Products study of the OH-initiated reaction of (γ)-caprolactone and (γ)-heptalactone at 298 K: Formation of peroxyacyl nitrates (nPANs).

Renne-Francia

Simposio; 26th International Symposium on Gas Kinetics and Related Phenomena; 2022

(γ)-caprolactone (C6H10O2) and (γ)-heptalactone (C7H12O2) are cyclic esters that have recently become compounds of atmospheric interest identified in urban areas(1). On the other hand, these oxygenated volatile organic compounds (OVOCs) are associated with the formation of secondary organic aerosols(SOA)(2). These lactones in the atmosphere can be degraded by OH radicals or other tropospheric oxidants as Cl atoms in marine enviroments or highly industrialized areas. In the present work, the main products for the reaction of gamma-caprolactone and gamma-heptalactone with OH radicals in absence of NO were identified and quantified. Furthermore, the gas-phase mechanisms for these reactions were postulated. The experiments were performed in a 480 L pyrex-glass chamber, with 3 m long and 60 cm inside diameter at (298 ± 2) K in one atmosphere of synthetic air. The OH radicals were generated from the photolysis of methyl nitrite (CH3ONO) at 310 nm, using in situ FTIR spectroscopy to monitor the reactants and products evolution.The identified products suggest that the mechanism of oxidation of both lactones proceeds through H atom abstraction that can be produced at three main positions of the lactone. According to SARs estimations, the positions with the highest H abstraction probability are the carbons near to the oxygen of the ether functionality of the ring. In our experimental conditions, the alkyl radicals formed after H abstraction react with O2 to generate peroxy radicals with further alkoxy radicals formation. The generated alkoxy radicals can: a) Generate succinic anhydride from abstractions at the (γ)-carbon to the oxygen of the ring with formation yields of (48±2) % and (35±2) % for (γ)-caprolactone and (γ)-heptalactone, respectively. Other possibility to produce succinic anhydride is from the abstraction at the (-CH2-) of the alkyl chain attached to the ring. b) Ring opening with further fragmentation in the presence of NO2 to generate peroxypropionyl nitrate (PPN) with yields of (50±3) % from the (γ)-caprolactone reaction and n-butyryl nitrate (nBPN) with the yield of (52±2) % generated from (γ)-heptalactone. nPANs (nBPN, PPN) could act as NOx tropospheric reservoirs and are hazardous compounds responsible of photochemical smog with negative impact on air quality and consequences for the health and the biota.