Densification processes of crystalline purine: Negative linear compressibility and pressure-induced phase transition

We investigated the densification mechanisms of crystalline purine at room temperature using powder and single-crystal x-ray diffraction under quasi-hydrostatic pressure. The ambient orthorhombic phase exhibits negative linear compressibility until it transforms at 0.7–1.1 GPa into a newly reported high-pressure polymorph. This phase transition can be explained by flipping and translation of hydrogen-bonded molecular arrays. Judging from interatomic distances, intermolecular N–H…N hydrogen bonds in the high-pressure phase are weakened but doubled, leading to an efficient densification. • The ambient phase of crystalline purine exhibits negative linear compressibility. • A new high-pressure monoclinic phase was identified above 0.7 GPa. • Molecular arrays are flipped and shifted in the high-pressure phase. • Intermolecular N–H…N hydrogen bonds are elongated but doubled in the high-pressure phase.