Terahertz Spectroscopy and Solid-State Density Functional Theory Simulations of the Improvised Explosive Oxidizers Potassium Nitrate and Ammonium Nitrate

Study on the crystallization behavior and conformation adjustment scale of poly(lactic acid) in the terahertz frequency range

The observed properties of crystalline polymers are determined by their internal structure, which in turn is the result of their different crystallization behaviors. Here, we investigate the crystallization behavior of poly(lactic acid) (PLA) by terahertz time-domain spectroscopy (THz-TDS) at varied temperatures. We find that the changes in the chain packing and conformation of PLA are characterized by THz spectroscopy. Combining X-ray diffraction (XRD) and infrared spectroscopy (IR), we attributed the blue-shift of the THz peak to the tightness of the chain packing, while its absorption enhancement is caused by the conformation transition. The effects of chain packing and chain conformation on the characteristic peak are phased. Furthermore, absorption discontinuities of the characteristic peaks of PLA crystallized at different temperatures are observed, which originated from differences in the degree of conformational transition caused by different thermal energies. We find that the crystallization temperature at which the absorption mutation of PLA occurs corresponds to the temperature at which the motion of the segment and molecular chain is excited, respectively. At these two temperatures, PLA exhibits different scales of conformational transitions leading to stronger absorption and larger absorption changes at higher crystallization temperatures. The results demonstrate that the driving force of PLA crystallization is indeed from changes in chain packing and chain conformation, and the molecular motion scale can also be characterized by THz spectroscopy.

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.

Temperature-dependent terahertz vibrational spectra of tetracycline and its degradation products

Terahertz (THz) spectroscopy has emerged as an attractive technique for qualitative and quantitative detection. Analysis of these chemicals in the THz range under various temperatures can yield detailed information on molecular vibrational modes, which is of utmost importance for effective detection. Here we report the use of THz time-domain spectroscopy (THz-TDS) to measure tetracyclines hydrochloride (TCH) and its degradation products including epitetracycline hydrochloride (ETCH), anhydrotetracycline hydrochloride (ATCH), and epianhydrotetracycline hydrochloride (EATCH) over the temperature range of 4.5-300 K for the first time. The results showed that these four tetracyclines exhibited numerous distinct spectral features in frequency-dependent absorption spectra, which demonstrated the qualitative capacity of THz-TDS. Through density functional theory (DFT) calculations and analysis of temperature-dependent absorption spectra, the origin of the observed terahertz absorption peaks of these four tetracyclines were well interpreted. This study could lay the foundation for high-performance analysis of biological and chemical molecules by THz spectroscopy, which is essential for sensing application.

Experimental and theoretical investigations of tartaric acid isomers by terahertz spectroscopy and density functional theory

The terahertz (THz) absorption spectra of l-, d-, and dl-tartaric acid have been measured in the frequency range from 0.2 to 2.0 THz by terahertz time-domain spectroscopy (THz-TDS). The characteristic absorption peaks of these three tartaric acid isomers were obtained, which showed remarkable difference between enantiomers (l- and d-tartaric acid) and the racemic compound (dl-tartaric acid) in their peak frequencies. In parallel with the experimental study, theoretical calculations on isolated-molecule and unit cell of tartaric acids using density functional theory (DFT) were also performed for simulating the experimental THz spectrum features, which were in good agreement with the experimental data. Results demonstrate that THz-TDS can distinguish the tiny diversity between tartaric acid chiral isomers and its racemic compound, and provided an effective method for molecular identification in biological and biomedical engineering.

Spectral Characterization and Molecular Dynamics Simulation of Pesticides Based on Terahertz Time-Domain Spectra Analyses and Density Functional Theory (DFT) Calculations

This work provides the experimental and theoretical fundamentals for detecting the molecular fingerprints of six kinds of pesticides by using terahertz (THz) time-domain spectroscopy (THz-TDS). The spectra of absorption coefficient and refractive index of the pesticides, chlorpyrifos, fipronil, carbofuran, dimethoate, methomyl, and thidiazuron are obtained in frequencies of 0.1–3.5 THz. To accurately describe the THz spectral characteristics of pesticides, the wavelet threshold de-noising (WTD) method with db 5 wavelet fucntion, 5-layer decomposition, and soft-threshold de-noising was used to eliminate the spectral noise. The spectral baseline correction (SBC) method based on asymmetric least squares smoothing was used to remove the baseline drift. Spectral results show that chlorpyrifo had three characteristic absorption peaks at 1.47, 1.93, and 2.73 THz. Fipronil showed three peaks at 0.76, 1.23, and 2.31 THz. Carbofuran showed two peaks at 2.72 and 3.06 THz. Dimethoate showed three peaks at 1.05, 1.89, and 2.92 THz. Methomyl showed five peaks at 1.01, 1.65, 1.91, 2.72, and 3.20 THz. Thidiazuron showed four peaks at 0.99, 1.57, 2.17, and 2.66 THz. The density functional theory (DFT) of B3LYP/6-31G+(d,p) was applied to simulate the molecular dynamics for peak analyzing of the pesticides based on isolated molecules. The theoretical spectra are in good agreement with the experimental spectra processed by WTD + SBC, which implies the validity of WTD + SBC spectral processing methods and the accuracy of DFT spectral peak analysis. These results support that the combination of THz-TDS and DFT is an effective tool for pesticide fingerprint analysis and the molecular dynamics simulations.

Advances in explosives analysis--part II: photon and neutron methods

The number and capability of explosives detection and analysis methods have increased dramatically since publication of the Analytical and Bioanalytical Chemistry special issue devoted to Explosives Analysis [Moore DS, Goodpaster JV, Anal Bioanal Chem 395:245-246, 2009]. Here we review and critically evaluate the latest (the past five years) important advances in explosives detection, with details of the improvements over previous methods, and suggest possible avenues towards further advances in, e.g., stand-off distance, detection limit, selectivity, and penetration through camouflage or packaging. The review consists of two parts. Part I discussed methods based on animals, chemicals (including colorimetry, molecularly imprinted polymers, electrochemistry, and immunochemistry), ions (both ion-mobility spectrometry and mass spectrometry), and mechanical devices. This part, Part II, will review methods based on photons, from very energetic photons including X-rays and gamma rays down to the terahertz range, and neutrons.

Anisotropic terahertz dielectric responses of sodium nitrate crystals

Terahertz (THz) spectroscopy has become an effective tool to characterize the low-frequency rotational and vibrational modes of molecules. In addition, novel THz dielectric responses and optical properties on the basis of molecular rotation and vibration have attracted lots of attention because of their potential application in THz devices. In this paper, the dielectric response of low-symmetric sodium nitrate crystals in the frequency range of 0.2-1.5 THz was experimentally demonstrated. Four absorption bands at 0.23, 0.47, 0.92, and 1.15 THz were observed in the dielectric spectra and were tentatively ascribed to the rotational motion of nitrate ions. Based on the molecular rotation mechanism, the dielectric anisotropy and dielectric resonance of the crystal were discussed in detail.