Spectroscopic and ab initio characterization of the conformational states of the bis(perfluoro‐ ethanesulfonyl)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(SO 2 C 2 F 5 ) 2 ] − , as was previously done for [N(SO 2 CF 3 ) 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 ∼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 Me 4 NBETI. 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 1 by 4.7 ± 0.7 kJ mol −1 .