(253b) Simulating NMR Relaxation Time Constants of Benzene in MOF-5 to Investigate Phase Coexistence

Authors: 
Braun, E. D., University of California, Berkeley
Schnell, S. K., Delft University of Technology
Reimer, J. A., University of California - Berkeley
Smit, B., University of Amsterdam
Capillary condensation has long been thought to be impossible in a microporous framework due to the small pore size making the system analogous to a one-dimensional Ising model. Steps in the adsorption isotherms of MOF-5, a model microporous metal-organic framework, have thus been difficult to explain as they are reminiscent of capillary condensation. Recently, molecular simulations have shown bulk-like vapor and liquid phases of benzene coexisting in MOF-5 with the domains extending across multiple unit cells, explaining these puzzling results. 1H NMR Relaxometry has provided experimental evidence for this phase coexistence as multiple relaxation time constants have been found for a sample of benzene in MOF-5, which have been hypothesized to be due to the multiple phases. We have conducted molecular dynamics simulations of the benzene in MOF-5 system and calculated the resulting NMR relaxation times; the time constants have been separated into contributions due to dipole-dipole interactions that are intramolecular between protons on a benzene molecule, intermolecular between the protons on different benzene molecules, and intermolecular between the benzene protons and the MOF-5 protons. These contributions can also be experimentally determined by using various amounts of deuterated benzene and deuterated MOF-5 to limit the effects of the various interactions.