Theoretical and experimental studies of intermediates in the atmospheric degradation of imines: Degradation of methyl methanimine and N-methyl ethanimine by OH radicals

Abstract

Amine-based CO2 capture has been suggested as a way to mitigate global warming. This project aims to improve the understanding of the atmospheric photo-oxidation of two of the simplest imines by OH radicals. Laboratory studies on methyl methanimine and N-methyl ethanimine have been performed in the smog chamber at the University of Oslo and the mechanism for the photo-oxidation of N-methyl ethanimine was further studied by using quantum chemical calculations. M06-2X density functional theory and explicitly correlated coupled cluster theory were used to determine the stationary points on the potential energy surface of the reaction of N-methyl ethanimine with OH. Hydrogen abstraction from the methylene site and from the N-methyl group are predicted to be the most important pathways. The calculated rate constant for the reaction between N-methyl ethanimine and OH at 298 K was kOH,t = 6.4 × 10-13 cm3 molecule1 s-1 when tunnelling was explicitly included and kOH,nt = 5.4 × 10-13 cm3 molecule-1 s-1 without including tunnelling. Subsequent reactions were modelled with M06-2X density functional theory, which predicted N-methyleneacetamide (CH3C(O)N=CH2) and N-ethylideneformamide (CH3CH=NCHO) as the major products. The laboratory studies show that both compounds are present as a trimer during the photooxidation laboratory experiments, meaning that their results cannot be used to verify the theoretical results. The laboratory experiments will have to be repeated whilst the smog chamber is heated to assure that the imines of interest are present as monomers.