Vibrational signatures in the THz spectrum of 1,3-DNB: A first-principles and experimental study

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 intraand intermolecular vibrational modes characteristic of energetic materials.

Vibrational Response of Felodipine in the THz Domain: Optical and Neutron Spectroscopy Versus Plane-Wave DFT Modeling

“…While introducing the use of the analytical response scheme by Gonze and Baroni [37][38][39][52][53][54][55], this paper provides a proven numerical recipe for an accurate modeling of low-wavenumber phonon frequencies. This strategy has been followed with a number of examples presented in the literature, using CASTEP [30][31][32][33][105][106][107][108][109], QUAN-TUM ESPRESSO [106,[110][111][112][113][114], ABINIT [115][116][117], or VASP [118,119], and has been used in the present study. Instead of advocating any of the presented strategies, we further highlight the main differences and limitations of each approach as both have their pros and cons.…”