Density Functional Theory Descriptors for Ionic Liquids and the Charge-Transfer Interpretation of the Haven Ratio

One of the few properties
common to all ionic liquids is their
inherent electrical conductivity, which makes them promising candidates
for advanced electrochemical applications. A central finding in this
respect is that the measured conductivity is almost always lower than
the one obtained from the Nernst–Einstein relation. There has
been much dispute about whether correlated motion, charge transfer,
or some sort of aggregation is the reason for this difference. In
this work, we apply density functional theory-based descriptors to
estimate the charge transfer in ionic liquids, which allow predictions
for a large number of systems with minimal effort. The theoretical
charge transfer was obtained from vertical ionization potentials and
electron affinities at the RB3LYP/6-311+G­(2d,p)//RB3LYP/6-31+G­(d,p)
level of theory. To be able to compare and classify the values obtained
with this approach, another measure for charge transfer, available
directly from the Nernst–Einstein relation, is introduced.
The two quantities show significant correlation for some subsets of
ionic liquids for which a sufficient amount of information is available.
Additionally, the purely theoretical charge-transfer values allow
for identifying interesting systems that should be the subject of
further investigation.