Molecular mobility of tert-butyl alcohol confined in a breathing MIL-53 (Al) MOF.

We present a detailed solid state NMR characterization of the molecular dynamics of tert -butyl alcohol (TBA) confined inside breathing metal-organic framework (MOF) MIL-53 (Al). 27 Al MAS NMR demonstrates that TBA adsorption induces the iX phase of MIL-53 framework structure with partially shrinked channels. 2 H solid-state NMR shows that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two [[EQUATION]] axes and librations of the molecule as a whole about the axis passing through the TBA C-O bond. These librations are realized by two distinct ways: the faster ones with characteristic time τ D ≈10 -9 s at 300 K, reflects the molecule orientation change during the translational jump diffusion along the channel; the slower one with τ l ≈10 -6 s at 300 K reflects the local librations at a single coordination site, representing framework hydroxyl groups. We provide an estimation of self-diffusion coefficient of the alcohol in the MOF: D = 3.4×10 -10 m 2 s -1 at 300 K. It is inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL-53 (Al).