The Complex Dance of the Two Conformers of Bis(trifluoromethanesulfonyl)imide as a Function of Pressure and Temperature

Abstract

Absorbance spectra of two ionic liquids, the short alkyl chain N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) and the longer chain N-trimethyl-N-hexylammonium bis(trifluoromethanesulfonyl)imide (TMHA-TFSI) are reported as a function of pressure and temperature. The occurrence of various phase transitions is evidenced by the changes in the relative concentration of the cisoid and transoid conformers of their common TFSI anion. The infrared spectrum of TMPA-TFSI was measured at 300 K with an applied pressure varying over the 0-5 GPa range. Above 0.2 GPa only the trans conformer is detected, suggesting the occurrence of a pressure induced crystallization. When pressure is applied to TMHA-TFSI at T = 310 K, both TFSI conformers subsist up to ∼11 GPa. However, the clear change of their intensity ratio observed around 2 GPa, suggests the onset of a glass phase as supported by measurements carried out at 4.2 GPa along a cooling/heating cycle. A careful analysis of the spectra collected along different p-T thermodynamic paths shows the occurrence of a cold crystallization at 295 K on heating from 139 K along the p = 0.5 GPa isobar. The rich phase diagrams of the two ionic liquids is the result of the competition among the anion-cation intermolecular interactions, the lower energy of trans-TFSI with respect to cis-TFSI and the smaller volume of cis-TFSI with respect to trans-TFSI.

Pressurizing the mixtures of two ionic liquids: Crystallization versus vetrification

“…After pressurization to 1 GPa, one can note the absence of the spectral line that, at room pressure, is centered around 327 cm −1 , due to cis ‐TFSI . This finding is consistent with infrared spectroscopy measurements performed on TMPA‐TFSI, which indicated that between 0.1 and 0.2 GPa, the system crystallizes into a structure containing trans ‐TFSI only …”

“…We want to remark that the present Raman measurements are collected with the DAC gasket hole completely filled by the sample, whereas in the case of the infrared spectroscopy measurements reported in Capitani et al, part of the gasket hole was filled with KBr, in order to avoid the saturation of the bands recorded in transmission mode. In the present study, we wanted to study the pure ILs and mixtures, without the addition of other compounds.…”

“…A combined study conducted by means of X‐ray diffraction and Raman spectroscopy on [bmim][PF 6 ] as a function of pressure and temperature evidenced a complex phase diagram with at least three crystalline phases, with different conformers distribution, structural arrangements, and disorder. Also TMHA‐TFSI displayed a rich phase diagram as a function of p and T , with at least two different crystalline phases and a large region with a glass phase …”

“…As to this last compound, it has been also reported that upon compression at room temperature, it undergoes a glass transition around 2 GPa. Conversely, a similar compound with a shorter alkyl chain, namely, N‐trimethyl‐N‐propylammonium bis(trifluoromethylsulfonyl)imide (TMPA‐TFSI), undergoes crystallization both on cooling below ~255 K at ambient pressure and upon compression above 0.2 GPa at ambient temperature . Both crystalline phases of TMPA‐TFSI retain only the trans conformer of TFSI.…”

New Experimental Evidences Regarding Conformational Equilibrium in Ammonium−Bis(trifluoromethanesulfonyl)imide Ionic Liquids

“…[17] However, in the intermediate concentration range no crystalline state develops, but only a glass phase. [19,20] In fact, TMPA-TFSI crystallizes at room temperature as soon as a pressure of 0.2 GPa is applied, retaining the trans conformer of TFSI. [17,18] The behavior of the two ILs and of their mixtures is even more complex when a pressure is applied.…”