The impact of anion electronic structure: similarities and differences in imidazolium based ionic liquids

In this paper the structural and energetic landscapes of ion-pair dimer conformers of 1,3-dimethylimidazolium based ionic liquids have been explored ([C1C1im][A])2, A = Cl(-), [NO3](-), [MeSO4](-), [OTf](-) and [BF4](-)). A common low-energy conformer has been selected for full electronic structure analysis. We have compared and contrasted each cluster based on the relative hydrogen bonding ability (β-value) of the anion, which varies experimentally as Cl(-) > [NO3](-) ≈ [MeSO4](-) > [OTf](-) ≈ [BF4](-). Correlations between experimental β-values, computed binding energies, charge transfer and various hydrogen bonding data have been made and outliers have been explained in terms of environmental effects present in the liquid phase. This is most evident in the structurally similar [MeSO4](-) and [OTf](-) anions that have very similar hydrogen bonding motifs, but significantly different β-values. Moreover, detailed analysis of the cluster molecular orbitals, for each anion, reveals a subtle interplay between two modes of interaction, an in-plane traditional H-bonding and inter-planar anion-π interaction. Inter-planar anion-π interactions are particularly prominent for the [NO3](-) cluster. We have rationalized how the full range of interactions could impact on the structuring of ILs at surfaces and the effect these may have on viscosity.