DOI: 10.1002/cphc.200900680 Combined THz, FIR and Raman Spectroscopy Studies of Imidazolium-Based Ionic Liquids Covering the Frequency Range 2–300 cm 1 Alexander Wulf, [a] Koichi Fumino, [a] Ralf Ludwig,* [a, b] and Philip F. Taday [c] The ubiquitous properties of ionic liquids (ILs) are governed by the type and strength of interaction between its constitu- ents. [1–6] Intermolecular forces between cation and anion deter- mine whether we find expanded network structures, larger clusters or ion-pairs in the liquid phase of this new liquid mate- rial. Studying and understanding these interactions is a real challenge in particular for ionic liquids. Here, the strength of interaction is given by a well balanced combination of Cou- lomb forces, local and directional hydrogen bonds as well as dispersion forces. We could show recently that tuning these ratios towards stronger Coulomb or hydrogen bonding contri- butions enables to tune the favoured properties of ionic liq- uids. [7, 8] In particular it was demonstrated that local and direc- tional hydrogen bonds can disrupt the symmetry of the Cou- lomb system resulting in fluidized ionic liquids. Obviously, the type and the strength of intermolecular forces are crucial for the understanding of ionic liquid properties. Thus it is highly desirable to have access to the frequency range typically de- scribing this kind of forces. In general, intermolecular interactions can be studied by all experimental methods covering the suitable frequency range between 2 and 300 cm 1 . That includes optical heterodyne-de- tected Raman-induced Kerr-effect spectroscopy, [9–16] dielectric relaxation spectroscopy (DRS), [16, 17] terahertz (THz) spectrosco- py, [16, 18–20] low-energy neutron scattering, [21] X-ray diffraction, [22] far infrared spectroscopy as well as Raman spectroscopy. [23–39] Beside the structural information some of these methods also provide dynamical properties. However, the observed spectra are usually difficult to describe and deconvoluted bands cannot be assigned clearly to specific vibrational motions with- out support by theoretical methods. Overall, all these methods have their strong points but also their shortcomings for inves- tigating this spectral region. Thus it is the purpose of this work to combine THz, far infrared (FIR) and Raman spectroscopy for studying in detail the low frequency range of imidazolium- based ionic liquids. The FIR spectra have been published earli- er, but are presented again for a reliable comparison with the results obtained from the other spectroscopic methods. [7] The interpretation of the experimental spectra will be supported by ab initio calculations on differently sized clusters of these compounds. In particular we can show that the spectroscopic methods are differently adequate to investigate intra- and in- termolecular forces in ionic liquids. Our THz spectra clearly in- dicate that no significant vibrational contribution occurs below 50 cm 1 . Recently, we presented the low frequency vibrational spectra of imidazolium-based ionic liquids in the range between 30 and 300 cm 1 obtained by far infrared spectroscopy. [7] We could show that the wavenumbers above 150 cm 1 can be as- signed to intramolecular bending and wagging modes of cat- ions and anions in the ionic liquid. The contributions below 150 cm 1 were attributed to the intermolecular interactions be- tween cations and anions describing the bending and stretch- ing vibrational modes of hydrogen bonds. This assignment was supported by DFT calculations giving wavenumbers for the bending and stretching modes of ion-pairs and ion-pair ag- gregates in this frequency region. Further proof for having de- tected the intermolecular interactions was coming from a nearly linear relation between the average binding energies of calculated IL aggregates and the measured wavenumbers for maxima of the low-frequency vibrational bands for a series of ionic liquids containing the same imidazolium cation but dif- ferent anions. However, uncertainties remained as to whether all intramolecular vibrational modes above 120 cm 1 are attrib- uted correctly because of too small IR intensities. Additionally the intermolecular vibrational modes below 30 cm 1 were not accessible by our FIR spectrometer. For that purpose we mea- sured additional Raman spectra as well as THz spectra of the same imidazolium ionic liquids using the same charges. Com- bining THz, FIR and Raman spectroscopy we could finally cover the complete frequency range between 2 and 300 cm 1 . These spectroscopic methods are differently suitable for detecting in- termolecular and intramolecular vibrational modes. The given combination allows the assignment of all contributions occur- ing in this frequency range. Herein we measured the THz and Raman spectra of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethyl- sulfonyl)imide ([C 2 mim][NTf 2 ]), 1-ethyl-3-methylimidazolium ethylsulfate ([C 2 mim][EtSO 4 ]), 1-ethyl-3-methyl imidazolium di- cyanamide ([C 2 mim][N(CN) 2 ]), and 1-ethyl-3-methylimidazolium thiocyanate ([C 2 mim][SCN]), respectively. The basic idea here is to have the same cation C 2 mim + in all ionic liquids through- out. Thus we should find similar contributions arising from the cations and different contributions stemming from the varying anions of the particular ionic liquid. The measured spectra are [a] A. Wulf, Dr. K. Fumino, Prof. Dr. R. Ludwig Institut für Chemie Abteilung Physikalische Chemie, Universität Rostock Dr.-Lorenz-Weg 1, 18059 Rostock (Germany) Fax: (+ 49) 381-498-6524 E-mail : ralf.ludwig@uni-rostock.de [b] Prof. Dr. R. Ludwig Leibniz-Institut für Katalyse an der Universität Rostock Albert-Einstein-Str. 29a, 18059 Rostock (Germany) [c] Dr. P. F. Taday TeraView Ltd, Platinium Building St John’s Innovation park, Cambridge (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900680. ChemPhysChem 2010, 11, 349 – 353 # 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 349