Rate Coefficients for the Gas-Phase Reaction of ( E)- and ( Z)-CF3CF═CFCF3 with the OH Radical and Cl-Atom

Atmospheric Chemistry of c-C5HF7 and c-C5F8: Temperature-Dependent OH Reaction Rate Coefficients, Degradation Products, Infrared Spectra, and Global Warming Potentials

c-C₅HF₇ (1H-heptafluorocyclopentene) and c-C₅F₈ (perfluorocyclopentene) are potent greenhouse gases presently used as replacement compounds in Si etching. A thorough understanding of their potential impact on climate and air quality necessitates studies of their atmospheric reactivity, radiative properties, and atmospheric degradation pathways. The predominant atmospheric removal process for these compounds is expected to be via reaction with the OH radical. In this study, rate coefficients, k, for the gas-phase reaction of the OH radical with c-C₅HF₇ and c-C₅F₈ were measured over a range of temperatures (242-370 K) and pressures (50-100 Torr, He) using a pulsed laser photolysis-laser-induced fluorescence technique. In addition, a complementary relative rate technique, employing multiple reference compounds, was used to study the reactions between 273 and 372 K at 100 Torr (He) total pressure. Reaction rate coefficients were found to be independent of pressure over this range of conditions with k₁(296 K) = (4.59 ± 0.10) × 10^-14 cm³ molecule^-1 s^-1 and k₁(T) = (4.00 ± 0.40) × 10^-13 exp(-(631 ± 30)/T) cm³ molecule^-1 s^-1 for c-C₅HF₇ and k₂(296 K) = (4.90 ± 0.14) × 10^-14 cm³ molecule^-1 s^-1 and k₂(T) = (3.59 ± 0.4) × 10^-13 exp(-(591 ± 25)/T) cm³ molecule^-1 s^-1 for c-C₅F₈. Stable end-products were measured following the OH radical-initiated degradation of c-C₅HF₇ and c-C₅F₈ in the presence of O₂. F(O)CCF₂CF₂CF₂CH(O), CF₂O, and CO₂ were observed as the major end-products in the oxidation of c-C₅HF₇ with molar yields of 0.64, 1.27, and 0.53, respectively. For c-C₅F₈, F(O)CCF₂CF₂CF₂CF(O), CF₂O, and CO₂ were observed with molar yields of 0.66, 0.63, and 0.43, respectively. The total carbon mass balance in both systems was 1.0 ± 0.15. The high yield of a C5-dicarbonyl end-product is consistent with a ring opening at the carbon-carbon double bond site for both c-C₅HF₇ and c-C₅F₈. A comparison of the present kinetic and degradation product results with previously published studies is presented. A rate coefficient upper limit for the gas-phase reaction of O₃ with c-C₅HF₇ and c-C₅F₈ of 1 × 10^-21 cm³ molecule^-1 s^-1 was measured as part of this work. Atmospheric lifetimes for c-C₅HF₇ and c-C₅F₈ are estimated to be 252 and 236 days, respectively. Infrared absorption spectra of c-C₅HF₇ and c-C₅F₈ were also measured and found to agree, to within 5%, with results from previous studies. The well-mixed and lifetime adjusted radiative efficiencies (RE, W m^-2 ppb^-1) and 100 year time horizon global warming potential (GWP) for c-C₅HF₇ are 0.35, 0.24, and 46.7 and for c-C₅F₈ are 0.38, 0.25, and 46.2, respectively.