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

Porous materials, particularly metal-organic frameworks (MOFs), hold great promise for ad- vanced applications, with one of the most exciting being passive water harvesting. In order to realize such goals, it is crucial to understand in detail the structures of these host-guest com- plexes. MIL-53(Al) is an exceptionally well-studied MOF, which 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 within the framework. We use terahertz (THz) spectroscopy, coupled with powder X-ray diffraction (PXRD) 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, which are extremely sensitive to the orientation of the adsorbed water molecules. This enables us to definitively determine the correct structure of MIL-53(Al)-np, while highlighting the extreme sensitivity of THz spectroscopy to the bulk structure of porous crystals, suggesting its potential as a robust complement to X-ray diffraction for precise characterization of host-guest complexes.