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

Isoreticular metal organic frameworks (IRMOFs) have shown high uptake capabilities for storage of H-2 (11.5 wt % at 77 K and 170 bar). A significant literature has employed fragment models and a single adsorbed H-2 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 H-2 center dot center dot center dot H-2 intermolecular interactions may influence the energetics. This study focuses upon IRMOF-1 (also known as MOF-5), and examines the individual H-2 stabilization energies at different sites using Moller 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 H-2 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 (alpha >> 3 > gamma > delta approximate to E). To examine site-to-site interactions that may occur in the high-pressure regime, 64 co-adsorbed H-2 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 cr orbitals of the H-2 bound at the gamma positions into the sigma* orbitals of H-2 bound at the alpha sites leads to stabilization of the interaction of H-2 at the y, 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.