ON THE HEATS OF FORMATION OF TRIFLUOROMETHYL RADICAL CF3 AND ITS CATION CF3+

The CF+ and CF3+ fragment ion yield curves from C2F4 have been remeasu red by photoionization mass spectrometry. Fits with appropriate model curves yield the appearance potentials AP(0)(CF3+/C2F4)=13.721+/-0.005 eV and AP(0)(CF+/C2F4)=13.777+/-0.005 eV and an accurate difference i n ionization potentials, IP(CF)-IP(CF3)=0.055+/-0.003 eV. With the exi sting photoelectron value IP(CF)=9.11+/-0.01 eV, this produces IP(CF3) =9.05(5)+/-0.01(1) eV. The CF3+ fragments from CF3Cl, CF3Br, and CF3I have also been remeasured, and their ion yield curves fitted with mode l functions. The experimentally derived AP(0)(CF3+/CF3Cl) <12.867+/-0. 008 eV has been found to be only an upper limit. The analogous CF3+ fr agment yield curves from CF3Br and CF3I produce AP(0)(CF3+/CF3Br)=12.0 95+/-0.005 eV and AP(0)(CF3+/CF3I)=11.384+/-0.005 eV, leading to D-0(C F3-Br)=70.1+/-0.3 kcal/mol (70.8+/-0.3 kcal/mol at 298 K) and D-0(CF3- I)=53.7+/-0.3 kcal/mol (54.3+/-0.3 kcal/mol at 298 K). Based on tabula ted values for Delta H-f(0)-(CF3Br) and Delta H-f(o)(CF3I), which appe ar to be inconsistent by similar to 1 kcal/mol, a compromise value of Delta H-f298(o)(CF3)=-111.4+/-0.9 kcal/mol (-110.7+/-0.9 kcal/mol at 0 K) is selected, resulting in Delta H-f(o) (298)(CF3+)=97.4+/-0.9 kcal /mol (98.1+/-0.9 kcal/mol at 0 K). Additionally, IP(CF4)=AP(0)(CF3+/CF 4)=14.67+/-0.04 eV can be inferred. From data on C2F4, Delta H-f298(o) (CF)=62.5+/-1.1 kcal/mol (61.7+/-1.1 kcal/mol at 0 K) can be deduced. Many earlier I literature values for appearance potentials of CF3+ fro m CF(3)X, leading to very low Delta H-f(o)(CF3+) and/or IP(CF3) values , are demonstrated to be in error. (C) 1997 American Institute of Phys ics.