Ionic-Equilibrium-Based Mechanism of ^•OH Conversion to Dichloride Radical Anion in Aqueous Acidic Solutions by...

A new mechanism for the dichloride radical anion (Cl₂^•-) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of ^•OH conversion into Cl₂^•- is determined by the equilibrium concentration of the ionic pair H₃O⁺·Cl^-. According to the proposed mechanism, the diffusional encounter of ^•OH and H₃O⁺·Cl^- is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 10¹² s^-1) to yield Cl^•, which then reacts with Cl^- to produce Cl₂^•-. The mechanism has been confirmed by the observed first-order growth of the Cl₂^•- absorption and a direct proportionality of the rate constant to the activities of H₃O⁺ and Cl^- ions. The salt effect on the rate of Cl₂^•- formation is due to the ionic strength effect on the equilibrium H₃O⁺ + Cl^- ⇄ H₃O⁺·Cl^-.