According to eq 7, the observed activation energy of ketyl formation represents £5 -£4, where E4 and £5 are the activation energies of reactions 4 and 5, respectively. Since £4 is thought to be small, the observed value of 12 kcal mol"1 almost corresponds to the activation energy for the escape of radicals from the solvent cage, which would be controlled by diffusion. Assuming the diffusion rate of radicals from the cage is the same as the self-diffusion rate of the solvent, the observed value of 12 kcal mol"1 should coincide with the activation energy of self-diffusion of ethanol. The average activation energy of self-diffusion in the temperature region 125 ~155 K is estimated to be about 11 kcal mol'1 from Figure 2. The resulting value agrees with the observed one. Thus, reaction 5 is concluded to be the rate-determining step for free ketyl radical formation in the low temperature region.In conclusion, quenching of triplet benzophenone in ethanol at low temperature, as well as at room temperature, is governed by the hydrogen abstraction reaction, which results in a radical pair in the solvent cage. The activation energy for the hydrogen abstraction reaction is determined to be 2.2 kcal mol'1 in the temperature region 93 ~131 K. Free ketyl radicals are formed as a result of the escape of the radical pair from the cage, and hence the ketyl forming process is diffusion controlled. At room temperature, the diffusion rate is fast and all of the radical pairs separate into free ketyl and solvent radicals. At low temperature, the reactions of the radical pair in the cage are predominant and the yield of ketyl is very low. The activation energy of ketyl formation coincides with that of self-diffusion of ethanol. References and Notes(1) A. Beckett and G.

No abstract

2435make the following recommendations: (1) Solutions of phen should be stored in the dark and kept for no more than 3 months. (2) As a precaution, buffer should be added before phen-the opposite of the current recommendation^.^^^^^ (3)With good phen development is immediate (<2 min), but 30 minl allows a good safety margin. (4) NH4F should not be used.17916) and thank F. Wilkinson and S. H. Peterson for comments on the ferrioxalate system. We thank R. B. Martin for use of his Cary 11 and 14, F. S. Richardson for use of his laser, and E. Sinn for a sample of 1,lO-phenanthroline N-oxide.