Conformations and Vibrational Assignments of the (Fluorosulfonyl)(trifluoromethanesulfonyl)imide Anion in Ionic Liquids

Investigations of the (fluorosulfonyl)­(trifluoromethanesulfonyl)­imide (FTFSI) anion, incorporated in various ionic liquids, by means of density functional theory (DFT) methods and differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Raman techniques are reported in this work. Theoretical studies using DFT methods (B3LYP/6-31G**) show that there are three likely anion geometries (syn, gauche, and anti) separated by less than 3 kJ·mol^–1. The energy barrier to conversion between the anti and syn/gauche conformers is between 10 and 14 kJ·mol^–1 and lower than 10 kJ·mol^–1 for rotations around the SNSF and SNSC dihedral angles, respectively. The FTFSI anion has a characteristic vibration at 730 cm^–1 assigned to the expansion and contraction of the entire anion that is sensitive to ionic interactions with metal cations. DSC, XRD, and Raman studies indicate that an alkali metal salt containing the FTFSI anion, KFTFSI, exists in two crystalline forms. Form II converts to form I via a solid–solid phase transition at 96.9 °C. The FTFSI expansion–contraction mode at 745 cm^–1 in KFTFSI form I shifts to 741 cm^–1 in form II. It can be hypothesized that this shift is due to the presence of different anion geometries or varying ionic interactions in the two crystalline forms