The Role of Triplet State Keto–Enol Tautomerism in the Photodeamination of Metamitron

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-12, Vol.115 (50), p.14397-14406
• Main Authors: Kouras-Hadef, Sofia, de Sainte-Claire, Pascal, ter Halle, Alexandra, Amine-Khodja, Amina, Richard, Claire
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Deoxygenation; Environment and Society; Environmental Sciences; Excitation; Lasers; Mathematical analysis; Nanostructure; Physical chemistry; Pulsed lasers; Solvents
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp208907f

Substituted 4-amino-1,2,4-triazin-5-ones undergo photodeamination through cleavage of the N–NH2 bond in the presence of oxygen and water. To elucidate the mechanism of this reaction, we investigated the photolysis of metamitron (4-amino-6-phenyl-3-methyl-1,2,4-triazin-5-one) by nanosecond laser flash photolysis, steady-state irradiation, and ab initio calculations. Upon pulsed laser excitation of deoxygenated aqueous metamitron, two transient species are clearly detected. The predictions of ab initio results are consistent with experimental results: (i) it is proposed here that the transient species are, respectively, the keto and diradical forms of the metamitron keto–enol tautomerism in the triplet state, and (ii) in water, the activation free energy barrier of enolization is drastically decreased. Thus, the formation of the diradical triplet is enabled in aqueous solvent. A detailed analysis of the intermediate structures that lead to the final products (HNO2 and deaminometamitron) is provided.