Simulating NMR Relaxation Time Constants of Benzene in MOF-5 to Investigate Phase Coexistence

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.