Atmospheric chemistry of hexa- and penta-fluorobenzene: UV photolysis and kinetics and mechanisms of the reactions of Cl atoms and OH radicals†
Photochemical reactors were used to study the kinetics and mechanisms of reactions of Cl atoms and OH radicals with hexa- and penta-fluorobenzene (C6F6, C6F5H) in 700 Torr total pressure of N2, air, or O2 diluent at 296 ± 2 K. C6F6 and C6F5H undergo ring-opening following 254 nm UV irradiation, but with small quantum yields (φ < 0.03). Reaction of Cl atoms with C6F6 proceeds via adduct formation, while the reaction of Cl atoms with C6F5H proceeds via hydrogen abstraction and adduct formation. C6F6–Cl and C6F5H–Cl adducts decompose rapidly (k ∼ 105–106 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 + C6F5H is estimated at (7.3 ± 5.7) × 10−16 cm3 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 cm3 molecule−1 for C6F6 and C6F5H, respectively. Reaction of the adducts with O2 occurs slowly with estimated rate constants of <7 and <4 × 10−18 cm3 molecule−1 s−1 for C6F6–Cl and C6F5H–Cl, respectively. The rate constants for reaction of OH radicals with C6F6 and C6F5H were determined to be (2.27 ± 0.49) × 10−13 and (2.56 ± 0.62) × 10−13 cm3 molecule−1 s−1, respectively. UV and IR spectra of C6F6 and C6F5H 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.