Reaction of Cl Atom with c -C 5 F 8 and c -C 5 HF 7 : Relative and Absolute Measurements of Rate Coefficients and Identification of Degradation Products.

Partially and fully fluorinated olefins are a class of compounds with relatively short atmospheric lifetimes and low 100-year global warming potentials, compared to their saturated predecessors, which are used or considered as refrigerants, propellants, solvents, and other end-uses. The cyclic unsaturated compounds c -C 5 F 8 and c -C 5 HF 7 are currently under consideration as etching agents for the semiconductor industry. In this study, we expand on our previous work on the reaction of the OH radical with c -C 5 F 8 and c -C 5 HF 7 and report the rate coefficients, k , for the gas-phase reaction of the Cl atom with c -C 5 F 8 and c -C 5 HF 7 over a range of temperature (245-367 K) and pressure (100-200 Torr of He or N 2 and 0 to 4.8 Torr O 2 ) using a pulsed laser photolysis-resonance fluorescence (PLP-RF) technique. In addition, a relative rate (RR) technique, employing multiple reference compounds, was used to study the Cl atom reactions at 296 K, 100 and 630 Torr (N 2 or air) total pressure. Reaction rate coefficients, k 1 , of the Cl atom reaction with c -C 5 F 8 were found to be independent of pressure, over the pressure range used in this work, with k 1 (296 K), derived as an average of results from the PLP-RF and RR techniques being (1.07 ± 0.02) × 10 -12 cm 3 molecule -1 s -1 and k 1 ( T ) = (7.76 ± 0.73) × 10 -13 × (exp[(98 ± 26)/T]) cm 3 molecule -1 s -1 , where the quoted error limits represent the 2σ data precision. Rate coefficients, k 2 , for the Cl atom + c -C 5 HF 7 reaction were measured to be k 2 (296 K) = (4.61 ± 0.10) × 10 -12 cm 3 molecule -1 s -1 and k 2 ( T ) = (7.42 ± 0.89) × 10 -13 × (exp[(540 ± 32)/ T ]) cm 3 molecule -1 s -1 . The Cl atom temporal profiles, observed with the PLP-RF technique, indicate that the Cl atom with c -C 5 F 8 and c -C 5 HF 7 reactions lead to adduct formation. The equilibrium constants for adduct formation were derived in this work, and a second-law analysis was used to obtain Δ H and Δ S values of -18.5 ± 0.4 kcal mol -1 , -30.9 ± 1.2 cal K -1 mol -1 , and -13.9 ± 0.5 kcal mol -1 , -27.6 ± 1.1 cal K -1 mol -1 for the c -C 5 F 8 and c -C 5 HF 7 reactions, respectively. The Cl-initiated degradation of c -C 5 F 8 and c -C 5 HF 7 in the presence of O 2 was studied and stable products were identified via infrared spectroscopy using experimental or theoretically derived spectra from our previous OH reaction work. For c -C 5 F 8 , FC(O)CF 2 CF 2 CF 2 C(O)F and FC(O)C(O)F were observed with molar yields of 0.80 and 0.10, respectively. For c -C 5 HF 7 , we observed the formation of HC(O)CF 2 CF 2 CF 2 C(O)F and HC(O)C(O)F with a combined molar yield of 0.72. Carbonyl difluoride, F 2 CO, was also a major product in the decomposition of c -C 5 F 8 and c -C 5 HF 7 . The oxidation mechanism of the Cl-initiated degradation of c -C 5 F 8 and c -C 5 HF 7 is discussed. Based on the combined findings from this and our previous work, the atmospheric implications from the use of c -C 5 F 8 and c -C 5 HF 7 are presented.