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

The cation coordination of the bis(trifluoromethanesulfone)imide (TFSI) ani on (CF3SO2)(2)N- has been studied using the molecular electrostatic potenti al (MESP) topography as a tool. The critical points of MESP are employed fo r searching the cation binding sites of the anion. Subsequent ab initio Har tree-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 fro m each of the different SO2 groups on the central nitrogen, with the anion having C-2 point group symmetry. Li+TFSI- conformers with the cation in coo rdination (a) with oxygen and nitrogen atoms of the anion or (b) with the t wo 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 degrees-132 degrees, which agrees well with the X-ray data on th e 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 separati on of 7-60 cm(-1) in the ion pair.