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.