| Résumé | Absolute rate constants have been determined in Freon solvents over a temperature range from ca. 190 to 300 K for H atom abstraction by (CF3)2NO· from 11 substrates and for the addition of this radical to CH2=CCl2. Some values found for log A (M-1 s-1) and Ea (in kcal/mol) are 5.8 ± 0.6 and 10.7 ± 0.7 for cyclopentane, 6.5 ± 0.3 and 5.7 ± 0.5 for 1,4-cyclohexadiene, 5.3 ± 0.3 and 6.9 ± 0.3 for benzaldehyde, 4.8 ± 0.2 and 3.3 ± 0.3 for 2,4,6-tri-tert-butylphenol, 5.9 ± 0.7 and 4.3 ± 0.7 for tri-n-butylstannane, and 5.3 ± 0.4 and 9.3 ± 0.5 for CH2=CCl2. The preexponential factors are uniformly smaller than the values generally considered "normal" for radical/H atom abstractions and radical/C=C double-bond additions, viz., 108.5±0.5 M-1 s-1. It is concluded that the low A factors are not due to tunneling. It is suggested that they are probably due to geometric constraints on the transition states. For a wide range of substrates exhibiting very different reactivities the absolute rate constant for H atom abstraction by (CF3)2NO· and by Me3COO· at ambient temperatures are virtually equal. |
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