Energetics and dynamics of H(2) adsorbed in a nanoporous material at low temperature

Molecular hydrogen adsorption in a nanoporous metal-organic framework structure (MOF-74) is studied via van der Waals density-functional calculations. The primary and secondary binding sites for H(2) are confirmed. The low-lying rotational and translational energy levels are calculated, based on the orientation and position dependent potential energy surface at the two binding sites. A consistent picture is obtained between the calculated rotational-translational transitions for different H(2) loadings and those measured by inelastic neutron scattering exciting the singlet to triplet (para to ortho) transition in H(2). The H(2) binding energy after zero-point energy correction due to the rotational and translational motions is predicted to be approximately 100 meV in good agreement with the experimental value of approximately 90 meV.