A computational investigation of twelve phenylurea herbicides including photoexcitation and structural relaxation

Abstract

Phenylurea herbicides are widely used to control unwanted plant growth, but can linger in the environment. Photodegradation is a key mechanism by which these compounds can be transformed. This paper investigates the direct photoabsorption and relaxation of twelve phenylurea compounds (chlorbromuron, chlortoluron, diuron, fenuron, fluometuron, isoproturon, linuron, methiuron, metobromuron, metoxuron, monolinuron, monuron) using density functional theory. Ground state structures are optimized at the B3LYP-D3/def2-SVP level of theory and excited states are computed using CAM-B3LYP-D3/def2-SVPD. These molecules are found to be mostly planar and structurally similar in the ground state, with the exception of methiuron which has a significant distortion. The vertical excitation energies are affected by the presence of halogen substituents, and trends in the relaxation of these states are observed. Specifically, halogenated compounds generally have a local excited state minimum corresponding to a stretched carbon-halogen bond, phenylureas containing a methoxy group on the nitrogen have a local minimum corresponding to a twisted urea group and/or a minimum corresponding to the breaking of the nitrogen–oxygen bond between the urea and the methoxy group. The results are discussed in the context of known photodegradation mechanisms for this class of compounds.