Aqueous photo-degradation of Flupyradifurone (FPD) in presence of a natural Humic Acid (HA): A quantitative solution state NMR analysis

The present study aims to characterize the molecular interaction and photo-degradation behaviour of halogen (-F and -Cl) containing novel insecticide, flupyradifurone, (FPD) in the presence of humic acid (KHA), isolated from Karwar, western Rajasthan, India. Time-dependent photo-degradation kinetics of UV-exposed FPD is quantified in the absence and presence of KHA employing one dimensional H-1 and 19F solution-state NMR. The pseudo first-order photo-degradation rate constant quantified for FPD showed a significant decrease in the presence of KHA. Interestingly, variable degradation rates are found from H-1 and F-19 NMR spectral changes indicating chemical moiety wise degradation of FPD. Further, NMR saturation transfer difference (STD), relaxation, and diffusion experiments have been employed for the complete evaluation of binding interactions of KHA with FPD and its major degradation product difluoroacetic acid (DFA). STD experiments revealed that FPD binds to KHA through the chlorinated pyridine ring while the -F containing moiety of FPD does not show any detectable interaction. Relevant kinetic parameters, namely association constants and Gibbs free energy of interaction are quantified at neutral pH conditions by diffusion experiments depicting a stronger and more spontaneous association of FPD with KHA (KA = 55.55 M-1) compared to that of DFA-KHA (KA = 10.90 M-1). Both FPD and DFA bind with KHA through weak non-covalent interaction indicating the encapsulation of these molecules within KHA hindering the photo-degradation process of FPD. Concomitantly, it is also envisaged that FPD will be more persistent in soil compared to DFA that may leach out to groundwater.

Automated summary

This study characterized the molecular interaction and photo-degradation behavior of halogen-containing insecticide FPD in the presence of humic acid KHA. The results showed a significant decrease in photo-degradation rate constant of FPD in the presence of KHA, with different degradation rates observed through H-1 and F-19 NMR spectral changes. FPD binds to KHA through the chlorinated pyridine ring, and exhibits a stronger association compared to DFA-KHA. Both FPD and DFA bind with KHA through weak non-covalent interactions, hindering the photo-degradation process of FPD.