The properties of the bonding between CO and ZIF-8 structures: a density functional theory study

The bonding properties of CO molecule with both defective and complete ZIF-8 structures are investigated by using density functional theory. Their interaction is explored by the geometry parameters, the natural bond orbital and atoms in molecules analyses, the vibrational frequency as well as the binding energy of CO molecule. The result shows that CO attaches directly to one of the 2-methylimidazolate ligands of complete ZIF-8 structure with forming a C(O)···HC–C2H2N hydrogen bond. For defective ZIF-8 structures, they form an ionic Zn–C (Zn–O) bond with a weak covalent nature through a strong σ binding orbital in C-side adsorption and donor–acceptor interaction in O-side adsorption situation, respectively. Compared to a negligible Lewis acidity of complete ZIF-8 structure, the Lewis acidity of defective structures is much stronger, and their strengths increase with the decrease in the coordination number of zinc cation. However, the net electrons accepted by zinc cations are found to decrease with the decrease in the coordination number. These quantitative results would help to understand the catalysis mechanism of some reactions incorporated in ZIF-8 and promote the acid catalysis research with respect to metal centers of MOFs and ZIFs.