THERMAL-DECOMPOSITION OF CF2HCL

The development of a multipass optical absorption (MPA) technique for detecting CF2 radicals behind reflected shock waves and its applicatio n for studying the thermal dissociation of CF2HCl in Kr are reported. In an earlier work, a Cl atom atomic resonance absorption spectrometri c (ARAS) study on the thermal decomposition of CF2Cl2 gave the stoichi ometric yield of two Cl atoms per dissociating CF2Cl2, indicating the overall process is CF2Cl2 = 2Cl + CF2. CF2 yields with CF2Cl2 as the t hermal source were then used to obtain the CF2 curve of growth. The ef fective absorption cross section is 2.86 x 10(-18) cm(2) at 249.8 nm. In the CF2HCl experiments, the yield was measured to be 1.01 +/- 0.06, confirming that the thermal decomposition pathway is molecular HCl el imination; i.e., CF2HCl (+ M) --> CF2 + HCl (+ M). Above 1900 K, C-Cl bond fission was measured to be <1%. Rate constants for the title reac tion were measured by observing the temporal formation of CF2 radicals . Also, rate constants were measured and the overall enthalpy change w as determined in incident shock waves using the laser schlieren (LS) t echnique. Over 1047 less than or equal to T less than or equal to 1731 K and 113 less than or equal to P less than or equal to 589 Torr, bot h the magnitude and the T dependence of the measured rate constants fr om the two techniques are in good agreement. A fit to combined sets of data is expressed to within +/- 54% by the Arrhenius equation: k = 2. 42 x 10(-9) exp(-20180K/T) cm(3) molecule(-1) s(-1). With an E(0) = 54 .85 kcal mol(-1) that is fixed by the low-temperature data, ab initio transition state properties, determined at the MP4(SDTQ) and QCISD(T) levels, were used in RRKM modeling of the rate constants. Excellent ag reement with the present and previous high-temperature density-depende nt data was obtained with constant [Delta E](down) = (265 +/- 20) cm(- 1). From the LS experiments, Delta H-0(0) = 52.1 kcal mol(-1), implyin g Delta(f)H(0,CF2)(0) = -39.5 kcal mol(-1) if the JANAF value for Delt a(f)H(0,CF2HCl)(0) is accepted. For the reverse insertion process this suggests a barrier of 2.8 kcal mol(-1).