Computational Study of the Reactions of Chlorine Radicals with Atmospheric Organic Compounds Featuring NH_x–π-Bond (x = 1, 2) Structures

Among 160 organic NH_x-containing compounds (x = 1, 2) detected in the atmosphere, there are about 80 species for which the molecules contain p−π conjugate substructures of NH_x–π-bonds. Here, chlorine radical (·Cl)-initiated reactions for formamide, N-methylformamide, ethenamine, and aniline, as their cases, were investigated by a quantum chemical method [CCSD­(T)/aug-cc-pVTZ//MP2/6-31+G­(3df,2p)] and kinetics modeling. The calculated overall rate constants are 5.5 × 10^–11, 2.3 × 10^–10, 2.7 × 10^–10, and 1.7 × 10^–10 cm³ molecule^–1 s^–1 for formamide, N-methylformamide, ethenamine, and aniline, respectively, and agree well with experimental values for available ones. Importantly, the results show that the reactions of two amides with ·Cl mainly lead to C-center radicals via ·Cl abstracting the −CHO hydrogen of amides. However, both ethenamine + ·Cl and aniline + ·Cl reactions mainly produce delocalized radicals with the radical center on the C-site and N-site via a ·Cl addition and the −NH_x hydrogen abstraction pathway, respectively. Therefore, this study reveals that reactions of organic NH_x-containing compounds with ·Cl have various reaction mechanisms, in contrast to our previous understanding that −NH_x hydrogen abstraction pathways, leading to N-center radicals, are the most favorable. The unveiled reaction mechanisms should be of significance for the risk assessment of atmospheric organic NH_x-containing compounds and enriching ·Cl chemistry.