Study of 2-H-heptafluoropropane and its thermal decomposition using UV photoelectron spectroscopy and ab initio molecular orbital calculations

The thermal decomposition of 2-H-heptafluoropropane, CF3CHFCF3, at low pressure, heavily diluted in argon, has been studied over the temperature range 600-2000 °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 CF3CF=CF2+ HF, is found to be the main decomposition reaction at lower temperatures, 600900 °C. At higher temperatures, 900-1200 °C, the decomposition reaction to give C2F4+ CF3H was found to become important. No evidence for CF3CHFCF3-CF3CHF → CF3, a reaction expected to be important from a shock tube study, performed at much higher pressures, or for CF3CHFCF3-CF3CF → CF3II was obtained, although for the latter reaction it is likely that CF3CF converts into C2F4under the conditions used before photoionization, in the ionization region of the photoelectron spectrometer. At higher temperatures C3F6decomposes to C2F4+ CF2, and C2F4decomposes to CF2. 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 CF3CF=CF2