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.