Pivotal role of water molecules in the photodegradation of pymetrozine: New insights for developing green pesticides

Photodegradation of the insecticide pymetrozine (PYM) was studied on surface of wax films, and in aqueous and nonaqueous phase. The half-life of PYM on the wax surface was approximately 250 times longer than in water. Scavenging experiments, laser flash photolysis, and spectra analysis indicated the first singlet excited state of PYM (S1 *(PYM)) to be the most important photoinduced species initiating the photodegradation. Quantum chemistry calculations identified significant molecular torsion and changes in the structure C-C=N-N of S1 *(PYM), and the absolute charges of the C=N atoms increased and the bond strength weakened. Free energy surface analysis, and O-18 labeling experiments further confirmed that the mechanism was two-step photoinduced hydrolysis. The first step is the hydrolysis of S1 *(PYM) at C=N upon reaction with 2-3 water molecules (one H2O molecule as the catalyst). The second step is an intramolecular hydrogen transfer coupled with the cleavage of C-N bond and formation of two cyclic products. During the interactions, water molecules experience catalytic activation by transferring protons, while there is a negligible solvent effect. Clarifying the detailed photodegradation mechanisms of PYM is beneficial for the development of green pesticides that are photostable and effective on leaf surfaces, and photolabile and detoxified in the aquatic environment.

Automated summary

The insecticide pymetrozine degrades much slower on wax surfaces compared to in water, with the first singlet excited state of PYM identified as the key photoinduced species initiating degradation. Quantum chemistry calculations revealed significant structural changes in S1 *(PYM), leading to a two-step photoinduced hydrolysis mechanism. Activated water molecules catalyze the process, while the solvent effect is minimal. Understanding the detailed photodegradation mechanisms of PYM is crucial for developing environmentally friendly pesticides.