Quantification and modeling of deactivation rate constant of singlet oxygen by low-molecular-weight dissolved organic...

Singlet oxygen (¹O₂) is a key reactive intermediate in aquatic photochemistry, but its deactivation by dissolved organic matter (DOM) remains poorly characterized. In this study, we quantitatively evaluated the deactivation rate constant of ¹O₂ by representative DOM compounds and developed predictive models for the corrected total deactivation rate constant ( [Formula: see text] ) based on experimentally measurable optical properties. Experiments with representative phenolic, amino, and carbohydrate compounds revealed that the deactivation rate constant of ¹O₂ by each of those compounds (k_Q) ranged from 8.76×10⁷ to 2.12×10⁹ s^-1 M^-1. The quantitative structure-activity relationship (QSAR) determined the key determinants of k_Q, which are molecular descriptors such as the HOMO-LUMO gap (an indicator of π-conjugation) and charge-related topological indices (indicators of electron-donating capacity). These findings elucidated the structural factors governing ¹O₂ deactivation. The mixture experiments indicated that the total ¹O₂ deactivation could be explained by the additive contribution of individual components. Additionally, random forest model based on optical properties, particularly a₂₅₄, E₄:E₆, S_350-400, and FI, described [Formula: see text] with high accuracy (R²=0.85). This model highlights that simple absorbance-derived optical properties can act as practical and reliable proxies for ¹O₂ quenching capacity of a certain range of DOM types. The proposed predictive framework provides a basis for accurately estimating the lifetime and reactivity of ¹O₂ in natural waters, thereby improving the prediction of contaminant fate and carbon dynamics in aquatic environments.