Shock wave and modeling study of the thermal decomposition reactions of pentafluoroethane and 2-H-heptafluoropropane

Abstract

The thermal decomposition reactions of CF 3 CF 2 H and CF 3 CFHCF 3 have been studied in shock waves by monitoring the appearance of CF 2 radicals. Temperatures in the range 1400-2000 K and Ar bath gas concentrations in the range (2-10) Â 10 À5 mol cm À3 were employed. It is shown that the reactions are initiated by C-C bond fission and not by HF elimination. Differing conclusions in the literature about the primary decomposition products, such as deduced from experiments at very low pressures, are attributed to unimolecular falloff effects. By increasing the initial reactant concentrations in Ar from 60 to 1000 ppm, a retardation of CF 2 formation was observed while the final CF 2 yields remained close to two CF 2 per C 2 F 5 H or three CF 2 per C 3 F 7 H decomposed. This is explained by secondary bimolecular reactions which lead to comparably stable transient species like CF 3 H, releasing CF 2 at a slower rate. Quantum-chemical calculations and kinetic modeling help to identify the reaction pathways and provide estimates of rate constants for a series of primary and secondary reactions in the decomposition mechanism.

The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules

“…Several methods, such as B3PW91/cc-pVDZ and M06-2X/cc-pVDZ, were able to provide valid descriptions of the transition states for the three reaction pathways, as judged by the intrinsic reaction coordinate (IRC) test. Our survey also was consistent with results from a larger set of calculations 18 for the CF 3 CHF 2 system. We decided to compare the rate constants from the B3PW91/cc-pVDZ and M06-2X/aug-cc-pVTZ calculations.…”

“…In addition to the four calculations for the E 0 values given in Table 2, we have results from M06-2X/aug-cc-pVTZ, which are 84, 88, and 95 kcal mol −1 for 1,1-HF, 1,2-HF, and H atom transfer, respectively. Cobos and co-workers 18 did several calculations, and their results from M06-2X/6-311+ +G(3df,3pd), CBS-QB3, Q3B3, and G4 are 84.8, 83.2, 81.5, and 81.5 for E 0 (1,1-HF); 88.2, 89.1, 87.7, and 87.9 for E 0 (1,2-HF); and 95.6, 91.1, 90.6, and 90.6 kcal mol −1 for E 0 (1,2-H atom). The more extended basis sets for the M06-2X calculations gave nearly the same results as for the calculations listed in Table 2.…”

“…Modeling of the system will need to employ a reliable bond dissociation energy and advanced treatment of the rate constant for dissociation. Calculated Arrhenius expressions from models similar to those used in our work for the decomposition reaction of CF 3 CHF 2 are given in ref , and those pre-exponential factors can be consulted. The transition state for H atom transfer has low frequencies, and the pre-exponential factor is larger than those for 1,1-HF and 1,2-HF elimination.…”

“…A third unimolecular pathway from CF 3 CHF 2 has been identified from the photoelectron spectra of CF 3 H in a high-temperature flow reactor and in conventional product analysis from a flow reactor. Computational studies , also have suggested that (H atom transfer) forming CF 3 H + :CF 2 could compete with 1,1-HF and 1,2-HF elimination. In the current study the three unimolecular rate constants were determined by comparing ratios of the three decomposition products to collisionally stabilized C 2 F 5 H as a function of bath gas pressure.…”

“…22,23 The dissociation reaction giving CF 3 and CHF 2 radicals was recognized as being in competition with 1,2-HF elimination in high temperature and in infrared multiphoton excitation experiments 20−25 of CF 3 CHF 2 . A recent shock-wave study 18 monitored the CF 2 concentration as a measure of the decomposition rate of CF 3 CHF 2 , since most of the secondary reactions lead to CF 2 at high temperatures. These authors also employed high-level electronic-structure calculations to explore primary reactions in addition to 1,2-HF elimination.…”