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Materials (Basel). 2016 Jul 15;9(7). pii: E578. doi: 10.3390/ma9070578.

H₂ Adsorbed Site-to-Site Electronic Delocalization within IRMOF-1: Understanding Non-Negligible Interactions at High Pressure.

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Department of Chemistry, Washington State University, Pullman, WA 99164, USA.
Department of Chemistry, Washington State University, Pullman, WA 99164, USA.
Department of Chemistry, Washington State University, Pullman, WA 99164, USA.


Isoreticular metal organic frameworks (IRMOFs) have shown high uptake capabilities for storage of H₂ (11.5 wt % at 77 K and 170 bar). A significant literature has employed fragment models and a single adsorbed H₂ to identify adsorption sites within IRMOFs, as well as the necessary adsorbate-adsorbent interactions needed to reach sufficient adsorption enthalpy for practical usage, however at high pressures it remains to be seen if H₂···H₂ intermolecular interactions may influence the energetics. This study focuses upon IRMOF-1 (also known as MOF-5), and examines the individual H₂ stabilization energies at different sites using Möller-Plesset perturbation theory and density functional theory alongside chemical models that consist of isolated fragment models and a cubic super cell cluster consisting of both the face- and edge-cube's of IRMOF-1. Optimization of twenty stable configurations of singly adsorbed H₂ in the super-cell cluster is observed to be essential to obtain energy ordering of the five primary sites consistent with experiment and prior benchmark calculations (α >> β > γ > δ ≈ ε). To examine site-to-site interactions that may occur in the high-pressure regime, 64 co-adsorbed H₂ within a super-cell cluster have been studied (a theoretical maximum of all adsorption sites, 14 wt %). There, delocalization and/or charge transfer of electrons is observed from the σ orbitals of the H₂ bound at the γ positions into the σ* orbitals of H₂ bound at the α sites leads to stabilization of the interaction of H₂ at the γ, by 1.4 kJ/mol, respectively (using M06-2X/LANL2DZ). This effect has been confirmed to be charge transfer, and not a manifestation of enhanced dispersion at high loading, through natural bond order (NBO) analysis and by comparisons of the square of off-diagonal NBO Fock matrix elements for both density functionals that account for dispersion interactions and Hartree-Fock calculations that ignore dispersion.


H2 adsorption; IRMOF-1; MOF-5; high-pressure interactions

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