UC San Diego

The breathing behavior of metal-organic frameworks (MOFs) is an exciting new phenomenon in which a framework significantly changes unit cell shape depending on adsorbent, change in pressure, or temperature. There are few force fields capable of computationally describing MOF breathing behavior, and they exist only for MIL-53. In this study, fully flexible force fields were created for the MOF "DMOF-1" (Zn₂(BDC)₂(DABCO)), which is known to breathe with the addition of benzene and isopropyl alcohol, and upon post-synthetic modification with amines. Over the course of the study, three flexible force fields were examined: a non-bonded force field, a restrained force field, and a fully-bonded force field. The non-bonded and restrained force fields represented the Zn-ligand bonds by electrostatic interactions, and did not reasonably simulate the DMOF-1 breathing behavior. The fully-bonded force field explicitly described the Zn-ligand interactions and was found to quantitatively reproduce experimental data of DMOF-1 and of DMOF-1 with benzene, and qualitatively reproduce DMOF-1 with isopropyl alcohol. The aminated DMOF-1 series DMOF-1-AM1 to DMOF-1-AM4 was not as successfully modeled, however. The fully-bonded force field incorrectly predicted the DMOF-1-AM1 and DMOF- 1-AM2 structures to be in a large pore configuration. This highlights a potential issue with force field transferability between different breathing systems for DMOF-1, a current problem with flexible MOF force fields. At present, it is not clear whether the difficulties in transferability to the aminated framework are due to insufficient representation of to the amine chains or a more fundamental problem describing the framework flexibility