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

Abstract

The CF ϩ and CF 3 ϩ fragment ion yield curves from C 2 F 4 have been remeasured by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP 0 ͑CF 3 ϩ /C 2 F 4 ͒ϭ13.721Ϯ0.005 eV and AP 0 ͑CF ϩ /C 2 F 4 ͒ϭ13.777Ϯ0.005 eV and an accurate difference in ionization potentials, IP͑CF͒ϪIP͑CF 3 ͒ϭ0.055Ϯ0.003 eV. With the existing photoelectron value IP͑CF͒ϭ9.11Ϯ0.01 eV, this produces IP͑CF 3 ͒ϭ9.05 5 Ϯ0.01 1 eV. The CF 3 ϩ fragments from CF 3 Cl, CF 3 Br, and CF 3 I have also been remeasured, and their ion yield curves fitted with model functions. The experimentally derived AP 0 ͑CF 3 ϩ /CF 3 Cl͒Ͻ12.867Ϯ0.008 eV has been found to be only an upper limit. The analogous CF 3 ϩ fragment yield curves from CF 3 Br and CF 3 I produce AP 0 ͑CF 3 ϩ /CF 3 Br͒ϭ12.095Ϯ0.005 eV and AP 0 ͑CF 3 ϩ /CF 3 I͒ϭ11.384Ϯ0.005 eV, leading to D 0 ͑CF 3 ϪBr͒ϭ70.1Ϯ0.3 kcal/mol ͑70.8Ϯ0.3 kcal/mol at 298 K͒ and D 0 ͑CF 3 ϪI͒ϭ53.7Ϯ0.3 kcal/ mol ͑54.3Ϯ0.3 kcal/mol at 298 K͒. Based on tabulated values for ⌬H f ‫ؠ‬ ͑CF 3 Br͒ and ⌬H f ‫ؠ‬ ͑CF 3 I͒, which appear to be inconsistent by ϳ1 kcal/mol, a compromise value of ⌬H f 298 ‫ؠ‬ ͑CF 3 ͒ϭϪ111.4Ϯ0.9 kcal/mol ͑Ϫ110.7Ϯ0.9 kcal/mol at 0 K͒ is selected, resulting in ⌬H f 298 ‫ؠ‬ ͑CF 3 ϩ ͒ϭ97.4Ϯ0.9 kcal/mol ͑98.1Ϯ0.9 kcal/mol at 0 K͒. Additionally, IP͑CF 4 ͒ϵAP 0 ͑CF 3 ϩ /CF 4 ͒ϭ14.67Ϯ0.04 eV can be inferred. From data on C 2 F 4 , ⌬H f 298 ‫ؠ‬ ͑CF͒ϭ62.5Ϯ1.1 kcal/mol ͑61.7Ϯ1.1 kcal/mol at 0 K͒ can be deduced. Many earlier literature values for appearance potentials of CF 3 ϩ from CF 3 X, leading to very low ⌬H f ‫ؠ‬ ͑CF 3 ϩ ͒ and/or IP͑CF 3 ͒ values, are demonstrated to be in error.

Femtosecond Coincidence Imaging of Multichannel Multiphoton Dynamics

“…Here a plethora of superexcited states belonging to series converging upon theà A state can be expected. In fact, Asher and Ruscic [22] observed a strong and broad autoionizing feature extending into this region. Since theà A 2 A 1 state is dissociative, the autoionizing state is also likely dissociative.…”

Vacuum-UV and electron dissociative ionisation studies of SF5Cl

“…They are determined from the first observation of signal above the background noise. At the resolution (0.3 nm) and step size used in these experiments, this is equivalent to extrapolation of the linear portion of the ion yield to zero signal, and more sophisticated fitting procedures for the threshold region are not appropriate [27]. For the major fragment ions, defined as those formed by a single bond cleavage, the AE 298 values can be converted into an upper limit of r H o 298 for the corresponding unimolecular reaction (Column 1) using the procedure of Traeger and McLoughlin [28], and are given in Column 4 (denoted r H o 298, exp ).…”

Fragmentation of Energy-Selected SF5CF3+ Probed by Threshold Photoelectron Photoion Coincidence Spectroscopy: Bond Dissociation Energy of SF5-CF3 and Its Atmospheric Implications

“…The greenhouse effect is usually associated with small polyatomic molecules such as CO 2 6 , which has a global warming potential (GWP) of 22,200 relative to CO 2 over a time horizon of 100 years. In a very recent paper, 1 Sturges et al…”

“…Close to the energy threshold, the cross-section for photodissociation is negligibly small. Thus, CF 4 has a dissociation energy (to CF 3 + F) of 5.61 eV, 2 but VUV photons with energies in excess of 12 eV are required to photodissociate CF 4 . 3 Likewise, the bond dissociation energy of SF 6 (to SF 5 + F) is 3.82 eV, 4 but photodissociation is not observed until the photon energy exceeds ca.…”