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

Atmospheric oxidation of NH_x-containing (x = 1, 2) compounds can produce N-center radicals, a precursor of toxic nitrosamines. The reaction rate constant (k_O2) with O₂ has been considered as an important parameter to determine the nitrosamines yield in the subsequent reactions of N-center radicals. However, available k_O2 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 k_O2 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 k_O2 values. Based on the mechanistic understanding of k_O2 values, a quantitative structure-activity relationship (QSAR) model of k_O2 values was developed. The model has satisfactory goodness-of-fit, robustness and predictive ability. The determined k_O2 values and the in silico methods provide a scientific base for assessing formation risk of toxic nitrosamines in the atmosphere.