Vibrational Signatures in the THz Spectrum of 1,3-DNB: A First-Principles and Experimental Study

TL;DR

This study combines first-principles calculations and THz spectroscopy to analyze vibrational modes in 1,3-DNB crystals, aiding in the detection of energetic materials through their unique vibrational signatures.

Contribution

It provides a detailed comparison of theoretical and experimental vibrational spectra of 1,3-DNB, highlighting the importance of van der Waals forces for accurate modeling.

Findings

Theoretical vibrational modes agree qualitatively with experimental data up to 2.5 THz.

Intra-molecular forces dominate higher frequency vibrational modes.

Including van der Waals dispersion improves the accuracy of theoretical predictions.

Abstract

Understanding the fundamental processes of light-matter interaction is important for detection of explosives and other energetic materials, which are active in the infrared and terahertz (THz) region. We report a comprehensive study on electronic and vibrational lattice properties of structurally similar 1,3-dinitrobenzene (1,3- DNB) crystals through first-principles electronic structure calculations and THz spectroscopy measurements on polycrystalline samples. Starting from reported x-ray crystal structures, we use density-functional theory (DFT) with periodic boundary conditions to optimize the structures and perform linear response calculations of the vibrational properties at zero phonon momentum. The theoretically identified normal modes agree qualitatively with those obtained experimentally in a frequency range up to 2.5 THz and quantitatively at much higher frequencies. The latter frequencies are set by intra-molecular forces. Our results suggest that van der Waals dispersion forces need to be included to improve the agreement between theory and experiment in the THz region, which is dominated by intermolecular modes and sensitive to details in the DFT calculation. An improved comparison is needed to assess and distinguish between intra- and intermolecular vibrational modes characteristic of energetic materials.