Experimental and Theoretical Studies on Reaction Kinetics, Mechanism, and Degradation of Quinoline-Based Herbicide with Hydroxyl Radical, Sulphate Radical Anion, and Hydrated Electron.

Abstract

The kinetic and mechanistic studies for the reaction of hydroxyl radical (^•OH), sulfate radical anion ( ${\text{SO}}_4^{•-}$ ), and hydrated electron $\left(e_{\text{aq}}^{-}\right)$ with quinoline-based herbicide, namely, 8-quinoline carboxylic acid (8QCA), have been performed using experimental and computational methods. Experimental studies are performed using pulse radiolysis technique at different pHs and corroborated with theoretical studies using ab initio molecular orbital calculations. At lower pH of 1, the 8QCA is protonated and reacts with ^•OH radical to generate transient spectrum with maxima at 340 and 420 nm. Similarly at higher pH of 9, the 8QCA is deprotonated and shows transient absorption maxima at 320 nm. At neutral pH, it exists as neutral species and reacts with ^•OH radical differently. Theoretically, individual rate coefficients for ^•OH radical addition reaction with each carbon atoms are evaluated including solvent effect and tunneling correction. Fukui index and individual rate constant determination confirm that C5 carbon atom is the most reactive site for the ^•OH radical addition reaction. The total rate constant evaluated theoretically and experimentally for the ^•OH radical reaction is equal to its diffusion-limit value. The ability of ^•OH radical to degrade 8QCA is found to be higher as compared to $e_{\text{aq}}^{-}$ .