Insights into the effects of alcohols on hydrated electron (e_aq⁻) generation from the p-benzoquinone/UV process

In this work, the effect of alcohols as hydrogen donors on hydrated electron (e_aq⁻) generation from p-benzoquinone (p-BQ) photolysis was investigated by quantum chemical calculations and experiments to further understand the underlying mechanisms associated within the p-BQ/UV process. Theoretically, the UV photolysis of p-BQ at 253.7 nm in the presence of H₂O could induce the formation of p-HOC₆H₄OH and hydroxy-p-benzoquinone, which was almost independent of the addition of CH₃OH. While, the first triplet state of hydroxy-p-benzoquinone preferred to react with CH₃OH (rather than H₂O) to release the corresponding semiquinone radical and hydroxymethyl radical ([rad]CH₂OH). These two radicals could induce the reduction of p-BQ to p-benzosemiquinone radical and thus enhanced the formation of p-HOC₆H₄OH as the precursor of e_aq⁻. Experimentally, the detection of e_aq⁻ generated in the process was accomplished by the degradation of monochloroacetic acid (MCAA) (the probe of e_aq⁻). With the addition of CH₃OH, the degradation of MCAA was accelerated in the p-BQ/UV process. A similar acceleration was also observed by the addition of ethanol or 2-propanol with the α-H. However, tert-butanol, which is without the α-H in the structure, didn't induce the acceleration. Since the quinone-like and alcoholic groups are widely distributed in the environment, these findings may improve the understanding of the photochemistry of quinones in the presence of alcohols with the α-H.