The structure of imidazolium-based ionic liquids: Insights from ion-pair interactions

A large number of ab-initio (B3LYP/6-31++G(d, p)) computed ion-pair structures have been examined in order to determine if such calculations are capable of offering insight into the physical properties of the liquid state, particularly viscosity and melting point. Ion pairings based around the 1-butyl-3-methylimidazolium (C(4)C(1)im) cations and a range of anions (Cl, BF4, and N(Tf)(2) where N(Tf)(2) is bis(trifluoromethylsulfonly)imide) were chosen because of the range of viscosities exhibited by the corresponding ionic liquids. We have used these results to build up a 'picture' of the ionic liquid structure which is consistent with molecular dynamics simulations and experimental evidence. However, further work is required to established if such an analysis could be predictive. Nevertheless, we establish clear relationships relating ion-pair association energy, a derived 'connectivity index', and the diversity of structures with viscosity and melting point. Our calculations indicate that ions in C(4)C(1)imCl form a strong, highly connected and regular array thus rationalizing the high viscosity and melting point. In contrast the ion-pairs of C(4)C(1)imN(Tf)(2) form a weakly interacting, highly disordered, and low connectivity network consistent with the low viscosity and melting point. C(4)C(1)imBF(4) lies midway between these two extremes.