Ab Initio Structure and Vibrational Frequencies of (CF₃SO₂)₂N⁻Li⁺ Ion Pairs

The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) anion (CF 3 SO 2 ) 2 N - 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 SO 2 groups on the central nitrogen, with the anion having C 2 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 SO 2 group as well as (c) with the oxygen and one of the fluorines at the CF 3 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 C 2 symmetry of the TFSI anion on coordination shows a separation of 7−60 cm -1 in the ion pair.