Femtosecond Raman-induced Kerr Effect Spectroscopy Research Articles

Low-Frequency Spectra of Hydrated Ionic Liquids with Kosmotropic and Chaotropic Anions.

In this study, we investigated the water concentration dependence of the intermolecular vibrations of two hydrated ionic liquids (ILs), cholinium dihydrogen phosphate ([ch][dhp]) and cholinium bromide ([ch]Br), using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The anions of the former and latter hydrated ILs are kosmotropic and chaotropic, respectively. We found that the spectral peak of ∼50 cm-1 shifted to the low-frequency side in hydrated [ch][dhp], indicating the weakening of its intermolecular interactions. In contrast, no change in the peak frequency of the low-frequency band at ∼50 cm-1 was observed with increasing water concentration in hydrated [ch]Br. The vibrational density of states (VDOS) spectra generated from molecular dynamics (MD) simulations were in qualitative agreement with the experimental results. Decomposition analysis of the VDOS spectra for each component revealed that the red shift of the low-frequency band in the hydrated [ch][dhp] upon water addition was essentially due to the contributions of anions and water rather than that of the cholinium cation. We also found from the low-frequency spectra of the two hydrated ILs that they differed in the concentration dependence of the 180 cm-1 band, which is assigned as a hindered translational motion of water molecules combined to form O···O stretching motions. From the relationship between the peak frequency of the low-frequency band and the bulk parameter, which is the square root of the surface tension divided by the density, we found that the peak frequency in the hydrated IL with kosmotropic [dhp]- depends on the bulk parameter, similar to the case for an aqueous solution of the typical deep eutectic solvent reline. However, the peak frequency of the hydrated IL with chaotropic Br- is constant with the bulk parameter.

Exploring the Microscopic Aspects of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Formamide, N-Methylformamide, and N,N-Dimethylformamide by Multiple Spectroscopic Techniques and Computations.

The microscopic aspects of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF) were investigated using spectroscopic techniques of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), FT-IR, and NMR. Molecular dynamics simulations and quantum chemistry calculations were also performed. According to fs-RIKES, the first moment of the low-frequency spectrum bands mainly originating from the intermolecular vibrations in the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems changed gradually with the molecular liquid mole fraction XML but that in the [MOIm][BF4]/NMF system was constant up to XNMF = 0.7 and then gradually increased in the range of XNMF ≥ 0.7. Excluding the contribution of the 2D hydrogen-bonding network due to the presence of FA in the low-frequency spectrum band, the XML dependence of the normalized first moment of the low-frequency band in the [MOIm][BF4]/FA and [MOIm][BF4]/NMF systems revealed that the normalized first moment did not remarkably change in the range of XML < 0.7 but drastically increased in XML ≥ 0.7. FT-IR results indicated that the amide C═O band shifted to the low-frequency side with increasing XML for the three mixtures due to the hydrogen bonds. The imidazolium ring C-H band also showed a similar tendency to the amide C═O band. 19F NMR probed the microenvironment of [BF4]- in the mixtures. The [MOIm][BF4]/NMF and [MOIm][BF4]/DMF systems showed an up-field shift of the F atoms of the anion with increasing XML, and the [MOIm][BF4]/FA system exhibited a down-field shift. Steep changes in the chemical shifts were confirmed in the region of XML > 0.8. On the basis of the quantum chemistry calculations, the observed chemical shifts with increasing XML were mainly attributed to the many-body interactions of ions and amides for the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems. Meanwhile, the long distance between the cation and the anion was due to the high dielectric medium for the [MOIm][BF4]/NMF system, which led to an up-field shift.

Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group

This study compared the physical properties, e.g., glass transition temperature, melting point, viscosity, density, surface tension, and electrical conductivity, and the low-frequency spectra under 200 cm-1 of three synthesized ionic liquids (ILs), triethylpentylphosphonium bis(fluorosulfonyl)amide ([P2225][NF2]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P222(2O2)][NF2]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P222(2S2)][NF2]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P222(2S2)][NF2] had the highest viscosity and glass transition temperature, whereas the [P222(2O2)][NF2] had the lowest. Among the three ILs, the [P222(2S2)][NF2] had the highest density and surface tension, and the [P222(2O2)][NF2] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P2225][NF2], molecular dynamics simulations successfully reproduced the line shapes of the experimental spectra and indicated that the RIKES spectrum was mainly due to the cation and cross-term and their rotational motions, whereas the THz-TDS spectrum was mainly due to the anion and its translational motion. This shows that it is desirable to utilize both fs-RIKES and THz-TDS methods to reveal molecular motions at the low-frequency domain. The [P222(2S2)][NF2] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P2225][NF2] and [P222(2O2)][NF2].

Low-frequency spectra of reline and its mixtures with water: A comparative study based on femtosecond Raman-induced Kerr effect spectroscopy and molecular dynamics simulations

In this study, we investigated the low-frequency spectra below 650 cm−1 of aqueous reline solutions with various concentrations via femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and molecular dynamics (MD) simulations. The density, surface tension, viscosity, and electrical conductivity of the aqueous reline solutions were also measured. In the low-frequency spectrum, the peak frequency due to intermolecular vibrations of the aqueous reline solutions proportionally shifted to lower frequency with increasing the water content (CH2O; wt%). The density of state (DOS) spectra of aqueous reline solutions obtained via MD simulations also showed a red shift of the low-frequency spectrum upon increasing CH2O. A decomposition analysis of the DOS spectra revealed that the peaks corresponding to all components, that is, choline cation, urea, chloride, and water, showed a lower frequency shift in the DOS spectrum with increasing CH2O. The plot of the peak frequency of the low-frequency spectrum vs the bulk parameter γ/ρ (γ: surface tension; ρ: density) in aqueous reline solutions showed two relations at a turning point CH2O = 10 wt%, which can be attributed to a change in the microscopic structure of the aqueous reline solution from a bulk-like reline structure to reline clusters.

Intermolecular Dynamics of Positively and Negatively Charged Aromatics and Their Isoelectronic Neutral Analogs in Aqueous Solutions.

In this study, we investigated the temperature dependence of intermolecular vibrations and orientational dynamics in the aqueous solutions of imidazole hydrochloride, imidazole, sodium triazolide, and triazole using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and steady-state Raman spectroscopy. The difference low-frequency Raman spectra under 250 cm-1 of the aqueous solutions relative to the neat water showed that the spectral shoulder in the high-frequency region at 60-100 cm-1, assigned to the libration of an aromatic ring, was higher in frequency for the imidazolium cation but lower for the triazolide anion than those of the respective neutral aromatics. The results of the ab initio quantum chemistry calculations of the clusters of the aromatics and water molecule(s) were consistent with the experimental spectra of the aqueous solutions. Further, the results of the temperature-dependent experiments showed that the signal intensity in the low-frequency region below 50 cm-1 increased for all solutions with an increase in temperature. In contrast, the spectral density in the high-frequency region above 80 cm-1 exhibited almost no shift for the 1.0 M solutions, while a significant red shift was observed for the 5.0 M solutions. In addition, the temperature-dependent densities, viscosities, and surface tensions were characterized for the aqueous aromatic solutions from 293 to 353 K.

Intermolecular Dynamics and Structure in Aqueous Lidocaine Hydrochloride Solutions.

We investigated the intermolecular dynamics and static structure in the aqueous solutions of lidocaine hydrochloride (LDHCl) in the concentration range of [LDHCl] = 0-2.00 M using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), small- and wide-angle X-ray scattering (SWAXS), and dynamic light scattering (DLS). For the fs-RIKES experiments, the concentration dependence of the difference low-frequency spectra of the aqueous LDHCl solutions relative to the neat water, which was mainly due to the intermolecular vibrations, was characterized using an exponential function with a characteristic concentration of ∼1 M. For the SWAXS experiments, we observed a manifestation of an excess scattering component centered within a range of 8-10 nm-1 in the aqueous LDHCl solutions. The results of Fourier inversion and further deconvolution analyses unambiguously demonstrated that lidocaines assemble into a nanometer-sized micelle-like structure with the innermost core (∼0.3 nm) and outer shell (∼0.5 nm), respectively. The DLS experiments also found nanometer-sized aggregates and further indicated evidence of the clusters of the aggregates. The results of viscosities, densities, and surface tensions of the solutions and the quantum chemistry calculations supported the unique features of the microscopic intermolecular interaction and the micelle-like aggregation.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Poly(ethylene glycol) by Femtosecond Raman-Induced Kerr Effect Spectroscopy

This is the first report on low-frequency spectra of ionic liquid (IL)/polymer mixtures using femtosecond Raman-induced Kerr effect spectroscopy. We studied mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) and poly(ethylene glycol) (PEG) with Mn = 400 (PEG400) at various concentrations. To elucidate the unique features of the IL/polymer mixture system, mixtures of PEG400 with a molecular liquid, 1-octhylimidazole (OIm), which is a neutral analog of the cation, were also studied. In addition, mixtures of [MOIm][BF4] with ethylene glycol (EG) and poly(ethylene glycol) with Mn = 4000 (PEG4000) were also investigated. The first moments of broad low-frequency spectra, mainly due to intermolecular vibrations for the [MOIm][BF4]/PEG400 and OIm/PEG400, increased slightly with increasing concentration of PEG400, indicating that microscopic intermolecular interactions, in general, are slightly enhanced. We also compared the [MOIm][BF4] mixtures with EG, PEG400, and PEG4000 at concentrations of 5 and 10 wt % PEG or EG. The low-frequency spectra of samples with the same concentrations were quite similar, but a comparison of the normalized spectra showed that the spectral intensity in the low-frequency region below ∼50 cm-1 of the [MOIm][BF4] mixtures with PEG400 and PEG4000 is somewhat lower than that of the [MOIm][BF4] mixtures with EG. Although the effect of the polymer is small compared to other polymer solution systems, this feature is attributed to a suppression of translational motion in the mixtures of [MOIm][BF4] with PEG compared to the mixtures of [MOIm][BF4] with EG due to the greater mass of PEG than EG. Density, surface tension, viscosity, and electrical conductivity were also estimated. From Walden plots, it was found that the [MOIm][BF4]/PEG4000 system showed more ideal electrical conductive behavior than the [MOIm][BF4]/PEG400 and [MOIm][BF4]/EG systems.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Methanol, Acetonitrile, and Dimethyl Sulfoxide: A Combined Study of Femtosecond Raman-Induced Kerr Effect Spectroscopy and Molecular Dynamics Simulations.

In this study, we examined the low-frequency spectra of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with methanol (MeOH), acetonitrile (MeCN), and dimethyl sulfoxide (DMSO), which were obtained by femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and molecular dynamics (MD) simulations. In addition, we estimated the liquid properties of the mixtures, such as density ρ, surface tension γ, viscosity η, and electrical conductivity σ. The line shapes of the low-frequency Kerr spectra of the three [MOIm][BF4] mixture systems strongly depend on the mole fraction of the molecular liquid, XML. The spectral intensity increases with increasing XML of the [MOIm][BF4]/MeCN system but decreases for the [MOIm][BF4]/MeOH and [MOIm][BF4]/DMSO systems. These behaviors of the spectral intensities reasonably agree with the vibrational density-of-states spectra when the polarizability anisotropies of MeOH, MeCN, DMSO, and ion species are considered. The characteristic frequencies (first moments, M1) of the low-frequency spectra of the three mixture systems are almost insensitive at XML = 0-0.6. However, the frequencies vary mildly at XML = 0.6-0.9 and dramatically at XML = 0.9-1. The XML-dependent M1 in the Kerr spectra are well reproduced by the MD simulations. Plots of M1 versus bulk parameter, (γ/ρ)1/2, for the three mixture systems show that the mixtures at XML = 0-0.6 behave like aromatic cation-based ionic liquids (ILs), those at XML = 0.9-1 are molecular liquids (MLs), and those at XML = 0.6-0.9 are transitioning between aromatic cation-based ILs and MLs. MD simulations show that the solvent molecules localized at the interface between the ionic and the alkyl group regions without forming large solvent networks at XML = 0-0.6. However, solvent networks or regions develop largely at XML = 0.6-0.9 and the constituent ions of the IL disperse in the MLs at XML = 0.9-1. The MD simulations corroborate the results obtained by fs-RIKES.

Ultrafast Dynamics in Nonaromatic Cation Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study

Abstract Herein, the data of the intermolecular vibrations of forty nonaromatic cation based ionic liquids (ILs) at 293 K measured by femtosecond Raman-induced Kerr effect spectroscopy are reported. The low-frequency spectra in the frequency range of 0.3–700 cm−1 were obtained by Fourier transform deconvolution analysis. The line shapes of the low-frequency spectra below ∼200 cm−1 were discussed on the basis of the ion species. The spectral intensity in nonaromatic cation based ILs was much lower than that in aromatic cation based ILs owing to the absence of the aromatic ring, i.e., the libration of the aromatic species had a strong spectral intensity in the low-frequency region. However, nonaromatic cation based ILs with a flat anion, such as dicyanamide and tricyanomethanide, showed stronger spectral intensity because of the libration of the anion. Other unique spectral features were also discussed in the context of the structure of the ion species. Liquid properties, such as density, viscosity, electrical conductivity, and surface tension, were also estimated. On comparing the low-frequency spectra with the bulk liquid properties of the nonaromatic cation based ILs, a mild linear relationship between the first moment of the low-frequency spectrum and a bulk parameter comprised of surface tension and density was observed.

Low-Frequency Vibrational Motions of Polystyrene in Carbon Tetrachloride: Comparison with Model Monomer and Dependence on Concentration and Molecular Weight

In this study, the low-frequency vibrational dynamics of polystyrene (PS) in CCl4 was investigated by femtosecond Raman-induced Kerr effect spectroscopy. Ethylbenzene (EBz) was also investigated as a model monomer of the polymer to elucidate the unique dynamical features of PS in solution. The broadened low-frequency spectrum of the PS/CCl4 in the frequency region below 150 cm-1 is significantly different from that of the EBz/CCl4. Difference spectra between the PS or EBz solutions and neat CCl4, normalized to an internal vibrational mode of CCl4, clearly show a much lower spectral intensity for the PS/CCl4 than the EBz/CCl4 in the low-frequency region below ca. 20 cm-1. This indicates that translational motions are suppressed in the PS/CCl4 compared to the EBz/CCl4. Moreover, the high-frequency motion at ca. 70 cm-1, mainly due to phenyl ring librations, occurs at higher frequency in PS (78 cm-1) than EBz (65 cm-1). In addition, the results of concentration-dependent experiments show that the first moment (M1) of the low-frequency difference spectra of both PS/CCl4 and EBz/CCl4 is almost independent of the concentration. The molecular weight dependence of the low-frequency spectrum in the PS/CCl4 shows that the M1 value of the low-frequency spectral band of PS shifts to higher frequencies when the molecular weight of PS increases up to Mw = ∼1000, which corresponds approximately to the decamer, and then remains constant upon further increasing the molecular weight.

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Pyrrolidinium-Based Ionic Liquids: Effects of Anion Species.

We investigated the temperature dependence of the intermolecular vibrational dynamics of pyrrolidinium-based ionic liquids (ILs) with 10 different anion species using femtosecond Raman-induced Kerr effect spectroscopy. The features of the temperature-dependent vibrational spectra vary with the different anions. In the case of the ILs with spherical top anions, such as tetrafluoroborate and hexafluorophosphate, and trifluoromethanesulfonate, the spectral intensity in the low-frequency region below 50 cm-1 increases with rising temperature, while that in the high-frequency region above 50 cm-1 remains almost unchanged. Similar temperature-dependent features were also found in the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts. However, the difference spectra at respective temperature relative to 293 K indicate that the spectra of the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts are more temperature-sensitive in the low-frequency region below 50 cm-1 compared to those of the tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate salts. The spectra of 1-butyl-1-methylpyrrolidinium-based ILs with dicyanamide and tricyanomethide anions show a characteristic temperature dependence; in addition to an increase of the spectral intensity in the low-frequency region below 50 cm-1, a red shift of the spectra in the high-frequency side above 50 cm-1 was observed with increasing temperature. This implies that the librational motions of planar dicyanamide and tricyanomethide anions contribute substantially to the low-frequency spectra. We also compared the temperature-dependent low-frequency spectra of 1-butyl-1-methylpyrrolidinium- and 1-(2-methoxyethyl)-1-methylpyrrolidinium-based ILs with some anions. Although the spectral shapes are slightly different in the range of 70-150 cm-1, which can be attributed to the intramolecular vibrational modes of the cations, the temperature dependence of the spectral shapes is quite similar, indicating that the ether substitution in the cation side groups has little effects on the temperature dependence of the low-frequency spectra. The fragilities of the ILs were also estimated from the temperature-dependent viscosities and the glass-transition temperatures. The fragility parameter seems to be correlated with the temperature dependence of the first moment of the low-frequency spectral bands mainly arising from the intermolecular vibrations of the ILs.

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Molten Bis(trifluoromethylsulfonyl)amide Salts: Effects of Cation Species.

In this study, we have investigated the effects of cation structures on the temperature dependence of the intermolecular vibrational dynamics of ionic liquids using femtosecond Raman-induced Kerr effect spectroscopy. The ionic liquids used in this study are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of the cations 1-butyl-3-methylimidazolium [C4MIm]+, 1-butyl-1-methylpyrrolidinium [Pyrr14]+, 1-butylpyridinium [C4Py]+, butyldiethylmethylammonium [N1224]+, triethyloctylammonium [N2228]+, and triethyloctylphosphonium [P2228]+. All of the ionic liquids show temperature-dependent low-frequency spectra. A difference in the temperature dependence between the spectra of the aromatic and nonaromatic cation based ionic liquids is especially significant. In the case of the aromatic cation based ionic liquids [C4MIm][NTf2] and [C4Py][NTf2], the spectral intensities in the low-frequency region below ca. 50 cm-1 increase and the high-frequency components at ca. 80 cm-1 shift to lower frequencies with rising temperature. In contrast, the ionic liquids based on nonaromatic cations only exhibit an increase in the low-frequency region below ca. 50 cm-1 with increasing temperature, while the high-frequency region of the spectra above ca. 50 cm-1 shows little change with variation of the temperature. These results suggest that the presence or absence of aromatic rings is the main factor in determining the temperature-dependent spectral features, particularly in the high-frequency region. We also found that the alkyl chain length and central atoms of the nonaromatic quaternary cations do not have much influence on the temperature-dependent spectral features. The first moments of the aromatic cation based ionic liquids are a little more sensitive to temperature than those of the nonaromatic cation based ionic liquids. The temperature-dependent viscosities and fragilities of the ionic liquids have also been examined.

Effects of aromaticity in cations and their functional groups on the temperature dependence of low-frequency spectrum

In this study, we investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3-200 cm-1 for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr]+, 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm]+, N-cyclohexylmethylpyridinium [CHxmPy]+, 1-benzyl-1-methylpyrrolidinium [BzMPyrr]+, 1-benzyl-3-methylimidazolium [BzMIm]+, and N-benzylpyridinium [BzPy]+ cations. The aim of this study is to better understand the effects of aromaticity in the cations' constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but the temperature-dependent spectrum of [CHxmMPyrr][NTf2] is different from that of other ILs. While [CHxmMPyrr][NTf2] shows spectral changes with temperature in the low-frequency region below 50 cm-1, the other ILs also show spectral changes in the high-frequency region above 80 cm-1 (above 50 cm-1 in the case of [BzMPyrr][NTf2]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.

Femtosecond Raman-Induced Kerr Effect Study of Temperature-Dependent Intermolecular Dynamics in Imidazolium-Based Ionic Liquids: Effects of Anion Species and Cation Alkyl Groups.

The temperature dependence of the intermolecular vibrational dynamics in imidazolium-based ionic liquids (ILs) with 10 different anions was studied by femtosecond Raman-induced Kerr effect spectroscopy. For all ILs investigated in this study, the intensity in the low-frequency region below 50 cm-1 increases, and the spectral density in the high-frequency region above 80 cm-1 decreases (and shows a redshift) with increasing temperature. The first phenomenon would be attributed to the activation of the translational vibrational motions, whereas the second one is ascribed to the slowing librational motion of the imidazolium ring with increasing temperature. Calculated spectra of the density of states for the intermolecular vibrations of 1-butyl-3-methylimidazolium hexafluorophosphate, which is one of the experiment samples studied here, obtained by molecular dynamics simulation agreed well with the experimental results and confirmed the spectral assignments. When we compared the difference spectra between spectra measured at various temperatures and the spectrum measured at 293 K, a clear difference was found in the ∼50 cm-1 region of the Kerr spectra of 1-butyl-3-methylimidazolium thiocyanate and 1-butyl-3-methylimidazolium dicyanamide from those of the other ILs. The difference might have originated from the librational motions of the corresponding anions. We also compared the temperature-dependent Kerr spectra of hexafluorophosphate salts of 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and 1-heptyl-3-methylimidazolium cations. These ILs showed a similar temperature dependence, which was not affected by the alkyl group length. The temperature-dependent viscosities and glass transition temperatures of the ILs were also estimated to determine their fragilities.

Understanding the vibrational mode-specific polarization effects in femtosecond Raman-induced Kerr-effect spectroscopy.

Optically heterodyne-detected femtosecond Raman-induced Kerr-effect spectroscopy (OHD-FRIKES) was observed in neat cyclohexane. In this Letter, an examination of the effect of the Raman pump ellipticity on the multiplex OHD-FRIKES spectra is discussed. The Raman pump ellipticity scanned OHD-FRIKES results reproduce anomalous observables from previous OHD-FRIKES experiments and suggest new methods of tracking transient vibrational mode polarization in complex systems.

Ultrafast Dynamics in Aromatic Cation Based Ionic Liquids: A Femtosecond Raman-Induced Kerr Effect Spectroscopic Study

Abstract We studied the ultrafast dynamics of 40 aromatic cation based ionic liquids (ILs) by means of femtosecond Raman-induced Kerr effect spectroscopy. The low-frequency Kerr spectra (ca. 0.3–700 cm−1) of the ILs were obtained from the Kerr transients by Fourier-transform deconvolution analysis. The low-frequency Kerr spectra in the frequency range less than 200 cm−1 coming mainly from the intermolecular vibrations for the ILs were discussed in terms of (i) anion dependence, (ii) imidazolium cations vs. pyridinium cations, (iii) alkyl group dependence, and (iv) effect of methylation in aromatic cations. Several liquid properties, such as density, viscosity, electrical conductivity, and surface tension, of the present sample ILs at 293 K were also estimated in this study. We clarified that the aromatic cation based ILs show a different relation of the first moment of the low-frequency spectral band to the bulk liquid parameter, which is the square root of surface tension divided by liquid density, from aprotic molecular liquids. The slope of the first moment to the bulk parameter for the aromatic cation based ILs is gentler than that for aprotic molecular liquids.

Effects of Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1-Methyl-3-octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect Spectroscopy.

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

Temperature Dependence of Low-Frequency Spectra in Molten Bis(trifluoromethylsulfonyl)amide Salts of Imidazolium Cations Studied by Femtosecond Raman-Induced Kerr Effect Spectroscopy.

In this study, the temperature dependence of the low-frequency spectra of liquid bis(trifluoromethylsulfonyl)amide salts of the monocations 1-methyl-3-propylimidazolium and 1-hexyl-3-methylimidazolium and the dications 1,6-bis(3-methylimidazolium-1-yl)hexane and 1,12-bis(3-methylimidazolium-1-yl)dodecane has been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The intensity in the low-frequency region below 20 cm(-1) in the spectra of the four ionic liquids increases with rising temperature. From a line-shape analysis of the broadened low-frequency spectra of the ionic liquids, it is clear that the lowest-frequency component, which peaks at approximately 5 cm(-1), contributes to the temperature dependence of the spectra. This implies that the activity of the intermolecular translational vibrational motion is increasing with rising temperature. It is also possible that decoupling in the crossover process between intermolecular vibrational motion and structural relaxation occurs as a result of a deterioration of the non-Markovian feature or the loss of memory caused by the higher temperature. The peak of the highest-frequency component, which is due mainly to the imidazolium ring libration, shifts to lower frequency with increasing temperature. This is attributed to weaker interactions of the ionic liquids at higher temperatures. Temperature-dependent viscosities from 293 to 353 K of the four ionic liquids have also been characterized.

Effects of Aromaticity in Cations and Their Functional Groups on the Low-Frequency Spectra and Physical Properties of Ionic Liquids.

We have critically investigated the low-frequency spectra of six ionic liquids (ILs) consisting of systematically different cations having benzyl moieties or comparable-sized saturated cyclohexylmethyl groups, by means of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The target ionic liquids are bis(trifluoromethylsulfonyl)amide ([NTf2](-)) salts of the 1-benzyl-3-methylimidazolium ([BzMIm](+)), 1-benzyl-1-methylpyrrolidinium ([BzMPyrr](+)), 1-benzylpyridinium ([BzPy](+)), 1-cyclohexylmethyl-3-methylimidazolium ([CHxmMIm](+)), 1-cyclohexylmethyl-1-methylpyrrolidinium ([CHxmMPyrr](+)), and 1-cyclohexylmethylpyridinium ([CHxmPy](+)) cations. The primary purpose of this study is to clarify the effects of charged and neutral aromatic moieties on the low-frequency spectrum and bulk properties such as liquid density, surface tension, shear viscosity, glass transition temperature, and melting point. We found that ILs with benzyl groups have larger surface tensions than those with the same cation bearing the cyclohexylmethyl group. The trend in the glass transition temperatures, comparing ILs having the same side group, is pyridinium > imidazolium > pyrrolidinium. The effects of a single aromatic moiety on the shear viscosity are inconclusive, although the viscosities of the ILs with aromatic moieties on both the cation and the benzyl group, i.e., [BzMIm][NTf2] and [BzPy][NTf2], are substantially lower than those of the other ILs at room temperature, as a consequence of their higher fragilities. In the low-frequency Kerr spectra in the frequency range of approximately 0.1 to 200 cm(-1) measured by fs-RIKES, the ILs possessing two aromatic groups show the largest relative intensity of the nuclear response to the electronic response. Both the charged and neutral aromatic rings show signals due to the ring libration; the neutral one appears at a lower frequency than the charged one. The relationship between the first moment of the broad low-frequency spectrum band and the bulk parameter consisting of the square root of the surface tension divided by the liquid density is obeyed by the cyclohexylmethyl derivatives whether the cation is aromatic or not, but not by the ILs with the neutral aromatic benzyl group. Quantum chemistry calculations have been also performed to understand the vibrational modes of the ionic species in the ILs.

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

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 [0.75CH3CONH2 + 0.25{f KSCN + (1 - f )NaSCN}] and [0.78CH3CONH2 + 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 [0.78CH3CONH2 + 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 interaction-induced) 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 [0.75CH3CONH2 + 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 [0.78CH3CONH2 + 0.22{f LiBr + (1 - f )LiNO3}] and [0.75CH3CONH2 + 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.