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

## TL;DR

This paper shows how low-frequency Raman and terahertz spectroscopy can detect and distinguish organic gunshot residues, which are important in forensic investigations.

## Contribution

The study introduces low-frequency vibrational spectroscopy as a novel method for identifying and quantifying organic gunshot residues.

## Key findings

- LFRS and THz-TDS can distinguish between ethyl centralite and methyl centralite based on unique spectral features.
- LFRS was found to be more sensitive for detecting these compounds, with specific peaks at 98.8 cm–1 and 111.7 cm–1.
- The strongest spectral features arise from torsional motions of phenyl rings in the molecular crystals.

## Abstract

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.