THERMAL-DECOMPOSITION OF CF2CL2

The thermal decomposition of CF2Cl2 (Freon-12) has been studied in ref lected shock waves using the Cl atom atomic resonance absorption spect roscopic (ARAS) detection technique. Experiments were performed with v arying [CF2Cl2](0) in Kr diluent, over the temperature range 1446-2667 K, at three post-shock densities, 3.2, 2.2, and 1.2 x 10(18) cm(-3). This is the first thermal rate study for CF2Cl2 (+M) --> CF2Cl + Cl(+M ). At 100% dissociation, the Cl atom yield, [Cl](infinity)/[CF2Cl2](0) , is 2.02 +/- 0.13, confirming that C-Cl bond fission is the dominant dissociation pathway for both the parent CF2Cl2 molecule and the produ ct CF2Cl radical, CF2Cl(+M) --> CF2 + Cl (+M). Molecular elimination t o give CF2 + Cl-2 does not occur. The data can be represented in secon d order with the Arrhenius expression k(d) = (1.34 +/- 0.57) x 10(-7) exp(-31663 +/- 714 K/T) cm(3) molecule(-1) s(-1) (+/-34% error at the one standard deviation level) for 1446 less than or equal to T less th an or equal to 2090 K. RRKM calculations have been applied to this rea ction, and the temperature and pressure dependences of the rate consta nts can be characterized with threshold energy, E(0) = 85 +/- 1 kcal m ol(-1), and energy transfer parameter, [Delta E(down)] = 1050 +/- 150 cm(-1). This threshold energy implies Delta(f)H(0 CF2Cl)(0) = -60.1 +/ - 1 kcal mol(-1) acid a threshold energy for CF2Cl dissociation of E(0 ) = 49.1 +/- 1 kcal mol(-1). This low band strength in CF2Cl is consis tent with the observed fast dissociation to produce the second Cl atom .