Spectroscopic and ab initio characterization of the conformational states of the bis(perfluoroethanesulfonyl)imide anion (BETI-)

Ab initio calculations were combined with infrared and Raman studies to spectroscopically distinguish the two conformers of the BETI- or bis(perfluoroethanesulfonyl)imide anion, [N(SO2C2F5)(2)](-), as was previously done for [N(SO2CF3)(2)](-), the TFSI- anion. BETI- is predicted to exist, as does TFSI-, in two conformational states of C-2 and C-1 symmetries, the former being more stable by about 6 kJ mol(-1). This conformational isomerism produces weak Raman splittings that can be resolved only at low temperatures. Thus, solutions of LiBETI with glymes cooled down to 113 K exhibit a very intense Raman doublet at similar to 745-740 cm(-1) characteristic of a quenched conformational equilibrium between the C-2 and C-1 conformers. Annealing of the (G3)(2):LiBETI solvate, where G3 is triglyme, leads to an ordered crystalline phase with all the anions in the C-2 conformation, as in the reference salt Me4NBETI. This conclusion cannot be extended to all the systems in which the BETI- anion interacts weakly with the cation, however, since the diglyme solvate, (G2)(2):LiBETI, contains both C-1 and C-2 anion conformers (in 2:1 ratio) at low temperatures independent of the sample's thermal history. The conformational splittings are larger in infrared, as illustrated by two absorption bands at 601 and 615 cm(-1) associated with the C-2 and C-1 anion conformers, respectively. It is possible to follow the relative intensities of these bands in a LiBETI solution with diglyme above room temperature up to 387 K. The C-2 conformer is found to be more stable than C, by 4.7 +/- 0.7 kJ mol(-1). Copyright (c) 2006 John Wiley & Sons, Ltd.