Inter-molecular vibrations of liquid 2-nitrotoluene probed by xTerahertz spectroscopy and revealed by molecular dynamics and first principles simulations.

Abstract

The deliberate addition of nitro-based chemical compounds to explosive formulations has become a regulatory requirement aimed at enhancing the traceability and detection of both commercial and improvised explosive devices. The intrinsic properties of terahertz radiation, including its ability to penetrate through a wide range of non-metallic materials and its sensitivity to molecular vibrations, make it a promising tool for explosive related compounds (ERCs) detection. In this study, terahertz time-domain spectroscopy (THz-TDS) is employed as a non-destructive analytical technique to investigate the terahertz spectral response of liquid 2-nitrotoluene (2-NT). The experimental spectral response of 2-NT was characterised under ambient conditions. In parallel, density functional perturbation theory (DFPT) and molecular dynamics (MD) simulations using the OPLS-AA force field were performed to interpret the experimental spectra. The vibrational spectra were obtained from the Fourier transform of the velocity autocorrelation function (VACF) and compared to DFPT calculations. We find that the low-frequency terahertz response of liquid 2-NT does not originate from intramolecular vibrations but from the collective intermolecular dynamics of the condensed phase, which are well reproduced by the MD simulations. These low-frequency features are robust with respect to the OPLS-AA 1-4 intramolecular scaling factor (α=0,0.3,0.5) and to the choice of thermodynamic ensemble, confirming their intermolecular origin.

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