5-Amino-1,2,4-triazol-3-one Degradation by Indirect Photolysis: A Density Functional Theory Study

Sunlight irradiation induces formation of reactive oxygen species (superoxide, hydroperoxyl radical, singlet oxygen, etc.), which readily take part in degradation of environmental pollutants. Being a primary ingredient in a suite of insensitive munition formulations, NTO (5-nitro-1,2,4-triazol-3-one) can be released onto training range soils and reduced to ATO (5-amino-1,2,4-triazol-3-one) by soil bacteria or iron-contained minerals. ATO can be dissolved in surface water and groundwater due to its good water solubility and then undergo further decomposition. A detailed investigation of possible mechanisms for ATO decomposition in water induced by superoxide, hydroperoxyl radical, and singlet oxygen as pathways for ATO environmental degradation was performed by computational study at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. Hydrolysis and degradation of ATO induced by superoxide are unlikely to occur due to the high activation energy or endergonicity of the processes. The hydroperoxyl radical causes rapid and reversible hydrogen transfer from ATO, while an attachment of the hydroperoxyl radical to ATO can induce decomposition of ATO, leading to its mineralization. Singlet oxygen shows a higher reactivity toward ATO than the hydroperoxyl radical. Decomposition of ATO was found to be a multistep process that begins with singlet oxygen attachment to the carbon atom of the C═N double bond. The intermediate that is formed undergoes recyclization, cycle opening, and sequential elimination of nitrogen gas, ammonia, and carbon(IV) oxide. Isocyanic acid, which arises intermediately, hydrolyzes into ammonia and carbon(IV) oxide. Calculated activation energies and high exergonicity of the studied processes support the contribution of singlet oxygen and the hydroperoxyl radical to ATO degradation into low-weight inorganic compounds in the environment.