Atmospheric Oxidation of N-Methyl Succinimide Initiated by OH Radicals: A Theoretical Study

N-methyl succinimide (NMS), the main atmospheric oxidation product of N-methylpyrrolidinone (a common industrial solvent), can expose developmental toxicity; however, the atmospheric chemistry of NMS has not been well-characterized yet. In this work, the kinetic mechanism of the OH-initiated NMS reaction is investigated for the first time by using ab initio calculations at the CCSD(T)/CBS//M06–2X/aug-cc-pVTZ level of theory combined with the master equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) rate model. The results show that H-abstractions at the −CH₃ moiety (to generate C₄H₄NO₂CH₂^•, P_α) and −C_ringH₂– moiety (to form ^•C₄H₃NO₂CH₃, P_β) dominate the OH-addition at the −C═O site. The simulated rate constants demonstrate a negative temperature dependence with the rate constant expression of k(T) = 1.21 × 10^–16 × T^0.98 × exp(1286.2 K/T) cm³ molecule^–1 s^–1 for T = 200 – 500 K at 760 Torr. A mechanism shift with temperature is also observed for the major abstraction pathway from the −CH₃ site (T < 280 K) to the −C_ringH₂– site (T > 280 K). A small positive pressure dependence was observed at low temperatures (T = 200 K), and it is insignificant under atmospheric conditions. Further analysis and calculations reveal that the title process is entropically driven, and the alkyl radicals from the abstraction pathways can be subsequently oxidized into N-formyl succinimide and 1-methylpyrrolidine-2,3,5-trione. It is suggested that NMS and its oxidation products are less harmful to human health than their parent compound, N-methylpyrrolidinone. Our results provide a theoretical understanding of the atmospheric mechanism and fate of NMP and other NMS-related processes, which is important for assessing and managing their impact on the troposphere and human health.