Understanding structural flexibility of the paddle-wheel Zn-SBU motif in MOFs: influence of pillar ligands

Abstract

Structural parameters and energies of secondary building units (SBUs) in Zn₂(C₈H₄O₄)₂·C₆H₁₂N₂ have been examined with DFT and CCSD(T) methods. The SBU structure exhibits flexibility due to close energies of formation of its D_4h, D₄, and D_2d forms. The potential energy surfaces (PESs) calculated for these systems testify that the presence of pillar ligands with negatively charged nitrogens (NCH or DABCO) can lead to almost barrier-free transitions between structural SBU forms. The mobility regions of the SBU motif have been determined.