On the Role of Enthalpic and Entropic Contributions on the Conformational Free Energy Landscape of MIL-101(Cr) Structural Building Units

11 October 2019, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

The thermostructural behavior of metal-organic framework (MOF) precursors is responsible for regulating the introduction of MOF structure 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. In particular, we calculate entropic and enthalpic contributions to the conformational free energy landscape of isolated MIL-101(Cr) half-SBUs, in water, in the presence and absence of ionic species (Na+ and F-), and in N, N-dimethylformamide (DMF). We find that the interplay between enthalpy and entropy determines the most probable conformational state for half-SBUs. Furthermore, we investigate the role of enthalpy and entropy in the conformational rearrangement of an SBU in water, noting that entropic contributions are essential to stabilize configurations that depart from those coherent with the MIL-101(Cr) crystal structure. Our analysis highlights the importance of explicitly considering entropic effects on the configurational ensembles of MOF building units, as such effects can significantly impact the relative stability of structurally different conformers, which ultimately can be responsible for the formation of defects during materials synthesis.

Keywords

MOF
enthalpy
entropy
metadynamics
Structural Building Units

Supplementary materials

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