Radical scavenging rate constants determined by spin relaxation times of electron spin polarized radicals as measured by free induction decay signals

Radical scavenging reaction rate constants were measured by monitoring the free induction decay (FID) of the unpaired electron of radicals by using laser-synchronized pulsed-EPR. This method probes large electron spin magnetization arisen from dynamic electron spin polarization (DEP), which remarkably enhances the EPR signal. DEP decays with the longitudinal spin relaxation time, which was observed by using the FID detection method. In the presence of a radical scavenger, the DEP decay time depends on both spin-lattice relaxation and the chemical reaction with the scavenger, the latter of which reduces radical concentration. The plots of DEP decay rates against the radical scavenger concentrations gave the pseudo-first-order reaction rate constants for various radicals. This procedure was applied to determine the radical scavenging reaction rate constants of hydroxycyclohexyl, 2-hydroxypropyl, diphenylphosphinoyl, and α,α-dimethoxybenzyl radicals. The measured rate constants show good agreement with the previously reported values determined by using another method monitoring electron spin echo (ESE) decays of the radicals. We discussed the advantageous and disadvantageous characters of the FID detection method with respect to the existing ESE method in the viewpoints of signal intensity, selectivity of radicals, the simpleness of the measurements, and so on.