Theoretical studies on the kinetics and mechanism of hydroxyl radical reaction with quinclorac and quinmerac herbicides in aqueous media

The kinetic and mechanistic studies for the reaction of hydroxyl radical with two quinoline based herbicides, namely, quinclorac and quinmerac has been performed using various computational methods in aqueous media. Geometry optimizations were performed using Density Functional Theory (DFT) methods including water as the solvent. Local reactivity parameters of these herbicides towards the ^•OH radical are predicted using condensed Fukui function. Single point energies of various species were calculated using double hybrid method, namely, B2PLYP–D for better accuracy. The pK_a values for these acid based herbicides allow them to exist in deprotonated form in aqueous condition. Hence, the calculations are also performed for the deprotonated or the anionic form apart from the neutral species. Individual rate coefficients for ^•OH radical addition reaction with each carbon atoms were evaluated using conventional transition state theory using one–dimensional tunneling corrections. The solvent effect on reaction is implemented through Collins–Kimball formulations. Both the approaches, namely, the Fukui index and individual rate constant determination confirms that the most reactive site for the ^•OH radical addition in these two herbicide is the carbon atom attached to the COOH group. The total rate constant for the ^•OH radical reaction with both neutral and anionic forms of these two herbicides are relatively high and equal to its diffusion-limit value. Evaluation of the ecotoxicities of the parent herbicides and their OH adducts is estimated using the structure–activity relationship concept.