Spin Dynamics of the Radicals Produced from Methane Sulfonic Acid and Acetic Acid by Hydroxyl Radicals in Aqueous Solution Studied by Means of Time-Resolved Electron Spin Resonance Spectroscopy.

Abstract

Fundamental information on the reactions of ·OH radical with alkyl sulfonic acid and carboxylic acid is important for understanding the degradation of the polymer electrolytes for fuel cells. In the present research, the spin dynamics of the organic radicals generated by the ·OH radical was investigated by means of a time-resolved electron spin resonance (TR-ESR) method with the pulsed laser irradiation on H2O2 in aqueous solution. The time profiles of the ESR signals of ·SO3-, ·CH2SO3-, ·CH2COOH, and ·CH2COO- have been analyzed along with the evaluation of characteristics of electron spins such as the g factor, hyperfine coupling constant A, lifetime of radicals, and electron spin relaxation time T1. The effects of the laser repetition frequency on the TR-ESR spectra, the pH dependence on the kinetic parameters, and the mechanism of chemically induced dynamic electron polarization (CIDEP) were discussed. The radical pair mechanism was suggested for the CIDEP, where the electron spins of these radicals are polarized by forming a free radical pair with another radical or the·OH radical, which have remained in the vicinity of photon absorption.