Molecular Level Understanding of the Free Energy Landscape in Early Stages of Metal–Organic Framework Nucleation

Abstract

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

On the Role of Enthalpic and Entropic Contributions to the Conformational Free Energy Landscape of MIL‐101(Cr) Secondary Building Units

“…[18,19] In particular, the rich conformational ensemble unveiled during the association of SBUs, highlights the importance of entropic contributions in defining the free energy landscape of MIL-101(Cr) precursors. [18] Moreover, the propensity of healing defects in the crystal lattice, as well as the formation of crystal-like SBUs is impacted by solution composition. The relative abundance of different isomers of half-SBUs precursors, as well as the presence of spectator ions emerge as key variables in this context.…”

“…[18] Furthermore, the AA SBU is the largest source of crystal-like growth units building the MIL-101(Cr) lattice. [18] This analysis allows for an evaluation of the conformational freedom that different intermediates experience in solution. This, in turn, provides a detailed description of their accessible structural fluctuations, and provides further insight into the nature of the ensemble of configurations sampled by precursors during self-assembly and nucleation.…”

“…[ 18,19 ] In particular, the rich conformational ensemble unveiled during the association of SBUs, highlights the importance of entropic contributions in defining the free energy landscape of MIL‐101(Cr) precursors. [ 18 ]…”

The role of molecular modelling and simulation in the discovery and deployment of metal-organic frameworks for gas storage and separation

“…They also found free energy barriers that are associated with structural rearrangements and solvent interactions that are disrupted in an amorphous-to-crystalline transition. Similarly, Kollias and co-workers[371] calculate free energies to explore the role that solvent and counterions play in the early-stage nucleation of MIL-101. They found that the stability of the early structures and the presence of amorphous vs. crystalline moieties can be affected by the identity of the solvent and ions present in solution.…”

Pathways for the formation of ice polymorphs from water predicted by a metadynamics method

“…Metadynamics (MTD) is an enhanced sampling method that increases the probability of reaching high free-energy states by adding a Gaussian bias potential to the Hamiltonian of a state 6,7 . The MTD method has been used to analyze reaction pathways [8][9][10][11][12][13][14][15] , the conformations of additives at crystal surfaces [16][17][18][19] , crystal structures [20][21][22][23][24][25] , crystal nucleation [26][27][28][29][30][31][32][33] , drug design [34][35][36][37][38] , and the transport mechanism of an ion in a sub-nanopore 39 . The MTD method affords a free-energy landscape in a space of collective variables (CVs) for a system of interest.…”