Chlorine Radical-Initiated Atmospheric Oxidation of Imines: Implications for Structural Influence on the Nitrosamine Formation

Abstract. Chlorine radical (·Cl) initiated oxidation of organic nitrogen compounds (ONCs) plays an important role in carcinogenic nitrosamines formation. Imines are important constituents of ONCs, primarily formed from the atmospheric oxidation of amines. However, ∙Cl-initiated atmospheric oxidation of imines remains poorly understood. Here, we studied the reaction mechanisms and kinetics of ·Cl-initiated oxidation for five representative imines (CH₂=NH, CH₃CH=NH, CH₃N=CH₂, (CH₃)₂C=NH, HN=CHCH₂OH) to elucidate their atmospheric fate and extend the limited available data of ONCs, thereby establishing a structure-activity relationship for the reactions. The calculated overall reaction rate constants (× 10^–11 cm³ molecule⁻¹ s⁻¹) of ∙Cl + CH₂=NH, ∙Cl + CH₃CH=NH, ∙Cl + CH₃N=CH₂, ∙Cl + (CH₃)₂C=NH, and ∙Cl + HN=CHCH₂OH are 4.5, 27.2, 7.32, 44.8 and 12.6, respectively, which are consistent with the available experimental values. Importantly, our results show that the ∙Cl-initiated reactions of the NH-containing imines mainly produce N-centered radicals. These N-centered radicals exhibit various fates under tropospheric conditions: mainly reacting with NO to form nitrosamines or with O₂ to form cyanide compounds, which differs substantially from the behavior of previously reported amines. The various fates of the N-centered radicals formed from imines originates from the difference in direct hydrogen abstraction reaction rate constants (k_O2) with O₂ and the reaction rate (k_NO) with NO, both of which are principally governed by the distinct molecular structure of N-centered radicals. The revealed reaction mechanism provides new insights into the atmospheric transformation and risks of imines, and enrich our understanding of ∙Cl/ONCs chemistry.