Interionic Vibrations of 1-Methyl-3-n-octylimidazolium

Article pubs.acs.org/JPCB

Intermolecular/Interionic Vibrations of 1‑Methyl-3‑n‑octylimidazolium Tetrafluoroborate Ionic Liquid and Benzene Mixtures Hideaki Shirota* Department of Nanomaterial Science and Department of Chemistry, Chiba University, 1-33 Yayoi, Inage-ku Chiba 263-8522, Japan S Supporting Information *

ABSTRACT: Intermolecular/interionic vibrational spectra of mixtures composed of 1-methyl-3-n-octylimidazolium tetrafluoroborate ionic liquid and benzene at mole fractions of 0 (i.e., neat ionic liquid), 0.2, 0.4, 0.6, 0.8, and 1.0 (i.e., neat benzene) have been investigated using femtosecond Raman-induced Kerr effect spectroscopy. The line shape of the low-frequency Kerr spectra obtained from Fourier transform analyses of the Kerr transients is strongly dependent on the composition of the mixture. By comparing the experimental spectra to calculated spectra based on experimental spectra of the neat liquids, it is evident that the spectrum is not achieved by simply combining those of the neat liquids and taking the mole fraction into consideration. Close examination of the spectral comparison results in a microscopic picture involving specific stacking of imidazolium and benzene rings that is not sufficiently stable to affect the ring librations. The quantum chemistry calculation results also support this proposal. No clear correlation between the first moment of the spectrum and the bulk parameter (i.e., the square root of the surface tension divided by the liquid density), which occurs for neat liquids, is evident.

1. INTRODUCTION Because ionic liquids (ILs) possess many fascinating and unique properties, research into this area is currently expanding extensively.1−10 One of the most fascinating and unique aspects of ILs is their dynamic and static microheterogeneity.11−22 The microheterogeneity of ILs indeed provides their unique properties, that is, their solubility in a wide variety of solutes. Essentially, a nonpolar solute has a high affinity to the nonpolar region of the IL, while a polar solute has a low affinity or even aversion to the nonpolar region. It is therefore important to study ILs at the microscopic or molecular level for both a better and deeper understanding of the nature of ILs and the application and design of ILs for industrial and laboratory uses. Presently, mixtures of ILs with solvents are receiving more attention for several reasons, including that mentioned above.23−33 It is particularly important to identify the localization region or microenvironment of solvents (or mimic solute) in the matrix of the IL because the microscopic environment and intermolecular interactions of a solute significantly influence the reaction dynamics, kinetics, and yield. Also, new information and knowledge on mixtures of ILs with solvents are promising to be a help for the design and modification of fluid properties for wider application of ILs. One of the most effective and useful methods of investigating the microscopic or molecular-level properties, including the microstructure and microscopic intermolecular interactions, in condensed phases is femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) (alternatively named femtosecond optical Kerr effect spectroscopy), which can capture intermolecular vibrations that appear below 150 cm−1.34,35 This spectroscopic technique was originally developed to investigate the © XXXX American Chemical Society

ultrafast dynamics, such as intermolecular vibrational dynamics and collective orientational dynamics, of simple molecular liquids.36−40 More recently, it has been used for complex condensed phases,41−44 including ILs.45−47 In the earlier stage of applying fs-RIKES to study ILs, neat ILs were targeted. Quitevis and co-workers reported the dependence of the low-frequency Kerr spectrum on the alkyl group of the imidazolium cation.48 They also studied the temperaturedependent low-frequency spectrum of an imidazolium-based IL with the bis(trifluoromethylsulfonyl)amide49,50 anion and the effect of mixing binary ILs on the interionic vibrational band.51,52 Wynne and co-workers demonstrated the effect of anion substitution of imidazolium-based ILs on the lowfrequency spectrum.53 Recently, they compared the lowfrequency spectrum of several imidazolium-based ILs obtained by fs-RIKES with the spectra obtained by terahertz time domain spectroscopy and dielectric relaxation measurements.54 We extensively investigated the effect of heavy atom substitution of the cation and anion on the interionic vibrations of both aromatic and nonaromatic ILs.55−58 In addition, we reported some comparative studies, such as comparing an IL with a neutral binary mixture composed of isoelectronic neutral mimic molecules of the cation and anion59 and comparing ILs with highly concentrated electrolyte solutions,60 to reveal the unique nature of ILs. The interionic vibrational spectra of some ILs, such as nonaromatic ILs,61 imidazolium-based ILs with siloxy or silyl groups,62 dicationic ILs,63,64 and imidazolium-based Received: March 11, 2013 Revised: May 30, 2013

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dx.doi.org/10.1021/jp402456g | J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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ILs with a wide variety of anions,65 were also discussed on the basis of the ionic/molecular structure. Some specific topics, such as the effects of heavy atom substitution and aromaticity on neat ILs, that were investigated via fs-RIKES were reviewed in refs 46 and 47. More recently, mixtures of ILs with CS2 and acetonitrile were investigated using collaboration fs-RIKES by Quitevis and coworkers and molecular dynamics (MD) simulations by Voth and co-workers.66−68 They found that nonpolar CS2 molecules are located at the nonpolar region of the IL under low concentration conditions, while polar acetonitrile molecules are located at the interfacial region between the nonpolar and ionic regions. We also explored mixtures of an imidazolium-based IL with H2O.69 A tiny spectral shift of the low-frequency spectrum occurred with the addition of H2O up to the H2O mole fraction of 0.6. We concluded that H2O molecules are localized at the interfacial region and only slightly weaken the interionic interactions. In this study, mixtures of 1-methyl-3-n-octylimidazolium tetrafluoroborate ([MOIm][BF4], Chart 1) with benzene were

Details of the femtosecond optical heterodyne-detected RIKES apparatus used in this study were already reported elsewhere.40,73,74 The light source in the current RIKES setup was a titanium sapphire laser (KMLabs Inc., Griffin) pumped by a Nd:VO4 diode laser (Spectra Physics, Millennia Pro 5sJ).74 The output power of the titanium sapphire laser was approximately 440 mW. The typical temporal response, which was the cross correlation between the pump and probe pulses, was measured using a 200 μm thick KDP crystal (type I) to be 33 ± 3 fs (full width at half-maximum). The scans with a high time resolution of 2048 points at 0.5 μm/step were performed for a short time window (6.8 ps). Intermediate time window transients (∼30 ps) and long time window transients (∼300 ps) were recorded at 5.0 and 50.0 μm/step, respectively. Pure heterodyne signals were achieved by combining the Kerr transients recorded at ∼1.5° rotations of the input polarizer to both positive and negative orientations to eliminate the residual homodyne signal. For the [MOIm][BF4]−benzene mixtures and neat [MOIm][BF4] samples, 3, 6, and 9 scans for each polarization measurement were averaged for the short, intermediate, and long time window transients, respectively. Only the short and intermediate time window transients were measured for neat benzene because of the fast relaxation of its diffusive overdamped decay (Figure 2c, vide infra). Prior to the femtosecond RIKES measurements, the samples were injected into a 3 mm optical path length quartz cell (Tosoh Quartz) using either a 0.2 or 0.02 μm Anotop filter (Whatman). All of the RIKES measurements were made at 292 ± 1 K. Ab initio quantum chemistry calculations were performed at the B3LYP/6-311++G(d,p) level of theory75,76 to obtain optimized structures of the model imidazolium (1,3-dimethylimidazolium, [DMIm]+), benzene, and [DMIm]+−benzene clusters as well as the stabilization energies of the [DMIm]+− benzene clusters using the Gaussian 03 program suite.77 The calculation provides a broad understanding of the interaction between the imidazolium ring and benzene; therefore, a simple imidazolium, [DMIm]+, is likely sufficient. To assess the effect of dielectric media, the IEF-PCM model was used.78−80 Acetonitrile was chosen as the dielectric medium because the polarity of ILs is similar to that of acetonitrile.1 The obtained atom coordinates of the optimized structures of [DMIm]+, benzene, and [DMIm]+− benzene clusters are summarized in the Supporting Information.

Chart 1

investigated using fs-RIKES. Some bulk physical properties such as density, shear viscosity, and surface tension were also measured for comparison with the RIKES results. The motivation for this study comes from the recent study of 1-dodecyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide and benzene mixtures using several techniques, including X-ray and neutron scattering, by Takamuku and co-workers;33 the results indicate a unique sandwich-type structure of the imidazolium ring and benzene via π−π interactions. Preceding this study, in fact, several groups also studied the structure of imidazoliumcation-based IL mixtures with benzene and pointed out the stable sandwich-type structure of the imidazolium ring and benzene in the mixture.70−72 Here, using fs-RIKES, we attempt to elucidate the microscopic aspects of an IL−benzene mixture, including the interionic/intermolecular vibrations and microscopic interionic/intermolecular interactions.

3. RESULTS Table 1 summarizes the values of d, η, and γ of the [MOIm][BF4]− benzene mixtures, neat [MOIm][BF4], and neat benzene at 292 K;

2. MATERIALS AND METHODS [MOIm][BF4] (Iolitec, 99%) was used after drying in vacuo (∼10−3 Torr) at 313 K for over 36 h. The H2O content of [MOIm][BF4] was estimated to be 25.3 ppm via Karl Fischer titration using a coulometer (Hiranuma, AQ-300). Benzene (Wako Pure Chemical, 99.7%) was used as received. Note that the mixture with the mole fraction of benzene (X) of 0.9 was not transparent and showed phase separation. The shear viscosities (η) of the [MOIm][BF4]−benzene mixtures, neat [MOIm][BF4], and neat benzene were measured using a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) equipped with a circulating water bath (Yamato, BB300) at 292.0 ± 0.2 K. The surface tensions (γ) of the samples were measured using a duNouy tensiometer (Yoshida Seisakusho) at 292.0 ± 0.3 K, and the densities (d) of the samples were obtained at 292.0 ± 0.3 K using a 2 mL volumetric flask.

Table 1. Density d, Shear Viscosity η, and Surface Tension γ of [MOIm][BF4]−Benzene Mixtures at 292 K X

da,b (g/dm3)

ηc,d (cP)

γe,f (mN/m)

0.0 (IL) 0.2 0.4 0.6 0.8 1.0 (benzene)

1.116 1.085 1.062 1.036 0.986 0.875

473 208 83.4 27.6 6.36 0.688

32.9 32.7 30.5 29.2 28.9 28.8

a Temperature: 292.0 ± 0.3 K. bError: ±1%. cTemperature: 292.0 ± 0.2 K. dError: ±5%. eTemperature: 292.0 ± 0.3 K. fError: ±3%.

they are also plotted with respect to X in Figure 1. The values for neat [MOIm][BF4] and benzene are in good agreement with reported values ([MOIm][BF4]69,81,82 and benzene40,74,83) at B



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similar temperatures. It is clear from Figure 1 that the dependence on X differs for each bulk property.

Figure 1. Dependence of (a) density, d, (b) shear viscosity, η, and (c) surface tension, γ, on the mole fraction of benzene, X, of [MOIm][BF4]−benzene mixtures.

Figure 2. Normalized Kerr transients of [MOIm][BF4]−benzene mixtures. (a) Short time window data and (b) long time- window data (dots). (c) Long time window transient for neat benzene (dots). Multiexponential fits from 3 ps for the long time window transients are represented by the solid lines.

Figure 2a shows the short-time-range Kerr transients for the [MOIm][BF4]−benzene mixtures, and Figure 2b and c shows the long-time-range Kerr transients for the [MOIm][BF4]− benzene mixtures with those of neat [MOIm][BF4] and neat benzene, respectively. The intensities of the Kerr transients are normalized to the signal intensity at t = 0 (electronic response). As shown in the figures, the relative intensity of the nuclear response to the electronic response increases with a larger X of benzene; therefore, benzene is more polarizable than [MOIm][BF4] in the nuclear part. The Kerr transients over 3 ps are fitted using a triexponential function, except for that of neat benzene, which is fitted with a biexponential function. The fits are included in Figure 2b and c, and the fit parameters are summarized in Table 2. The slowest relaxation (or α-relaxation) in typical ILs is in the nanosecond range;84−86 thus, the present fs-RIKES system cannot observe the entire relaxation process. Accordingly, the faster dynamics (i.e., intermolecular/interionic vibrational dynamics) is the focus of this study, although the present system captures a trace of the slow relaxation process (i.e., the a0 component).

The Fourier transform Kerr spectra obtained from the Kerr transients for the [MOIm][BF4]−benzene mixtures, neat [MOIm][BF4], and neat benzene are shown in Figure 3. Note that the contributions of the slow overdamped relaxation process (i.e., the intermediate and slow exponential components for the [MOIm][BF4]−benzene mixtures and neat [MOIm][BF4] and the slow exponential component for neat benzene) are subtracted from these spectra. The Fourier transform deconvolution analysis method was originally developed by McMorrow and Lotshaw;87,88 details of the analysis method used in this study have been reported elsewhere.40,47 As seen in Figure 3, the Kerr spectra in this study are well-resolved up to 700 cm−1. The band at ∼607 cm−1 is attributed to the degenerate ring deformation modes of benzene.73 The broad band below 150 cm−1 is mainly due to intermolecular/interionic vibrations. It is evident from the figure that the intensity of the low-frequency broad band increases with increasing X. C



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Table 2. Triexponential Fit Parameters for Kerr Transients in [MOIm][BF4]−Benzene Mixtures X

a0

0.0 (IL) 0.2 0.4 0.6 0.8 1.0 (benzene)

0.000138 0.000097 0.000093 0.000105 0.000057 0

± ± ± ± ±

a1 0.000094 0.000011 0.000048 0.000007 0.000006

0.004051 ± 0.000428 0.010048 ± 0.000144 0.009777 ± 0.000475 0.016754 ± 0.000632 0.034558 ± 0.001070 0.05273 ± 0.00263

τ1 (ps)

a2

± ± ± ± ± ±

0.000723 ± 0.000084 0.001865 ± 0.000094 0.003002 ± 0.000267 0.004600 ± 0.000035 0.011588 ± 0.000027 0.04118 ± 0.00087

1.68 1.19 1.66 1.61 1.37 1.15

0.17 0.09 0.11 0.08 0.04 0.04

τ2 (ps) 10.01 7.38 6.54 6.25 5.69 3.24

± ± ± ± ± ±

2.38 0.47 0.45 0.41 0.11 0.02

τ3 (ps)

a3 0.000267 0.000535 0.000953 0.001644 0.001500

± ± ± ± ±

0.000068 0.000054 0.000026 0.000071 0.000044

62.93 83.30 79.87 42.22 45.80

± ± ± ± ±

18.10 8.19 4.80 1.94 1.75

where aL, ωL, and ΔωL are the amplitude, peak frequency, and bandwidth parameters, respectively. The former two functions are mainly attributed to intermolecular/interionic vibrations, while the latter one corresponds to an intraionic vibrational band of the [MOIm]+ cation.69 Figure 4a and b shows the low-

Figure 3. Fourier-transform Kerr spectra of [MOIm][BF4]−benzene mixtures. (a) Entire frequency range spectra and (b) magnification of the low-frequency region.

The broad low-frequency spectra are replicated via line shape analysis with a sum of Ohmic, antisymmetrized Gaussian, and Lorentzian functions. This model function was first used for the low-frequency Kerr spectrum of simple molecular liquids by Chang and Castner.89 The Ohmic function is given by ⎛ ω⎞ IO(ω) = aOω exp⎜ − ⎟ ⎝ ωO ⎠

Figure 4. Low-frequency Kerr spectra and line shape analysis results of (a) neat [MOIm][BF4] and (b) a [MOIm][BF4]−benzene mixture with X = 0.6. Black dots denote the Kerr spectra, red lines denote entire fits, blue areas denote the Ohmic components (eq 1), green areas denote antisymmetric Gaussian components (eq 2), and brown areas denote the Lorentzian (intramolecular vibration) components (eq 3).

(1)

where aO and ωO are the amplitude and characteristic frequency parameters, respectively, of the Ohmic line shape. The antisymmetrized Gaussian function is given by 3

IG, i(ω) =

frequency Kerr spectra and line shape analysis results for a neat [MOIm][BF4] and the [MOIm][BF4]−benzene mixture with X = 0.6, respectively. As displayed in Figure 4, the function provides a good replicate of the complex low-frequency spectral shape. The line shape analysis results are summarized in Table 3. To semiqualitatively discuss the low-frequency Kerr spectra with respect to the bulk properties, the first moment (M1) of the spectrum is estimated by

⎡ −2(ω − ω )2 ⎤ ⎡ −2(ω + ω )2 ⎤ G, i G, i ⎥ − aG, i exp⎢ ⎥ ⎢⎣ ⎥⎦ ⎥⎦ ΔωG, i 2 ΔωG, i 2 ⎣⎢

∑ aG, i exp⎢ i=1

(2)

where aG,i, ω G,i, and Δω G,i are the amplitude, characteristic frequency, and bandwidth parameters, respectively, for the ith antisymmetrized Gaussian function. The Lorentzian function is aL IL(ω) = (ω − ωL)2 + ΔωL 2 (3)

M1 =

∫ ωI(ω) dω ∫ I(ω) dω

(4)

where I(ω) is the frequency-dependent spectral intensity estimated from the fit analysis (i.e., the sum of eqs 1 and 2). D


1.42 0.77 0.07 0.10 0.16 4.47 9.49 3.29 3.83 8.81 0.42 28.11 24.13 13.77 13.76 14.15 48.03 0.048 0.112 0.056 0.088 0.277 0.010 ± ± ± ± ± ± 3.51 5.58 1.47 2.48 6.78 2.28

27.23 29.90 19.59 18.72 21.37 31.93

2.28 2.82 1.15 1.80 4.26 1.58

0.251 0.270 0.259 0.351 0.482 1.695 ± ± ± ± ± ± ± ± ± ± ± ± 0.048 9.47 0.121 8.75 0.037 7.57 0.095 6.95 0.356 7.06 0.081 13.99 ± ± ± ± ± ± 0.001 0.001 0.001 0.001 0.001 0.002

4.20 5.03 3.51 3.38 4.02 5.59

0.04 0.03 0.23 0.03 0.04 0.07

0.131 0.195 0.175 0.254 0.356 0.568 ± ± ± ± ± ± ± ± ± ± ± ± 0.056 0.078 0.149 0.249 0.377 0.426 68.9 67.0 65.0 63.2 60.6 59.0 0.0 (IL) 0.2 0.4 0.6 0.8 1.0 (benzene)

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The integration range for estimating M1 is 0−1000 cm−1. Figure 5 shows plots of M1 versus X for the [MOIm][BF4]−benzene

a M1 = (∫ ωI(ω) dω/∫ I(ω) dω). I(ω) is the normalized Kerr spectrum by line shape analysis. The integration is made from 0 to 1000 cm−1. bωL in the mixtures is fixed at 175.15 cm−1 for fits that is the value for the neat IL.

175.15 ± 0.65 38.77 ± 0.64 175.15 40.21 ± 0.34 175.15 48.06 ± 0.42 175.15 41.10 ± 0.55 175.15 40.96 ± 0.96 ± ± ± ± ± ± ± ± ± ± 70.64 77.06 86.06 84.38 80.99 1.64 1.31 0.07 0.09 0.15 ± ± ± ± ± 89.10 78.74 61.95 60.54 60.41 0.009 0.012 0.001 0.001 0.002 ± ± ± ± ± 0.267 0.399 0.673 0.909 1.188 5.95 7.86 1.64 1.99 4.30 0.33 ± ± ± ± ± ± ± ± ± ± ± ±

59.20 60.83 36.94 34.81 37.44 90.35

ΔωG,2 (cm−1) ωG,2 (cm−1) aG,2 ΔωG,1 (cm−1) ωG,1 (cm−1) aG,1 ωO (cm−1) aO M1a (cm−1) X

Table 3. Fit Parameters for Fourier Transform Kerr Spectra of [MOIm][BF4]−Benzene Mixtures

aG,3

ωG,3 (cm−1)

46.10 54.81 87.41 74.85 71.99

aL ΔωG,3 (cm−1)

2.21 0.83 1.56 2.00 3.30

ωLb (cm−1)

ΔωL (cm−1)


Figure 5. X dependence of M1 for [MOIm][BF4]−benzene mixtures.

mixtures; M1 monotonically decreases with increasing X of the mixture. Figure 6 shows the calculated stable structures (T-shape one and slipped parallel one) of the [DMIm]+−benzene at the

Figure 6. Calculated structures of a model imidazolium (1,3dimethylimidazolium) and benzene based on the B3LYP/6-311++G(d,p) level of theory. The left-hand side shows the T-shape structure (optimized structure), and right-hand side shows the slipped parallel structure (local minimum).

B3LYP/6-311++G(d,p) level using the IEF-PCM model (acetonitrile). Note that the T-shaped structure was calculated as the optimized structure; at the initial condition, it was the parallel structure in this calculation. The interaction energy (Ei) is estimated by Ei = Ec − (E[DMIm]+ + E Bz)

(5)

where Ec is the energy of the cluster, E[DMIm] is the energy of the model cation, and EBz is the energy of benzene. The energies of [DMIm]+, benzene, and the [DMIm]+−benzene clusters as well as the interaction energies in the gas phase and in acetonitrile are summarized in Table 4. It is known that the +

Table 4. Calculated Energies of [DMIm]+, Benzene, and [DMIm]+−Benzene Clusters and Interaction Energies of [DMIm]+−Benzene Clusters E[DMIm]+, EBz, Ec (kJ/mol) [DMIm]+ benzene [DMIm]+−benzene (T-shape) [DMIm]+−benzene (slipped parallel) E

Ei (kJ/mol)

gas

acetonitrile

gas

acetonitrile

−801585.47 −609933.35 −1411546.21

−801792.03 −609949.74 −1411736.56

−27.39

−5.21

−1411536.38

−1411741.96

−17.56

−0.19



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stable structure of an N-alkylaromatic cation (not protonated), such as N-methylpyridinium, and the benzene cluster features parallel alignment; however, the pyridinium cation (protonated) and benzene show a stable T-shape structure. Also, the interaction energies of the clusters in the gas phase are quite strong.90 From the present quantum chemistry calculations, it is evident that the T-shape structure of the [DMIm]+−benzene cluster is stable, and the interaction energy in the gas phase is large (−27.39 kJ/mol) but decreases under acetonitrile dielectric condition (−5.21 kJ/mol). We can also find the stable slipped parallel structure for the [DMIm]+−benzene cluster in the gas phase (−17.56 kJ/mol), but this is not so stable in the acetonitrile dielectric medium (−0.19 kJ/mol).

4. DISCUSSION 4.1. X Dependence of Density, Shear Viscosity, and Surface Tension. As shown in Figure 1, the X dependence of d, η, and γ differs; d decreases monotonically with increasing X to X = 0.8, at which point the slope becomes steeper (Figure 1a), while η decreases exponentially with respect to X (Figure 1b). The X dependences of d and η are quite similar to that reported for imidazolium-based IL and benzene mixtures.91,92 It is predictable that the η of the IL steeply decreases with the addition of benzene because the η of [MOMIm][BF4] is much higher than that of benzene (Table 1). In the case of d, the mixture becomes immiscible when X = 0.9; these two characteristics might be related. According to the work by Takamuku and co-workers,33 the C−H out-of-plane bending mode of benzene and chemical shifts of benzene in the 1H and 13 C NMR spectra showed a break point at X ≈ 0.8 for the 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and benzene mixture; they concluded that benzene molecules begin to settle at the nonpolar region at this concentration. It is plausible that the density is related to the intermolecular interactions of benzene, which should be very different in the ionic region and nonpolar region, and thus, a unique X dependence is observed in the [MOMIm][BF4] mixture. In the case of γ, there is a weak critical point in the range of X = 0.2−0.4. Satnos and Baldelli reported the X dependence of γ in the 1-butyl-3-methylimidazolium tetrafluoroborate and benzene mixture.93 They found that γ decreases with increasing X, similar to that in the present study, but the variation is more monotonic. Because the present cation has a relatively long alkyl group, a critical point should become apparent compared to an IL whose cation has a relatively short alkyl group. 4.2. Features of the Low-Frequency Spectrum. As seen in Figure 3, the shape of the low-frequency spectrum of neat [MOMIm][BF4] is monomodal, while that of neat benzene is bimodal. The spectral shape gradually changes with increasing X, and the bimodal shape becomes evident at high benzene concentration (X = 0.8). To understand the effect of X on the spectral shape, we compare the experimental spectra with spectra that are obtained from the simple X-weighted summation of the spectra for neat [MOMIm][BF4] and benzene at a specific X value, as follows Ical(ω) = (1 − X )I[MOMIm][BF4](ω) + XIBz(ω)

Figure 7. Comparison between the experimental (solid lines) and calculated spectra (broken lines) of [MOIm][BF4]−benzene mixtures, X = (a) 0.8, (b) 0.6, (c) 0.4, and (b) 0.2.

of microsegregation or microphase separation in the mixtures. However, as is clearly evident in Figure 7, the calculated spectrum does not match the experimental spectrum at any concentration. This means that the microscopic structure and intermolecular/interionic interactions in the mixtures differ from those in the neat liquids. Because the intensity of the lowfrequency Kerr spectrum is predominantly from imidazolium ring libration,47,53 the results indicate that the imidazolium ring and benzene mix well at the molecular scale. Careful comparison of the experimental and calculated spectra (Figure 7) reveals that the difference in the spectral

(6)

where Ical(ω), I[MOMIm][BF4](ω), and IBz(ω) are the calculated, experimentally obtained [MOMIm][BF4], and experimental benzene spectra, respectively. Figure 7 displays comparisons of the experimental and calculated low-frequency spectra. If the experimental and calculated spectra match (i.e., show additivity), it indicates a trace F



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intensity in the low-frequency region (∼5−30 cm−1) is greater than that at the near peak (∼60 cm−1). Stratt and co-workers reported calculated low-frequency Kerr spectra of liquid benzene based on MD simulations and instantaneous normalmode analysis.94,95 According to their results, the translational component arising from interaction-induced (or collision-induced) motion appears in the low-frequency region (50 cm−1). For aromatic ILs, the spectrum intensity at the near peak and higher-frequency region is mainly attributed to the aromatic ring.47,53 Therefore, it is plausible that the mixture effect is more significant in the translational component than that in the rotational component. Previously, we compared the low-frequency Kerr spectra of polymer solutions with those of model monomer solutions and found that the spectral intensity for the polymer solutions was lower than that for the model monomer solutions, especially in the low-frequency region, when the concentration is identical.44,96,97 This is attributed to the decreased activity of the translational motion due to the heavier solute of the polymer compared to the model monomer. A similar feature was also confirmed in 1-butyl-3-methylimidazolium hexafluoropnitogenate ILs (1-butyl-3-methylimidazolium with the anions of hexafluorophosphate, hexafluoroarsenate, and hexafluoroantimonate).56 MD simulations of the three ILs showed that the translational motions of the ions become less active with larger/heavier anions.57 These results indicate that the translational motion of benzene is suppressed in the present mixtures due to the heavier mass of the cation and/or strong interactions with neighboring species. Another notable feature of the spectra of the mixture is the width. Figure 8 shows normalized experimental and calculated

clear from Figure 8 that the calculated spectrum is narrower than the experimental spectrum. The spectra of mixtures with different concentrations (i.e., X = 0.4 and 0.6) also show this feature. There are several possible origins for the spectral broadening. Because the samples are mixtures, an inhomogeneous broadening likely occurs due to the nature of mixing of the IL with benzene. In contrast to the inhomogeneous broadening, it would also be possible for the homogeneous broadening for the fast lifetime of the intermolecular/interionic interaction often caused by the stronger interaction. Another possible reason for the broadening of the lowfrequency Kerr spectrum of [MOIm][BF4]−benzene mixtures could be the increased intensity on the high-frequency side of the spectrum arising from modes present in the mixture. Recent MD simulation results by Shimizu,98 as well as MD simulation by Harper and Lynden-Bell,71 pointed out the importance of the interaction due to the quadrupole and ion in the imidazolium-cation-based IL and benzene mixture on the miscibility. Thus, it could happen that the quadrupole−ion interaction influences modes in the high-frequency region that is attributed to the ring librations and leads to broadening of the low-frequency spectrum. Unlike a fifth-order nonlinear spectroscopy, however, it is limited to assign the origin of the broadening of the spectrum measured by fs-RIIKES.99−101 However, the present RIKES results clearly show that the line shape of the low-frequency spectrum of [MOIm][BF4]− benzene mixtures cannot be reproduced by the simple weighted average spectrum of neat [MOIm][BF4] and benzene and the spectral broadening in the IL−benzene mixture system, for the first time. In addition, we should also consider the effect of the microstructure of the rings. If the sandwich-type structure of the rings33 is rather stable at the time scale of 100 fs, the librational motion of the sandwiched rings would be slower than that of the isolated ones. In fact, a low-frequency shift of the lowfrequency Kerr spectrum (i.e., slowing of the molecular/ionic motion) in 1-butyl-3-methylimidazolium tricyanomethide was observed, which implies that the cation and anion are stacked.65 However, such evidence was not observed in the present mixtures. Also, the quantum chemistry calculations show that the T-shape structure is stable (Figure 6), and the interaction energy of the complex weakens with the dielectric medium, as shown in Table 4. Furthermore, it is expected that the presence of an anion weakens the interactions between the rings. These results suggest that the ring interaction is not sufficiently strong to attenuate the librational motions of the aromatic rings and that the influence of the heterogeneity is greater than the effect of the interactions on the low-frequency spectrum. As discussed above, the molecular-level aspect (trace of the T-shape structure) in this RIKES study is somewhat different from the microscopic picture (sandwich-type structure) of other preceding works.33,70,72 Recent MD simulation of 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)amide and benzene mixture with X = 0.75 by Shimizu showed the spatial distribution of the C2 carbon of the imidazolium ring near the benzene ring and found that the C2 carbon is located on the benzene ring, but there are two spatial distributions of the C2 carbon of the imidazolium ring on one side of the benzene ring.98 One can think that if the picture by the MD simulation by result is close to the real situation, it is not surprising that we face the disagreement of the present RIKES result with the diffraction studies.33,70,72

Figure 8. Comparison of the normalized experimental (solid lines) and calculated (broken lines) spectra of [MOIm][BF4]−benzene mixtures with X = (a) 0.8 and (b) 0.2.

low-frequency spectra of [MOIm][BF4]−benzene mixtures with X = 0.8 and 0.2 to closely examine the spectral width. It is G



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In fact, a similar feature of the effect of X on the bulk parameter (γ/d)1/2, namely, almost X independence of (γ/d)1/2, was also identified in the case of the [OMIm][BF4]−H2O mixture.69 The microscopic interactions in the [OMIm][BF4]−H2O mixture are, however, somewhat different from those of the [OMIm][BF4]−benzene mixture. In the [OMIm][BF4]−H2O mixture, M1 changes only slightly with X, that is, a ∼2 cm−1 red shift with a concentration change from X = 0 (neat [OMIm][BF4]) to 0.6.69 We concluded that the H2O molecules are located at the interface between the ionic and nonpolar regions and do not break the ionic network much in the low H2O concentration region, especially in the [OMIm][BF4]−H2O mixture. In contrast, the present results indicate that the benzene molecules interfere with the interior of the ionic region, as discussed above. Thus, the microscopic interactions in the [OMIm][BF4]−benzene mixture weaken with increasing X but do not show a linear correlation with the bulk parameters. As discussed previously,47 the heterogeneity likely breaks a sort of scaling rule because of the specific characteristics of the microscopic region. However, one can simply think that the high-frequency shift of M1 with increasing X is affected by the nature of the low-frequency Kerr spectra of the neat solvents (68.9 cm−1 for [BMIm][BF4] and 59.0 cm−1 for benzene) and the fraction of the mixture. A similar feature was actually observed in CS2− alkane mixtures.103,104 Unlike simple monocomponent solvent systems, estimating the strength of the microscopic intermolecular (interionic) interaction is not straightforward. Because the low-frequency Kerr spectrum shape of the mixtures cannot be reproduced by the simple summation of the spectra of the two neat liquids as shown in Figure 7, it is no doubt that the effect of the intermolecular/interionic interactions on the spectrum shape is not small. However, it should be noticed that this simple mixture effect cannot be ignored with the lowfrequency shift of M1 with the larger X for the [BMIm][BF4]− benzene.

On the other hand, another possible reason why the RIKES does not capture a trace of the sandwich-type structure, if there is one, might be related to the interaction between the imidazolium and benzene rings. If both the T-shape and parallel structures exist in the mixture, the intermolecular interactions are likely different, as shown in the quantum chemistry calculation results; the T-shape pair should be much stronger than the parallel pair. It can be expected that the stronger intermolecular interaction in the T-shape structure unlikely slows down the ring librational motions because of the perpendicular structure of the rings. Also, unchanged interaction between the rings in the parallel shape should make it hard to affect the ring libration. In a sense, the major driven structure that determines in advance the T-shape and the parallel structure fits well in the liquid mixture geometrically if both the T-shape and parallel structures exist in the mixture. 4.3. Comparison with Bulk Properties. Previously, we showed a linear correlation between M1 of the low-frequency broad band and (γ/d)1/2 for aprotic molecular liquids.40 While the slope differs from that for the aprotic molecular liquids, a linear correlation between M1 of the low-frequency broad band and (γ/d)1/2 for aromatic ILs has also been confirmed.47 These plots are based on the simple consideration that an intermolecular vibrational band could be expressed similarly to a harmonic oscillator; however, this relationship essentially implies some connection between the microscopic scale (i.e., intermolecular vibrational band) and bulk scale (i.e., bulk properties) of the target systems. Here, we will elucidate this connection for the [MOIm][BF4]−benzene mixtures. Figure 9 shows plots of M1 versus (γ/d) 1/2 for the [MOIm][BF4]−benzene mixtures. The relationships of aprotic

5. CONCLUSIONS In this study, the intermolecular/interionic vibrations of [MOIm][BF4]−benzene mixtures have been investigated using fs-RIKES and are discussed on the basis of the Fourier transform Kerr spectra. The spectral shape of the mixtures depends strongly on X but cannot be simply explained by the addition of the spectra of neat [MOIm][BF4] and benzene considering X. From probing comparisons of the spectra from the RIKES experiments and a simple summation of the neat spectra with respect to X, it was determined that the translational motion is likely less active for the real mixture than that in the ideal (calculated) mixture. Also, the experimental spectrum is broader than that of the spectrum calculated from the neat IL and benzene spectra; this indicates that the vibrational bands due to librations of the imidazolium and benzene rings broaden. We have also observed that M1 of the low-frequency spectrum monotonically shifts to a lower frequency with increasing X; however, in contrast to that in neat liquids, this does not correlate with the bulk properties, (γ/d)1/2.

Figure 9. Plots of M1 versus (γ/d)1/2 for the [MOIm][BF4]−benzene mixtures (open triangles: neat [MOIm][BF4]; open squares: neat benzene; filled circles: [MOIm][BF4]−benzene mixtures). The corresponding plots for aromatic ILs (solid line)47 and aprotic molecular liquids (broken line)40,74,102 are shown for comparisons.

molecular liquids40,74,102 and aromatic ILs47 are also shown to identify the differences. It is evident that the relationship for the [MOIm][BF4]−benzene mixture system is completely different from those of the aromatic ILs and aprotic molecular liquids; thus, M1 is independent of (γ/d)1/2 because (γ/d)1/2 is rather independent of X. Also, that of benzene is a little different from those of the neat IL and mixtures; there is a large (γ/d)1/2 compared to that of the neat IL and its mixtures. Thus, it is evident that the microscopic intermolecular interactions in the [OMIm][BF4]−benzene mixtures do not directly correlate with the bulk properties, unlike in the neat systems.

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ASSOCIATED CONTENT

* Supporting Information S

Atomic coordinates based on the quantum chemistry calculations at the B3LYP/6-311++G(d,p) level of theory and complete author list for ref 77. This material is available free of charge via the Internet at http://pubs.acs.org. H



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(20) 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. (21) Samanta, A. Dynamic Stokes Shift and Excitation Wavelength Dependent Fluorescence of Dipolar Molecules in Room Temperature Ionic Liquids. J. Phys. Chem. B 2006, 110, 13704−13716. (22) Pal, T.; Biswas, R. Rank-Dependent Orientational Relaxation in an Ionic Liquid: An All-Atom Simulation Study. Theor. Chem. Acc. 2013, 132, 1348/1−1348/12. (23) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192−5200. (24) Koddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. The Association of Water in Ionic Liquids: A Reliable Measure of Polarity. Angew. Chem., Int. Ed. 2006, 45, 3697−3702. (25) Roth, C.; Peppel, T.; Fumino, K.; Kockerling, M.; Ludwig, R. The Importance of Hydrogen Bonds for the Structure of Ionic Liquids: Single-Crystal X-ray Diffraction and Transmission and Attenuated Total Reflection Spectroscopy in the Terahertz Region. Angew. Chem., Int. Ed. 2010, 49, 10221−10224. (26) Jeon, Y.; Sung, J.; Kim, D.; Seo, C.; Cheong, H.; Ouchi, Y.; Ozawa, R.; Hamaguchi, H.-o. Structural Change of 1-Butyl-3methylimidazolium Tetrafluoroborate + Water Mixtures Studied by Infrared Vibrational Spectroscopy. J. Phys. Chem. B 2008, 112, 923− 928. (27) Jeon, Y.; Sung, J.; Seo, C.; Lim, H.; Cheong, H.; Kang, M.; Moon, B.; Ouchi, Y.; Kim, D. Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy. J. Phys. Chem. B 2008, 112, 4735−4740. (28) Chang, H.-C.; Jiang, J.-C.; Liou, Y.-C.; Hung, C.-H.; Lai, T.-Y.; Lin, S. H. Effects of Water and Methanol on the Molecular Organization of 1-Butyl-3-methylimidazolium Tetrafluoroborate as Functions of Pressure and Concentration. J. Chem. Phys. 2008, 129, 044506/1−044506/6. (29) Danten, Y.; Cabao, M. I.; Besnard, M. Interaction of Water Highly Diluted in 1-Alkyl-3-methyl Imidazolium Ionic Liquids with the PF6− and BF4− Anions. J. Phys. Chem. A 2009, 113, 2873−2889. (30) Masaki, T.; Nishikawa, K.; Shirota, H. Microscopic Study of Ionic Liquid-H2O Systems: Alkyl-Group Dependence of 1-Alkyl-3methylimidazolium Cation. J. Phys. Chem. B 2010, 114, 6323−6331. (31) 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. (32) Shimomura, T.; Fujii, K.; Takamuku, T. Effects of the AlkylChain Length on the Mixing Sate of Imidazolium-Based Ionic Liquid− Methanol Solutions. Phys. Chem. Chem. Phys. 2010, 12, 12316−12324. (33) Shimomura, T.; Takamuku, T.; Yamaguchi, T. Clusters of Imidazolium-Based Ionic Liquid in Benzene Solutions. J. Phys. Chem. B 2011, 115, 8518−8527. (34) 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. (35) 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. (36) 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. (37) 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. (38) 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.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. Toshiyuki Takamuku (Saga University, Japan) is acknowledged for helpful discussion. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (Grant in Aid for Young Scientists (A): 21685001).

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REFERENCES

(1) Ionic Liquids in Synthesis, 2nd ed.; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2008. (2) Electrochemical Aspects of Ionic Liquids; Ohno, H., Ed.; WileyInterscience: Hoboken, NJ, 2005. (3) Seddon, K. R. Ionic Liquids for Clean Technology. J. Chem. Technol. Biotechnol. 1997, 68, 351−356. (4) Wilkes, J. S. A Short History of Ionic Liquids  From Molten Salts to Neoteric Solvents. Green Chem. 2002, 4, 73−80. (5) Welton, T. Ionic Liquids in Catalysis. Coord. Chem. Rev. 2004, 248, 2459−2477. (6) Weingaertner, H. Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies. Angew. Chem., Int. Ed. 2008, 47, 654−670. (7) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. Rev. 2008, 37, 123−150. (8) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Lquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621−629. (9) Wishart, J. F. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2009, 2, 956−961. (10) Castner, E. W., Jr.; Wishart, J. F. Spotlight on Ionic Liquids. J. Chem. Phys. 2010, 132, 120901/1−120901/9. (11) 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. (12) 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− 164521/6. (13) 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 Dynamics of 1-Alkyl-3methylimidazolium Bis{(trifluoromethane)sulfonyl}amide Ionic Liquids: Structural and Dynamic Evidence of Nanoscale Segregation. J. Phys.: Condens. Matter 2009, 21, 424121/1−424121/9. (14) 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. (15) 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. (16) Wang, Y. T.; Voth, G. A. Unique Spatial Heterogeneity in Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 12192−12193. (17) Wang, Y.; Voth, G. A. Tail Aggregation and Domain Diffusion in Ionic Liquids. J. Phys. Chem. B 2006, 110, 18601−18608. (18) Canongia Lopes, J. N. A.; Pádua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B 2006, 110, 3330−3335. (19) Canongia Lopes, J. N. A.; Gomes, M. F. C.; Pádua, A. A. H. Nonpolar, Polar, and Associating Solutes in Ionic Liquids. J. Phys. Chem. B 2006, 110, 16816−16818. I



Article

(57) Ishida, T.; Nishikawa, K.; Shirota, H. Atom Substitution Effects of [XF6]− in Ionic Liquids. 2. Theoretical Study. J. Phys. Chem. B 2009, 113, 9840−9851. (58) 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. (59) 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. (60) Fujisawa, T.; Nishikawa, K.; Shirota, H. Comparison of Interionic/Intermolecular Vibrational Dynamics between Ionic Liquids and Concentrated Electrolyte Solutions. J. Chem. Phys. 2009, 131, 244519/1−244519/14. (61) Shirota, H.; Funston, A. M.; Wishart, J. F.; Castner, E. W., Jr. Ultrafast Dynamics of Pyrrolidinium Cation Ionic Liquids. J. Chem. Phys. 2005, 122, 184512/1−184512/12. (62) 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. (63) 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. (64) Ishida, T.; Shirota, H. Dicationic versus Monocationic Ionic Liquids: Distinctive Ionic Dynamics and Dynamical Heterogeneity. J. Phys. Chem. B 2013, 117, 1136−1150. (65) 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. (66) 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. (67) 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, 034502/1−034502/12. (68) Bardak, F.; Xiao, D.; Hines, L. G., Jr.; 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. (69) Shirota, H.; Biswas, R. Intermolecular/Interionic Vibrations of 1-Methyl-3-n-octylimidazolium Tetrafluoroborate Ionic Liquid and H2O Mixtures. J. Phys. Chem. B 2012, 116, 13765−13773. (70) 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. (71) 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. (72) 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. (73) Shirota, H. Ultrafast Molecular Dynamics of Liquid Aromatic Molecules and the Mixtures with CCl4. J. Chem. Phys. 2005, 122, 044514/1−044514/12. (74) Shirota, H. Intermolecular Vibrations and Diffusive Orientational Dynamics of Cs Condensed Ring Aromatic Molecular Liquids. J. Phys. Chem. A 2011, 115, 14262−14275. (75) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (76) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−589.

(39) Zhong, Q.; Fourkas, J. T. Optical Kerr Effect Spectroscopy of Simple Liquids. J. Phys. Chem. B 2008, 112, 15529−15539. (40) 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. (41) Loughnane, B. J.; Farrer, R. A.; Scodinu, A.; Reilly, T.; Fourkas, J. T. Ultrafast Spectroscopic Studies of the Dynamics of Liquids Confined in Nanoporous Glasses. J. Phys. Chem. B 2000, 104, 5421− 5429. (42) Farrer, R. A.; Fourkas, J. T. Orientational Dynamics of Liquids Confined in Nanoporous Sol−Gel Glasses Studied by Optical Kerr Effect Spectroscopy. Acc. Chem. Res. 2003, 36, 605−612. (43) Hunt, N. T.; Jaye, A. A.; Meech, S. R. Ultrafast Dynamics in Complex Fluids Observed Through the Ultrafast Optically-Heterodyne-Hetected Optical-Kerr-Effect (OHD-OKE). Phys. Chem. Chem. Phys. 2007, 9, 2167−2180. (44) Shirota, H.; Castner, E. W., Jr. Ultrafast Dynamics in Aqueous Polyacrylamide Solutions. J. Am. Chem. Soc. 2001, 123, 12877−12885. (45) Castner, E. W., Jr.; Wishart, J. F.; Shirota, H. Intermolecular Dynamics, Interactions, and Solvation in Ionic Liquids. Acc. Chem. Res. 2007, 40, 1217−1227. (46) 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; pp 201−224. (47) 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, 1217−1227. (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-3methylimidazolium Bis((trifluoromethyl)sulfonyl)imides. J. Phys. Chem. A 2002, 106, 7579−7585. (49) Rajian, J. R.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. Temperature-Dependence of the Low-Frequency Spectrum of 1Pentyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide Studied by Optical Kerr Effect Spectroscopy. Chem. Phys. Lett. 2004, 393, 372−377. (50) 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. (51) Xiao, D.; Rajian, J. R.; Li, S. F.; Bartsch, R. A.; Quitevis, E. L. Additivity in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures: Implications for Nanostructural Organization. J. Phys. Chem. B 2006, 110, 16174−16178. (52) Xiao, D.; Rajian, J. R.; Hines, L. G., Jr.; Li, S.; Bartsch, R. A.; Quitevis, E. L. Nanostructural Organization and Anion Effects in the Optical Kerr Effect Spectra of Binary Ionic Liquid Mixtures. J. Phys. Chem. B 2008, 112, 13316−13325. (53) Giraud, G.; Gordon, C. M.; Dunkin, I. R.; Wynne, K. The Effects of Anion and Cation Substitution on the Ultrafast Solvent Dynamics of Ionic Liquids: A Time-Resolved Optical Kerr-Effect Spectroscopic Study. J. Chem. Phys. 2003, 119, 464−477. (54) 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. (55) 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. (56) Shirota, H.; Nishikawa, K.; Ishida, T. Atom Substitution Effects of [XF6]− in Ionic Liquids. 1. Experimental Study. J. Phys. Chem. B 2009, 113, 9831−9839. J



Article

(77) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (78) Cances, E.; Mennucci, B.; Tomasi, J. A New Integral Equation Formalism for the Polarizable Continuum Model: Theoretical Background and Applications to Isotropic and Anisotropic Dielectrics. J. Chem. Phys. 1997, 107, 3032−3041. (79) Mennucci, B.; Tomasi, J. Continuum Solvation Models: A New Approach to the Problem of Solute’s Charge Distribution and Cavity Boundaries. J. Chem. Phys. 1997, 106, 5151−5158. (80) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Ab Initio Study of Ionic Solutions by a Polarizable Continuum Dielectric Model. Chem. Phys. Lett. 1998, 286, 253−260. (81) Seddon, K. R.; Stark, A.; Torres, M. J. Viscosity and Density of 1-Alkyl-3-methylimidazolium Ionic Liquids. ACS Symp. Ser. 2002, 819, 34−49. (82) Sanchez, L. G.; Espel, J. R.; Onink, F.; Meindersma, G. W.; Haan, A. B. d. Density, Viscosity, and Surface Tension of Synthesis Grade Imidazolium, Pyridinium, and Pyrrolidinium Based Room Temperature Ionic Liquids. J. Chem. Eng. Data 2009, 54, 2803−2812. (83) CRC Handbook of Chemistry and Physics, 89th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2008. (84) 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. (85) 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. (86) Nicolau, B. G.; Sturlaugson, A.; Fruchey, K.; Ribeiro, M. C. C.; Fayer, M. D. Room Temperature Ionic Liquid−Lithium Salt Mixtures: Optical Kerr Effect Dynamical Measurements. J. Phys. Chem. B 2010, 114, 8350−8356. (87) 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. (88) McMorrow, D.; Lotshaw, W. T. Intermolecular Dynamics in Acetonitrile Probed with Femtosecond Fourier-Transform Raman Spectroscopy. J. Phys. Chem. 1991, 95, 10395−10406. (89) 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. (90) 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. (91) Huo, Y.; Xia, S.; Ma, P. Densities of Ionic Liquids, 1-Butyl-3methylimidazolium Hexafluorophosphate and 1-Butyl-3-methylimidazolium Tetrafluoroborate, with Benzene, Acetonitrile, and 1-Propanol at T = (293.15 to 343.15) K. J. Chem. Eng. Data 2007, 52, 2077−2082. (92) Qiao, Y.; Yan, F.; Xia, S.; Yin, S.; Ma, P. Densities and Viscosities of [Bmim][PF6] and Binary Systems [Bmim][PF6] + Ethanol, [Bmim][PF6] + Benzene at Several Temperatures and Pressures: Determined by the Falling-Ball Method. J. Chem. Eng. Data 2011, 56, 2379−2385. (93) Santos, C. S.; Baldelli, S. Gas−Liquid Interface of Hydrophobic and Hydrophilic Room-Temperature Ionic Liquids and Benzene: Sum Frequency Generation and Surface Tension Studies. J. Phys. Chem. C 2008, 112, 11459−11467. (94) Ryu, S.; Stratt, R. M. A Case Study in the Molecular Interpretation of Optical Kerr Effect Spectra: Instantaneous-NormalMode Analysis of the OKE Spectrum of Liquid Benzene. J. Phys. Chem. B 2004, 108, 6782−6795. (95) Tao, G.; Stratt, R. M. Why Does the Intermolecular Dynamics of Liquid Biphenyl so Closely Resemble that of Liquid Benzene? Molecular Dynamics Simulation of the Optical-Kerr-Effect Spectra. J. Phys. Chem. B 2006, 110, 976−987.

(96) Shirota, H.; Castner, E. W., Jr. Molecular Dynamics and Interactions of Aqueous and Dichloromethane Solutions of Polyvinylpyrrolidone. J. Chem. Phys. 2006, 125, 034904/1−034904/14. (97) Shirota, H.; Ushiyama, H. Hydrogen-Bonding Dynamics in Aqueous Solutions of Amides and Acids: Monomer, Dimer, Trimer, and Polymer. J. Phys. Chem. B 2008, 112, 13542−13551. (98) Shimizu, K.; Gomes, M. F. C.; Pádua, A. A. H.; Rebelo, L. P. N.; Canongia Lopes, J. N. A. 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. (99) Tanimura, Y.; Mukamel, S. Two-Dimensional Femtosecond Vibrational Spectroscopy of Liquids. J. Chem. Phys. 1993, 99, 9496− 9511. (100) Tominaga, K. Off-Resonant Fifth and Seventh Order TimeDomain Nonlinear Spectroscopy on Vibrational Dephasing in Liquids. Adv. Multi-Photon Processes Spectrosc. 1998, 11, 127−207. (101) Kubarych, K. J.; Milne, C. J.; Lin, S.; Astinov, V.; Miller, R. J. D. Diffractive Optics-Based Six-Wave Mixing: Heterodyne Detection of the Full χ(5) Tensor of Liquid CS2. J. Chem. Phys. 2002, 116, 2016− 2042. (102) Shirota, H.; Kato, T. Intermolecular Vibrational Spectra of C3v CXY3 Molecular Liquids, CHCl3, CHBr3, CFBr3, and CBrCl3. J. Phys. Chem. A 2011, 115, 8797−8807. (103) McMorrow, D.; Thantu, N.; Melinger, J. S.; Kim, S. K.; Lotshaw, W. T. Probing the Microscopic Molecular Environment in Liquids: Intermolecular Dynamics of CS2 in Alkane Solvents. J. Phys. Chem. 1996, 100, 10389−10399. (104) Steffen, T.; Meinders, N. A. C. M.; Duppen, K. Microscopic Origin of the Optical Kerr Effect Response of CS2−Pentane Binary Mixtures. J. Phys. Chem. A 1998, 102, 4213−4221.

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