Study of pentafluoroethane and its thermal decomposition using UV photoelectron spectroscopy and Ab initio molecular orbital calculations

The UV photoelectron spectrum of CF3CHF2 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 CF3CHF2+ to CHF 2- + CF3 occurs. This has a calculated thermodynamic onset of (12.89 ± 0.20) eV. Recommendations are made for the heats of formation, ΔHØf,298, of CF 3CHF2 and CF3CHF2+, based on the results of the ab initio calculations. Pyrolysis of flowing CF 3CHF2 diluted in argon shows evidence of production of C2F4 and HF at lower temperatures and CF2 and CF3H 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 CF3CHF2 as well as the reaction C2F4 →2CF2.