The Raman vibrational frequencies in the finger print region (700-1600 cm-1) have been calculated for 2,4-
dinitrotoluene, 2,6-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT). The Raman vibrational intensities
and frequencies for these molecules have been calculated using B3LYP Density Functional Theory method with
6-311+G** and Sadlejs medium-sized polarized basis sets (Sadlej pVTZ). The normal mode assignments in the
finger print region were carried out by Normal Coordinate Analysis, where localized and de-localized coordinates
were used to facilitate an accurate description of the vibrational modes. The Raman intensities were calculated
from the Raman scattering cross sections using the ab initio calculated Raman scattering activities. Comparison
of these intensities using different basis sets indicates that the Sadlej pVTZ basis sets increase the calculated
intensities for the NO2 symmetric stretching and bending frequencies by more than 15 % relative to 6-311+G**
basis. The potential energy distribution for the symmetric and asymmetric NO2 stretches indicates that 2-NO2
and 6-NO2 couple strongly in 2,6-DNT and 2,4,6-TNT, while 2-NO2 and 4-NO2 groups couple weakly in 2,4-
DNT. These findings suggest that the coupling strength of 2-NO2, 6-NO2 and 4-NO2 groups can be used to distinguish between dinitro and trinitro toluenes.
Landmines have been a part of war technology for many years. As a result of the continued and indiscriminate use in approximately 90 countries landmines pose a severe and ever growing problem and a daily risk. Raman Spectroscopy is capable of providing rich information about the molecular structure of the sample and pinpoint detection of many chemicals, both of organic and inorganic nature. The presence of landmines in soils can be detected by Raman Spectroscopy sensing in a Point Detection modality, using characteristic vibrational signals of each explosive present in landmines. Detection of 2,4-DNT in sand and studies on how the vibrational signatures of 2,4-DNT is modified by interacting with soil particles and environmental conditions is reported. Raman Microspectrometers equipped with 514 nm and 785 nm laser excitation lines were used. The work focused in how the spectroscopic signatures of DNT in contact with Ottawa Sand are affected by the presence of humidity, pH, temperature, UV light and reaction times. Samples of mixtures of sand/2,4-DNT were analyzed by Raman Spectroscopy at 10, 50 and 100% water content and temperatures in range of 40-80 °C. Mixtures were also analyzed at different pH: 4, 7 and 10 and under ultraviolet light at 254 nm. Raman spectra were taken as a function of time in an interval from 24 to 336 hours (two weeks). Characteristic signals of 2,4-DNT were analyzed in different ranges 100-3800 cm-1, 600-1200 cm-1, 300-1700 cm-1 and 2800-3500 cm-1. The effect of these variables was measured during 45 consecutive days. It was confirmed that the decrease of characteristic vibrational signatures of 2,4-DNT can be attributed to increase of the degradation of 2,4-DNT by the simulated environmental conditions. Spectroscopic characterization of degradation products, both in contact with sand as well as airborne is under way. These results will make possible the development of highly sensitive sensors for detection of explosives materials and correlated with their degradation products in landmines.
2,4,6-Trinitrotoluene, commonly known as TNT, is an explosive used in military shells, bombs, landmines, grenades, demolition operations, and underwater blasting. It is produced in the United States only at military facilities. Accidental releases of TNT and residues in battle fields have contaminated groundwater, soil, and sand at numerous sites around the world. TNT exists in two physical forms at room temperature: droplets and crystals. The spectroscopic information conveyed depends on its physical form and the substrate on which it is deposited. Vibrational spectroscopy is a powerful tool that can be used to characterize TNT in its diverse forms. Crystallization of TNT from different solvents (acetonitrile, methanol, and water) was carried out to subsequently measure the vibrational spectra. The important nitroaromatic compound exhibits a series of unique characteristic bands that allow its detection and spectroscopic characterization. The spectroscopic signatures of neat TNT samples were determined with Raman Microspectroscopy and Fourier Transform Infrared (FTIR) Microscopy. The Raman spectra of neat TNT are dominated by strong bands at about 1365 and 2956 cm-1. The intensity and even the presence of these bands are found to be remarkably dependent on TNT form and source.
TNT and DNT are important explosives used as base charges of landmines and other explosive devices. They are often combined with RDX in specific explosive formulations. Their detection in vapor phase as well as in soil in contact with the explosives is important in landmine detection technology. The spectroscopic signatures of nitroaromatic compounds in neat forms: crystals, droplets, and recrystallized samples were determined by Raman Microspectroscopy (RS), Fourier Transform Infrared Microscopy (FTIR) and Fiber Optics Coupled - Fourier Transform Infrared Spectroscopy (FOC-FTIR) using a grazing angle (GA) probe. TNT exhibits a series of characteristic bands: vibrational signatures, which allow its detection in soil. The spectroscopic signature of neat TNT is dominated by strong bands about 1380 and 2970 cm-1. The intensity and position of these bands were found remarkably different in soil samples spiked with TNT. The 1380 cm-1 band is split into a number of bands in that region. The 2970 cm-1 band is reduced in intensity and new bands are observed about 2880 cm-1. The results are consistent with a different chemical environment of TNT in soil as compared to neat TNT. Interactions were found to be dependent on the physical source of the explosive. In the case of DNT-sand interactions, shifts in vibrational frequencies of the explosives as well as the substrates were found.
Raman Spectroscopy is a well established tool for vibrational spectroscopy analysis. Interactions of explosives with different substrates can be measured by using quantitative vibrational signal shift information of scattered Raman light associated with these interactions. A vibrational spectroscopic study has been carried out on 2,4-DNT and 2,6-DNT crystals. Raman Microscopy spectrometers equipped with 514 nm and 785 nm laser excitation lines were used. The samples were recrystallized on different solvents (water, methanol and acetonitrile) and allowed to interact with soil samples. The interaction with sand and soil samples doped with the nitroaromatic compounds showed significant shifts in its peaks. The above information was used to detect DNT in soil using Raman Microscopy. These results will make possible the development of highly sensitive sensors for detection of explosives materials.