Photolysis of fungicide triadimefon: A combined experimental and theoretical investigation on homolytic C[sbnd]O and C[sbnd]N bonds dissociation mechanisms

Abstract

Among the most frequently used fungicides, triazole has been widely applied in agriculture on many crops. Extensive usage and low effective utilization result in the widespread occurrence of this kind of chemicals in the environment. Known as an important decomposition pathway in aquatic environment, photochemical transformation is believed to significantly affect the fate of organic contaminants. Herein, we reported the detailed photodegradation mechanism of a typical triazole, triadimefon (TDF), based on a combined experimental and theoretical strategy. Our results reveled that TDF could undergo a relative fast photodegradation with a pseudo-first-order rate constant of 0.14 h−1. Photoproducts identification using high-resolution mass spectrometry (HRMS) analyses indicated two different photolysis pathways of TDF, namely, the homolytic CO and CN bonds dissociation processes. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to explore these two processes. Theoretical results showed that the CO bond dissociation could occur in either the S1 or T1 state, with the energy barrier being 9.0 and 10.7 kcal/mol, respectively, while the CN bond dissociation was only possible to occur in the T1 state, with a higher energy barrier of 24.3 kcal/mol. In addition, spin-orbit coupling calculations indicated that the triplet state could be effectively populated through S1 → T3 intersystem crossing. The present study highlighted the potential application of DFT and TDDFT methods as stable methods in photochemical study.