Combined THz, FIR and Raman Spectroscopy Studies of Imidazolium‐Based Ionic Liquids Covering the Frequency Range 2–300 cm −1
2009; Wiley; Volume: 11; Issue: 2 Linguagem: Inglês
10.1002/cphc.200900680
ISSN1439-7641
AutoresAlexander Wulf, Koichi Fumino, Ralf Ludwig, Philip F. Taday,
Tópico(s)Acoustic Wave Resonator Technologies
ResumoCombination of spectroscopic methods: THz, FIR and Raman spectroscopy (see figure) allow the assignment of all intra and inter molecular vibrational modes in imidazolium-based ionic liquids in the low frequency range between 2 and 300 cm−1. Although THz spectroscopy is very sensitive for this frequency range, clearly no significant contribution could be detected below 50 cm−1. The ubiquitous properties of ionic liquids (ILs) are governed by the type and strength of interaction between its constituents.1–6 Intermolecular forces between cation and anion determine whether we find expanded network structures, larger clusters or ion-pairs in the liquid phase of this new liquid material. 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 Coulomb 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 contributions enables to tune the favoured properties of ionic liquids.7, 8 In particular it was demonstrated that local and directional hydrogen bonds can disrupt the symmetry of the Coulomb 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 describing 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-detected Raman-induced Kerr-effect spectroscopy,9–16 dielectric relaxation spectroscopy (DRS),16, 17 terahertz (THz) spectroscopy,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 without support by theoretical methods. Overall, all these methods have their strong points but also their shortcomings for investigating 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 earlier, 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 intermolecular forces in ionic liquids. Our THz spectra clearly indicate 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 assigned to intramolecular bending and wagging modes of cations and anions in the ionic liquid. The contributions below 150 cm−1 were attributed to the intermolecular interactions between cations and anions describing the bending and stretching 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 aggregates in this frequency region. Further proof for having detected 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 different anions. However, uncertainties remained as to whether all intramolecular vibrational modes above 120 cm−1 are attributed 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 measured additional Raman spectra as well as THz spectra of the same imidazolium ionic liquids using the same charges. Combining 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 intermolecular and intramolecular vibrational modes. The given combination allows the assignment of all contributions occuring in this frequency range. Herein we measured the THz and Raman spectra of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-ethyl-3-methylimidazolium ethylsulfate ([C2mim][EtSO4]), 1-ethyl-3-methyl imidazolium dicyanamide ([C2mim][N(CN)2]), and 1-ethyl-3-methylimidazolium thiocyanate ([C2mim][SCN]), respectively. The basic idea here is to have the same cation C2mim+ in all ionic liquids throughout. 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 shown in Figures 1–2, 3 along with the earlier measured FIR spectra of the same ionic liquids. The low frequency ranges covered by our experimental set ups of the different spectroscopic methods are the following: 2 and 100 cm−1 for THz spectroscopy, 30 and 300 cm−1 for FIR spectroscopy and 70 to 300 cm−1 for Raman spectroscopy. Thus overall we are able to study the spectra between 2 and 300 cm−1 corresponding to 0.06 and 9 THz, respectively. First we briefly review the discussion for the recently measured FIR spectra of the imidazolium-based ionic liquids as shown in Figure 2.7 In the light of these results we then discuss the new Raman and THz spectra. Terahertz (THz) spectra of imidazolium-based ionic liquids [C2mim][SCN], [C2mim][N(CN)2], [C2mim][EtSO4], and [C2mim][NTf2]. Far infrared (FIR) spectra of imidazolium-based ionic liquids [C2mim][SCN], [C2mim][N(CN)2], [C2mim][EtSO4], and [C2mim][NTf2].7 Raman spectra of imidazolium-based ionic liquids [C2mim][SCN], [C2mim][N(CN)2], [C2mim][EtSO4], and [C2mim][NTf2]. For the ionic liquid [C2mim][N(CN)2] a detailed interpretation of the FIR spectrum was given. Figure 4 shows the measured spectrum deconvoluted into Voigt functions along with the ab initio calculated low frequencies of the ([C2mim][N(CN)2])x clusters with x=2,4,6,8. The main features of the measured spectrum are reproduced by the calculated and deconvoluted vibrational bands. The band at about 250 cm−1 can be assigned to the out-of-plane bending mode of the CH3(N) methyl group in the imidazolium cation (C2mim+). This contribution is therefore shown up in the measured spectra of all ionic liquids as indicated in Figure 4. The intense vibrational band at 170 cm−1 can be referred to the intramolecular bending mode of the N(CN)2− anion. Consequently, this vibrational contribution is missing in all the other ionic liquid spectra. The most interesting bands occur below 150 cm−1. The calculated frequencies of differently sized IL clusters suggest that the main intensity at about 120 cm−1 can be clearly attributed to the stretching modes of the hydrogen bonds +C-H..A−, where CH can be either C(2)H or C(4/5)H. The vibrational bands about 50–60 cm−1 are mainly stemming from the corresponding bending modes of these hydrogen bonds and are weaker in intensity. The stretching mode of the hydrogen bond describes the particular cation–anion interaction and will be discussed later related to the spectra of the other ionic liquids.7, 8 The FIR spectra of the other ionic liquids we assigned as follows. The vibrational band at about 250 cm−1 represents the out-of-plane bending mode of the CH3(N) methyl group in the imidazolium cation (C2mim+) of each ionic liquid throughout. Whereas for [C2mim][N(CN)2] the vibrational band at 170 cm−1 can be assigned to the bending mode of the anion, this band is consequently missing for all the other ionic liquids. Obviously, for [C2mim][SCN] and [C2mim][EtSO4] no significant intramolecular vibrational modes of the anions can be detected in the FIR spectra. Instead, for the ionic liquid [C2mim][NTf2] additional bands occur between 150 and 250 cm−1 which can be referred to intramolecular vibrations of the complex anion for example, the double peak slightly above 200 cm−1 represents the wagging modes of O=S=O groups in NTf2−. Measured low-frequency FTIR spectrum of [C2mim][N(CN)2] deconvoluted into four main vibrational bands which can be assigned to the bending mode of the cation–anion bend (δCH…︁A), the cation–anion stretch (νCH…︁A), the anion bend (δNCNCN) and the cation bend (δCH3(N)), respectively. Additionally the measured spectrum is compared to ab initio calculated vibrational modes of corresponding IL clusters [C2mim][N(CN)2]x with x=2, 4, 6, 8. It can be seen that the major vibrational bands are reflected by the calculated frequencies which are corrected for the harmonic approximation.7 The shapes and intensities of the measured spectra below 150 cm−1 show significant differences. These contributions could be assigned to the stretching and bending vibrational bands of the hydrogen bonds +CH⋅⋅⋅A−. Obviously the interaction between the cation and anion is of significantly different strength. The maxima of these bands shift to lower wavenumbers in the order SCN−, N(CN)2−, EtSO4− and NTf2−. Such a trend suggests a decrease in interaction energy following this series. Increasing strength of a hydrogen bond +CH⋅⋅⋅A− results in lengthened covalent bonds CH and shortened hydrogen bonds +CH⋅⋅⋅A−. The weaker force constants for the CH bonds lead to lower wavenumbers and thus redshifted vibrational bands. This was shown for the CH stretch vibrational region of imidazolium-based ionic liquids. It could be demonstrated that in the above given order of anions the intramolecular frequencies CH are shifted to the red.36, 37 The opposite behaviour is expected for the stretching modes of hydrogen bonds. Increasing hydrogen bond strength results in shorter bond distances and larger force constants. The stronger the hydrogen bond, the larger the wavenumber and the corresponding intensity of the vibrational band. This is shown in the FIR spectra. The wavenumbers and absorption strength, both decrease in the order SCN−, N(CN)2−, EtSO4− and NTf2−. The maxima occur at 117.6 cm−1, 113.5 cm−1, 106.4 cm−1 and 83.5 cm−1, respectively. In the foregoing paper we also pointed out that the resulting wavenumbers for the vibrational modes are determined by the force constants as well as by the reduced masses via , where c is the speed of light, k the force constant and μ the reduced mass.7, 8 However the ab initio calculations clearly showed that the shifts to lower wavenumbers are mainly given by decreasing force constants and only to minor contributions by increasing reduced masses. Although the masses of the anions in the ionic liquids [C2mim][SCN] and [C2mim][NTf2] differ significantly, the reduced masses contributing to the low vibrational modes are close. Whereas the SCN− moves completely, NTf2− is only partly involved in the vibrational motion resulting in reduced mass of comparable size. Here we should point out that the spectra obtained from OKE or DRS in this low frequency range are usually fitted by a set of Brownian and Gaussian oscillators,16, 17 as well as α- and β-relaxation processes. For example, Turton et al.16 fitted a Cole–Cole mode, two Brownian oscillators and a Gaussian oscillator for the IL [C2mim][N(CN)2] in the frequency range we explicitly refer to intermolecular interactions. These oscillators account for librations (rocking or hindered rotations) as well as short-range intermolecular vibrations. It is reported that in general the low frequency range can be satisfactorily fitted by the Brownian (damped-harmonic) oscillator. In some cases Gaussian oscillators provide a better fit by implying inhomogeneity of the modes. Here we want to emphasize that although different methods and procedures are used to fit the low-frequency spectra, the interpretation of the spectra do not have to be necessarily in contradiction. In both cases intermolecular interactions are the dominant contributions, no matter whether they are described as hindered translational motions or damped-harmonic oscillations. Reduced Raman spectra of imidazolium-based ionic liquids [C2mim][SCN], [C2mim][N(CN)2], [C2mim][EtSO4], and [C2mim][NTf2]. The characteristic contributions of the cations are indicated by bars. Unfortunately, the Raman spectra could not be really used for the discussion of the intermolecular interactions. Firstly, our experimental setup does not allow reliable measurements below 100 cm−1 and secondly, the corrections may modify unrealistically the spectra in this region. However it has been shown by Hamaguchi that Raman spectra can be also obtained properly for this frequency range.39 He measured the cation dependence of imidazolium-based ionic liquids [Cnmim][PF6] (with n=4,6,8) but could not find a significant dependence on the alkyl chain lengths of the cation. From our interpretation of the FIR spectra this is not really surprising because the wavenumbers and intensities are mainly determined by the strength of interaction between cation and anion. Overall it can be concluded that the Raman spectra nicely show the intramolecular contributions of cations and anions. Usually those are better pronounced than in the FIR spectra. The intermolecular vibrational frequency range can be probably best explored by THz spectroscopy.16, 18–20 Yamamoto et al. measured the terahertz complex dielectric spectra of imidazolium-based ionic liquids by THz time-domain spectroscopy (THz–TDS) in the frequency range from 5 to 140 cm−1 corresponding to 0.15 to 4.2 THz, respectively.20 They studied [Cnmim][TfO] and [Cnmim][BF4] with n=2,4 and found differences between the two types of liquid in the THz dielectric shapes. They could show that all the intramolecular vibrations of the anions are located above 140 cm−1. Yamamoto et al. concluded that the interion vibrations rather than the intramolecular vibrations dominantly contribute to the THz dielectric spectra. Although we used other ionic liquids including different anions this statement is in agreement with our interpretation of the FIR and Raman spectra. Our THz spectra were measured with a commercial instrument. The transmission spectra between 2 and 100 cm−1 are shown in Figure 1. The noise level becomes larger above 80 cm−1. To demonstrate that the spectra below this wavenumber can be reproduced nicely, the spectra were measured three times and are all given in the figure. These spectra have been in particular measured because the FIR spectra only barely cover the frequency range down to 30 cm−1. It has been argued in the literature that significant vibrational contributions may occur between 2 and 50 cm−1. However, our THz spectra clearly show that this is not the case. Because any pattern in the spectra can be better seen, we additionally plotted the frequencies on a logarithmic scale (see Supporting Information). Up to 4 cm−1, we obtained flat curves indicating no vibrational contribution throughout for all ionic liquids. Starting at 4 cm−1 the intensities of the spectra continuously increase and can be finally fit to the given FIR spectra in Figure 1. Thus the intensities measured by THz spectroscopy in this frequency range stem from vibrational contributions at higher wavenumbers already seen by FIR spectroscopy. That THz spectroscopy is generally sensitive for ionic liquids in this frequency range is shown by the measured spectrum of [C2mim][NTf2]. It shows a maximum at about 80 cm−1 in nice agreement to the deconvoluted contribution of the measured FIR spectra given at 83.5 cm−1 (see Figure 2) Overall, the THz spectra draw two interesting conclusions. Firstly, as expected, they show the same features as the FIR spectra in the shared frequency range between 30 and 100 cm−1. And secondly, the THZ spectra give no further vibrational bands below 30 cm−1. At least the latter statement is true for the imidazolium-based ionic liquids in their liquid state used in this study. Here, we present a combined THz, FIR and Raman spectroscopy of imidazolium-based ionic liquids covering the full frequency range between 2 and 300 cm−1 corresponding to 0.06 to 9 THz. It could be shown that THz spectroscopy nicely covers the frequency range between 2 and 100 cm−1, that FIR is most sensitive for the cation–anion interaction between 50 and 120 cm−1 and that Raman spectroscopy better gives the intramolecular vibrational modes of cations and anions between 120 and 300 cm−1. This combination of spectroscopic methods allows an assignment of all intra- and intermolecular vibrational modes in the low frequency range. In particular we could show by THz spectroscopy that there are no contributions below 50 cm−1 at least for imidazolium-based ionic liquids in their liquid state. All findings could be supported by ab initio calculations on ionic liquids clusters of different size. Understanding the strength of interactions between cations and anions now allows studying relations with other ionic liquids properties such as enthalpies of vaporization which strongly depend on these forces. The studied ionic liquids were purchased from Iolitec GmbH (Denzlingen, Germany) with a stated purity of>98 %. All substances were dried in vacuum (p=8 10−3 mbar) for approx. 36 h. The water content was then determined by Karl-Fischer titration and was found to be 336 ppm in 1-ethyl-3-metyhlimidazolium ethylsulfate ([C2mim][EtSO4]), 228 ppm in 1-ethyl-3-methyl imidazolium dicyanamide ([C2mim][N(CN)2]), 220 ppm in 1-ethyl-3-methylimidazolium thiocyanate ([C2mim][SCN]) and 113 ppm in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) respectively. Further purification was not carried out. The THz spectroscopy measurements were made using a TeraView TPS spectra 3000 transmission spectrometer. For all measurements the sample compartment of the spectrometer was purged with dry nitrogen at 10 litres per minute. Transmission terahertz spectra were collected from 2 cm−1 to 120 cm−1 at a resolution of 2 cm−1. Each rapid-scan spectrum, the average of 18000 co-added scans, took 60 s to record. Three spectra of each sample were recorded. A liquids cell consisting of z-cut quartz windows and a 300 μm spacer was used for all THz measurements. The FTIR measurements were performed with a Bruker Vertex 70 FTIR spectrometer. The instrument is equipped with an extension for measurements in the far infrared. This equipment consists of a multilayer mylar beam splitter, a room temperature DLATGS detector with preamplifier and polyethylene (PE) windows for the internal optical path. The accessible spectral region for this configuration lies between 30 and 680 cm−1. The Raman measurements were carried out with a Bruker RAM II FT-Raman module coupled to the optical bench of a Bruker VERTEX 70 FTIR spectrometer. A Nd:YAG laser operating at 1064 nm wavelength with a maximum power output of 1500 mW served as excitation source. The Stokes part of the Raman scattered radiation was detected by a liquid-nitrogen-cooled Ge diode. The spectral resolution was 2 cm−1. The appropriate liquid cell consisting of quartz glass SUPRASIL had a path length of 10 mm and was provided by Hellma. The sample temperature was maintained by an external Haake DC 30/K 20 bath chiller and recorded with a NiCrNi thermocouple attached directly to the cell. Ab initio calculations have been performed at the Hartree–Fock level with the Gaussian 98 program.41 using the internal stored 3–21G basis set. The basis set superposition error (BSSE) corrected binding energies and average binding energies per ion of [C2mim][N(CN)2], [C2mim][SCN], [C2mim][EtSO4], and [C2mim][NTf2] cluster comprising six ion-pairs are given in an earlier paper.7 The vibrational frequencies for the aggregates ([C2mim][N(CN)2])x with x=2,4,6,8 were corrected by the standard factor 0.89. The cluster frequencies could not be calculated on higher ab initio levels than RHF/3–21G due to computational limitations. This work was supported by the German Science Foundation (DFG) via the priority programme SPP 1191 and additional support from the Sonderforschungsbereich SFB 652. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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