EPR studies on the kinetics of the α-hydroxyethyl radical generated by Fenton-like chemistry

Flow systems, either stopped or continuous, have long been at the core of kinetic studies of chemical reactions. Such flow systems need to be coupled with appropriate spectroscopic or otherwise techniques for the detection of the chemical species studied. If paramagnetic species are formed or consumed during the investigated reaction, electron paramagnetic resonance (EPR) with its nanomolar sensitivity can be the spectroscopic method of choice. However, not much literature is available on the application of EPR to quantitatively study kinetics of chemical reactions in the liquid state. Herein, we report the characterisation of the commercially available mixing resonator ER 4117 MX from Bruker using the TEMPO-dithionite reaction as a standard. Furthermore, this setup was used to study the kinetics of the Fenton-like system of TiCl₃/H₂O₂ and ethanol forming the α-hydroxyethyl radical. Potential contributions of reactions with O₂, H₂O₂, Ti^3+/4+, and self-recombination in the decay of the α-hydroxyethyl radical were investigated and the bimolecular decay was shown to be the dominant decay pathway, with a decay rate constant of 6.6×¹⁰⁸ M^-1 s^-1. This study shows the effectiveness and capabilities of EPR as a direct, sensitive and in-situ method in kinetic studies.