Identification and quantification of polymorphism in the pharmaceutical compound diclofenac acid by terahertz spectroscopy and solid-state density functional theory

Polymorph detection and quantification in crystalline materials is a principle interest of the pharmaceutical industry. Terahertz (THz) spectroscopy can be used for such analytical applications since this technique is sensitive to the intermolecular interactions of molecules in the solid state. Understanding the fundamental nature of the lattice vibrational motions leading to absorptions in THz spectra is challenging, but may be achieved through computational approaches. In this study, the THz spectra of two diclofenac acid polymorphs were obtained by THz spectroscopy, and the vibrational characters of the observed absorptions were analyzed using solid-state density functional theory (DFT). The results demonstrate the quantitative capacity of THz spectroscopy and the reliability and utility of solid-state DFT in the calculation of low-frequency vibrational motions.