On the Mechanism of the Iodide–Triiodide Exchange Reaction in a Solid-State Ionic Liquid

Efficient charge transport has been observed in iodide-based room-temperature ionic liquids when doped with iodine. To investigate preferred pathways for the iodide (I<sup>–</sup>)-to-triiodide (I<sub>3</sub><sup>–</sup>) exchange reaction and to clarify the origin of this high ionic conductivity, we have conducted electronic structure calculations in the crystal state of 1-butyl-3-methylimidazolium iodide ([BMIM]­[I]). Energy barriers for the different stages of the iodine-swapping process, including the reorientation of the I<sup>–</sup>···I<sub>3</sub><sup>–</sup> moiety, were determined from minimum energy paths as a function of a reaction coordinate. Hirshfeld charges and structural parameters, such as bond lengths and angles, were monitored during the reaction. Several bond-exchange events were observed with energy barriers ranging from 0.17 to 0.48 eV and coinciding with the formation of a twisted I<sup>–</sup>···I<sub>3</sub><sup>–</sup> complex. Striking similarities were observed in the mechanics and energetics of this charge-transfer process in relation to solid-state superionic conductors.