On the heats of formation of trifluoromethyl radical CF3 and its cation CF3+

Abstract

The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP0(CF3+C2F4)=13.721±0.005 eV and AP0(CF+/C2F4)=13.777±0.005 eV and an accurate difference in ionization potentials, IP(CF)−IP(CF3)=0.055±0.003 eV. With the existing photoelectron value IP(CF)=9.11±0.01 eV, this produces IP(CF3)=9.055±0.011 eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP0(CF3+/CF3Cl)<12.867±0.008 eV has been found to be only an upper limit. The Analogous CF3+CF3+ fragment yield curves from CF3Br and CF3I produce AP0(CF3+/CF3Br)=12.095±0.005 eV and AP0(CF3+/CF3I)=11.384±0.005 eV, leading to D0(CF3−Br)=70.1±0.3 kcal/mol (70.8±0.3 kcal/mol at 298 K) and D0(CF3−I)=53.7±0.3 kcal/mol (54.3±0.3 kcal/mol at 298 K). Based on tabulated values for ΔHf∘(CF3Br) and ΔHf∘(CF3I), which appear to be inconsistent by ∼1 kcal/mol, a compromise value of ΔH298 f∘(CF3)=−111.4±0.9 kcal/mol (−110.7±0.9 kcal/mol at 0 K) is selected, resulting in ΔH298f∘(CF3+)=97.4±0.9 kcal/mol (98.1±0.9 kcal/mol at 0 K). Additionally, IP(CF4)≡AP0(CF3+/CF4)=14.67±0.04 eV can be inferred. From data on C2F4, ΔH298f∘(CF)=62.5±1.1 kcal/mol (61.7±1.1 kcal/mol at 0 K) can be deduced. Many earlier literature values for appearance potentials of CF3+ from CF3X, leading to very low ΔHf∘(CF3+) and/or IP(CF3) values, are demonstrated to be in error.