Low-Frequency\nVibrational Spectra of 4‑Fluorophenol\nStudied by THz Spectroscopy and Solid-State Density Functional Theory

Hydrogen bonds serve as important intermolecular interactions\norganizing\nthe spatial arrangement of molecular crystals. Determining the hydrogen-bond\norientations remains a challenging task. In the previous XRD study,\nthe authors assumed a single crystal structure of 4-fluorophenol in\nwhich molecules form hexamer-ring clusters via anticlockwise hydrogen\nbonding. However, the existence of the other structure, which adopts\nclockwise hydrogen bonding, remains uncertain and warrants further\nexploration. We unveil distinctive fingerprint information associated\nwith both structures by using high-resolution terahertz spectroscopy.\nThe distinct structures result in different intramolecular geometries\nof 4-fluorophenol regarding the O–H bond configurations, which\nare manifested by a noticeable peak splitting in the 20–200\ncm<sup>–1</sup> frequency range. This result illustrates the\nsensitivity of THz spectroscopy to hydrogen-bond conformational polymorphism.\nOur findings represent a significant advancement in utilizing high-resolution\nTHz spectroscopy to resolve hydrogen-bond orientations in molecular\ncrystals with possible broad applicability across diverse hydrogen-bonded\norganic and inorganic framework materials.