Synchrotron far-infrared spectra for the characterisation of molecular crystals of forensic interest: Amphetamine, methamphetamine, MDA, MDMA and substituted methcathinones

Abstract

This combined laboratory and theoretical investigation focussed on a suite of crystalline phenethylamine-class molecules of forensic interest: amphetamine sulfate, levo- and dextro-methamphetamine hydrochloride, 3,4-methyl​enedioxy-methamphetamine (MDMA) hydrochloride, 3,4-methylenedioxy-​amphetamine (MDA) hydrochloride, and the hydrochloride salts of two substituted (4-fluoro and 4-methyl) methcathinones. Far-infrared (far-IR) spectra across the 30–600 cm−1 wavenumber range were recorded at the Australian Synchrotron light source. Resolved absorption profiles enabled vibrational features to be assigned via comparison to periodic density functional theory (p-DFT) performed at the B3LYP-D3/6-311 G(d) level. The calculations accurately reproduce the experimental spectra (frequencies and peak intensities) for ordered molecular crystal-type samples, such as methamphetamine stereoisomers and two substituted methcathinones. Significant band broadening for amphetamine sulfate was observed, likely due to crystal disorder, while MDMA and MDA showed convolved stretching modes from comparatively impure samples. For all seven molecular crystals investigated, previously unpublished peak assignments and relative band intensities have now been systematically compiled; completing the spectral region that lies between the more thoroughly studied terahertz and mid-IR regions. These results can be incorporated in spectral libraries and databases to enable far-IR statistical analyses to be performed on forensic samples of mixed or impure compounds.