Crystallographic symmetry increasing in the pressure-induced phase transition of the hydrogen-bonded organic crystal 1-methylhydantoin.

This article focuses on the high-pressure phase transition behavior of 1-methylhydantoin (1-MH), an organic supramolecular polymorphic compound with pharmaceutical relevance. Using in situ high-pressure Raman spectroscopy and synchrotron angular dispersive x-ray diffraction (ADXRD) combined with first-principles calculations, the study demonstrates that 1-MH undergoes a reversible phase transition from a low-symmetry monoclinic phase I to a high-symmetry orthorhombic phase II between 4.0 and 14.1 GPa. The mechanism of this transition is attributed to interlayer contraction and rearrangement of the hydrogen-bonding network driven by enhanced hydrogen-bond interactions under pressure. These findings provide theoretical insight into rare pressure-induced transitions from low- to high-symmetry phases in organic supramolecular crystals, with implications for understanding pharmaceutical polymorphism.