The effect of high pressure on two forms (α, β) of p-aminobenzoic acids (PABA) is studied in a diamond anvil cell using in situ Raman spectroscopy. Previous research showed that α-PABA undergoes a phase transition, and β-PABA is transformed to α-PABA at high temperatures. In the present study, we investigate whether a new polymorph or a transformation between the two polymorphs occurs upon the application of pressure. Experimental results reveal that the two forms remain stable up to ∼13 GPa. Ab initio calculations are performed to account for the changes in unit cell parameters, molecular arrangements, and hydrogen bonds. Polymerization is observed in type B molecules of α-PABA through the calculated geometric parameters of hydrogen bonds. Based on a systematic comparison of the subtle structural changes, anisotropic characteristic, and various interactions of the two polymorphs, we propose that the stability of α-form crystals is associated with the special dimer structure. The stability of the β-form is attributed to hydrogen-bonded networks with four-membered ring construction.

In this study, malonamide (C₃H₆N₂O₂) was compressed under up to 10.4 GPa of pressure in a diamond anvil cell at room temperature.

High-pressure polymorphism is a developing interdisciplinary field. Pressure up to 20 GPa is a powerful thermodynamic parameter for the study and fabrication of hydrogen-bonded polymorphic systems. This review describes how pressure can be used to explore polymorphism and surveys the reports on examples of compounds that our group has studied at high pressures. Such studies have provided insight into the nature of structure–property relationships, which will enable crystal engineering to design crystals with desired architectures through hydrogen-bonded networks. Experimental methods are also briefly surveyed, along with two methods that have proven to be very helpful in the analysis of high-pressure polymorphs, namely, the ab initio pseudopotential plane–wave density functional method and using Hirshfeld surfaces to construct a graphical overview of intermolecular interactions.