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

D. functional theory methods in combination with vibrational spectroscopy are used to investigate possible variants of mol. structure of the ion pairs of several imidazolium-based ionic liqs. (ILs). Multiple stable structures are detd. 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 liq. 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 assocn. 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 sepd. and to regard the couple of the ions as an anharmonic oscillator. Such a model correlates the mol. structure of various ILs and their m.ps. without involving the energy of the interaction between the cations and anions but explains structure-m.p. correlations on the grounds of quasy-elastic properties.