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.

Figure 1

Structures of MIL-53(Al)-np. (a) The well-referenced Cc structure with only one pore per unit cell, and (b) the DFT-optimized P2₁/c structure suggested here, with two pores per unit cell.

Figure 2

Experimental (blue) and simulated PXRD patterns for MIL-53(Al)-np.

Figure 3

50 K experimental THz-TDS spectrum (blue) compared to the simulated spectra in the Cc (top) and P2₁/c (bottom) space groups.

Figure 4

Distribution of the orientation of the symmetry-independent water molecules from AIMD simulations, and the corresponding static orientation of the P2₁/c water molecules from Ortiz (dashed vertical lines) and from the DFT-optimized structure (solid vertical lines). The latter provides a better match to the AIMD distributions.

Figure 5

Visualization of the normal mode displacement vectors (red) for two representative modes in MIL-53(Al)-np.