Quantum dynamics of adsorbed normal- and para-H2, HD, and D2 in the microporous framework MOF-74 analyzed using infrared spectroscopy

Low-temperature diffuse reflectance infrared spectroscopy is used to measure the quantum dynamics of molecular hydrogen and its isotopologues adsorbed in the microporous material MOF-74. At least two distinct adsorption sites are revealed by increasing the concentration of H2 within the material and monitoring the successive appearance of absorption bands with decreasing bathochromic shifts (or redshifts ) from the unperturbed (gas-phase) vibrational frequencies. Under conditions wherein both the primary and neighboring secondary sites are occupied, additional frequency shifts are observed that confirm the interactions among the adsorbed molecules are non-negligible. Ortho to para conversion of the adsorbed normal-H2 is observed to occur within minutes and this process is accelerated when both primary and secondary sites are occupied. Translational sidebands are also observed, providing an estimation of 124 cm-1 for the frequency of the center-of-mass motion of H2 at the primary adsorption site. The frequency shifts of the absorption bands of HD and D2 diverge from the predictions of a simple isotope effect, emphasizing the importance of the zero-point energy contribution