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

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Ruggiero, Michael T 
Sibik, Juraj 
Zeitler, J Axel 
Korter, Timothy M 

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.

Computer Simulation, Crystallization, Glutamic Acid, Hydrogen Bonding, Models, Chemical, Molecular Structure, Terahertz Spectroscopy, Thermodynamics, X-Ray Diffraction
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J Phys Chem A
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American Chemical Society (ACS)
Engineering and Physical Sciences Research Council (EP/J007803/1)
National Science Foundation (Grant ID: CHE-1301068), Royal Society (International Exchanges Scheme), Royal Society of Chemistry (JWT Jones Travelling Fellowship), Engineering and Physical Sciences Research Council (Grant ID: EP/J007803/1)
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