Solution and Solid Trinitrotoluene (TNT) Photochemistry: Persistence of TNT-like Ultraviolet (UV) Resonance Raman Bands

Abstract

We examined the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state trinitrotoluene (TNT) and its solution and solid-state photochemistry. Although TNT photodegrades with a solution quantum yield of ϕ ∼ 0.015, the initial photoproducts show DUVRR spectra extraordinarily similar to pure TNT, due to the similar photoproduct enhancement of the -NO2 stretching vibrations. This results in TNT-like DUVRR spectra even after complete TNT photolysis. These ultraviolet resonance Raman spectral bands enable DUVRR of trace as well as DUVRR standoff TNT detection. We determined the structure of various initial TNT photoproducts by using liquid chromatography-mass spectrometry and tandem mass spectrometry. Similar TNT DUVRR spectra and photoproducts are observed in the solution and solid states.

Review of explosive detection methodologies and the emergence of standoff deep UV resonance Raman

“…TNT photodegrades upon exposure to near‐UV (<400 nm) irradiation from sunlight through multiple pathways as shown by Clark et al and Kunz et al TNT shows weak UV absorption in the near UV . TNT excited deeper into the UV, for example, at 229 nm forms similar photoproducts as seen in sunlight illumination studies . This photolysis leads to photoproducts consisting of an aromatic ring with amine, hydroxyl, and carboxylic acid substituents .…”

“…Most explosives have their first allowed electronic transitions in the deep UV below 260 nm . Exciting explosive molecules in the deep UV results in resonance enhancement, which increases the Raman cross sections and the spectral sensitivity . The UVRR spectra show increased S/N ratios because of these increased Raman cross sections.…”

“…Standoff measurements of explosives will be impacted by the photochemistry that accompanies UV excitation . For example, TNT is well known to photochemically degrade upon exposure to the near UV irradiation from sunlight (<400 nm) .…”

“…UV excitation within the absorption bands of explosives causes photolysis, which decreases the explosive molecule's concentration such that the analyte Raman spectral band intensities decrease. To quantitate this phenomenon, we have examined the photochemistry of NaNO 3 , TNT, RDX, and NH 4 NO 3, and are currently studying the photochemistry of PETN…”

“…The Raman band at 1210 cm −1 selectively decreases and disappears over time, indicating the complete photolysis of TNT. The 1210 cm −1 Raman band, which is a TNT marker band, derives from a symmetric aromatic ring breathing mode whose origin requires the symmetry of the TNT aromatic ring . This band intensity is directly proportional to the TNT concentration in the illuminated sample.…”