Theoretical analysis of the OH-initiated atmospheric oxidation reactions of imidazole

• Published in: Physical chemistry chemical physics : PCCP 2024-09, Vol.26 (36), p.2357-23587
• Main Authors: Golin Almeida, Thomas, Martí, Carles, Kurtén, Theo, Zádor, Judit, Johansen, Sommer L
• Format: Article
• Language: English
• Subjects: Earth atmosphere; Hydroxyl radicals; Imidazole; Nitrogen compounds; Oxidation; Reaction kinetics; Reaction mechanisms
• ISSN: 1463-9076; 1463-9084; 1463-9084
• DOI: 10.1039/d4cp02103g

Imidazoles are present in Earth's atmosphere in both the gas-phase and in aerosol particles, and have been implicated in the formation of brown carbon aerosols. The gas-phase oxidation of imidazole (C 3 N 2 H 4 ) by hydroxyl radicals has been shown to be preferentially initiated via OH-addition to position C5, producing the 5-hydroxyimidazolyl radical adduct. However, the fate of this adduct upon reaction with O 2 in the atmospheric gas-phase is currently unknown. We employed an automated approach to investigate the reaction mechanism and kinetics of imidazole's OH-initiated gas-phase oxidation, in the presence of O 2 and NO x . The explored mechanism included reactions available to first-generation RO 2 radicals, as well as alkoxyl radicals produced from RO 2 + NO reactions. Product distributions were obtained by assembling and solving a master equation, under conditions relevant to the Earth's atmosphere. Our calculations show a complex, branched reaction mechanism, which nevertheless converges to yield two major closed-shell products: 4 H -imidazol-4-ol (4 H -4ol) and N , N ′-diformylformamidine (FMF). At 298 K and 1 atm, we estimate the yields of 4 H -4ol and FMF from imidazole oxidation initiated via OH-addition to position C5 to be 34 : 66, 12 : 85 and 2 : 95 under 10 ppt, 100 ppt and 1 ppb of NO respectively. This work also revealed O 2 -migration pathways between the α- N -imino peroxyl radical isomers. This reaction channel is fast for the first-generation RO 2 radicals, and may be important during the atmospheric oxidation of other unsaturated organic nitrogen compounds as well. Imidazole's gas-phase oxidation by OH radicals leads to the formation of two major closed-shell products, 4 H -imidazol-4-ol and N , N ′-diformylformamidine.