The assembly mechanism of hierarchical materials controlled by the choice of solvent and presence of spectator ions. In this paper, we use enhanced sampling molecular dynamics methods to investigate these effects on the configurational landscape of metal-linker interactions in the early stages of synthesis, using MIL-101(Cr) as a prototypical example. Microsecond-long welltempered metadynamics (WTmetaD) simulations uncover a complex free energy structural landscape, with distinct crystal (C) and non-crystal (NC) like configurations and their equilibrium population. In presence of ions (Na+, F-), we observe a complex effect on the crystallinity of secondary building units (SBUs), by encouraging/suppressing salt bridges between C configurations and consequently controlling the percentage of defects. Solvent effects are assessed by introducing N, N dimethylformamide (DMF) instead of water, where SBU adducts are appreciably more stable and compact. These results shed light on how solvent and ionic strength impact the free energy of assembly phenomena that ultimately control material synthesis
Ab initio molecular dynamics simulations combined with enhanced sampling techniques are becoming widespread methods to investigate chemical phenomena in catalytic systems. These techniques automatically include finite temperature effects, anharmonicity, and...
The thermo-structural behavior of metal-organic framework (MOF) precursors is responsible for regulating the introduction of defects in MOF structures during synthesis. In this paper, factors affecting the flexibility of MIL-101(Cr) half-secondary building units (half-SBUs) are evaluated in solution using enhanced sampling methods. In particular, entropic and enthalpic contributions to the conformational free energy landscape of isolated MIL-101(Cr) half-SBUs are calculated in water, in the presence and absence of ionic species (Na + and F −), and in N, N-dimethylformamide (DMF). This analysis leads to the observation that the interplay between enthalpy and entropy determines the most probable conformational state for half-SBUs. This observation extends to the most relevant SBU intermediate, in which conformational entropy plays a key role in stabilizing configurations that differ from those found in the MIL-101(Cr) crystal structure. The findings highlight the importance of explicitly considering entropic effects, associated with finite-temperature sampling when estimating the relative stability of different conformers of SBUs.
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