Terahertz spectroscopy of 2,4-dinitrotoluene over a wide temperature range (7-245 K)

Dual spectroscopic detection of THz energy modes of critical chemical compounds

Precise identification and sensing of organic and inorganic molecular systems are key factors in several applications in present industrial and scientific domains. While high energy modes, due to electronic interactions, are mostly impervious to the initial thermodynamic or chemical conditions, the low energy modes are sensitive to such alterations which makes them suitable for quality control purpose with sensitive spectral identification methods. Here we report for the first time, several low frequency peaks of specific nitrogen-based compounds and their derivatives, using the dual spectroscopic approach of Terahertz Time-Domain Spectroscopy (THz-TDS) and THz Raman Spectroscopy (THz-RS). Two different isomeric molecular systems have also been investigated to assess both the selectivity and specificity of low energy modes in their identification and spectral correlation in terms of molecular interactions. This information of low frequency modes can be utilized readily by pharmaceutical and agri-food industries, chemical engineering and crystal growth communities in identification, detection, quality control and industrial waste management.

Characteristic Spectral Features of Terra Preta (TP) in the 5-15 Terahertz Range

Terra preta is a fertile anthropogenic soil found in the Amazon basin. One of the most significant differences between the terra preta and surrounding soils is that terra preta is rich in aromatic carbons. Previous infrared investigations of terra preta were reported at energies above 1000 cm^-1 where many other forms of carbon also have absorption lines. No measurements have been reported below 800 cm^-1, where many absorptions associated with aromatic carbons occur in the absence of aliphatic carbon lines. We employ Fourier transform infrared spectroscopy between 150 cm^-1 and 500 cm^-1. A comparison was made between the spectra of terra preta, several pure aromatic compounds, organic fertilizers developed to replicate terra preta and several Australian soils, some of which containing char from bushfires. The spectra in the 150-500 cm^-1 range were very similar between terra preta and the organic fertilizers, while they were very different for the natural soils. These findings indicate that the content of aromatic carbons in terra preta and organic fertilizers is different than in natural soils containing the bushfire chars, but also soils produced entirely by bacterial and fungal activities. This point to the importance of the preparation conditions of the biochars, which are essential ingredients of terra preta and organic fertilizers used in this study.

Examination of proline, hydroxyproline and pyroglutamic acid with different polar groups by terahertz spectroscopy

Hydroxyproline (HYP) and pyroglutamic acid (PGA), as amino acid derivatives, are highly similar in structure to proline (Pro). However, their low-frequency vibrations show significant differences in the range of 0.25-2.6 THz. Therefore, this study investigated the reasons for the differences combined with terahertz time domain spectroscopy (THz-TDS) and density functional theory (DFT). The results show that HYP and PGA have stronger absorption of terahertz waves due to the existence of polar substituents. Furthermore, the absorption peaks of HYP and PGA are significant red shifted and blue shifted, respectively. We believe that this is caused by the change in the strength of intermolecular hydrogen bonds. Our findings demonstrate that dipole and hydrogen bond effects play a significant role in low-frequency vibrations.

The necessity of periodic boundary conditions for the accurate calculation of crystalline terahertz spectra

Terahertz vibrational spectroscopy has emerged as a powerful spectroscopic technique, providing valuable information regarding long-range interactions - and associated collective dynamics - occurring in solids. However, the terahertz sciences are relatively nascent, and there have been significant advances over the last several decades that have profoundly influenced the interpretation and assignment of experimental terahertz spectra. Specifically, because there do not exist any functional group or material-specific terahertz transitions, it is not possible to interpret experimental spectra without additional analysis, specifically, computational simulations. Over the years simulations utilizing periodic boundary conditions have proven to be most successful for reproducing experimental terahertz dynamics, due to the ability of the calculations to accurately take long-range forces into account. On the other hand, there are numerous reports in the literature that utilize gas phase cluster geometries, to varying levels of apparent success. This perspective will provide a concise introduction into the terahertz sciences, specifically terahertz spectroscopy, followed by an evaluation of gas phase and periodic simulations for the assignment of crystalline terahertz spectra, highlighting potential pitfalls and good practice for future endeavors.

Terahertz spectral investigation of temperature induced polymorphic transformation of 2,2dinitroethylene-1,1-diamine

The dynamic process of polymorphic transformations provides deep insights into the understanding of both the performance of explosives and its packing sensitivity. While terahertz spectroscopy is a sensitive tool that can be used to characterize its intermolecular forces. In this paper, terahertz time-domain spectroscopy (THz-TDS) is employed to investigate the polymorphic transformations of the explosive 2,2dinitroethylene-1,1-diamine (FOX-7) under a heating process. Solid-state density functional theory (DFT) uncovers the physical mechanisms that contribute to the polymorphic transformation. This is achieved by assessing the calculated spectra relative to the actual spectra from experiment. The results suggest that the THz spectral features that arise represent a redistribution of the intermolecular forces during the polymorphic transformations. This research will assist the understanding of the evolving properties of FOX-7, under external stimuli, which is vital knowledge for an explosive substance that can combust or detonate.

Terahertz response of DL-alanine: experiment and theory

The terahertz (THz) spectrum of dl-alanine has been measured for the first time at cryogenic temperatures and with a pure sample. Several sharp absorptions are observed, over a wide frequency range (0.8-4.8 THz), at 8 K. The sample structure and purity were confirmed with both Raman spectroscopy and X-ray diffraction. Temperature dependent spectra revealed redshifting, with increasing temperature, for all modes except one at 2.70 THz. This mode exhibits blueshifting until ≈120 K, where it starts to redshift. A Bose-Einstein distribution has been used to model the frequency shift with temperature for the four lowest energy modes. Strong correlations between the fits and data indicate that these modes are caused by phonon excitation in an anharmonic potential. Density functional theory has also been used to identify the origin of these low frequency modes. They are attributed to large scale molecular vibrations.

Low-temperature dependence on the THz spectrum of CL-20/TNT energetic cocrystal by molecular dynamics simulations

Based on the unique advantages of terahertz (THz) spectrum on the detection of energetic cocrystals, the low-temperature dependent THz spectra of CL-20/TNT cocrystal were investigated by using molecular dynamics (MD) simulations from 5 to 296 K, as well as three different crystal faces, (001), (120), and (010). When the temperature decreases below 95 K, we have observed two new peaks for CL-20/TNT cocrystal, at 4.58 and 5.99 THz, respectively. Also, the THz peaks below 1.5 THz gradually disappear under cooling from 296 to 5 K, and they should originate from the lattice thermal vibrations. THz absorption peaks of CL-20/TNT cocrystal reveal frequency shifting, linearly dependent on temperature. Four of them are red shift and other two are blue shift of THz vibrational peaks of CL-20/TNT cocrystal with the temperature increase. The frequency shifts can be attributed to the effects of lattice thermal expansion on inter-/intramolecular vibrational modes as well as their coupling. From the temperature-dependent THz spectra of different crystal faces, we further confirm the response of different kinds of intermolecular interactions on the THz spectrum of CL-20/TNT cocrystal. Graphical abstractThe intermolecular interactions and peak positions of THz spectra of CL-20/TNT cocrystal in the range of 0-6 THz versus temperature.

Low frequency terahertz spectrum of acetylsalicylic acid over a wide temperature range investigated by FTIR spectroscopy

The temperature dependence of the terahertz spectrum of acetylsalicylic acid over an extended temperature range as well as the theoretical modeling results are reported. Several absorption bands which were not observed at room temperature emerged as the sample was cooled down to 14.9 K. All the bands shifted to higher energies upon cooling. The bands which exhibited an anomalous behavior in the frequency shift have a rate of change in the range (4-5) × 10^-3 cm^-1 K^-1-1 while bands which displayed an expected behavior have the rate of change in the range (1-3) × 10^-2 cm^-1 K^-1-1. There is a fluctuation in both the intensity and the width of the bands implying composite bands. There is no fundamental correspondence between the frequency shifts, band widths and band intensities hence making analysis of all the three variables necessary to understand the extent to which the bands have been resolved. A good agreement was observed between the experiment and the model of acetylsalicylic acid crystal. The model of an acetylsalicylic acid crystal with PBEsol DFT functional proved to be better than the previously used model of an acetylsalicylic acid dimer using a B3LYP/6-31++G** DFT functional.

Superficial and Fundamental Correspondences in the Terahertz/IR (6-15 THz) Absorption Spectra of Aspirin and Benzoic Acid

The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200-500 cm^-1 (6-15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental.

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

Trace detection of explosives has been an ongoing challenge for decades and has become one of several critical problems in defense science; public safety; and global counter-terrorism. As a result, there is a growing interest in employing a wide variety of approaches to detect trace explosive residues. Spectroscopy-based techniques play an irreplaceable role for the detection of energetic substances due to the advantages of rapid, automatic, and non-contact. The present work provides a comprehensive review of the advances made over the past few years in the fields of the applications of terahertz (THz) spectroscopy; laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy; and ion mobility spectrometry (IMS) for trace explosives detection. Furthermore, the advantages and limitations of various spectroscopy-based detection techniques are summarized. Finally, the future development for the detection of explosives is discussed.

A terahertz study of taurine: Dispersion correction and mode couplings

The low-frequency characteristics of polycrystalline taurine were studied experimentally by terahertz (THz) absorption spectroscopy and theoretically by ab initio density-functional simulations. Full optimizations with semi-empirical dispersion correction were performed in spectral computations and vibrational mode assignments. For comparison, partial optimizations with pure density functional theory were conducted in parallel. Results indicate that adding long-range dispersion correction to the standard DFT better reproduces the measured THz spectra than the popular partial optimizations. The main origins of the observed absorption features were also identified. Moreover, a coupled-oscillators model was proposed to explain the experimental observation of the unusual spectralblue-shift with the increase of temperature. Such coupled-oscillators model not only provides insights into the temperature dynamics of non-bonded interactions but also offers an opportunity to better understand the physical mechanisms behind the unusual THz spectral behaviors in taurine. Particularly, the simulation approach and novel coupled-oscillators model presented in this work are applicable to analyze the THz spectra of other molecular systems.