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 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.