Conformational isomerism and phase transitions in tetraethylammonium bis(trifluoromethanesulfonyl)imide Et4NTFSI

Solid phase transitions are detected by DSC at 277 and 322 K in the tetraethylammonium bis(trifluoromethanesulfonyl)imide salt (Et4NTFSI) before the melting point at 377 K. Over this temperature range, the experimental Raman spectra of the solid and liquid phases of Et4NTFSI can satisfactorily be reproduced using the calculated frequencies and intensities of the cation in its all-trans (D2d) or trans–gauche (S4) conformation, and of the anion in its transoid (C2) or cisoid (C1) conformation. The Raman spectra of the low-temperature ordered phase III show that the cation and anion adopt the D2d and C2 conformations, respectively, predicted by DFT calculations to be of lower energy than the corresponding S4 and C1 ones. Above the first order phase transition near 277 K, ∼70% of the cations are transformed into the S4 conformation while the population of C2 anions decreases progressively (phase II). Another disordered phase seems to occur between 322 and 377 K where the cations and anions reach comparable populations of their two possible conformations. In this phase I, the cation D2d form is found to be more stable than the S4 form by 4.4±0.3 kJ mol−1 and the anion C2 form is more stable than the C1 form by 7±1 kJ mol−1. In the liquid phase, the cation keeps comparable populations of the D2d and S4 conformers, whereas ∼70% of the anions adopt the C1 conformation. Finally, by quenching the liquid or the disordered phases, a supercooled solid phase II' is easily obtained. It is composed of C2 anions and a mixture of ∼80/20 S4/D2d cations that corresponds roughly to the extrapolation of the phase II populations.