High-Pressure Crystallization and Structural Transformations in Compressed <i>R</i>,<i>S</i>‑Ibuprofen

<i>R</i>,<i>S</i>-Ibuprofen dissolved in chiral and achiral solvents recrystallizes at varied pressure and temperature in the same racemic form I of monoclinic space group <i>P</i>2₁/<i>c</i>. At 4.0 GPa the crystal is compressed to 78% of its ambient-pressure volume. The main structural transformation is the compression of intermolecular van der Waals contacts, increasing the effects of intermolecular interactions in the crystal environment, which results in the more skew position of OH···O bonded carboxyl groups and more stabile H-atoms in the bistable H-bonds. Consequently, the H-atoms remain ordered in all pressure ranges investigated, despite the compression of the H-bonds by over 0.1 Å at 4 GPa. The crystal compression and thermal expansion obey the reverse relationship rule of temperature and pressure effects; however, the compression is considerably nonlinear due to the strong reduction of void volume up to about 0.8 GPa