Theoretical Calculation For Second-order Reaction Rate Constant (k_OH) Of Per-and Polyfluoroalkyl Substances （PFASs） With·OH In The Atmosphere

Per-and polyfluoroalkyl substances（PFASs）are a large and diverse group of compounds that are widely used as additives in products and continue to be released into the atmosphere as pollutants throughout the whole life cycle of the products.The reaction between·OH and pollutants is an important degradation pathway for pollutants in the atmosphere.Second-order reaction rate constant of pollutants reacting with·OH(k _OH)is an essential parameter to characterize the atmospheric persistence of pollutants.However,considering the time consuming of experimental measurement of k _OH and the lack of the k _OH data for PFASs,it is difficult to meet the needs of atmospheric persistence assessment and management by only relying on experimental data.It is feasible necessary to screen an efficiently and accurate theoretical calculation method to predict the k OH values of PFASs.In this study,a variety of quantum chemical methods were employed to select a method that could accurately predict the atmospheric k _OH values of PFASs,and quantitative structure-activity relationship（QSAR）models were constructed based on experimental and calculated k _OH values of PFASs.Main research contents and results are as follows:（1）A quantum chemical method was selected to accurately calculate the atmospheric k _OHvalues of PFASs.With the development of quantum chemical methods and the improvement of computer computing power,appropriate quantum chemical methods can efficiently and accurately predict the k _OH values of PFASs.In this study,considering the carbon chain length and the diversity of functional groups,15 PFASs with their experimental k _OH values were selected as references.M06-2X/CC-PVDZ,a density functional theory（DFT）method,was used to find the transition states between·OH and PFASs,and optimized the structures of reactants,pre-reaction complexes,transition states,post-reaction complexes and products for their thermodynamic parameters.The suitable DFT method was selected from 16 DFT calculation methods to calculate single point energy.In addition,the transition state theory（TST）was used to predict k _OH through selecting the appropriate transmission coefficient.The result shows that DFT method of M06-2X-D3/jul-cc-p VTZ combined with the TST method modified by Wigner transmission coefficient can accurately predict the k_OH of PFASs.（2）The atmospheric k_OH values of PFASs were calculated by quantum chemical method,and a data set of the atmospheric k_OH values of PFASs was constructed.The experimental atmospheric k _OH values of 80 PFASs combined with and k_OH values of 43 highly concerned PFASs calculated using the selected quantum chemistry method,were employed to construct several QSAR models for the atmospheric k _OH values prediction of PFASs.The results show that hydrofluoroolefins and n:2 fluorotelomer acrylate react with·OH were faster than the others.Perfluorooctane sulfonamide is difficult to react with·OH,and its atmospheric half-life is much higher than that of perfluorooctane sulfonate and perfluorooctane sulfonate.The performance of random forest in the constructed QSAR models was the best,whose coefficient of determination（R²）of testing set was 0.845,and R² of the 5-fold cross-validation was 0.888.The k _OH values calculated by quantum chemical methods filled in the data gaps in the kinetics of PFASs in the atmosphere,and the constructed QSAR models provide a basic tool for predicting the k _OH values of PFASs in the atmosphere.In summary,this study established an accurate method for PFASs atmospheric k _OHprediction by using a variety of quantum chemistry methods and QSAR modeling methods,which could help fill in the lack of PFASs atmospheric k _OH data and provided theoretical and data support for evaluating its atmospheric persistence.