Molecular hydrogen "pairing" interaction in a metal organic framework system with unsaturated metel centers (MOF-74)

Infrared (IR) absorption spectroscopy measurements of molecular hydrogen in MOF-74-M (M = metal center) are performed as a function of temperature and pressure [to 45 kTorr (60 bar) at 300 K, and at lower pressures in the 20-200 K range] to investigate the nature of H₂ interactions with the unsaturated metal centers. A small shift (∼ -30 cm^-1 with respect to the unperturbed H₂ molecule) is observed for the internal stretch frequency of H₂ molecules adsorbed on the metal site at low loading. This finding is in contrast to much larger shifts (∼ -70 cm^-1) observed in previous studies of MOFs with unsaturated metal centers (including MOF-74) and the general assumption that H2 stretch shifts depend on adsorption energies (FitzGerald et al., Phys. Rev. B 2010, 81, 104305). We show that larger shifts (∼ -70 cm^-1) do occur, but only when the next available site ("oxygen" site) is occupied. This larger shift originates from H₂-H₂ interactions on neighboring sites of the same pore, consistent with the short distance between H₂ in these two sites ∼2.6 Å derived from an analysis of neutron diffraction experiments of D₂-D₂ at 4 K (Liu et al., Langmuir 2008, 24, 4772-4777). Our results at 77 K and low loading can be explained by a diffusion barrier against pair disruption, which should be enhanced by this interaction. Calculations indicate that the vibrational shifts do not correlate with binding energies and are instead very sensitive to the environment (interaction potential and H₂-H₂ interactions), which complicates the use of variable temperature IR methods to calculate adsorption energies of specific adsorption sites.