Orientational ortho-H2 pair interactions in the microporous framework MOF-5

Infrared spectroscopy is used to observe the orientational fine structure arising from ortho-H-2 adsorbed at the primary site of the microporous framework MOF-5. The Q(1)(1) vibrational transition shows at least two symmetrically spaced fine structure bands on either side of the main band. These grow in relative intensity with increasing H-2 concentration indicative of interacting H-2 pairs. This interpretation is strongly supported by D-2 addition experiments, which cause a large increase in intensity of the fine structure bands with only minimal change in themain band. The spectra are analyzed in terms of H-2-H-2 electric quadrupole-quadrupole interactions. Consistent with this approach we observe no fine structure bands for the Q(1)(0) vibrational transition arising from para-H-2, which does not possess a quadrupole moment.