Dramatic Effect of the Electrostatic Parameters on H₂ Sorption in an M-MOF-74 Analogue

Simulations of H₂ sorption were performed in Cu-MOF-74, a recent addition to the M-MOF-74 series. Electronic structure calculations revealed that the Cu²⁺ ions exhibit an unusually low partial positive charge distribution in Cu-MOF-74, which is a direct consequence of the Jahn-Teller effect. This is in contrast to the charge environment for the metal ions in some of the other M-MOF-74 variants as determined in previous work [ Pham, T.; et al. J. Phys. Chem. C 2015, 119, 1078-1090 ]. Because of the low magnitude of the partial charges of the Cu²⁺ ions in Cu-MOF-74, this MOF displays the lowest H₂ uptake and Q_st values of the M-MOF-74 series, which is consistent with what was observed experimentally for H₂ sorption in this series of MOFs. Control simulations of H₂ sorption in a nonphysical Cu-MOF-74 variant were performed in which a set of calculated partial charges, appropriate for one of the other M-MOF-74 analogues, were used. These simulations utilize a much higher partial positive charge for the metal ions and, as a result, a different shape for the simulated H₂ sorption isotherms was obtained compared to that using the normal force field. This shape was not representative of the experimental isotherm for Cu-MOF-74, and thus, confirms the notion that the electrostatic parameters on the metal ions are the key to understanding the H₂ sorption behavior in this MOF. Examining the distribution of the induced dipoles and the Cu²⁺-H₂ distance via simulated annealing and executing two-dimensional quantum rotation calculations have also verified that the H₂-metal interaction in Cu-MOF-74 is the weakest in the M-MOF-74 series. This study shows the power of using computational modeling to explain certain experimental observables and trends in a series of MOFs.