Theoretical Infrared and Terahertz Spectra of an RDX/Aluminum Complex

Spectral Considerations for Standoff Infrared Detection of RDX on Reflective Aluminum

This paper examines infrared spectroscopic effects for the standoff detection of an explosive material, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), inkjet printed on an aluminum surface. Results of a spectroscopic study are described, using multiple optical setups. These setups were selected to explore how variations in the angles of incidence and collection from the surface of the material result in corresponding variations in the spectral signatures. The goal of these studies is to provide an understanding of these spectral changes since it affects standoff detection of hazardous materials on a reflective substrate. We demonstrate that variations in spectral effects are dependent on the relative surface concentration of the deposited RDX. We also show that it is reasonable to use spectroscopic data collected in a standard laboratory infrared spectrometer outfitted with a variable angle reflectometer set at 0° as reference spectra for data collected in a standoff configuration. These results are important to provide a systematic approach to understanding infrared (IR) spectra collection using standoff systems in the field, and to allow for comparison between such data, and data collected in the laboratory. Although the precise results are constrained to a specific material system (thin layers on a reflective substrate), the approach and general discussion provided are applicable to a broad range of IR standoff sensing techniques and applications.

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.

Demonstration of a Human Color Vision Mimic in the Infrared

Color vision results from the interaction of retinal photopigments with reflected or transmitted visible light. The International Commission on Illumination (CIE) developed the CIE color-matching chart, which separates colors on the basis of the interaction of their spectral profiles with three retinal photopigments in the human eye. We report the development of an infrared chromaticity (CIE-IR) chart, which mimics the CIE chart, in order to discriminate between different chemicals on the basis of the interactions of their IR signatures with three different IR optical filters, instead of the retinal photopigments in the human eye. Our results demonstrate that the CIE-IR chart enables separation of different classes of chemicals, as the visible CIE chart does with color, except for those in the IR spectral region. Such results clearly show that the biomimetic sensing method based on human color vision is in fact a true analogue to color vision and that the proposed CIE-IR chart can be used as a classification method unique to this biomimetic sensing modality.

Tandem mass spectrometry and density functional theory of RDX fragmentation pathways: Role of ion-molecule complexes in loss of NO3 and lack of molecular ion peak

RATIONALE

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is an explosive compound that finds a wide range of military and civilian applications. RDX has been a target in environmental matrices by gas chromatography/tandem mass spectrometry (GC/MS/MS). MS/MS in negative chemical ionization (NCI) mode of RDX provides important fragmentation patterns that are useful for structural elucidation. The fragmentation patterns are needed for proper identification of precursor and product ions in analytical methods that depend on MS/MS approaches for a reliable identification of RDX.

METHODS

This study focuses on the MS fragmentation mechanisms of RDX in NCI mode using both MS/MS and density functional theory (DFT). The DFT studies were performed at the B3LYP/6-311G(d,p) level of theory.

RESULTS

The DFT results showed that NCI of RDX leads to the formation of an anion-molecule complex that was energetically more stable than the RDX anion. The fragmentation proceeds through two pathways, leading to the loss of NO(2) and NO(3). The loss of NO(3) takes place in an anion-molecule complex leading to the formation of characteristic nitroso group fragment ions. Using the fragmentation schemes, important ion structures are proposed including structures for m/z 160, 129, 102, and 86.

CONCLUSIONS

The results demonstrate the importance of both charge-induced and charge-remote dissociations in RDX pathways. The ion structures identified along the pathways could be used as targets in analytical methods for reliable identification purposes.

Vibrational spectra of an RDX film over an aluminum substrate from molecular dynamics simulations and density functional theory

We report calculated vibrational spectra in the range of 0-3,500 cm(-1) of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) molecules adsorbed on a model aluminum surface. A molecular film was modeled using two approaches: (1) density functional theory (DFT) was used to optimize a single RDX molecule interacting with its periodic images, and (2) a group of nine molecules extracted from the crystal structure was deposited on the surface and interacted with its periodic images via molecular dynamics (MD) simulations. In both cases, the molecule was initialized in the AAA conformer geometry having the three nitro groups in axial positions, and kept that conformation in the DFT examination, but some molecules were found to change to the AAE conformer (two nitro groups in axial and one in equatorial position) in the MD analysis. The vibrational spectra obtained from both methods are similar to each other, except in the regions where collective RDX intermolecular interactions (captured by MD simulations) are important, and compare fairly well with experimental findings.