Atmospheric chemistry of (Z)-CF3CHCHCl: products and mechanisms of the Cl atom, OH radical and O3 reactions, and role of (E)–(Z) isomerization

Abstract

The chemical mechanisms of the OH radical, Cl-atom and O₃ initiated oxidation of (Z)-CF₃CHCHCl were studied at 296 ± 1 K in 10–700 Torr air of N₂/O₂ diluent. Cl atoms add to the CC double bond: 12 ± 5% to the terminal carbon and 85 ± 5% to the central carbon. In 700 Torr of air the products are CF₃CHClCHO, HCOCl, CF₃COCl, CF₃CHO, (E)-CF₃CHCHCl, CF₃C(O)CHCl₂, and CF₃CHClCOCl. The yield of (E) isomer was dependent on total pressure, but independent of O₂ partial pressure; consistent with isomerization occurring via Cl atom elimination from the chemically activated rather than the thermalized CF₃CHCHCl–Cl adduct. The rate constant for (Z)-CF₃CHCHCl + Cl was measured at low pressure (10–15 Torr) and found to be indistinguishable from that determined at 700 Torr total pressure, whereas the low pressure rate constant for (E)-CF₃CHCHCl was 36% smaller. G4MP2 ab initio calculations showed that the (E) isomer is 1.2 kcal mol⁻¹ more stable than the (Z) isomer. Cl atom elimination from the adduct will preferentially form the (E) isomer and hence the rate of CF₃CHCHCl loss will be more sensitive to pressure for the (Z) than the (E) isomer. Reaction of (Z)-CF₃CHCHCl with OH radicals gives CF₃CHO, HCOCl, (E)-CF₃CHCHCl, and HCl. A significant chlorine atom elimination channel was observed experimentally, and supported by computational results. The oxidation products of the reaction of O₃ with (Z)- and (E)-CF₃CHCHCl were determined with no evidence of isomerization. The results are discussed with respect to the atmospheric chemistry and environmental impact of (Z)- and (E)-CF₃CHCHCl.