J . Phys. Chem. 1989, 93, 3167-3169 currentlyaccepted values for IP(Fe0) and D0,(FeO). Creating iron ions directly by laser vaporization significantly populates excited electronic states. If the vaporization is followed by > 1O5 quenching collisions with He, no evidence for Fe+(4F) is observed. At higher laser power, a small fraction of Fe' (> ksY. These assumptions are consistent with the fact that the activation energies E2, E3, and E8 are negligibly small while E,, E5, E6, and E7 are not, so that the rate constants of the latter set become comparatively large at elevated temperatures. Then from ( l ) , (8), and (9) we obtain the expression k3(klk6k7A2/k3k5kg)2/3 = d[C,F6] /dt Using the values for E, and E5 given by (10) and ( l l ) , the published value E7 = 31.3 kcal/mol,I0 and the experimental activation energy value of 55.0 kcal/mol for C2F6 production at high temperatures, we deduce that E6
= 27.8 kcal/mol
(12)
This result is entirely plausible in view of the value of E7 and the similarity of the chlorine abstraction reactions R6 and R7. In summary, the analysis indicates that all of the reactions R1 through R8 are required to explain the data over both temperature ranges. Despite the mathematical complexity of the proposed mechanism, a solution is discernible which conforms with experiment and values for the activation energies E,, ES,and E6 are derived. The reaction mechanism proposed here is consistent with the results of Dever and Grunwaldl but not necessarily with those of Horowitz et aL2 where extremely high laser intensities are involved, greater by a factor of lo6 than those used here, and where for example fluorine elimination or abstraction reactions could occur. It is of interest to inquire if analogous reaction patterns occur in CF3Br and CFJ. C W laser studies made at our laboratory in the past on these compounds6**2 were confined to low temperatures where C2F6 was the only halocarbon product detected. Due to the low dissociation energies of the Br and I compounds, C2F6 production at high temperatures were too fast to measure easily and that regime was left unexplored. However, high temperatures were certainly reached in the TEA C 0 2 laser experiments of Jalenak and Nogar13 on CF3Br, where in analogy with CF3C1 the products CF, and CF2Br2were observed in addition to C2F6.
which is precisely the value expected from the enthalpy of the reaction R1. For the high-temperature regime where CF4 is also produced, (1) and (2) lead to the relation
Acknowledgment. We are grateful for the support of the Department of Chemistry and Biochemistry, Department of Physics, and the Office of Research and Development at SIUC.
(1 1) Kondratiev, V. N . Rate Conrtants of Gas Phase Reactions (translated by Holtschlag, L. J.); Fristrom, R. M., Ed.; NSRDS Publication; National Bureau of Standards: Washington DC, 1972;.pp 95-96.
(12) Choudhury, T. K. Ph.D. Dissertation, Southern Illinois University, Carbondale, IL 1986. (13) Jalenak, W. A.; Nogar, N. S. Chem. Phys. 1979, 41, 407.
Registry No. C F Q , 75-72-9; CF3, 2264-21-3; CF2CI, 1691-89-0.
