Reaction of Cl Atom with c-C5F8 and c-C5HF7: 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-C₅F₈ and c-C₅HF₇ 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-C₅F₈ and c-C₅HF₇ and report the rate coefficients, k, for the gas-phase reaction of the Cl atom with c-C₅F₈ and c-C₅HF₇ over a range of temperature (245-367 K) and pressure (100-200 Torr of He or N₂ and 0 to 4.8 Torr O₂) 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 (N₂ or air) total pressure. Reaction rate coefficients, k₁, of the Cl atom reaction with c-C₅F₈ were found to be independent of pressure, over the pressure range used in this work, with k₁(296 K), derived as an average of results from the PLP-RF and RR techniques being (1.07 ± 0.02) × 10^-12 cm³ molecule^-1 s^-1 and k₁(T) = (7.76 ± 0.73) × 10^-13 × (exp[(98 ± 26)/T]) cm³ molecule^-1 s^-1, where the quoted error limits represent the 2σ data precision. Rate coefficients, k₂, for the Cl atom + c-C₅HF₇ reaction were measured to be k₂(296 K) = (4.61 ± 0.10) × 10^-12 cm³ molecule^-1 s^-1 and k₂(T) = (7.42 ± 0.89) × 10^-13 × (exp[(540 ± 32)/T]) cm³ molecule^-1 s^-1. The Cl atom temporal profiles, observed with the PLP-RF technique, indicate that the Cl atom with c-C₅F₈ and c-C₅HF₇ 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 ΔH and ΔS 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-C₅F₈ and c-C₅HF₇ reactions, respectively. The Cl-initiated degradation of c-C₅F₈ and c-C₅HF₇ in the presence of O₂ was studied and stable products were identified via infrared spectroscopy using experimental or theoretically derived spectra from our previous OH reaction work. For c-C₅F₈, FC(O)CF₂CF₂CF₂C(O)F and FC(O)C(O)F were observed with molar yields of 0.80 and 0.10, respectively. For c-C₅HF₇, we observed the formation of HC(O)CF₂CF₂CF₂C(O)F and HC(O)C(O)F with a combined molar yield of 0.72. Carbonyl difluoride, F₂CO, was also a major product in the decomposition of c-C₅F₈ and c-C₅HF₇. The oxidation mechanism of the Cl-initiated degradation of c-C₅F₈ and c-C₅HF₇ is discussed. Based on the combined findings from this and our previous work, the atmospheric implications from the use of c-C₅F₈ and c-C₅HF₇ are presented.