Terahertz Spectroscopy of 2,4-Dinitrotoluene over a Wide Temperature Range (7–245 K)

Abstract

Previous THz spectroscopy of the TNT explosive precursor, 2,4-dinitrotoluene (DNT), has been restricted to room temperature (apart from one set of data at 11 K). Here, for the first time, we investigate the spectrum as the temperature is systematically varied, from 7 to 245 K. Many new features appear in the spectrum on cooling below room temperature. As well as the five absorption lines observed previously, we observe five additional lines. In addition, a new room-temperature line at 8.52 THz (281 cm(-1)) is observed. Six of the lines red-shift with temperature and four of them blue-shift. The blue shift is explained by interplay between intramolecular and intermolecular hydrogen bonds. The variation in line width and line intensity with temperature is not systematic, although a conspicuous decrease in line intensity with temperature is observed in all cases. Modeling with hybrid PBE0 and TPSSh functionals helps identify absorption modes.

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…95,96 However, many recent studies have used ab initio gas-phase simulations to assign experimental condensed phase terahertz spectra, with varying levels of apparent success. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] It is important to note that in almost all of these cases the discussed 'agreement' with experiment is largely based upon the position of calculated vibrational transitions, with little focus placed upon the accuracy of the predicted mode-types -critical for gaining deeper insight into the way terahertz motions influence bulk material properties, for example. Therefore, this work aims to asses how gas-phase simulations reproduce terahertz spectra, as well as predicting the corresponding vibrational mode-types.…”

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…29 However, due to the complexity and (assumed) increased computational cost of periodic boundary condition simulations, gas-phase calculations involving clusters of molecules have been employed in studies describing terahertz vibrations, with varying levels of apparent success. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] It is important to note that, in general, such assessment is based solely on the position of the calculated transitions, with little focus on the actual motions involved or their accuracy. However, there is a lack of comparison in the literature between these methods, and therefore this work aims to assess how gas-phase simulations perform in regard to reproducing not only the experimental terahertz spectra, but also the prediction of the vibrational mode-types.…”

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

“…Explosives, drugs, and other biologics have unique absorption characteristics in the terahertz frequency range. [62,63,64] These spectral fingerprints are collectively due to the intermolecular and intramolecular vibrations [65,66]. The large-amplitude vibration modes detected in THz spectroscopy provide an in-depth understanding of the molecular structure dynamics by directly interrogating the motions responsible for conformational isomerization.…”