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 particula r, 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 fragmentati on 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 fo und to occur at less than or equal to 20.87(+0.03)/(-0.07) eV at 0 K b y fitting with a model curve that incorporates ''fluctuations'' associ ated with second-generation fragments. This onset leads to Delta H deg rees(f 298)(CF2O) greater than or equal to -149.1(+1.4)/(-0.7) kcal/mo l, 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 ret arded by competition with lower energy processes, the two-step process derives its strength from the FCO+ fragment, which assumes the role o f a pseudoparent. Thus, the onset of CO++2F is expected to appear reas onably close to the thermochemical threshold. Such an interpretation i ndicates that recently calculated nb initio values of -145.3+/-1.7 kca l/mol and -145.6+/-1.0 kcal/mol are very likely too high by 3-4 kcal/m ol. In addition, the adiabatic ionization potential of CF2O was refine d 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.