Ab initio calculations were combined with infrared and Raman studies to distinguish spectroscopically the two conformers of the bis(trifluoromethanesulfonyl)imide anion, (TFSI − ). Spectra of crystalline LiTFSI complexes with organic ligands, where the anion adopts a known conformational state, are presented to confirm the calculated spectra. Several regions are identified where either the infrared or the Raman spectra contain separate bands for the two conformers. The conformational equilibrium between the transoid and cisoid rotamers is then illustrated from the infrared spectra of solutions of LiTFSI in aprotic solvents. The transoid form is found to be more stable than the cisoid form by about 2.2 kJ mol −1 , in good agreement with the present and earlier theoretical predictions. It is also shown that the IR and Raman spectral changes coming from conformational isomerism have to be carefully distinguished from those due to ionic interactions.

The vibrational properties of the TFSI- anion solvated in a polymer or in water have been studied by comparing its IR and Raman spectra with those of the HTFSI molecule. Ab initio self-consistent field Hartree−Fock calculations have also been performed on the free ion or molecule to investigate their Mulliken charges, equilibrium geometry, and internal force constants. Both experimental and theoretical approaches confirm a pronounced delocalization of the negative charge on the nitrogen and oxygen atoms and a marked double-bond character of the SNS moiety for the anion. This double-bond character is decreased for the HTFSI molecule, leading to rather distinct frequencies for some specific vibrations such as the stretching motions of the SO2 and SNS groups. The agreement between experimental and calculated spectra is much better for HTFSI than for TFSI-. Tentative explanations are proposed for this difference.

The solvation of the lithium ion in LiTFSI-doped ionic liquids based on alkyl-substituted imidazolium cations and bis(trifluoromethanesulfonyl)imide anions (TFSI-) was investigated by infrared and Raman spectroscopies. The spectral changes occurring for some TFSI- vibrations sensitive to the lithium coordination were analyzed with the help of DFT calculations. In addition, the vibrations of the lithium ion in its solvating cage were found to produce a broad IR absorption band centered at 374 cm(-1). For low to moderate LiTFSI mole fractions, 0.08 < x < 0.2, the [Li(TFSI)2]- solvating cage was found to involve bidentate coordinations of Li+ with two oxygen atoms of one anion in the trans (C2) conformation and two oxygen atoms of the other anion in the cis (C1) conformation. At higher LiTFSI concentration, up to x = 0.5, the lithium ion-TFSI coordination number progressively becomes less than 2, indicating the possible formation of aggregates.