Complex Structural and Dynamical Interplay of Cyano-Based Ionic Liquids.

Abstract

We carried out ab initio molecular dynamics simulations for the three cyano-based ionic liquids, 1-ethyl-3-methylimidazolium tetracyanoborate ([C2C1Im][B(CN)4]), 1-ethyl-3-methyl-imidazolium dicyanamide ([C2C1Im][N(CN)2]), and 1-ethyl-3-methylimidazolium thiocyanate ([C2C1Im][SCN]). We found that the [SCN]-based ionic liquid is much more prone to π-π stacking interactions as opposed to the other two ionic liquids, contrary to the fact that all liquids bear the same cation. Hydrogen bonding is strong in the dicyanamide- and the thiocyanate-based ionic liquids and it is almost absent in the tetracyanoborate liquid. The anion prefers to stay on-top of the imidazolium ring with the highest priority for the [N(CN)2](-) anion followed by the [B(CN)4](-) anion. We find that experimental viscosity trends cannot be correlated to the hydrogen bond dynamics which is fastest for [B(CN)4](-) followed by [SCN](-) and [N(CN)2](-). For the dynamics of the cation on-top of itself, we find the order of [B(CN)4](-) followed by [N(CN)2](-) and finally by [SCN](-). Interestingly, this trend correlates well with the viscosity, suggesting a relation between the cation-cation dynamics and the viscosity at least for these cyano-based ionic liquids. These findings, especially the apparent correlation between cation-cation dynamics and the viscosity, might be useful for the suggestion of better ionic liquids in electrolyte applications.