Phase Transitions in p-Toluamide Polycrystals Under High Pressure by Raman Spectroscopy

Raman spectroscopy data of p -toluamide (PTA) polycrystals in the spectral region of 100–3500 cm −1 under high pressure have been collected up to 14.8 GPa at room temperature. Raman spectra exhibited distinct changes during compression, including the appearance of new Raman bands, disappearance of original Raman bands, and discontinuous changes in Raman shifts. The pressure dependence of the Raman shifts of PTA has been obtained. Detailed Raman spectral analysis suggested that compression induces three structural phase transitions of PTA within the investigated pressure range: phase I to II at 1.8 GPa, phase II to III at 5.5 GPa, and phase III to IV at 13.1 GPa. Moreover, the effect of pressure on the N  H stretching vibrational modes has been analyzed based on the hydrogen bonds rearrangements among PTA molecules. This study provides a comprehensive understanding of PTA vibrational mode assignments and valuable insights into the amide structure stability for medicinal and pharmacological applications.