Theoretical studies on the mechanism, kinetics, and degradation pathways of auxin mimic herbicides by •OH radical in aqueous media

The kinetics and mechanism in the oxidative degradation pathways of the • OH radical reaction with seven auxin mimic aromatic acid-based herbicides were investigated with the help of various theoretical methods. Various global and local reactivity parameters such as ionization energy, molecular hardness, electrophilicity, condensed Fukui function, and total energies were determined to predict the reactivity of these herbicides towards the • OH radical. Geometry optimization was performed at the CAM–B3LYP/6–311 + G(d) level of theory including the solvent effect using the polarizable continuum model (PCM) incorporating the integral equation formalism (IEF) with water as solvent. Single point energies of various species were calculated at ROMP2/aug–cc–pVDZ level of theory for better accuracy. The pK a values for these acid-based herbicides allow them to exist in the deprotonated form in aqueous condition. Hence, the calculations are also performed for the deprotonated form apart from the neutral species. The most reactive site for the • OH radical reaction is predicted and validated for neutral and deprotonated species. Once the most reactive site is known, the reaction rate constants are calculated theoretically by the traditional transition–state theory using one-dimensional tunneling corrections. The solvent effect on the reaction rate constant is implemented through Collins–Kimball formulations.