Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy
has continued use as a powerful tool to
characterize ionic liquids since the literature on room temperature
molten salts experienced the rapid increase in number of publications
in the 1990’s. In the past years, infrared (IR) and Raman spectroscopies
have provided insights on ionic interactions and the resulting liquid
structure in ionic liquids. A large body of information is now available
concerning vibrational spectra of ionic liquids made of many different
combinations of anions and cations, but reviews on this literature
are scarce. This review is an attempt at filling this gap. Some basic
care needed while recording IR or Raman spectra of ionic liquids is
explained. We have reviewed the conceptual basis of theoretical frameworks
which have been used to interpret vibrational spectra of ionic liquids,
helping the reader to distinguish the scope of application of different
methods of calculation. Vibrational frequencies observed in IR and
Raman spectra of ionic liquids based on different anions and cations
are discussed and eventual disagreements between different sources
are critically reviewed. The aim is that the reader can use this information
while assigning vibrational spectra of an ionic liquid containing
another particular combination of anions and cations. Different applications
of IR and Raman spectroscopies are given for both pure ionic liquids
and solutions. Further issues addressed in this review are the intermolecular
vibrations that are more directly probed by the low-frequency range
of IR and Raman spectra and the applications of vibrational spectroscopy
in studying phase transitions of ionic liquids.