Falloff Curves of the Reaction CF3 (+M) → CF2 + F (+M)

High-Temperature Fluorocarbon Chemistry Revisited

The thermal dissociation reactions of C₂F₄ and C₂F₆ were studied in shock waves over the temperature range 1000-4000 K using UV absorption spectroscopy. Absorption cross sections of C₂F₄, CF₂, CF, and C₂ were derived and related to quantum-chemically modeled oscillator strengths. After confirming earlier results for the dissociation rates of C₂F₄, CF₃, and CF₂, the kinetics of secondary reactions were investigated. For example, the reaction CF₂ + CF₂ → CF + CF₃ was identified. Its rate constant of 10¹⁰ cm³ mol^-1 s^-1 near 2400 K is markedly larger than the limiting high-pressure rate constant of the dimerization CF₂ + CF₂ → C₂F₄, suggesting that the reaction follows a different path. When the measurements of the thermal dissociation CF₂ (+Ar) → CF + F (+Ar) are extended to temperatures above 2500 K, the formation of C₂ radicals was shown to involve the reaction CF + CF → C₂F + F (modeled rate constant 8.0 × 10¹² (T/3500 K)^1.0 exp(-4400 K/T) cm³ mol^-1 s^-1) and the subsequent dissociation C₂F (+Ar) → C₂ + F + (Ar) (modeled limiting low-pressure rate constant 3.0 × 10¹⁶ (T/3500 K)^-4.0 exp(-56880 K/T) cm³ mol^-1 s^-1). This mechanism was validated by monitoring the dissociation of C₂ at temperatures close to 4000 K. Temperature- and pressure-dependences of rate constants of reactions involved in the system were modeled by quantum-chemistry based rate theory.

Shock wave and modelling study of the dissociation kinetics of C2F5I

The thermal dissociation of C2F5I was studied in shock waves monitoring UV absorption signals from the reactant C2F5I and later formed reaction products such as CF, CF2, and C2F4. Temperatures of 950-1500 K, bath gas concentrations of [Ar] = 3 × 10-5-2 × 10-4 mol cm-3, and reactant concentrations of 100-500 ppm C2F5I in Ar were employed. Absorption-time profiles were recorded at selected wavelengths in the range 200-280 nm. It was found that the dissociation of C2F5I → C2F5 + I was followed by the dissociation C2F5 → CF2 + CF3, before the dimerization reactions 2CF2 → C2F4 and 2CF3 → C2F6 and a reaction CF2 + CF3 → CF + CF4 set in. The combination of iodine atoms with C2F5 and CF3 had also to be considered. The rate constant of the primary dissociation of C2F5I was analyzed in the framework of statistical unimolecular rate theory accompanied by a quantum-chemical characterization of molecular parameters. Rates of secondary reactions were modelled as well. Experimental rate constants for the dissociations of C2F5I and C2F5 agreed well with the modelling results. The comparably slow dimerization 2CF2 → C2F4 could be followed both by monitoring reactant CF2 and product C2F4 absorption signals, while CF3 dimerization was too fast to be detected. A competition between the dimerization reactions of CF2 and CF3, the recombination of CF2 and CF3 forming C2F5, and CF-forming processes like CF2 + CF3 → CF + CF4 finally was discussed.