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.