Mechanism and Kinetics of Diuron Oxidation Initiated by Hydroxyl Radical: Hydrogen and Chlorine Atom Abstraction Reactions.

Diuron is a herbicide that has been classified as an environmental pollutant because of its harmful effects on living beings and environment. In the present work, the OH-initiated oxidation reaction of diuron is investigated by performing electronic structure calculations based on density functional theory (DFT) methods, M06-2X, ωB97X-D, and MPWB1K using the 6-31G(d,p) basis set. The CBS-QB3 method is used to validate the results obtained from the DFT methods. All possible initial hydrogen and chlorine atom abstraction reaction pathways involved in the oxidation of diuron were studied, and the favorable reaction pathways were found by analyzing the potential energy surface and thermochemistry of the reactions. The results obtained from the present work show that hydrogen atom abstraction from methyl and amine groups of diuron are energetically favorable, which leads to the formation of diuron radical intermediate and water molecule. The rate constant is calculated for most favorable reaction pathways by using canonical variation transition state theory (CVT) with small curvature tunnelling (SCT) correction over the temperature range 298-1000 K. The atmospheric lifetime of diuron is found to be around 15 days. The subsequent reaction of most favorable diuron radical intermediate with other atmospheric reactive species, such as O2, H2O, HO2, and NOx (x = 1, 2) radicals are studied. The time-dependent DFT calculation is performed to study the photolysis of diuron and favorable diuron radical intermediates. This study provides thermochemical and kinetic data for the oxidation of diuron initiated by OH radical through H atom abstraction reaction.