1H NMR Study of Molecular Motion of Benzene and n-Decane Confined in the Nanocavities of Metal-Organic Frameworks

The molecular motion of benzene and n-decane confined in the nanocavities of [Zn4O(O2CC6H4CO2)(3)](n) (IRMOF-1) was investigated in terms of the temperature dependence of the proton spin-lattice relaxation time (H-1 T-1). Both substances exhibited two components of T,, below 216 K for benzene and 181 K for n-decane, indicating that the guest molecules are localized separately in large and small cavities. For n-decane below 181 K, methyl reorientation is excited although the rate of reorientation in large and small cavities differed, whereas the overall motion was frozen. A thermal anomaly accompanies the translational diffusion of n-decane and is associated with melting of the confined n-decane. For benzene, translational diffusion was also excited at the thermal anomaly at 216 K, indicating melting of benzene molecular assemblies in IRMOF-1. Below 216 K, the molecular motions in the large and small cavities differed. In the large cavities, benzene undergoes isotropic reorientation, which is unaffected by the thermal anomaly at 150 K, whereas in small cavities in-plane C-6 reorientation occurs, and the isotropic reorientation and the intracavity diffusion are successively excited as the temperature increases. In particular, the acceleration of the isotropic reorientation of benzene accompanies the thermal anomaly at 150 K, corresponding to the partial melting of molecular assemblies in the small cavities.