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.