Prediction of Structure and Properties of Boron-Based Covalent Organic Frameworks by a First-Principles Derived Force Field

We present a method to theoretically predict structures in arbitrary network topologies for all currently known boron based covalent organic frameworks (COFs). This is particularly useful because these materials are accessible experimentally only as polycrystalline powders. The method is based on a new fully flexible molecular mechanics force field. The consistent parameter set is derived by a genetic algorithm optimization approach from first-principles reference computed data. To achieve high accuracy, the convergence with respect to the level of theory is carefully controlled for this reference. The force field is used to investigate the relative stability of the two high symmetry topologies ctn and bor. Interestingly, for all systems, the ctn topology is found to be energetically more stable. This preference is observed experimentally, too, with the single exception of COF-108, which forms the bor topology. This exception can thus be attributed to the different synthesis conditions, demonstrating that other topologies might be accessible in principle for all COFs. The force field is further used to compute first benchmark surface areas for ideal systems, thermal expansion coefficients, elastic constants, and CO2 adsorption isotherms for all systems in both topologies, which are experimentally unavailable. Our force field opens the way for theoretical structure prediction and prescreening of properties for these fascinating materials.