  We report an investigation of the "missing-linker phenomenon" in the Zr-based
metal-organic framework UiO-66 using atomistic forcefield and quantum chemical
methods. For a vacant benzene dicarboxylate ligand, the lowest energy charge
capping mechanism involves acetic acid or Cl-/H2O. The calculated defect free
energy of formation is remarkably low, consistent with the high defect
concentrations reported experimentally. A dynamic structural instability is
identified for certain higher defect concentrations. In addition to the changes
in material properties upon defect formation, we assess the formation of
molecular aggregates, which provide an additional driving force for ligand
loss. These results are expected to be of relevance to a wide range of
metal-organic frameworks.
