Polymorphism in Paracetamol: Evidence of Additional Forms IV and V at High Pressure

The structural phase stability of N-(4-hydroxyphenyl) acetamide (paracetamol) has been studied at ambient temperature up to 23 GPa using Raman spectroscopy. Spectral changes have provided further evidence for a highly kinetically driven Form I -> II transition that occurs as a mixed phase from 4.8 to 6.5 GPa, and might complete as early as 7 GPa. Upon further compression to 8.1 GPa, a drastic shift in spectral signature was observed providing the first evidence for a previously undiscovered Form IV of paracetamol. Additional shifts in mode intensities were observed near 11 GPa indicating a potential restructuring of the hydrogen bonding network and/or structural modification to a potentially new Form V. Phase boundaries at 7 and 8 GPa were confirmed under hydrostatic conditions using Raman spectroscopy. Spectral changes indicate that the transition Form IV -> V occurs near 11 GPa. Multiple ab initio harmonic frequency calculations at different levels of theory were performed with a B3LYP/6-31G** being used to provide a more robust mode assignment to our experimentally obtained Raman modes. High pressure X-ray diffraction (XRD) was performed up to 21 GPa, which provided further evidence for a highly kinetically driven Form I -> II transition in agreement with our Raman measurements. In addition, the XRD provided further evidence for the existence of Form IV near 8 GPa and Form V near 11 GPa with Form V persisting up to 21 GPa.