Study of 2-H-Heptafluoropropane and Its Thermal Decomposition Using UV Photoelectron Spectroscopy and ab Initio Molecular Orbital Calculations

Abstract

The thermal decomposition of 2-H-heptafluoropropane, CF(3)CHFCF(3), at low pressure, heavily diluted in argon, has been studied over the temperature range 600-2000 degrees C using photoelectron spectroscopy. Comparison of the results obtained has been made with results of recent electronic structure calculations of possible decomposition pathways and results of a shock tube study. The most favored reaction thermodynamically, to produce CF(3)CF=CF(2) + HF, is found to be the main decomposition reaction at lower temperatures, 600-900 degrees C. At higher temperatures, 900-1200 degrees C, the decomposition reaction to give C(2)F(4) + CF(3)H was found to become important. No evidence for CF(3)CHFCF(3) --> CF(3)CHF + CF(3), a reaction expected to be important from a shock tube study, performed at much higher pressures, or for CF(3)CHFCF(3) --> CF(3)CF + CF(3)H was obtained, although for the latter reaction it is likely that CF(3)CF converts into C(2)F(4) under the conditions used before photoionization, in the ionization region of the photoelectron spectrometer. At higher temperatures C(3)F(6) decomposes to C(2)F(4) + CF(2), and C(2)F(4) decomposes to CF(2). Ab initio calculations have been performed of the adiabatic and vertical ionization energies of possible primary pyrolysis products to assist assignment of the photoelectron spectra recorded for heated flowing gas samples. A comparison is made between the threshold photoelectron spectrum and the photoelectron spectrum of CF(3)CF=CF(2).

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

“…One would be tempted to conclude from these results that reaction (1) followed by the faster reaction (3) dominates C 2 F 5 H decomposition. Likewise, one might conclude that C 3 F 7 H decomposition is dominated by reaction (5) followed by the faster dissociation of C 3 F 6 to 3 CF 2 . However, our detailed studies of the concentration dependence of the kinetics of CF 2 formation showed that this conclusion is premature and a more complicated mechanism of radical reactions has to be considered.…”

“…The product yields measured in ref. 5, therefore, do not correspond to those relevant for high pressure practical applications.…”

“…9-11 for C 2 F 5 H and in ref. 5 and 12 for C 3 F 7 H. In addition, however, we also need to consider the secondary processes which finally lead to CF 2 . Again quantumchemical calculations here are helpful to identify the most relevant pathways.…”

“…[1][2][3][4][5] and work cited therein). The use of these substances is not unproblematic as they have long atmospheric lifetimes and even may enhance combustion (see ref.…”

“…10 over the range 1273-1373 K at atmospheric pressure. Much lower pressures (of the order of 10 À4 to 10 À3 Torr) were applied in a heated flow inside a UV photoelectron spectrometer, 5,11 allowing for an in situ analysis of primary reaction products. However, no attention was paid to possible pressure dependencies of the unimolecular dissociation steps.…”