Magnitude and Directionality of Interaction in Ion Pairs of Ionic Liquids: Relationship with Ionic Conductivity

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Abstract
The intermolecular interaction energies of nine ion pairs of room temperature ionic liquids were studied by MP2/6-311G** level ab initio calculations. The magnitude of the interaction energies of 1-ethyl-3-methylimidazolium (emim) complexes follows the trend CF3CO2- > BF4- > CF3SO3- > (CF3SO2)2N- ∼ PF6- (−89.8, −85.2, −82.6, −78.8, and −78.4 kcal/mol, respectively). The interaction energies of BF4- complexes with emim, ethylpyridinium (epy), N-ethyl-N,N,N-trimethylammonium ((C2H5)(CH3)3N), and N-ethyl-N-methylpyrrolidinium (empro) are not very different (−85.2, −82.8, −84.6, and −84.4 kcal/mol, respectively), while the size of the orientation dependence of the interaction energies follows the trend emim > epy ∼ (C2H5)(CH3)3N > empro. Comparison with the experimental ionic conductivities shows that the magnitude and directionality of the interaction energy of the ion pairs play a crucial role in determining the ionic dissociation/association dynamics in the ionic liquids. The electrostatic interaction is the major source of attraction between ions. The induction contribution is small but not negligible. The hydrogen bonding with the C2−H of imidazolium is not essential for the attraction in the ion pair. The interaction energy of the BF4- complex with 1-ethyl-2,3-dimethylimidazolium (em2im) (−81.8 kcal/mol) is only 4% smaller than that of the emim complex.