The infrared spectra of protic ionic liquids: performances of different computational models to predict hydrogen bonds and conformer evolution

Abstract

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

Temperature and Pressure Dependence of the Infrared Spectrum of 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate Ionic Liquid

“…These results are in good agreement with previous investigations on the same ions [32]. Recently, it has been shown that a good agreement between the calculated and the experimental infrared spectrum of the protic ionic liquid DEMA-TfO can be obtained by exploiting DFT calculations on the ionic couple with the ωB97X-D functional, including empirical dispersion corrections and the presence of a polar solvent (dimethylformamide, εr = 37.22) [49]. Here, we want to investigate whether this better agreement is visible also in the case of the presently investigated non-protic IL.…”

“…The calculated angle formed by the C2, H2 and O1 atoms is 136.6 • , a value slightly lower than the experimental value of 149 • , which, however, was reported for the solid phase of EMI-TfO [51]. It must be noted that the most evident change induced by considering the presence of a solvent in the DFT calculations is enlargement of the distance between anion and cation in similar ionic couples containing TfO [49], and, indeed, this approach allows to correctly evaluate the H2-O1 distance.…”

“…2020, 10, x FOR PEER REVIEW 4 of 11 Figure 2 reports the infrared absorption spectrum of EMI-TfO measured on cooling from 330 K down to 120 K and subsequent heating from 120 K up to 330 K, in the frequency range between 500 and 1800 cm −1 . In all spectra, the most intense lines centered around 518, 574, 638, 1031, 1167, 1226, 1266 cm −1 are known to be due to the vibrations of theTfO anions [49]. The less intense lines between 680 and 1000 cm −1 , as well as the absorption bands above 1300 cm −1 , are due to the vibration modes of the EMI cation [32].…”

“…For the ionic couple EMI-TfO, the ωB97X-D functional, considering both the empirical dispersion corrections and the presence of a polar solvent (dimethylformamide, ε r = 37.22), was used. Indeed, this last combination of functional, dispersion forces and polar solvent has recently provided a good description of the absorbance spectra of diethyl methyl ammonium methanesulfonate (DEMA-MS) and diethyl methyl ammonium trifluoromethanesulfonate (DEMA-TfO) [49]. The introduction of the polar solvents represents a further improvement compared to previous similar DFT calculations [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50].…”

“…On further increasing the temperature, around 280 K all absorption bands return to the shape they had in the liquid phase at room temperature before the thermal cycle. The splitting of absorption bands or the abrupt disappearance or appearance of spectral lines has been largely attributed to the occurrence of solid-liquid or solid-solid transformations [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]49]. In this framework, the overall behavior as a function of temperature of EMI-TfO can be attributed to the occurrence of crystallization on cooling between 220 and 200 K and the occurrence of a solid-solid phase transition on heating around 260 K and finally to melting around 280 K. Indeed, this phase changes are in agreement with differential scanning calorimetry measurements reported in Ref.…”