Reaction Mechanisms and Kinetics of the OH-Initiated Oxidation of N-Methylethanimine (CH3CH═NCH3): Relevance to Atmospheric and Astrochemical Chemistry

Abstract

The atmospheric and astrochemical oxidation chemistry of imines remains poorly understood despite their importance as intermediates in nitrogen-containing organic and prebiotic chemistry. In this work, we report the first comprehensive conformer-specific theoretical investigation of the OH-initiated oxidation of N-methylethanimine (CH₃CH═NCH₃) under both terrestrial atmospheric and interstellar medium (ISM) conditions. Ab initio/DFT calculations, i.e., CCSD(T)/6-311++(3df,3pd)//ωB97X-D/6-311++(3df,3pd), combined with microcanonical variational transition state theory (μVTST) and Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) simulations reveal that the reaction proceeds through strongly stabilized prereactive complexes (PRCs), followed by competing OH-addition and H-abstraction pathways. The calculated rate coefficients were found to be in the range of 10^–11 to 10^–12 cm³ molecule^–1 s^–1 between 200 and 400 K, consistent with analogous imine systems reported in previous studies. The estimated atmospheric lifetime of approximately 16 days suggests that CH₃CH═NCH₃ is a relatively moderate live intermediate; nevertheless, its oxidation chemistry may contribute to the formation of hazardous nitrogen-containing species and volatile organic compounds. The pressure-dependent kinetic analysis under ISM conditions predicts efficient low-temperature formation of acetaldehyde (CH₃CHO) and NHCH₃ radicals with effective association rate coefficients in the range of 10^–9 to 10^–10 cm³ molecule^–1 s^–1. The present work suggests that OH-initiated oxidation of N-methylethanimine may generate intermediates that could contribute to acetaldehyde formation through subsequent oxidation steps in interstellar environments and provides mechanistic insight into the possible role of imine oxidation pathways in atmospheric nitrogen chemistry.