It is possible that the smaller ( ) values should be associated with a more complex transition probability model than the stepladder case. However, this is just speculation at this time. To fit the data, large collision diameters (Sam) are required for the C« and C8 fluoroalkanes. Dividing Sam into and t/k can be done in several ways, and we chose to follow the trend suggested by second virial coefficient data.14 Rabinovitch and coworkers13 successfully used a constant t/k value of 160°and an increase in of 0.54 Á per CF2 group to obtain collision diameters for thermal activation studies of perfluoroalkanes and CH3NC. Their collision diameters corresponding to vam [ (2•2)*]'/ are 6.7 and 7.3A for CeFi4 and C8Fi8, respectively. The difference from our choices (8.0 and 8.9 A) could be reduced by using a larger t/k for CF3CH3; however, this would not alleviate our problem of fitting the relative rate constants in the series C^e-CeFis. 963 ConclusionsUnimolecular rate constants for elimination of HF from chemically activated CH3CF3 and CH3CH2F fit the pattern for other chloro-, bromo-, and fluoroethanes. Consequently, we conclude that the elimination reactions at 90-100 kcal mol-1 are adequately described by RRKM theory. The amount of energy removed from CH3CF3* per collision with perfluoroalkane bath gases ranged from 7 to 10 kcal mol-1; the parent molecule also followed a similar collisional deactivation36 pattern and did not show an unusually high efficiency.Acknowledgment. We wish to thank Dr. J. W. Root for sending us the data cited in ref 35 and for giving us permission to quote the results. Mr. E. E. Siefert assisted with some of the energy transfer computations. This work was supported by the National Science Foundation under Grants GP-9245 and -27536X and by the Kansas State University Bureau of General Research.

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