Atmospheric Chemistry of N-Methylmethanimine (CH₃N═CH₂): A Theoretical and Experimental Study

Abstract

The OH-initiated photo-oxidation of N -methylmethanimine, CH 3 N=CH 2 , was investigated in the 200 m 3 EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the =CH 2 group and π-system C-addition, whereas H-abstraction from the −CH 3 group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH 3 N=CH 2 photolysis and the CH 3 N=CH 2 + O 3 reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH 3 N=CH 2 was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH 3 N=CH 2 monomer–trimer equilibrium, surface reactions, and particle formation, find CH 2 =NCHO and CH 3 N=CHOH/CH 2 =NCH 2 OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k ( T ) = 5.70 × 10 –14 × ( T /298 K) 3.18 × exp(1245 K/ T ) cm 3 molecule –1 s –1 with k 298 = 3.7 × 10 –12 cm 3 molecule –1 s –1 . The atmospheric fate of CH 3 N=CH 2 is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH 3 NH 2 + CH 2 O will constitute a dominant loss process. N 2 O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.

Atmospheric Oxidation of Imine Derivative of Piperazine Initiated by OH Radical

“…19 As for the product of OH radical addition to the imine carbon g1, the C−C bond scission reaction forming an imidic acid (g2) is the major sink. This outcome is similar to what is observed to follow OH addition to formaldimine 19 and N-methylmethanimine, 23 however in those cases the imidic acid is produced via subsequent Habstraction by O 2 . The pathway revealed here for THPyz is not unlike the β-scission reactions known to be important to alkoxyl radicals (RO • ), and may be a significant sink to other aminyl radicals (RN • ) as well, especially those derived from OHaddition to imines.…”

“…The oxidation kinetics of the simplest imine, formaldimine, and the simplest Schiff base, N -methylmethanimine (MIM), initiated by OH radical have been previously investigated by Bunkan et al (2014; 2022). , These studies revealed that the reaction may initially proceed via H-abstraction from C or N atoms, but also via OH radical addition to the C-terminus of the imine double bond (Scheme ), similar to what is observed with alkenes. Addition to the N-terminus was found to be a negligible pathway.…”

“…This reaction pathway is perhaps important in the atmospheric oxidation of any Z-isomer of a C-alkyl-substituted Schiff base aldimine (R–NCHR′), as long as initial H-abstraction from the imine carbon is competitive. While a fast 1,5 H-shift is possible for a peroxyl radical derived from an E-isomer, as observed for the oxidation of N -methylmethanimine, the constrained orientation of the peroxyl group in a Z-isomer derived analogue makes it much less prone to react via unimolecular channels. Oxidation of non-Schiff base aldimines (HNCHR′) initiated by the same H-abstraction step would likely react further via O 2 -addition/HO 2 -elimination instead, generating a nitrile function (NCR′), as observed for formaldimine + OH reaction .…”

“…Piperazine is an efficient and widely employed CCS amine solvent, whose initial oxidation by OH radical has so far been reported to have a low potential for generating nitrosamines and nitramines. , Its major channel of oxidation is formation of its imine derivative 1,2,3,6-tetrahydropyrazine (THPyz) as shown in Scheme , with branching ratio values of 0.2–0.6 and 0.9 found by computational and experimental studies. − The atmospheric fate of this imine product however has received no attention. In fact, just a few studies have focused on the atmospheric chemistry of imines in general, − despite consistently appearing as major products of amine oxidation. − …”

An Experimental and Master Equation Investigation of Kinetics of the CH2OO + RCN Reactions (R = H, CH3, C2H5) and Their Atmospheric Relevance

“…For OH + stabilized dioxazole reaction, the OH addition rate is likely to be faster for addition to the imine carbon due to its electropositivity, but lower for addition to the nitrogen due to the instability of the carbon centered radical. 55 Hydrogen-abstraction from the CH 2 group(s) is a viable bimolecular OH radical reaction channel, since it leads to the formation of resonance-stabilized radical(s). We estimate a total bimolecular rate coefficient about 10 −11 cm 3 molecule −1 s −1 for the OH + stabilized dioxazole reaction, leading to dioxazole lifetime of a few days in the atmosphere.…”