Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: a femtosecond Raman-induced Kerr effect spectroscopic study.

Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: A femtosecond Raman-induced Kerr effect spectroscopic study Ranjit Biswas, Anuradha Das, and Hideaki Shirota Citation: The Journal of Chemical Physics 141, 134506 (2014); doi: 10.1063/1.4897207 View online: http://dx.doi.org/10.1063/1.4897207 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low-frequency modes of the benzoic acid dimer in chloroform observed by the optical Kerr effect J. Chem. Phys. 135, 134504 (2011); 10.1063/1.3635778 Glasslike behavior in aqueous electrolyte solutions J. Chem. Phys. 128, 161102 (2008); 10.1063/1.2906132 Temperature evolution of the low-frequency optical Kerr effect spectra of liquid benzene in quasicrystalline approach J. Chem. Phys. 116, 4563 (2002); 10.1063/1.1449867 Ultrafast Raman-induced Kerr-effect of water: Single molecule versus collective motions J. Chem. Phys. 113, 4674 (2000); 10.1063/1.1288690 Molecular dynamics of thiophene homologues investigated by femtosecond optical Kerr effect and low frequency Raman scattering spectroscopies J. Chem. Phys. 109, 10948 (1998); 10.1063/1.477791

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THE JOURNAL OF CHEMICAL PHYSICS 141, 134506 (2014)

Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: A femtosecond Raman-induced Kerr effect spectroscopic study Ranjit Biswas,1,a) Anuradha Das,1 and Hideaki Shirota2,a) 1

Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, J. D. Block, Sec. III, Salt Lake, Kolkata 700 098, West Bengal, India 2 Department of Nanomaterial Science and Department of Chemistry, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan

(Received 22 August 2014; accepted 23 September 2014; published online 7 October 2014) In this study, we have investigated the ion concentration dependent collective dynamics in two series of deep eutectic solvent (DES) systems by femtosecond Raman-induced Kerr effect spectroscopy, as well as some physical properties, e.g., shear viscosity (η), density (ρ), and surface tension (γ ). The DES systems studied here are [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] with f = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. γ of these DES systems shows near insensitivity to f, while ρ shows a moderate dependence on f. Interestingly, η exhibits a strong dependence on f. In the low-frequency Kerr spectra, obtained via the Fourier transform of the collected Kerr transients, a characteristic band at ∼70 cm−1 is clear in [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] DES especially at the larger f. The band is attributed to the intermolecular hydrogen bond of acetamide. Because of less depolarized Raman activities of intermolecular/interionic vibrational motions, which are mostly translational (collision-induced or interactioninduced) motions, of spherical ions, the intermolecular hydrogen-bonding band is clearly observed. In contrast, the intermolecular hydrogen-bonding band is buried in the other intermolecular/interionic vibrational motions, which includes translational and reorientational (librational) motions and their cross-terms, in [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system. The first moment (M1 ) of the intermolecular/interionic vibrational band in these DES systems is much higher than that in typical √ neutral molecular liquids and shows a weak but contrasting dependence on the bulk parameter γ /ρ. The time constants for picosecond overdamped Kerr transients in both the DES systems, which are obtained on the basis of the analysis fitted by a triexponential function, are rather insensitive to f for both the DES systems, but all the three time constants (fast: ∼1–3 ps; intermediate: ∼7–20 ps; and slow: ∼100 ps) are different between the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] and [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] systems. These results indicate that the intermolecular/interionic interactions in DES systems is strongly influenced by the ionic species present in these DES systems. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4897207] I. INTRODUCTION

Solvents that are less hazardous to environment occupy the center stage for industrial applications in the modern era. Bio-compatibility and bio-degradability are the key qualities that render such solvents with the minimum ecological foot-print. If this combines with cost-effectiveness for preparation, storage, and recycle then these solvents are endorsed as “green solvents.” Deep eutectic solvents (DES) that are liquids at approximately 300 K but composed of components with individual melting temperatures much above 300 K, offer one such class of solvents that can be employed for a variety of applications.1–5 These multicomponent mixtures are molten at room temperature because of entropy-driven depression of freezing points and allow inter-component hydrogen-bond (H-bond) interactions. Mixtures of alkylamides with electrolytes are examples5, 6 of a) Electronic addresses: [email protected] and [email protected]

0021-9606/2014/141(13)/134506/11/$30.00

low-cost DES systems possessing multi-tasking abilities with potential for industrial applications.3, 7 For effective use of DES as a designer medium for tailor-made chemical reactions, exploration of structure-dynamics relationship is critically important because outcome of a reaction is often influenced by the reactant-medium interaction and medium dynamics.8–12 A few fast spectroscopic measurements combined with suitable computer simulations13–18 have just begun to characterize the medium-dynamics and nature of solutemedium coupling in these (alkylamide + electrolyte) systems. However, no ultrafast measurements have been carried out so far for these DES systems. Existing viscoelastic,19, 20 dielectric,5, 6, 21 nuclear magnetic,19 and ultrasonic19, 22 relaxation measurements of (alkylamide + electrolyte) systems indicated presence of microscopic inhomogeneity in solution structure even at T ≥ 300 K. Interestingly, dielectric relaxation measurements suggested formation of charged domains via ion aggregation which exist in equilibrium with amorphous and liquid

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amide environments.21 Presence of spatial heterogeneity in DES made of acetamide (CH3 CONH2 ) and lithium bromide/nitrate (LiBr/NO3 ) was later reflected in the excitation wavelength dependence of fluorescence emission of a dissolved dye15 although similar measurements for (CH3 CONH2 + Na/KSCN)17 DES did not reveal any such excitation wavelength dependence. Subsequent dynamic fluorescence Stokes shift and anisotropy measurements for both (CH3 CONH2 + LiBr/NO3 ) and (CH3 CONH2 + Na/KSCN) systems revealed strong fractional viscosity dependence of solute solvation and rotation rates at temperatures much above the respective glass transition temperatures, suggesting presence of a strong dynamic heterogeneity in these media. Molecular dynamics simulations carried out later employing realistic interaction potentials reported stretched exponential (or multi-exponential) relaxations for amide dynamic structure factors in several of these (amide + electrolyte) DES system,13 providing a justification for explaining the observed viscosity dependence in terms of spatially varying relaxation rates. In addition, multi-probe fluorescence measurements showed probe lifetime dependence for the extents of both the excitation wavelength dependence of solute emission and solute-medium decoupling.13, 14 All these results, therefore, support the interpretation of earlier experimental results19–21, 23 in terms of solution heterogeneity and domain formation. In such a scenario, applications of ultrafast vibrational spectroscopic techniques can provide useful molecular level information on microscopic structure, interaction, and dynamics of these molten multi-component mixtures containing ions, and strengthen the understanding gained from the existing fluorescence measurements and simulations. Note that even traditional vibrational spectroscopic measurements, such as FT-IR, far-IR, and Raman measurements for these DES are non-existent although these measurements are regularly used for studying intramolecular and interactioninduced dynamics of systems possessing H-bond interactions. Femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) is a third order nonlinear spectroscopy and can observe the molecular motions in the low-frequency range that is typically 0.1–300 cm−1 corresponding to the time scale of approximately 100 fs–300 ps.24, 25 Liquid dynamics in this time scale is typically collective in nature and includes intermolecular vibrations, librations, and collisionor interaction-induced motions (often bestowed as origin of β-relaxation in glassy systems), dipolar solvent reorientation (rendering α-relaxation in glassy systems), and crossover processes between them. The dynamics, especially the intermolecular vibrations, are strongly influenced by the microscopic intermolecular interaction and structure of the target system. Therefore, fs-RIKES measurements provide a microscopic insight into the collective dynamics of condensed phases. Note fs-RIKES has been used for studies of ultrafast dynamics in simple molecular liquids and binary mixtures at the early stage,26–31 but now expanded to more complex systems.32–37 Room temperature ionic liquids, which possess a kinship to the present (acetamide + electrolyte) DES, have been extensively studied via fs-RIKES recently.35, 36, 38–44

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In this study, we have explored the ultrafast intermolecular dynamics employing fs-RIKES in the two following DES systems: [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }]. We expect that the fs-RIKES measurements would provide new insight into the modification of amide-amide interactions by ions and the impact of solution heterogeneity on collective intermolecular dynamics. Note fs-RIKES detects the time-dependent fluctuations in the collective polarizability anisotropy tensor auto-correlation function (detection of isotropic part is also possible by controlling the polarization condition)45–47 and thus makes the ion effects (via polarizability difference) on amide dynamics amenable to such measurements. Interestingly, recent fluorescence measurements14 have revealed ion influence on the fractional viscosity dependence of solute solvation and rotation rates in (CH3 CONH2 + LiX) DES. This observation has subsequently been explained in terms of ion-dependent non-exponential relaxation of simulated acetamide dynamic structure factors. Hence, fs-RIKES measurements proposed here will supply new evidences on molecular-level aspects, complement the existing fluorescence study (even though the time scale is different), and stimulate further experiments and simulations for these multi-component systems. We will also discuss the lowfrequency spectral features of the present DES systems together with the other systems including ionic and molecular liquids. II. EXPERIMENTS AND DATA ANALYSIS

Acetamide (>98%), KSCN (>98%), NaSCN (>99%), and LiNO3 (>99% typical), all from Kanto Chemical, were used as received. For reasons explained later, LiBr were obtained from two different sources (Kanto Chemical, >95%, and TCI, >99%) and used as received. Following the procedure reported earlier,17, 18 required amount of each component for a given DES system was added in a sample vial and the resultant mixtures were warmed at ∼323 K and stirred more than 12 h. Shear viscosities (η) of the DES systems were measured by a reciprocating electromagnetic piston viscometer (Cambridge Viscosity, ViscoLab 4100) equipped with a circulating water bath (Yamato, BB300) at 313.0 ± 0.2 K. Surface tensions (γ ) of the samples were measured using a duNouy tensiometer (Yoshida Seisakusho) at 313.0 ± 1.5 K. Densities (ρ) of the samples were measured by using an automated temperature-controlled density-cum-sound analyzer (Anton Paar, model DSA 5000) at 314.0 ± 0.5 K. Because the femtosecond optical heterodyne-detected RIKES setup used here were already reported in detail,31, 48, 49 we describe here the set-up briefly. The light source was a titanium sapphire laser (KMLabs Inc., Griffin) pumped by a Nd:YVO4 diode laser (Spectra Physics, Millennia Pro 5sJ). The output power of the titanium sapphire laser was approximately 420 mW. The typical temporal response, which was the cross-correlation between the pump and probe pulses measured using a 200-μm-thick KDP crystal (type I), was 35 ± 3 fs (full width at half maximum). The scans with high


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time resolution of 2048 points at 0.5 μm/step were performed for a short time window (6.8 ps). Long time window transients (∼300 ps) were recorded with a data acquisition of 30.0 μm/step. Pure heterodyne signals were achieved by combining the transients recorded at ∼1.5◦ rotations of the input polarizer on both positive and negative orientations to eliminate the residual homodyne signal. Three scans and 10 scans for each polarization measurement were averaged respectively for the short and long time window transients. Prior to the fs-RIKES measurements, the samples were injected into a 3-mm optical-path-length quartz cell (Tosoh Quartz) using a 0.2-μm Anotop filter (Whatman). All the RIKES measurements were made at 313 ± 2 K. The temperature was regulated with a heating tape and a transformer. Data analyses of the recorded Kerr transients were performed following the standard method reported in detail previously.31, 36 The overdamped Kerr transients from 3 to 300 ps were fitted by a triexponential function with an offset parameter: a0 + a1 exp(−t/τ 1 ) + a2 exp(−t/τ 2 ) + a3 exp(−t/τ 3 ). The Fourier transform deconvolution analysis has originally been established by Lotshaw and McMorrow.50, 51 In the present study, the Kerr spectrum was well resolved up to ∼600 cm−1 (vide infra). To focus on the intermolecular/interionic vibrational dynamics, the contribution of the diffusive relaxation components (the intermediate and slow exponential components) was removed from the entire Fourier-transform low-frequency Kerr spectrum. The resulting spectra were further analyzed to represent the spectral shapes. Although two major fit models, the multimode Brownian oscillator model52, 53 and sum of Ohmic (or BucaroLitovitz) and antisymmetrized Gaussian functions,54, 55 had been frequently used to perform the line shape analysis of the low-frequency Kerr spectrum in liquids, we employed here a sum of an Ohmic and three antisymmetrized Gaussian functions because such a description was found to be adequate for concentrated electrolyte solutions56 and binary mixtures of ionic liquid with neutral solvents, such as water57 and benzene.58 This model function was also used earlier to fit the low-frequency Kerr spectrum for ionic liquid mixtures with CS2 59, 60 and acetonitrile.61 Note that the other model (multimode Brownian oscillator model) also adequately described the low-frequency Kerr spectrum in systems containing charge-charge interactions such as ionic liquids.40, 62 Purpose of the line shape analysis in this study is to present the spectral shape and semi-quantitative discussion, but not to establish or modify the model of the line shape function used for studying the low-frequency spectra in liquids and solutions.

III. RESULTS AND DISCUSSION A. Bulk properties

Table I summarizes the values of η, ρ, and γ for the DES investigated here and the f dependences of these values are depicted in Fig. 1. Note that η decreases with the increase of potassium content in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system although the ρ increases. This is in contrast to what one expects from the ρ–η relationship for hard sphere systems.63 However, the trend reverses for the

J. Chem. Phys. 141, 134506 (2014) TABLE I. Shear viscosities η, liquid densities ρ, and surface tensions γ at 313 K for DES systems. ηa,b (cP)

f

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

ρ c,d (g cm−3 )

γ e,f (mN m−1 )

A. [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] 187.3 1.162 52.3 123.2 1.179 52.8 95.40 1.186 52.2 71.83 1.190 52.9 58.53 1.194 52.2 49.26 1.199 52.5 B. [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] 125.3 1.172 53.2 183.7 1.184 52.8 279.3 1.195 53.8 444.1 1.209 53.2 711.9 1.223 54.3 1032 1.237 55.0

313.0 ± 0.2 K. ±5%. c 313.0 ± 0.5 K. d ±0.5%. e 313.0 ± 1.5 K. f ±3%. a

b

[0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] DES as both ρ and η increase upon successive replacement of nitrate by bromide. We note here that the f-dependences for ρ and η observed here agree qualitatively with earlier report for the thiocyanate containing DES64 but differ from that for the lithium containing one.65 Importantly, the f-dependence for these properties remains unaltered even when LiBr from different sources (Kanto Chemical and TCI) was used in preparing the lithium containing DES in this study to make sure no source dependence. γ appears to be nearly insensitive to both f and electrolyte identity in these DES systems and the value fluctuates weakly over an average value of ∼53 mN/m. Note this value of γ for these systems is rather close to that of neat formamide,66 and this similarity parallels with the observed proximity in fluorescence emission energies16, 18 of a dipolar probe dissolved in these systems and in formamide. For composition dependence of bulk properties (for example, ρ or η) of these DES systems, ionic radius or volume is an important parameter because intermolecular force or interaction depends on inter-particle separation. In the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}], K+ replaces Na+ upon increase of f. As the radii of Na+ and K+ are 1.02 and 1.38 Å, respectively (volumes being 4.45 and 11.68 Å3 , respectively67 ), it is plausible that the variation in cation size may be responsible for imparting an f-dependence to shear viscosity in the corresponding DES. Such a correlation between cation size and shear viscosity has indeed been observed for room temperature ionic liquids although they are not spherical.35, 68, 69 On the other hand, the f-dependence of shear viscosity in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] does not follow the above mentioned ion size correlation as ionic radius of Br− is larger (1.96 Å) than that of NO3 − (1.79 Å).67 This may be understood as follows. The mass and size ratios between Br− and NO3 − are ∼2.6 and ∼1.1, respectively. The same ratios for Na+ and K+ are


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FIG. 1. Shear viscosities η, liquid densities ρ, and surface tensions γ at 313 K for DES systems, [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f ) NaSCN}] (blue squares) and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f ) LiNO3 }] (red circles).

∼1.7 and ∼1.35, respectively. Since viscosity is related to momentum transfer between two adjacent liquid layers and the liquid layer structure is in turn determined by the interparticle interactions,14, 70, 71 the larger mass ratio dominates over the size ratio in determining the f-dependence of viscosity for the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] DES. A model simulation study in this regard can be helpful in understanding the f dependence of viscosity of these DES. Regarding the f-dependence of solution density, the densities of the electrolytes should be compared. The densities of NaSCN, KSCN, LiNO3 , and LiBr are 1.74, 1.88, 2.38, and 3.46 g/cm3 , respectively.66 Therefore, the f-dependence of density in the two DES systems can be accounted for by the densities of the salts. Note the surface tension exhibits no clear f-dependence for these DES. This indicates that acetamide molecules, not ions, dominate the surface of these DES systems. This also explains why the surface tension for these mixtures is very similar to that of formamide.

FIG. 2. Short time window Kerr transients (−0.5 – 5.8 ps) of DES systems, (a) [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and (b) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }], and long time window Kerr transients (−1 – 300 ps) of DES systems, (c) [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and (d) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }]. Offsets are included in the long time window Kerr transients.

B. Kerr transients and spectra

Fig. 2 shows the Kerr transients with both the short time (high resolution data) and long time (wide time range data) windows for these DES. Fits using a triexponential


103.7 ± 11.2 115.3 ± 14.3 91.4 ± 19.0 115.0 ± 26.3 99.0 ± 33.3 93.7 ± 18.7 0.001036 ± 0.000042 (0.072) 0.000747 ± 0.000027 (0.069) 0.000548 ± 0.000057 (0.047) 0.000403 ± 0.000028 (0.047) 0.000191 ± 0.000026 (0.030) 0.000288 ± 0.000027 (0.072) 8.09 ± 1.23 7.54 ± 1.01 9.24 ± 1.40 7.87 ± 1.23 7.75 ± 1.01 8.17 ± 3.02 + 0.22{f LiBr + (1 – f )LiNO3 }] 0.002096 ± 0.000314 (0.146) 0.001977 ± 0.000281 (0.183) 0.001675 ± 0.000153 (0.144) 0.001417 ± 0.000191 (0.167) 0.001361 ± 0.000153 (0.212) 0.000493 ± 0.000184 (0.122) 0.011239 ± 0.001710 (0.782) 0.008092 ± 0.001520 (0.748) 0.009437 ± 0.004250 (0.809) 0.006683 ± 0.002550 (0.786) 0.004864 ± 0.001890 (0.758) 0.003246 ± 0.000074 (0.806) 0.000048 ± 0.000027 0.000037 ± 0.000026 0.000046 ± 0.000023 0.000038 ± 0.000026 0.000055 ± 0.000015 0.000054 ± 0.000012 0.0 0.2 0.4 0.6 0.8 1.0

B. [0.78CH3 CONH2 1.47 ± 0.18 1.43 ± 0.15 1.10 ± 0.22 1.21 ± 0.25 1.22 ± 0.26 1.56 ± 0.33

89.1 ± 27.8 99.0 ± 27.2 86.0 ± 26.7 75.8 ± 14.0 81.8 ± 11.8 99.8 ± 9.7 0.000414 ± 0.000124 (0.053) 0.000777 ± 0.000171 (0.096) 0.000716 ± 0.000244 (0.079) 0.001090 ± 0.000236 (0.124) 0.001509 ± 0.000228 (0.143) 0.001200 ± 0.000073 (0.098) 20.2 ± 2.6 20.3 ± 7.0 21.9 ± 5.4 18.5 ± 5.1 19.0 ± 5.9 16.0 ± 4.2 0.006010 ± 0.000099 (0.760) 0.006441 ± 0.000128 (0.792) 0.007080 ± 0.000136 (0.779) 0.006403 ± 0.000153 (0.728) 0.007911 ± 0.000218 (0.750) 0.008561 ± 0.000253 (0.701) 0.000045 ± 0.000017 0.000046 ± 0.000033 0.000055 ± 0.000026 0.000063 ± 0.00021 0.000059 ± 0.000026 0.000064 ± 0.000021 0.0 0.2 0.4 0.6 0.8 1.0

A. [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] 2.69 ± 0.10 0.001481 ± 0.000097 (0.127) 3.06 ± 0.16 0.000912 ± 0.000143 (0.112) 2.80 ± 0.12 0.001296 ± 0.000019 (0.142) 2.80 ± 0.17 0.001305 ± 0.000018 (0.148) 2.52 ± 0.14 0.001133 ± 0.000018 (0.107) 2.16 ± 0.09 0.002452 ± 0.000096 (0.201)

a3 (a3 /(a1 + a2 + a3 )) τ 2 (ps) a2 (a2 /(a1 + a2 + a3 )) τ 1 (ps) a1 (a1 /(a1 + a2 + a3 )) a0 f

function for the long time Kerr transients are also displayed in Figs. 2(c) and 2(d). The fit parameters are summarized in Table II. The fact that an offset parameter is necessary to represent the long time Kerr transients in these DES systems strongly indicates existence of a relaxation process much slower than that could be detected by the present experimental setup. In fact, nanosecond relaxation components have already been observed in dynamic fluorescence Stokes shift measurements of these systems.15, 17 Also, many ionic liquids, which resemble in shear viscosity of these systems, possess similar slow relaxation processes with timescales in the subnanosecond to nanosecond regime.72–74 These observations therefore support the presence of nanosecond or slower relaxation component in these multi-component mixtures. Because the amplitude of the long relaxation process, which is expressed here by the intermediate and slow exponential components, in these systems captured by the present RIKES measurements is very small and the time window covered is short compared to the relaxation time, we focus primarily on relatively faster intermolecular/interionic vibrational dynamics in this study. The Kerr spectra with the frequency range of 0–600 cm−1 for the systems are shown in Figs. 3(a) and 3(b). Clear intramolecular vibrational bands are observed at 457 and 573 cm−1 . Previously, the vibrational bands of acetamide in aqueous solutions were observed at 455 and 574 cm−1 by fs-RIKES, and they were assigned to the vibrational mode of the in-plane N–C–C bend, and the overlapping band of the out-of-plane N-C-C bending mode and the in-plane N– C=O bending mode based on the ab initio quantum chemistry calculations.75 We assign the bands at 457 and 573 cm−1 in the present DES systems in the same manner as the previous report.75 Because the intensities and shapes of these bands do not vary with f and are similar for the two different DES systems, and because there are no other intramolecular vibrational bands near these frequencies, these assignments are most likely plausible. The broad band spreading over 0 to 200 cm−1 arises from the intermolecular vibrations that is commonly observed in molecular liquids and mixtures, as well as in ionic systems including ionic liquids and electrolyte solutions. The magnification of the low-frequency bands is shown in Figs. 3(c) and 3(d). The intermolecular/interionic vibrational band in liquids essentially includes translational (collisioninduced or interaction-induced) and reorientational (librational) motions and their coupling motion (cross-term). In the case of spherical molecules or ions, the reorientational motion is silent. The spectral shape differs between these two DES systems and also changes gradually with f for a given system. Namely, the spectral shape of the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system is trapezoidal but that of the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system indicates a characteristic band at approximately 75 cm−1 . This spectral structure becomes clearer at larger f in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system. Prior to a discussion on the line shape analysis of the low-frequency broadened spectra in liquids, we should note that each function is hard to assign a vibrational mode. This is

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τ 3 (ps)


TABLE II. Triexponential fit parameters for Kerr transients in DES systems.

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FIG. 3. Fourier transform Kerr spectra with the frequency range of 0– 600 cm−1 of DES, (a) [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and (b) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }]. Magnifications of the frequency range of 0–300 cm−1 are shown in (c) [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and (d) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }].

because molecular fluctuations provide different intermolecular vibrational modes and intermolecular vibrational modes are often coupled to each other. For example, some computational studies have shown that the cross-terms between trans-

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lational and reorientational motions have a negative contribution to the low-frequency spectrum of common liquids76–79 and ionic liquids.80–82 In addition, the line shapes of the translational and reorientational motions are not describable by simple Gaussian, Ohmic functions or Brownian oscillators. Thus, assignment of each function to a specific vibrational mode in the low-frequency spectral band is somewhat tricky. However, we believe that a primary understanding of intermolecular vibrations in terms of the first moment and spectral width is often useful for a qualitative visualization of the dynamics in molecular liquids. We should also note exception cases. Some specifically coordinated or fixed molecular systems such as HB systems, e.g., 7-azaindole dimer,83, 84 acetic acid dimer,85 exhibit intermolecular vibrational modes. In contrast to the collision-induced (or interaction-induced) motion, librational motion, and their coupling motion, assignments of these bands are likely possible. Because the present DES systems includes 3 or 4 species (acetamide, cation(s), and anion(s)), let us clarify the contributions of components in the DES systems to the low-frequency Kerr spectra. As mentioned above, the spherical species (Li+ , Na+ , K+ , and Br− ) show no spectral densities due to the librational motions. They can contribute to the spectral densities by the collision-induced or interaction induced motions, but the magnitudes of the spectral densities are not large in comparison with the other components (acetamide, SCN− , and NO3 − ).37, 39, 56, 62, 86 SCN− and NO3 − contribute to low-frequency Kerr spectra significantly.43, 87 Turton et al. measured ethylammonium nitrate by fs-RIKES.87 According to the report, the major contribution to the spectral density in ∼70 cm−1 is simply assigned to the libration.87 Fukazawa et al. studied 1-butyl3-methylimidazolium cation based ionic liquids with six different anions including SCN− by fs-RIKES. The strong spectral density appears in ∼50 cm−1 for the SCN salt.43 In a linear molecule CS2 , both the translational and reorientational as well as the cross-terms populate the entire low-frequency Kerr spectrum.76 Thus, it is logical to think that the spectral contribution of the SCN− to the low-frequency Kerr spectrum comes from all these motions. Although pure acetamide liquid is not reported so far, the low-frequency spectrum of aqueous acetamide solution was measured by fs-RIKES.75 The spectral shape has a hump in approximately 70–100 cm−1 that is assigned to N–H · · · O=C hydrogen-bonding mode. Ab initio quantum chemistry calculations showed that the mode is sensitive to the number of water molecules interacted to acetamide.75 The low-frequency spectra of liquid amide derivatives, such as formamide and methylformamide, were studied by several groups.86, 88–93 Characteristic bands at ∼100 cm−1 in the liquids are attributed to HB bands. Because the low-frequency spectrum includes “intermolecular” vibrations, the spectral bands in the present DES systems cannot be simply identified on the basis of the above reports. However, we believe that this summary would be a help to discuss the low-frequency Kerr spectra of the present complex DES systems. Representative results from our line shape analyses for the low-frequency spectra are shown in Fig. 4. Similar quality fits have also been obtained for other compositions as well.


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FIG. 4. Fourier transform Kerr spectra and fits with the frequency range of 0– 300 cm−1 of DES, (a) [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] (f = 0.4), (b) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] (f = 0.0), and (c) [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] (f = 1.0). Black dots denote the Fourier transform Kerr spectra, red lines denote the entire fits, green areas denote Ohmic functions, blue areas denote anti symmetrized Gaussian functions, and red areas denote anti-symmetrized Gaussian functions (hydrogen bonding mode).

As illustrated in Fig. 4, the fit model function adequately describes the low-frequency Kerr spectrum of these DES systems. Fit parameters obtained from the line shape analyses are summarized in Table III. Since the spectral shape in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system is indicating a characteristic band at ∼75 cm−1 , an antisymmetrized Gaussian function with a peak at ∼70 cm−1 is necessary to represent the low-frequency spectrum in the this system, especially at larger f. This band is estimated from the free spectral fit in the [0.78CH3 CONH2 + 0.22LiBr] mixture (that is, f = 1), and subsequent fits for other compositions has been constrained with this band. A characteristic band at ∼70–100 cm−1 has often been observed in fs-RIKES measurements of N–H HB systems, such as in aqueous75, 91, 92, 94, 95

J. Chem. Phys. 141, 134506 (2014)

and ionic86 amides, and 7-azaindole solutions.83, 84 Previously, a band at 76 cm−1 in aqueous acetamide solutions has been assigned to the intermolecular hydrogen bond mode between acetamide and water.75 In the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system, there is no clear trend in the f-dependences of the fit parameters (aG3 and ωG3 ) of the HB band (third anti-symmetrized Gaussian component). This indicates that the band in the DES systems is not affected by the anion species. In addition, Palombo and Meech studied the concentration dependence of formamide solutions of NaI and KI fs-RIKES and observed the weak concentration dependences of the band frequency and width.86 They assigned the band to the libration of trans-HB molecules based on MD simulation and normal mode analysis of liquid formamide.96 Accordingly, we assign the band at 70 cm−1 in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system to the intermolecular hydrogen bonding mode of acetamide–acetamide. Note, on the other hand, that a libration band with a peak around 70 cm−1 may also arise due to the presence of NO3 − in such mixtures.87 However, the clear presence of this band in the nitrate-free system, [0.78CH3 CONH2 + 0.22LiBr] (that is at f = 1), provides unambiguous support in favor of assigning this band to the intermolecular H-bonding mode. Some questions, however, may arise: why does the structure of the low frequency spectrum in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system becomes less prominent as f decreases? Why is such a structure absent in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system? In the present polarization condition for the RIKES setup, the signal arises from the depolarized Raman active motions and thus the measured Kerr transients actually depict the polarizability anisotropy relaxations. On the basis of the selection rule, the reorientational motions, such as libration, of totally symmetric molecules and ionic species become silent, but translational motion, such as collision-induced and interaction-induced motions of the amides and ions contribute to the Kerr signals in the present polarization condition (anisotropic).48, 97, 98 Note that even the translational motion of a totally symmetric molecule or ion is active for the polarizability anisotropy relaxation, the Kerr signal due to the nuclear response (not the electronic hyperpolarizability response) is much weak compared to non-totally symmetric molecule or ion that has a polarizability anisotropy. In these DES systems, Na+ , K+ , and Br− are spherical, but SCN− , NO3 − , and acetamide possess polarizability anisotropy. We can therefore expect that the low-frequency Kerr spectrum of [0.78CH3 CONH2 + 0.22LiBr] system is dominated by the signal from acetamide and thus the intermolecular HB band is clearly observed. On the other hand, [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system at all compositions contains high concentration of SCN− which has a polarizability anisotropy (∼7 Å3 based on the B3LYP/ 6-311++G(d,p) level calculations),43 and thus the anion contribute significantly to the low-frequency Kerr spectrum of this system. As a result, the intermolecular HB band is buried under the intermolecular/interionic vibrational spectrum. The f-dependence of the spectral shape of the low-frequency Kerr spectrum of the [0.78CH3 CONH2 + 0.22{f LiBr


32.75 ± 1.65 35.72 ± 0.99 35.84 ± 0.84 32.90 ± 0.57 33.46 ± 0.48 32.19 ± 0.79 136.03 ± 0.48 133.75 ± 0.41 133.80 ± 0.37 128.91 ± 0.29 127.97 ± 0.26 132.21 ± 0.35 B. [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] 0.3042 ± 0.0607 12.94 ± 2.94 40.54 ± 1.40 0.7692 ± 0.0074 63.25 ± 0.76 0.2534 ± 0.0449 13.22 ± 2.67 40.41 ± 1.31 0.6311 ± 0.0046 67.73 ± 0.62 0.1925 ± 0.0266 13.59 ± 2.15 39.20 ± 1.13 0.5548 ± 0.0038 66.61 ± 0.57 0.1694 ± 0.0254 13.69 ± 2.36 40.02 ± 1.21 0.4553 ± 0.0019 72.88 ± 0.40 0.1306 ± 0.0129 14.15 ± 1.62 37.56 ± 0.95 0.3966 ± 0.0015 73.91 ± 0.36 0.0934 ± 0.0030 16.55 ± 0.75 31.78 ± 0.71 0.3757 ± 0.0021 69.37 ± 0.51

0.0147 ± 0.0014 70.26 0.0225 ± 0.0013 70.26 0.0198 ± 0.0010 70.26 0.0244 ± 0.0008 70.26 0.0236 ± 0.0007 70.26 0.0197 ± 0.0007 70.26 ± 0.28

52.14 ± 1.11 44.80 ± 2.05 64.69 ± 0.37 64.38 ± 0.40 61.71 ± 0.36 45.27 ± 2.10 43.50 ± 2.65 40.87 ± 1.75 44.85 ± 2.70 42.10 ± 1.86 41.49 ± 1.51 39.61 ± 2.59 A. [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] 0.1352 ± 0.0431 7.58 ± 2.69 18.34 ± 2.09 0.2605 ± 0.0706 15.95 ± 5.03 0.1620 ± 0.0646 7.10 ± 3.13 18.18 ± 2.28 0.2335 ± 0.0258 19.13 ± 2.60 0.1815 ± 0.0977 6.20 ± 3.72 18.61 ± 2.07 0.3116 ± 0.1190 14.02 ± 6.03 0.1780 ± 0.0946 6.46 ± 3.82 18.88 ± 2.33 0.2632 ± 0.0395 17.35 ± 3.10 0.1486 ± 0.0290 7.47 ± 1.61 17.43 ± 1.30 0.2852 ± 0.0409 16.04 ± 2.71 0.1406 ± 0.0973 7.13 ± 5.36 17.86 ± 3.88 0.2750 ± 0.0616 16.65 ± 4.42

0.7709 ± 0.0136 0.8824 ± 0.0352 0.6422 ± 0.0026 0.6609 ± 0.0029 0.6802 ± 0.0029 0.8853 ± 0.0354

ωG3 (cm−1 ) ωG2 (cm−1 )

5.07 ± 0.03 5.24 ± 0.04 5.56 ± 0.04 5.48 ± 0.05 5.42 ± 0.06 4.33 ± 0.06 0.1115 ± 0.0006 0.0831 ± 0.0004 0.0601 ± 0.0002 0.0475 ± 0.0002 0.0336 ± 0.0002 0.0346 ± 0.0028 89.5 90.2 90.4 90.8 91.5 92.5 0.0 0.2 0.4 0.6 0.8 1.0

145.7 145.0 143.1 141.7 139.8 138.1

2.30 ± 0.03 2.08 ± 0.02 2.19 ± 0.02 2.23 ± 0.03 2.40 ± 0.02 2.72 ± 0.07 0.2038 ± 0.0019 0.2664 ± 0.0024 0.2481 ± 0.0015 0.2336 ± 0.0018 0.2363 ± 0.0013 0.2015 ± 0.0021 86.6 86.0 84.5 83.9 83.6 84.1 0.0 0.2 0.4 0.6 0.8 1.0

142.7 143.8 143.1 141.3 141.5 140.6

ωO (cm−1 )

aG1

ωG1 (cm−1 )

ωG1 (cm−1 )

aG2

ωG2 (cm−1 )

aG3

J. Chem. Phys. 141, 134506 (2014)

aO M1 (cm−1 ) ω (cm−1 ) f

TABLE III. Fit parameters for Kerr spectra in DES systems.

139.13 ± 0.61 142.61 ± 0.92 128.54 ± 0.28 127.00 ± 0.30 128.98 ± 0.26 139.49 ± 0.98


ωG3 (cm−1 )

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+ (1 – f )LiNO3 }] system, where the fraction of the non-spherical anion is increasing with decreasing f (Fig. 3(d)), supports this consideration. If we further closely observe the low-frequency Kerr spectra of the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] systems in Fig. 3, the difference in the spectrum between the two systems appears to be the intensity of the low-frequency region below 50 cm−1 relative to the intermediate frequency region at ∼75 cm−1 . This difference may be understood from the nature of the anionic species involved. Namely, the relative intensity of the low-frequency region below 50 cm−1 is rather high (or trapezoid line shape) in the case of the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system, but it is not as high (or structured with a hump at ∼75 cm−1 ) in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system. The [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system further shows a slight f-dependence: 0.465 (0.167 at 10 cm−1 and 0.359 at 75 cm−1 : Fig. 3(d) includes the offset of 1.0) in the f = 1 mixture (0.78CH3 CONH2 + 0.22LiBr mixture, which contains only spherical anion) and 0.634 (0.429 at 10 cm−1 and 0.677 at 75 cm−1 ) in the f = 0 mixture (0.78CH3 CONH2 + 0.22LiNO3 mixture, which contains non spherical anion). In contrast, the relative intensity in the low-frequency region of the spectrum for the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system looks rather independent on f as seen in Fig. 3(c). Therefore, among the present DES systems, the anionic species significantly influences the line shape of the low-frequency spectrum because of the nature of the structure and polarizability anisotropy. The first moments M1 , defined as M1 = ωI(ω)dω/ I(ω)dω where I(ω) denotes the frequency-dependent spectral intensity obtained from the fit analysis, of the lowfrequency bands of the mixtures are also listed in Table III. Values of the width (full-width-at-half-maximum, FWHM) of the low frequency spectral band, ω, are also summarized in this table. Dependence on f of M1 and ω for the two DES systems are illustrated in Fig. 5. Note that ω discussed here is of the entire broad band, not each fit component. Our focus here is understanding the features of the entire spectrum itself. It is clear that M1 decreases as f increases for the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system, while an opposite dependence is observed for the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system. Note, however, that the f-dependence of these quantities is not strong as they vary by a few percent only upon changing f. We will discuss the f-dependence of M1 of the low-frequency Kerr spectrum together with a bulk parameter in Sec. III C. We note further that the M1 for these DES systems is quite large compared to that for most common aprotic nonaromatic molecular liquids.31 Since the vibrational mode, for example, the harmonic oscillator in the simplest case, depends on the reduced mass and force constant (density or molecular weight and surface tension in the case of the intermolecular vibrational band in simple liquids31 ), they or one of them should critically influences the feature of the higher frequency M1 in these multi-component systems. Overall, the liquid densities (or molecular weights of the components) for


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J. Chem. Phys. 141, 134506 (2014)

C. Comparison of low-frequency spectrum with bulk parameter

FIG. 5. Plots of (a) M1 and (b) ω of low-frequency Kerr spectrum versus salt composition f for [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] (blue squares) and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] (red circles).

Previously, a linear relationship between M1 of the lowfrequency Kerr spectrum due √ to the intermolecular vibrations and a bulk parameter, γ /ρ, has been found for aprotic molecular liquids.31 Construction of such a correlation is based on the harmonic oscillator description of the intermolecular vibrational band. The latter is a collective quantity and thus may have a link to collective liquid properties such as surface tension and density as these quantities (γ and d) may be used in place of force constant and reduced mass. Because M1 of the low-frequency spectrum is a measure of the collective intermolecular interactions, this relationship appears to reflect a scaling feature. However, it should be remembered that this relationship becomes effective for homogeneous liquids but not in the case of highly inhomogeneous liquids and solutions, such as, aromatic ionic liquids36 and ionic liquid mixtures with neutral solvents.57, 58 In this section, we examine the relationship between M1 of the low-frequency Kerr √ spectrum and γ /ρ in these DES systems. √ Fig. 6(a) shows the plots of M1 versus γ /ρ for the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] systems. In the figure, the relations for aprotic molecular liquids31, 49, 101 and aromatic and nonaromatic ionic liquids36 are √ also shown for a comparison. As seen in Fig. 6(a), γ /ρ for the present DES systems varies over a small range.

the present DES systems are not particularly high compared to common aprotic liquids31, 66 and ionic liquids.99, 100 Thus the larger value of M1 in the present DES systems is likely coming from the microscopic intermolecular/interionic interaction. It is not surprising that the microscopic intermolecular/ interionic interaction in the present DES systems is likely to have strong effects on the low-frequency spectrum (M1 ) compared to typical aprotic molecular liquids and ionic liquids, probably because of HB network of acetamide and ions. Note the f-dependence of spectral width, ω, is also different for these DES systems for example, ω is nearly insensitive to f for the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system while ω for the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system exhibits slight narrowing. This narrowing with f could be due to the observed intermolecular hydrogen-bonding vibrational band. As seen earlier, the intermolecular hydrogen-bonding band becomes clearer with the increase of f for the [0.78CH3 CONH2 + 0.22{fLiBr + (1 – f )LiNO3 }] system. The increase of the amplitude of the HB band naturally causes the narrower band width at FWHM for the entire low-frequency broad band. However, the f-dependence of the width for the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system is very small. Furthermore the spectral width for the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system is nearly unchanged by varying f, as pointed out above. Therefore the intermolecular vibrational structure in these DES systems does not vary much with f indicating the presence of heterogeneity in these systems rather constant for all the salt contents in the present DES systems.

FIG.√6. Plots of M1 of low-frequency Kerr spectrum versus bulk parameter γ /ρ for [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] (blue squares) and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] (red circles) with (a) wide range and (b) magnification in the data points for the DES systems. Black, green, and purple lines denote the relationships for aprotic molecular liquids, aromatic ionic liquids, and nonaromatic ionic liquids, respectively.


134506-10


However, it is clear that the data for the present DES systems are far away from the correlations established for aprotic molecular liquids and aromatic ionic liquids but reside closer to the correlation shown by nonaromaic ionic liquids. When the DES data are presented on√a magnified scale (Fig. 6(b)), a weak dependence of M1 on γ /ρ with opposite trend for these DES systems can be detected. √ The dependence of M1 on γ /ρ for these DES systems shown in Fig. 6(b) suggests that cations can influence the lowfrequency spectrum (M1 ) through microscopic intermolecular/interionic interactions. The smaller volume of Na+ than K+ imparts stronger interaction of cation with the anion and acetamide molecules in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system, producing larger M1 upon lowering of f. Similarly, the smallest size of Li+ produces the relatively larger value of M1 in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system. It is not clear why the anion effect is negligibly small although their volumes are different. In the case of nonaromatic ionic liquids, anion affects more critically M1 than cation does.62, 69 The present DES systems are not only ionic but hydrogen-bonded also. Presence of ionic species in these systems may make the HBs or HB network to respond in a more complicated manner than in simple and pure ionic systems like ionic liquids. In near future, the anion effect on the low-frequency spectrum in DES systems will be focused using anions having largely different volumes. IV. CONCLUSIONS

In this study, we have reported the ultrafast dynamics data for acetamide and inorganic salt DES systems, [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] and [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] systems with f of 0, 0.2, 0.4, 0.6, 0.8, and 1.0, by means of fs-RIKES, for the first time. The dynamics obtained by fs-RIKES has been discussed based on the Fourier transform low-frequency Kerr spectra. The spectral shape of the low-frequency Kerr spectrum in the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system shows a unique band at ∼70 cm−1 due to the intermolecular HB mode, but the band is buried in the low-frequency broad band in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system. This difference is caused by the difference in the activity of the intermolecular/interionic vibrational motions of the constituent ions in the DES systems. Namely, the Kerr spectral intensities coming from the spherical ions, such as Li+ , Na+ , K+ , and Br− , are much weaker than that in non-spherical ions and molecules because of the symmetric nature of ionic species. As a result, the relative intensity in the intermolecular hydrogenbonding band is strong in the case of the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] systems with the larger f compared to that with the smaller f and that is weak in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system. We have compared M1 of the low-frequency spectral band which is mainly attributed to the intermolecu√ lar/interionic vibrations with the bulk parameter γ /ρ. M1 of the low-frequency spectrum in the DES systems is higher than that in common neutral molecular liquids, and the re-

J. Chem. Phys. 141, 134506 (2014)

√ lation between M1 and γ /ρ in the DES systems is rather close to that in nonaromatic ionic liquids but far from that of aprotic molecular liquids and aromatic ionic liquids. Although the variable range in the present samples is not wide, the f-dependence of M1 is confirmed in the [0.75CH3 CONH2 + 0.25{f KSCN + (1 – f )NaSCN}] system. In contrast, the [0.78CH3 CONH2 + 0.22{f LiBr + (1 – f )LiNO3 }] system shows almost constant M1 to the variation of f. The results indicate that the cation species affects the microscopic interaction in the DES systems more effectively than the anion species does, but they are not significant. It would be interesting to explore the inter-relationship between dynamic heterogeneity and slow dynamics102, 103 and experimental detection of slower relaxation components in these systems via expanding the present RIKES detection system to longer timescales. ACKNOWLEDGMENTS

R.B.’s stay at Chiba was supported by Center for Frontier Science, Chiba University. This study was partially supported by the Tokyo Ohka Foundation for the Promotion of Science and Technology. 1 Q.

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