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Effects of Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1‑Methyl-3octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect Spectroscopy Hideaki Shirota,*,† Shohei Kakinuma,† Yu Itoyama,‡ Tatsuya Umecky,‡ and Toshiyuki Takamuku*,‡ †

Department of Nanomaterial Science and Department of Chemistry, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan

J. Phys. Chem. B 2016.120:513-526. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/14/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: The microscopic aspects of the two series of mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4])−benzene and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide ([MOIm][NTf2])−benzene were investigated by several spectroscopic techniques such as attenuated total reflectance IR (ATR-IR), NMR, and fs-Raman-induced Kerr effect spectroscopy (fs-RIKES). All three different spectroscopic results indicate that the anions more strongly interact with the cations in the [MOIm][BF4]−benzene mixtures than in the [MOIm][NTf2]−benzene mixtures. This also explains the different miscibility features between the two mixture systems well. The xC6H6 dependences of the chemical shifts and the C−H out-of-plane bending mode of benzene are similar: the changes are large in the high benzene concentration (xC6H6 > ∼0.6) compared to the low benzene concentration. In contrast, the linear xC6H6 dependences of the first moments of the low-frequency spectra less than 200 cm −1 were observed in both the [MOIm][BF4]−benzene and [MOIm][NTf2]−benzene systems. The difference in the xC6H6 dependent features between the chemical shifts and intramolecular vibrational mode and the intermolecular/interionic vibrational bands might come from the different probing space scales. The traces of the parallel aromatic ring structure and the T-shape structure were found in the ATR-IR and NMR experiments, but fs-RIKES did not observe a clear trace of the local structure. This might imply that the interactions between the imidazolium and benzene rings are not strong enough to librate the imidazolium and benzene rings together. The bulk properties, such as miscibility, density, viscosity, and surface tension, of the two ionic liquid-benzene mixture series were also compared to the microscopic aspects.

1. INTRODUCTION Ionic liquids (ILs) possess several unique features compared to conventional molecular liquids. One of the most unique features of ILs is their microheterogeneous (segregate) structure.1−12 Because most of the cations have moderately long alkyl groups, such 1-alkyl-3-methylimidazolium-based ILs are amphiphilic and show the microheterogeneous structure arising from the nonionic (alkyl groups) and ionic parts. In fact, the microstructural nature of ILs provides their relatively high solubilities in a wide variety of solutes. It may be simply expected that a nonpolar solute is stablized in the nonpolar region of an IL but a polar solute or an ionic solute interacts with the ionic region, though it is not that simple in reality. The high solubilities of ILs in a wide variety of solutes result from their structures, but the mixing scheme of ILs and neutral solvents is more complicated than conventional mixtures of neutral solvents. It is particularly important to understand the localizations or microenvironments of solvents (or mimic solutes) in the matrices of the ILs. This is because the microenvironments © 2015 American Chemical Society

and intermolecular interactions of solutes significantly affect the solute transportations and reaction dynamics, kinetics, and yields. Accordingly, IL mixtures with neutral solvents are currently getting more attention.13−16 In this study, the microscopic pictures at the molecular level of two series of mixtures, 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4])−benzene and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide ([MOIm][NTf2])− benzene, studied by several different spectroscopic techniques such as attenuated total reflectance IR (ATR-IR), NMR, and femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), were compared. Figure 1 shows the structure of [MOIm]+, [BF4]−, and [NTf2]− with the notation of the atoms within the imidazolium cation. In fact, the mixtures of imidazolium-based ILs and benzene are complicated systems. Received: November 6, 2015 Revised: December 19, 2015 Published: December 23, 2015 513

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

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The Journal of Physical Chemistry B

Figure 1. Structures of [MOIm]+, [BF4]−, and [NTf2]− with the notation of the atoms within the imidazolium cation.

Figure 2. Phase diagram of the [MOIm][BF4]−benzene system, together with those of the [MOIm][NTf2]−benzene and [HMIm][NTf2]−benzene systems.25,26 The temperatures of phase separation for the systems are depicted by red, blue, and double circles, respectively. The dashed lines represent expectation, and the solid lines are drawn for clarity of the border between one- and two-phase states.

Unique structures between the imidazolium ring and benzene, such as parallel structure and T-shape structure, were proposed by molecular dynamics (MD) simulations,17−19 ab initio calculations,20 X-ray and neutron scatterings,21−23 ATR-IR,23 NMR,23 and fs-RIKES.19,20 However, there is no report that directly compares the effects of different anions on the microscopic structures of the mixtures yet. Further advantage of this study is collaborating several spectroscopic techniques such as ATR-IR, NMR, and fs-RIKES. ATR-IR observes intramolecular (or intraionic) vibrational modes, NMR probes nuclei of ions and molecules, and fs-RIKES detects intermolecular (or interionic) vibrational dynamics. It is promising that the different probing space scales of the three spectroscopic techniques provide a bigger and deeper picture of the imidazolium-based ILs and benzene mixtures. We also estimated the degrees of miscibility and some physical properties such as liquid density ρ, viscosity η, and surface tension γ of the mixtures in this study.

and the neat benzene were measured with a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) equipped with a circulating water bath (Yamato, BB300) at 293.0 ± 0.2 K. The γ values of the mixtures and neat liquids were measured with a duNouy tensiometer (Yoshida Seisakusho) at 293.0 ± 0.3 K. The data of ρ, η, and γ for the [MOIm][BF4]−benzene mixtures at 292 K were reported in the previous report.20 2.3. ATR-IR Spectroscopy. At room temperature, ATR-IR experiments with a single reflectance were made on the benzene mixtures with the [MOIm][BF4] and [MOIm][NTf2] as a function of xC6H6. An FT-IR spectrometer (JASCO, FT/IR-6100) equipped with an ATR diamond prism (JASCO, PKS-D 470 with ATR PRO450-S) was used. ATR-IR spectra of the n-octane− benzene mixtures at various xC6H6 were also recorded on the same spectrometer for comparison. The absorption spectra were accumulated for 16 times per sample with a wavenumber resolution of 4.0 cm−1. The spectra of the sample mixtures were measured for three times at least. The wavenumbers of the C−H out-of-plane bending δC6H6 of benzene molecules were determined from each spectrum and averaged to obtain the final values. 2.4. NMR Spectroscopy. 1H and 13C NMR spectra of the [MOIm][BF4]−benzene and [MOIm][NTf2]−benzene mixtures at 298.2 K were measured as a function of xC6H6 using a 400 MHz FT-NMR spectrometer (Agilent Technologies, 400 MHz NMR System). An external double reference tube (Shigemi) was inserted into the sample tube (Shigemi, PS-001−7). 23 Hexamethyldisiloxane (HMDS) (Wako Pure Chemicals, the first purity grade, >97%) was used as a reference substance for 1 H and 13C atoms. The observed chemical shifts were corrected for the volume magnetic susceptibility of a sample solution using an external double reference method as described elsewhere.23,27−30 2.5. fs-RIKES. fs-RIKES is a third order nonlinear spectroscopy and has been used to investigate the ultrafast dynamics (or low-frequency spectrum) in liquids and mixtures.31−37 Currently, this technique is also applied for neat ILs3,38−52 and IL mixtures with neutral solvents.19,20,53−56 The details of the femtosecond optical heterodyne-detected RIKES apparatus used in this study were already reported elsewhere.37,57 The light source in the most recent RIKES apparatus was a titanium sapphire laser (KMLabs Inc., Griffin) pumped by a Nd:YVO4 diode laser (Spectra Physics, Millennia Pro 5sJ).58 The output

2. EXPERIMENTAL METHODS 2.1. Reagents and Sample Mixtures. [MOIm][BF4] (99%) was purchased from Iolitec and used after drying in vacuo at 313 K over 36 h. The water content of the [MOIm][BF4] estimated by a Karl Fischer coulometer (Hiranuma AQ-300) was 25 ppm.20 [MOIm][NTf2] was synthesized with the conventional method previously reported.24 The water content of the [MOIm][NTf2] was determined to be less than 100 ppm using a Karl Fischer coulometer (Mettler Toledo, DL32). Benzene (Wako Pure Chemicals, grade for HPLC, >99.7%) was used without further purification. n-Octane (the extra purity grade, >98%) was purchased from Wako Pure Chemicals. All sample mixtures were prepared by weighing ILs or n-octane and benzene at various benzene mole fractions xC6H6 required. 2.2. Miscibility, Densities, Viscosities, and Surface Tensions. The degrees of miscibility for the [MOIm][BF4]− benzene mixtures at various xC6H6 were examined at several temperatures as follows. The sample mixtures were sealed in a glass tube with the inner diameter of 1.0 mm. Then, the glass tube was immersed in a temperature-controlled silicone oil bath. Cloud points were regarded as the temperature of phase separation for the mixtures. As seen in Figure 2, the phase separation temperatures of the [MOIm][BF4]−benzene system are plotted in the phase diagram previously reported for benzene mixtures with [MOIm][NTf2] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([HMIm][NTf2]).25,26 The ρ values of the samples were estimated by a 2 mL volumetric flask at 293.0 ± 0.3 K. The η values of the [MOIm][NTf2]−benzene mixtures, the neat [MOIm][NTf2], 514

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

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Table 1. Density ρ, Viscosity η, and Surface Tension γ for [MOIm][NTf2] and Benzene Mixtures and Neat [MOIm][NTf2] and Benzene at 293 K

power of the titanium sapphire laser was approximately 430 mW, and the reputation rate was ca. 85 MHz. The temporal response, which was the cross-correlation between the pump and probe pulses measured using a 200-μm-thick KDP crystal (type I), was typically 36 ± 3 fs (full width at half-maximum). The scans with a high time resolution of 2048 points at 3.34 fs/step were made for a short time window (6.8 ps). Intermediate time window transients (approximately 30 ps) and long time window transients (approximately 300 ps) were recorded with a data acquisition of 16.7 and 167 fs/step, respectively. Pure heterodyne signals were achieved by combining the Kerr transients recorded at ± ∼1.5° rotations of a quarter wave plate in the probe beam path to eliminate the residual homodyne signal. For the [MOIm][NTf2]−benzene mixtures and neat [MOIm][NTf2], 3, 6, and 9 scans for each polarization measurement were averaged respectively for the short, intermediate, and long time window transients. Only the short and intermediate time window transients were measured for neat benzene because of its fast relaxation of the diffusive overdamped decay. Before the RIKES measurements the sample mixtures were injected into a 3 mm optical-path-length quartz cell (Tosoh Quartz) using either a 0.2-μm or 0.02-μm Anotop filter (Whatman). All the RIKES measurements were made at 293 ± 1 K. The RIKES measurements for the [MOIm][BF4]−benzene mixtures made at 292 K were reported previously,20 but the spectra of the [MOIm][BF4]−benzene mixtures are shown in this report to compare with the spectra of the [MOIm][NTf2]−benzene mixtures to see the effects of anions.

a

3. RESULTS AND DISCUSSION 3.1. Miscibility of the Mixtures. Figure 2 shows the phase diagram of the [MOIm][NTf2]−benzene system previously reported, together with that of the benzene system of the [HMIm][NTf2] with the shorter hexyl chain for comparison.25,26 In the same section, the phase separation temperatures for the [MOIm][BF4]−benzene system determined in the present study are plotted against the benzene mole fraction xC6H6. The upper critical solution temperature (UCST) for the [MOIm][NTf2]−benzene system is found at 374.36 K and xC6H6 = 0.9712. In contrast, the [HMIm][NTf2] system shows the hourglass shape of the demixing region. At the room temperature of 298 K, the demixing ranges of the two systems are 0.913 ≤ xC6H6 ≤ 0.998 and 0.872 ≤ xC6H6 ≤ ∼ 0.997, respectively. Hence, the range of the [HMIm][NTf2]−benzene system is wider toward the lower xC6H6 compared to the [MOIm][NTf2]−benzene system. Despite the small number of the plots, Figure 2 reveals that the demixing range of the [MOIm][BF4]−benzene system becomes wider toward the lower xC6H6 than that of the [MOIm][NTf2] system. Additionally, the shape of the demixing region for the [MOIm][BF4] system would be an hourglass type as shown by the dashed-lines for expectation, while that of the [MOIm][NTf2] system is the UCST type. Thus, the miscibilities of the [MOIm]+−based ILs into benzene depend on the anions, that is, [BF4]− may inhibit the interactions between the IL and benzene molecules. 3.2. Bulk Properties: Density, Viscosity, and Surface Tension. Table 1 lists the data of ρ, η, and γ of the [MOIm][NTf2]−benzene mixtures with the xC6H6 of 0.2, 0.4, 0.6, 0.8, and 0.9 and the neat [MOIm][NTf2] (xC6H6 = 0) and benzene (xC6H6 = 1). The values of ρ, η, and γ of the neat [MOIm][NTf2] and benzene measured in this study are in good agreement with the reported values within experimental error if we take the effect of temperature into consideration.37,59−62

xC6H6

ρa,b (g/mL)

ηc,d (cP)

γe,f (mN/m)

0 ([MOIm][NTf2]) 0.2 0.4 0.6 0.8 0.9 1 (neat benzene)

1.329 1.298 1.260 1.209 1.111 1.022 0.873

113 74.4 46.2 23.5 7.34 3.21 0.641

31.7 31.5 31.4 31.3 31.0 30.1 29.3

293.0 ± 0.3 K. b ±1%. c293.0 ± 0.2 K. d ±5%. e293.0 ± 0.3 K. f ±3%.

Figure 3. xC6H6 dependences of (a) the density ρ, (b) the viscosity η, and (c) the surface tension γ of the [MOIm][NTf2] mixtures (blue circles) and the [MOIm][BF4] mixtures (red circles). The data of the [MOIm][BF4] mixtures are taken from ref 20.

Figure 3 shows the xC6H6 dependences of ρ, η, and γ of the [MOIm][NTf2]−benzene mixtures together with the data of the [MOIm][BF4]−benzene mixtures that were reported previously.20 The ρ values decrease gradually with the larger xC6H6 and they dramatically decrease with the larger xC6H6 at xC6H6 = ∼0.8 for both the [MOIm][NTf2]−benzene and [MOIm][BF4]−benzene systems. The η values of the [MOIm][NTf2]−benzene and [MOIm][BF4]−benzene mixtures at the same xC6H6 are quite similar at more than 0.6 of xC6H6. There is a clear difference in the xC6H6 dependence of γ between the two systems. The xC6H6 dependence of γ in the [MOIm][NTf2]− benzene mixtures changes at xC6H6 = ∼0.8, but that in the [MOIm][BF4]−benzene mixtures changes at xC6H6 = ∼0.2−0.4. 515

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

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The Journal of Physical Chemistry B This implies that the surface structure and the intermolecular (and interionic) interaction are more influenced with the smaller amount of benzene in the [MOIm][BF4] mixtures than in the [MOIm][NTf2] ones. 3.3. ATR-IR Spectroscopy. Figure 4 panels a and b display the IR spectra of the C−H out-of-plane bending δC6H6 band of

among benzene molecules may be strengthened in the n-octane mixtures below this mole fraction.64 This is because n-octane molecules may not strongly interact with benzene due to the weak dispersion force between them. For the IL−benzene mixtures, the blue-shift of the δC6H6 against the decrease in the xC6H6 is much more significant compared to that in the n-octane mixtures. This suggests that benzene molecules may be affected by stronger interactions than the C−H···π and π···π interactions among benzene molecules. The π-conjugated imidazolium ring may interact with benzene molecules still through the π···π interaction. However, the imidazolium ring has the positive charge though the benzene ring is electrically neutral. Thus, the interactions between the imidazolium and benzene rings may be stronger compared to those between the nonpolar benzene molecules. Furthermore, the C−H···π interaction between the imidazolium ring hydrogen atoms and the benzene ring may also be stronger than that between benzene molecules due to the positive charge of the imidazolium ring hydrogen atoms. For example, the ab initio calculations with a CCSD(T) level electron correlation correction revealed that the C−H···π and π···π interactions between the pyridinium and benzene rings are stronger than those between the pyridine and benzene rings.66 Thus, both C−H···π and π···π interactions of the imidazolium ring with the benzene ring should be classified into the C−H-to-plane and the plane-to-plane of the cation···π interactions, respectively. Recently, the stable structures and interaction energies of clusters of 1,3-dimethylimidazolium cation and benzene in gas phase and acetonitrile dielectric medium (IEF-PCM) were calculated based on the B3LYP/ 6-311++G(d,p) level of theory.20 It was found that the T-shaped structure is more stable than the slipped parallel structure at the B3LYP/6-311++G(d,p) level of theory. As seen in Figure 5, the anion dependence of the wavenumber of the δC6H6 is observed between the two IL mixtures. The wavenumbers for the [NTf2]−−IL mixtures are larger than that for the [BF4]−−IL mixtures in the xC6H6 range from 0.2 to 0.8. This suggests that benzene molecules may more strongly interact with the imidazolium ring in the [NTf2]−−IL mixtures compared to the [BF4]−−IL mixtures. It is likely that the inherent interaction of the imidazolium ring with [BF4]− significantly prevents the benzene ring from interacting with the imidazolium due to the steric hindrance. This may be caused by the higher electron density of [BF4]− than [NTf2]−. In other words, [NTf2]− may more loosely interact with the imidazolium ring in pure IL compared to [BF4]−. 3.4. NMR Spectroscopy. To clarify the interactions between the imidazolium ring and benzene molecules in terms of the change in the electron density within ILs and benzene molecules against the increase in the xC6H6, 1H and 13C NMR measurements were conducted on the benzene mixtures with the [MOIm][BF4] and [MOIm][NTf2] as a function of xC6H6. The 1H and 13C NMR spectra for the [BF4]−−IL and [NTf2]−−IL mixtures are shown in Figures 6 and 7, respectively, together with the assignments for the peaks observed. Note that the chemical shifts discussed here are based on the external double reference method23,27−30 as mentioned in section 2.4. Recently the importance of the correction for the volume magnetic susceptibilities of the sample solutions has been noticed in discussions of the microscopic structures of the IL mixture systems.23,64,67−73 Figure 8 shows the 1H and 13C chemical shifts of the imidazolium ring hydrogen and carbon atoms as a function of xC6H6. For the pure ILs (xC6H6 = 0), the 1H and 13C chemical shifts

Figure 4. ATR-IR spectra of the C−H out-of-plane bending δC6H6 band of benzene molecules in the benzene mixtures with (a) [MOIm][BF4], (b) [MOIm][NTf2], and (c) n-octane as a function of xC6H6.

benzene molecules63 in the benzene mixtures with the [MOIm][BF4] and [MOIm][NTf2] as a function of xC6H6, respectively. For comparison, those in the n-octane−benzene mixtures are depicted in Figure 4c. The δC6H6 band of benzene in all of the three mixtures gradually weakens and blue-shifts with decreasing xC6H6. The blue-shift for the two IL−benzene mixtures with the decrease in the xC6H6 is more remarkable compared to that for the n-octane−benzene mixtures. In Figure 5, the wavenumbers of the δC6H6 of benzene molecules for the three mixtures are plotted against the xC6H6.

Figure 5. Wavenumbers of the C−H out-of-plane bending δC6H6 of benzene molecules in the benzene mixtures with the [MOIm][BF4], [MOIm][NTf2], and n-octane as a function of xC6H6 represented by red, blue, and opened circles, respectively. The estimated standard deviations σ are indicated as error bars.

The δC6H6 for the n-octane mixtures markedly blue-shifts with decreasing xC6H6 to ∼0.6 (increasing n-octane content). The blueshift then becomes moderate below xC6H6 ≈ 0.6. As discussed in the previous reports,23,64 this feature of the δC6H6 suggests that the inherent structure of benzene is markedly formed in the n-octane mixtures above xC6H6 ≈ 0.6 because of the large amount of benzene. Benzene molecules interact with themselves through the C−H···π and π···π interactions in the pure liquid as shown by the X-ray diffraction study.65 The C−H···π and π···π interactions 516

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

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Figure 6. 1H and 13C NMR spectra for the [MOIm][BF4]−benzene mixtures as a function xC6H6, together with the assignments for peaks, in panels (a) and (b), respectively. In panel (c), 13C NMR spectrum for the [MOIm][BF4] is expanded for clarity in the chemical shift range from 18 to 35 ppm.

Figure 8. 1H and 13C NMR chemical shifts for the imidazolium ring in the benzene mixtures with the [MOIm][BF4] and [MOIm][NTf2] as a function of xC6H6 represented by red and blue circles, respectively. The estimated standard deviations σ are indicated as error bars.

The imidazolium and benzene rings interact with each other by the plane-to-plane and the C−H-to-plane of the cation···π interactions, as described in the ATR-IR section. Thus, the imidazolium ring hydrogen and carbon atoms may be shielded by the benzene ring when the imidazolium and benzene rings interact with each other by the plane-to-plane of the cation···π interaction. Most of the chemical shifts of the imidazolium ring atoms for the [BF4]−−IL mixtures are larger than those for the [NTf2]−−IL mixtures in the xC6H6 range from 0 to the lowest xC6H6 for the immiscible range (Figure 8). This arises from the stronger interaction between the imidazolium ring and [BF4]− than [NTf2]− as discussed above. As shown in the figure, the difference of the chemical shifts for each atom in both the [BF4]−−IL and [NTf2]−−IL mixtures becomes larger with increasing xC6H6. In other words, the chemical shifts of the imidazolium ring atoms in the [NTf2]−−IL mixtures are more significantly decreased as the xC6H6 increases compared to those in the [BF4]−−IL mixtures. These NMR results suggest that the plane-to-plane of the cation···π interaction between the imidazolium and benzene rings in the [NTf2]−−IL mixtures is stronger than that in the [BF4]−−IL mixtures. Consequently, the NMR results still reveal that the steric hindrance of the stronger interaction between the imidazolium ring and [BF4]− prevents benzene molecule from approaching the imidazolium ring, as discussed on the ATR-IR results. The shielding of the imidazolium ring hydrogen atoms at positions 4 and 5 in both the IL−benzene mixtures against the increase in the xC6H6 is slightly stronger than the hydrogen atom at position 2. On the contrary, the imidazolium ring carbon atoms at positions 4 and 5 are more weakly shielded in the IL−benzene mixtures with increasing xC6H6. These features are common for both ILs. Probably, the imidazolium ring at positions 4 and 5 is influenced by the interaction with benzene molecules through the C−H-toplane of the cation···π interaction rather than the plane-to-plane. Figure 9 illustrates the 1H and 13C NMR chemical shifts of the methyl and methylene groups at positions 6 and 7, which are directly bound to the imidazolium ring nitrogen atoms, in both the IL−benzene mixtures as a function xC6H6. The H6 and H7 atoms indicate the same behavior as the imidazolium ring hydrogen atoms. Thus, the hydrogen atoms of the methyl and methylene groups are gradually shielded in both the IL−benzene mixtures with increasing xC6H6. The shielding of the H6 and H7

Figure 7. 1H and 13C NMR spectra for the [MOIm][NTf2]−benzene mixtures as a function xC6H6, together with the assignments for peaks, in panels (a) and (b), respectively. In panel (c), 13C NMR spectrum for the [MOIm][NTf2] is expanded for clarity in the chemical shift range from 18 to 35 ppm.

of the H2, H4, H5, and C2 atoms are clearly different between the [MOIm][BF4] and [MOIm][NTf2]. This means a different magnitude of interactions between the imidazolium ring and the anions in the pure ILs. The hydrogen and carbon atoms of the [MOIm][BF4] are more deshielded than those of the [MOIm][NTf2]. Especially, the difference of the chemical shifts for the C2 atom between the two ILs is the most significant among the atoms. [BF4]− with the higher charge density may more strongly interact with the imidazolium ring compared to [NTf2]− with the lower charge density. This is consistent with the discussion on the ATR-IR results. The imidazolium ring at position 2 most strongly interacts with the anion because of the highest positive charge in the ring.74,75 However, the interaction between the ring and the anions may not be the conventional linear C−H···anion hydrogen bond. This is because the C2 atom is more sensitive to the anions compared to the H2 atom. The anions may widely distribute around the imidazolium ring, particularly, at position 2, as shown in the previous report.67,76,77 For the IL−benzene mixtures (xC6H6 > 0), the imidazolium ring hydrogen and carbon atoms are gradually shielded with increasing xC6H6. As discussed in the previous reports,23,64 the shielding of the ring atoms with the increase in the xC6H6 is caused by the ring current effect of the π electrons of the benzene plane. 517

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

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atoms clearly show the anion dependence. This may be because the 1H and 13C NMR chemical shifts involve the information on the various states of benzene molecules in the mixtures. The 1 H and 13C chemical shifts for benzene molecules below xC6H6 ≈ 0.7 may mainly give the information on the interaction between the imidazolium and benzene rings because of the low benzene content. Thus, the almost constant chemical shifts suggest that benzene molecules are stably accommodated in the inherent IL structure through the cation···π interactions between the imidazolium and benzene rings in the mixtures below this mole fraction. In contrast, the chemical shifts for benzene molecules above xC6H6 ≈ 0.7 may mainly indicate the interaction among benzene molecules because of the high benzene content. Hence, the inherent benzene structure through the C−H···π and π···π interactions may evolve in the mixtures above the mole fraction, as discussed in the ATR-IR results. This leads to the shielding of the benzene hydrogen and carbon atoms. The slightly smaller 13C chemical shifts of benzene molecules in the [NTf2]−−IL mixtures below xC6H6 = 0.7 than those of the [BF4]−−IL mixtures might be attributed to the stronger cation···π interaction between the imidazolium and benzene rings in the former. The reason why the anion dependence cannot be observed in the 1H chemical shifts of benzene may be the smaller number of electrons for the hydrogen atom than the carbon atom. 3.5. Low-Frequency Spectra by fs-RIKES. Figure 10a shows the logarithmic plots for the Kerr transients of the mixtures of the [MOIm][NTf2] and benzene and the neat liquids. The multiexponential fits (a0 + ∑in= 1ai exp(−t/τi), where a0 is the offset amplitude, ai is the amplitude of the ith component, and τi is the time constant of the ith component) for the Kerr transients from 3 to 300 ps (except for benzene, 18.5 ps) are also shown in the figure. The fit parameters are summarized in the Supporting Information. It is noted that the offset amplitude is attributed to a slow structural relaxation component. It is known that neat ILs show the structural relaxation of subnano to nanoseconds.78−80 Because of the limitation of the fsRIKES apparatus used in this study, the slow relaxation cannot be resolved. However, the low frequency spectra can be reasonably discussed when we focus on the intermolecular vibrational dynamics which usually occurs at a time faster than something on the order of picoseconds.40 The measured Kerr transients were further analyzed by the Fourier transform deconvolution procedure, which was developed by McMorrow and Lotshaw,81,82 to obtain the Kerr spectra. The details of the analysis procedure made in this study were reported in previous reports.38,39 The obtained Fourier transform spectra with the frequency range of 0−650 cm−1 of the [MOIm][NTf2]−benzene mixtures and the neat liquids are shown in Figure 10b and the magnification of the frequency range of 0−250 cm−1 is also shown in Figure 10c. Note that the picosecond relaxation components (the slower three exponential and offset components for the mixtures, the slower biexponential and offset components for the neat [MOIm][NTf2], and the slow component for the neat benzene) are subtracted from the entire spectra to focus on the contribution of the intermolecular (and/or interionic) vibrational dynamics. To see the difference in the spectra between the [MOIm][NTf 2 ]−benzene and [MOIm][BF4]−benzene mixtures, the Fourier transform spectra for the [MOIm][BF4]−benzene mixtures that was reported previously are shown in Figure 11.20

Figure 9. 1H and 13C NMR chemical shifts for the imidazolium alkyl groups and benzene molecules in the benzene mixtures with the [MOIm][BF4] and [MOIm][NTf2] as a function of xC6H6 represented by red and blue circles, respectively. Opened circles give the values for the pure benzene. The estimated standard deviations σ are indicated as error bars.

atoms in the [NTf2]−−IL mixtures is more significant than that in the [BF4]−−IL mixtures. These results also arise from the ring current effect of the benzene molecule due to the plane-to-plane of the cation···π interaction between the imidazolium and benzene ring, because the methyl and methylene groups are in the vicinity of the ring. The methyl carbon C6 atom shows the same tendency, although the shielding of the atoms in both IL−benzene mixtures against the increase in the xC6H6 is small. The chemical shifts of the H6, H7, and C6 atoms for the two pure ILs (xC6H6 = 0) almost overlap each other. This makes sure thus that the differences of the chemical shifts of the H2, H4, H5, and C2 atoms between the pure ILs arise from the interaction between the imidazolium ring and the anions as discussed above. However, the chemical shifts of the methylene C7 atom for pure ILs are different from each other. The value of the [NTf2]−−IL is larger than that of the [BF4]−−IL. Thus, the methylene carbon atom in the latter is more shielded compared to the former. This may be because the stronger interaction between the imidazolium ring and BF4− more significantly draws the electron density from the long alkyl group. The methyl C6 atom might also be affected by the interaction between the imidazolium ring and the anions. However, there are only three hydrogen atoms, but not the large numbers of electrons that should be drawn. Hence, the chemical shifts of the C6 atom for both ILs almost agree with each other. The shielding of the C7 atom in the [NTf2]−−IL mixtures with increasing xC6H6 is more significant compared to that in the [BF4]−−IL mixtures. This still shows the stronger interaction between the imidazolium and benzene rings in the former. Figure 9 also shows the chemical shifts of the benzene hydrogen and carbon atoms against the xC6H6. The 1H chemical shifts of both the IL−benzene mixtures as a function of xC6H6 overlap each other. The values moderately decrease with increasing xC6H6 to ∼0.7. The decreases in the values becomes remarkable above xC6H6 ≈ 0.7. The changes in the 13C chemical shifts of benzene molecules with increasing xC6H6 are similar to the 1H chemical shifts. All of the chemical shifts for benzene show the inflection point at xC6H6 ≈ 0.7. However, these NMR results for benzene are not sensitive to the anions, while the ATR-IR spectroscopy for benzene and the NMR for the imidazolium ring 518

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Figure 11. (a) Fourier transform Kerr spectra with the wide frequency range of 0−650 cm−1 and (b) magnification of low-frequency Kerr spectra from 0 to 250 cm−1 for the [MOIm][BF4]−benzene mixtures. The figures are taken from ref 20 with modification.

band at ca. 20 cm−1 for the neat [MOIm][NTf2] was assigned as likely being the librational motion of the [NTf2]− anion and/or the coupling of the translational and reorientational motions.88 The high frequency bump at ca. 80 cm−1 that is clearly observed in both the [MOIm][NTf2] and [MOIm][BF4] is mainly the imidazolium ring libration.88 The bimodal spectral line shape of the mixtures with the larger xC6H6 up to 0.6 becomes ambiguous and monomodal; however, further increasing xC6H6 increases the spectral intensity in the low-frequency region of ca. 10 cm−1, and the line shape of the spectrum of liquid benzene is trapezoidal (or gently bimodal). Unlike intramolecular vibrations, assignments and understandings of the low-frequency broad spectral bands in liquids are not straightforward. This is because the intermolecular vibrational motions are strongly coupled. The present systems are even more complicated due to the ternary mixture systems: cation, anion, and benzene. It is thus helpful to refer to MD simulation studies. The low-frequency Kerr spectrum of liquid benzene was calculated by Ryu and Stratt.91 The decomposition analysis of the spectrum in their study showed that the reorientational motion (libration) contributes to the low frequency spectrum entirely but the translational motion appears with the peak at approximately 10−20 cm−1. In the case of imidazolium-based IL with [NTf2]−, the contribution of the cation to the low-frequency Kerr spectrum is larger than that of the anion.92 Note that because the low-frequency spectra measured with fs-RIKES in this study are equivalent to the depolarized Raman spectra multiplied the Bose−Einstein occupation factor, it is fair to compare these calculated spectra91,92 but is inappropriate to make a comparison with the simple density-of-state spectra. On the basis of the calculated spectra,91,92 we find the spectral densities of the low-frequency

Figure 10. (a) Normalized Kerr transients, (b) Fourier transform Kerr spectra with the wide frequency range of 0−650 cm−1, and (c) magnification of low-frequency Kerr spectra from 0 to 250 cm−1 for the [MOIm][NTf2]−benzene mixtures.

The line shapes of the low-frequency spectra were then analyzed by the sum of Ohmic,83 antisymmetrized Gaussian,84 and Lorentz functions. Ohmic and antisymmetrized Gaussian functions are predominantly for the intermolecular (or interionic) vibrational band, but Lorentz functions are for intramolecular (or intraionic) vibrational modes. Figure 12 shows the line shape analysis results. The line shape of the neat [MOIm][NTf2] is clearly bimodal. This is in contrast to the line shape of the low-frequency spectrum of the neat [MOIm][BF4] being rather monomodal as shown in Figure 11.20 The lowfrequency bump at ca. 20 cm−1 observed in the neat [MOIm][NTf 2 ] is commonly observed in both aromatic19,40,41,43,45−47,52,85−89 and nonaromatic42,87,90 cationbased ILs with the [NTf2]− anion. Previously, the low-frequency Kerr spectra of 1-butyl-3-methylimidazolium cation-based ILs with several different anions were critically compared, and the 519

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Figure 12. Line shape analysis results of the low-frequency spectra of the [MOIm][NTf2]−benzene mixtures. Black dots and red solid lines denote the data and entire fits, respectively. Blue areas, green areas, and purple areas denote components of Ohmic, antisymmetrized Gaussian, and Lorentz functions, respectively. Sums of Ohmic and antisymmetrized Gaussian functions are also shown by red broken lines.

spectra measured in the present mixture systems are largely due to aromatic ring libration. We further analyzed the spectra to see the effect of xC6H6 on the spectral line shape. The simple xC6H6-weighted calculated spectra, which are kinds of ideal mixture spectra, for the neat [MOIm][NTf2] and benzene at each xC6H6 were calculated by Ical(ω) = (1 − xC6H6)I[MOIm][NTf2](ω) + xC6H6IC6H6(ω) (1)

where I[MOIm][NTf2](ω) and IC6H6(ω) are the spectra of the [MOIm][NTf2] and benzene with subtracted clear intramolecular vibrational modes. Figure 13 compares the spectra obtained by the fits to the data and the calculated spectra. It is clear from Figure 13 that the experimental spectra do not match with the calculated spectra. The similar feature was also observed in the [MOIm][BF4]−benzene system.20 We can also see that the intensity of the calculated spectrum is stronger than that of the experimental spectrum at each xC6H6. As discussed previously,20 the microscopic structure and intermolecular/ interionic interactions in the [MOIm][NTf2]−benzene mixtures are different from that in the neat liquids. Because the intensities of the low frequency Kerr spectra of the imidazolium-based ILs come predominantly from imidazolium ring libration,38,52,89 the results indicate that the imidazolium ring and benzene mix well at the molecular scale. The spectra are normalized and compared to find the mixing effect on the spectral width for the [MOIm][NTf2]−benzene mixtures, as shown in Figure 14. It is clear from the figure that the spectral width (full width at half-maximum) of the real spectrum of the [MOIm][NTf2]−benzene mixture of xC6H6 = 0.2 is broader than that of the ideal mixture one, but in the other concentrations the spectral widths of the real spectra are almost the same with that of the ideal mixture ones. This feature is different from the [MOIm][BF4]−benzene mixtures reported previously: the spectral width of the real spectra that is based on the experimentally obtained ones of the [MOIm][BF4]−benzene mixtures are broader than that of the calculated spectra, which is calculated from the spectra of neat IL and benzene with the weighted xC6H6, at all of the xC6H6 examined (Figure 14).20 It is not surprising that the inhomogeneous broadening due to the

Figure 13. Comparison between the real (solid lines) and calculated (broken lines) low-frequency spectra of the [MOIm][NTf2]−benzene mixtures in xC6H6 = (a) 0.2, (b) 0.6, and (c) 0.8. The real spectra are based on the fits to the experimental data (sums of Ohmic and antisymmetrized Gaussian functions, Figure 12) and the real spectra are xC6H6 weighed ones of neat liquid samples.

microheterogeneous structure occurs in the mixture systems. However, the [MOIm][NTf2]−benzene mixture system is complicated: the [MOIm][NTf2]−benzene mixtures in the xC6H6 > 0.2 do not show the spectral broadening but the [MOIm][BF4]−benzene mixtures always show the spectral broadening. One might think that the intermolecular vibrations 520

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Figure 14. Intensity normalized spectra for real (solid lines) and calculated (dotted lines) spectra of the [MOIm][NTf2]−benzene mixtures in xC6H6 = (a) 0.2, (b) 0.6, and (c) 0.8. Spectra of the [MOIm][BF4]−benzene mixtures in xC6H6 = (d) 0.2, (e) 0.6, and (f) 0.8 are also shown for a comparison.20

are more homogeneous in the [MOIm][NTf2]−benzene mixtures than the [MOIm][BF4]−benzene mixtures. This might be because the [MOIm][NTf2] and benzene mix together well (homogeneously mix) compared to the [MOIm][BF4] and benzene. As discussed in sections 3.3 and 3.4, the interaction between the cation and anion in the [MOIm][BF4]−benzene mixtures is stronger than that in the [MOIm][NTf2]−benzene mixtures. In addition, the two phase region (phase separation) in the [MOIm][BF4]−benzene system is wider than that in the [MOIm][NTf2]−benzene system as shown in Figure 2. Accordingly, the results of the widths of the low-frequency spectra of the [MOIm][BF4]−benzene and the [MOIm][NTf2]−benzene seem to be in good agreement with the results of the ATR-IR, NMR, and miscibility. To see the mixing effect on the low-frequency spectrum further, Figure 15 shows the differential spectra of the xC6H6weighted calculated (ideal) spectra from the real spectra for the [MOIm][NTf2]−benzene mixtures and the [MOIm][BF4]− benzene mixtures. Note that the fit functions were used for the [MOIm][NTf2]−benzene mixtures but the experimentally obtained spectra were directly used for the [MOIm][BF4]− benzene mixtures instead, a clear intraionic vibrational band at ca. 120 cm−1 overlaps with the broad interionic/intermolecular vibrational band in the [MOIm][NTf2] mixtures, but there is no clear intraionic vibrational band in this frequency region for the [MOIm][BF4] mixtures. It is also noted that the differential spectra here are kinds of loss spectra due to mixing the ILs with benzene. In Figure 15, we find several important points. First, the xC6H6 dependences of the differential spectra of the [MOIm][NTf2]−benzene mixtures and the [MOIm][BF4]−benzene mixtures are qualitatively similar: the intensity increases with increasing xC6H6 until xC6H6 ≈ 0.6 and that decreases with further

Figure 15. Differential spectra between the real and calculated spectra for the (a) [MOIm][NTf2]−benzene mixtures and (b) [MOIm][BF4]− benzene mixtures.

increasing xC6H6. Second, the peaks of the loss spectra of both the [MOIm][NTf2]−benzene mixtures and the [MOIm][BF4]− benzene mixtures shift to the high frequency side with increasing xC6H6. Third, the sharp and strong intensity in the low frequency 521

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The Journal of Physical Chemistry B region of approximately 5−10 cm−1 (and triangular spectral shape) is obvious in the mixtures having xC6H6 ≤ ∼0.6 for both systems. Let us discuss these features of the xC6H6 dependences of the differential spectra step by step. Because the intensities of the differential spectra are always positive in the mixtures with any xC6H6, the intermolecular/interionic vibrations are deactivated or suppressed by mixing the ILs and benzene. The deactivation/ suppression of the intermolecular/interionic vibrational motions in the mixtures most strongly occur at the almost equimolar condition (xC6H6 ≈ 0.6). The spectral intensity of the broad spectrum band in the low frequency region less than 150 cm−1 of liquid benzene is much larger than that of the ILs as shown in Figure 10b,c and Figure 11, and thus this indicates that the intermolecular vibrational motion in liquid benzene is more active than that in the ILs. When benzene molecules are added into the ILs, the intermolecular vibrational motions of the benzene molecules are likely suppressed because of the IL’s larger moment of inertia and larger density. In a similar way, the interionic vibrational motions in ILs are likely activated or accelerated when benzene is added. However, the loss in the spectral intensity by benzene should be larger than the gain in the spectral intensity by ILs because the spectral intensity for the neat benzene is much larger than that for the ILs (ca. 5−6 times for both the mixture systems of the [MOIm][NTf2] and the [MOIm][BF4]20 (Figures 10 and 11, respectively)). If we assume benzene molecules interact with the imidazolium rings primarily, it is not surprising that the magnitude of the spectral loss is the largest in the mixture at near equimolar concentration. Regarding the line shape of the loss spectra, it might be worth comparing the present results with the line shapes of the translational and reorientational motions and their cross-term for liquid benzene revealed by MD simulations performed by Ryu and Stratt.91 The line shapes of the loss spectra of the mixtures in the xC6H6 ≤ 0.6 shown in Figure 15 are actually quite similar to the line shape of the component of the translational motion and the cross term for liquid benzene. This suggests that the deactivated/ suppressed intermolecular/interionic vibrational motions are translational and/or coupling motions. The low-frequency spectrum in liquids and mixtures is probing the microscopic structure and interactions. In the previous study of the [MOIm][BF4]−benzene mixtures,20 we concluded that the T-shape structure of the imidazolium ring and benzene rather than the parallel or sandwich structure mainly contributes to the low-frequency Kerr spectra. There were two main reasons. The first reason was based on the quantum chemistry calculations of 1,3-dimethylimidazolium cation and benzene clusters. The T-shape structure of the cluster is more stable than the parallel structure cluster.20 The other one was that the low-frequency spectra of near equimolar mixtures (xC6H6 = 0.4 and 0.6) were not particularly narrow. It was expected that the low-frequency spectrum should be narrow (means the libration of the stacked rings is slow), if the imidazolium ring and benzene made the stacking structure. In this study, we did not observe a specific change of the low-frequency spectra for the [MOIm][NTf2]−benzene mixtures at near equimolar concentration (xC6H6 = 0.4 and 0.6) shown in Figures 10 and 12, as well as the previous study of the [MOIm][BF4]−benzene mixtures (Figure 11).20 Thus, there is no clear evidence of the specific (cluster) structures of imidazolium cations in the low-frequency spectra. It might be

plausible that the imidazolium and benzene rings librate independently or do not interfere with each other because of the relatively weak interaction between the imidazolium and benzene. In other words, the interaction between the imidazolium and benzene rings is not strong enough to librate together even if the rings stack in parallel. To see the semiquantitative feature of the xC6H6 dependence of the low-frequency spectrum, the first moments M1 of the spectra of the mixtures and neat liquids have been estimated. M1 is defined as M1 = ∫ ωI(ω) dω/∫ I(ω) dω, where I(ω) is the frequency-dependent intensity of the spectra subtracted the contributions of the picosecond relaxation process and clear intraionic vibrational modes for the mixtures and the neat liquids. The reason why we use the M1 instead of the peak of the spectrum is that the spectrum (especially of the neat [MOIm][NTf2]) is not clearly monomodal, and the discussion based on the peak is inappropriate. Figure 16 shows the plots of M1 vs xC6H6

Figure 16. xC6H6 dependences of the first moment M1 of the lowfrequency spectral band for the [MOIm][NTf2]−benzene mixtures (blue circles) and the [MOIm][BF4]−benzene mixtures (red symbols). The data of the [MOIm][BF4] mixtures are reproduced from ref 20. Linear fits are also shown.

for the [MOIm][NTf2]−benzene mixtures and the [MOIm][BF4]−benzene mixtures. As shown in Figure 16, M1 depends linearly on xC6H6 but the slope for the [MOIm][BF4]−benzene system is steeper than that for the [MOIm][NTf2]−benzene system. Because the M1 for the neat [MOIm][NTf2] is smaller than that for the neat [MOIm][BF4], the difference in the slope between the two mixture systems is rather reasonable. In contrast to the low-frequency spectra, the ATR-IR and NMR results showed that the changes in the wavenumbers of the C−H bending mode and the chemical shifts by verifying the xC6H6 are large in the high benzene concentration region like xC6H6 > 0.6 compared to the low benzene concentration region. The difference in the xC6H6 dependence might arise from the difference in the probing space scale.

4. CONCLUSIONS In this study, we investigated the [MOIm][BF4]−benzene and [MOIm][NTf2]−benzene mixtures by ATR-IR, NMR, and fsRIKES. The miscibility and physical properties such as density, viscosity, and surface tension of the two series of the mixtures were also compared. The ATR-IR experiments, the C−H out-ofplane bending mode of benzene molecule in the mixtures was monitored. The peak of the band blue-shifts with the decrease in 522

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the xC6H6 for both the two mixture systems. The change in the peak of the C−H out-of-plane bending mode of benzene molecule in the [MOIm][NTf2]−benzene system is more significant than that in the [MOIm][BF4]−benzene system. This may indicate that the benzene molecule interacts with imidazolium ring more strongly in the [MOIm][NTf2]−benzene system than in the [MOIm][BF4]−benzene system. In the NMR experiments, it was found that the difference of the chemical shifts for the C2 atom of the imidazolium ring between the [MOIm][NTf 2 ]−benzene system and the [MOIm][BF4]−benzene system was large compared to the other atoms of the imidazolium cation, and the chemical shift in the [MOIm][BF4]−benzene system appears in the low magnetic field side compared to the [MOIm][NTf2]−benzene system. This indicates that the anion would exist in near the C2 atom of the imidazolium ring and the interaction between the cation and the anion in the [MOIm][BF4]−benzene mixtures is stronger than that in the [MOIm][NTf2]−benzene mixtures. As well as the ATR-IR results, the changes of the chemical shifts by verifying the xC6H6 are large in the high benzene concentration region like xC6H6 > 0.6 compared to the low benzene concentration region, particularly in the [MOIm][NTf2]− benzene system. Thus, the interactions between the imidazolium ring and benzene are stronger in the [MOIm][NTf2]−benzene system than in the [MOIm][BF4]−benzene system. fs-RIKES, which measures the low-frequency spectra that applies to the intermolecular/interionic vibrational dynamics and thus probes a bigger length scale compared to NMR and ATRIR, was also applied to study the [MOIm][NTf2]−benzene system in this study, and the present results were compared to the previously reported results of the [MOIm][BF4]−benzene system.20 The low-frequency spectrum of [MOIm][NTf2]− benzene with an xC6H6 of 0.2 is broadened in comparison with the ideal mixture spectrum that is the sum of spectra of the neat [MOIm][NTf2] and benzene with the xC6H6 being weighed, but the widths of the spectra of the other concentration mixtures are quite similar to that of the ideal mixtures. This is different from the feature of the [MOIm][BF4]−benzene mixtures: the lowfrequency spectra of the [MOIm][BF4]−benzene mixtures with any xC6H6 are broader than that of the ideal mixtures with the same xC6H6. This implies that the interionic/intermolecular vibrational bands of the [MOIm][BF4]−benzene mixtures would be more inhomogeneously broadened for the more heterogeneous structures than that of the [MOIm][NTf2]− benzene mixtures. The features observed in fs-RIKES seem to be in good agreement with the results of the ATR-IR and NMR. The loss spectra due to mixing the ILs with benzene of the two series of mixtures are qualitatively similar. In the xC6H6 < ∼0.6, the translational motions and/or the coupling motions between the translational and reorinentational motions seem to be deactivated or suppressed. Unlike the ATR-IR and NMR results, the first moment of the low-frequency band looks linearly dependent on xC6H6 in both mixture systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported partly by Grants-in-Aid (Nos. 15K05377 (H.S.), 22550018 and 26410018 (T.T.)) from the Japan Society for the Promotion of Science and Dean’s Grant for Progressive Research Projects from Saga University. The NMR measurements for the sample solutions were conducted at the Analytical Research Center for Experimental Sciences of Saga University.

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REFERENCES

(1) Triolo, A.; Russina, O.; Bleif, H.-J.; Di Cola, E. Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 4641−4644. (2) Triolo, A.; Russina, O.; Fazio, B.; Appetecchi, G. B.; Carewska, M.; Passerini, S. Nanoscale Organization in Piperidinium-Based Room Temperature Ionic Liquids. J. Chem. Phys. 2009, 130, 164521/1-6. (3) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R.; Xiao, D.; Hines, L. G., Jr.; Bartsch, R. A.; Quitevis, E. L.; Plechkova, N.; Seddon, K. R. Morphology and Intermolecular Dyamics of 1-Alkyl-3-methylimidazolium Bis{(trifluoromethane)sulfonyl}amide Ionic Liquids: Structural and Dynamic Evidence of Nanoscale Segregation. J. Phys.: Condens. Matter 2009, 21, 424121/1-9. (4) Triolo, A.; Russina, O.; Caminiti, R.; Shirota, H.; Lee, H. Y.; Santos, C. S.; Murthyd, N. S.; Castner, E. W., Jr. Comparing Intermediate Range Order for Alkyl- vs. Ether-Substituted Cations in Ionic Liquids. Chem. Commun. 2012, 48, 4959−4961. (5) Santos, C. S.; Annapureddy, H. V. R.; Murthy, N. S.; Kashyap, H. K.; Castner, E. W., Jr.; Margulis, C. J. Temperature-Dependent Structure of Methyltributylammonium Bis(trifluoromethylsulfonyl)amide: X ray Scattering and Simulations. J. Chem. Phys. 2011, 134, 064501/1-10. (6) Kashyap, H. K.; Santos, C. S.; Daly, R. P.; Hettige, J. J.; Murthy, N. S.; Shirota, H.; Castner, E. W., Jr.; Margulis, C. J. How Does the Ionic Liquid Organizational Landscape Change when Nonpolar Cationic Alkyl Groups Are Replaced by Polar Isoelectronic Diethers? J. Phys. Chem. B 2013, 117, 1130−1135. (7) Shigeto, S.; Hamaguchi, H.-o. Evidence for mesoscopic local structures in ionic liquids: CARS signal spatial distribution of Cnmim[PF6] (n = 4,6,8). Chem. Phys. Lett. 2006, 427, 329−332. (8) Wang, Y.; Voth, G. A. Unique Spatial Heterogeneity in Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 12192−12193. (9) Wang, Y.; Voth, G. A. Tail Aggregation and Domain Diffusion in Ionic Liquids. J. Phys. Chem. B 2006, 110, 18601−18608. (10) Canongia Lopes, J. N. A.; Padua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B 2006, 110, 3330−3335. (11) Canongia Lopes, J. N.; Gomes, M. F. C.; Padua, A. A. H. Nonpolar, Polar, and Associating Solutes in Ionic Liquids. J. Phys. Chem. B 2006, 110, 16816−16818. (12) Bhargava, B. L.; Devane, R.; Klein, M. L.; Balasubramanian, S. Nanoscale Organization in Room Temperature Ionic Liquids: A Coarse Grained Molecular Dynamics Simulation Study. Soft Matter 2007, 3, 1395−1400. (13) Heintz, A. Recent Developments in Thermodynamics and Thermophysics of Non-Aqueous Mixtures Containing Ionic Liquids. A Review. J. Chem. Thermodyn. 2005, 37, 525−535. (14) Chowdhury, S. A.; Scott, J. L.; MacFarlane, D. R. Ternary Mixtures of Phosphonium Ionic Liquids + Organic Solvents + Water. Pure Appl. Chem. 2008, 80, 1325−1335. (15) Barghamadi, M.; Best, A. S.; Bhatt, A. I.; Hollenkamp, A. F.; Musameh, M.; Rees, R. J.; Ruther, T. Lithium-Sulfur Batteries-the

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b10917. Lists of fit parameters for the Kerr transients and the Kerr spectra of the [MOIm][NTf2]−benzene mixtures (PDF) 523

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The Journal of Physical Chemistry B Solution is in the Electrolyte, but is the Electrolyte a Solution? Energy Environ. Sci. 2014, 7, 3902−3920. (16) Reddy, P. N.; Padmaja, P.; Subba Reddy, B. V.; Rambabu, G. Ionic Liquid/Water Mixture Promoted Organic Transformations. RSC Adv. 2015, 5, 51035−51054. (17) Harper, J. B.; Lynden-Bell, R. M. Macroscopic and Microscopic Properties of Solutions of Aromatic Compounds in an Ionic Liquid. Mol. Phys. 2004, 102, 85−94. (18) Shimizu, K.; Gomes, M. F. C.; Padua, A. A. H.; Rebelo, L. P. N.; Lopes, J. N. C. On the Role of the Dipole and Quadrupole Moments of Aromatic Compounds in the Solvation by Ionic Liquids. J. Phys. Chem. B 2009, 113, 9894−9900. (19) Xue, L.; Tamas, G.; Matthews, R. P.; Stone, A. J.; Hunt, P. A.; Quitevis, E. L.; Lynden-Bell, R. M. An OHD-RIKES and Simulation Study Comparing a Benzylmethylimidazolium Ionic Liquid with an Equimolar Mixture of Dimethylimidazolium and Benzene. Phys. Chem. Chem. Phys. 2015, 17, 9973−9983. (20) Shirota, H. Intermolecular/Interionic Vibrations of 1-Methyl-3-noctylimidazolium Tetrafluoroborate Ionic Liquid and Benzene Mixtures. J. Phys. Chem. B 2013, 117, 7985−7995. (21) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Liquid Clathrate Formation in Ionic Liquid−Aromatic Mixtures. Chem. Commun. 2003, 476−477. (22) Deetlefs, M.; Hardacre, C.; Nieuwenhuyzen, M.; Sheppard, O.; Soper, A. K. Structure of Ionic Liquid-Benzene Mixtures. J. Phys. Chem. B 2005, 109, 1593−1598. (23) Shimomura, T.; Takamuku, T.; Yamaguchi, T. Clusters of Imidazolium-Based Ionic Liquid in Benzene Solutions. J. Phys. Chem. B 2011, 115, 8518−8527. (24) Nockemann, P.; Binnemans, K.; Driesen, K. Purification of Imidazolium Ionic Liquids for Spectroscopic Applications. Chem. Phys. Lett. 2005, 415, 131−136. (25) Lachwa, J.; Szydlowski, J.; Makowska, A.; Seddon, K. R.; Esperanca, J. M. S. S.; Guedes, H. J. R.; Rebelo, L. P. N. Changing from an Unusual High-Temperature Demixing to a UCST-Type in Mixtures of 1-Alkyl-3-methylimidazolium Bis{(trifluoromethyl)sulfonyl}amide and Arenes. Green Chem. 2006, 8, 262−267. (26) Makowska, A.; Siporska, A.; Szydlowski, J. Isotope Effects on Miscibility of 1-Alkyl-3-methylimidazolium Bis(trifluoromethyl)sulfonyl Imides with Aromatic Hydrocarbons. Fluid Phase Equilib. 2009, 282, 108−112. (27) Mizuno, K.; Tamiya, Y.; Mekata, M. External Double Reference Method to Study Concentration and Temperature Dependences of Chemical Shifts Determined on a Unified Scale. Pure Appl. Chem. 2004, 76, 105−114. (28) Mizuno, K.; Imafuji, S.; Ochi, T.; Ohta, T.; Maeda, S. Hydration of the CH Groups in Dimethyl Sulfoxide Probed by NMR and IR. J. Phys. Chem. B 2000, 104, 11001−11005. (29) Momoki, K.; Fukazawa, Y. Bulbed Capillary External Referencing Method for Proton Nuclear Magnetic Resonance Spectroscopy. Anal. Chem. 1990, 62, 1665−1571. (30) Momoki, K.; Fukazawa, Y. Bulbed Capillary External Referencing Method Using a Superconducting Magnet NMR Instrument. Anal. Sci. 1994, 10, 53−58. (31) McMorrow, D.; Lotshaw, W. T.; Kenney-Wallace, G. A. Femtosecond Optical Kerr Studies on the Origin of the Nonlinear Responses in Simple Liquids. IEEE J. Quantum Electron. 1988, 24, 443− 454. (32) Lotshaw, W. T.; McMorrow, D.; Thantu, N.; Melinger, J. S.; Kitchenham, R. Intermolecular Vibrational Coherence in Molecular Liquids. J. Raman Spectrosc. 1995, 26, 571−583. (33) Kinoshita, S.; Kai, Y.; Ariyoshi, T.; Shimada, Y. Low Frequency Modes Probed by Time-Domain Optical Kerr Effect Spectroscopy. Int. J. Mod. Phys. B 1996, 10, 1229−1272. (34) Castner, E. W., Jr.; Maroncelli, M. Solvent Dynamics Derived from Optical Kerr Effect, Dielectric Dispersion, and Time-Resolved Stokes Shift Measurements: An Empirical Comparison. J. Mol. Liq. 1998, 77, 1−36.

(35) Smith, N. A.; Meech, S. R. Optically-Heterodyne-Detected Optical Kerr Effect (OHD-OKE): Applications in Condensed Phase Dynamics. Int. Rev. Phys. Chem. 2002, 21, 75−100. (36) Zhong, Q.; Fourkas, J. T. Optical Kerr Effect Spectroscopy of Simple Liquids. J. Phys. Chem. B 2008, 112, 15529−15539. (37) Shirota, H.; Fujisawa, T.; Fukazawa, H.; Nishikawa, K. Ultrafast Dynamics in Aprotic Molecular Liquids: A Femtosecond RamanInduced Kerr Effect Spectroscopic Study. Bull. Chem. Soc. Jpn. 2009, 82, 1347−1366. (38) Shirota, H. Comparison of Low-Frequency Spectra between Aromatic and Nonaromatic Cation Based Ionic Liquids Using Femtosecond Raman-Induced Kerr Effect Spectroscopy. ChemPhysChem 2012, 13, 1638−1648. (39) Shirota, H.; Fukazawa, H. Atom Substitution Effects in Ionic Liquids: A Microscopic View by Femtosecond Raman-Induced Kerr Effect Spectroscopy. In Ionic Liquids: Theory, Properties, New Approaches; Kokorin, A., Ed.; InTech: Rijeka, Croatia, 2011; Chapter 9; pp 201−224. (40) Shirota, H.; Kakinuma, S. Temperature Dependence of LowFrequency Spectra in Molten Bis(trifluoromethylsulfonyl)amide Salts of Imidazolium Cations Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy. J. Phys. Chem. B 2015, 119, 9835−9846. (41) Shirota, H.; Ishida, T. Microscopic Aspects in Dicationic Ionic Liquids through the Low-Frequency Spectra by Femtosecond RamanInduced Kerr Effect Spectroscopy. J. Phys. Chem. B 2011, 115, 10860− 10870. (42) Shirota, H.; Fukazawa, H.; Fujisawa, T.; Wishart, J. F. Heavy Atom Substitution Effects in Non-Aromatic Ionic Liquids: Ultrafast Dynamics and Physical Properties. J. Phys. Chem. B 2010, 114, 9400−9412. (43) Shirota, H.; Wishart, J. F.; Castner, E. W., Jr. Intermolecular Interactions and Dynamics of Room Temperature Ionic Liquids That Have Silyl- and Siloxy-Substituted Imidazolium Cations. J. Phys. Chem. B 2007, 111, 4819−4829. (44) Shirota, H.; Castner, E. W., Jr. Physical Properties and Intermolecular Dynamics of an Ionic Liquid Compared with its Isoelectronic Neutral Binary Solution. J. Phys. Chem. A 2005, 109, 9388−9392. (45) Xiao, D.; Hines, L. G., Jr.; Li, S.; Bartsch, R. A.; Quitevis, E. L.; Russina, O.; Triolo, A. Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids. J. Phys. Chem. B 2009, 113, 6426−6433. (46) Xiao, D.; Rajian, J. R.; Cady, A.; Li, S.; Bartsch, R. A.; Quitevis, E. L. Nanostructural Organization and Anion Effects on the Temperature Dependence of the Optical Kerr Effect Spectra of Ionic Liquids. J. Phys. Chem. B 2007, 111, 4669−4677. (47) Rajian, J. R.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. TemperatureDependence of the Low-Frequency Spectrum of 1-Pentyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide Studied by Optical Kerr Effect Spectroscopy. Chem. Phys. Lett. 2004, 393, 372−377. (48) Hyun, B. R.; Dzyuba, S. V.; Bartsch, R. A.; Quitevis, E. L. Intermolecular Dynamics of Room-Temperature Ionic Liquids: Femtosecond Optical Kerr Effect Measurements on 1-Alkyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)imides. J. Phys. Chem. A 2002, 106, 7579−7585. (49) Sonnleitner, T.; Turton, D. A.; Hefter, G.; Ortner, A.; Waselikowski, S.; Walther, M.; Wynne, K.; Buchner, R. Ultra-Broadband Dielectric and Optical Kerr-Effect Study of the Ionic Liquids Ethyl and Propylammonium Nitrate. J. Phys. Chem. B 2015, 119, 8826−8841. (50) Sonnleitner, T.; Turton, D. A.; Waselikowski, S.; Hunger, J.; Stoppa, A.; Walther, M.; Wynne, K.; Buchner, R. Dynamics of RTILs: A Comparative Dielectric and OKE Study. J. Mol. Liq. 2014, 192, 19−25. (51) Turton, D. A.; Hunger, J.; Stoppa, A.; Hefter, G.; Thoman, A.; Walther, M.; Buchner, R.; Wynne, K. Dynamics of Imidazolium Ionic Liquids from a Combined Dielectric Relaxation and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation. J. Am. Chem. Soc. 2009, 131, 11140−11146. (52) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, K. The Effects of Anion and Cation Substitution on the Ultrafast Solvent Dynamics of 524

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

Article

The Journal of Physical Chemistry B Ionic Liquids: A Time-Resolved Optical Kerr-Effect Spectroscopic Study. J. Chem. Phys. 2003, 119, 464−477. (53) Shirota, H.; Biswas, R. Intermolecular/Interionic Vibrations of 1Methyl-3-n-octylimidazolium Tetrafluoroborate Ionic Liquid and H2O Mixtures. J. Phys. Chem. B 2012, 116, 13765−13773. (54) Bardak, F.; Xiao, D.; Hines, L. G.; Son, P.; Bartsch, R. A.; Quitevis, E. L.; Yang, P.; Voth, G. A. Nanostructural Organization in Acetonitrile/ Ionic Liquid Mixtures: Molecular Dynamics Simulations and Optical Kerr Effect Spectroscopy. ChemPhysChem 2012, 13, 1687−1700. (55) Yang, P.; Voth, G. A.; Xiao, D.; Hines, L. G., Jr.; Bartsch, R. A.; Quitevis, E. L. Nanostructural Organization in Carbon Disulfide/Ionic Liquid Mixtures: Molecular Dynamics Simulations and Optical Kerr Effect Spectroscopy. J. Chem. Phys. 2011, 135, 034502/1-12. (56) Xiao, D.; Hines, L. G., Jr.; Bartsch, R. A.; Quitevis, E. L. Intermolecular Vibrational Motions of Solute Molecules Confined in Nonpolar Domains of Ionic Liquids. J. Phys. Chem. B 2009, 113, 4544− 4548. (57) Shirota, H. Ultrafast Molecular Dynamics of Liquid Aromatic Molecules and the Mixtures with CCl4. J. Chem. Phys. 2005, 122, 044514/1-12. (58) Shirota, H. Intermolecular Vibrations and Diffusive Orientational Dynamics of Cs Condensed Ring Aromatic Molecular Liquids. J. Phys. Chem. A 2011, 115, 14262−14275. (59) Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties. J. Phys. Chem. B 2006, 110, 19593−19600. (60) Carrera, G. V. S. M.; Afonso, C. A. M.; Branco, L. C. Interfacial Properties, Densities, and Contact Angles of Task Specific Ionic Liquids. J. Chem. Eng. Data 2010, 55, 609−615. (61) Almantariotis, D.; Gefflaut, T.; Padua, A. A. H.; Coxam, J.-Y.; Gomes, M. F. C. Effect of Fluorination and Size of the Alkyl Side-Chain on the Solubility of Carbon Dioxide in 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide Ionic Liquids. J. Phys. Chem. B 2010, 114, 3608−3617. (62) CRC Handbook of Chemistry and Physics, 89 ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, 2008. (63) Cole, A. R. H.; Michell, A. J. Solvent Effects on Out-of-Plane C-H Bending Absorption of Benzene Derivatives. Spectrochim. Acta 1964, 20, 747−758. (64) Shimomura, T.; Inoue, S.; Kadohata, S.; Umecky, T.; Takamuku, T. SANS, ATR-IR, and 1D- and 2D-NMR Studies of Mixing States of Imidazolium-Based Ionic Liquid and Aryl Solvents. Phys. Chem. Chem. Phys. 2013, 15, 20565−20576. (65) Narten, A. H. Diffraction Pattern and Structure of Liquid Benzene. J. Chem. Phys. 1968, 48, 1630−1634. (66) Tsuzuki, S.; Mikami, M.; Yamada, S. Origin of Attraction, Magnitude, and Directionality of Interactions in Benzene Complexes with Pyridinium Cations. J. Am. Chem. Soc. 2007, 129, 8656−8662. (67) Fujii, K.; Soejima, Y.; Kyoshoin, Y.; Fukuda, S.; Kanzaki, R.; Umebayashi, Y.; Yamaguchi, T.; Ishiguro, S.-i.; Takamuku, T. Liquid Structure of Room-Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Bis-(trifluoromethanesulfonyl)imide. J. Phys. Chem. B 2008, 112, 4329−4336. (68) Takamuku, T.; Honda, Y.; Fujii, K.; Kittaka, S. Aggregation of Imidazolium Ionic Liquids in Molecular Liquids Studied by Small-Angle Neutron Scattering and NMR. Anal. Sci. 2008, 24, 1285−1290. (69) Shimomura, T.; Fujii, K.; Takamuku, T. Effects of the Alkyl-Chain Length on the Mixing State of Imidazolium-Based Ionic LiquidMethanol Solutions. Phys. Chem. Chem. Phys. 2010, 12, 12316−12324. (70) Takamuku, T.; Shimomura, T.; Sadakane, K.; Koga, M.; Seto, H. Aggregation of 1-Dodecyl-3-methylimidazolium Nitrate in Water and Benzene Studied by SANS and 1H NMR. Phys. Chem. Chem. Phys. 2012, 14, 11070−11080. (71) Umecky, T.; Takamuku, T.; Matsumoto, T.; Kawai, E.; Takagi, M.; Funazukuri, T. Effects of Dissolved Water on Li+ Solvation in 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid Studied by NMR. J. Phys. Chem. B 2013, 117, 16219− 16226.

(72) Takamuku, T.; Hoke, H.; Idrissi, A.; Marekha, B. A.; Moreau, M.; Honda, Y.; Umecky, T.; Shimomura, T. Microscopic Interactions of the Imidazolium-Based Ionic Liquid with Molecular Liquids Depending on Their Electron-Donicity. Phys. Chem. Chem. Phys. 2014, 16, 23627− 23638. (73) Marekha, B. A.; Kalugin, O. N.; Bria, M.; Idrissi, A. Probing Structural Patterns of Ion Association and Solvation in Mixtures of Imidazolium Ionic Liquids with Acetonitrile by Means of Relative 1H and 13C NMR Chemical Shifts. Phys. Chem. Chem. Phys. 2015, 17, 23183−23194. (74) Elaiwi, A.; Hitchcock, P. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y.M.; Welton, T.; Zora, J. A. Hydrogen Bonding in Imidazolium Salts and Its Implications for Ambient-Temperature Halogenoaluminate(III) Ionic Liquids. J. Chem. Soc., Dalton Trans. 1995, 3467−3472. (75) Hardacre, C.; Holbrey, J. D.; McMath, S. E. J.; Bowron, D. T.; Soper, A. K. Structure of Molten 1,3-Dimethylimidazolium Chloride Using Neutron Diffraction. J. Chem. Phys. 2003, 118, 273−278. (76) Takamuku, T.; Kyoshoin, Y.; Shimomura, T.; Kittaka, S.; Yamaguchi, T. Effect of Water on Structure of Hydrophilic Imidazolium-Based Ionic Liquid. J. Phys. Chem. B 2009, 113, 10817− 10824. (77) Kanzaki, R.; Mitsugi, T.; Fukuda, S.; Fujii, K.; Takeuchi, M.; Soejima, Y.; Takamuku, T.; Yamaguchi, T.; Umebayashi, Y.; Ishiguro, S.i. Ion-Ion Interaction in Room Temperature Ionic Liquid 1-Ethyl-3methylimidazolium Tetrafluoroborate Studied by Large Angle X-ray Scattering Experiment and Molecular Dynamics Simulations. J. Mol. Liq. 2009, 147, 77−82. (78) Cang, H.; Li, J.; Fayer, M. D. Orientational Dynamics of the Ionic Organic Liquid 1-Ethyl-3-Methylimidazolium Nitrate. J. Chem. Phys. 2003, 119, 13017−13023. (79) Li, J.; Wang, I.; Fruchey, K.; Fayer, M. D. Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis. J. Phys. Chem. A 2006, 110, 10384−10391. (80) Sturlaugson, A. L.; Fruchey, K. S.; Fayer, M. D. Orientational Dynamics of Room Temperature Ionic Liquid/Water Mixtures: WaterInduced Structure. J. Phys. Chem. B 2012, 116, 1777−1787. (81) McMorrow, D.; Lotshaw, W. T. The Frequency Response of Condensed-Phase Media to Femtosecond Optical Pulses: SpectralFilter Effects. Chem. Phys. Lett. 1990, 174, 85−94. (82) McMorrow, D.; Lotshaw, W. T. Intermolecular Dynamics in Acetonitrile Probed with Femtosecond Fourier-Transform Raman Spectroscopy. J. Phys. Chem. 1991, 95, 10395−10406. (83) Bucaro, J. A.; Litovitz, T. A. Rayleigh Scattering: Collisional Motions in Liquids. J. Chem. Phys. 1971, 54, 3846−3853. (84) Chang, Y. J.; Castner, E. W., Jr. Fast Responses from ″Slowly Relaxing″ Liquids: A Comparative Study of the Femtosecond Dynamics of Triacetin, Ethylene glycol, and Water. J. Chem. Phys. 1993, 99, 7289− 7299. (85) Xiao, D.; Hines, L. G., Jr.; Holtz, M. W.; Song, K.; Bartsch, R. A.; Quitevis, E. L. Effect of Cation Symmetry on the Low-Frequency Spectra of Imidazolium Ionic Liquids: OKE and Raman Spectroscopic Measurements and DFT Calculations. Chem. Phys. Lett. 2010, 497, 37− 42. (86) Shirota, H.; Castner, E. W., Jr. Why Are Viscosities Lower for Ionic Liquids with -CH2Si(CH3)3 vs -CH2C(CH3)3 Substitutions on the Imidazolium Cations? J. Phys. Chem. B 2005, 109, 21576−21585. (87) Fujisawa, T.; Nishikawa, K.; Shirota, H. Comparison of Interionic/Intermolecular Vibrational Dynamics between Ionic Liquids and Concentrated Electrolyte Solutions. J. Chem. Phys. 2009, 131, 244519/1-14. (88) Fukazawa, H.; Ishida, T.; Shirota, H. Ultrafast Dynamics in 1Butyl-3-Methylimidazolium-Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study. J. Phys. Chem. B 2011, 115, 4621−4631. (89) Shirota, H.; Matsuzaki, H.; Ramati, S.; Wishart, J. F. Effects of Aromaticity in Cations and Their Functional Groups on the LowFrequency Spectra and Physical Properties of Ionic Liquids. J. Phys. Chem. B 2015, 119, 9173−9187. 525

DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526

Article

The Journal of Physical Chemistry B (90) Shirota, H.; Funston, A. M.; Wishart, J. F.; Castner, E. W., Jr. Ultrafast Dynamics of Pyrrolidinium Cation Ionic Liquids. J. Chem. Phys. 2005, 122, 184512/1-12. (91) Ryu, S.; Stratt, R. M. A Case Study in the Molecular Interpretation of Optical Kerr Effect Spectra: Instantaneous-Normal-Mode Analysis of the OKE Spectrum of Liquid Benzene. J. Phys. Chem. B 2004, 108, 6782−6795. (92) Ishida, T.; Shirota, H. Dicationic versus Monocationic Ionic Liquids: Distinctive Ionic Dynamics and Dynamical Heterogeneity. J. Phys. Chem. B 2013, 117, 1136−1150.

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DOI: 10.1021/acs.jpcb.5b10917 J. Phys. Chem. B 2016, 120, 513−526