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A. Domínguez-Vidal, N. Kaun, M.J. Ayora-Cañada, B. Lendl:
"Probing Intermolecular Interactions in Water/Ionic Liquid Mixtures by Far-infrared Spectroscopy";
Journal of Physical Chemistry B, 111 (2007), 17; S. 4446 - 4452.

Far-infrared spectra in the range from 600 to 20 cm-1 of two hydrophilic (1-ethyl-3-methylimidazolium
tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate) and one hydrophobic (1-butyl-3-
methylimidazolium hexafluorophosphate) ionic liquids and their mixtures with water at different concentrations
are reported. Shifts of the librational water bands depending on the nature of the anion are found to be related
to the strength of the interaction between the water molecules and the anions. For both hydrophilic ionic
liquids, the librational band is centered around 460 cm-1, whereas for the hydrophobic ionic liquid, it is
shifted to 388 cm-1, indicating less hindered rotation of single water molecules. Multivariate curve resolution,
paying special attention to the spectral range from 50 to 350 cm-1, was used to investigate the presence of
different species with increasing water concentration. For both hydrophilic ionic liquids, a band located at
153 cm-1 was resolved into two different contributions. A small contribution at 202 cm-1 can be attributed
to intermolecular interactions between water molecules forming dimers. The major contribution (centered at
148 cm-1) corresponds to water molecules that do not bond to each other via H-bonding. It is therefore
assigned to a hindered translation arising from the stretching of the hydrogen bond between BF4
- anions and
water molecules. Formation of water dimers in the hydrophobic ionic liquid does not occur. Furthermore, the
spectral contribution of the stretching of H-bonds between water molecules and PF6
- cannot be unambiguously
detected, which indicates an extremely weak interaction between water molecules and this anion.

Projektleitung Bernhard Lendl:
Verstehen von Ionischen Flüssigkeiten als neue Lösungsmittel für die Grüne Chemie