Atmospheric chemistry of hexa- and penta-fluorobenzene: UV photolysis and kinetics and mechanisms of the reactions of Cl atoms and OH radicalsElectronic supplementary information (ESI) available. See DOI: 10.1039/c8cp05540h

• Main Authors: Sulbaek Andersen, Mads P, Lengkong, Jonathan W, Wallberg, Jens, Hasager, Freja, Vo, Karen, Andersen, Simone Thirstrup, Kjaergaard, Henrik G, Wallington, Timothy J, Nielsen, Ole John
• Format: Journal Article
• Published: 21.11.2018
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c8cp05540h

Photochemical reactors were used to study the kinetics and mechanisms of reactions of Cl atoms and OH radicals with hexa- and penta-fluorobenzene (C 6 F 6 , C 6 F 5 H) in 700 Torr total pressure of N 2 , air, or O 2 diluent at 296 ± 2 K. C 6 F 6 and C 6 F 5 H undergo ring-opening following 254 nm UV irradiation, but with small quantum yields ( < 0.03). Reaction of Cl atoms with C 6 F 6 proceeds via adduct formation, while the reaction of Cl atoms with C 6 F 5 H proceeds via hydrogen abstraction and adduct formation. C 6 F 6 -Cl and C 6 F 5 H-Cl adducts decompose rapidly ( k ∼ 10 5 -10 6 s −1 ) reforming the reactants, and react with Cl atoms to form products. The fraction of adduct reacting with Cl atoms increases with steady state Cl atom concentration, resulting in an increasing apparent effective Cl atom rate constant. The rate constant for the H-abstraction channel for Cl + C 6 F 5 H is estimated at (7.3 ± 5.7) × 10 −16 cm 3 molecule −1 s −1 . Establishment of the equilibrium between the adducts and the aromatic reactant + Cl occurs rapidly with equilibrium constants of K ([adduct]/[aromatic][Cl]) = (1.96 ± 0.11) × 10 −16 and (9.28 ± 0.11) × 10 −17 cm 3 molecule −1 for C 6 F 6 and C 6 F 5 H, respectively. Reaction of the adducts with O 2 occurs slowly with estimated rate constants of <7 and <4 × 10 −18 cm 3 molecule −1 s −1 for C 6 F 6 -Cl and C 6 F 5 H-Cl, respectively. The rate constants for reaction of OH radicals with C 6 F 6 and C 6 F 5 H were determined to be (2.27 ± 0.49) × 10 −13 and (2.56 ± 0.62) × 10 −13 cm 3 molecule −1 s −1 , respectively. UV and IR spectra of C 6 F 6 and C 6 F 5 H at 296 ± 1 K were collected and calibrated. Results are discussed in the context of available literature data for reactions of Cl atoms and OH radicals with halogenated aromatic compounds. Chamber studies show that the atmospheric fates of aromatic Cl- and OH adducts are very different.