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

Abstract

NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment and dissolved in surface water and groundwater due to its good water solubility. Singlet oxygen is an important reactive oxygen species produced in the aquatic environment under sunlight irradiation. A detailed investigation of the possible mechanism for NTO decomposition in water induced by singlet oxygen as one of the pathways for NTO environmental degradation was performed by a computational study at PCM­(Pauling)/M06-2X/6-311++G­(d,p) level. Decomposition of NTO was found to be a multistep process that may begin with singlet oxygen attachment to the carbon atom of the CN double bond. The formed intermediate undergoes cycle opening, and nitrogen gas, nitrous acid, and carbon (IV) oxide elimination. Isocyanic acid, arisen transiently, hydrolyzes into ammonia and carbon (IV) oxide. The obtained results show a significant increase in reactivity of the anionic form of NTO as compared to its neutral form. The calculated activation energies and high exothermicity of the studied processes support the contribution of singlet oxygen to NTO degradation into low-weight inorganic compounds in the environment.

Role of Hydroxyl Radical in Degradation of NTO: DFT Study

“…Reaction of water addition to INT32 required an activation energy of 25.17 kcal/mol (Figure ). Our previous calculations showed the important catalytic role of protons, presented in the environment, for the hydrolytic decomposition of isocyanic acid into ammonia and carbon­(IV) oxide by significantly lowering the activation barrier and, therefore, increasing the rate of hydrolysis . In the current study, the decomposition of isocyanic acid induced by hydroxyl radical was examined (Figure ).…”

“…It should be noted that decomposition of NTO in natural waters may be potentially induced by different reactive species that are present there. According to experimental data the main pathway for NTO decomposition in natural water is photolysis, and therefore the main reactive species that may induce the degradation of NTO are the reactive oxygen species, generated in water solution under UV-irradiation, that are singlet oxygen, hydroxyl radical, hydroperoxyl radical, superoxide, etc. − The role of molecular singlet oxygen in photochemical degradation of NTO was studied in detail by us previously while the possibility of superoxide- and hydroperoxyl radical-induced mineralization of NTO is being investigated by our group.…”

“…Understanding of the mechanism of photochemical degradation of NTO is necessary in order to predict and control its environmental fate and persistence. Reactions of degradation of NTO in the triplet state and decomposition of NTO induced by singlet oxygen were modeled recently. , In the present study we examine pathways of NTO mineralization induced by hydroxyl radical, a highly reactive species with an oxidation potential of 2.8 V . Hydroxyl radical is ubiquitous in the environment and plays crucial roles in water remediation and environmental cleanup .…”

Degradation of NTO induced by superoxide and hydroperoxyl radicals: a comprehensive DFT study

“…In recent work, we showed that acidic catalysis may considerably decrease the activation energy for the hydrolysis of isocyanic acid to 9.51 kcal mol −1 . 10…”

“…Reactions of direct photolysis of NTO and its decomposition induced by singlet oxygen were modeled recently. 9,10 In the present study, we examine the ability of superoxide and hydroperoxyl radicals to take part in degradation of NTO.…”