Magnitude and directionality of interaction in ion pairs of ionic liquids: Relationship with ionic conductivity

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)(2)N- similar to 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-trimetliylamnioniuni ((C2H5)(CH3)(3)N), 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 similar to (C2H5)(CH3)(3)N > 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 C-2-H of imidazolium is not essential for the attraction in the ion pair. The interaction energy of the BF4- complex with I-ethyl,2,3-dimethylimidazolium (em2im) (-81.8 kcal/mol) is only 4% smaller than that of the emim complex.