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–)-to-triiodide (I3–) 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–···I3– 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–···I3– complex. Striking similarities were observed in the
mechanics and energetics of this charge-transfer process in relation
to solid-state superionic conductors.