Polymorphism in paracetamol: evidence of additional forms IV and V at high pressure

J Phys Chem A. 2014 Aug 7;118(31):6068-77. doi: 10.1021/jp411810y. Epub 2014 Jul 24.

Abstract

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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Acetaminophen / chemistry*
  • Hydrogen Bonding
  • Kinetics
  • Molecular Structure
  • Pressure*
  • Spectrum Analysis, Raman
  • Temperature
  • Vibration

Substances

  • Acetaminophen