Role of Singlet Oxygen in the Degradation of Selected Insensitive Munitions Compounds: A Comprehensive, Quantum Chemical Investigation

Abstract

DNAN (2,, NTO (3-nitro-1,2,4triazol-5-one), and NQ (nitroguanidine) are important energetic materials used in military applications. They may find their way to the environment during manufacturing, transportation, storage, training, and disposal. A detailed investigation of possible mechanisms for reactions of the nitrocompounds with singlet oxygen, one of the potential methods for their degradation, was performed by computational study using the PCM(Pauling)/M06-2X/6-311++G(d,p) approach. Obtained results suggest that reactivity of the investigated munitions compounds toward singlet oxygen follows the order: DNAN > NTO(anion) > NQ ≫ NTO. DNAN is involved in [4 + 2]-addition with oxygen, and further formation of diepoxide or epoxyketone through diradical intermediates have been predicted. The NTO may undergo intramolecular rearrangement with formation of peroxide compound or nitrite radical elimination, and NQ may be transformed into urea.

Role of Molecular Singlet Oxygen in Photochemical Degradation of NTO: DFT Study

“…The reaction of singlet oxygen with NTO starts from an addition of 1 O 2 to the carbon atom C5 of the C5�N1 double bond, which was found to be the most energy-favorable pathway for the initial attack of 1 O 2 onto NTO. 31 A detailed study of this reaction shows that its energy barrier depends on the orientation of 1 The modeled pathways and Gibbs free energy calculations allow us to suggest that decomposition of NTO induced by the singlet oxygen in aqueous solution may lead to its mineralization (Figure 1). The energetically most favorable reactive channel is NTO…”

“…An initial attack of 1 O 2 onto NTOa occurs on the carbon atom C5 of the C5�N1 double bond, as it was found earlier. 31 The orientation of 1 O 2 above the triazolone cycle leads to the transition state TS(NTOa → INT1a) with the lowest energy of 13.34 kcal/mol. The activation energy for interaction of a singlet oxygen with NTOa is lower than for the reaction of 1 O 2 with NTO (20.02 kcal/mol), implying a higher reactivity of the deprotonated anionic form of NTO toward the singlet oxygen, which was predicted by the analysis of the frontier orbital energies (Table 1).…”

“…Recently, different pathways for the initial reaction of NTO with singlet oxygen were modeled to evaluate the ability of its oxidation in aqueous solution. 31 It was shown that singlet oxygen may contribute to the photooxidation of NTO. The most energy-favorable direction of the reaction was found to be a singlet oxygen attachment to the C5 atom of NTO with subsequent nitrite elimination.…”

“…Computational modeling is an efficient approach to study reaction with short-lived species. Recently, different pathways for the initial reaction of NTO with singlet oxygen were modeled to evaluate the ability of its oxidation in aqueous solution . It was shown that singlet oxygen may contribute to the photooxidation of NTO.…”

Reduction of 1,2,3-trichloropropane (TCP): pathways and mechanisms from computational chemistry calculations

“…RCDs that only include the free energies of stable species are useful for comparing the energetics of reaction pathways, but they can also include transition states, which then provide insight into reaction barriers and therefore kinetics. Recent examples of RCD interpretation in the literature on contaminant fate include determination of the main decomposition pathways for diverse substances of pressing environmental concern such as insensitive munition compounds 81 and PFAS (per-and poly-uoroalkyl substances). 82 In general, almost all published RCDs appear to be drawings based on qualitative considerations of the relative energies or graphs that are manually compiled.…”