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

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.