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

Abstract

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

Mechanisms of Iodide–Triiodide Exchange Reactions in Ionic Liquids: A Reactive Molecular-Dynamics Exploration

“…However, in the solid phase, the rigidity of the crystal lattice imposes certain restrictions on the angle of approach/departure of the entering/leaving I − , which forces the reacting I 3 − to rotate before the bond-exchange process. The reorientation of I 3 − in the solid phase imposes an energy penalty, as discussed in detail in [24], which is absent in the isotropic liquid phase.…”

“…As in ref. [24], a system of 32 [BMIM][I]– ion pairs was established as a 2 × 2 × 2 crystal-state supercell; adding an extra I 2 molecule adjacent to an iodide (I−) anion, forming a triiodide ion (I3−), led to an electroneutral orthorhombic simulation box comprising 834 atoms. Under PBC, respective x -, y -, z - box dimensions were 16.561, 21.577 and 23.997 Å (coinciding with a -, b -, c - crystallographic directions), consistent with ambient pressure [26].…”

“…A rebalance function then ensures overall charge neutrality by dispersing any excess negative charge throughout the remaining I − anions of the system in a uniform way (i.e., regardless of position). Although heuristic, this scheme captures essential details in a reassuring manner: for reorientations [24], the central atom in a triiodide ion retains a charge of around −13e, and reaches ~−13e upon alternating chemical “personality” during a bond-exchange event from the outer ion in contact with the approaching I− ion (with charge of ~−12e) [24]. Should the highly improbable event arise of the next-nearest iodine atom is outside of the 8 Å cut-off, r 2 in Equation 1 is set at 8 Å.…”

“…Recently, Density Functional Theory (DFT) has proven an important tool in investigating I−/I3−-charge-transfer mechanisms in RTILs, both for gas-phase cation-anion clusters [23] and in the extended solid state under periodic boundary conditions (PBC) [24], primarily from nudged-elastic-band-type calculations as a function of simple reaction coordinates. Thapa and Park determined limiting bond-exchange potential-energy barriers of ~0.8–1 eV for 1-ethyl-3methylimidazolium (EMI) clusters with iodine ions, depending on inter-ion separations [23]; Grossi et al found a limiting barrier of ~0.5 eV in solid-state 1-butyl-3-methylimidazolium iodide ([BMIM][I]) with added iodine [24].…”

“…Recently, Density Functional Theory (DFT) has proven an important tool in investigating I−/I3−-charge-transfer mechanisms in RTILs, both for gas-phase cation-anion clusters [23] and in the extended solid state under periodic boundary conditions (PBC) [24], primarily from nudged-elastic-band-type calculations as a function of simple reaction coordinates. Thapa and Park determined limiting bond-exchange potential-energy barriers of ~0.8–1 eV for 1-ethyl-3methylimidazolium (EMI) clusters with iodine ions, depending on inter-ion separations [23]; Grossi et al found a limiting barrier of ~0.5 eV in solid-state 1-butyl-3-methylimidazolium iodide ([BMIM][I]) with added iodine [24]. Although DFT acts as both a sensible and a computationally tractable initial foray into elucidating charge transport in disordered, liquid or glassy, iodide-based RTIL, fully deterministic (unbiased), long-time molecular dynamics (MD) of iodine-“doped” RTILs becomes a necessary, even essential, tool in the quest for sampling mass-transfer-limited bond-exchange reactions, where temperature-dependent diffusion limits intrinsic I−/I3−-charge transfer, sampled over many RTIL configurations and phase space.…”