Examination of l-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy

J Phys Chem A. 2016 Sep 29;120(38):7490-5. doi: 10.1021/acs.jpca.6b05702. Epub 2016 Sep 19.

Abstract

The ability of l-glutamic acid to crystallize in two different forms has long been the subject of study due to its commercial importance. While a solvent-mediated phase transformation between the α and β polymorphs is the prevailing theory, recent reports indicate a thermal solid-solid transformation between the two may be possible. However, determining accurate thermodynamic stabilities of these crystals has been challenging. Here new low-temperature single-crystal X-ray diffraction data coupled to solid-state density functional theory simulations have enabled a detailed description to be achieved for the energetic parameters governing the stabilization of the two l-glutamic acid solids. The temperature-dependent Gibbs free-energy curves show that α-glutamic acid is the preferred form at low temperatures (<222 K) and the β form is most stable at ambient temperatures. Terahertz time-domain spectroscopy was utilized to evaluate the quality of the intermolecular force modeling as well as to provide characteristic low-frequency spectral data that can be used for quantification of polymorph mixtures or crystal growth monitoring.

MeSH terms

  • Computer Simulation*
  • Crystallization
  • Glutamic Acid / chemistry*
  • Hydrogen Bonding
  • Models, Chemical*
  • Molecular Structure
  • Terahertz Spectroscopy*
  • Thermodynamics
  • X-Ray Diffraction

Substances

  • Glutamic Acid