The optimal binding sites of CH4 and CO2 molecules on the metal-organic framework MOF-5: ONIOM calculations

Optimal binding sites and its corresponding binding energies between MOF-5 clusters and small guest molecules, CH4 and CO2, were investigated using the ONIOM method with different levels of quantum chemical calculations. The clusters were validated using three different sizes of the MOF-5 clusters, SINGLE, DOUBLE and TRIPLE consisting of (Zn4O)2(COOCH3)10(COO)2C6H4, (Zn4O)3(COOCH3)14(COO)4(C6H4)2 and (Zn4O)4(COOCH3)18(COO)6(C6H4)3 units, respectively. Guest molecules were assigned to lie in the configurations parallel (∥) and perpendicular (⊥) to linker (LINK) and corner (CORN) domains of the clusters. The ONIOM(MP2/6-31G∗∗:HF/6-31G∗∗) with the corrections due to the basis set superposition errors was found to be the optimal choice for the investigation of these systems. Strong effects of cluster size were found for the CO2/MOF-5 complexes, i.e., the SINGLE cluster is sufficient to represent interactions with CH4, but the interaction with CO2 requires the TRIPLE model. The optimal binding sites of guest molecules as well as their orientations in the cavity of the MOF-5 are CORN⊥ for both CH4 and CO2 with the corresponding binding energies of −3.64 and −9.27 kJ/mol, respectively.