Thermodynamic and Kinetic Insights into Single Electron Transfer Pathway of Reactive Halogen Species: Is the Reduction Potential a Reliable Indicator for Radical Reactivity?

Abstract

Single electron transfer (SET) is the most important reaction mechanism of reactive halogen species (RHS) (e.g., Cl^•, Cl₂^•–, Br^•, and Br₂^•–) with aromatic contaminants. However, the key parameters governing the reactivity of RHS via the SET pathway remain unclear. This study comprehensively investigated the SET pathway of the RHS with 51 aromatic contaminants by quantum-chemical calculations. The calculated free energy changes of the SET pathway (ΔG_aq,SET^R) followed the order Cl^• < Cl₂^•– < Br^• < Br₂^•–, which was consistent with the order of their reduction potentials. However, the free energy of activation of the SET pathway (ΔG_aq,SET^⧧) followed a different order of Cl^• < Br^• < Cl₂^•– < Br₂^•–, which agreed with the order of their rate constants. This inconsistent trend between ΔG_aq,SET^R and ΔG_aq,SET^⧧ for RHS reactions was attributed to the difference in reorganization energy, which is essentially related to the vertical electron affinity and radius of RHS. Therefore, the rate constants of SET reactions of RHS depend not only on their reduction potential but also on their vertical electron affinity and radius. These findings unravel the key factors determining the reactivity of RHS via the SET pathway and have profound implications for understanding other SET processes in water.