Aggregated ion pairs of [MIM+][N(CN)2−]2 ionic liquid: A quantum chemical study in solvents with different dielectric constants

The aggregated ion pairs of [MIM+][N(CN)2−] ionic liquid have been studied using MP2, B3LYP, M05-2X and M06-2X methods in conjunction with the 6-311++G(2d,2p) basis set. Five aggregated ion pairs (2-IP1-5) were characterized on the potential energy surface of interaction of two cations with two anions. The geometrical parameters, Gibbs free energy of formation in gas and solution phases (ΔG° and ΔGsln°) and solvation Gibbs free energy were calculated for aggregates of [MIM+][N(CN)2−]. The range of ΔG° was 3.3 to −10.4 kcal/mol at MP2/6-311++G(2d,2p) level of theory. The ΔG° value first significantly decreases with increasing solvent dielectric constant and then changes smoothly when the dielectric constant becomes greater than 20.0. The results of the temperature dependence of ΔG° revealed that the tendency for aggregation decreases as the temperature increases. Frequency analysis showed a significant red-shift in the N–H stretching vibrational wave number of 2-IP1 and 2-IP2 aggregates. Population analysis shows that the charge transfer (CT) taking place from anion N(CN)2− to cation MIM+ upon complex formation.