Low-Frequency Vibrational Spectra of 4-Fluorophenol Studied by THz Spectroscopy and Solid-State Density Functional Theory

Hydrogen bonds serve as important intermolecular interactions organizing the spatial arrangement of molecular crystals. Determining the hydrogen-bond orientations remains a challenging task. In the previous XRD study, the authors assumed a single crystal structure of 4-fluorophenol in which molecules form hexamer-ring clusters via anticlockwise hydrogen bonding. However, the existence of the other structure, which adopts clockwise hydrogen bonding, remains uncertain and warrants further exploration. We unveil distinctive fingerprint information associated with both structures by using high-resolution terahertz spectroscopy. The distinct structures result in different intramolecular geometries of 4-fluorophenol regarding the O-H bond configurations, which are manifested by a noticeable peak splitting in the 20-200 cm^-1 frequency range. This result illustrates the sensitivity of THz spectroscopy to hydrogen-bond conformational polymorphism. Our findings represent a significant advancement in utilizing high-resolution THz spectroscopy to resolve hydrogen-bond orientations in molecular crystals with possible broad applicability across diverse hydrogen-bonded organic and inorganic framework materials.