Terahertz Spectroscopy Unambiguously Determines the Orientation of Guest Water Molecules in a Structurally Elusive Metal–Organic Framework

Abstract

Porous materials, particularly metal−organic frameworks (MOFs), hold great promise for advanced applications. MIL-53(Al) is an exceptionally well-studied MOF that exhibits a phase transition upon guest capture�in this case, water�resulting in a dramatic change in the pore volume. Despite extensive studies, the structure of the water-loaded narrow-pore phase, MIL-53(Al)-np, remains controversial, particularly with respect to the positions of the adsorbed water molecules. We use terahertz spectroscopy, coupled with powder X-ray diffraction and density functional theory simulations, to unambiguously resolve this controversy. We show that the low-frequency (<100 cm −1 ) vibrational spectrum depends on weak long-range forces that are extremely sensitive to the orientation of the adsorbed water molecules. This enables definitively determining the correct structure of MIL-53(Al)-np while highlighting the extreme sensitivity of terahertz spectroscopy to bulk structure, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host−guest complexes.

Terahertz Signatures of the Methane Replacement Reaction in Hydroquinone Clathrates

“…The final step in our analysis is informed by recent studies on HQ clathrates, which suggested that CO 2 –CH 4 replacement reactions are generally incomplete, leaving both guest molecules present in the final system. , In fact, ref reports that the replacement reaction from CO 2 to CH 4 resulted in only 6% to 9% of CO 2 molecules replaced. However, if even a small proportion of CO 2 molecules are replaced in the clathrate, we expect a measurable impact on the resulting spectra because terahertz spectroscopy is so sensitive to the identity and orientation of the guest molecules. , …”