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

Infrared spectroscopy is used to observe the orientational fine structure arising from ortho-H2 adsorbed at the primary site of the microporous framework MOF-5. The Q1(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 H2 concentration indicative of interacting H2 pairs. This interpretation is strongly supported by D2 addition experiments, which cause a large increase in intensity of the fine structure bands with only minimal change in the main band. The spectra are analyzed in terms of H2-H2 electric quadrupole-quadrupole interactions. Consistent with this approach we observe no fine structure bands for the Q1(0) vibrational transition arising from para-H2, which does not possess a quadrupole moment.