Interactions of Perfluoroalkyltrifluoroborate Anions with Li Ion and Imidazolium Cation: Effects of Perfluoroalkyl Chain on Motion of Ions in Ionic Liquids

Seiji Tsuzuki, Tatsuya Umecky, Hajime Matsumoto, Wataru Shinoda, Masuhiro Mikami

J. Phys. Chem. B 114, 11390-11396 (2010).

The stabilization energies (Eform) for the formation of the perfluoroalkyltrifluoroborate complexes with Li+ and 1-ethyl-3-methylimidazolium cation (emim+) were calculated at the MP2/6-311G** level. The Eform calculated for the Li[BF4], Li[BF3CF3], Li[BF3C2F5], Li[BF3C3F7] and Li[BF3C4F9] complexes were -144.1, -139.3, -137.4, -136.3 and -135.4 kcal/mol, respectively. The Eform calculated for the [emim][BF4], [emim][BF3CF3], [emim][BF3C2F5], [emim][BF3C3F7] and [emim][BF3C4F9] complexes were -85.2, -81.2, -79.7, -79.7 and -79.2 kcal/mol, respectively. The electrostatic interactions are the major source of the attraction in the complexes, while the contribution of the induction interactions to the attraction is not negligible. The interactions of the perfluoroalkyltrifluoroborate anions with Li+ and emim+ are substantially weaker than those of the BF4- due to the weaker electrostatic interactions. The analysis of the interactions suggests that the weaker interactions between the BF3CF3- and emim+ compared with that between the BF4- and emim+ are the cause of the lower viscosity of the [emim][BF3CF3] ionic liquid compared with the [emim][BF4] ionic liquid. The order of experimental selfdiffusion coefficients of the cations and anions in the ionic liquids is BF4- < BF3CF3- ~ BF3C2F5- > BF3C3F7- > BF3C4F9-, which is well reproduced by the molecular dynamic simulations. The analysis of the rotational relaxation of emim+ suggests that the translational diffusion of cations and anions is associated with the rotational diffusion of emim+.