Ab initio kinetics of the CH3NH + NO2 reaction: formation of nitramines and N-alkyl nitroxides

Abstract

This study provides a detailed understanding of how the reaction between CH₃NH, one of the primary products of the CH₃NH₂ + OH/Cl reactions, and NO_x occurs in the atmosphere since the reaction is expected to be a dominant sink for the tropospheric CH₃NH radical. First, we focus on the reaction of the aminyl radical CH₃NH with NO₂, complementing the known reaction between CH₃NH and NO, to provide the overall picture of the CH₃NH + NO_x system. The reaction was meticulously examined across the extended range of temperature (298–2000 K) and pressure (0.76–76 000 torr) using quantum chemistry calculations and kinetic modeling based on the framework of the Rice–Ramsperger–Kassel–Marcus (RRKM)-based master equation. Highly correlated electronic structure calculations unveil that the intricate reaction mechanism of the CH₃NH + NO₂ reaction, which can proceed through O-addition or N-addition to form NO₂, encompasses numerous steps, channels, and various intermediates and products. The temperature-/pressure-dependent kinetic behaviors and product distribution of the CH₃NH + NO₂ reaction are revealed under atmospheric and combustion conditions. The main products under atmospheric conditions are found to be CH₃NHO and NO, as well as CH₃NHNO₂, while under combustion conditions, the primary products are only CH₃NHO and NO. Given its stability under ambient conditions, CH₃NHNO₂, a nitramine, is believed to have the potential to induce DNA damage, which can ultimately result in severe cancers. Secondly, by building upon prior research on the CH₃NH + NO system, this study shows that the reaction of CH₃NH with NO_x holds greater importance in urban areas with elevated NO_x emissions than other oxidants like O₂. Furthermore, this reaction occurs swiftly and results in the creation of various compounds, such as the carcinogenic nitrosamine (CH₃NHNO), carcinogenic nitramine (CH₃NHNO₂), CH₃NNOH, (CH₃NN + H₂O) and (CH₃NHO + NO).