Structure of functionalized porous metal-organic frameworks by molecular orbital methods

The molecular structures of the Zn4O(RCOO)6, R=CH3 and Ph, complexes, the model compounds (CH3COO)5(Zn4O)(OOC-C6H3X-COO)6(Zn4O)(CH3COO)5, X=H, Br and NH2, which represent a capped edge of the unit cell and the methyl capped unit cell of IRMOF-1, -2 and -3 were calculated with AM1 and PM3, with HF/6-31G and HF/6-31G**, with B3LYP/6-31G and B3LYP/6-31G** and with the two-layers ONIOM(HF/6-31G:PM3) methods. Comparisons with crystallographic results show that these structures were fairly well reproduced by the semiempirical methods and very well reproduced by the HF and B3LYP methods. Electronic correlation effects and polarization functions in the basis sets play minor roles in the results for the calculated structures, probably due to their rigidity. Vibrational frequencies and intensities for the Zn4O(OOC)6 core of the Zn4O(CH3COO)6 complex were (semi-) quantitatively described by the AM1, PM3 and HF/6-31G methods. The two-layers ONIOM(HF/6-31G:PM3) method is appropriate for structural calculations and improvements at the ab initio or DFT level yields a methodology that can properly describe the interaction between a sorbate and the edge of the IRMOF cavities.