Abstract
Synchrotron far-infrared (far-IR) spectra were measured for a representative set of pharmaceutical-type molecular crystals: phenylethylamine hydrochloride salt, ephedrine base and its hydrochloride salt, and pseudoephedrine base and its hydrochloride salt. Resolved spectral profiles in the 30-600 cm(-1) range allowed for comparison of absorption bands to simulated spectra obtained by periodic density function theory (DFT) code. The B3LYP-D3/6-311G(d) level of theory returned highly accurate predictions of the experimental far-IR spectra for well-ordered crystalline samples, including an agreement between relative peak intensities almost unprecedented for organic crystal systems of this size. Simulated far-IR spectra for the phenylethylamine hydrochloride and ephedrine base samples do not match to the same degree; potentially due to higher disorder within the solid-state. These comparisons to theory generally allowed for confident assignment of prominent absorption bands to low-lying vibrational modes. The experimental spectra (including those for related illicit drug compounds: amphetamine sulphate, methamphetamine hydrochloride, MDA hydrochloride, and MDMA hydrochloride), peak assignments, and relative band intensities are compiled and evaluated for their inclusion in spectral libraries and their application to chemometric analysis for mixed compounds of pharmaceutical and forensic interest.