A Density Functional Theory/Time-Dependent Density Functional Theory Investigation into the Structural Origins of Environmental Persistence and Mobility in the Herbicide Benzobicyclon and Its Hydrolysates

As critical tools in combating global weed resistance, the environmental fate of 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibiting herbicides is a subject of significant concern. This study utilizes the novel herbicide benzobicyclon (BZB) and its hydrolysates (BH, BOH) as a model system, employing an experimentally validated density functional theory (DFT) framework to uncover the structural origins of their environmental behavior at the atomic and electronic levels. Our calculations reveal that BZB's exceptional photostability originates from its higher electronic excitation energy (<i>E</i>_ex = 4.15 eV) and a delocalized lowest unoccupied molecular orbital (LUMO), which together create energetic and spatial barriers against photodegradation. In contrast, its metabolites BH (3.85 eV) and BOH (3.72 eV) are less stable due to lower excitation energies and LUMOs localized on degradable functional groups. Furthermore, by calculating the Gibbs free energy of solvation (Δ<i>G</i>_solv), we quantitatively reproduced the experimentally observed solubility trend (BOH > BH > BZB), confirming that polar groups introduced via hydrolysis are the fundamental cause of their enhanced aqueous mobility. More importantly, this work introduces a novel hypothesis linking chiroptical properties to biological activity: BZB's predicted strong electronic circular dichroism (ECD) signal is a spectral fingerprint of its unique, 'preorganized' three-dimensional conformation, which is optimized for stereoselective fitting within the HPPD target enzyme's active site. In summary, this research not only elucidates the environmental behavior mechanisms of BZB and its metabolites but also provides a computational framework that can be used to predict and guide the rational design of the next generation of environmentally safer herbicides.