Interactions of Perfluoroalkyltrifluoroborate Anions with Cations: Effects of Perfluoroalkyl Chain Length on Motion of Ions in Ionic Liquids

摘要

The stabilization energies (E_(form)) for the formation of the perfluoroalkyltrifluoroborate complexes with Li~+ and l-ethyl-3-methylimidazolium cation (emim~+) were calculated at the MP2/6-311G** level. The order of the E_(from) calculated for the Li~+ complexes is Li[BF_4] Li[BF_3CF_3] > Li[BF_3C_2F_5] ~ Li[BF_3C_3F_7] ~ Li[BF_3C_4F_9]. The E_(from) calculated for the emim~+ complexes have the same order, while they are substantially smaller than those for the Li~+ complexes. The calculations suggest that the weaker interactions of the BF_3CF_3~- with emim~+ compared with the BF_4~- are the cause of the lower viscosity of the [emim][BF_3CF_3] ionic liquid compared with the [emim][BF_4] ionic liquid. The trend of the experimentally observed perfluoroalkyl chain length dependence of the self-diffusion coefficients of ions is well reproduced by molecular dynamics simulations. The correlation of the rotational relaxation of the emim~+ and the self-diffusion coefficients of the cation and anion suggests that the translational motion of ions in the ionic liquids is associated with the rotational motion of the emim~+.