Free-Radical and Non-Free-Radical Based Reaction Pathways of Iodide Oxidation by Hydrogen Peroxide in Acid Solution–Ab Initio Calculations

Abstract

The oxidation of iodine ions (I^–) by hydrogen peroxide (H₂O₂) under acidic conditions was simulated using ab initio calculations. I₂ was formed in three radical-free reactions, where the initial HOI-generating step was identified as the rate limiting step with an energy barrier of 24.49 kcal/mol. The subsequent O₂-yielding decomposition of H₂O₂ with HOI was thermodynamically spontaneous, but kinetically slow due to a substantial energy barrier of 41.34 kcal/mol. In addition to small molecules, five common free radicals were also identified: HO^•, H₄O₂I^•, HOO^•, I^•, and ${\text{I}}_{2}^{ - \bullet}$. HO^• and H₄O₂I^• were produced by homolytic decomposition of the transient H₅O₃I with an energy barrier of 3.20 kcal/mol firstly. HO^• and HOO^• were also observed to participate in several reactions which yielded O₂. Overall, radical-mediated reaction had significantly lower energy barrier for O₂ generation than the non-free radical reaction pathways. In addition to the generation of O₂, other species such as H₂O₂, I₂, ${\text{I}}_{3}^{ - }$, and HOOI were also formed through the reactions between free radicals and small molecules.