Sorption-Induced Breathing in the Flexible Metal Organic Framework CrMIL-53: Force-Field Simulations and Electronic Structure Analysis

Density functional theory (DFT) and force-field-based calculations have been carried out on the breathing metal-organic framework MIL-53(Cr) in both its large- and narrow-pore forms. In its sorbate-free form, the large-pore structure appears to be the global minimum. We develop a hybrid force field combining ionic-model potentials, used for modeling inorganic solids, with molecular mechanics terms for the organic part. This gives an energy difference-of close to 30 kJ mol(-1) between the large- and narrow-pore forms. Calculations in which water molecules are introduced into the structures illustrate how the energetics of physisorption are able to drive the pore breathing process, with the water molecules, being more strongly stabilized in the narrow-pore form, favoring pore closure at loadings of more than one molecule per unit cell.