Quantum dynamics of adsorbed H2 in the microporous framework MOF-5 analyzed using diffuse reflectance infrared spectroscopy

Diffuse reflectance infrared spectroscopy is used to measure the quantum dynamics of molecular hydrogen adsorbed in the microporous material MOF-5. Low-temperature spectra reveal at least three distinct binding sites. The induced redshifts in the vibrational mode frequencies allow the estimation of site-specific binding energies ranging from 2.5 to 4 kJ/mol. Splittings in the rovibrational sidebands are consistent with the existing theories and indicate that H2 is relatively freely rotating even at temperatures as low as 10 K. Ortho to para conversion of the adsorbed H2 is observed to occur over the course of several hours. A translational sideband of 84 cm-1 arises from the center-of-mass motion of H2 at the primary adsorption site and indicates that the zero-point energy is a substantial fraction of the binding energy of this site