The detection of trace level of explosives is a challenging field of great importance to national security and landmine detection. Chemical signatures of buried landmines are in a very complex environment. External physical conditions that affect explosive vapors and particles in soil can affect the explosive chemical signature. The chemical spectroscopic signature of the RDX in clay soil environments has been investigated by means of reflectance FT-IR microspectroscopy. The soil obtained from the University of Puerto Rico at Mayaguez was treated using the textural mechanical method in order to separate the clay from all the other components in the soil. B3LYP/6-311G** calculations performed on the low energy conformers of RDX helped to determine its most stable conformations, their symmetry, and vibrational spectra. The FT-IR technique confirmed the existence of two different RDX solid phases, known as the α-RDX and β-RDX, which have different symmetries and revealed significant differences in their spectra. The IR microspectroscopic study showed that the RDX-Clay mineral complex and its interactions can be detected using the FT-IR technique at a low concentration of 1000 part-per-millions. Variations in the clay's pH revealed changes in the RDX-Clay complex spectroscopic signature. These results also indicate that the interaction between the RDX and the clay minerals affects mainly the ring breathing, the C-N vibrations and the NO2 groups of the explosive molecules. It is suggested that the electron donor nitrogen atoms from RDX are interacting with the electron acceptor oxygen atoms of the edge sites of the clay's surface.
Computational algorithms have been very useful to study molecular interactions between explosives and different types of soils. In this work ab initio molecular orbital calculations were employed to study the interaction of 2,4,6-trinitrotoluene (TNT) with the basal siloxane surface of clay minerals. The intermolecular interaction energy, the vibration frequencies and efficient computational algorithms have been tested for the complex of TNT with the siloxane surface site of clay minerals. Two cluster models have been developed to represent the TNT on the siloxane surface of clay minerals. They have been employed in order to determine the changes in the spectroscopic signature of TNT. The results obtained provide information about the interaction energy of TNT on clays. The binding energy between the TNT and the basal siloxane surface was -38 kJ/mol, obtained with MP2//HF/6-31+G(d) level of theory and basis set, respectively. The calculated interaction has their minimal at separation between the two molecules of 3.5 Å. The theoretical IR spectra of the interaction was obtained with DFT//HF methods and the 6-31+G(d) basis set. The calculation predicted a shifting effect in NO<sub>2</sub> bands, due to the interaction. The results are in excellent agreement with available experimental data. Further, result of such theoretical studies could contribute to an understanding of the interaction energy of the other kinds of explosives that may be occurring in other environments.
Experimental studies have shown that a key factor affecting the bioavailability and biodegradability of nitroaromatic compounds (NAC's) in subsurface environments is their sorption onto clay minerals. This study present the recent ab initio quantum mechanical calculations on the interaction of 2,4-DNT (DNT) with the basal siloxane site surface of kaolinite, a clay mineral. Theoretical calculations of the low energy conformation of DNT interacting with the siloxane site surface of clay minerals were performed in order to obtain their properties adsorbed on soil environments as well as the structure of the adsorbed molecule. The calculations also yielded the way of orientation and the effect of the adsorption. This study was performed using DFT//HF and MP2//HF methods taking into account the contribution of the Coulombic (CEb) and dispersion (DEb) energies, to obtain the binding energies between DNT and siloxane surface. A comparison of the CEb and DEb energies shows that the stabilization of DNT at the siloxane sites, using a small molecular model (single tetrahedra), is mainly provided by dispersion interaction energy. Considering the accuracy and cost of the computation methods the 6-31+G* basis set produced the best representation of the interaction energy (42 kJ/mol) using the MP2//HF level of theory for the DNT-Siloxane surface. These theoretical calculations give a good prediction of the interaction between the 2,4-DNT molecule with soil clay minerals. The computational results are compared with the experimental results obtained with the FT-IR microscopic technique.
2,4,6-trinitrotoluene (TNT) is the most used explosive as main charge in landmines. There have been found contamination of soil and groundwater with munitions residues of TNT due to buried landmines. We are investigating the molecular structure, vibration behavior and the binding energy of TNT with the siloxane surface site of clay minerals in order to determine the spectroscopic signature of TNT in soil. Two different molecular symmetry structures were found with density functional theory (DFT) B3LYP method with 6-31G, 6-31G*, 6-311G, 6-311G*, and 6-311+G** basis sets from the Gaussian 98 systems of programs. Different deformations of the phenyl ring and distortions of the nitro and methyl groups with the ring were observed. In both structures, C<sub>1</sub> and C<sub>s</sub>, the nitro groups in positions 2 and 6 are out of plane with the phenyl ring due to steric interaction with the methyl group while the nitro group in position 4 is planar to the phenyl ring. The difference between the two structures is the internal rotation of the methyl group and 2, 6-nitro groups. Comparison of the calculated energies of the two structures in several basis sets reveals that the lowest-energy geometry for the TNT structure corresponds to C<sub>s</sub> symmetry with B3LYP/6-311+G**. FTIR spectra of TNT are presented and assigned assisted by B3LYP/6-311+G** result. The lowest-energy molecular structure of TNT was interacted with the basal siloxane surface of clay minerals to determine the binding energy (E<sub>b</sub>) between them. The binding energy was obtained by optimizing the vertical distance, the rotational and inclination angles between TNT and siloxane surface using the B3LYP hybrid functional with different basis sets.
Among the many different signature compounds emitted from a landmine in the vapor phase, 2,4-dinitrotoluene (2,4-DNT) is the most common nitroaromatic compound in terms of detecting buried landmines, although it is a byproduct in the synthesis of TNT. 2,4-DNT is used as an ingredient in mining explosives and also prevalent on the soil surface but is somewhat seasonally dependent. The B3LYP hybrid functional was used to obtain the lowest-energy structure of both 2,4 and 2,6-DNT. Increasing basis sets from the 3-21G up to the 6-31++G (d, p) are used to predict structural parameters, vibrational frequencies, IR intensities and Raman activities for the explosives molecules. The calculated energies show that the 2,4-dinitrotoluene isomer is more stable than 2,6-dinitrotoluene isomer due to the lesser levels of steric effects between the nitro groups and the methyl group. The optimized structures were interacted with the siloxane site of clay minerals, using the density functional level of theory and the basis sets used to optimize the geometry of the DNT molecules. The binding energy (E<sub>b</sub>) between the optimized molecules and the basal siloxane site surface of clay minerals was calculated at distances in a range between 2.5 to 8.5 Å.
The chemical spectroscopic signature of the RDX-clay mineral complex has been investigated by means of reflectance FT-IR micro spectroscopy. The mechanical analysis method was used to separate the clay from the other soil components. The soil was obtained from the University of Puerto Rico at Mayagüez (UPRM) campus backyard. B3LYP/6-311G** calculations performed on RDX helped to determine the most stable conformations, their symmetry, and vibrational spectra. The FTIR technique confirmed the existence of two different RDX solid phases, known as the α-RDX and β-RDX, which have different symmetries and revealed significant differences in their spectra. The IR microspectroscopic study showed that the RDX-clay mineral complex and its interactions can be detected using the FTIR technique at a low concentration of 1000 part-per-millions. The results also suggest that the vibrational modes presenting changes in the different vibrational spectra correspond to the C-N and NO<sub>2</sub> groups. In comparison with α-RDX spectrum, the complex exhibits three bands at 740, 754 and 792 cm<sup>-1</sup>. A 12 cm<sup>-1</sup> red shift is observed in this last band assign to the C-N stretching and NO<sub>2</sub> scissoring vibrations in the equatorial position. Differences in the spectra were also seen in the shifted bands at 942 and 953 cm<sup>-1</sup>. These vibrational modes are assigned to the ring breathing and N-N stretching vibration in the axial position for the -phase. Comparison of the spectra of the α-RDX, the β-RDX and the RDX mixed with clay in the range from 1190 to 1700 cm<sup>-1</sup> clearly indicated that the FTIR technique can be used to study the interaction between RDX and clay. The results also indicate that the interaction between the RDX and the clay minerals affects mainly the NO<sub>2</sub> groups of the explosive molecules. It is suggested that the electron donor nitrogen atoms from RDX are interacting with the electron acceptor oxygen atoms of the siloxane surface that is present in the majority of clays.
RDX, a high power explosive used as the main charge in some landmines, was investigated in our laboratory in order to determine the spectroscopic signature to be used in its identification by means of ion mobility spectrometry (IMS) and FTIR. Density functional theory (DFT) was also used to predict structural parameters and vibrational frequencies. It was confirmed that RDX has two conformers known as the β and α-RDX phases. There are several conformers depending on the position of the NO<sub>2</sub> groups with respect to the triazine ring. This is important in order to determine whether RDX will have affinity for soil and/or the different materials in the ground or will be carried out by water once it starts to leak from the container holding the explosives. Different amounts of RDX were deposited on soil, aluminum plates, glass, and vinyl polymeric films. For IMS studies, the surfaces were rubbed with filter paper and the RDX was desorbed directly from the filter to the instrument inlet port. In the case of the FT-IR studies the samples were examined using an ATR coupled FTIR system. The FTIR spectra showed significant differences between the α and β phases of RDX.