Intermolecular interactions in Li⁺-glyme and Li ⁺-glyme-TFSA^- complexes: Relationship with physicochemical properties of [Li(glyme)][TFSA] ionic liquids

The stabilization energies (ΔE_form) calculated for the formation of the Li⁺ complexes with mono-, di- tri- and tetra-glyme (G1, G2, G3 and G4) at the MP2/6-311G* level were -61.0, -79.5, -95.6 and -107.7 kcal mol^-1, respectively. The electrostatic and induction interactions are the major sources of the attraction in the complexes. Although the ΔE_form increases by the increase of the number of the O⋯Li contact, the ΔE_form per oxygen atom decreases. The negative charge on the oxygen atom that has contact with the Li⁺ weakens the attractive electrostatic and induction interactions of other oxygen atoms with the Li⁺. The binding energies calculated for the [Li(glyme)]⁺ complexes with TFSA^- anion (glyme=G1, G2, G3, and G4) were -106.5, -93.7, -82.8, and -70.0 kcal mol^-1, respectively. The binding energies for the complexes are significantly smaller than that for the Li⁺ with the TFSA^- anion. The binding energy decreases by the increase of the glyme chain length. The weak attraction between the [Li(glyme)]⁺ complex (glyme=G3 and G4) and TFSA ^- anion is one of the causes of the fast diffusion of the [Li(glyme)]⁺ complex in the mixture of the glyme and the Li salt in spite of the large size of the [Li(glyme)]⁺ complex. The HOMO energy level of glyme in the [Li(glyme)]⁺ complex is significantly lower than that of isolated glyme, which shows that the interaction of the Li ⁺ with the oxygen atoms of glyme increases the oxidative stability of the glyme. The interactions in Li⁺-glymes-TFSA^- complexes: The interactions of Li⁺ with glymes (tri- and tetra-glyme) are strong (-96 and -108 kcal mol^-1), while the interactions of the [Li(glyme)]⁺ complexes with TFSA^- (-82 and -70 kcal mol^-1) are weaker than that between Li⁺ and TFSA ^- (-137 kcal mol^-1).