Thermal Decomposition of CFCl3

Abstract

The thermal decomposition of CFCl3 (CFC-11) has been studied in reflected shock waves using the Cl-atom atomic resonance absorption spectroscopy (ARAS) detection technique. The first thermal rate measurements for CFCl3 (+M) → CFCl2 + Cl (+M) are reported. The experimental Cl-atom concentration profiles show two distinct rates of formation. The initial fast process gives a Cl-atom yield of 2, and this is followed by slow secondary processes that are density and temperature dependent. The final Cl-atom yield is greater than 2[CFCl3]0. This behavior confirms that C−Cl bond scission is the dominant dissociation pathway for both CFCl3 and the product radical, CFCl2, as observed in an earlier study from this laboratory on the related CF2Cl2 decomposition. Profile fits require the fast subsequent dissociation of CFCl2, and therefore, the short-time kinetics can be best explained as being due to C−Cl bond breaking in the parent, CFCl3. The temperature and density dependences of the later time Cl-atom profiles suggest that the slow secondary rate can be ascribed to reactions involving the carbene diradical, CFCl. The Cl-atom data were analyzed with detailed kinetics modeling calculations. Experiments were performed with varying [CFCl3]0 (15.23, 7.877, 5.159, and 2.496 ppm) in Kr diluent at three (3.1 × 1018, 2.1 × 1018, and 1.2 × 1018 cm-3) post-shock densities. An Arrhenius fit to the experimental CFCl3 dissociation rates over the T-range 1279−1950 K gives k = (2.82 ± 1.22) × 10-8 exp(−26420 ± 674 K/T) cm3 molecule-1 s-1, with ±36% error at the one standard deviation level. Comparing this expression to earlier results from this laboratory on CF3Cl, CF2Cl2, and CCl4 suggests that the C−Cl bond strength in CFCl3 should be between those for CF2Cl2 and CCl4. The temperature and pressure dependence of the rate constants, i.e., the falloff from the low-pressure limit, have been characterized with Rice−Ramsperger−Kassel−Marcus (RRKM) calculations using E0 = (76.5 ± 0.5) kcal mol-1 with 〈ΔE〉down = (800 ± 215) cm-1. This E0 implies = −20.3 kcal mol-1, and subsequently = (58 ± 2) kcal mol-1 for CFCl2 → CFCl + Cl.