Abstract
CF 3 OH , an important and controversial by-product of atmospheric decomposition of CF3CFH2 (HFC-134a) and other hydrofluorocarbons, has been examined by photoionization mass spectrometry. The ionization onset is characterized by a broad FranckâCondon distribution, arising primarily from a substantial elongation of the CâO bond upon ionization. An upper limit to the adiabatic ionization potential (IP) of â©œ13.08±0.05 eV has been established. The appearance potentials (APs) of the first two fragments have been accurately determined by fitting with appropriate model functions as AP0(CF2OH+/CF3OH)â©œ13.830±0.005 eV and AP0(CF3+/CF3OH)â©œ13.996±0.005 eV. While the exact nature of the lowest-energy fragment (nominally CF2OH+) is not clear, the CF3+ fragment threshold leads unambiguously to ÎHf 298â(CF3OH)â©Ÿâ217.2±0.9 kcal/mol and D298(CF3âOH)â©œ115.2±0.3 kcal/mol. With previously derived ÎHf 298â(CF3O)=â151.8â1.1+1.7 kcal/mol, this yields D298(CF3OâH)=117.5â1.4+1.9 kcal/mol, very close to, or only slightly weaker than the OâH bond energy in water: D298(CF3OâH)âD298(HOâH)=â1.8â1.4+1.9 kcal/molâ0 kcal/mol. Similarly, with the recently redetermined value for ÎHfâ(CF2O), this implies a 298 K reaction enthalpy for the 1,2-elimination of HF from CF3OH of 2.8â1.1+1.7 kcal/mol. CF3OF and CF3OCl have also been examined by photoionization. CF3OF produces a very weak parent, with an apparent adiabatic IP(CF3OF)â©œ12.710±0.007 eV. An analysis of the CF3+ and CF2O+ fragments from CF3OF, when combined with literature data, suggests ÎHf 298â(CF3OF)=â176.9â1.3+1.8 kcal/mol. The fitted value for the appearance potential of CF3+ from CF3OCl, AP0(CF3+/CF3OCl)â©œ12.85±0.01 eV, leads to ÎHf 298â(CF3OCl)â©Ÿâ175.6±1.0 kcal/mol, D298(CF3âOCl)â©œ88.4±0.3 kcal/mol, and D298(CF3OâCl)â©œ52.8â1.5+2.0 kcal/mol.
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