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Entropy Control of the Cross-Reaction between Carbon-Centered and Nitroxide Radicals

2001; American Chemical Society; Volume: 123; Issue: 12 Linguagem: Inglês

10.1021/ja0036460

1943-2984

Jens Sobek, Rainer Martschke, Hanns Fischer,

Free Radicals and Antioxidants

Absolute rate constants for the cross-coupling reaction of several carbon-centered radicals with various nitroxides and their temperature dependence have been determined in liquids by kinetic absorption spectroscopy. The rate constants range from <2 x 10(5) M(-)(1) s(-)(1) to 2.3 x 10(9) M(-)(1) s(-)(1) and depend strongly on the structure of the nitroxide and the carbon-centered radical. Grossly, they decrease with increasing rate constant of the cleavage of the corresponding alkoxyamine. In many cases, the temperature dependence shows a non-Arrhenius behavior. A model assuming a short-lived intermediate that is hindered to form the coupling product by an unfavorable activation entropy leads to a satisfactory analytic description. However, the behavior is more likely due to a barrierless single-step reaction with a low exothermicity where the free energy of activation is dominated by a large negative entropy term.