On the heat of formation of carbonyl fluoride, CF2O

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Abstract
In light of the recent controversy surrounding its heat of formation, CF2O was reexamined by photoionization mass spectrometry. In particular, the CO+ fragment ion yield curve from CF2O was interpreted in terms of a retarded CO++F2 process, and a more facile two-step fragmentation to CO++2F. The former process produces a weak, slowly growing tail region without a clear onset, while the latter occurs at higher energy and causes a pronounced growth with a conspicuous onset, which was found to occur at ≤20.87+0.03/−0.07 eV at 0 K by fitting with a model curve that incorporates ‘‘fluctuations’’ associated with second-generation fragments. This onset leads to ΔHf° 298(CF2O) ≥−149.1+1.4/−0.7 kcal/mol, and indicates that the older experimental values for this quantity are too low by at least 3–4 kcal/mol. While the F2 elimination is retarded by competition with lower energy processes, the two-step process derives its strength from the FCO+ fragment, which assumes the role of a pseudoparent. Thus, the onset of CO++2F is expected to appear reasonably close to the thermochemical threshold. Such an interpretation indicates that recently calculated ab initio values of −145.3±1.7 kcal/mol and −145.6±1.0 kcal/mol are very likely too high by 3–4 kcal/mol. In addition, the adiabatic ionization potential of CF2O was refined to 13.024±0.004 eV, while the 0 K appearance potential of the FCO+ fragment was found by fitting to be ≤14.752±0.005 eV. Together with the suggested value for ΔHf°(CF2O), these two onsets lead to ΔHf° 298(CF2O+)=151.2+1.4/−0.7 kcal/mol and ΔHf° 298(FCO+)=173.5+1.4/−0.7 kcal/mol.

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