High-Pressure Polymorphs of n-Butanol Governed by Hydrogen Bonding

This study reports the discovery of two new high-pressure polymorphs of n-butanol, C4H9OH, HP-I, and HP-II, expanding the variety of known crystalline structures of this monohydroxy alcohol. Using high-pressure single-crystal and powder X-ray diffraction in diamond anvil cells, we observed crystallization of n-butanol at 1.4 GPa, adopting the P21/c space group, which differs from the already known low temperature, LT, P1 structure. Upon regrowth of a single crystal at 2.4 GPa, another high-pressure structure was observed. HP-I and HP-II structures show different molecular arrangements while retaining the P21/c space group. The powder diffraction experiment, within the pressure range from 1.5 to 3.5 GPa, did not show any phase transition from HP-I to HP-II. Raman spectroscopy and DFT calculations confirmed distinct hydrogen-bonding properties in all three LT, HP-I, and HP-II polymorphs while having similar infinite chains of hydrogen bonds. The results highlight the critical role of hydrogen bonding in governing polymorphism under varying pressure conditions.