Ionic-Equilibrium-Based Mechanism of •OH Conversion to Dichloride Radical Anion in Aqueous Acidic Solutions by Kinetic and Theoretical Studies

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–.