283600-87-3Relevant articles and documents
Kinetics of nitroxide radical trapping. 2. Structural effects
Bowry,Ingold
, p. 4992 - 4996 (1992)
Laser flash photolysis and kinetic competition product demonstrated that in isooctane at ambient temperature the rate constant for coupling of carbon-centered with presistent nitroxides, kT, upon the of steric hindrance to coupling and upon the extent of resonance stabilization of the carbon radical. Sterically induced reductions in the magnitude of kT are observed for changes in both the structure of the nitroxide the structure of the carbon radical. Thus, for any particular carbon kT is largest for the Bredt's rule protected nitroxides, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) and nortropane-N-oxyl, while for the "usual" di-tert-alkyl nitroxides kT decreases along the series, 1,1,3,3-tetra-methylisoindoline-2-oxyl ≥ 2,2,5,5-tetramethylpiperidin-1-oxyl (Tempo) > di-tert-butyl nitroxide, i.e., kT decreased on going from a five-membered ring to a six-membered ring to a noncyclic structure. Cyclopropyl triphenylmethyl are trapped at the fastest and slowest rates, respectively, the corresponding kT value 3.0 × 109 and 1.2 × 108 M-1 s-1 for ABNO and 2.1 × 109 6 M-1 s-1 for Tempo. Steric effects in the carbon radicals are for Tempo than for ABNO. For example, the ratio of kT's for the trapping of nonyl tert-butyl is 1.7 for Tempo but 1.3 for ABNO, while for the trapping of benzyl cumyl the ratio of kT's is 4.1 for Tempo 0.9 for ABNO. The effect of resonance stabilization can be illustrated by the kT values for three sterically unhindered primary radicals, n-nonyl, benzyl, 2-naphthyhmethyl, viz., 1.2 × 109, 4.8 × 108, and 5.7 × 107 M-1 s-1, respectively, for Tempo and 2.2 × 109, 1.2 × 109, and 8.1 × 108 M-1 s-1, respectively, for ABNO.
Entropy control of the cross-reaction between carbon-centered and nitroxide radicals
Sobek,Martschke,Fischer
, p. 2849 - 2857 (2007/10/03)
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 5 M-1 s-1 to 2.3 × 109 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.
