Hydrogen Transfer from Ketyl Radicals to Nitroxyl Radicals

The transfer of an H atom from a ketyl radical (KR) to a nitroxyl radical is the sole reaction occurring between benzophenone or acetone KR and nitroxyl radicals (NR). The rate constants (kH) of the reaction of the KR of substituted benzophenones with the NR-4-hydroxy-2,2,6,6-tetramethylpiperidine-1oxyl and 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl in various solvents were measured by the pulse photolysis method. In low-viscosity solvents (up to 1-2 cP), the values of kH are not limited by the diffusion of the reagents. The kH values decrease with increase in the Hammet's σ-constants of the substituents in the KR and with decrease in the reduction potential of the NR, which indicates a charge transfer from the KR to NR in the transition state of the reaction. A cyclic structure was proposed for the transition state. The reaction is characterized by a low isotopic effect (kH/kD=1.4–1.5). The dependence of log kH on the solvation parameter of the solvent Et(30) is V-shaped in character.