Conformational evolution of TFSI⁻ in protic and aprotic ionic liquids

Abstract

We here report on the conformational evolution of the bis(trifluoromethanesulfonyl)imide anion (TFSI − ) in protic and aprotic TFSI − -based ionic liquids as a function of temperature. The investigation is performed by Raman spectroscopy in the spectral ranges 240-380 cm −1 and 715-765 cm −1 , where the interference from bands due to the cations is negligible. The contribution from each TFSI − conformation, i.e. the cisoid (C 1 ) and the transoid (C 2 ), is quantified in order to estimate the enthalpy of conformational change, H, which is found to be in the range 3.4-7.3 kJ/mol in the liquid state. Conformational information is for the first time determined from the 740 cm −1 band, which previously mainly has been used as an indicator of ion-ion interactions. The similarity in H values obtained from the two spectral ranges demonstrates the validity of using also the 740 cm −1 band for the quantification of the TFSI conformational evolution.

A Study of the Conformers of the (Nonafluorobutanesulfonyl)imide Ion by Means of Infrared Spectroscopy and Density Functional Theory (DFT) Calculations

“…The ratio of the conformers' concentration, r, in a liquid is proportional to the ratio of the intensities of the spectral bands attributable to specific conformers [12]. In the present case, we can distinguish a band at 616 cm −1 due to the low energy tt IM14 and a band at 626 cm −1 which is attributable to the higher energy tg IM14 conformer.…”

“…where Ix designates the integrated IR (Infrared) intensity of the band centered at wavenumber x [12]. The following equation holds for the equilibrium constant, K, between conformers.…”

Graft copolymer electrolytes for high temperature Li-battery applications, using poly(methyl methacrylate) grafted poly(ethylene glycol)methyl ether methacrylate and lithium bis(trifluoromethanesulfonimide)

“…Furthermore, Fujii et al [20] obtained a similar broad peak for the EMITFSI IL system, and concluded it to consist of contributions due to both cisoid (C 1 ) and transoid (C 2 ) TFSI conformers. Thus the TFSI conformation in LiTFSI (only C 2 ) is altered into an equilibrium between these two conformers-as has also been suggested in many previous studies of TFSI in many other matrices [21,22] supported by the small energy differences between the conformers [21,23] and the low energy barriers for the needed dihedral angle changes [24]. The severe peak overlap in the spectral regions useful for quantifying the relative amounts of the TFSI conformers, however, hinders any further analysis.…”

A Raman spectroscopic approach to investigate the production of biodiesel from soybean oil using 1-alkyl-3-methylimidazolium ionic liquids with intermediate chain length

“…In any case, the observed shift is rather small (below 1 cm À1 ) indicating that the change in interaction strength is marginal. For the TFSI anion we also observe an increased population of the cis (C 1 ) conformations with respect to the trans (C 2 ) [36], Fig. 9A.…”

“…It can be appreciated that despite the presence of several compounds in solution, the spectra are of sufficient good quality. The spectral region 100-800 cm À1 is dominated by the signatures of the ionic liquid, in particular the vibrational modes of the TFSI anion [36]. In the range 800-1000 cm À1 the modes sensitive to the alkyl conformations are found [37], however greatly overlapping with the features assigned to the enzyme [38].…”