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Mechanism and predictive model development of reaction rate constants for N-center radicals with O2

Liu, Cong, Ma, Fangfang, Elm, Jonas, Fu, Zihao, Tang, Weihao, Chen, Jingwen, Xie, Hong-Bin
Chemosphere 2019
chemical bonding, energy, free radicals, kinetics, models, molecular conformation, nitrosamines, oxidation, oxygen, quantitative structure-activity relationships, risk, toxicity
Atmospheric oxidation of NHx-containing (x = 1, 2) compounds can produce N-center radicals, a precursor of toxic nitrosamines. The reaction rate constant (kO2) with O2 has been considered as an important parameter to determine the nitrosamines yield in the subsequent reactions of N-center radicals. However, available kO2 values of N-center radicals are limited. Here, a three-step scheme including mechanistic analysis and kinetics calculation of the reactions of 28 various N-center radicals with O2, and model development was taken to solve the kO2 data shortage. Mainly employed tools include highly cost-expensive coupled-cluster theory (CCSD(T)), kinetic model and statistics. The results indicate that the direct H-abstraction pathway is the most favorable for the reactions of all considered N-center radicals with O2. The specific molecular conformation and the C–H bond energy of the N-center radicals are two important factors to determine kO2 values. Based on the mechanistic understanding of kO2 values, a quantitative structure-activity relationship (QSAR) model of kO2 values was developed. The model has satisfactory goodness-of-fit, robustness and predictive ability. The determined kO2 values and the in silico methods provide a scientific base for assessing formation risk of toxic nitrosamines in the atmosphere.