Atmospheric Oxidation Mechanism and Kinetics of Indole Initiated by ·OH and ·Cl: A Computational Study

Abstract

Abstract. The atmospheric chemistry of organic nitrogen compounds (ONCs) is of great importance for understanding the formation of carcinogenic nitrosamines and ONC oxidation products might influence atmospheric aerosol particle formation and growth. Indole is a polyfunctional heterocyclic secondary amine with global emission quantity almost equivalent to that of trimethylamine, the amine with the highest atmospheric emission. However, the atmospheric chemistry of indole remains unclear. Herein, the reactions of indole with ·OH/·Cl, and subsequent reactions of resulting indole-radicals with O2 under 200 ppt NO and 50 ppt HO2· conditions, were investigated by a combination of quantum chemical calculations and kinetics modeling. The results indicate that ·OH addition is dominant pathway for the reaction of ·OH with indole. However, both ·Cl addition and H-abstraction are feasible for the corresponding reaction with ·Cl. All favorably formed indole-radicals further react with O2 to produce peroxy radicals, which mainly react with NO and HO2· to form organonitrates, alkoxy radicals and hydroperoxide products. Therefore, the oxidation mechanism of indole is distinct from that of previously reported amines, which primarily form highly oxidized multifunctional compounds, imines or carcinogenic nitrosamines. In addition, the peroxy radicals from the ·OH reaction can form N-(2-formylphenyl)formamide (C8H7NO2), for the first time providing evidence for the chemical identity of the C8H7NO2 mass peak observed in the ·OH + indole experiments. More importantly, this study is the first to demonstrate despite forming radicals by abstracting an H-atom at the N-site, carcinogenic nitrosamines were not produced in the indole oxidation reaction.

A new advance in the pollution profile, transformation process, and contribution to aerosol formation and aging of atmospheric amines

“…121 However, Xue et al found that cOH addition was the dominant pathway for the reaction of cOH with indole which is a polyfunctional heterocyclic secondary amine. 122 Amines are also attacked by O 3 . Gai et al believed that amines might react with O 3 through the electrophilic reaction mechanism.…”

“…There are some experimental and computational studies exploring its initial steps. [122][123][124][125][127][128][129] In 2003, Rudić et al rst revealed that hydrogen-abstraction occurred with roughly equal probability from both the carbon and nitrogen sites of MA in the reaction of cCl with MA through isotope experiments. 127 Sz} ucs et al also found that the hydrogen-abstraction of carbon and nitrogen sites of MA was exothermic low-barrier, but the substitution reactions were endothermic high-barrier using an accurate composite ab initio approach.…”

“…123 For unsaturated amines, they may also undergo addition reactions via a cCl addition. 122,125 For example, Xie et al considered that both ethenamine+cCl and aniline+cCl reactions mainly produced delocalized radicals with the radical center on the C-site and N-site via a cCl addition and the hydrogenabstraction from the nitrogen site, respectively. 125 Besides, the reaction of ethenamine+cCl rst showed that the products of hydrogen-abstraction from the N-site could be favorably formed via addition intermediates in addition to the direct Habstraction pathway.…”

“…Recently, Xue et al also found that both cCl addition and hydrogen-abstraction from N-sites were feasible for the reaction of indole with cCl, while cOH addition was the dominant pathway in the case of the cOH reaction. 122 Obviously, the reaction of cCl with amines is quite different from that of cOH. Hence, to assess the fate of atmospheric amines more accurately, it is necessary to consider both the atmospheric concentrations of cCl and cOH and their initial reaction steps with atmospheric amines.…”

“…Their group further found that the delocalized character of N-center radicals formed by indole is the reason why they wouldn't further form nitrosamine. 122 It is evident that the potential, efficiency and stability of N-center radical forming nitrosamines are highly dependent on the structure and the functional groups of parent amines.…”