QSAR Studies On Reaction Rate Constants Of Selected Organic Pollutants With·OH

The reaction of organic pollutants with the hydroxyl radical (·OH) is a dominant degradation process in the troposphere and various aqueous systems. Thus, the reaction rate constants with·OH (koH) are important for evaluating the persistence of organic pollutants in the environment, and important endpoints for environmental risk assessment. However, all the experimental methods for determining koH are time-consuming, laborious, costly and can not satisfy the need of environmental risk assessment of organic chemicals. Thus it is crucial to develop quantitative structure-activity relationship (QSAR) models for koH prediction. Therefore, following the OECD guidelines of QSAR development, QSAR models were developed for predicting gas-phase and aqueous-phas kOH of organic compounds. The main molecular structure factors and temperature governing kOH were also discussed. Furtheromore, a direct computational method was developed for predicting gas-phase and aqueous-phase kOH based on the Transition State Theory (TST).A QSAR model for predicting the gas-phase kOH of organic compounds containing C, H, N,O, S, F, Cl, Br, I, and Si atoms in various functional groups was developed based on theorical molecular structural descriptors and Partial Least Squares (PLS) regression. For the established model, the correlation coefficient square (R2) was 0.878, the root mean squared error (RMSE) was 0.391 log units, and the leave-many-out cross-validated Q2cv was 0.865, indicating good robustness and predictivity. The predictive capability was also evaluated by external validation with Q2EXT = 0.872. The main factors governing the gas-phase kOH were the compactness of the molecule (Ds), the highest occupied molecular orbital energy (EHOMO) and the number of halogen atoms (nx).In order to discuss the influence of temperature on the gas-phase reactions of organic pollutants with·OH, a predictive model was developed for gas-phase logkOH, based on a training set consisting of 20 polycyclic aromatic hydrocarbons (PAHs),6 polychlorinated dibenzo-ρ-dioxins/dibenzofurans (PCDD/Fs),18 polychlorinated biphenyls (PCBs), and 7 polybrominated diphenyl ethers (PBDEs), over a temperature range of 294-432 K. It was confirmed that the temperature was an important factor governing the gas-phas kOH-The temperature-depended model indicated that 1/T and logT totally accounted for 22.8% variance of log kOH, thus the temperature was an important factor governing the gas-phase kOHIn order to predict the aqueous-phase kOH, a QSAR model was developed for aqueous-phase kOH of phenols, alkanes and alcohols, based on the mechanisms of aqueous-phase·OH reacting with organic compounds, i.e.·OH addition to aromatic rings as well as H-atom abstraction from aliphatic C-H and O-H bonds. The established model yielded satisfactory performance:the correlation coefficient square (R2) was 0.905, the root mean squared error (RMSE) was 0.139, the leave-many-out cross-validated Q2CV was 0.806, and the external validated Q2EXT was 0.922 log units. The main molecular structural factors governing aqueous-phase kOH were the highest occupied molecular orbital energy (EHOMO), molecular solvent assessiable area (MSA), average net atomic charges on hydrogen atoms (QH) and dipole moment(μ). Sub-models and mechanism domain validated the hypothesis of the mechanisms. The gas-phas kOH and aqueous-phase kOH were highly correlated (the correlation coefficient square= 0.709) and both were mainly influenced by the highest occupied molecular orbital energy (EHOMO).However, the key factors governing gas-phase kOH and aqueous-phase kOH were different. For the gas-phase, the key molecular structural parameter is the compactness of the molecule (DS); and for aqueous reactions, the most important parameter is the molecular solvent assessiable area (MSA).In order to broaden the applicability domain for the models of aqueous-phase kOH, some pharmaceuticals and personal care products (PPCPs) were selected as the target compounds for experiments. Their aqueous-phase kOH values were measured. Based on their experimental kOH, a new model was developed for predicting the aqueous-phase kOH of those compounds containing N and S atoms. The results further indicated that the highest occupied molecular orbital energy (EHOMO) was the key factor governing the aqueous-phase kOH of compounds containing N and S atoms.In order to develop a direct computational method for predicting gas-phase and aqueous-phase kOH, the density functional theory (DFT) and the second-order Mφller-Plesse perturbation theory (MP2) were employed to optimize the geometries of the reactants, intermediates, transition states, and products based on the Transition State Theory (TST). Reaction energies were gotten through minimum energy paths (MEP) computation. In order to deduce calculations,11 small molecular compounds were selected, i.e., CH4, CH3CH3, CH3COCH3, CH3COOH, CH3COCH3 and hydrochlorofluorocarbon (HCFCs), etc. It was confirmed that the computated and experimental kOH values were closer.