Experimental and theoretical studies on aqueous-phase reactivity of hydroxyl radicals with multiple carboxylated and hydroxylated benzene compounds.

In this study, we shed light on the initial addition of hydroxyl radicals (HO˙) to multiple carboxylated and hydroxylated benzene compounds in aqueous-phase advanced oxidation processes (AOPs). We analyze the experimentally measured transient spectra near neutral pH using quantum mechanical-based time-dependent density functional theory (TD-DFT). The ab initio DFT method was first used to find and optimize aqueous-phase transition state structures, then the TD-DFT was used to analyze molecular orbitals (MOs) of the optimized transition state structures to reveal the functional groups that are responsible for the individual absorption peaks. The initial addition of HO˙ to the benzene ring produced hydroxycyclohexadienyl radicals. Then, HO-adducts are generated from dimerization or disproportionation of hydroxycyclohexadienyl radicals and represent their transient spectral peaks at approximately 350 nm and 250 nm. As reaction proceeds, the HO-adducts are decreased depending on the subsequent reactions. These investigations into the experimental transient spectra coupled with the theoretical analysis using the TD-DFT enable us to visualize an initial transformation of organic compounds induced by the aqueous phase HO˙ oxidation. Moreover, the experimental reaction rate constants and the theoretically calculated aqueous phase free energies of activation provide quantitative insights into the addition of HO˙ to multiple carboxylated and hydroxylated benzene compounds.