Abstract
H-2 NMR and quasi-elastic neutron scattering techniques have been used to study the rotational dynamics of the 1,4-benzene-dicarboxylate (BDC) linkers in the porous cubic UiO-66(Zr) metal-organic framework (MOF). The rotation of the benzene rings in the BDC linkers is at the limit of detection of the neutron technique, but it fits perfectly on the H-2 NMR time scale. The aromatic rings in the UiO-66 framework exhibit the lowest rotational barrier compared to other MOFs, the activation energy for pi-flips being 30 kJ mol(-1). However, instead of having well-defined flipping rates like in MOF-5, MIL-47, or MIL-53, UiO-66(Zr) shows a distribution of flipping correlation times, probably due to local disorder in the structure. Because of the rotational motion of the benzene rings, the effective size of the microporous windows in UiO-66(Zr) appears to be temperature dependent.