J. Phys. Chem. A 2010, 114, 685–686
Comment on “New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids”
685
SCHEME 1: Nomenclature for the Alkyl-Substituted Imidazolium Cation As Used in Reference 1
A. Wulf, K. Fumino, and R. Ludwig* Institut fu¨r Chemie, Physikalische und Theoretische Chemie, UniVersita¨t Rostock, 18051 Rostock, Germany ReceiVed: August 19, 2009 In a recent Letter, Lasse`gues et al. reported a new interpretation of the CH stretching vibrations in imidazolium-based ionic liquids.1 For weakly coordinating anions they could show that the C-H groups of the imidazolium ring interact by Fermi resonance with the overtones and the combination of two inplane ring vibrations R1 and R2 (see Scheme 1).1 The new assignment is based on isotopic substitution and anharmonic frequency calculations for the gas phase cations. As a result there is no need to invoke hydrogen bonding between the CH and the anions. This interpretation is in strong contradiction to the common view that the C-H bonds of the imidazolium ring are involved in hydrogen bonding and that the C(2)-H vibrational band is further red-shifted compared to that of C(4,5)-H caused by stronger interaction with the anion.2-4 In view of this controversy, we have critically reviewed the literature, scrutinized our earlier results, and carried out further experimental verifications, which we present here. Our data suggest that the new interpretation of the CH stretching vibrations is an overstatement. The Fermi resonances with the overtones and the combination of the two in-plane ring vibrations contribute to the spectra in this frequency range, but hydrogen bonding is essential for explaining not only IR but also NMR spectra of imidazolium-based ionic liquids. It was already pointed out by Seddon et al. that hydrogen bonding plays a crucial role in imidazolium-based ionic liquids.5 Local and directional hydrogen bonds between the ring C-H bonds and the oxygen of the [NTf2] anion were already reported from single crystal X-ray diffraction.6 As for water, it is a reliable assumption that most of the hydrogen bonds persist under phase transition from the solid into the liquid. For testing the statement by Lasse`gues et al. that the contributions below 3120 cm-1 are stemming from Fermi resonance with the overtones and the combination of R1 and R2, we measured the FTIR spectra of the imidazolium-based ionic liquids containing the same 1-ethyl-3-methylimidazolium [EMI] cation but varying anions thiocyanate [SCN]-, dicyanamide [N(CN)2]-, tricyanomethanide [C(CN)3]-, and tetracyanoborane [B(CN)4]-, respectively (see Scheme 2). The idea here is to keep the cation constant while increasing the volume and thus lowering the charge density of the anions. As expected, the ring vibrations R1 and R2 are nearly constant in shape and position between 1560 and 1580 cm-1 (Figure 1). The same is essentially true for the C-H contributions at about 3160 cm-1, which only differ by about 9 cm-1 with varying anions. Following the argument by Lasse`gues et al., we should expect similar bands caused by Fermi resonance between the overtones and combinations bands of the ring with that of the C-H band for all ionic liquids throughout. This is obviously not the case. Instead, we see a strong red shift of the C-H contributions in * Corresponding author. E-mail:
[email protected].
SCHEME 2: Nomenclature Used for the Anions Containing an Increasing Number of Cyano Groups
the range between 3040 and 3120 cm-1 in the order from [B(CN)4]- to [SCN]-. Additionally, these contributions are increasing on the debt of the C-H contributions between 3140 and 3180 cm-1. Smaller volumes and thus increasing charge density of the anions lead to stronger hydrogen bonds via C(2)-H, as suggested by the “old” interpretation.2-4 Lasse`gues et al. draw their conclusions from calculated gas phase structures only. And indeed, for the isolated cation there is no difference in wavenumber and intensity expected between C(2)-H and C(4/5)-H. But this becomes true in the liquid phase and already in larger calculated aggregates. We recently presented B3LYP/6-31+G* calculated clusters for [CnMIM][SCN]x with n ) 1, 2, 4, 6 and x ) 1-4.7 In Figure 2 it is shown that most of the calculated C(4,5)-H frequencies can be found between 3120 and 3200 cm-1, whereas those of C(2)-H are clearly red-shifted and given between 3020 and 3120 cm-1. This behavior is reflected in the measured IR spectrum. In conclusion, the cluster calculations clearly indicate that local and directional hydrogen bonds are present in imidazolium-based ionic liquids and that the H-bonds via C(2)-H are particularly preferred. And, finally, we emphasize that other spectroscopic methods also support the traditional view. From NMR it is well-known that the proton chemical shift of C(2)-H is further downfield shifted than those of C(4/5)-H caused by stronger hydrogen bonding.8 In [EMIM][SCN] the C(2)-H proton chemical shift is found at 9.291 ppm, those of C(4,5)-H at 7.910 and 7.996 ppm, respectively (Figure 3).7 The stronger downfield proton chemical shift for C(2)-H is related to the red shift of the
10.1021/jp9080146 2010 American Chemical Society Published on Web 12/16/2009
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J. Phys. Chem. A, Vol. 114, No. 1, 2010
Comments
Figure 3. Relation between calculated vibrational frequencies C-H and calculated chemical shifts of the ring protons in [CnMIM][SCN]x clusters (with n ) 1, 2, 4, 6) of different size (x ) 1-3). If we plug in the accurately measured proton chemical shifts (vertical dashed lines), we can predict vibrational frequencies (triangles down) that can be assigned to C(4,5)-H and C(2)-H contributions, respectively.
Figure 1. IR spectra of the four indicated ILs in the region of the imidazolium C-H stretching vibrations (left) and R1, R2 ring vibrations (right).
Figure 2. B3LYP/6-31+G* calculated imidazolium C-H stretch frequencies in [CnMIM][SCN]x clusters (with n ) 1, 2, 4, 6) of different size (x ) 1-3). The intensities are chosen arbitrarily because the calculated values can differ by more than a factor of 100 and cannot be visualized in one plot. For comparison the measured spectrum of [EMIM][SCN] is shown. It is seen that the calculated C(2)-H vibrational bands are significantly red-shifted compared to the C(4,5)-H contributions as interpreted for the measured spectrum.
corresponding IR vibrational band depending on the interaction strength between cation and anion. Recently we showed that IR and NMR properties of ionic liquids tell us the same thing.7 Vibrational frequencies and NMR chemical shifts react in comparable manner to changes in the chemical environment. For [EMIM][SCN] we obtained a relation between C-H stretching frequencies and the C-H proton chemical shifts derived from calculated properties on IL aggregates (νCH ) A - B (δ 1H)3 with A ) 3258.3 cm-1 and B ) 0.227 893 cm-1 ppm-3) (Figure 3). Taking the accurately measured proton chemical shifts, we could predict average frequencies for C(4,5)-H and C(2)-H stretching frequencies at 3148 and 3082
cm-1, respectively. That is in the spectral range that can be expected for the C(4,5)-H and C(2)-H stretches from the “old” of interpretation. This view is further supported by far-infrared measurements of ionic liquids providing a direct measure of H-bonds.4 By switching on and off the H-bond ability via C(2)-H, we could show that local and directional H bonds formed between cations and anions lower the charge symmetry. H-bonds introduce “defects” into the Coulomb network and fluidize ionic liquids, resulting in decreased melting points and reduced viscosities. It could be further shown by DFT and NBO calculations that the anion-cation interactions are described by characteristic ratios between Coulomb forces and hydrogen bonds.9 These ratios can be tuned toward increasing hydrogen bond contributions, which is reflected in important macroscopic properties of ionic liquids such as enthalpies of vaporization and viscosities. In a more recent study, the enhanced anion-cation interaction due to hydrogen bonding is demonstrated.10 By excluding reduced mass effects for the specific anion-cation combinations, the characteristic frequency shifts could be referred to the number and strength of H-bond abilities. In conclusion, all these data suggest that not only IR but also NMR spectra cannot be explained by excluding local and directional hydrogen bonding in imidazolium-based ionic liquids in particular via C(2)-H. However, it is highly desirable to separate the vibrational contributions stemming either from hydrogen bonding or from Fermi resonance with the overtones and combinations bands. References and Notes (1) Lasse`gues, J.-C.; Gronding, J.; Cavagnat, D.; Johansson, P. J. Phys. Chem. A 2009, 113, 6419. (2) Yokoczki, A; Kasprzak, D. J.; Shiflett, M. B. Phys. Chem. Chem. Phys. 2007, 36, 5018. (3) Ko¨ddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. ChemPhysChem 2006, 7, 1994. (4) Fumino, K.; Wulf, A.; Ludwig, R. Angew. Chem., Int. Ed. 2008, 47, 3830. 8731. (5) Aakero¨y, C. B.; Seddon, K. R. Chem. Soc. ReV. 1993, 82, 397. (6) Holbrey, J.; Reichert, W. M.; Rogers, R. D. Dalton Trans. 2004, 2267. (7) Fumino, K.; Wulf, A.; Ludwig, R. ChemPhysChem. 2007, 8, 2265. (8) Tubbs, J. D.; Hoffmann, M. M. J. Solution Chem. 2004, 33, 381. (9) Fumino, K.; Wulf, A.; Ludwig, R. Phys. Chem. Chem. Phys. 2009, 11, 8790. (10) Fumino, K.; Wulf, A.; Ludwig, R. Angew. Chem., Int. Ed., in press.
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