Far-Infrared Spectroscopy as a Probe for Polymorph Discrimination

Pharmaceutical crystalline polymorph and amorphous form detection and quantification is a standard requirement in the pharmaceutical industry. Infrared (IR) spectroscopy provides an important probe for the characterization of polymorphs. Nonetheless, characterization and discrimination among polymorphs using mid-IR spectroscopy is not always possible in part because the technique mainly probes vibrational modes arising from functional groups in the sample. In the present work, far-IR spectroscopy is demonstrated for the discrimination of polymorphs. This region is influenced by delocalized lattice vibrational modes derived from intermolecular forces and packing arrangements in the crystal structure. A total of 10 polymorphic pharmaceuticals were prepared to conduct a critical evaluation of the question, does this far-IR region add value for polymorph differentiation? It is demonstrated that the far-IR region offers high discriminating power for polymorphs compared to the mid-IR spectral region. In addition, structural similarity and dissimilarity in polymorphic packing arrangements can be derived from this analysis.