Molecular Hydrogen "Pairing" Interaction in a Metal Organic Framework System with Unsaturated Metal 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 H-2 interactions with the unsaturated metal centers. A small shift (similar to -30 cm(-1) with respect to the unperturbed H-2 molecule) is observed for the internal stretch frequency of H-2 molecules adsorbed on the metal site at low loading. This finding is in contrast to much larger shifts (similar to -70 cm(-1)) observed in previous studies of MOFs with unsaturated metal centers (including MOF-74) and the general assumption that H-2 stretch shifts depend on adsorption energies (FitzGerald et al., Phys. Rev. B2010, 81, 104305). We show that larger shifts (similar to -70 cm(-1)) do occur, but only when the next available site ("oxygen" site) is occupied. This larger shift originates from H-2-H-2 interactions on neighboring sites of the same pore, consistent with the short distance between H-2 in these two sites similar to 2.6 angstrom derived from an analysis of neutron diffraction experiments of D-2-D-2 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 H-2-H-2 interactions), which complicates the use of variable temperature IR methods to calculate adsorption energies of specific adsorption sites.