Study On Photochemical Degradation Of Prothioconazole In Aqueous Solution

Prothioconazole is a broad-spectrum triazolinthione fungicide which is widely used to control fungal diseases of a wide range of crops.Most researches prothioconazole focused on its synthesis method,mechanism of action and toxicology,studies about the photochemical degradation dynamics and mechanism of prothioconazole is rare.In this study,the photochemical degradation dynamics of prothioconazole in aqueous solution was carried out,and the effects of different factors on the photolysis kinetics of prothioconazole were clarified.The degradation products were resolved using UPLC-QTOF/MS,and its photochemical degradation mechanism was studied with theoretical computational chemistry.The main results were as follows.1.Photochemical degradation dynamics of prothioconazole in water solution The photolysis half-lives of prothioconazole in aqueous solutions under xenon,UV and Hg lamp irradiation were 173.29 min,21.66 min and 11.18 min.Prothioconazole photolysis half-lives in buffer solutions at p H 4.0,7.0 and 9.0 under xenon lamp irradiation were 693.15min,231.05 min and 99.02 min,the photolysis of prothioconazole was promoted by NO₃^-with half-lives of 115.53 min,77.02 min and 69.32 min at NO₃^-concentrations of 1.0,2.0and 5.0 mg/L,respectively.2.Identification of photochemical degradation products in water solutions of prothioconazoleThe photolysis products of prothioconazole were resolved using a Waters ACQUITY UPLC I-Class Xevo G2-XS QTof system in combination with UNIFI software.The product T1 with a retention time of 6.12 min was the most abundant photolysis product with a measured mass charge ratio of 312.07 and a molecular formula of C₁₄H₁₅Cl₂N₃O.The product T1 was verified to be prothioconazole-desthio by standard comparison.The measured mass charge ratio of the product T2 with a retention time of 3.42 min was 310.06with the molecular formula C₁₄H₁₆Cl N₃OS.The measured mass charge ratio of the product T3 with a retention time of 3.23 min was 361.16 with the molecular formula C₁₄H₁₅Cl₂N₃O₂S;the measured mass charge ratio of the product T4 with a retention time of 3.09 min was294.10 with the molecular formula C₁₄H₁₃Cl₂N₃.3.Photochemical degradation mechanism of prothioconazole based on DFT calculationResults based on DFT calculations show that the carbon-sulphur,carbon-chlorine,carbon-nitrogen and carbon-oxygen bonds of the prothioconazole molecule have high absolute natural charge values and long bond lengths and are the active sites for the photodegradation reaction of the prothioconazole molecule.Light excites electron leap between the highest occupied orbital and the lowest vacant orbital of the prothioconazole molecule.Electrons from bonding orbitals leap to anti-bonding orbitals,resulting in lower bonding levels,reduced stability and easier dissociation of chemical bonds such as carbon-sulphur,carbon-chlorine and carbon-nitrogen bonds.Based on the structural identification of the photolysis products and results based on DFT calculations show that the photolysis pathways of prothioconazole are mainly desulphurisation to product T1,dechlorination to product T2,hydroxyl substitution to product T3 and further hydroxyl elimination of product T1 to T4.DFT calculation of the energy change of the reaction process show that light excitation reduced the activation energy of the desulfurization reaction by 21.80 k J/mol;the activation energy of the dechlorination reaction by 66.40 k J/mol;the activation energy of the hydroxyl substitution reaction by 11.80 k J/mol;and the activation energy of the hydroxyl elimination reaction by75.20 k J/mol.Through DFT calculation,the active site of the photochemical degradation reaction of prothioconazole and its energy change during the degradation and conversion were defined,which provides a research basis for its structural modification to improve the photochemical stability of prothioconazole.