A photoionization study of trifluoromethanol, CF3OH, trifluoromethyl hypofluorite, CF3OF, and trifluoromethyl hypochlorite, CF3OCl

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 Franck–Condon distribution, arising primarily from a substantial elongation of the C–O bond upon ionization. An upper limit to the adiabatic ionization potential (IP) of ⩽13.08±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)⩽13.830±0.005 eV and AP0(CF3+/CF3OH)⩽13.996±0.005 eV. While the exact nature of the lowest-energy fragment (nominally CF2OH+) is not clear, the CF3+ fragment threshold leads unambiguously to ΔHf 298∘(CF3OH)⩾−217.2±0.9 kcal/mol and D298(CF3–OH)⩽115.2±0.3 kcal/mol. With previously derived ΔHf 298∘(CF3O)=−151.8−1.1+1.7 kcal/mol, this yields D298(CF3O–H)=117.5−1.4+1.9 kcal/mol, very close to, or only slightly weaker than the O–H bond energy in water: D298(CF3O–H)–D298(HO–H)=−1.8−1.4+1.9 kcal/mol≈0 kcal/mol. Similarly, with the recently redetermined value for ΔHf∘(CF2O), this implies a 298 K reaction enthalpy for the 1,2-elimination of HF from CF3OH of 2.8−1.1+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)⩽12.710±0.007 eV. An analysis of the CF3+ and CF2O+ fragments from CF3OF, when combined with literature data, suggests ΔHf 298∘(CF3OF)=−176.9−1.3+1.8 kcal/mol. The fitted value for the appearance potential of CF3+ from CF3OCl, AP0(CF3+/CF3OCl)⩽12.85±0.01 eV, leads to ΔHf 298∘(CF3OCl)⩾−175.6±1.0 kcal/mol, D298(CF3–OCl)⩽88.4±0.3 kcal/mol, and D298(CF3O–Cl)⩽52.8−1.5+2.0 kcal/mol.