Study of Pentafluoroethane and its Thermal Decomposition Using UV Photoelectron Spectroscopy and Ab Initio Molecular Orbital Calculations

Abstract

The UV photoelectron spectrum of CF(3)CHF(2) has been recorded and assigned using EOM-CCSD calculations. For the first band, the adiabatic ionization energy (AIE) and vertical ionization energy (VIE) have been measured as (12.71 +/- 0.05) and (13.76 +/- 0.02) eV, respectively. The measured AIE is higher than the recommended value from state-of-the-art ab initio calculations of (12.26 +/- 0.02) eV because of a large geometry change on ionization, mainly arising from a significant increase in the C-C bond length, which results in poor Franck-Condon factors in the adiabatic region. The experimental VIE also shows poor agreement with the computed value of 14.05 +/- 0.06 eV because, in the higher energy region of the first photoelectron band, dissociation of CF(3)CHF(2)(+) to CHF(2)(+) + CF(3) occurs. This has a calculated thermodynamic onset of (12.89 +/- 0.20) eV. Recommendations are made for the heats of formation, DeltaH(f,298)(slashed circle), of CF(3)CHF(2) and CF(3)CHF(2)(+), based on the results of the ab initio calculations. Pyrolysis of flowing CF(3)CHF(2) diluted in argon shows evidence of production of C(2)F(4) and HF at lower temperatures and CF(2) and CF(3)H at higher temperatures. The relative temperature dependence of the observed photoelectron bands associated with these molecules is interpreted in terms of two decomposition reactions of CF(3)CHF(2) as well as the reaction C(2)F(4) --> 2CF(2).

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

“…21) that k 19 is about a factor of 3 below k 19,N at [Ar] = 5 Â 10 À5 mol cm À3 and T = 1300 K while it is a factor of 5 below at T = 1500 K and a factor of 10 below at 1800 K. The opposing conclusions about a dominance of the HF elimination reaction (15) from ref. 11 and about that of C-C bond breaking from the present work (and ref. 14) are easily explained by the described falloff effects.…”

“…for C 2 F 5 H in ref. [9][10][11], and for C 3 F 7 H in ref. 5 and 12. The present work extends such calculations.…”

“…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.…”

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

“…The 1,2-HF and 1,1-HF channels were identified by measuring the CF 2 CF 2 product and the adduct between CF 3 (F)­C: and trans- 2 - butene, respectively. 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.…”

Estimation of the ionization potential for polyhalogenated hydrocarbons by weakest bound potential method

“…Polyhalogenated hydrocarbons including chlorofluorocarbons and bromofluorocarbons have been widely applied to industrial production, and also they result in some environmental problems. Thus, the relationship between their structure and properties has gained more and more attention . As a fundamental property of polyhalogenated hydrocarbons, their ionization potential (IP) relates to some important thermodynamic and kinetic properties and has played a significant role in theory research .…”