THERMAL-DECOMPOSITION OF CFCL3

The thermal decomposition of CFCl3 (CFC-11) has been studied in reflec ted shock waves using the Cl-atom atomic resonance absorption spectros copy (ARAS) detection technique. The first thermal rate measurements f or CFCl3 (+M) --> CFCl2 + Cl (+M) are reported. The experimental Cl-at om concentration profiles show two distinct rates of formation. The in itial 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 con firms 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. Profil e fits require the fast subsequent dissociation of CFCl2, and therefor e, the short-time kinetics can be best explained as being due to C-Cl bond breaking in the parent, CFCl3. The temperature and density depend ences of the later time Cl-atom profiles suggest that the slow seconda ry rate can be ascribed to reactions involving the carbene diradical, CFCl. The Cl-atom data were analyzed with detailed kinetics modeling c alculations. Experiments were performed with varying [CFCl3](0) (15.23 , 7.877, 5.159, and 2.496 ppm) in Kr diluent at three (3.1 x 10(1)8, 2 .1 x 10(18), and 1.2 x 10(18) cm(-3)) post-shock densities. An Arrheni us fit to the experimental CFCl3 dissociation rates over the T-range 1 279-1950 K gives k = (2.82 +/- 1.22) x 10(-8) exp(-26420 +/- 674 K/T) cm(3) molecule(-1) s(-1), with +/-36% error at the one standard deviat ion level. Comparing this expression to earlier results from this labo ratory 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 E(0) = (76.5 +/- 0.5) kcal mol (-1) with [Delta E](down) = (800 +/- 215) cm(-1). This E(0) implies De lta(f)H(0,CFCl2)degrees = -20.3 kcal mol(-1), and subsequently Delta H -0 degrees = (58 +/- 2) kcal mol(-1) for CFCl2 --> CFCl + Cl.