It is shown here that carbamazepine (CBZ) would undergo direct photolysis and reaction with • OH as main phototransformation pathways in surface waters. Environmental lifetimes are expected to vary from a few weeks to several months, and predictions are in good agreement with the available field data. Acridine (I) and 10,11-dihydro-10,11-trans-dihydroxy-CBZ (V) are the main quantified phototransformation intermediates upon direct photolysis and • OH reaction, respectively. The photochemical yield of mutagenic I from CBZ is in the 3-3.5% range and, being similar for both direct photolysis and • OH reaction, it would undergo limited variation with environmental conditions. In contrast, the yield of V would vary in the 4-8.5% range depending on the conditions, because V is formed from CBZ by • OH (9.0% yield) more effectively than upon direct photolysis (1.4% yield). Other important photointermediates, mostly formed from CBZ upon • OH reaction are an aromatic-ring-dihydroxylated CBZ (VI) and N,N-di(2-carboxyphenyl)urea (VII). Compounds VI and VII are formed by photochemistry and are not reported as human metabolites, thus they could be used as tracers of CBZ phototransformation in surface waters. Interestingly, VI has recently been detected in river water.
Substituted 4-amino-1,2,4-triazin-5-ones undergo photodeamination through cleavage of the N-NH(2) 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 (HNO(2) and deaminometamitron) is provided.
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