Switched external magnetic field CIDNP studies of coupling reaction of carbon-centered radicals with TEMPO

Abstract

CIDNP in a switched external magnetic field (SEMF) is applied to study reactions of carbon-centered radicals with the persistent nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidinyl-N-oxyl) in benzene. The carbon-centered radicals were generated by laser pulse photolysis of the corresponding ketone precursors RC(O)R with well-known photochemistry in the presence of TEMPO. Theoretical background of SEMF CIDNP was adapted to the system under study and the analytical solution of the corresponding set of kinetic equations was found. In the case of low initial concentration of radicals R the SEMF CIDNP kinetics are expected to be monoexponential with decay equal to k_c[Y] + 1/T₁, where k_c — coupling rate constant between alkyl and nitroxyl radicals, [Y] — concentration of nitroxyl radicals, and T₁ — electron-nuclear relaxation time of transient radicals. Analysis of SEMF CIDNP kinetics with varied concentrations of nitroxide allowed us to derive the corresponding k_c values, which were compared with the results of laser flash photolysis experiments found in the literature. Good agreement between different methods is achieved.