Hydrated electron (eaq−) generation from p-benzoquinone/UV: Combined experimental and theoretical study

Abstract

A p-benzoquinone (p-BQ)/UV process to induce hydrated electron (eaq−) generation was predicted by quantum chemical calculations and validated by experiment in this work. Theoretically, the photolysis of p-BQ under UV irradiation at 253.7nm could induce water to generate eaq− with a molar ratio of 1:2 via the direct triplet mechanism, in which 1,4-addition reaction of the first triplet state of p-BQ with water was the key step. Experimentally, monochloroacetic acid (MCAA) (the probe of eaq−) was used to detect eaq− generated in the p-BQ/UV process. The generation efficiency showed a positive linear dependence on the p-BQ concentration, which illustrated the crucial role of p-BQ on the generation of eaq−. During the photolysis, p-hydroquinone was the primary intermediate for the generation of eaq−. Kinetically, the energy barriers of the eaq− generation from p-HOC6H4OH, p-HOC6H4O− and p-−OC6H4O− were 100.8kcalmol−1, 46.5kcalmol−1 and 5.6kcalmol−1, respectively. Both the experimental and theoretical results show that the generation of eaq− was much more efficient from the anions than that from p-HOC6H4OH. The findings in the present study may help to understand the mechanism of eaq− generation from natural organic matters (NOM), since quinone-like groups are usually contained in NOM.