| Abstract | 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 <1 × 106 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. |
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