Application of Density Functional Theory and Vibrational Spectroscopy Toward the Rational Design of Ionic Liquids

Abstract

Density functional theory methods in combination with vibrational spectroscopy are used to investigate possible variants of molecular structure of the ion pairs of several imidazolium-based ionic liquids (ILs). Multiple stable structures are determined with the anion positioned (a) near to the C2 atom of the imidazolium ring, (b) between N1 and C5, (c) between N3 and C4, and (d) between C4 and C5. Chloride and bromide anions in vacuum also occupy positions above or below the imidazolium ring, but in the condensed state these positions are destabilized. In comparison with the halides that almost equally occupy the positions (a-d), tetrafluoroborate and hexafluorophosphate anions strongly prefer position (a). The position and the type of the anion influence the conformation of the side chains bound to the imidazolium N1 atom, which are able to adopt in vacuum all usual staggered or eclipsed conformations, although in the liquid state some of the conformations are present only as minor forms if at all. Vibrations of the cations depend both on the conformational changes and on the association with the anion. The formation of the ion pairs influences mainly stretching and out-of-plane vibrations of the imidazolium C-H groups and stretching vibrations of the perfluoroanions. Other modes of the ions retain their individuality and practically do not mix. This allows "interionic" vibrations to be separated and to regard the couple of the ions as an anharmonic oscillator. Such a model correlates the molecular structure of various ILs and their melting points without involving the energy of the interaction between the cations and anions but explains structure-melting point correlations on the grounds of quasy-elastic properties.

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

“…As an example, the observed melting point change for ionic liquids with different anions was explained in terms of the change in the interaction energy and the relative position depending on the anion. 14,40 Many calculation studies found that the relative position of the anion with respect to the cation could change with the anion type 9,[14][15][16]20,21 and, in case of imidazolium-based ionic liquid, proposed that the interaction between the anion and H-C(2) is crucial in determining this structure. 15,18,19,40,41 The recent simulated IR spectrum in the 2800-3200 cm -1 range was shown to change with anion position.…”

“…Previous reports also suggested the BF 4 -and PF 6 -anions are located out of the imidazolium ring plane. 9,15,20,24 More definitely, any specific interaction or proximity with the imidazolium ring proton is unlikely, as that would be reflected as a peak shift or change in , the iodine anion should be near H-C(2) to promote deuterium exchange. We further suggest it would lie close to the plane made by the imidazolium ring, such that I -‚‚‚H-C(2) forms a near-linear hydrogen bond.…”

“…26 Figure 2 shows the ATR-IR spectra of [ 4 ] have been studied, and the peaks were assigned in previous report. 9 For [BMIM][I], although the IR spectrum has not been reported as yet, the Raman spectroscopic study of [BMIM][Cl] could be consulted for peak assignment. 23 Apparently, the overall absorption peaks are bigger for [BMIM] [I].…”

“…4,16,[18][19][20][21][22][23] By contrast, there have been several simulation studies and experiments which proposed that the difference in the relative position is very small for different anion types. 9,15,24 Thus, even for prototypical imidazolium ionic liquid, this issue is still unresolved and awaits further investigation, especially as the relative position between the cation and the anion is one of the most important structural information.…”

Interaction of Water Vapor with the Surfaces of Imidazolium-Based Ionic Liquid Nanoparticles and Thin Films

“…71−81 Spectral bands in the region from 3200 to 3000 cm −1 are attributed to the coupled aromatic ring C−H stretching vibrations of the imidazolium hydrogen atoms, while those between 3000 and 2850 cm −1 can be assigned to the aliphatic C−H stretches of the CH 2 and CH 3 groups of the ethyl chain and of the CH 3 group bonded directly to the [C 2 MIM + ] ring. [71][72][73][74][75][76][77][78]81 4 ] thin films to water vapor in the sealed ATR cell at different levels of RH at 295 K. After each RH adjustment, spectra were collected until the band intensities stopped changing to ensure that equilibrium was attained. It typically took ∼20 min for the system to equilibrate.…”