Mechanism of radiation-induced reactions in aqueous solution of coumarin-3-carboxylic acid: Effects of concentration, gas and additive on fluorescent product yield

Abstract

The radiation-induced reactions of a water-soluble coumarin derivative, coumarin-3-carboxyl acid (C3CA), have been investigated in aqueous solutions by pulse radiolysis with a 35 MeV electron beam, final product analysis following 60Co γ-irradiations and deterministic model simulations. Pulse radiolysis revealed that C3CA reacted with both hydroxyl radicals (•OH) and hydrated electrons (e− aq) with near diffusion-controlled rate constants of 6.8 × 109 and 2.1 × 1010 M−1 s−1, respectively. The reactivity of C3CA towards O2• − was not confirmed by pulse radiolysis. Production of the fluorescent molecule, 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), was confirmed by final product analysis with a fluorescence spectrometer coupled to a high performance liquid chromatography (HPLC) system. Production yields of 7OH-C3CA following 60Co γ-irradiations depended on the irradiation conditions and ranged from 0.025 to 0.18 (100 eV) −1. Yield varied with saturating gas, additive and C3CA concentration, implying the presence of at least two pathways capable of providing 7OH-C3CA as a stable product following the scavenging reaction of C3CA with •OH, including a peroxidation/elimination sequence and a disproportionation pathway. A reaction mechanism for the two pathways was proposed and incorporated into a deterministic simulation, showing that the mechanism can explain experimentally measured 7OH-C3CA yields with a constant conversion factor of 4.7% from •OH scavenging to 7OH-C3CA production, unless t-BuOH was added.