Ab Initio structure and vibrational frequencies of (CF₃SO₂)₂N^-Li⁺ ion pairs

Abstract

The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF₃SO₂)₂N^- has been studied using the molecular electrostatic potential (MESP) topography as a tool. The critical points of MESP are employed for searching the cation binding sites of the anion. Subsequent ab initio Hartree-Fock calculations using the 6-31G(d) basis for the Li⁺TFSI^- ion pair engenders seven local minima on the potential energy surface of the complex. In the lowest energy conformer, Li⁺ coordinates with two oxygens, one from each of the different SO₂ groups on the central nitrogen, with the anion having C₂ point group symmetry. Li⁺TFSI^- conformers with the cation in coordination (a) with oxygen and nitrogen atoms of the anion or (b) with the two oxygens of the same SO₂ group as well as (c) with the oxygen and one of the fluorines at the CF₃ end, typically 48-114 kJ mol^-1 higher in energy than the lowest minimum, have also been obtained. The SNS bond angle in the free anion as well as that in the Li⁺TFSI^- ion pair turns out to be in the range 126°-132°, which agrees well with the X-ray data on the crystal hydrate structure for the HTFSI. A comparison of the vibrational spectra of the free TFSI anion and the Li⁺TFSI^- ion pair conformers reveals that the S-N stretching vibration at 815 cm-1 can be used as a probe to distinguish the different ion pair conformers. A near doublet at the 703 cm^-1 in the C₂ symmetry of the TFSI anion on coordination shows a separation of 7-60 cm^-1 in the ion pair.