PNNL

Abstract

Thermostructural behavior of Metal-Organic Frameworks (MOFs) is responsible for regulating the introduction of defects during synthesis. In this paper, we evaluate factors affecting the flexibility of MIL-101(Cr) half – secondary building units (half-SBUs) in solution using enhanced sampling methods. Also, we calculate entropic and enthalpic contributions to the free energy in the space of the mass-weighted radius of gyration (Rgyr) in water, in presence and absence of Na+ and F-, and in N, N-dimethylformamide (DMF). Furthermore, the size of the solvation shell that provides the same potential energy profile as in a highly solvated environment is assessed in order to optimize computational effort. The principal axes of inertia and the corresponding moments are calculated in order to evaluate molecular symmetry. Free energy profiles with respect to the total moment of inertia agree with those projected on Rgyr; rendering the latter as an appropriate proxy for rotational inertia. At last, we depart from half-SBUs and study the rearrangement of an SBU, where we conclude that the free energy profile is driven by entropy. This analysis highlights the importance of studying entropic effects on the configurational freedom of MOF building units as they can regulate the formation of defects during synthesis of materials consisting of flexible building units.