Polyiodine and polyiodide species in an aqueous solution of iodine + KI: Theoretical and experimental studies

In the presence of KI, iodine crystals dissolve rapidly in an aqueous solution forming triiodide ions (I3^-) and other neutral species. The experimental evidence does not support the formation of polyiodide ions such as I₅^-, I₇^-, etc. in the solution. However, there is a strong evidence suggesting the formation of polyiodine species, I_2x where x = 2, 3, etc., stabilized by H⁺ ions in the solution. Ab initio results are presented for structures and energies of some of these species with an x value of up to 4. Each geometry optimization was done at the HF/LANL2DZ level followed by a single-point energy calculation at the MP2/LANL2DZ level. These calculations suggest that the isolated polyiodine species are not stable and, among the complexed species, the protonated clusters are most stable.