Interactions of perfluoroalkyltrifluoroborate anions with li ion and imidazolium cation: Effects of perfluoroalkyl chain on motion of ions in ionic liquids

The stabilization energies (E_form) 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 E_form values calculated for the Li[BF₄], Li[BF₃CF₃], Li[BF₃C ₂F₅], Li[BF₃C₃F₇], and Li[BF₃C₄F₉] complexes were -144.1, -139.3, -137.4, -136.3, and -135.4 kcal/mol, respectively. The E_form values calculated for the [emim][BF₄], [emim][BF₃CF₃], [emim][BF₃C₂F₅], [emim][BF₃C ₃F₇], 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, whereas 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^- because of the weaker electrostatic interactions. The analysis of the interactions suggests that the weaker interactions between the BF₃CF ₃^- and emim⁺ compared with those 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 self-diffusion coefficients of the cations and anions in the ionic liquids is BF₄^- < BF₃CF₃^- ∼ BF₃C₂F₅^- > BF₃C ₃F₇^- > BF₃C₄F ₉^-, 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⁺.