On the heat of formation of carbonyl fluoride, CF2O

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++F-2 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 less than or equal to 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 Delta H degrees(f 298)(CF2O) greater than or equal to -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 F-2 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 nb 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 less than or equal to 14.752+/-0.005 eV. Together with the suggested value for Delta H degrees(f)(CF2O), these two onsets lead to Delta H degrees(f 298)(CF2O+) = 151.2(+1.4)/(-0.7) kcal/mol and Delta H degrees(f 298)(FCO+) = 173.5(+1.4)/(-0.7) kcal/mol. (C) 1996 American Institute of Physics.