Pulsed EPR Study of the Initial Steps of Radical-Scavenging Reactions of C70 with Diphenylphosphine Oxide, Hydroxycyclohexyl, and 2-Hydroxypropyl Radicals

Abstract

The radical scavenging reactions of C70 were investigated using time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) methods. In the diphenylphosphine oxide (DPPO) radical and C70 system, structural isomers of the adduct radicals of C70 appeared in the TR-EPR spectrum with clear hyperfine structures due to the phosphorus atom. Four isomers were identified through the analysis of the hyperfine coupling constants. The assignment of the adduct radicals was confirmed by the semiempirical calculation of the relative addition reaction rate constants that produce each isomer. The radical scavenging rate constants, ksca, of C70 in toluene were determined for DPPO, hydroxylcyclohexyl, and 2-hydroxypropyl radicals through electron spin echo observations using the pulsed-EPR method. The ksca values were nearly 109 mol-1 dm3 s-1 and were almost equal to the diffusion-controlled rate constant in toluene. This proves that C70 acts as an excellent radical scavenger. In addition, the radical addition rate constants, kadd, of C70 for varying carbon atoms in C70 were obtained by considering the peak intensity ratio of the adduct radicals in the TR-EPR spectrum. In this study, we demonstrated that the large number of carbon atoms in pentagons (five-membered rings) is responsible for the high reactivity of fullerenes.