Thermal Decomposition of CF₂HCl

Abstract

The development of a multipass optical absorption (MPA) technique for detecting CF 2 radicals behind reflected shock waves and its application for studying the thermal dissociation of CF 2 HCl in Kr are reported. In an earlier work, a Cl atom atomic resonance absorption spectrometric (ARAS) study on the thermal decomposition of CF 2 Cl 2 gave the stoichiometric yield of two Cl atoms per dissociating CF 2 Cl 2 , indicating the overall process is CF 2 Cl 2 ) 2Cl + CF 2 . CF 2 yields with CF 2 Cl 2 as the thermal source were then used to obtain the CF 2 curve of growth. The effective absorption cross section is 2.86 × 10 -18 cm 2 at 249.8 nm. In the CF 2 HCl experiments, the yield was measured to be 1.01 ( 0.06, confirming that the thermal decomposition pathway is molecular HCl elimination; i.e., CF 2 HCl (+ M) f CF 2 + HCl (+ M). Above 1900 K, C-Cl bond fission was measured to be <1%. Rate constants for the title reaction were measured by observing the temporal formation of CF 2 radicals. Also, rate constants were measured and the overall enthalpy change was determined in incident shock waves using the laser schlieren (LS) technique. Over 1047 e T e 1731 K and 113 e P e 589 Torr, both the magnitude and the T dependence of the measured rate constants from 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 × 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 agreement with the present and previous high-temperature density-dependent data was obtained with constant 〈∆E〉 down ) (265 ( 20) cm -1 . From the LS experiments, ∆H 0 0 ) 52.1 kcal mol -1 , implying ∆ f H 0,CF 2 0 ) -39.5 kcal mol -1 if the JANAF value for ∆ f H 0,CF 2 HCl 0 is accepted. For the reverse insertion process this suggests a barrier of 2.8 kcal mol -1 .

Thermal decomposition of CH3I using I-atom absorption

“…With the Table III model and kcal , their Ar data can be rationalized with whereas their N 2 data require . Even though we have noted a high value for N 2 in the CF 2 HCl decomposition [4], this latter result is surprising since the implied ␤ c values (1300 K) for Ar : Kr : N 2 are in the ratio 1.0 : 1.2 : 3.4. We have also tried to reconcile the data of Kodama and Ooi [10] with the Table III model.…”

“…Two different unimolecular theoretical methods, the details of which are completely described elsewhere [1,2,4], were used to model the present rate data. The necessary molecular parameters are given in Table III.…”

Thermal Decomposition Studies of Halogenated Organic Compounds

“…[ls], CH3Cl [10,11], C3H3Cl [19], CFC13 [16,20], CF2C12 [16,21], CF3Cl [16,22], CF2HCl [13], CF3I [12], CH3I [23], C2H5I [24], C6H5I [25], and CC120 [26]. The methods used are nearly identical in all cases, and for the purposes of discussion, we will illustrate the method with CH3I data -ln(IdIo) (It and Io refer to transmitted and incident I-atom resonance light intensities, respectively), is measured as CH3I decomposes.…”

“…ARAS techniques for detecting C1-[lO,ll] and I-atom [12] concentrations have been used for most of the studies. However, in one study [13], multi-pass optical absorption of CF2, with White cell optics 11141, was used as the detection method. In all experiments, the photometer system was radially located 6 cm from the end plate, and the optical path length was either 9.94 or 8.745 cm depending on whether a round or square shock tube was used for a particular investigation.…”

Thermal decomposition of CF3Br using Br-atom absorption

“…Hence, the present theory and experiments are also completely consistent with this earlier work. Lastly, we have recently commented on how halogen substitution affects the energy transfer parameter in halomethanes [1,4,5] and have specifically compared the dissociations of CH 3 I and CF 3 I [26] because the respective threshold energies are almost identical. Though the fraction of molecules with energy exceeding the threshold was greater in CF 3 I, the ␤ c values necessary to explain the both sets only differed by 1.8.…”