Reaction mechanism and kinetics of the degradation of bromoxynil initiated by OH radical

Abstract

Bromoxynil is a selective foliage herbicide used to control weeds. The degradation of bromoxynil in the atmosphere takes place dominantly via reaction with OH radicals. In this work, the OH initiated reactions of bromoxynil is studied using density functional theory methods M06-2X, B3LYP and MPW1K with 6-311++G(d,p) basis set. The relative energy of the reactive species is also calculated at CCSD(T)/6-311+G(d,p) level of theory. The OH initiated reaction of bromoxynil is found to proceed through H-atom abstraction and OH addition reactions, leading to the formation of six intermediates. The reactions subsequent to the principal oxidation steps are studied and the different reaction pathways are modeled. The radicals formed in the initial and subsequent reactions have a greater ability to undergo self-coupling, which yields dioxin and dioxepine products. These products are shown to be highly toxic and carcinogenic. The kinetics of the most favorable initial reactions are studied using canonical variational transition state theory with small curvature tunneling corrections over the temperature range of 278–350 K. This study provides thermochemical and kinetic data for the oxidation of bromoxynil in the atmosphere and demonstrates the formation of significant pollutants through oxidation reactions and lifetime of bromoxynil in the atmosphere.