₁₃ inorganic unit shows that upon cooling below 140 K the motion of benzene changes from isotropic reorientation to rotation about its six-fold axis. Computational simulation suggests that benzene occupies a site close to the Zn₄O₁₃ unit at low temperatures.