Characterizing Organic Gunshot Residues with Low-Frequency Raman and Terahertz Vibrational Spectroscopies

Low-frequency (10-300 cm^-1) vibrational spectroscopy is a promising method for enhancing the detection of solid-state organic gunshot residues (OGSRs) which serve as vital trace evidence in crime scene investigations. The use of low-frequency Raman spectroscopy (LFRS) and terahertz time-domain spectroscopy (THz-TDS) allows for the measurement of material-specific lattice vibrations that originate not only from individual molecules, but also from motions between molecules in the solid-state. Together, these vibrations yield unique spectral profiles for compound recognition and characterization. In this study, LFRS and THz-TDS data for two common and structurally similar OGSRs are presented and analyzed: 1,3-diethyl-1,3-diphenylurea (ethyl centralite) and 1,3-dimethyl-1,3-diphenylurea (methyl centralite). Despite their similarities, both exhibit distinctive Raman and THz spectra, and the data have been interpreted using solid-state density functional theory simulations. The computational results show that the vibrations in these molecular crystals that lead to the strongest spectral features all involve torsional motions of the phenyl rings rather than intermolecular motions, such as translations. To demonstrate the ability of LFRS and THz-TDS to differentiate between OGSRs, measurements were also made of binary mixtures of ethyl centralite and methyl centralite. For these specific OGSRs, LFRS was found to be the more sensitive technique with peaks at 98.8 cm^-1 (ethyl centralite) and 111.7 cm^-1 (methyl centralite) that are suitable for reliable detection and quantification. These spectral features should serve as practical markers in future analytical studies of alkylated diphenylurea compounds, while the overall approach highlights the unexplored potential of low-frequency vibrational spectroscopy in forensic science.