DFT calculations with the ωB97-D functional reproduce hydrogen bonding features of the far-infrared spectra of diethylmethylammonium methanesulfonate and diethylmethylammonium trifluoromethanesulfonate.

The thermal behaviour of selected ionic liquids, potentially useful as electrolyte components in electrochemical devices, formed by ammonium or imidazolium cations combined with bis(trifluoromethanesulfonyl)imide (C 2 F 6 NO 4 S 2 , TFSI) or bis(fluorosulfonyl)imide (F 2 NO 4 S 2 , FSI) anions, was investigated through thermogravimetric analysis (TGA). The liquid with the FSI anion decomposes at lower temperatures than those based on TFSI. Among ionic liquids with the TFSI anion, the stability in scanning mode with different cations decreases in the order 1-ethyl-3-methylimidazolium). Moreover, the vapour pressure of the six ionic liquids was obtained by means of long-lasting isothermal heating at temperatures between 150 and 300 °C. The highest pressure values were obtained in the order N1114 FSI > EMI TFSI > N122(2O1) TFSI > N1114 TFSI > N111(2O1) TFSI > N1113 TFSI.

Knowledge of all the intermolecular forces occurring in ionic liquids (ILs) is essential to master their properties. Aiming at investigating the weaker hydrogen bonding in aprotic liquids, the present work combined computational study and far-infrared spectroscopy on four imidazolium-based ILs with different anions. The DFT calculations of the ionic couples, using the wB97X-D functional and considering both the empirical dispersion corrections and the presence of a polar solvent, show that, for all samples, the lowest energy configurations of the ion pair present H atoms, directly bound to C atoms of the cation and close to O atoms of the anion, capable of creating moderate to weak hydrogen bonding with anions. For the liquids containing anions of higher bonding ability, the absorption curves generated from the calculated vibrational frequencies and intensities show absorption bands between 100 and 125 cm−1 corresponding to the stretching of the hydrogen bond. These indications are in complete agreement with the presently reported temperature dependence of the far-infrared spectrum, where the stretching modes of the hydrogen bonding are detected only for samples presenting a moderate interaction and become particularly prominent at low temperatures. Moreover, from the analysis of the infrared spectra, the occurrence of various phase transitions as a function of temperature was detected, and the difference in the average energy between the H-bonded and the dispersion-governed molecular configurations was evaluated.

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The intra-and intermolecular interactions in etherfunctionalized ionic liquids (ILs) are studied by means of infrared (IR) spectroscopy measurements of N-ethoxyethyl-Nmethylpiperidiniumbis(fluorosulfonyl)imide (P 1,2O2 -FSI) and Nethoxyethyl-N-methylmorpholiniumbis(fluorosulfonyl)imide (M 1,2O2 -FSI). The temperature dependence of the spectra in the medium IR range allows the study of the anion conformer distribution and its variation during phase transitions. In particular, it is found that for both ILs the trans conformer of FSI is more stable than the cis conformer, and the enthalpy differences between them are calculated and are found to decrease upon the addition of a Li salt. The results obtained in the far IR range, combined with ab initio calculation of the ionic couple performed using the B3LYP-D functional and considering both empirical dispersion corrections and the presence of a polar solvent, provide evidence for the occurrence of a hydrogen bonding between the O atom of the anion and its closest H atoms directly linked to a C atom of the cation. The comparison with samples having the same cations but with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion, that is, M 1,2O2 -TFSI and P 1,2O2 -TFSI, as well as with samples having cations without the ether-functionalization neither in the ring nor in the side chain, such as N-propyl-N-methylpyrrolidinium-FSI (PYR 13 -FSI) and 1-butyl-1-methylpyrrolidinium-TFSI (PYR 14 -TFSI), indicates that the occurrence of such highly directional interaction between anion and cation is better observable in the etherfunctionalized samples, in particular in those containing FSI as an anion.