Infrared (IR) absorption spectroscopy measurements, performed at 300 K and high pressures (27-55 bar) on several prototypes of metal organic framework (MOF) materials, reveal that the MOF ligands are weakly perturbed upon incorporation of guest molecules and that the molecular hydrogen (H2) stretch mode is red-shifted (30-40 cm-1) from its unperturbed value (4155 cm-1 for ortho H2). For MOFs of the form M(bdc)(ted)0.5 (bdc = 1,4-benzenedicarboxylate; ted = triethylenediamine), H2 molecules interact with the organic ligands instead of the saturated metal centers located at the corners of the unit cell. First-principles van der Waals density functional calculations identify the binding sites and further show that the induced dipole associated with the trapped H2 depends sensitively on these sites. For M(bdc)(ted) 0.5 systems, the strongest dipole moment is of the site that is in the corner of the unit cell and is dominated by the interaction with the benzene ligand and not by the metal center. For MOFs of the M3[HCOO] 6 type with relatively short ligands (i.e., formate) and 1-D pore structures, there is a weak dependence of H2 vibrational frequency on the cations, due to a small change in the unit cell dimension. Furthermore, translational states of ∼±100 cm-1 are clearly observed as side bands on the H2 stretch mode in these 1-D channels interconnected by very small apertures. The H2 stretch IR integrated areas in all the MOFs considered in this work increase linearly with H 2 pressure, consistent with isotherm measurements performed in similar conditions. However, the IR intensity varies substantially, depending on the number of benzene rings interacting with the H2 molecules. Finally, there is no correlation between H2 binding energies (determined by isotherm measurements) and the magnitude of the H2 stretch shift, indicating that IR shifts are dominated by the environment (organic ligand, metal center, and structure) rather than the strength of the interaction. These results highlight the relevance of IR spectroscopy to determine the type and arrangement of ligands in the structure of MOFs.
ASJC Scopus subject areas
- Colloid and Surface Chemistry