vibrational spectroscopy of ionic liquids et al. 20… · 82 on vibrational spectroscopy of ionic...

1 Vibrational Spectroscopy of Ionic Liquids2 Vitor H. Paschoal, Luiz F. O. Faria, and Mauro C. C. Ribeiro*

3 Laboratorio de Espectroscopia Molecular, Departamento de Química Fundamental, Instituto de Química, Universidade de Sao Paulo,4 Av. Prof. Lineu Prestes 748, Sao Paulo 05508-000, Brazil

5 *S Supporting Information

6 ABSTRACT: Vibrational spectroscopy has continued use as a powerful tool to7 characterize ionic liquids since the literature on room temperature molten salts8 experienced the rapid increase in number of publications in the 1990’s. In the past years,9 infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and10 the resulting liquid structure in ionic liquids. A large body of information is now available11 concerning vibrational spectra of ionic liquids made of many different combinations of12 anions and cations, but reviews on this literature are scarce. This review is an attempt at13 filling this gap. Some basic care needed while recording IR or Raman spectra of ionic14 liquids is explained. We have reviewed the conceptual basis of theoretical frameworks15 which have been used to interpret vibrational spectra of ionic liquids, helping the reader16 to distinguish the scope of application of different methods of calculation. Vibrational17 frequencies observed in IR and Raman spectra of ionic liquids based on different anions18 and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader19 can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions20 and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further21 issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR22 and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

23 CONTENTS

25 1. Introduction A26 2. Experimental Details C27 3. Computational Details E28 4. Vibrational Spectroscopy of Pure Ionic Liquids in29 the Mid-Frequency Range H30 4.1. Vibrational Frequencies of Anions H31 4.1.1. Small Symmetric Anions H32 4.1.2. More Complex Fluorinated Anions K33 4.1.3. Alkylsulfates and Hydrogen Sulfate N34 4.1.4. Carboxilates O35 4.2. Vibrational Frequencies of Cations Q36 4.2.1. Imidazolium Q37 4.2.2. Pyridinium U38 4.2.3. Pyrrolidinium U39 4.2.4. Piperidinium V40 4.2.5. Ammonium W41 4.2.6. Other Cations Y42 4.3. Applications Y43 5. Vibrational Spectroscopy of Pure Ionic Liquids in44 the Low-Frequency Range AC45 6. Vibrational Spectroscopy for Studying Ionic46 Liquid Phase Transitions AH47 7. Vibrational Spectroscopy of Ionic Liquid Solu-48 tions AL49 8. Concluding Remarks AR50 Associated Content AS51 Supporting Information AS52 Author Information AS

53Corresponding Author AS54ORCID AS55Notes AS56Biographies AS57Acknowledgments AS58References AS

1. INTRODUCTION

59Vibrational and NMR spectroscopies are fundamental tools to60characterize ionic liquids. The vibrational spectroscopy61techniques of infrared (IR) and Raman permeate the literature62on essentially all of the actual or potential applications63envisaged for ionic liquids. IR and Raman spectroscopies64have provided deep insights on the nature of ionic interactions,65the role played by anion−cation hydrogen bonds, molecular66conformations, and their modifications as pressure and67temperature is varied in the normal liquid phase, during68phase transition to crystalline or amorphous (glassy) solid69phases, after vaporization, etc. The user of a vibrational70spectroscopy technique needs to have in hand reliable71interpretation of vibrational spectra, in particular, assignment72of experimental frequencies to vibrational motions of the73common ions of ionic liquids. This information is usually

Special Issue: Ionic Liquids

Received: July 15, 2016

Review

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© XXXX American Chemical Society A DOI: 10.1021/acs.chemrev.6b00461Chem. Rev. XXXX, XXX, XXX−XXX

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74 available from studies which are more dedicated to theoretical75 and computational issues related to vibrational spectroscopy.76 Quantum chemistry methods are now routinely used to77 calculate vibrational frequencies to be compared to exper-78 imental data. However, strong ionic interactions may imply that79 comparison between experimental and calculated frequencies80 by a relatively fast ab initio calculation is far from81 straightforward. In fact, scrutinizing the large body of literature82 on vibrational spectroscopy of ionic liquids, eventually there is83 no full agreement between different authors for the same84 system. Other schemes have been considered for assigning85 vibrational spectra of ionic liquids (e.g., taking advantage of86 molecular dynamics simulations of liquids). On the other hand,87 even though vibrational spectroscopy has a long history in88 studying high temperature molten salts and some ions are89 common species for both high and room temperature molten90 salts (e.g., relatively simple polyatomic anions), sometimes91 previous knowledge on vibrational spectroscopy of molten salts92 is not fully appreciated within the context of ionic liquids.93 Moreover, vibrational spectroscopy is well appropriate for94 studying intermolecular interactions not only in pure ionic95 liquids but also in ionic liquids mixtures, solutions of molecular96 or ionic solutes in ionic liquids, gas absorption, etc.97 This review addresses the above-mentioned issues and98 others, aiming to be useful for users who employ IR and99 Raman spectroscopies combined with other techniques while100 investigating ionic liquids and also for those who are involved101 in assignments of vibrational spectra of ionic liquids.102 Papatheodorou et al.1,2 published important reviews on103 Raman spectroscopy of high temperature molten salts, but104 previous reviews on vibrational spectroscopy of ionic liquids are105 scarce. In 2007, Berg3 published a review on Raman106 spectroscopy and ab initio calculations of ionic liquids. Berg’s107 review focused on the spectroscopic signatures of molecular108 conformations achieved by the ions, mainly 1-alkyl-3-109 methylimidazolium and N,N-dialkylpyrrolidinium cations, and110 the bis(trifluoromethanesulfonyl)imide anion, [NTf2]

−. These111 Raman spectroscopic studies have been reviewed more recently112 by Saha et al.4 The more specific issue of vibrational113 spectroscopy of ionic liquid surfaces using linear and nonlinear114 techniques has also been reviewed recently.5 In this review, we115 will address both IR and Raman studies encompassing a wide116 group of cations and anions commonly used in forming ionic

f1 117 liquids. Figure 1 shows molecular structures of several cations118 and anions whose vibrational spectra will be discussed in this119 review, together with notation used throughout this work.120 This review addresses many issues in which vibrational121 spectroscopy has given fundamental contributions for our122 current understanding about interactions and structure of ionic123 liquids along the last two decades. The review focusesd on124 linear IR and Raman spectroscopies, being beyond the scope of125 techniques such as time-resolved IR spectroscopy and coherent126 anti-Stokes Raman scattering (CARS) which have recently been127 applied for investigating ionic liquids.6−11 On the other hand,128 we will address far-infrared (FIR) and low-frequency Raman129 spectroscopy studies of ionic liquids. Thus, results obtained130 from optical Kerr effect (OKE) spectroscopy, being the131 counterpart of low-frequency Raman spectroscopy, will be132 mentioned, even though OKE is a time-resolved spectroscopy133 technique, because it has been the most common technique134 probing the low-frequency vibrations of ionic liquids.135 The review is organized as follows. We provide experimental136 details in section 2 since most of spectra shown below have

137been especially obtained for the purpose of this review. Raman138spectroscopy of high temperature molten salts demand139nontrivial experimental skills for building furnaces and sample140containers for handling corrosive and air-sensitive melts.2 In141contrast, IR and Raman measurements of ionic liquids are more142easily carried out since the liquid sample may be simply143accommodated in quartz tubes. Nevertheless, some warnings144on the experimental side are timely. All of IR and Raman145spectra recorded in this work are available for the reader in146TXT files. The usefulness of vibrational spectroscopy is heavily147linked to the calculation of vibrational frequencies, so that148section 3 reviews the methods which have been applied for149calculating vibrational spectra of ionic liquids. These methods150include classical normal coordinate analysis, ab initio calculation151for an isolated ion or for a cluster of ions, and classical or ab152initio molecular dynamics simulations of liquids. The scope of153section 3 is not technical details of all of these methods; instead154the section focuses on the conceptual basis of them, helping the155reader to distinguish the assumptions and the need for different156methods of calculation. Section 4 reviews vibrational spectros-157copy studies of pure ionic liquids, being the longest section of158this work. This section discusses the assignment of vibrational159frequencies of the most common anions (section 4.1) and160cations (section 4.2) which form ionic liquids. These two161sections are already plenty of applications of vibrational162spectroscopy in studying pure ionic liquids, but further163applications are given in section 4.3. It is our hope that the164vibrational assignments discussed thoroughly in this section165helps the reader to interpret vibrational spectra when working166with a given ionic liquid based on a different combination of167anions and cations. We separated in section 5 the discussion of168the low-frequency range, as this range directly manifests the169liquid structure and intermolecular dynamics of ionic liquids.170Studies concerning the low-frequency range of vibrational171spectra of ionic liquids have been developed along two different172research lines, one by workers using far-infrared (FIR) and173other by workers using low-frequency Raman spectroscopy, in

Figure 1. Molecular structures of some cations and anions whosevibrational spectra are discussed in this review. The Ri, Rj, Rk, and Rlindicate a hydrogen atom or an alkyl chain group. We adopt the usualnotation of cations throughout this work (e.g., 1-alkyl-3-methylimida-zolium, [CnC1im]

+, where n is the number of carbon atoms in the alkylchain, and 1-alkyl-2,3-dimethylimidazolium, [CnC1C1im]

+). Deriva-tives of ammonium cations are also indicated by the length of the alkylchains (e.g., [C4C1C1C1N]

+ means butyl-trimethylammonium and[C3NH3]

+ is the propylammonium cation which forms protic ionicliquids). Similar notation follows for derivatives of pyridinium,pyrrolidinium, and piperidinium and usual notation for the anions.

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.6b00461Chem. Rev. XXXX, XXX, XXX−XXX

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174 particular OKE spectroscopy. Section 5 is an attempt at putting175 into a proper perspective the results from FIR and low-176 frequency Raman spectroscopy that have been obtained along177 the past decade. The accumulated knowledge on the nature of178 vibrational motions and characteristic frequencies in the normal179 liquid phase discussed in sections 4 and 5 is then applied in180 section 6 for studying phase transitions, in particular181 crystallization and glass transition experienced by ionic liquids182 under low temperature or high pressure. In section 7, we move183 to ionic liquids solutions. Since ionic liquids as solvents184 encompass a very large range of applications, from gas185 molecules to cellulose, and in each of these areas vibrational186 spectroscopy has given its contribution, section 7 gives some187 representative examples of the capability of vibrational188 spectroscopy in shedding light on solute−ionic liquid189 interactions. Section 8 closes the review with some concluding190 remarks.

2. EXPERIMENTAL DETAILS

191 IR and Raman spectra of several ionic liquids were recorded for192 specific purposes of this review. The spectra are available as193 TXT files. The ionic liquids used in this work were purchased194 from different suppliers (e.g., Iolitec, Solvionic, Aldrich, and195 Merck). Most of the purchased ionic liquids have purity196 superior to 98%, and they were used without further197 purification except for the drying process as discussed below.198 Fourier transform IR spectra were recorded with a Bruker199 Alpha equipment with a DTGS detector and KBr optics. IR200 spectra measured by transmission were obtained from thin201 films of liquid samples between KRS-5 windows. In the case of202 solid samples, we used the attenuated total reflection (ATR)203 Platinum accessory (Bruker) with diamond crystal and a single204 reflection. Spectral resolution was 2 cm−1. Optical effects can205 result in differences between transmission and reflection206 measurements.12−15 ATR is a technique appropriate for207 quantitative IR spectroscopy since it allows for better control208 of sample size and thickness.16,17 Moreover, ATR Fourier209 transform IR spectrum can be used to obtain optical210 constants12−14,16,18,19 (i.e., frequency-dependent refractive211 index and extinction coefficient), as shown by Buffeteau et

f2 212 al.15 for different ionic liquids. Figure 2 illustrates differences in213 relative intensities and vibrational frequencies that can be found214 between transmission and ATR measurements of IR spectra of215 a given ionic liquid, [C4C1im][CF3SO3]. A combination of

216transmission and ATR infrared spectroscopy has been used by217Burba et al.20−22 as a methodology to infer about charge218organization in the series [CnC1im][CF3SO3], n = 2−8. They219considered the intense IR band of the anion symmetric220stretching mode, νs(SO3), with maximum at 1031 cm−1. The221approach is based on a first estimate of the dipole moment222derivative from a transmission measurement according to the223dipolar coupling theory, which assumes a quasilattice224organization of the ions. A second estimate of the dipole225moment derivative is obtained from optical constants obtained226from an ATR measurement, which is not based on the227quasillatice model. The ratio between these two values of dipole228moment derivatives indicates the degree of quasilattice229structure in the ionic liquid. Burba et al.21,22 showed that230such charge ordering decreases with increasing length of the231alkyl chain in [CnC1im][CF3SO3]. It should be noted, however,232that the method also relies on assigning the asymmetric shape233of the νs(SO3) IR band as the result of longitudinal optic−234transverse optic (LO-TO) splitting23,24 to be considered within235the dipolar coupling theory.20−22 All of the IR spectra recorded236in this work will be reported in transmittance, except Figures 2237and 5 where IR spectra are shown in absorbance.238Raman spectra were obtained with a Horiba-Jobin-Yvon239T64000 triple monochromator spectrometer equipped with240CCD. The need of triple or double monochromator, or a single241monochromator spectrometer with notch and bandpass filters242suitable to reduce the Rayleigh scattering line, is particularly243important for the low-frequency range, 5 < ω < 100 cm−1, to be244discussed in section 5. Raman spectra were obtained in 180°245scattering geometry. There was no selection of polarization of246scattered light for most of the Raman spectra shown in this247review, otherwise some polarized and depolarized Raman248spectra will be shown as mentioned in text. The excitation line249used was the 647.1 nm line of a mixed argon−krypton250Coherent laser, typically with 200 mW of output power.251Fluorescence background is a known issue in Raman252spectroscopy of (high temperature) molten salts.2 Even though253most ionic liquids are colorless, we use a red laser line to excite254Raman spectra in order to reduce eventual fluorescence255 f3background. For instance, Figure 3 shows Raman spectra256obtained with excitations at 514.5 and 647.1 nm for the same257sample of a colorless ionic liquid, [C4C1im][PF6]. The same258laser power was used to obtain the spectra of Figure 3, and no259baseline correction was done, so that it is clear that fluorescence260background is strongly reduced when using the 647.1 nm laser261line. Whenever fluorescence precluded obtaining suitable

Figure 2. IR spectra of [C4C1im][CF3SO3] obtained in transmittance(black) and ATR (red). The IR spectra have their intensitiesnormalized by the most intense band for comparison purpose.

Figure 3. Raman spectra of [C4C1im][PF6] obtained using the 514.5nm (black) and the 647.1 nm (red) laser line.



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262 Raman spectrum with the 647.1 nm laser line, we used a FT-263 Raman Bruker RFS-100 spectrometer with a 1064 nm exciting264 radiation (Nd:YAG laser Coherent Compass 1064−500N) and265 Germanium detector.266 A relatively stringent spectral resolution is eventually267 required since the complex molecular structures of the ions

f4 268 result in overlapping of bands. Figure 4 illustrates the effect of

269 spectral resolution in the Raman spectrum of [C4C1im][NTf2]270 within the spectral range of the anion SO2 wagging mode. This271 figure exhibits a single broad band at ∼400 cm−1 when using272 poor spectral resolution, but the band is resolved into two273 peaks at 397 and 404 cm−1 when using better spectral274 resolution. In this work, Raman spectra were obtained with275 spectral resolution of 2 cm−1, taking into consideration276 compromise on good signal-to-noise ratio.

277All the samples used in this work were submitted to a drying278process under high vacuum (10−5 mbar) at ∼60 °C for 48 h279prior the analysis. The samples were then manipulated inside a280drybox with argon atmosphere. It has been shown that small281amount of water can have significant effect on ionic liquids282properties.25−28 Karl Fischer coulometric titration is the usual283method to quantify water, but IR spectroscopy has been applied284to measure water content in ionic liquids. Andanson et al.27

285used the O−H stretching mode of water in the range 3400−2863800 cm−1, and Fadeeva et al.28 used a combination band of287water at ∼5250 cm−1 to measure the water content in ionic288liquids. Cammarata et al.29 showed by using IR spectroscopy289that interactions between water and the ions are dominated by290anion−water interactions in the case of nonprotic ionic liquids291based on imidazolium cations. In the case of a mixture of292liquids, it is worth noting that accurate analysis of band293intensity should take into account the fact that the effective294path length eventually changes in ATR-IR measurements295because the refractive index of the sample depends on the296 f5concentration of the solution.30 Figure 5 (panels A−C)297illustrate the evolution of the drying process as monitored by298IR spectroscopy for three ionic liquids with the same299[C4C1im]+ cation but anions with distinct coordination300strength or basicity, [NTf2]

−, [CF3SO3]−, and [CH3COO]

−.301The Karl Fischer analyses of [C4C1im][NTf2], [C4C1im]-302[CF3SO3], and [C4C1im][CH3COO] indicated water concen-303tration of 284, 835, and 22353 ppm, respectively, for the304samples as taken straight from their flask prior to any drying305attempt. Water content dropped to 214, 298, and 18795 ppm306after 48 h under high-vacuum at room temperature, and when307the sample was simultaneously warmed to ∼60 °C, the water308content achieved 45, 118, and 8580 ppm, respectively. This309trend is manifested in the IR spectra of Figure 5 (panels A−C)310by the comparison between relative intensities of water bands311(3400−3800 cm−1) and ionic liquid bands (2800−3200 cm−1).312The drying protocol using high vacuum at room temperature313seems enough for a less viscous ionic liquid and less

Figure 4. Raman spectra of [C4C1im][NTf2] in the range of SO2wagging mode recorded with different spectrometer resolutions.Raman spectrum obtained with 0.5 cm−1 of spectral resolution isshown with the intensity multiplied by a factor of 20.

Figure 5. IR spectra of (A) [C4C1im][NTf2], (B) [C4C1im][CF3SO3], and (C) [C4C1im][CH3COO] before and after the drying processes showingthe spectral range of water bands (3400−3800 cm−1). The asterisk in each panel marks the ionic liquid band used to normalize IR intensities withinthe spectral range shown in the figure. (C) also compares transmission (red) and ATR (green) measurements of IR spectra of the same sample of[C4C1im][CH3COO] after the drying process. (D) shows the molar absorption coefficient Em (black, scale at right), the real part of the refractiveindex n (green, scale at left), and the imaginary part of the dielectric constant ε″ (red, scale at left) of liquid H2O as provided by Bertie and Lan.31



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314 coordinating anion such as [C4C1im][NTf2]. Otherwise heating315 is needed to reduce the viscosity and facilitate the drying316 process for more viscous ionic liquids and more strongly317 coordinating anions, [C4C1im][CF3SO3] and [C4C1im]-318 [CH3COO].29 It is worth mentioning that increase of319 temperature implies a certain degree of decomposition as320 discussed by Gurau et al.32 for [C4C1im][CH3COO]. Even in a321 situation where the minor amount of impurity is generated, this322 heating effect may have a direct consequence in Raman323 spectroscopy because of eventual increase in fluorescence324 background. The IR spectra reported by Andanson et al.27 for325 [C4C1im][NTf2] and [C4C1im][CF3SO3] and by Thomas et326 al.33 and Marekha et al.34 for [C4C1im][CH3COO] exhibit less327 intense water bands in comparison with the spectra shown in328 Figure 5. However, it should be noted that these authors329 reported IR spectra obtained by ATR while we report330 transmission IR spectra. In fact, Figure 5C compares trans-331 mission and ATR spectra of [C4C1im][CH3COO], the latter332 exhibiting lower intensity of water bands in comparison with333 the former, being the spectra normalized by an ionic liquid334 band. The strong variation of the real part of the refractive335 index, n(ω), within the range of wavenumbers ω of an IR336 absorption (i.e., the so-called anomalous dispersion as337 illustrated in Figure 5D for liquid water (green line),31 might338 imply significant differences between ATR and transmission339 measurements of IR spectra.12−14 The molar absorption340 coefficient Em, given by Em = A10/Cd, where d is the path341 length, C is the molar concentration, and A10 is the decadic342 absorbance following the Beer’s Law, is related to the imaginary

343 part of the refractive index, ω=πω ω E2.303 ( )kC m

4 ( ) .18,19,35 On

344 the other hand, Stuchebryukov and Rudoy12 showed that an345 ATR spectrum can be considered as the spectrum of the346 imaginary part of the dielectric constant, ε″(ω). Since the347 complex dielectric constant, ε = ε′ + iε″, and refractive index, n 348 = n + ik, are related by the fundamental equation ε = n 2, then349 ε′(ω) = n2(ω) − k2(ω) and ε″(ω) = 2n(ω)k(ω). As a350 consequence of the anomalous dispersion of n(ω) around an IR351 band, the frequency of the ε″(ω) spectrum is shifted toward the352 low-frequency side of the band. This is clearly seen in the353 comparison between ATR and transmission IR spectra of354 [C4C1im][CF3SO3] shown in Figure 2. Relative intensities of355 IR bands in ATR and transmission measurements are also356 affected by the different dependence on the optical constants.357 This is evident in Figure 5D, which shows Em(ω) and ε″(ω)358 spectra provided by Bertie and Lan31 from an investigation of359 the optical constants of liquid water. It is clear from Figure 5D360 the difference of intensities in the O−H stretching region for361 each spectrum. Therefore, the word of caution is that362 transmission IR spectrum might indicate that an apparent363 “dry” sample of ionic liquid still contains a significant amount of364 water, depending on the ionic liquid.365 Raman spectra as a function of temperature were obtained in366 this work for some ionic liquids. We used an OptistatDN

367 cryostat (Oxford Instruments) filled with liquid nitrogen, thus368 allowing for the achievement of temperatures as low as 77 K at369 which ionic liquids are in crystalline or glassy phases. Raman370 spectra were also obtained for some ionic liquids under371 pressure within the GPa range by using a diamond anvil cell372 (DAC).36,37 Spectra as a function of pressure at room373 temperature were obtained with the already mentioned374 Horiba-Jobin-Yvon T64000 spectrometer having a coupled375 Olympus BX41 microscope. The Raman spectra were excited

376with the 647.1 nm laser line focused into the sample by a 20×377Leica objective. We used a DAC from Almax EasyLab, model378Diacell LeverDAC-Maxi, having a diamond culet size of 500379μm. The Boehler microDriller (Almax EasyLab) was used to380drill a 250 μm hole in a stainless steel gasket (10 mm diameter,381250 μm thick) preindented to ∼150 μm. Pressure calibration382has been done by the usual method of measuring the shift of383 f6the fluorescence line of ruby.38,39 Figure 6 illustrates the

384pressure-dependent fluorescence emission spectra of ruby in385[Pyr14][NTf2]. The inset is a photograph of the gasket hole in386between the diamonds of the DAC containing the liquid387sample and the ruby spheres used for pressure calibration. In388fact, Faria et al.40 showed that the characteristic [NTf2]

− Raman389band at 741 cm−1 can be used for pressure calibration. The390pressure induced frequency shift of this vibrational mode of391[NTf2]

− exhibits a linear variation, ca. 4.2 cm−1/GPa,392depending on the ionic liquid for pressures up to ∼2.5 GPa,393so that within this pressure range the ionic liquid can be used as394pressure-transmitting medium and pressure marker.

3. COMPUTATIONAL DETAILS395Polyatomic ions usually involved in forming ionic liquids range396from highly symmetric small ions to complex flexible organic397ions, so that different theoretical methods have been used for398the calculation of vibrational frequencies and normal mode399assignment. These include classical normal coordinates analysis400relying on an assumed intramolecular force field, ab initio401quantum chemistry calculations at different levels of theory for402an isolated ion, ion pair or cluster of ions, and molecular403dynamics simulation (classical or ab initio) to calculate an404appropriate time correlation function and then its Fourier405transform to result in the vibrational spectrum. Different406methods are chosen not merely on the basis of molecular407structure complexity but also on the level of analysis one wishes408and approximations allowed for. All of these theoretical409methods have been used for assigning vibrational spectra of410the most common ions to be discussed in section 4, so that it is411useful to provide here a brief account of these methods for412future reference along the review.413Classical normal coordinate analysis is based on a ball-and-414spring model for the nuclei vibrations with no consideration of415electronic degrees of freedom.41,42 Taking the [SCN]− anion as

Figure 6. Pressure dependence of the fluorescence emission spectra ofruby in the ionic liquid [Pyr14][NTf2]. Spectra are normalized by themost intense peak. The inset shows a photograph of a DAC chambercontaining the ionic liquid sample and ruby spheres used for pressurecalibration.



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416 an example, a quadratic force field proposed by Baddiel and417 Janz43 involves internal coordinates for stretching of C−N and418 C−S bonds, ΔrCN and ΔrCS, and bending of the SCN angle,419 Δθ,

θ= Δ + Δ + Δ Δ + Δθk r k r k r r k2V CN CN2

CS CS2

CN,CS CN CS2

420 (1)

421 with corresponding force constants kCN, kCS, and kθ, and422 interaction constant kCN,CS. Force constants are second423 derivatives of the potential energy function at the equilibrium424 configuration, and they are optimized in order to reproduce the425 experimental vibrational frequencies. A normal mode Qα is a426 coordinate in which all atoms execute harmonic vibrations with427 the same frequency leading to a Hamiltonian function, H = T +428 V, where T is the kinetic energy, as a collection of independent429 oscillators without cross-terms between different coordinates:

∑ λ= +α

α α α=

H Q Q12

( )N

1

32 2

430 (2)

431 where the eigenvalue λα of mode α is related to the square of its432 vibrational frequency, λα = (2πνα)

2, and the coordinates for433 convenience are weighted by the square root of atomic masses.434 The number of nuclei is N, and the number of actual vibrational435 motions, excluding rotational and translational motions of zero436 frequency, is 3N − 6 in general or 3N − 5 for linear molecules.437 If this classical Hamiltonian function is used to build the438 Hamiltonian operator in the quantum chemistry treatment, the439 set of independent harmonic oscillators implies that the total440 vibrational energy is the sum of the well-known textbook result441 for each normal mode, Eα = hνα (vα + 1/2), where h is the442 Planck constant, vα is the vibrational quantum number, vα = 0,443 1, 2···, and να is the same as the classical vibrational frequency.444 Each Qα is a linear combination of the original coordinates of445 displacements for all of the atoms of the molecule. The linear446 combination can be done in terms of the 3N atomic Cartesian447 coordinates, in terms of internal coordinates of bond-length448 and angle variations, or in terms of symmetry coordinates, Sj. In449 the so-called method of GF matrixes of Wilson, Decius, and450 Cross,41 the symmetry coordinate is a new coordinate system451 mixing the internal coordinates, so that each Sj belongs to a452 given symmetry species of the point group of the molecule. The453 advantage of proposing symmetry coordinates relies on the fact454 that only Sj of the same symmetry species will mix together in455 the composition of a given normal mode Qα. Furthermore,456 defining symmetry coordinates makes easier the resolution of457 the secular equation for obtaining the eigenvalues because it458 factors into diagonal blocks for each symmetry species.459 In the context of ionic liquids, the classical normal coordinate460 analysis is more appropriate for simple ions, typically highly461 symmetric anions. Most of vibrational frequency calculations of462 ionic liquids referred to in section 4 are based instead on463 quantum chemistry methods. The electronic structure problem464 of the molecule is solved within a given approximation for the465 electronic wave function, and the minimum energy config-466 uration of nuclei is found. For this configuration, one obtains467 the 3N × 3N Hessian matrix (i.e., the matrix containing second468 derivatives of potential energy in terms of the nuclei Cartesian469 coordinates). Diagonalization of the Hessian matrix gives the470 eigenvalues λα along the diagonal, and the eigenvector471 corresponding to each frequency gives the composition of Qα

472 in terms of atomic displacements.

473There are some accounts in the literature of ionic liquids474concerning the performance of quantum chemistry methods for475calculating different properties (e.g., anion−cation binding476energy44,45 or gas−ion interactions for studying gas capture by477ionic liquids).46 On the other hand, there are no benchmark478studies for ab initio calculations of vibrational spectra of ionic479liquids, although some works provide specific comparisons of480vibrational frequencies calculated with different levels of481theory.47−49 Density functional theory (DFT) is one of the482most widely used level of theory for the calculation of483vibrational frequencies of ionic liquids since it provides a484satisfactory compromise between accuracy and computational485time. Overall, the DFT/B3LYP with 6-311++G(d,p) basis set486gives good results without being computationally expensive.487Second-order Møller−Plesset perturbation theory (MP2) has488been also extensively used in the context of vibrational489spectroscopy of ionic liquids. We used these methods to490calculate vibrational frequencies for specific purposes of this491review.492The ultimate aim of calculating vibrational frequencies is the493assignment of vibrational motions to observed frequencies.494Most of the works dealing with calculation of vibrational495spectra of ionic liquids have interpreted the normal mode496composition by direct visualization of atomic displacements on497the computer screen. This is a helpful and easy way of assigning498the vibrations, although not a quantitative one. Furthermore, it499might be misleading because large displacement of hydrogen500atoms eventually is only a matter of the small mass of hydrogen501but not implying large contribution to the normal mode energy.502Potential energy distribution (PED) in the classical normal503coordinate analysis is accomplished by evaluating the percent504weight of each symmetry coordinate Sj contributing to the505potential energy of the normal mode Qα. The PED can also be506written in terms of internal coordinates, an approach which is507chemically appealing as one understands the vibrations in terms508of changes in bond lengths and angles of valence, out-of-plane,509and torsional. Jamroz50 made available the computer code510VEDA (vibrational energy distribution analysis) which takes the511output of the commonly used Gaussian51 package of quantum512chemistry. The matrixes containing atomic displacements and513force constants in terms of Cartesian coordinates are available514along the calculation of vibrational frequencies with the515Gaussian program. The VEDA program then uses the516molecular structure, automatically sets internal coordinates,517and evaluates how much each one contributes to the energy of518a given normal mode. We show in Figure 23 atomic519displacements for two normal modes of [C4C1C1im]+

520calculated by the Gaussian program that illustrate the large521amplitude of motions of hydrogen atoms, although PED522calculated by the VEDA program indicate they contribute little523to the normal modes energies.524Harmonic vibrational frequencies are obtained in these ab525initio quantum chemistry calculations. No negative eigenvalue526(i.e., no imaginary frequency) is obtained as long as the527minimum energy molecular structure has been correctly528identified. However, theoretical harmonic frequencies are529usually higher than experimentally observed, so that scaling530factors multiplying the frequencies are needed to bring531calculations into agreement with the experiment. Taking 1-532alkyl-3-methylimidazolim cations as examples, Talaty et al.52

533and Heimer et al.53 found for [CnC1im][PF6] and [CnC1im]-534[BF4], n = 2, 3, and 4, it was necessary to multiply harmonic535frequencies calculated by the DFT/B3LYP level of theory by



F


536 scaling factors within the range 0.963−0.967. Katsyuba et al.54537 proposed sets of different scaling factors within the range of538 0.889−1.0144 for each kind of normal mode (stretch, bend,539 torsion, and out of plane) of the [C2C1im]+ cation. In order to540 go beyond the calculation of harmonic frequencies and to avoid541 empirical scaling factors, anharmonicity should be included.542 The concept of independent normal coordinates is no longer543 rigorously valid when anharmonicity is considered because544 there appears additional potential terms in the Hamiltonian545 function of eq 2 mixing together different coordinates,546 ∑α,β,γgαβγQαQβQγ + ∑α,β,γ,δhαβγδQαQβQγQδ + ..., where gαβγ547 and hαβγδ are anharmonic constants corresponding, respectively,548 to third and fourth derivatives of the potential energy.42

549 Nevertheless, one still thinks in terms of normal modes because550 anharmonic effects are taken into account as perturbations on551 the states and energies of the harmonic model according to552 perturbation theory of quantum chemistry. The harmonic553 frequency of a given normal mode continues being related to554 the force constant as in the harmonic case, but the correction555 on energy levels mixes gαβγ and hαβγδ in complicated expressions556 depending on the order of the expansion of the potential557 energy function and the order of perturbation theory.42,55,56

558 Barone et al.57 made available a code including cubic and559 quartic anharmonicity constants at second-order perturbation560 theory (VPT2) to correct harmonic vibrational frequencies561 calculated by the Gaussian program. Anharmonic frequencies562 have been indeed calculated by the method of Barone for ionic563 liquids based on 1-alkyl-3-methylimidazolium cations.58 The564 vibrational frequency shift when anharmonicity is included in565 the calculation of an isolated 1-alkyl-3-methylimidazolium566 cation might be comparable in magnitude to the effect of567 considering a cation−anion pair, in particular for those568 vibrations involving C−H stretching motions. Including569 anharmonicity not only downshifts vibrational frequencies but570 also allows for addressing important effects on the experimental571 spectra (e.g., Fermi resonance).42 This has been found572 particularly important in the high-frequency range of vibrational573 spectra of 1-alkyl-3-methylimidazolium cations because this574 spectral range is prone to Fermi resonance between overtones575 or combination bands of ring vibrations with the C−H576 stretching modes.58,59

577 The harmonic model of expanding the potential energy578 function to quadratic terms in coordinates has also been579 proposed for an assembly of molecules forming a liquid. If580 liquid phase configurations are generated along a computer581 simulation, either by Monte Carlo (MC) or molecular582 dynamics (MD),60 the Hessian matrix can be evaluated for583 each configuration. Eigenvalues and eigenvectors resulting from584 the diagonalization of the Hessian matrix give frequencies and585 composition of normal coordinates for an instantaneous586 configuration of the liquid, so that these are called587 instantaneous normal modes (INM).61,62 Part of the INM588 has imaginary frequency because a liquid configuration of589 thousands of particles is not a minimum energy configuration.590 Nevertheless, the fraction of imaginary frequency INM also591 carries physical information; for instance, it has been related to592 ionic diffusion coefficients.63−65 The INM method was applied593 in MD simulations of (high temperature) molten salts (e.g.594 ZnCl2,

66−68 BeCl2,69 and cryolitic NaF-AlF3 mixtures)70 but

595 not yet for (room temperature) ionic liquids. In atomic molten596 salts, the positive frequency INM covering the low-frequency597 range are direct probes of the intermolecular dynamics as598 experimentally accessible by far-infrared and low-frequency

599Raman spectroscopy (see section 5). The INM analysis of600molten ZnCl2 was useful to disentangle collective soundlike601modes and localized vibrations of a molecular-like structure of602ZnCl4 tetrahedral.67,68 In the case of BeCl2 and NaF-AlF3603mixtures, INM of frequencies up to ∼1000 cm−1 are obtained604because of relatively stable [BeCl2]n oligomers

69 and AlFn(n−3)−

605polyhedrals.70

606Dynamical effects are not included in a quantum chemistry607calculation of vibrational spectrum once it is carried out in a608minimum energy configuration of an isolated ion, an ionic pair,609or a cluster of ions. In contrast, the theoretical framework of610time correlation function71−74 is appropriated for calculating611vibrational spectra by MD simulations of liquids. A time612correlation function measures how a system property at given613time, A(t), correlates with another property at a previous time,614B(0). In an autocorrelation function, CA(t) = < A(0)·A(t)>,615where <···> indicates an average according to statistical616mechanics, CA(t) starts from < |A|2> at time zero and reaches617 f7the long time value < |A|>2. Figure 7 illustrates the behavior of a618time correlation function for the atomic velocities, Cv(t) = <619vi(0)·vi(t)>, calculated by MD simulation of [C2C1im][NTf2].620The intra- and intermolecular potential function we used in this621simulation is the CL&P model proposed by Lopes and Padua

Figure 7. Upper panel: power spectra, P(ω), calculated according toeq 3 by classical MD simulation of [C2C1im][NTf2] at 400 K anddensity 1.05 g cm−3. The MD simulation considered 500 ion pairs andthe CL&P force field.75 The total P(ω) (black line) has been split intocontributions of atoms belonging to anions (red line) and cations(green line). Intensity was normalized by the most intense band ofeach P(ω). The inset highlights the low-frequency range of the totalP(ω). Bottom panel: the normalized time correlation functions ofvelocity, Cv(t), obtained from the Fourier transform of thecorresponding power spectrum. The inset shows the total Cv(t) in awider time range.



G


622 for MD simulations of ionic liquids.75 Figure 7 shows Cv(t),623 including all of the atoms i, and also Cv(t) calculated separately624 for atoms belonging either to cations or anions. The Cv(t) is625 shown normalized by its initial value, decaying to the average626 value of velocity which is zero. Figure 7 also shows the so-called627 power spectrum, P(ω) (i.e., the density of vibrational states).628 Given any time-dependent property A(t), its power spectrum629 PA(ω) can be obtained as the Fourier transform of the630 corresponding time correlation function, CA(t) = < A(0)·A(t)>.631 An alternative method to calculate PA(ω) more efficiently632 follows from the spectral density of A(t):74,76−78

∫ω = | |ω

−∞

∞−P A t( ) ( ) eA

i t 2

633 (3)

634 Then, the CA(t) can be obtained by Fourier transforming the635 PA(ω).636 The Cv(t) shown in the inset of the bottom panel of Figure 7637 exhibits an overall damped oscillatory decay within the638 picosecond time range, arising from rattling dynamics of ions639 within a temporary cage made of neighboring ions. This640 intermolecular dynamics is manifested in the P(ω) within the641 frequency range below ca. 100 cm−1 (see inset in the top panel642 of Figure 7), which is the range accessible by far-IR and low-643 frequency Raman spectroscopy. The CL&P model considers644 flexible ions, so that the very fast oscillations in Cv(t) (main645 figure at the bottom panel of Figure 7) arise from the646 intramolecular vibrations. Accordingly, P(ω) shows high-647 frequency peaks assigned to cation and anion intramolecular648 vibrations.649 Vibrational frequencies in P(ω) manifest condensed phase650 and anharmonicity effects, as long as an anharmonic intra-651 molecular potential function is included in the model. It is652 worth noting, however, that the CL&P model75 considers653 harmonic terms for stretching and bending motions. On the654 other hand, it is well-known that proper coupling between655 intra- and intermolecular degrees of freedom, and the656 consequent vibrational frequency shift and vibrational relaxa-657 tion in the liquid with respect to the gas phase, is heavily658 dependent on anharmonic terms in the intramolecular659 potential.79−81 Calculations of P(ω) have been done by ab660 initio MD simulations,82,78 which do not rely on an empirical661 parametrized force field, instead the electronic structure is662 solved along the simulation run giving the forces that move the663 nuclei to the new configuration. Therefore, anharmonicity of664 vibrations are taken into account when P(ω) of ionic liquids are665 calculated by ab initio MD simulations.666 The power spectrum shown in Figure 7 is not a theoretical667 IR or Raman spectrum.78 P(ω) exhibits all of vibrations668 included in the model, and it has to be weighted by how much669 the intra- and intermolecular dynamics fluctuate the electric670 dipole moment or the polarizability in order to represent the IR671 or the Raman spectrum, respectively. In other words, IR and672 Raman activities are determined by the coupling between673 mechanical vibrations and electronic molecular properties. It is674 a formal result of nonequilibrium statistical mechanics that the675 IR spectrum, IIR(ω), is proportional to the Fourier transform of676 the time correlation function of fluctuations of the electric677 dipole moment of the whole system:74,77,83−85

∫ω ∝ ⟨ · ⟩ ω

−∞

∞I t e tM M( ) (0) ( ) di t

IR678 (4)

679M(t) may be approximated at first order as the sum of680individual molecular dipole moments, although induced681interaction effects imply that M(t) is a collective property.86

682An expression analogous to eq 4 follows for the Raman683spectrum as the Fourier transform of the time correlation684function of polarizability fluctuation. The alternative method of685eq 3 also applies to the calculation of IR or Raman spectrum686from the time-dependence of dipole moment or polarizability.687Vibrational spectra resulting from time correlation functions of688dipole moment or polarizability fluctuations calculated by ab689initio MD simulations show reasonable agreement to690experimental spectra of ionic liquids.33 This approach for691calculating vibrational spectra of liquids has been implemented692in the TRAVIS (Trajectory Analyzer and VISualizer) package,87

693which includes several routines for analyzing trajectories694generated by computer simulations.695The time correlation function approach for vibrational696spectroscopy allows for a direct comparison between simulation697and experiment in terms of peak positions, band shapes, and698relative intensities, but the ultimate goal of the normal mode699assignment is not yet reached. One possibility is to take a few700liquid configurations generated by the MD simulation for701energy minimization using, for instance, the conjugate gradient702method.88,89 In contrast to the INM analysis discussed above,703this approach calculates the Hessian matrix and performs the704normal-mode analysis for those quenched configurations. In the705time correlation function approach, however, the actual nature706of the molecular vibration responsible for a given band should707be retrieved from the spectrum obtained from the Fourier708transform. A generalized normal coordinates approach90,91 has709been applied for an anharmonic Hamiltonian as it is the case in710ab initio MD simulations of ionic liquids. The approach is711based on a generalization of <vi(0)·vi(t)>, which is a single712particle time correlation function as it involves the property of a713given particle i. The generalization accounts for calculating the714collective counterpart of mass-weighted velocities, <mi

1/2vi(0)·715mj

1/2vj(t)>, whose Fourier transform gives a power spectrum716Pij(ω), including cross-correlations between different particles.717The matrix relating Cartesian to the generalized normal718coordinates is obtained according to a recipe which minimizes719off-diagonal terms of Pij(ω) for all of the frequencies. The720procedure brings the collective power spectrum as close as721possible to a diagonal form leading to a representation of722vibrations in terms of normal modes. This approach for723calculating normal modes from computer simulation includes724anharmonicity and condensed phase effects, and it has been725applied to assign vibrational spectra of [C2C1im][CH3COO]726and its mixture with CO2 and water.33

4. VIBRATIONAL SPECTROSCOPY OF PURE IONIC727LIQUIDS IN THE MID-FREQUENCY RANGE

4.1. Vibrational Frequencies of Anions

7284.1.1. Small Symmetric Anions. Vibrational spectroscopy729is of long usage for studying (high temperature) molten salts of730polyatomic inorganic anions, some of which are common731species in ionic liquids. Reliable assignment of vibrational732frequencies is needed in order to use IR and Raman733spectroscopies as useful tools for unravelling molecular734conformations, intermolecular interactions, hydrogen bonds,735etc. The highly symmetric structures of inorganic anions allow736for more straightforward assignment of vibrational frequencies737in comparison with the ionic liquids forming organic cations.



H


738 There is accumulated knowledge on how vibrational739 frequencies of anions depend on cation charge and size,740 solid−liquid phase transition, temperature, and dilution in741 solvents of different dielectric constants. Vibrational frequency742 of stretching motion of simple anion decreases when the cation743 is replaced by another with lower polarizing power (i.e., the744 ratio between charge and ionic radius of the cation).92 In terms745 of mechanical models of balls and springs,80,79 vibrational746 frequency shift in the condensed phase is a subtle balance747 between attractive and repulsive intermolecular forces con-748 tributing, respectively, to negative and positive shift in749 comparison with the vibrational frequency of the free species.750 The very fact that anion vibrational frequencies increase with751 strength of interaction with the cation points out the role752 played by short-range repulsive forces on the probe oscillator.92

753 As ionic liquids are usually made of bulky organic cations, the754 anion stretching mode is expected at lower frequency than

755molten salts based on alkali cations. For instance, the756vibrational frequency of the totally symmetric stretching757mode of the nitrate anion, νs(NO3), in molten alkali nitrates758(at T ∼ 400 °C) exhibits a significant downward shift from7591067 to 1043 cm−1 when the counterion is changed along the760sequence of Li+, Na+, K+, Rb+, and Cs+.93 Accordingly, νs(NO3)761is observed at 1041 cm−1 in molten [C4C1im][NO3] at 313 K.762Analogous effect of cation polarizing power is found in the763CN stretching mode of [SCN]−. In molten LiSCN, NaSCN,764and KSCN, ν(CN) follows the trend 2083, 2074, and 2068765cm−1.43 Arguing beyond mechanical springs−and−balls models,766Chabanel et al.94 claimed that strongly polarizing cations767stabilize the anion σ orbitals. This effect of strengthening the768CN bond is partially counterbalanced by the effect of resonance769structures, −S−CN ↔ [SCN]−, that softens the CN770bond and becomes predominant as larger is the cation.771Accordingly, the ν(CN) mode is observed at 2054 cm−1 in

Table 1. Fundamental Frequencies of Some Highly Symmetric Anions Commonly Used in Ionic Liquidsa

[NO3]− [BF4]

− [PF6]−

IR Raman D3h IR Raman Td IR Raman Oh

1346 vs νas (E′) 1062 vs νas (F2) 843 vs 864 wb νas (F1u)1041 wb 1041 vs νs (A1′) 764 mb 764 vs νs (A1) 741 mb 740 vs νs (A1g)830 m γ (A2″) 522 m 521 w δ (F2) 565 wb 568 m ν (Eg)708 w 706 m δ (E′) 352 w δ (E) 558 s 560 wb δ (F1u)

470 wb 471 m δ (F2g)[SCN]− [N(CN)2]

−

IR Raman C∞v IR Raman C2v

2056 vs 2054 s ν(CN) (Σ+) 2192 s 2192 s νs(CN) (A1)737 w 738 m ν(CS) (Σ+) 2133 vs 2133 w νas(CN) (B2)471 w δ (Π) 1309 s νas(N−C) (B2)

904 w 904 w νs(N−C) (A1)666 m δ(CNC) (A1)

524 wb γs(N−CN) (A2)508 w γas(N−CN) (B1)496 w δas(N−CN) (B2)

[C(CN)3]− [B(CN)4]

−

IR Raman D3h IR Raman Td

2209 vwb 2209 vs νs(CN) (A1′) 2223 vs νs(CN) (A1)2165 vs 2163 s νas(CN) (E′) 2223 m νas(CN) (F2)1257 m 1257 m δ((CN)C(CN)) + ν(C−CN) (E′) 938 s ν(B−C) (F2)647 wb 646 m νs(C−C) (A1′) 521 w δ(BCN) (E)563 s 565 wb δ(CCN) (A2″) 496 m δ(BCN) (F2)

483 w δ(CCN) (E″) 483 m ν(B(CN)4) (A1)aValues (cm−1) correspond to frequencies observed in IR and Raman spectra at room temperature for ionic liquids with the [C4C1im]

+ cation for[NO3]

−, [BF4]−, and [PF6]

− anions and the [C2C1im]+ cation for the cyanate anions. ν, stretch; δ, bend; γ, out-of-plane; s, symmetric; as,

antisymmetric; w, weak; m, medium; s, strong; v, very. bInactive according to the symmetry of isolated species.

Figure 8. IR (red, transmittance scale at right) and Raman (black) spectra of [C4C1im][BF4] (left panel) and [C4C1im][PF6] (right panel) at roomtemperature in the range of the totally symmetric stretching mode of the anion.



I


772 the ionic liquid [C2C1im][SCN]. Since a vast literature is773 available reporting vibrational frequencies for simple anions

t1 774 with different counterions in solid or liquid phases,95 Table 1775 gives for reference purpose the values actually observed for776 some highly symmetrical anions in ionic liquids based on the777 [C2C1im]+ or [C4C1im]+ cations.778 Symmetry considerations are more easily applied for779 vibrations of relatively rigid inorganic anions than for the780 complex cations of ionic liquids. Vibrational spectroscopy was a781 powerful tool to unravel the presence of complex anionic782 species in the early period of highly hygroscopic ionic liquids783 based on mixing AlCl3 and organic cations chloride. Using784 Raman spectroscopy, Takahashi et al.96 showed that in the785 basic solution of AlCl3/[C2C1im]Cl, x(AlCl3) = 0.54, the786 prevailing anionic species is [AlCl4]

− belonging to the Td point787 group, whereas in acidic solution, x(AlCl3) = 0.67 and the788 prevailing species is [Al2Cl7]

− belonging to the C2 point group789 (acidic melts have AlCl3:organic salt mole ratio greater than 1).790 Grondin et al.58 suggested that the octahedral symmetry of791 the [PF6]

− anion is perturbed in the ionic liquid [C4C1im]-792 [PF6] since the totally symmetric stretching mode (A1g), which793 is IR inactive on the basis of the Oh point group of the isolated794 [PF6]

−, is actually observed in the IR spectrum. Analogous795 effect has been found by Katsyuba et al.,54 who observed the796 [BF4]

− band corresponding to the νs(A1) mode in the IR797 spectrum of [C2C1im][BF4]. These findings are shown in

f8 798 Figure 8, which compares IR and Raman spectra of799 [C4C1im][BF4] and [C4C1im][PF6] in the range where the800 anion totally symmetric mode is observed. Environmental effect801 on the anion symmetry is an issue in vibrational spectroscopy802 of molten salts,92 nitrate certainly being the most investigated803 anion.93,97,98 Experimental evidence of perturbation on the D3h804 symmetry of [NO3]

− is the IR band of the νs(A1′) or the805 Raman band of the γ(A2″) mode and split of νas(E′) because of806 the degeneracy lift. The split of νas(E′) is seen in molten LiNO3807 but not in molten NaNO3, KNO3, RbNO3, and CsNO3. These808 condensed phase effects in the [NO3]

− vibrations might be809 understood on the basis of relatively strong ion-pairing leading810 to C2v symmetry when the cation has high polarizing power.811 For less polarizing cations, one considers that the D3h symmetry812 of [NO3]

− is retained, but the environmental effects are enough813 to breakdown selection rules for the isolated nitrate.814 Accordingly, Table 1 indicates that the νs(A1′) mode is815 observed as a weak band in the IR spectrum of [C4C1im]-816 [NO3]. In the case of [C4C1im][PF6], even though νs(A1g) is817 observed in the IR spectrum, no lifting of degeneracy of anion818 bands has been observed in the IR and Raman spectra.58

819 Therefore, Grondin et al.58 proposed that [PF6]− preserves a

820 quasi-octahedral symmetry in the ionic liquid. We found the821 νas(F2) mode of [BF4]

− as a broad band with maximum at 1062822 cm−1 in the IR spectrum of [C4C1im][BF4]. In contrast,823 Holomb et al.99 found that the νas(F2) mode splits into three824 peaks (1015, 1033, and 1045 cm−1) in the IR spectrum of825 [C4C1im][BF4]. We found in the Raman spectrum of826 [C4C1im][BF4] a band at 493 cm−1, which may be the result827 of splitting of the F2 bending mode of [BF4]

−. This assignment828 is supported by ab initio calculations performed in this work829 (MP2 level of theory, aug-cc-pVDZ basis set) for a [BF4]

−

830 anion under the perturbation of an external positive charge.831 Symmetry reduction is not the only condensed phase effect832 leading to IR or Raman activity of an otherwise inactive833 vibration according to the point group of the isolated molecule.834 The so-called interaction-induced contribution to the molecular

835dipole or polarizability adds further mechanism of fluctuation of836electrical properties beyond vibrational and rotational dynamics837of the probe molecule.86 Being (o)μi(t) and

(o)αi(t), respectively,838dipole and polarizability of the isolated molecule i, where time839dependence comes from vibration and rotation of the molecule,840the actual dipole and polarizability of the molecule in the liquid841have additional contributions, (I)μi(t) and (I)αi(t), induced by842interactions with neighbor molecules. Let Qi be an IR inactive843normal mode of molecule i (i.e., the transition dipole ∂(o)μi/∂Qi844is zero for the isolated molecule) but Raman active (i.e.,845nonzero ∂

(o)αi/∂Qi). If the molecule i is surrounded by dipolar846molecules j, the dipole-induced dipole (DID) mechanism at847this order is enough to account for an interaction-induced848contribution to the dipole of i, (I)μi = Σj≠i (∂(o)αi/∂Qi)·849T(rij)·

(o)μj, where rij is the distance vector between the850molecules, and the dipole−dipole tensor is T(rij) =851(4πεo)

−1(3rij·rij − rij2·I)r−5, where I is the unitary matrix. In

852general, the total dipole and polarizability must be obtained in a853self-consistent way because the electric properties of all of the854molecules contain permanent and interaction-induced parts.86

855In the case of Raman spectra of ionic systems, Madden et856al.100,101 showed that other mechanisms might contribute to the857fluctuating polarizability besides the DID mechanism (e.g.,858distortion of a given ion by the Coulomb field of other ions and859short-range overlap interactions).860Liquid carbon disulfide is a well-known example of861interaction−induced effect contributing to vibrational spec-862tra:102−104 the doubly degenerate bending and the antisym-863metric stretching modes, which are Raman inactive for an864isolated CS2 molecule, become active in the Raman spectrum of865the liquid phase. Additional signature of interaction−induced866effect is an exponential long tail, e−ω/Δ, where Δ is a parameter,867in a Raman band corresponding to a normal mode which is868allowed by symmetry of the isolated molecule.102,103 Depolar-869ized Raman spectra have been interpreted by assuming there is870time separation between fast rattling dynamics of molecules871within the cage of neighbors, resulting in interaction-induced872effect as intermolecular distances are changed, and slow873reorientational dynamics of the molecule as a whole. The874short-time rattling dynamics contributes to the high-frequency875tail of the band, whereas the relatively slow reorientational876dynamics contributes to the center of the band. Unfortunately,877there are overlaps of bands in vibrational spectra of ionic878liquids, being difficult to address whether the high-frequency879range of a Raman band exhibits exponential shape. A particular880band that could be considered in this respect is the stretching881mode ν(CN) of [SCN]− because it appears in a spectral range882 f9free of overlaps. Figure 9 shows the depolarized ν(CN) Raman883band of [C2C1im][SCN] at room temperature. The ν(CN)884Raman band indeed exhibits a long tail which extends far from885the band-center. The inset of Figure 9 makes clear that the tail886exhibits the exponential shape, strongly suggesting that887interaction-induced mechanisms should not be ruled out. It888would be interesting for future work attempts to disentangle889symmetry reduction and interaction-induced effects in vibra-890tional spectroscopy of ionic liquids.891Classical intramolecular force fields for computing vibrational892frequencies are available for some ionic liquid forming anions893(e.g., [NO3]

−,98 [SCN]−,43 and [C(CN)3]−).105 On the other

894hand, the complex molecular structures of the ions typically895involved in ionic liquids prompt for quantum chemistry as the896more appropriate approach to calculate vibrational frequencies.897Hipps and Aplin105 accounted for the vibrations of [C(CN)3]

−



J


898 in the potassium salt with a quadratic force field with force899 constants for internal coordinates of bond displacements and900 angles based on ab initio calculation. The carbon−carbon901 distance and force constant obtained for [C(CN)3]

− are similar902 to values for benzene, indicating significant resonance903 stabilization in [C(CN)3]

−.105 We have found bands at 1231904 and 1230 cm−1 in IR and Raman spectra, respectively, of905 [C2C1im][C(CN)3]. In accordance with the assignment906 suggested by Hipps and Aplin for simpler [C(CN)3]

−

907 salts,105 this is a combination band intensified by Fermi908 resonance with the fundamental E′ mode observed at 1257909 cm−1 in IR and Raman spectra of [C2C1im][C(CN)3]. The910 occurrence of this Fermi resonance was indeed confirmed by911 anharmonic ab initio calculations of an isolated [C(CN)3]

−

912 anion performed in this work (MP2/VPT2 level of theory, aug-913 cc-pVDZ basis set). The calculation showed that the 1230 cm−1

914 band is the combination of A1′ δ(CCN) and E′ νs(C−CN)915 modes calculated at 606 and 641 cm−1, respectively.916 The vibrational frequency of the totally symmetric CN917 stretching mode decreases along the sequence [B(CN)4]

−,918 [C(CN)3]

−, and [N(CN)2]− as the electronic delocalization

919 increases by 2223, 2209, and 2192 cm−1, respectively, in ionicf10 920 liquids with the same [C2C1im]+ cation (see Figure 10).106 In

921 an IR spectroscopy study of solid [NH4][N(CN)2], Sprague et922 al.107 assigned the nontotally symmetric mode νas(CN) at923 higher frequency than the totally symmetric mode νs(CN). It924 is a rule of thumb in vibrational spectroscopy assigning the925 nontotally symmetric stretching at higher frequency than the926 totally symmetric stretching of a given moiety. However, the

927opposite is true for [N(CN)2]− and [C(CN)3]

−. The IR,928polarized, and depolarized Raman spectra shown in Figure 10929for [C2C1im][N(CN)2] and [C2C1im][C(CN)3] in the spectral930range covering νas(CN) and νs(CN) modes support the931assignment. Crystallographic analysis108 of Li[N(CN)2]932showed that the N−C bond length (131 pm) is in between933typical values for single and double bonds, whereas the CN934bond length (116 pm) is in between double and triple bonds.935Therefore, Reckeweg et al.108 concluded that the electronic936structure is better represented as the resonance hybrid −N937CN−CN ↔ NC−NCN−, rather than NC−938N−−CN. On the basis of this resonance, it is reasonable that939a vibration will be of lower energy when the two CN moieties940oscillate antisymmetrically. The IR spectrum of [C2C1im][N-941(CN)2] exhibits another band at 2227 cm−1 (just out of the942spectral window shown in Figure 10), which has been943assigned109 to a combination band of νs(N−C) and νas(N−944C) modes whose frequency is shifted and intensity is enhanced945because of the Fermi resonance with the fundamental of the946νas(CN) mode. In the case of [B(CN)4]

−, the Raman active947νs(CN) and the IR active νas(CN) are observed at the same948frequency.110

9494.1.2. More Complex Fluorinated Anions. The bis-950(trifluoromethanesulfonyl)imide, [NTf2]

−, is one of the most951popular anions, resulting in low melting point salts when952combined with the majority of common organic cations.953Assignment of vibrational frequencies of [NTf2]

− has been an954issue in the literature because the lithium salt of [NTf2]

− is955commonly used in polymer electrolytes. Rey et al.111 provided a956detailed analysis of the [NTf2]

− normal modes on the basis of957quantum chemistry calculations for its C2 conformer. In a958subsequent IR and Raman spectroscopic study, Herstedt et959al.112 were able to distinguish bands characteristic of C2 and C1

960conformers of [NTf2]−, commonly called transoid and cisoid,

961respectively, in solutions of Li[NTf2] in ethers CH3O-962(CH2CH2O)nCH3, where n = 1, 2, 3, and 4. A complete963table of experimental versus calculated vibrational frequencies,964internal force constants, and potential energy distribution of965[NTf2]

− normal modes can be found in these papers.111,112

966Some of the bands assigned to [NTf2]− vibrations actually

967observed in IR and Raman spectra of a typical ionic liquid,968 f11[C2C1im][NTf2], are marked in Figure 11. Tables containing969the [NTf2]

− vibrational frequencies are available in many970papers,111−123 but values strongly depend on the counterion,971 t2physical state, and solvent. Thus, Table 2 lists the actual values972observed in [C2C1im][NTf2] at room temperature. The973eigenvectors of [NTf2]

− normal modes exhibit a complex974pattern involving displacements of many atoms,112 so that the975assignment in Table 2 is only a simplification of the potential

Figure 9. Depolarized Raman spectrum of the ionic liquid [C2C1im]-[SCN] at room temperature in the range of the ν(CN) normal mode.Intensity has been normalized by the maximum of the band. The insetshows the logarithm of the high frequency side of the band, where thered line is a linear fit highlighting the exponential tail, e−ω/Δ, with Δ =36.7 cm−1.

Figure 10. IR (red, transmittance scale at right) and Raman spectra (black; full line, polarized; dashed line, depolarized) of ionic liquids[C2C1im][N(CN)2] (left), [C2C1im][C(CN)3] (middle), and [C2C1im][B(CN)4] (right) at room temperature.



K


976 energy distribution among the different internal coordinates977 given in ref 111.978 It is clear from Figure 11 that the Raman spectrum of979 [C2C1im][NTf2] is dominated by anion bands because of large980 polarizability fluctuation resulting from anion vibrations. The981 remaining bands assigned to the cation are easily identified by982 comparison with the spectrum of an atomic anion counterpart983 (e.g., [C2C1im]Cl). It has been a successful application of984 Raman spectroscopy unveiling the coexistence of transoid and

985cisoid [NTf2]− conformers in the normal liquid phase of ionic

986liquids. Ionic liquids in which [NTf2]− conformers have been

987found by vibrational spectroscopy include systems based on988derivatives of imidazolium,119,3,124,125 pyrrolidinium126,127

989(either protic or nonprotic derivatives of these cations),116,118

990piperidinium,120 and ammonium.122,123,128 Quantum chemistry991calculations indicate that the spectral ranges 260−370 cm−1 and992620−660 cm−1 are well-suited for finding bands that character-993ize the transoid or the cisoid conformation. These spectral994ranges of the Raman spectrum of [C2C1im][NTf2] are995 f12highlighted in Figure 12. The antisymmetric SO2 out-of-plane996bending at 623 cm−1 characterizes the transoid conformer,997whereas the S−N−S bending at 650 cm−1 characterizes the998cisoid conformer. These are very weak Raman bands so that the999260−370 cm−1 range is more appropriated for identifying the1000[NTf2]

− conformation. In particular, the SO2 rocking at 3261001cm−1 is characteristic of the cisoid conformer. The quantum1002chemistry assignments marked in Figure 12 are supported by1003remarkable spectral changes eventually observed in crystalline1004phases as bands belonging to a given conformer might be1005absent. Following phase transitions of ionic liquids by1006vibrational spectroscopy will be discussed in section 6. The1007most intense Raman band at 741 cm−1, corresponding to a1008normal mode in which the [NTf2]

− anion breathes as a whole,1009exhibits an asymmetric band shape that has been also assigned1010to transoid and cisoid conformers, giving two components with1011a small difference of ∼3 cm−1.116 On the other hand, it will be

Figure 11. IR (red, transmittance scale at right) and Raman spectra(black) of [C2C1im][NTf2] at room temperature. Some bandsassigned to the [NTf2]

− normal modes are indicated by arrows.

Table 2. Vibrational Frequencies of Some Fluorinated Anions Commonly Used in Ionic Liquidsa

[NTf2]− [N(SO2F)2]

− [CF3SO3]−

IR Raman IR Raman IR Raman

120 t(CF3) 291 τ(SO2F) 210 ρ(CF3)165 326 δop(SO2F) 312278 ρ(CF3) 359 δop(SO2F) 348 ρ(SO3)297 454 456 δsci(SOF) + δ(OSNSO) 518 518 δas(SO3)313 ρ(SO2) 482 482 δsci(SOF) + δop(SO2F) 573 573 δas(CF3)326 ρ(SO2) 524 524 δsci(SO2) + ν(SF) 638 640 δs(SO3)340 τ(SO2) 572 568 δip(O2SNSO2) + ν(SF) + δip(SO2F) 755 755 δs(CF3)351 τ(SO2) 725 727 δsci(SO2) + ν(SF) + δsci(SNS) 1031 1034 νs(SO3)397 ω(SO2) 831 831 ν(SF) + ρ(SO2) + νs(SNS) 1162 1167 νas(CF3)

408 404 ω(SO2) 1218 1217 νs(SO2) 1225 1226 νs(SO3)514 1365 1361 νas(SO2) 1261 1259 νas(SO3)

551 δs(SO2) 1383 1380 νas(SO2) 1281 1281 νas(SO3)571 571 δa(CF3) 1388 νas(SO2)602 590 δa,ip(SO2)619 623 δa,op(SO2)651 650 δ(SNS)741 741 νs(SNS)762 764 νs(SNS)790 796 ν(CS)10581139 1136 νs,ip(SO2)11951231

1243 νs(CF3)1333 1337 νa,op(SO2)1352 1353

aValues (cm−1) correspond to frequencies observed in IR and Raman spectra at room temperature for ionic liquids with the [C2C1im]+ cation for

[NTf2]− and [N(SO2F)2]

− anions and the [C4C1im]+ cation for [CF3SO3]

−. ν, stretch; δ, bend; δsci, scissoring; ω, wagging; τ, twisting; ρ, rocking; t,torsion; s, symmetric; as, antisymmetric; ip, in plane; and op, out-of-plane.



L


1012 discussed that the 741 cm−1 band is very sensitive to1013 coordination of [NTf2]

− to small cations of high polarizing1014 power (see section 7).1015 The closely related bis(fluorosulfonyl)imide anion, [N-1016 (SO2F)2]

−, also exhibits equilibrium between C1 and C21017 conformers in ionic liquids. Fujii et al.129 discussed the1018 signature of conformational equilibrium of [N(SO2F)2]

− in1019 vibrational spectra of ionic liquids containing the cations 1-1020 ethyl-3-methylimidazolium129 and N-methyl-N-propyl-pyrroli-

f13 1021 dium.130 Figure 13 shows vibrational spectra of [C2C1im][N-

1022 (SO2F)2] with the anion bands indicated by arrows. Assign-1023 ment of [N(SO2F)2]

− normal modes has not been provided in1024 refs 129 and 130, but it is available in the work of Matsumoto et1025 al.131 concerning polymorphism in crystals of Na+, K+, and Cs+

1026 salts of [N(SO2F)2]−.

1027 On the basis of quantum chemistry calculations at the DFT/1028 B3LYP level of theory, Fujii et al.129,130 proposed that Raman

1029bands of [N(SO2F)2]− in the range of 250−400 cm−1 are

1030asymmetric because of the presence of both the [N(SO2F)2]−

1031 f14conformers. The bands seen in Figure 14 at 291, 326, and 359

1032cm−1 in the Raman spectrum of [C2C1im][N(SO2F)2]1033correspond to SO2F out-of-plane bending for cisoid conformer

Figure 12. Raman spectrum of [C2C1im][NTf2] in the spectral ranges covering the bands characteristics of [NTf2]− in transoid (marked #) and

cisoid (marked *) conformations. Optimized structures of transoid and cisoid conformers of [NTf2]− are shown.

Figure 13. IR (red, transmittance scale at right) and Raman spectra(black) of [C2C1im][N(SO2F)2] at room temperature. Some bandsassigned to [N(SO2F)2]


Figure 14. Raman spectrum of [C2C1im][N(SO2F)2] at roomtemperature. Vibrational frequencies and relative intensities ofRaman bands calculated by the DFT/B3LYP level of theory areindicated for [N(SO2F)2]

− at C1 (cisoid, blue lines) and C2 (transoid,red lines) conformation. Optimized structures of transoid and cisoidconformers of [N(SO2F)2]

− are shown.



M


1034 or SO2F twisting for transoid conformer, SO2 rocking for cisoid1035 conformer or SO2F out-of-plane bending for transoid con-1036 former, and SO2F in-plane bending for cisoid conformer or1037 SO2F out-of-plane bending for transoid conformer, respec-1038 tively.131 The calculated frequencies for each [N(SO2F)2]

−

1039 conformer are relatively close, so that the more strong1040 experimental evidence in favor of conformational equilibrium1041 in the ionic liquid is the temperature dependence of the spectral1042 pattern.129,130,132 The relative intensities of components1043 belonging to transoid conformer decrease with increasing1044 temperature providing support to the calculations that transoid1045 is the lower energy conformer between them.129,130

1046 Giffin et al.133 prepared pyrrolidinium ionic liquids with an1047 anion whose molecular structure lies in between [NTf2]

− and1048 [ N ( S O 2 F ) 2 ]

− , n a m e l y , ( fl u o r o s u l f o n y l )1049 (trifluoromethanesulfonyl)imide, [N(SO2F) (CF3SO2)]

−. The1050 most intense Raman band of [N(SO2F) (CF3SO2)]

− at 7301051 cm−1 is the counterpart of the characteristic band due to the1052 expansion and contraction modes of [NTf2]

−. Giffin et al.133

1053 emphasized that this band of [N(SO2F) (CF3SO2)]− is broader

1054 than analogous band of [NTf2]− proper to distribution of three

1055 rotamers of [N(SO2F) (CF3SO2)]−. The authors of ref 133

1056 provided a table of experimental versus calculated vibrational1057 frequencies and normal mode assignment for different1058 conformers of [N(SO2F) (CF3SO2)]

−, but overlap of bands1059 in the spectral range 280−400 cm−1 implies that distinguishing1060 the presence of [N(SO2F) (CF3SO2)]

− conformers by Raman1061 spectroscopy is a more challenging task than [NTf2]

−

1062 conformers.1063 The trifluoromethanesulfonate (triflate) anion, [CF3SO3]

−,1064 was the subject of many vibrational spectroscopy studies1065 because it has been extensively used in polymer electro-1066 lytes.134−137 One recurrent issue in these works is to unveil,1067 from frequency shift and band split, ionic pairing between1068 [CF3SO3]

− and alkali metal cations. Here again vibrational1069 frequencies of some normal modes are very dependent on the1070 polarizing power of the cation. For instance, δs(CF3) and1071 νs(SO3) are observed at 755 and 1034 cm

−1, respectively, in the1072 Raman spectrum of [C4C1im][CF3SO3]. These frequencies are1073 close to values of “free” [CF3SO3]

− in 2-methyltetrahydrofuran1074 solution,138 whereas these modes are observed at 767 and 10531075 cm−1 for [CF3SO3]

− in aggregates with the strongly polarizing1076 Li+ cation. In a quantum chemistry investigation of [CF3SO3]

−

1077 coordinated to Li+, Gejji et al.139 calculated the potential energy1078 distribution of normal modes among the internal coordinates of1079 [CF3SO3]

−. Huang et al.138 compared the nature of the normal1080 modes of free and lithium-coordinated [CF3SO3]

−. Potential1081 energy distribution indicates less delocalized normal modes for1082 [CF3SO3]

− than [NTf2]− or [N(SO2F)2]

−, as one would expect1083 from the electronic structure of [CF3SO3]

−. Since the1084 [CF3SO3]

− vibrational frequencies are very sensitive to the1085 local environment experienced by the anion,140−143 we show in

f15 1086 Figure 15 IR and Raman spectra of a common [CF3SO3]−

1087 based ionic liquid. Vibrational frequencies belonging to1088 [CF3SO3]

− normal modes actually observed in spectra of1089 [C4C1im][CF3SO3] are listed in Table 2. Assignments are1090 available from the previous works on polymer electrolytes and1091 reconsidered by Schwenzer et al.144 within the context of ionic1092 liquids. A distinctive feature of a study published by Akai et1093 al.145 was recording IR spectrum of the ionic pair after1094 evaporating [C2C1im][CF3SO3] and trapping in a cryogenic1095 neon matrix. Of course the bands in the IR spectrum of matrix-1096 isolated ionic pair become much sharper than the normal liquid

1097phase spectrum shown in Figure 15. This allowed a fine1098comparison between experimental and calculated frequencies at1099the DFT/B3LYP level of theory for two different arrangements1100of the [C2C1im]+−[CF3SO3]

− pair.145 The experimental1101matrix-isolated IR spectrum of [C2C1im][CF3SO3] was more1102consistent with a local arrangement in which the anion SO31103group points toward the cation C2 atom with five anion−cation1104hydrogen bonds.145

1105The vibrations of tris(pentafluoroethyl)-trifluorophosphate,1106[FAP]−, in the ionic liquid [C2C1im][FAP] have been1107discussed in an IR and Raman study by Mao and Damodaran146

1108and in an IR study by Voroshylova et al.147 These two works1109provide a full list of observed frequencies of [C2C1im][FAP]1110and assignments based on the potential energy distribution of1111normal modes calculated for the ionic pair at the DFT/B3LYP1112level of theory. Concerning those bands belonging to [FAP]−

1113normal modes, there are some disagreement on assignments1114proposed in these works. For example, the intense IR band at11151209 cm−1 was assigned to ν(CC) by Mao and Damodaran,146

1116but to νas(CF2) by Voroshylova et al.147 Two isomers of1117[FAP]− are possible, meridional and facial, each one with1118several conformers. The calculations of Voroshylova et al.147

1119indicated IR bands at ∼800 and ∼700 cm−1 as characteristic1120features of meridional and facial isomers, respectively, and1121mixture of anion conformers in [C2C1im][FAP].11224.1.3. Alkylsulfates and Hydrogen Sulfate. Few works1123have discussed vibrational spectra of pure ionic liquids1124containing alkylsulfate anions, [R−O−SO3]

−, in spite of the1125relevance of this class of anion forming ionic liquids. IR and1126Raman spectra of 1-ethyl-3-methylimidazolium ethylsulfate,1127[C2C1im][C2SO4], have been first discussed by Kiefer et al.,115

1128and in a subsequent paper by Dhumal et al.148 The IR1129frequencies listed in these two works agree with each other;1130however, there is systematic mismatching between them for the1131Raman frequencies of [C2C1im][C2SO4]. For instance, the1132most intense Raman band of [C2SO4]

− is reported at 10721133cm−1 in ref 115, whereas it is reported at 1060 cm−1 in ref 148.1134The latter is most probably the correct value, being close to11351062 cm−1 as found in aqueous solution of Na[C2SO4].

149 In1136light of disagreement of reported frequencies for [C2SO4]

−,1137 f16Figure 16 shows IR and Raman spectra obtained in this work1138for [C2C1im][C2SO4] at room temperature. Raman frequencies1139of [C2SO4]

− we obtained agree with values given in ref 148.1140Dhumal et al.148 calculated vibrational frequencies at the1141DFT/B3LYP level of theory for isolated ions and the ionic pair.

Figure 15. IR (red, transmittance scale at right) and Raman spectra of[C4C1im][CF3SO3] (black) at room temperature. Some bandsassigned to the [CF3SO3]




N


1142 Assignment of vibrational frequencies on the basis of1143 comparison between calculated and experimental data is1144 particularly difficult for this system because strong anion−1145 cation interaction imply significant frequency shifts. For1146 instance, the most intense and sharp Raman band at ∼10601147 cm−1, which is assigned to S−O symmetric stretching of1148 [C2SO4]

− in ref 149, is assigned to C−O stretching in ref 148.1149 In this work, we calculated vibrational frequencies of an ionic1150 pair made of [C2SO4]

− and tetramethylammonium cation at1151 the DFT/B3LYP level of theory with a basis set 6-311+1152 +G(d,p). Our results are inline with the assignment of Dhumal1153 et al.,148 that is, the Raman bands observed at 1061.5 and 9591154 cm−1 in Figure 16 are assigned to C−O and S−O stretching1155 modes, respectively. We also found that the vibrational1156 frequencies of the main bands of methylsulfate, [C1SO4]

−, are1157 essentially the same in the ionic liquid [C4C1im][C1SO4]1158 (spectra not shown) since differences between [C2C1im]-1159 [C2SO4] and [C4C1im][C1SO4] spectra arise from different1160 alkyl chain lengths. Vibrational frequencies of stretching modes1161 of C−H bonds of alkylsulfate anions overlap the same high-1162 frequency range of corresponding vibrations of 1-alkyl-3-1163 methylimidazolium cations discussed in the next section.1164 At first sight, one would expect more simple spectra for ionic1165 liquids based on hydrogen sulfate (or bisulfate), [HSO4]

−, since1166 it could be considered the first of the [R−O−SO3]

− series.1167 However, hydrogen bonding implies nontrivial features in1168 vibrational spectra of molten bisulfates. Bisulfate belongs to the1169 class of relative simple polyatomic anions with a large number1170 of spectroscopic studies concerning alkali cation molten1171 salts150−152 or aqueous solution.153,154 Within the more recent

1172context of ionic liquids, Kiefer and Pye155 used IR and Raman1173spectroscopies and quantum chemistry calculations to charac-1174terize the conformations of ions in [C6C1im][HSO4],1175concluding that [HSO4]

− occurs in the trans conformation.1176Kiefer and Pye155 found a group of bands which they assigned1177to sulfuric acid in the IR (903, 1158, and 2963 cm−1) and1178Raman (959, 1157, and 1364 cm−1) spectra of [C6C1im]-1179[HSO4]. They did not conclude whether the sulfuric acid1180originated from proton transfer, like the case of 1-alkyl-3-1181methylimidazolium acetate, or from a residual of synthesis. On1182the other hand, sulfuric acid bands have not been found either1183in the IR spectrum of [C4C1im][HSO4] reported by Schwenzer1184et al.144 or in the Raman spectra of [C2C1im][HSO4] and1185[C4C1im][HSO4] reported by Ribeiro.

156 The [HSO4]− Raman

1186bands observed in the ionic liquids at 416 [δas(SO3)],1187581[δs(SO3)], 845 [ν(S−OH)], and 1046 cm−1 [νs(S1188O)]156 nicely match the corresponding bands in the (high1189temperature) molten salt KHSO4.

151 Another νs(SO) band1190at 1010 cm−1 has also been found, whose intensity increases as1191temperature decreases, and assigned to [HSO4]

−, engaged in1192chains of hydrogen-bonded anions.156 Signature of anion−1193anion hydrogen bonding has been found in vibrational spectra1194of alkali bisulfate crystals.157−160 It had been proposed decades1195ago that simple [HSO4]

− molten salts are very viscous because1196structures of hydrogen-bonded anions existing in the crystalline1197phase remain in the liquid phase just above the melting1198temperature.161 Therefore, it was proposed that the very high1199viscosity of [HSO4]

− ionic liquids in comparison with1200alkylsulfates for a given 1-alkyl-3-methylimidazolium cation is1201due to anion−anion, rather than anion−cation, hydrogen1202bonding.156

12034.1.4. Carboxilates. Vibrational spectroscopy studies of1204carboxylate-based ionic liquids are heavily linked to applications1205in gas absorption to be discussed in section 7. Here we focus on1206vibrational frequencies of acetate in pure ionic liquids based on12071-alkyl-3-methylimidazolium cations. IR and Raman spectra of1208[C2C1im][CH3COO] and [C4C1im][CH3COO] have been1209discussed by Thomas et al.33 and Cabaco et al.,162 respectively.1210Raman frequencies of [CH3COO]

− reported in these works are1211 t3listed in Table 3 showing some inconsistencies between them.1212Ito and Bernstein163 discussed IR and Raman spectra of1213aqueous solutions of formate, oxalate, and acetate; the1214vibrational frequencies of the latter are given in Table 3 for1215comparison purposes. Assignment of νs(COO) and νas(COO)1216modes in ionic liquids [CnC1im][CH3COO] is cumbersome1217because of overlaps with bands of imidazolium ring modes in1218the same spectral range. A large number of works address1219vibrational spectra of acetate in alkali salts and metal

Figure 16. IR (red, transmittance scale at right) and Raman (black)spectra of [C2C1im][C2SO4] at room temperature. Some bandsassigned to [C2SO4]


Table 3. Fundamental Frequencies (cm−1) and Assignments Reported in the Literature for Raman Spectra of the Acetate Anionin Ionic Liquids. Raman Frequencies of Na[CH3COO] Aqueous Solution are Given for Comparison Purposesa

Thomas et al.33 [C2C1im][CH3COO] Cabaco et al.162 [C4C1im][CH3COO] Ito and Bernstein163 Na[CH3COO](aq)

454 ρ(COO) 471 ρip(COO), B1

635 δ(OCO)+ν(CC) 637 δ(OCO) 650 δ(OCO), A1

899 δ(OCO)+ν(CC) 902 ν(CC) 926 ν(CC), A1

1334 νs(COO) 1326 δ(CH3) 1344 δ(CH3), A1

1382 νs(COO) 1413 νs(COO), A1

1567 νas(COO) 1582 νas(COO) 1556 νas(COO), B1

2917 νs(CH3) 2936 νs(CH3), A1

aFor comparison purposes, corresponding Raman frequencies of Na[CH3COO] aqueous solution are given with their assignment and symmetry. ν,stretch; δ, bend; ρ, rocking; s, symmetric; as, antisymmetric; ip, in plane.



O


1220 complexes.163−166 However, direct comparison with the1221 frequencies observed in ionic liquids is not straightforward1222 since [CH3COO]

− frequencies are very sensitive to coordina-1223 tion. Furthermore, theoretical calculations for [CnC1im]

+ and1224 [CH3COO]

− in the gas phase might lead to proton transfer and1225 formation of the N-heterocyclic carbene. This is regarded as an1226 important step along the process of CO2 absorption, but the1227 ionic environment in the liquid phase stabilizes the ions.167 The1228 theoretical tool employed by Thomas et al.33 was ab initio1229 molecular dynamics simulation of 36 pairs of [C2C1im]

+−1230 [CH3COO]

− ions, whereas Cabaco et al.162 performed DFT/1231 B3LYP calculations of a cluster made of two pairs of1232 [C4C1im]+−[CH3COO]− ions. In order to identify the1233 [CH3COO]

− bands, it is useful to compare vibrational spectra1234 of acetate-based and other ionic liquids with the same cation.1235 Cabaco et al.152 compared IR and Raman spectra of1236 [C4C1im][CH3COO], [C4C1im][BF4], and [C4C1im][PF6].

f17 1237 In Figure 17, we provide a comparison of IR and Raman spectra

1238 of [C4C1im]Br and [C4C1im][CH3COO], where bands that1239 Cabaco et al.162 assigned to [CH3COO]

− are indicated by1240 arrows. The Raman bands at 1334 and 1567 cm−1 assigned by1241 Thomas et al.33 to the acetate anion actually belongs to1242 imidazolium ring modes as discussed in the next section (see

t4 1243 Table 4). Therefore, we follow Cabaco et al.162 assignment of1244 [CH3COO]

− bands. However, it is worth mentioning that we1245 found the most intense [CH3COO]

− Raman band at 911 cm−1

1246 in the Raman spectrum of [C4C1im][CH3COO].1247 It is worth noting the difference Δ = νas(COO) − νs(COO)1248 = 200 cm−1 in the ionic liquid [C4C1im][CH3COO]. In the1249 context of coordination chemistry, the magnitude of Δ has1250 been used as a signature of the coordinating structure of acetate1251 to cations,164−166,168 taking the value of Δ in Na-1252 [CH3COO]

163,169 as a reference of purely ionic interaction.1253 In the case of unidentate coordination (i.e., only one oxygen1254 atom of [CH3COO]

− coordinating the metal cation), the1255 equivalence of oxygens is removed, implying large Δ, typically1256 Δ > 200 cm−1. In contrast, bidentate coordination (i.e.,1257 chelating structure) implies small Δ, typically Δ < 150 cm−1.165

1258 The value of Δ in [C4C1im][CH3COO] suggests dominance of1259 unidentate interaction of acetate to imidazolium cation. The1260 magnitude of Δ has also been commented by Tanzi et al.170 in1261 an IR and Raman investigation carried out for ionic liquids with

1262three different carboxylate anions (formate, propanoate, and1263butanoate) and a common cation (choline). The values of Δ in1264formate and propanoate ionic liquids are higher, whereas it is1265slightly smaller in butanoate than corresponding values in the1266sodium salts. The work of Tanzi et al.170 provided vibrational1267frequencies of these carboxylate anions and assignments on the1268basis of DFT calculations for isolated ions or ionic pairs and by1269ab initio MD simulations of clusters of ions. The sequence of1270calculations for isolated ions, ionic pairs, and clusters highlight1271the spectral signature of strong hydrogen bonds between1272carboxylate anions and the choline cation.1273The Raman band shape of the ν(CC) mode is also a probe of1274the coordinating structure of acetate in solution.171 Cabaco et1275al.162 considered the asymmetry of the ν(CC) Raman band of1276[C4C1im][CH3COO] at 902 cm−1, which exhibits a high1277frequency tail due to a low-intensity component at 909 cm−1, as1278an indication of a fraction of anions in the bidentate1279coordination. A jointed neutron diffraction and MD simulation1280study of [C2C1im][CH3COO] by Bowron et al.172 indeed1281suggested dominance of unidentate over bidentate coordination1282of [CH3COO]

− to the hydrogen atoms bounded to the1283imidazolium ring of [C2C1im]

+. It will be discussed in the next1284section that an anion hydrogen-bonded to 1-alkyl-3-methyl-1285imidazolium cations has significant effect on vibrational1286frequencies of stretching of C−H ring bonds of the cation.1287The Raman band at 2917 cm−1 assigned to the νs(CH3) mode1288of [CH3COO]

− (see Table 3) overlaps the spectral range of1289C−H stretching modes of the butyl chain of [C4C1im]

+ (see1290Figure 19 below).1291Vibrations of the trifluoroacetate anion have been discussed1292by Cabaco et al.173 These authors provided a list of vibrational1293frequencies and assignments for [CF3COO]

− in [C4C1im]-1294[CF3COO] together with corresponding values for the simple1295salt Na[CF3COO] in aqueous solution.174 Some effects of1296replacing H by F atoms in going from [CH3COO]

− to1297[CF3COO]− are worth noting on the most important1298vibrations of the anion. In the case of [C4C1im][CF3COO],1299νas(COO) and νs(COO) are observed at 1691 and 1406 cm−1,1300respectively.173 The difference between them (Δ = 285 cm−1),

Figure 17. IR (red) and Raman (black) spectra of [C4C1im]-[CH3COO] at room temperature. For comparison purposes, IR(green) and Raman (blue) spectra of [C4C1im]Br in the liquid phaseare shown. Arrows indicate bands assigned to [CH3COO]

− normalmodes.

Table 4. Assignment According Different Authors for SomeCharacteristic Imidazolium Ring Vibrations Numbered asFigure 18a

# IR Raman assignment

1 1021 1021 ref 58: ν(CNring) + δ(CNCring) + δ(CH)ref 53: νring,ip,sref 54: breathing + ν (N−CH2) + ν (N−CH3)

2 1337 1337 ref 58: νip(C(2)H3N) + νip(CNring) + w(CH2)ref 53: ν[CN CH3(N)] + ν[CN CH2(N)] + νring,ip,sref 54: breathing + ν(N−CH2) + ν(N−CH3)

3 1378 1385 ref 58: νas(C(2,4)N) + δ(CH) + δs(CH3)ref 53: δs[HCH CH3(N)] + ν[CN CH2(N)] + νring,ip,sref 54: νas(C(2)N(1)C(5)) + δs(CH3)

4 1416 ref 58: νas(C(2,5)N) + δ(CH2) + δ(C(2,4,5)H)+ν(C(2)N)

ref 53: νring,ip,as + ν[CN CH2(N)]+ δs[HCH butyl]ref 54: νring + δ(CH2)

5 1568 1564 ref 58.:ν(C(2)N) + δ(CH) + ν(CCring)ref 53: νring,ip,as + ν[CN CH2(N)] + ν[CN CH3(N)]ref54: νas(N(1)C(2)N(3)) + r(C(2)−H)

av, stretching; δ, in plane bending; w, wagging; r, rocking; s,symmetric; as, antisymmetric; ip, in-phase.



P


1301 being significantly larger than in Na[CF3COO] (Δ = 2461302 cm−1),165 is also an indication of unidentate interaction1303 between [CF3COO]

− and the imidazolium cation. In contrast1304 to the asymmetric Raman band shape of ν(CC) of1305 [CH3COO]

−, the ν(CC) mode of [CF3COO]− gives a single

1306 inhomogeneously broadened Gaussian profile at 824 cm−1 in1307 the Raman spectrum of [C4C1im][CF3COO].

173 Another1308 worth mentioning difference between [C4C1im][CH3COO]1309 and [C4C1im][CF3COO] is the Raman band at 1203 cm−1 in1310 the latter due to the CF3 stretching mode.173

4.2. Vibrational Frequencies of Cations

1311 4.2.1. Imidazolium. Vibrations of 1-alkyl-3-methylimidazo-1312 lium cations have been discussed in more detail than other1313 ionic liquid forming cations. Assignment of vibrational1314 frequencies has been done with the help of quantum chemistry1315 calculations for the isolated cation or ion pairs. Berg3 used MP21316 level of theory to calculate vibrational frequencies of [C4C1im]

+

1317 with two different conformations of the butyl chain. A well-1318 suited level of theory to calculate vibrational frequencies is1319 DFT/B3LYP with a typical basis set, say 6-31+G(d,p). DFT/1320 B3LYP has been used by Dhumal et al.119 for isolated ions and1321 ionic pairs of [C2C1im]+ and [Tf2N]

−, by Talaty et al.52 for1322 ionic pairs of [CnC1im][PF6] and by Heimer et al.53 for1323 [CnC1im][BF4], n = 2, 3, and 4, and by Katsyuba et al.54 for1324 [C2C1im][BF4]. Once harmonic frequencies follow from these1325 standard calculations, scaling factors are needed for agreement1326 between calculated and experimental data. Grondin et al.58

1327 calculated harmonic and also anharmonic frequencies of1328 [C1C1im]+ and [C2C1im]+ using the second-order perturbative1329 method proposed by Barone56,57 in order to avoid such scaling1330 factors of harmonic frequencies.1331 Long tables of vibrational frequencies calculated for 1-alkyl-3-1332 methylimidazolium cations are available in these papers. The1333 composition of normal modes is naturally complicated because1334 of the electronic structure of imidazolium cations. Furthermore,1335 visual inspection of the eigenvectors on computer screen1336 implies that description of the same normal coordinate may1337 vary among different authors. Grondin et al.58 provided the1338 distribution of potential energy allowing for more rigorous1339 assignment of normal modes in terms of internal coordinates.1340 In the following, we will distinguish three spectral ranges in1341 vibrational spectra of 1-alkyl-3-methylimidazolium cations. The1342 high-frequency range, 2800−3200 cm−1, exhibits a complex1343 pattern of overlapped bands proper to several C−H stretching1344 modes. The 800−1600 cm−1 range includes characteristic1345 bands of imidazolium ring vibrations. The 400−800 cm−1

1346 range, which also contains ring vibrations, is interesting because1347 it provides insight on conformations of alkyl chains. (The low-1348 frequency range probing intermolecular dynamics is the issue of1349 section 5.)

f18 1350 Figure 18 shows IR and Raman spectra of [C4C1im]Br and1351 [C6C1im]Br in the spectral range covering imidazolium ring1352 vibrations. (There are also out-of-plane ring vibrations within1353 600−650 cm−1.) It is clear from this figure that vibrational1354 spectra in this range are essentially the same irrespective of the1355 length of the alkyl chain. Table 4 illustrates different ways that1356 authors describe the nature of vibrations of the bands marked1357 1−5 in the Raman spectra of Figure 18.1358 The stretching of C−H bonds of 1-alkyl-3-methylimidazo-1359 lium cations can be separated into two groups of bands: 2800−1360 3000 cm−1, belonging to CH modes of alkyl chains attached to

f19 1361 the imidazolium ring, and 3000−3200 cm−1, belonging to CH

1362 f19of the imidazolium ring. Figure 19 shows for [C4C1im][PF6]1363and [C4C1im][CF3SO3] such separation of two groups of

1364bands of CH stretching modes. The comparison made in1365Figure 19 illustrates the well-known finding that this spectral1366range is a signature of ionic interactions:54,119,175,176 the1367imidazolium ring CH stretching modes shift to lower1368wavenumbers for ionic liquids containing stronger interacting1369anions. Furthermore, the band at ∼3115 cm−1 in the IR1370spectrum of [C4C1im][CF3SO3] is much broader than1371[C4C1im][PF6], suggesting stronger hydrogen bonding in the1372former. Out-of-plane ring CH vibrations within 700−900 cm−1

1373are also sensitive to strength of anion−cation interactions,1374shifting to lower wavenumber with increasing anion basicity.58

1375Using these γ(CH) vibrations as probes of ionic interactions is1376better accomplished with IR than Raman spectroscopy because1377of higher IR activity. Figure 19 also illustrates stronger anion1378dependence for the ring CH than alkyl CH stretching modes, as1379expected from the preferred location of anions around the1380imidazolium polar head. Furthermore, the doubled-peaked1381feature observed within the 3100−3200 cm−1 range is1382commonly assigned to stretching of C(2)−H (the low frequency1383component) and C(4),(5)−H (the high frequency component).1384This interpretation is supported by quantum chemistry1385calculations of ionic pair54,119,53,175,177,48 putting the anion1386either in front of the C(2) or the C(4),(5) atoms of the

Figure 18. IR and Raman spectra in the 1000−1600 cm−1 range of[C4C1im]Br (green and blue) and [C6C1im]Br (red and black) inliquid phase at room temperature. Table 4 gives the assignment ofRaman bands marked 1−5.

Figure 19. IR (red, transmittance scale at right) and Raman (black)spectra in the CH stretching range of [C4C1im][PF6] (full lines) and[C4C1im][CF3SO3] (dashed lines) at room temperature.



Q


1387 imidazolium ring. These findings are considered as evidence in1388 favor of anion−cation hydrogen bonding, the C(2)−H···A−

1389 arrangement being preferred over C(4),(5)−H···A−.1390 The interpretation of this spectral range in terms of different1391 strengths between C(2)−H···A− and C(4),(5)−H···A− hydrogen1392 bonds has been disputed by Lassegues et al.59 These authors1393 claim that the double-peaked feature above 3100 cm−1 is1394 instead a Fermi resonance,42 resulting from mixing of1395 vibrational states of fundamental CH ring vibrations with1396 overtone and combination bands of ring modes whose1397 fundamental transitions are observed in the 1550−1585 cm−1

1398 range (see Figure 18). Lassegues et al.59 grounded their1399 interpretation by isotopic substitution of each hydrogen atom1400 bonded to the ring in order to clear out the spectral range of1401 3100−3200 cm−1, leaving the corresponding C−D stretching1402 within the 2300−2400 cm−1 range. In accordance with their1403 proposition, the band at ∼3170 cm−1 (see Figure 19) includes1404 all C(2),(4),(5)−H vibrations, and the band at ∼3115 cm−1 is1405 overtone and combination bands of ring modes enhanced by1406 Fermi resonance. The alternative assignment put forward by1407 Lassegues et al.59 roused a lively debate in the literature178,179

1408 because it implies there is no need of anion−cation hydrogen1409 bonding to explain the vibrational spectra nor C(2)−H being1410 stronger interacting site than C(4),(5)−H. Wulf et al.178

1411 strengthened the usual point of view with further quantum1412 chemistry calculations showing that C(2)−H modes are always1413 obtained at lower frequency than C(4),(5)−H modes in many1414 different ion pairs. However, the cyanate-anions chosen by1415 Wulf et al.178 are strongly interacting species, whereas1416 Lassegues et al.179 provided a reminder that their interpretation1417 concerned imidazolium ionic liquids containing less coordinat-1418 ing anions (e.g., [NTf2]

−, [BF4]−, and [PF6]

−). Nevertheless,1419 there is consensus that strongly coordinated anions shift CH1420 stretching modes to lower wavenumber,178,179,58 even though1421 definitive assignment of this spectral range is an open issue in1422 vibrational spectroscopy of imidazolium ionic liquids. Irre-1423 spective of the assignment of vibrational spectra, yet a more1424 delicate issue is the nature of hydrogen bond between1425 imidazolium cations and anions.180

1426 Vibrational spectroscopy has been a powerful tool to reveal1427 molecular conformations in ionic liquids. Studies in this1428 direction followed after the discovery of crystal polymorphism1429 in [C4C1im]Cl by Hayashi et al.181 and Holbrey et al.182 These1430 authors concluded from X-ray diffraction that [C4C1im]Cl1431 forms two crystalline phases differing in the conformation of1432 the butyl chain. Hayashi et al.181 found that these polymorphs1433 exhibit different patterns in the 500−800 cm−1 range of the1434 Raman spectrum, where a group of bands (625, 730, an 7901435 cm−1) and (500, 600, an 700 cm−1) is characteristic of each1436 crystal. All of these bands appear in the Raman spectrum of1437 liquid phase of [C4C1im]Cl, so that one concludes there is a1438 mixture of conformers in the ionic liquid.181,183,184 Hamaguchi1439 and Ozawa185 reviewed their early Raman spectroscopy studies1440 on conformational changes of [CnC1im]+ in ionic liquids with1441 Cl−, Br−, I−, [BF4]

−, and [PF6]−. In a previous review, Berg3

1442 offered a detailed analysis on how alkyl chain conformation of1443 [CnC1im]+ adds fingerprints in Raman spectra of ionic liquids.1444 We also recommend a very detailed work recently published by1445 Endo et al.186 concerning DFT calculations of conformational1446 flexibility and vibrational frequencies of typical ionic liquid1447 forming cations (imidazolium, pyridinium, pyrrolidinium, and1448 piperidinium). It should be noted that this issue has been1449 addressed mainly by Raman spectroscopy, since features

1450indicating the [CnC1im]+ conformation are not so evident in

1451 f20the IR spectrum.99 Figure 20 illustrates the frequency range

1452exhibiting bands that characterize anti−anti (AA, 620 and 7351453cm−1) and gauche−anti (GA, 600 and 697 cm−1) conforma-1454tions of [C4C1im]

+. Quantum chemistry calculations show that1455these bands belong to ring deformations coupled to CH21456rocking motions3,185 so that the actual vibrational frequency1457is sensitive to the butyl chain conformation. As emphasized by1458Berg,3,47 Figure 20 also shows a mixture of gauche and anti1459conformers in the longer alkyl chain [C6C1im]

+ cation. Kiefer1460and Pye155 considered other bands as signatures of three1461conformers of [C6C1im]

+ in [C6C1im][HSO4] (e.g., bending1462modes in 340−510 cm−1 and C−C stretching modes in 900−14631100 cm−1 ranges).1464Nine conformers of [C4C1im]

+ have been the relative1465energies calculated by quantum chemistry methods.187,188 In1466a re-examination of the Raman spectrum of [C4C1im][BF4],1467Holomb et al.99 were able to discriminate four conformers1468(GG, GA, TA, and AA) coexisting in the liquid phase by1469considering additional bands covering wider spectral range1470(200−1200 cm−1). Further studies by Umebayashi et al.,188

1471Katayanagi et al.,189 and Singh et al.190 focused on the anion1472dependence of relative intensities of the 600 and 620 cm−1

1473bands. The ratio of intensities I600/I620 increases in the1474sequence [C4C1im]I, [C4C1im]Br, and [C4C1im]Cl, so that1475one concludes that gauche conformation is stabilized for1476stronger interacting halide anions.1477Umebayashi et al.49 and Lassegues et al.125 showed that1478[C2C1im]

+ conformers with the planar or nonplanar ethyl chain1479can be distinguished by characteristic bands in the spectral1480 f21range of 200−500 cm−1 of the Raman spectrum. Figure 211481shows this spectral range of the Raman spectrum of [C2C1im]1482Cl with the bands proposed by these authors as signatures of1483the [C2C1im]

+ conformation. Endo and Nishikawa191 found

Figure 20. Raman spectra of [C4C1im]Br (blue) and [C6C1im]Br(black) in the liquid phase at room temperature. For comparisonpurpose, Raman spectrum of [C2C1im]Cl (red) at T = 360 K is shown.Bands marked * (620 and 735 cm−1 in [C4C1im]Br) characterize theAA conformer; bands marked # (600 and 697 cm−1 in [C4C1im]Br)characterize the GA conformer. Optimized structures of AA and GAconformers are shown.



R


1484 two isomers (symmetric and asymmetric) of 1-isopropyl-3-1485 methylimidazolium cation in the ionic liquids [i-C3C1im]Br and1486 [i-C3C1im]I. Raman bands considered as signatures of the1487 symmetric conformation of [i-C3C1im]+ are found at ∼315 and1488 688 cm−1 and for the asymmetric conformation at ∼340 and1489 699 cm−1.191 The spectral pattern of Raman spectra of ionic1490 liquids may become very different after phase transition when1491 some bands characterizing mixture of conformers in the liquid1492 phase are absent in the crystalline phase. Section 6 discusses the1493 usage of vibrational spectroscopy for studying phase transition1494 of ionic liquids.1495 Methylation at the C(2) position of imidazolium cations,1496 leading to [CnC1C1im]

+, blocks the most important site for C−1497 H···A− hydrogen bonding. Despite eliminating this site for1498 hydrogen bonding, for a given anion the ionic liquid based on1499 [CnC1C1im]+ is more viscous than the [CnC1im]+ counterpart.1500 The reason for this effect is not fully understood, and different1501 explanations have been proposed.192−195 Concerning vibra-1502 tional spectroscopy, the most significant difference between1503 [CnC1im]+ and [CnC1C1im]+ is seen in the spectral range of1504 ring vibrations. Noack et al.196 discussed IR and Raman spectra1505 of ionic liquids based on [CnC1im]+ and [CnC1C1im]+, n = 2

f22 1506 and 4, with the [NTf2]− anion. Figure 22 compares vibrational

1507 spectra of [C4C1im][NTf2] and [C4C1C1im][NTf2]. Taking

1508the [NTf2]− band marked with asterisk in Figure 22 as the

1509intensity pattern, it is remarkable how strong the [C4C1C1im]+

1510 f23Raman band is at 1515 cm−1. Figure 23 shows atomic

1511displacements calculated in this work at the DFT/B3LYP1512level of theory for two normal modes given new spectral1513features in IR (1540 cm−1) and Raman (1515 cm−1) spectra of1514[C4C1C1im][NTf2]. The small mass of hydrogen implies large1515displacements of hydrogen atoms in the eigenvectors displayed1516in Figure 23. However, in terms of potential energy distribution1517calculated with the VEDA program,50 the [C4C1C1im]

+ IR1518band at 1540 cm−1 involves mainly stretching of N(1)−C(2) and1519N(3)−C(2) bonds (29%) and bending of the methyl group and1520the C(5)−N(3)−C(2) angle (23%). The Raman band at 15151521cm−1 involves stretching of C(2)−CH3, C(4)−C(5), and N(1)−1522C(2) bonds (54%) and bending of the N(1)−C(2)−N(3) angle1523(13%). Hunt et al.197 showed that the electronic structure of1524imidazolium cations is better represented by the double bond1525C(4)C(5) and delocalization in the N(1)−C(2)−N(3) bonds of1526the ring.197 Therefore, the 1515 cm−1 Raman band of1527[C4C1C1im]

+ is very intense because of vibrations of N(1)−1528C(2)−N(3) and C(2)−CH3 moieties with large polarizability1529fluctuation. Endo et al.198 discussed the effect of methylation of1530the C(2) position on the spectral ranges of C−H stretching1531modes and 550−800 cm−1. Raman frequencies of C(4),(5)−H1532stretching modes change when [C4C1im]

+ is replaced by1533[C4C1C1im]+, shifting to lower wavenumber when the1534counterion is Cl−, Br−, or I−, but shifting to a higher1535wavenumber when the counterion is [BF4]

− or [PF6]−. The

1536relative intensities of bands that characterize AA and GA1537conformers (see previous Figure 20) change in going from1538[C4C1im]

+ to [C4C1C1im]+. These findings were considered as

1539the consequence of the local arrangement of anions around1540[C4C1C1im]

+ being different from [C4C1im]+.198

1541Fewer works have been dedicated to detailed assignment of1542vibrational frequencies of protic imidazolium ionic liquids (i.e.,1543monoalkylimidazolium with a free N−H bond). Moschovi et1544al.118 discussed IR and Raman spectra as a function of1545temperature of [C1im][NTf2], which melts at 325 K, where1546[C1im]

+ is the 1-H-3-methylimidazolium cation. Moschovi et1547al.118 addressed the issue of vibrational signatures of [NTf2]

−

1548conformers and the effect of the hydrogen bond on the N−H1549and C−H vibrations of the imidazolium ring. This work1550provides full tables of observed IR and Raman frequencies and1551vibrational assignments for [C1im][NTf2]. An expected differ-1552ence in comparison with 1,3-dialkylimidazolium cations is the1553occurrence of the N−H stretching mode of [C1im]

+ at higher

Figure 21. Raman spectrum of [C2C1im]Cl at T = 360 K. Bandsmarked * belong to the planar conformer, whereas bands marked #belong to the nonplanar conformer.

Figure 22. IR and Raman spectra of [C4C1C1im][NTf2] (green andblue lines) and [C4C1im][NTf2] (red and black lines) at roomtemperature. The band marked with asterisk is assigned to an anionnormal mode. Bands marked with arrows (Raman, 1515 cm−1; IR,1540 cm−1) belonging to [C4C1C1im]

+ have the atomic displacementsof the corresponding normal modes shown in Figure 23.

Figure 23. Atomic displacement of the two normal modes of[C4C1C1im]

+ calculated at the DFT/B3LYP level of theory for theisolated cation. The harmonic vibrational frequencies calculated at1533 and 1560 cm−1 correspond to experimental bands at 1515 and1540 cm−1, respectively, of [C4C1C1im][NTf2] (bands marked byarrows in Figure 22).



S


1554 frequency than the C−H stretching modes. The ν(N−H)1555 mode is observed at 3287 cm−1 in the Raman spectrum of solid1556 [C1im][NTf2], becoming a very weak band at 3281 cm−1 upon1557 melting. The ν(N−H) mode gives intense IR bands both in1558 solid and liquid phases of [C1im][NTf2], 3271 and 3273 cm−1,1559 respectively, although broader in the liquid.118 Moschovi et1560 al.199 went further in reporting IR and Raman spectra for a1561 series of protic ionic liquids [Cnim][NTf2], n = 0−12. Insights1562 on conformation of the alkyl chain of [Cnim]+ cations have1563 been obtained from the Raman bands in the 820−900 cm−1

1564 range, corresponding to deformation motions of the CH3 group1565 at the end of the chain.199 It has been found that vibrational1566 frequencies of N−H and C−H stretching modes in protic1567 [Cnim][NTf2],

199 and also C−H modes in nonprotic1568 [CnC1im][NTf2],

200 decrease as n increases, but these findings1569 have not been considered as signatures of stronger anion−1570 cation hydrogen bonding. These authors considered instead the1571 role played by an intramolecular effect, that is, the positive1572 charge on the imidazolium ring is diminished by the electron1573 donor inductive effect of longer alkyl chain, so that the strength1574 of anion−cation interaction does not increase even though1575 polar/nonpolar segregation becomes better defined in ionic1576 liquids with the imidazolium cation of the long chain. In a1577 subsequent work, Moschovi et al.201 provided very detailed1578 analyses and comparisons of vibrational spectra for nonprotic,1579 [CnC1im][NTf2], and protic, [Cnim][NTf2], ionic liquids.1580 Inserting a functional group in the side chain of 1-alkyl-3-1581 methylimidazolium cations might result in ionic liquids with1582 improved properties for some specific tasks (e.g., gas1583 absorption). Despite vibrational spectroscopy being a powerful1584 tool to characterize molecular interactions in these solutions1585 (see section 7), detailed spectroscopic analyses and assignments1586 of vibrations for these imidazolium derivatives are more sparse1587 than the usual 1-alkyl-3-methylimidazolium cations. Once the1588 vibrational frequencies of [CnC1im]+ and anions are well-1589 established, the most important band of the added functional

f24 1590 group is easily identified. Figure 24 shows molecular structures1591 of some derivatives whose characteristic vibrations we show in

f25 1592 Figure 25.

1593 Xuan et al.202 studied 1-allyl-3-methylimidazolium dicyana-1594 mide and 1-allyl-3-methylimidazolium chloride with assign-1595 ments based on potential energy distributions of normal modes1596 calculated for ion pairs at the DFT/B3LYP level of theory.1597 Figure 25A shows IR and Raman spectra of 1-allyl-3-1598 methylimidazolium dicyanamide in the spectral range where1599 the CC stretching mode is observed at 1647 cm−1 in the1600 Raman spectrum. Xu et al.203 assigned a few IR bands of 1-allyl-1601 3-methylimidazolium bicarbonate in agreement with the more1602 complete assignment of ref 202. The quantum chemistry1603 calculations of Xuan et al.202 indicated three stable conformers

1604of the 1-allyl-3-methylimidazolium cation, but no spectroscopic1605signatures have been proposed for these conformers.1606Shirota et al.204 discussed the vibrational spectra of some1607ionic liquids containing benzyl-substituted cations, and Xue et1608al.205 compared the spectra of 1-benzyl-3-methylimidazolium1609bis(trifluoromethanesulfonyl)imide and a solution of benzene1610in [C1C1im][NTf2]. However, these works focused on the low-1611frequency range obtained by femtosecond Raman-induced Kerr1612effect spectroscopy. Shirota et al.204 provided the observed1613vibrational frequencies for bands below 700 cm−1, and they1614distinguished the vibrations belonging to anions or cations. We1615show in Figure 25B part of the vibrational spectra, including the1616most characteristic new feature of 1-benzyl-3-methylimidazo-1617lium dicyanamide. The sharp band observed at 1003 cm−1 in1618the Raman spectrum of the ionic liquid is the counterpart to the1619totally symmetric breathing mode of benzene.1620There is still no detailed analysis of vibrational spectra of an1621ionic liquid based on CN-functionalized imidazolium cation.1622Figure 25C shows the characteristic band of the CN stretching1623vibration in 1-(3-cyanopropyl)-3-methylimidazolium bis-1624(trifluoromethanesulfonyl)imide, [NC-C3C1im][NTf2], at1625room temperature. The vibrational frequency of the ν(CN)1626mode at 2251 cm−1 in [NC-C3C1im]

+ lies at a significantly1627higher wavenumber than cyanate-anions (see Figure 10 and1628Table 1). The ν(CN) frequency in [NC-C3C1im]

+ is indeed1629close to the value in the molecular liquid acetonitrile (22531630cm−1).1631Knorr et al.206,207 discussed temperature effect on the IR1632band corresponding to O−H stretching mode of 1-(2-1633hydroxyethyl)-3-methylimidazolium tetrafluoroborate, [HO-1634C2C1im][BF4]. The ν(OH) spectral range is shown in Figure163525D for [HO-C2C1im][BF4] at room temperature, where the1636strong feature at ∼3552 cm−1 in the IR spectrum belongs to the1637O−H stretching motion in O−H···F hydrogen bonded to1638[BF4]

−. Knorr et al.206,207 found that the low-frequency tail in1639the IR band grows in intensity with decreasing temperature,1640becoming a well-resolved band at ∼3402 cm−1 at 233 K. With1641the support of quantum chemistry calculations of clusters of

Figure 24. Molecular structures of some functionalized imidazoliumcations.

Figure 25. Characteristics IR (red, transmittance scale at right) andRaman (black) bands of (A) 1-allyl-3-methylimidazolium dicyanamide,(B) 1-benzyl-3-methylimidazolium dicyanamide, (C) 1-(3-cyanoprop-yl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and (D)1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate at roomtemperature.



T


1642 ions, this low-frequency component has been assigned to the1643 O−H vibration engaged in O−H···O hydrogen bond with the1644 neighbor cation.206,207 Katsyuba et al.208 observed a weak band1645 at ∼3425 cm−1 in the IR spectrum of [HO-C2C1im][PF6], and1646 they also assigned it to ν(OH) in hydrogen bonding with −OH1647 groups of neighbor cations. Hydrogen bonding between like-1648 charge species has been also suggested for the choline cation1649 from the analysis of the O−H stretching region of IR spectra of1650 [Cho][NTf2].

209 Knorr et al.206,207,209 pointed out that these1651 are the first spectroscopic evidence of what Weinhold and1652 Klein210 called an antielectrostatic hydrogen bond (i.e., a1653 hydrogen bond between ions of the same charge). It is worth1654 remembering, however, that signature of anion−anion hydro-1655 gen bond between [HSO4]

−, previously found in IR and Raman1656 spectra of crystals of the halide salts157−160 has also been found1657 in Raman spectra of [C2C1im][HSO4] and [C4C1im][HSO4]1658 as discussed in the previous section.156

1659 4.2.2. Pyridinium. Gale et al. discussed Raman211 and IR212

1660 spectra of N-butylpyridinium in the early period of room1661 temperature molten salts based on mixtures of an organic1662 cation chloride and AlCl3. Tait and Osteryoung213 found that1663 the main difference in IR spectra between basic and acidic1664 AlCl3/N-butylpyridinium chloride mixtures (acidic melts have1665 AlCl3:organic salt mole ratio greater than 1) occurs in the C−H1666 stretching region, in particular enhancement in the intensity of1667 bands of aliphatic relative to aromatic C−H stretching modes,1668 suggesting loss in ring aromaticity with decreasing acidity.1669 Tables of experimental frequencies of cation vibrations are1670 available;213 however, the most important issue of a vibrational1671 spectroscopy study of those systems was identifying complex1672 species AlCl4

− and Al2Cl7− rather than a detailed analysis of

1673 cation normal modes.211,212

f26 1674 Figure 26 shows the fingerprint regions of IR and Raman1675 spectra of 1-butyl-4-methylpyridinium bromide, [Py1,4][Br],

1676 and 1-butyl-4-methylpyridinium tetrafluoroborate, [Py1,4][BF4],1677 where the few [BF4]

− bands are marked with asterisks. The1678 vibrational modes of N-ethylpyridinium have been discussed by1679 Zhao et al.214 for [BF4]

− and [CF3COO]− salts, and by Zheng

1680 et al.215 for the Br− salt. Sashina et al.216 discussed IR and1681 Raman spectra of halides salts of N-alkylpyridinium for the1682 series from two to ten carbon atoms in the alkyl chain. These1683 authors paid attention to the high-frequency range of 2800−1684 3200 cm−1 of C−H stretching modes. This spectral range for

1685N-alkylpyridinium cations can be separated in stretching1686vibrations of C−H bonds of the alkyl chain (2800−30001687cm−1) and the ring (above 3000 cm−1). Frequency shift of C−1688H stretching of N-alkylpyridinium to a lower wavenumber can1689also be related to stronger anion−cation interaction.216

1690It has not been addressed in these works on N-1691alkylpyridinium cations whether vibrational spectroscopy1692could be used to identify different conformation of the alkyl1693chain. Thus, we calculated harmonic vibrational frequencies at1694the DFT/B3LYP level of theory for gauche and anti conformers1695 f27of an isolated [Py1,4]

+ cation (see molecular structures in Figure1696 f2727). The calculations suggest there is indeed a doublet of

1697Raman bands (887 and 909 cm−1 in liquid [Py1,4][BF4]; 8921698and 913 cm−1 in solid [Py1,4]Br) that can be used to1699discriminate each conformer. These bands correspond to1700Raman active modes with frequencies calculated at 886 and1701917 cm−1, respectively, for gauche and anti [Py1,4]

+ conformers.1702Figure 27 highlights this spectral range of the Raman spectra of1703[Py1,4][BF4] and [Py1,4]Br together with peak position and1704relative Raman intensities obtained from the DFT calculation.1705This figure indicates there is a mixture of gauche and anti1706[Py1,4]

+ conformers in both solid [Py1,4]Br and liquid1707[Py1,4][BF4]. Recently, Endo et al.186 provided a detailed1708theoretical analysis by DFT calculations on the conformational1709flexibility and the corresponding vibrational frequencies of the1710most common organic cations forming ionic liquids. In the case1711of pyridinium derivatives, the calculations suggest that the1712480−510 and 740−820 cm−1 regions of the Raman spectrum1713could also be used to identify different conformers.17144.2.3. Pyrrolidinium. The relationship between vibrational1715frequency and molecular conformation has been studied in1716more detail for 1,1-dialkylpyrrolidinium than pyridinium1717cations. Castriota et al.114 reported Raman spectra in the1718250−1700 cm−1 range of ionic liquids based on 1-methyl-1-

Figure 26. IR (red) and Raman (black) spectra of 1-butyl-4-methylpyridinium tetrafluoroborate, [Py1,4][BF4], at room temper-ature. Bands marked with asterisk belong to anion vibrations. Forcomparison purposes, IR and Raman spectra of solid [Py1,4]Br areshown by green and blue lines, respectively.

Figure 27. Raman spectra of liquid [Py1,4][BF4] (black line) and solid[Py1,4]Br (blue line) at room temperature in the region includingbands assigned to different [Py1,4]

+ conformers. Vibrationalfrequencies and relative intensities calculated for [Py1,4]

+ are indicatedby green (886 cm−1, gauche) and red (917 cm−1, anti) lines. Atomicdisplacement vectors calculated for these normal modes are shown foreach conformer.



U


1719 propylpyrrolidinium, [Pyr1,3]+, with the anions [NTf2]

− and I−.1720 However, assignment of [Pyr1,3]

+ vibrations was not given1721 because the issue was coordination of Li+ by [NTf2]

− in a 2:11722 mixture of [Pyr1,3][NTf2] with Li[NTf2].

114 The analogous1723 effect of a strong polarizing cation on the [NTf2]

− vibrations1724 has been discussed by Rocher et al.217 in [Pyr1,4][NTf2]/AlCl31725 mixtures as a function of Al3+ concentration. The clear1726 signature provided by Raman spectroscopy about coordination1727 of [NTf2]

− to Li+ will be discussed in section 7. The vibrational1728 motions of [Pyr1,3]

+ and [Pyr1,4]+ have been discussed in

1729 several works.122,3,127,130,217,218 Besides gauche or anti con-1730 formation of the butyl chain, additional issues concerning1731 [Pyr1,4]

+ include equatorial or axial position (eq or ax) of the1732 butyl chain relative to ring carbon atoms, and envelope or twist1733 conformation of the nonplanar ring. By comparing exper-1734 imental Raman spectrum and DFT/B3LYP calculation within1735 the whole 200−1600 cm−1 range, Fujimori et al.218 concluded1736 that in [Pyr1,4][NTf2] the butyl chain and the ring are at all anti1737 and envelope conformations, respectively, but there is mixture1738 of eq and ax [Pyr1,4]

+ conformers.1739 It is difficult to characterize the conformers when many1740 experimental and calculated frequencies have to be compared,1741 instead of a small frequency range including a few character-1742 istics bands. Comparison with related systems with a single1743 conformer in crystalline phase (e.g., [Pyr1,4]Br) helps1744 identifying appropriate bands as the fingerprint of a given1745 conformer.218 Bands observed at 486, 586, and 655 cm−1 in the1746 Raman spectrum of solid [Pyr1,4]Br closely match the1747 frequencies calculated for [Pyr1,4]

+ in ax−anti-envelopef28 1748 conformation. Figure 28 shows IR and Raman spectra of

1749 [Pyr1,4][NTf2] in the range of 860−950 cm−1 used by Fujimori1750 et al.218 in order to reveal coexisting eq and ax [Pyr1,4]

+

1751conformers in the liquid phase. Although there is overlap of1752bands in this region, quantum chemistry calculations indicate1753that the 883 cm−1 Raman band is a signature of the ax1754conformer, whereas the 890−930 cm−1 range includes bands of1755both eq and ax conformers. Furthermore, the intensities of1756components at 892, 905, and 930 cm−1 decreases with1757increasing temperature, while the intensity of the 883 cm−1

1758band remains the same, also indicating a mixture of eq and ax1759[Pyr1,4]

+ conformers in [Pyr1,4][NTf2].218 Umebayashi et al.127

1760also concluded for mixtures of eq and ax conformers in1761[Pyr1,3][NTf2] and [Pyr1,4][NTf2] on the basis of the DFT/1762B3LYP calculations and the temperature dependence of Raman1763spectra in the range shown in Figure 28. Fujii et al.130 paid1764attention to the 250−400 cm−1 range of the Raman spectrum1765of [Pyr1,3][N(SO2F)2], in which cation bands at 350 and 3801766cm−1 again indicate a mixture of [Pyr1,3]

+ conformers. There is1767an interesting contrast between these studies on pyrrolidi-1768nium218,127 and the above discussion of imidazolium ionic1769liquids: there is mixture of anti and gauche conformation of the1770butyl chain in [C4C1im]

+ ionic liquids, but anti conformation of1771[Pyr1,4]

+ predominates with negligible population of the gauche1772conformer. However, when [Pyr1,4]

+ is functionalized upon1773insertion of an ether group function in the long alkyl chain then1774IR spectroscopy with support of DFT calculations indicate that1775the side chain acquires gauche geometry.219 A distinctive aspect1776of works by Vitucci et al.121 and Mao et al.117 is that these1777authors considered the IR spectrum, rather than the Raman1778spectrum, of [Pyr1,4][NTf2]. In order to assign the IR bands by1779DFT calculations, Vitucci et al.121 considered the isolated ion,1780whereas Mao et al.117 considered an ionic pair. Tables1781containing assignment of [Pyr1,4]

+ frequencies are avail-1782able.117,121 In particular, Figure 6 of ref 121 provides1783visualization of atomic displacements for the normal modes1784corresponding to the bands shown in Figure 28.17854.2.4. Piperidinium. Derivatives of piperidinium encom-1786pass an important class of ionic liquid forming cations, but1787assignment of vibrational spectra has been much less discussed1788for piperidinium than imidazolium or pyrrolidinium cations.1789Shukla et al.120 compared calculated and experimental IR and1790Raman spectra of N-butyl-N-methylpiperidinium bis-1791(trifluoromethanesulfonyl)imide, [Pip1,4][NTf2] and [Pip1,4]Br,1792the latter being a solid with a relatively high melting point (2411793 f29°C). Figure 29 shows IR and Raman spectra of [Pip1,4][NTf2]1794and [Pip1,4]Br. As the [NTf2]

− bands dominate the spectra of1795[Pip1,4][NTf2], we provide spectra for solid [Pip1,4]Br in order

Figure 28. IR (red, transmittance scale at right) and Raman (black)spectra of [Pyr1,4][NTf2] at room temperature in the region used tocharacterize different conformers of [Pyr1,4]

+. The asterisk marks the883 cm−1 Raman band characteristic of the ax conformer. Optimizedstructures are shown for an isolated [Pyr1,4]

+ cation with the butyl-chain in the equatorial (eq) or axial (ax) position in relation to thering. In both the structures, the butyl chain is in anti and the ring inenvelope conformations.

Figure 29. IR and Raman spectra of liquid [Pip1,4][NTf2] (red andblack) and solid [Pip1,4]Br (green and blue) at room temperature.



V


1796 to highlight the [Pip1,4]+ bands. Conformations of piperidi-

1797 nium, like pyrrolidinium cations, include ring flexibility (chair,1798 twist, or boat), eq or ax position of the butyl chain, and1799 rotational isomers of the butyl chain. The DFT/B3LYP1800 calculations of Shukla et al.120 indicated that the more stable1801 conformation is chair for the piperidinium ring and gauche for1802 the butyl chain. The characteristic Raman bands of cisoid and1803 transoid conformations of [NTf2]

− were discussed by Shukla et1804 al.,120 but they did not identify vibrations that could be used as1805 signatures of cation conformation. Shimizu et al.132 identified1806 characteristic bands of [Pip1,4]

+ conformers in the range of1807 700−800 cm−1 of the Raman spectrum of [Pip1,4][N(SO2F)2].1808 With the support of DFT/B3LYP calculations for the chair1809 form of [Pip1,4]

+, Shimizu et al.132 proposed that eq and ax1810 conformations of [Pip1,4]

+ can be identified by the Raman1811 bands at 704 and 710 cm−1, respectively, in the crystalline1812 phases of [Pip1,4][N(SO2F)2]. In the liquid phase, the Raman1813 spectrum indicates a mixture of eq and ax conformers of1814 [Pip1,4]

+.1815 4.2.5. Ammonium. A large variety of ionic liquids can be1816 prepared from mono-, bi-, tri-, and tetraalkylammonium1817 cations, [C iNH3]

+, [C iC jNH2]+, [C iC jCkNH]+, and

1818 [CiCjCkClN]+, respectively, with lengths of alkyl chains

1819 indicated by the number of carbon atoms i, j, k, and l.1820 Vibrational frequencies of the prototype [NH4]

+ of Td

1821 symmetry in halide salts were already reviewed by Herzberg,42

1822 3033 (A1), 1685 (E), 3134 (F2), and 1397 (F2) cm−1, the actual

1823 values being dependent on the counterion and temper-1824 ature.143,220,221 On the other hand, in the context of ionic1825 liquid forming cations, carbon chains of different lengths result1826 in asymmetric species, and the alkyl substituents give1827 conformational flexibility. Those cations with one or more1828 hydrogen atoms directly bounded to the nitrogen are called1829 protic ionic liquids with properties dependent on relatively1830 strong anion−cation hydrogen bonds,222 whereas those based1831 on tetraalkylammonium cations are nonprotic ionic liquids. Let1832 us focus first on tetraalkylammonium ionic liquids, for which1833 detailed assignment of cation vibrations became available1834 recently.121,122,223

1835 Vitucci et al.121 discussed IR spectra of [C1C1C1C3N][NTf2]1836 and [C1C1C1C6N][NTf2] with the help of quantum chemistry1837 calculation at the DFT/B3LYP level of theory. The IR spectra1838 exhibit of course the high-frequency bands of C−H stretching1839 modes at 2800−3200 cm−1, but the fingerprint region is mainly1840 dominated by the [NTf2]

− bands. In accordance with Figure1841 11, spectral windows appropriate to characterize the cations1842 without [NTf2]

− bands include the ranges of 800−1050 cm−1

1843 and 1350−1550 cm−1. The work of Vitucci et al.121 concernedf30 1844 the IR spectrum, so that we show in Figure 30 for completeness

1845 these spectral ranges for both IR and Raman spectra for a1846 closely related system, [C4C1C1C1N][NTf2]. The range of

18471350−1550 cm−1 corresponds to C−H bending modes, and the1848range of 800−1050 cm−1 corresponds to normal modes with1849displacements of many atoms of the cation. The complex1850patterns of atomic displacements calculated for the normal1851modes of [C1C1C1C3N]

+ and [C1C1C1C6N]+ can be found in

1852ref 121. In a subsequent work, Vitucci et al.122 discussed1853spectral changes after crystallization of [C1C1C1C3N][NTf2]1854and [C1C1C1C6N][NTf2] at ∼250 K.1855The possibility of revealing alkyl chain conformation of1856tetraalkylammonium cations by combined usage of IR spec-1857troscopy and quantum chemistry calculations is a more1858challenging task in comparison with the above discussion on18591-alkyl-3-methylimidazolium cations. Palumbo et al.223 used the1860DFT/B3LYP level of theory to calculate vibrational frequencies1861for six different conformers of [C1C1C1C3N]

+. An appropriate1862spectral window for comparison between calculation and1863experiment is the 900−1070 cm−1 range of the IR spectrum1864of [C1C1C1C3N][NTf2]. The calculations indicated that the IR1865feature at 1000 cm−1 (a vibration mixing C−C, C−N1866stretching, and CH2 wagging) is a characteristic band of the1867lowest energy [C1C1C1C3N]

+ conformer.223 Occurrence of1868several bands in this spectral range indicates a mixture of1869conformers in the liquid phase, whereas only the lowest-energy1870conformer is retained in the crystalline phase accompanied by1871concomitant change in the spectral pattern.223 The application1872of vibrational spectroscopy to investigate phase transitions of1873ionic liquids will be reviewed in section 6.1874Doman ska and Bogel-Lukasik,224 in a study focusing on1875thermodynamic properties of ammonium salts, provided the IR1876frequencies of solid phases of tetraalkylammonium bromide1877salts in which one of the alkyl chains has been functionalized1878with a terminal −OH group. Aparicio et al.225 used the high-1879frequency range of the IR spectrum, 3100−3600 cm−1, in order1880to monitor water uptake from the atmosphere by ionic liquids1881based on 2-hydroxyethyltrimethylammonium and tris(2-1882hydroxyethyl)methylammonium cations, but detailed assign-1883ment of cation vibrations was not an issue. A discussion on the1884nature of vibrations was given by Arkas et al.226 for N,N-di(2-1885hydroxyethyl)-N-methyl-N-alkylammonium bromides (alkyl1886chain from dodecyl to octadecyl), however, within the context1887of liquid-crystal phase transition.1888Concerning −OH-functionalized ammonium cations, more1889detailed spectroscopy studies have been reported for choline1890(i.e., hydroxyethyl-trimethylammonium, [Cho]+). In the case of1891nonprotic tetraalkylammonium ionic liquids, an indication of1892weak interaction comes from finding that simple juxtaposition1893of vibrational frequencies calculated for isolated anion and1894cation give reasonable agreement to experimental spectra.121

1895On the other hand, this is not the expectation for systems with1896the possibility of strong hydrogen bonds such as choline cation1897with carboxylate170 or amino acid anions.227 Vibrational

Figure 30. IR (red) and Raman (black) spectra of [C4C1C1C1N][NTf2] at room temperature in different regions covering cation bands.



W


1898 frequencies of the choline cation have been discussed by Tanzi1899 et al.170 in ionic liquids with formate, propanoate, or butanoate.1900 By comparing quantum chemistry calculations for the isolated1901 choline and the choline−anion pair, these authors found the1902 expected hydrogen bond induced frequency shift: ν(OH) shifts1903 to a lower wavenumber, whereas HOC bending and CO1904 stretching and torsion modes shift to a higher wavenumber.170

1905 The more powerful tool to calculate vibrational spectra of these1906 strongly hydrogen-bonded ionic liquids is ab initio MD1907 simulation, since the method takes into account many different1908 ionic arrangements resulting from the liquid dynamics.170,227

1909 Tanzi et al.170 showed that Raman spectra of choline1910 carboxilates exhibit a large variability of O−H stretching1911 frequencies within 3200−3400 cm−1, whereas the less1912 perturbed C−H stretching modes of choline cover narrower1913 range centered at ∼2985 cm−1. This spectral separation1914 between O−H and C−H stretching modes is clear in IR1915 spectra reported by Harmon et al.228 for the simple salts1916 choline bromide and choline iodide. The O−H stretching band1917 in the IR spectrum of [Cho][NTf2] exhibits a tail with1918 components at 3431 and 3474 cm−1 which have been assigned1919 as the signature of hydrogen bonding between neighboring1920 choline cations.209 On the other hand, the IR spectrum1921 reported by Campetella et al.227 for choline with the amino acid1922 anion alanine, where there is also overlap with the alanine N−H1923 stretching modes, shows a broad band covering the whole1924 2500−3500 cm−1 range. The environmental-induced modifica-1925 tions of IR spectra have also been discussed by Perkins et al.229

1926 for the eutectic mixture of choline chloride and urea (1:2 molar1927 ratio). The ν(OH) IR band in pure choline chloride is a1928 relatively sharp band at 3256 cm−1, which overlaps the N−H1929 stretching modes of urea upon formation of the mixture.1930 It is difficult to correlate vibrational frequencies in the1931 spectral range of 700−1700 cm−1 for simple choline halides228

1932 with the frequencies observed in ionic liquids with strongly1933 interacting carboxilate and amino acid anions. For instance, the1934 IR band at 1088 cm−1 for choline carboxylate,170 which agrees1935 with the value for choline halides,228 was assigned to the1936 choline CO stretching mode in ref 170, but the corresponding1937 band at 1091 cm−1 in choline alanine was assigned to CH rock1938 and twisting.227 Thus, it would be interesting to mark the1939 choline vibrational frequencies in IR and Raman spectra of an1940 ionic liquid with a less interacting anion. Once the [NTf2]

−

1941 frequencies were already discussed in the previous section, thenf31 1942 we show IR and Raman spectra of [Cho][NTf2] in Figure 31

1943 with the choline bands indicated by arrows. Harmon et al.228

1944 assigned the choline normal modes within 700−1200 cm−1,1945 assuming a C3v point group for the (CH3)3N−CH2− moiety.1946 Some of the IR frequencies reported for choline halides228 have1947 a correspondent feature in the spectra shown in Figure 31 for1948 [Cho][NTf2].1949 The first example of the series of protic alkylammonium ionic1950 liquids is monomethylammonium, [C1NH3]

+. Raman spectra1951 and X-ray powder diffraction as a function of temperature have1952 been reported by Bodo et al.230 for [C1NH3][NO3], which1953 melts at 381 K. Calculation of vibrational spectra have been1954 done by first using classical MD simulation of clusters of ion1955 pairs, ([C1NH3][NO3])n, n up to 8, in order to generate1956 reasonable configurations, which were then optimized by1957 quantum chemistry calculation at the DFT/B3LYP level of1958 theory. This approach aims for a better representation of the1959 hydrogen bond network and the condensed phase spectrum.1960 The high-frequency range of the Raman spectrum of liquid

1961[C1NH3][NO3] exhibits relatively sharp bands of C−H1962stretching modes in the range of 2800−3050 cm−1 and N−H1963stretching modes seen as broad bands at 3100−3300 cm−1

1964because of a distribution of cation−anion hydrogen bonds of1965different strengths.230 The intense [NO3]

− Raman bands1966dominate the spectrum in the 400−1700 cm−1 range, where1967few bands (C−N stretching and CH2 bending) have been1968assigned to [C1NH3]

+.230

1969This approach of comparing experimental spectra with DFT1970calculations of clusters of ions was continued by Bodo et al.231

1971for the sequence [C2NH3][NO3], [C3NH3][NO3], and1972 f32[C4NH3][NO3]. Figure 32 shows IR and Raman spectra of

1973[C3NH3][NO3]. Several groups of Raman bands were assigned1974to [CnNH3]

+ vibrations:231 Cn−N bending (416−450 cm−1),1975Cn−N symmetric stretching (870−890 cm−1), CH2 and CH31976wagging (∼990 cm−1), asymmetric C−C and C−N stretching1977(∼950 cm−1), CHn rocking (1175−1195 cm−1), CHn scissoring1978(∼1300 cm−1), and CH2 and CH3 bending (1450−1460 cm−1).

Figure 31. IR (red, transmittance scale at right) and Raman (black)spectra of [Cho][NTf2] at room temperature. Some bands assigned tocholine are indicated by arrows.

Figure 32. IR (red, transmittance scale at right) and Raman (black)spectra of [C3NH3][NO3] at room temperature. Bands indicated byblue arrows at 828 and 868 cm−1 characterize gauche and anticonformers. Optimized structures of propylammonium in gauche andanti conformations are shown.



X


1979 A specific comment concerns the band located at 870−8901980 cm−1, which Henderson et al.232 assigned instead to C−C1981 stretching and found at 875 cm−1 in the Raman spectrum of1982 [C2NH3][NO3]. This assignment is in line with a detailed1983 analysis performed by Hagemann and Bill233 of the Raman1984 spectra of the simpler salts [C2NH3]Cl and [C2NH3]Br. These1985 authors provided a full list of Raman frequencies and1986 assignments for ethylammonium halides, including the1987 complicated pattern of bands intensified by Fermi resonance1988 in the high-frequency range of C−H and N−H stretching1989 modes. It is worth mentioning that the ν(CC) mode at 10471990 cm−1 in [C3NH3]Cl is an useful probe of local structure along1991 the phase transition,232,234 but it is hidden by the intense anion1992 band νs(NO3) in [CnNH3][NO3]. Nevertheless, there is nice1993 agreement between experiment and theory,231 in particular the1994 finding that cation frequencies within 400−1700 cm−1 depend1995 on the alkyl chain length of [CnNH3]

+.1996 Faria et al.235 calculated vibrational frequencies for different1997 conformers of [C3NH3]

+ at the DFT/B3LYP level of theory.1998 Temperature and pressure induced crystallization of [C3NH3]-1999 [NO3] helped Faria et al.235 to propose that Raman bands at2000 828 and 868 cm−1 characterize, respectively, gauche and anti2001 conformers of [C3NH3]

+ (i.e., there is a mixture of conformers2002 in the liquid phase of [C3NH3][NO3]) (see Figure 32). The2003 high-frequency spectral range, which is separated into bands2004 belonging to C−H and N−H stretching modes, is barely2005 affected by the alkyl chain length because hydrogen bonds2006 remain essentially the same between the polar head of2007 [CnNH3]

+ and the nitrate anion.231 Bodo et al.236 have also2008 compared the experimental Raman spectrum of liquid2009 [C2NH3][NO3] with the power spectrum obtained from the2010 Fourier transform of the time correlation function of atomic2011 velocities calculated by ab initio MD simulation. IR frequencies2012 have been reported by Luo et al.237 for derivatives containing2013 longer alkyl chains (trioctylammonium) and the more complex2014 triphenylammonium (and also the trialkylphosphonium2015 counterpart) triflate ionic liquids.2016 4.2.6. Other Cations. In Berg’s review on Raman2017 spectroscopy of ionic liquids,3 he provided preliminary results2018 for a system based on the tetramethylguanidinium cation,2019 [TMGH]+. More detailed analysis of vibrations of [TMGH]+

2020 have been performed by Berg et al.238 and Horikawa et al.239 for2021 [TMGH][NTf2], and by Berg et al. for [TMGH]Cl240 and2022 [TMGH]Br.241 These works provide lists of vibrational2023 frequencies experimentally observed and calculated by quantum2024 chemistry methods.2025 Carper et al.242 reported IR and Raman spectra of2026 trimethylsulfonium dicyanamide, [C1C1C1S][N(CN)2]. The2027 authors provided vibrational assignment based on quantum2028 chemistry calculations for an ionic pair and a dimer

f33 2029 ([C1C1C1S][N(CN)2])2.242 We show in Figure 33 the

2030 fingerprint range of IR and Raman spectra of other2031 alkylsulfonium ionic liquid, [C2C2C2S][NTf2], with some2032 cation bands indicated by arrows.

4.3. Applications

2033 The discussion of vibrational frequencies and normal modes2034 assignments for important anions and cations in the previous2035 sections already stated some of the applications of vibrational2036 spectroscopy in studies of ionic liquids. This section aims an2037 overall view of these and further applications of vibrational2038 spectroscopy of pure ionic liquids.

2039One of the most fundamental applications of molecular2040spectroscopy42 is the calculation of thermodynamic properties2041of the ideal polyatomic gas model, for which exact expressions2042exist for the partition function of translational, rotational, and2043vibrational degrees of freedom. Focusing here on the2044vibrational contribution, the knowledge of vibrational frequen-2045cies allows for the calculation of thermodynamic functions2046according to fundamental expressions of statistical mechanics2047(e.g., for the heat capacity of harmonic oscillators):74,243

∑=−=

− ⎛⎝⎜

⎞⎠⎟

C

Nk

hv

kTe

e( 1)j

Nj

hv kT

hv kTV,vib

1

3 6 2 /

/ 2

j

j

2048(5)

2049where k is the Boltzmann constant, h is the Planck constant, T2050is temperature, N is the number of atoms, and the summation2051extends to the 3N − 6 normal modes (3N − 5 for linear2052molecules) each one with vibrational frequency νj. Paulechka et2053al.244 calculated heat capacity, energy, and entropy for the ideal2054gas phase of [C4C1im][PF6]. Blokhin et al.245 calculated these2055thermodynamic properties for [C4C1im][NTf2], and Paulechka2056et al.244 extended the calculations for the series [CnC1im]-2057[NTf2], n = 2, 4, 6, and 8. Following a combined usage of2058experimental IR frequencies and calculated by quantum2059chemistry methods, these works give full lists of vibrational2060frequencies νj that have actually been considered in the2061thermodynamic calculations. Some low frequencies are not2062included in the summation because they correspond to internal2063rotations, which are commonly represented by an empirical2064cosine potential function for hindered rotations.74,243 Rota-2065tional contributions to thermodynamic properties take into2066account the moments of inertia given in these works.244−246

2067The quantum chemistry calculations giving the full set of2068vibrational frequencies needed in evaluating thermodynamic2069properties have been done for a cation−anion pair because it is2070considered that the gas phase of ionic liquids is made of ion2071pairs rather than isolated ions.247,248 Dong et al.249 were able to2072obtain in situ IR spectrum of [C2C1im][NTf2] in the gas phase2073after vacuum distillation of the liquid at 150 °C. Relative2074intensities of some bands change in the gas phase in2075comparison with the liquid phase spectrum, a finding2076reproduced by calculations of IR spectra of clusters of different2077sizes.249 Obi et al.250 and Cooper et al.251 also concluded for2078vaporization of [C2C1im][NTf2] as ion pairs, but these workers2079obtained the IR spectrum after trapping the vapor in helium2080nanodroplets at very low temperature250 or supersonic jet-

Figure 33. IR (red, transmittance scale at right) and Raman (black)spectra of [C2C2C2S][NTf2] at room temperature. Some bandsassigned to [C2C2C2S]

+ are indicated by arrows.



Y


2081 cooled in helium carrier gas.251 The role played by methylation2082 of the imidazolium ring on the strength of interactions of the2083 anion to the imidazolium cation was addressed by Fournier et2084 al.252 by comparing IR spectra of clusters of [C4C1im][BF4]2085 and [C4C1C1im][BF4]. Both trapping techniques of helium2086 nanodroplets and matrix isolation were used by Hanke et al.253

2087 to reveal individual ion contributions to the IR spectra of2088 [C1C1C1C4N][NTf2]. Booth et al.

254 identified in the gas phase2089 several conformers of [C2C1im][NTf2] ion pairs with different2090 strengths of interaction with the anion according to the2091 magnitude of vibrational frequency shift of the C(2)−H2092 stretching mode. Berg et al.255 obtained in situ gas phase2093 Raman spectrum of a protic ionic liquid, 1-methylimidazolium2094 acetate. Comparison between spectra of the gas phase ionic2095 liquid and 1-methylimizadole and acetic acid indicated that2096 proton transfer takes place upon vaporization, so that the gas2097 phase of this protic ionic liquid is made of the starting neutral2098 molecules.255 However, this is not a general rule for protic ionic2099 liquids as demonstrated by Horikawa et al.,239 who obtained IR2100 spectra of several protic ionic liquids after vaporization and2101 trapping in a cryogenic neon matrix. Comparison between2102 spectra of the evaporated ionic liquid and the parent acids and2103 bases leads to the conclusion that vaporization as an ionic pair2104 is preferred as larger is the difference in pKa of the parent acids2105 and bases.239 Returning to the issue of calculating thermody-2106 namic properties of the ideal gas phase of nonprotic2107 imidazolium ionic liquids,245,246 the agreement between2108 calculation under the assumption of ionic pairs and2109 experimental data gives further support for the physical picture2110 that nonprotic ionic liquids in the gas phase are not made of2111 isolated anions and cations.2112 Vibrational spectroscopy is a complementary tool for thermal2113 analysis and mass spectrometry256 to reveal whether thermal2114 decomposition of ionic liquids takes place and the nature of2115 decomposition products. Berg et al.241 proposed that the2116 Raman spectrum of 1,1,3,3-tetramethylguanidinium chloride,2117 [TMGH]Cl, in the vapor phase at 225 °C was consistent with2118 the presence of [TMGH]+Cl− ion pairs rather than the neutral2119 molecules 1,1,3,3-tetramethylguanidine and HCl. A Raman2120 band at 2229 cm−1 was assigned to the N−H stretching mode2121 experiencing significant lower frequency shift because of the2122 N−H···Cl− hydrogen bond. In a subsequent work, however,2123 Berg et al.241 corrected this interpretation since this band was2124 observed in exactly the same position in the Raman spectrum of2125 vapor phase of [TMGH]Br. Thus, it was proposed instead that2126 this band belongs to CN stretching mode of dimethylcyana-2127 mide, (CH3)2N−CN, resulting from thermal decomposition of2128 [TMGH]Cl and [TMGH]Br.241 In the temperature-jump2129 experiments of Chambreau et al.,257 the IR spectrum is2130 recorded for the vapor phase generated after a discharge on a2131 filament within a small sample of the ionic liquid. For instance,2132 characteristic IR bands of CH3NCS indicated formation of this2133 species upon heating of [C4C1im][SCN], and Chambreau et2134 al.257 proposed reaction mechanisms of thermal decomposition2135 for several ionic liquids based on 1-alkyl-3-methylimidazolium2136 cations and cyanate-anions. Liaw et al.258 obtained IR spectra of2137 [C2C1im][C2SO4], [C6C1im]Cl, and [C4C1im][NTf2] before2138 and after the flash point and after ignition. Changes on the2139 spectral pattern after ignition were considered as evidence of2140 thermal decomposition,258 so that these authors claimed that2141 flammability of these ionic liquids is related to thermal2142 decomposition rather than vaporization of the liquid.

2143Vibrational assignments discussed in previous sections2144indicated there are some frequencies that characterize2145molecular conformations, whose difference of conformer2146energies according to quantum chemistry calculations might2147be relatively small in comparison with thermal energy available2148at room temperature. Vibrational spectroscopy has been2149extensively used to estimate the relative population of2150conformers and to obtain the thermodynamic functions2151characterizing conformational changes in ionic liquids. The2152procedure considers equilibrium between conformers, conf1 ⇆2153conf2, with equilibrium constant K = [conf2]/[conf1], and takes2154Raman intensities, Iconf1 and Iconf2 (i.e., areas of appropriate2155Raman bands), as proportional to the concentration of2156conformers, Iconf1 = Jconf1[conf1] and Iconf 2 = Jconf2[conf2],2157where Jconf1 and Jconf2 are the Raman scattering cross sections2158corresponding to the vibration of each conformer. From the2159experimental intensity ratio Iconf2/Iconf1, one obtains the relative2160population of conformers as long as one takes into account the2161ratio of Raman scattering cross sections Jconf2/Jconf2, which can2162be estimated by quantum chemistry methods. By recording2163Raman spectra as a function of temperature, one obtains the2164temperature dependence of ln(Iconf2/Iconf1) and the enthalpy of2165conformational change by traditional application of the van’t

2166Hoff equation, = − Δ

( )K H

Rd ln

dT

o

1 , with the assumption that Raman

2167scattering cross sections Jconf1 and Jconf2 do not depend on2168temperature. The ratio Jconf2/Jconf1 must be known in order to2169obtain the entropy of conformational change because the2170intercept of an Arrhenius plot of Raman intensities depends on2171both ΔSo and Jconf2/Jconf1:

= − Δ + Δ +II

HRT

SR

J

Jln ln2

o oconf

conf1

conf2

conf1 2172(6)

2173Umebayashi et al.49 considered Raman bands of [C2C1im]-2174[BF4] and [C2C1im][CF3SO3] in the spectral range shown in2175Figure 21 and obtained ΔHo ≈ 2 kJ mol−1 for the nonplanar ⇆2176planar equilibrium of [C2C1im]

+ (i.e., the nonplanar is slightly2177more stable than the planar conformer). This figure of ΔHo

2178obtained from Raman spectroscopy was practically the same as2179calculated by quantum chemistry methods for the two2180[C2C1im]

+ conformers.49 In the case of [C4C1im]+, Holomb

2181et al.99 considered the intensities of Raman bands at 808, 825,2182883, and 905 cm−1 as characteristics of four conformers2183gauche−gauche, gauche−anti, anti−gauche, and anti-anti,2184respectively, and obtained the relative populations21850.07:0.48:0.17:0.28 in the ionic liquid [C4C1im][BF4].2186Thermodynamic functions of conformational changes of2187[C4C1im]

+ were obtained by Umebayashi et al.188 from the2188temperature-dependence of Raman intensities of bands in the2189spectral range shown in Figure 20. These authors calculated2190ΔHo, ΔSo, and ΔGo by considering an equilibrium between2191only two [C4C1im]

+ conformers, anti ⇆ gauche, for a sequence2192of ionic liquids containing chloride, bromide, and iodide.2193Umebayashi et al.188 found that both the conformers coexist in2194comparable quantities (i.e., ΔGo close to zero), even though2195ΔHo drives the preference for the gauche [C4C1im]

+ conformer2196as the halide anion gets smaller. This spectroscopic approach2197for obtaining thermodynamic properties of conformational2198changes has been used for other cations (e.g., 1-isopropyl-3-2199methylimidazolium,191 N-alkyl-N-methylpyrrolidinium,126,127

2200and [C1C2C2C3N]+).223



Z


2201 Figure 12 showed Raman bands that are good markers of2202 different conformers of [NTf2]

−, so that Raman spectroscopy2203 has been used to quantify thermodynamic properties related to2204 conformational changes of this anion. Assuming the [NTf2]

−

2205 equilibrium as transoid⇆ cisoid, Fujii et al.124 used the 398 and2206 407 cm−1 Raman bands as markers of transoid and cisoid2207 conformers, respectively. These authors obtained ΔHo = 3.5 kJ2208 mol−1 from the Raman spectra of [C2C1im][NTf2] in close2209 agreement with ab initio calculations of the isolated anion, 2.2−2210 3.1 kJ mol−1, depending on the level of theory. Lassegues et2211 al.125 considered the spectral range of 260−360 cm−1 shown in2212 Figure 12 and found that the relative population of transoid2213 [NTf2]

− conformer is 75% in [C2C1im][NTf2] at room2214 temperature but a slightly higher value for the enthalpy, ΔHo

2215 = 4.5 kJ mol−1. Essentially the same ΔHo for conformational2216 change of [NTf2]

− was obtained in N-alkyl-N-methylpyrrolidi-2217 nium ionic liquids.126,127 A distinctive feature of a Raman2218 spectroscopic study performed by Martinelli et al.116 for some2219 nonprotic and protic ionic liquids was that ΔHo of [NTf2]

−

2220 conformational change was also obtained from the intense2221 Raman band at 740 cm−1. This band exhibits a slight2222 asymmetric shape toward the low-wavenumber side, so that2223 two components at 738 and 741 cm−1 resulting from the curve2224 fit have been assigned to cisoid and transoid [NTf2]

−

2225 conformers, respectively. The analysis of the temperature2226 dependence of the band shape gave ΔHo in reasonable2227 agreement with the value obtained from the analysis of the2228 260−360 cm−1 spectral range.116 Plots ln(Itrans/Icis) versus T

−1

2229 for bands that characterize transoid and cisoid conformers also2230 offer spectroscopic signatures of the glass transition of glass-2231 forming ionic liquids. Palumbo et al.259 considered IR bands in2232 the range of 500−650 cm−1 to characterize [NTf2]

− conformers2233 in an ammonium-based ionic liquid: the linear Arrhenius plot2234 above the glass transition temperature (Tg ∼ 210 K) changes2235 slope at Tg and becomes constant indicating that the relative2236 concentration of [NTf2]

− conformers does not change any2237 longer in the glassy phase.2238 Moschovi et al.118 paid attention to appropriate corrections2239 on the raw spectral data prior the analysis of equilibrium by2240 Raman spectroscopy. As already warned by Papatheodorou et2241 al.2 in their review on molten salts, these corrections include2242 accounting for polarization by using the isotropic component of2243 the Raman spectrum, the excitation wavelength dependence,2244 and the Boltzmann population factor. In the liquid phase of2245 protic [C1im][NTf2], which melts at Tm ∼ 325 K, Moschovi et2246 al.118 found significantly higher enthalpy for conformational2247 change of [NTf2]

− when these corrections are taken into2248 account, 8.5 kJmol−1, in comparison with the analysis2249 performed using raw spectra (6.0 kJ mol−1). Taking a sequence2250 of protic ionic liquids [Cnim][NTf2] with increasing length of2251 the alkyl chain (n up to 12), Moschovi et al.199 claimed that the2252 population of [NTf2]

− conformers and the corresponding ΔHo

2253 are signatures of the relative importance of anion−cation2254 interaction by Coulombic forces in comparison with other2255 contributions to intermolecular forces (hydrogen bond, van der2256 Waals, and dispersion due to π−π interaction). These authors2257 proposed that as the alkyl chain of [Cnim]+ increases, leading to2258 segregation of nonpolar and polar domains, then [NTf2]

−

2259 experiences a local environment in which the transoid2260 conformation is favored. The population of cisoid conformer2261 increases with temperature proper to the positive ΔHo value,2262 which however becomes less positive the longer the [Cnim]

+

2263 alkyl chain.199 On the other hand, the way that the transoid

2264over cisoid ratio Itrans/Icis increases with the chain length is2265different for protic and nonprotic ionic liquids: Itrans/Icis reaches2266a plateau when n = 4 in [Cnim][NTf2], whereas it steadily2267increases up to n = 12 in [CnC1im][NTf2].

201

2268In the case of [N(SO2F)2]−, Fujii et al.129 obtained

2269essentially the same enthalpy of conformational change as2270[NTf2]

− from the analysis of the temperature dependence of2271the Raman bands of [C2C1im][N(SO2F)2] within 280−3802272cm−1 (see this spectral range in Figure 14). If the [N(SO2F)2]

−

2273anion is in the ionic liquid based on the N-methyl-N-propyl-2274pyrrolidinium cation, the ΔHo of the [N(SO2F)2]

− conforma-2275tional change is slightly higher, 6.8 kJ mol−1.130

2276Investigating molecular conformations in condensed phase is2277a common issue for molecular dynamics (MD) simulations of2278ionic liquids. Classical MD simulation relies on an assumed2279potential energy function including intermolecular interactions,2280which are normally described by Lennard-Jones potential and2281Coulombic interactions between partial atomic charges and an2282intramolecular force field accounting for molecular vibra-2283tions.260 The validation of a proposed model is accomplished2284by comparing thermodynamic, structural, and dynamical2285properties calculated by MD simulations and experimental2286data.261 Nevertheless, vibrational spectra are the source of data2287for reasonable parameters of force constants needed for the2288intramolecular terms of the potential function. The well-known2289force field of Canongia Lopes and Padua (CL&P)75 has been2290tested taking into account information on conformers2291distribution available from vibrational spectroscopy. Focusing2292here on the intramolecular part of the CL&P model, Vintra, this2293includes bond stretching, r, angle bending, θ, and torsion of2294dihedral angles, ψ:

∑ ∑

∑ ∑

θ θ

ψ

= − + − +

+ −

θ

ψ

=

Vk

r rk

km

2( )

2( )

2[1 ( 1) cos( )]

r

m

m m

intrabonds

eq2

angleseq

2

dihedrals 1

4,

2295(7)

2296where re and θeq are equilibrium bond length and angle. Force2297constants of stretching and bending, kr and kθ, used in MD2298simulations of ionic liquids are usually taken from previous2299force fields (e.g., AMBER and OPLS-AA). It is worth noting2300that due to the harmonic oscillator model for stretching and2301bending modes, the condensed phase effect on vibrational2302frequency shift is not the issue being addressed by this kind of2303model. In fact, proper coupling between intra- and2304intermolecular degrees of freedom is very dependent on2305anharmonicity of the probe oscillator.80,79 On the other hand,2306special attention was given in the CL&P model for kψ,m2307parameters. The dihedral angle terms should give potential2308energy profiles of torsion consistent with quantum chemistry2309calculations and the distribution of conformers resulting from2310analyses of Raman spectra. For instance, in the case of 1-alkyl-3-2311methylimidazolium cations,262 the relative population of gauche2312and anti conformers calculated by MD simulations was2313compared with the analysis of the 600−700 cm−1 spectral2314range, which includes the Raman bands that characterize cation2315conformers.2316Understanding macroscopic properties of ionic liquids on the2317basis of intermolecular interactions is a major aim that2318vibrational spectroscopy shares with other experimental2319techniques and theoretical methods. IR spectroscopy provides2320one of the most characteristic experimental evidence of the X−



AA


2321 H···Y hydrogen bond: the vibrational frequency of the X−H2322 stretching mode shifts to lower wavenumber (red shift) in2323 comparison with the isolated X−H oscillator.263−267 Other2324 spectroscopic features usually accompanying the ν(X−H) red2325 shift on hydrogen bond formation include the enhancement of2326 IR intensity and broadening of this band and the increase of2327 vibrational frequency (blue-shift) of the bending mode δ(X−2328 H). Applying vibrational spectroscopy for studying pure ionic2329 liquids aims insights on the strength and atomic sites involved2330 in hydrogen bonds and, within a more general viewpoint, on2331 the overall nature of anion−cation interactions. Katsyuba et2332 al.48 provided a detailed comparison between experimental2333 frequencies and ab initio calculations for different ion pair2334 configurations of 1-alkyl-3-methylimidazolium cations and2335 [BF4]

− or [PF6]−. Frequencies related to C−H stretching and

2336 out-of-plane vibrations of the imidazolium ring and ν(B−F)2337 orν(P−F) are sensitive to ion pair formation. Katsyuba et al.48

2338 advocate that IR bands belonging to ν(C(4)−H) and ν(C(5)−2339 H) modes (∼3160 cm−1) are separated from the red-shifted2340 ν(C(2)−H) mode (∼3120 cm−1) because the C(2)−H···F2341 interaction is stronger than C(4),(5)−H···F interactions.2342 This assignment of C−H stretching modes,54,119,53,175,177,48

2343 and the alternative interpretation that the doublet of bands in2344 the high-frequency range of 1-alkyl-3-methylimidazolium2345 vibrations is due to Fermi resonance,52,168,169 have already2346 been addressed in section 4.2.1. In a subsequent work,2347 Katsyuba et al.208 preferred to call these bands asν(Caromatic−2348 H) being aware of the role played by Fermi resonance in the2349 spectral range of C−H stretching modes. These authors2350 investigated imidazolium ionic liquids with a terminal −OH2351 group in the side chain [HO-C2C1im]+ (see Figure 25) and2352 different fluorinated anions. Vibrational frequencies of both2353 ν(Caromatic−H) and ν(O−H) decrease, and the bands become2354 broader, as the anion basicity increases.208 It is worth noting2355 that the intensity of the lower frequency ν(Caromatic−H)2356 component (3100−3130 cm−1) increases in relation to the2357 higher frequency component of the doublet (3150−31702358 cm−1) with increasing strength of anion−cation hydrogen2359 bond.208 Taking for granted the assignment of these bands2360 asν(C(2)−H) and ν(C(4),(5)−H), respectively, these findings2361 have been considered signatures of stronger H-bonding2362 through the more acid C(2)−H site of dialkyl-substituted2363 imidazolium cations. Some empirical relationships between2364 vibrational frequency shift, Δν = ν(XHfree) − ν(XHbounded), and2365 the enthalpy of hydrogen bond formation, − ΔHHB, are well-2366 known.265,266 Katsyuba et al.208 estimated −ΔHHB for hydrogen2367 bonding between [HO-C2C1im]+ and fluorinated anions from2368 both ν(Caromatic−H) and ν(O−H) modes, covering a rather2369 large range depending on the anion basicity, 1.6−10.0 kJ mol−1.2370 In contrast, Moschovi et al.199 kept the same [NTf2]

− anion,2371 while investigating how the hydrogen bond strength depends2372 on the alkyl chain length of protic imidazolium cations,2373 [Cnim]+. They found that vibrational frequencies of both2374 ν(Caromatic−H) and ν(N−H) modes of [Cnim]+ are red-shifted2375 as n increases.199 Garaga et al.200 obtained the same results for2376 ν(Caromatic−H) modes in nonprotic [CnC1im][NTf2] ionic2377 liquids. However, these findings were assigned to the2378 intramolecular inductive effect of longer alkyl chain rather2379 than an effect on the strength of the anion−cation hydrogen2380 bond.2381 The works of Katsyuba et al.48,208 and Moschovi et al.199,201

2382 mentioned above illustrate the application of IR and Raman2383 spectroscopies to infer about structural features both in

2384nonpolar and polar domains of ionic liquids. In fact, the2385combined usage of vibrational spectroscopy and quantum2386chemistry calculations covers a large literature of ionic2387liquids,99,117,120,146,148,52,53,202,242,268 part of it already consid-2388ered in the previous sections. On the other hand, in an attempt2389to correlate with melting temperature, Katsyuba et al.48 put2390forward the proposition that it is the anharmonicity of the2391intermolecular anion−cation vibration, rather than the2392interaction energy of an ionic pair, that determines the melting2393temperature of ionic liquids. Their conclusion was based on2394quantum chemistry calculation of the potential energy curve as2395a function of anion−cation distance. Direct evidence of the2396intermolecular anion−cation vibration is the realm of far-IR and2397low-frequency Raman spectroscopies to be discussed in the2398next section.2399Some works went further on quantum chemistry calculations2400of vibrational spectra by considering clusters of ions instead of a2401single ionic pair. Dong et al.269 calculated the IR spectrum of2402[C2C1im][BF4] at the DFT/B3LYP level of theory for clusters2403of ions ([C2C1im][BF4])n with increasing size, n = 2, 3, 4, and24045. The overall appearance of theoretical IR spectrum (peak2405positions, band shapes, and relative intensities, including the2406FIR range) gets closer to the experimental spectrum of liquid2407[C2C1im][BF4] with increasing cluster size.

269 Thus, there is an2408incipient hydrogen bond network making the five ion pairs2409cluster a microscopic model of bulk [C2C1im][BF4]. The need2410for considering ion clusters in order to the full appearance of2411vibrational spectra being recovered by ab initio calculations is2412particularly demanding for protic ionic liquids. Bodo et al.230

2413calculated the Raman spectrum of methylammonium nitrate by2414considering clusters whose structures were first generated by2415classical MD simulations and then optimized at the DFT/2416B3LYP level of theory. It is worth remembering that such2417calculations of clusters still concern harmonic vibrational2418frequencies, so that scaling factors are needed for better2419agreement between calculation and experiment. Nevertheless,2420the Raman spectra calculated for clusters ([C1NH3][NO3])n, n2421= 2, 4, 6, and 8, exhibit distribution of frequencies that become2422broader as the cluster size increases.230 In a subsequent work,2423Bodo et al.231 extended the calculations of Raman spectra for2424the series ethyl-, propyl-, and butylammonium nitrate. Thus,2425the ab initio calculation of reasonable structures of clusters of2426ions captures some key features of ionic interactions that are2427important for the calculation of vibrational frequencies, in2428particular the strength distribution of anion−cation hydrogen2429bonds.2430The nitrate anion can be used as an example of taking an2431anion vibration, νs(NO3), rather than C−H, N−H, or O−H2432stretching modes, as a probe of the difference in width in the2433distribution of hydrogen bond strengths between protic and2434 f34nonprotic ionic liquids. Figure 34 shows the νs(NO3) Raman2435band in [C4C1im][NO3] and [C3NH3][NO3]. Stronger anion−2436cation interaction in [C3NH3][NO3] in comparison with2437[C4C1im][NO3] implies +3.5 cm−1 shift of νs(NO3) and a2438remarkable broadening and asymmetry of the band because of2439the disturbed hydrogen bond network in the protic ionic liquid.2440Full width at half height (fwhh) of the νs(NO3) Raman band in2441[C4C1im][NO3] and [C3NH3][NO3] are 5.0 and 11.0 cm−1,2442respectively. For comparison purposes, fwhh is ∼15.0 cm−1 in2443molten NaNO3 at 600 K and ∼6.0 cm−1 in [NO3]

− aqueous2444solution at room temperature.270 DFT calculation performed2445by Faria et al.235 showed that νs(NO3) vibrational frequencies



AB


2446 of anions in a cluster ([C3NH3][NO3])4 indeed cover the2447 spectral range of bandwidth of the experimental spectrum.2448 Ionic liquids whose vibrational spectra have been interpreted2449 with the help of ab initio MD simulations include those based2450 on 1-alkyl-3-methylimidazolium with cyanate anions or2451 acetate,33 alkylammonium with nitrate236 or bromide,89 and2452 choline with carboxylates170 or amino acid anions.227 Ab initio2453 MD simulations of ionic liquids go further than calculations of2454 minimum energy structures by taking into account the liquid2455 phase dynamics and anharmonicity of vibrations. Wendler et2456 al.82 found that the calculation of liquid phase IR spectra of2457 imidazolium ionic liquids exhibit a spectral pattern significantly2458 different from the single ion pair calculation. The calculation of2459 ion pair strongly favors the anion pointing toward the C(2)−H2460 bond of the imidazolium ring, whereas ab initio MD simulation2461 explores a distribution of anions around cations. Extreme2462 examples of this situation is [C2C1im][CH3COO], for which2463 quantum chemistry calculation of a single pair leads to the2464 formation of hydrogen-bonded carbene−acetic acid with an IR2465 band at 2167 cm−1, which is however not seen in the2466 experimental spectrum nor in the spectrum calculated by ab2467 initio MD simulation of the liquid phase.33 Wendler et al.82

2468 found that including eight ionic pairs in the calculations was2469 enough to capture the most important dynamics on the point2470 of view of vibrational spectroscopy (i.e., the short-time2471 dynamics of ions rattling inside the cages made by the2472 neighbors).2473 Bodo et al.236 calculated the vibrational frequencies of2474 [C2NH3][NO3] by ab initio MD simulation of clusters of 6 or2475 24 ionic pairs. The experimental Raman spectrum of [C2NH3]-2476 [NO3] was compared to the power spectrum obtained by2477 Fourier transforming the autocorrelation function of atomic2478 velocities so that Raman intensities were not reproduced in the2479 fingerprint region. In this region there are different kinds of2480 vibrations with very different Raman activities, so that they2481 should be properly weighted by polarizability fluctuations. In2482 the case of choline alanine, Campetella et al.227 found2483 reasonable agreement between experiment and calculation in2484 the fingerprint region of the IR spectrum since they calculated2485 the Fourier transform of the autocorrelation function of2486 molecular dipole moments. On the other hand, the power2487 spectrum calculated for [C2NH3][NO3] reproduces the spectral2488 pattern in the region of C−H and N−H stretching modes (see

2489Figure 5 in ref 236). In other words, the high-frequency range2490of the Raman spectrum of [C2NH3][NO3] essentially reflects2491the density of vibrational states: ν(N−H) contributes with a2492broad background band between 3000−3250 cm−1 proper to2493hydrogen bonding of various strengths, while ν(C−H) give2494relatively sharp peaks on this background.236

5. VIBRATIONAL SPECTROSCOPY OF PURE IONIC2495LIQUIDS IN THE LOW-FREQUENCY RANGE2496In the previous section, insights on anion−cation interactions2497relied on the analysis of vibrational frequency shifts and2498changes in intensities and band shapes in mid-infrared and2499Raman spectra. Attempt to obtain data more directly related to2500intermolecular vibrations is the realm of far-infrared (FIR)2501spectroscopy, 10 < ω < 400 cm−1.271 It is natural that a smaller2502number of studies on ionic liquids has been reported using FIR2503spectroscopy because of more demanding components (e.g.,2504beamsplitter and detector for this spectral range).272 Never-2505theless, interesting results have been obtained by FIR2506spectroscopy, in particular, evidence in the spectral range2507below 150 cm−1 of anion−cation hydrogen bonds. In the case2508of Raman spectroscopy, the range ω < 150 cm−1 is usually2509called the low-frequency Raman spectrum. Raman spectrom-2510eters with double or triple monochromator, or else single stage2511spectrometer with special bandpass filters, are needed to2512achieve the frequency range close to the exciting laser line.2513However, the intermolecular vibrational dynamics is hidden2514under the very intense quasi-elastic scattering which dominates2515the low-frequency Raman spectrum of a liquid. On the other2516hand, the quasi-elastic intensity decreases as the liquid is2517cooled, so that intermolecular vibrations become apparent in2518Raman spectra of supercooled or glassy phases of ionic liquids.2519The revision of vibrational spectroscopy studies on crystal-2520lization and glass transition of ionic liquids is the issue of the2521next section.2522Fumino et al.273 reported the first FIR spectroscopic study2523showing evidence of +C−H···A− hydrogen bonding for a series2524of ionic liquids with the [C2C1im]

+ cation and different anions.2525An overview of similarities and differences in FIR spectra of2526 f35aprotic and protic ionic liquids is provided by Figure 35 taken2527from a subsequent work by Fumino et al.274 (We recommend2528the review published by Fumino and Ludwig275 focusing on2529their FIR spectroscopy studies of ionic liquids.) Assignments of2530the featureless bands in FIR spectra demand quantum2531chemistry calculations for clusters of ions in order to2532disentangle the intermolecular anion−cation vibrations from2533intramolecular modes eventually covering the same frequency2534range.2535Low-frequency vibrations expected for anion−cation hydro-2536gen bond are the stretching of one ion against the other, ν(C−2537H···A), and the bending, δ(C−H···A). The ν(C−H···A) in2538imidazolium ionic liquids has been assigned to a FIR band2539around 100 cm−1, depending on the anion. Stronger anion−2540cation hydrogen bonding, according to the magnitude of red-2541shift of high-frequency C−H stretching modes (see previous2542section), has the counterpart in the FIR spectrum as the2543frequency of the intermolecular vibration shifts to higher2544wavenumber.274 Curve fit by Voigt functions showed that the2545low-frequency tail of this broad FIR band overlaps the band2546assigned to δ(C−H···A) at 50−60 cm−1.275 On the other hand,2547Buffeteau et al.15 refrained from assigning the FIR band of2548imidazolium ionic liquids to a specific anion−cation vibration.2549Instead, these authors called the spectral feature in the FIR

Figure 34. Raman spectra in the range of the νs(NO3) mode of[C3NH3][NO3] at room temperature (red) and the molten phase of[C4C1im][NO3] at 313 K (black). Intensities of Raman spectra havebeen normalized.



AC


2550 spectra as the density of states by analogy to low-frequency2551 spectra of glass forming liquids (see eq 10 below). Schwenzer et2552 al.144 assigned anion−cation hydrogen bond vibrations in FIR2553 spectra of [C4C1im][CF3SO3] and [C4C1im][HSO4] to bands2554 observed at 80 and 103 cm−1, respectively. A distinctive feature2555 in the FIR spectrum of [C4C1im][HSO4] are additional bands2556 that Schwenzer et al.,144 with the support of MD simulations2557 and NMR spectroscopy, assigned to hydrogen-bonded2558 [HSO4]

− anions. The occurrence of anion−anion hydrogen2559 bonding in [HSO4]

− ionic liquids is in line with the proposition2560 of Ribeiro156 based on the analysis of high-frequency Raman2561 bands of [HSO4]

−.2562 Fumino et al.276,277 argued that strong hydrogen bonds in the2563 protic ionic liquids alkylammonium nitrate imply higher2564 frequencies for anion−cation intermolecular vibrations. The2565 similarity between FIR spectra of alkylammonium nitrate and2566 water led Fumino et al.276 to the proposition of a three-2567 dimensional hydrogen bond network in these protic ionic2568 liquids. In the case of imidazolium ionic liquids, Fumino et2569 al.193 proposed that [C2C1im][NTf2] is less viscous than2570 [C2C1C1im][NTf2], even though the former has the C(2)−H2571 site available for stronger hydrogen bond because hydrogen2572 bonds act like defects perturbing the charge symmetry of the2573 Coulomb network. However, it is yet an unsettled issue why2574 methylation of the most acidic hydrogen of the imidazolium2575 ring results in more viscous ionic liquid. It has also been2576 proposed that methylation of the C(2) site of the imidazolium2577 ring restricts ionic mobility because of loss of variation of2578 anion−cation arrangements,192 higher potential energy bar-2579 rier,194 or reduced free volume.195

2580 In order to use the vibrational frequency shift seen in Figure2581 35 as a signature of the strength of +C−H···A− hydrogen bond,2582 one must rule out that it is not a trivial effect of changing the

2583reduced mass among different ion pairs. Yamada et al.278 found2584that the maximum of the FIR band for [CnC1im]X, n = 3, 4,2585and 6, does not depend on the alkyl chain length and follows2586the trend 72.5, 92, and 122 cm−1 for the halide anions I−, Br−,2587and Cl−, respectively. Yamada et al.278 argued that these2588frequencies are on the same proportion of the inverse of the2589square root of reduced mass for the vibration of anion and2590imidazolium ring. However, Fumino et al.274,279−281 compared2591the position of the FIR band for a large set of ionic liquids and2592showed that only the mass effect does not account for the2593vibrational frequency shift. These authors performed quantum2594chemistry calculations for clusters of ionic pairs and correlated2595FIR frequencies with binding energies, thus supporting the2596interpretation of frequency shifts as the result of different2597anion−cation force constants.273,274,281−283,280,277,284 Further-2598more, Fumino et al.282 showed that the vibrational frequency of2599the main spectral feature in the FIR spectrum correlates with2600the enthalpy of vaporization of the ionic liquid.2601A step forward along the combined usage of FIR spectros-2602copy and DFT calculations was the attempt at separating the2603relative contributions of hydrogen bonding and dispersion2604forces out of the total energy, which is predominantly2605Coulombic energy.284−286 Fumino et al.284 measured FIR2606spectra as a function of temperature (303−353 K) for protic2607ammonium ionic liquids with appropriately chosen alkyl chain2608lengths and anions in order to play with the relative strengths of2609hydrogen bonding and dispersion forces. Bands assigned to2610δ(C−H···A) and ν(C−H···A) were seen in the FIR spectrum of2611[C6C6C6NH][CF3SO3] as two resolved peaks around 70 and2612130 cm−1, respectively, as temperature decreases. When2613temperature increases, the FIR spectrum is dominated by a2614single broad band with maximum at 100 cm−1, which has been2615assigned to the anion interacting with the cation alkyl chains2616(i.e., an ion pair determined by dispersion forces). The2617temperature dependence of relative intensities of FIR bands2618assigned to local arrangements dominated by hydrogen bonds2619and dispersion forces allowed for a van’t Hoff analysis of the2620equilibrium between these two kinds of ion pairs.284 The van’t2621Hoff plot indicated that the hydrogen-bonded ion pair is2622favored by ∼34 kJ mol−1 over the dispersion-interaction2623dominated structure. This experimental finding pointed out the2624need for proper consideration of dispersion corrected methods2625in DFT calculations of ion pair energies.284−286

2626Fumino et al.193 and Buffeteau et al.287 addressed the effect of2627methylation of the C(2)−H position of the imidazolium ring by2628comparing the FIR spectra of [C2C1im][NTf2] and2629[C2C1C1im][NTf2]. FIR bands of [C4C1im][NTf2] and2630[C4C1C1im][NTf2] were found respectively at 83.5 and 79.02631cm−1 by Fumino et al.193 and 82.5 and 73 cm−1 by Buffeteau et2632al.287 Analogous effect of methylation of the imidazolium ring is2633seen in the FIR spectra of [C4C1im][BF4] and [C4C1C1im]-2634[BF4] shown in Figure 35. In addition to the red shift of2635vibrational frequency, Fumino et al.193 found that the intensity2636of the FIR band decreases because of weakening of the anion−2637cation hydrogen bonding as the stronger binding site C(2)−H is2638switched off in [C2C1C1im][NTf2]. In contrast, Buffeteau et2639al.287 have not found significant difference in the FIR intensity2640between [C4C1im][NTf2] and [C4C1C1im][NTf2] after proper2641normalization by the intensities of high-frequency intra-2642molecular bands. Buffeteau et al.287 also compared FIR spectra2643of [C4C1im][NTf2] and [C4C1C1C1N][NTf2] and they found2644essentially the same intensity and vibrational frequency for both2645the systems. Therefore, Buffeteau et al.287 reached the opposite

Figure 35. FIR spectra of some aprotic ionic liquids based onimidazolium cations with [BF4]

− or [NO3]− and the protic ionic liquid

propylammonium nitrate. The arrow on the top of the low-frequencyfeature indicates the band assigned to the stretching mode of anion−cation hydrogen-bonded. Bands in the gray area belong to intra-molecular normal modes. Reproduced with permission from ref 274.Copyright 2009 PCCP Owner Societies.



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2646 conclusion, claiming that the characteristic broad band in FIR2647 spectra should not be assigned exclusively to stretching mode of2648 the anion−cation hydrogen bond. These authors preferred2649 instead assigning the density of states revealed by the FIR2650 spectrum as a kind of envelope over the solidlike lattice2651 vibrations. This physical picture is alike the one resulting from2652 low-frequency Raman spectroscopy studies of phase transitions2653 of ionic liquids to be discussed in the next section.2654 Wulf et al.288 compared FIR to low-frequency Raman2655 spectra, and also Terahertz (THz) spectra, for the same set2656 of ionic liquids ([C2C1im][SCN], [C2C1im][N(CN)2],2657 [C2C1im][C2SO4], and [C2C1im][NTf2]) previously studied2658 by Fumino et al.273 It has been found that the spectral pattern2659 and optical constants obtained from FIR spectra of imidazolium2660 ionic liquids agree with data obtained by THz spectrosco-2661 py.278,283,287,288 However, the experimental setup of Wulf et2662 al.288 did not allow obtaining reliable Raman spectra below 1002663 cm−1 so that the comparison between FIR and low-frequency2664 Raman was restricted to intramolecular bending modes of2665 cations or anions observed in the range of 150−300 cm−1.2666 Low-frequency Raman spectroscopy investigations of inter-2667 molecular dynamics encompass a large literature with a special2668 focus on glass-forming liquids289−293 and among them several2669 (high temperature) molten salts (e.g., alkali halides,294−296

2670 ZnCl2,297,298 BiCl3,

298,299 mixtures ZnCl2−AlCl3,300 etc.).1,2 An

2671 early study of the low-frequency range was reported by Ribeiro2672 et al.301 for the low-temperature molten salt tetra(n-butyl)-2673 ammonium croconate, [C4C4C4C4N]2[C5O5]·4H2O, which is a2674 glass-forming liquid with Tg = 293 K. The first low-frequency2675 Raman spectroscopy studies concerning nowadays typical ionic2676 liquids were reported by Ribeiro302 for [C4C1im][PF6] as a2677 function of temperature and by Iwata et al.8 for [C4C1im][BF4],2678 [C4C1im][PF6], [C6C1im][PF6], and [C8C1im][PF6] at room2679 temperature.2680 In these works, the low-frequency Raman spectrum is2681 conveniently reported after discounting a thermal population2682 factor from the raw experimental spectrum, I(ω), in the so-2683 called susceptibility representation, χ″(ω):

χ ω ωω

″ =+

In

( )( )

( ) 12684 (8)

2685 where

ω =−ωℏn

e( )

11kT/

2686 (9)

2687 The motivation behind the reduction of the low-frequency2688 Raman spectrum is the relation between I(ω) and the density2689 of vibrational states g(ω):303,304

ω ωω

ω ω= +I

nC g( )

( ) 1( ) ( )

2690 (10)

2691 where C(ω) is the light-vibration coupling. The spectral pattern2692 of g(ω) corresponds to the power spectrum alluded for in2693 section 3, and C(ω) accounts for how the low-frequency2694 vibrations modulate the material polarizability.

f36 2695 Figure 36 shows the raw low-frequency Raman spectrum of2696 [C4C1im][NTf2] at room temperature, and the resulting χ″(ω)2697 (inset in the top panel). The relatively sharp bands above 1002698 cm−1 belongs to intramolecular modes of the [NTf2]

− anion2699 (see Figure 11 and Table 2). If the frequency-dependence of2700 C(ω) were linear then χ″(ω) would give directly the density of2701 vibrational states. The actual frequency dependence of C(ω) is

2702experimentally accessible by comparing the Raman spectrum2703with g(ω) obtained by neutron scattering spectroscopy.298,305

2704Another common representation of the low-frequency2705spectrum is the reduced Raman intensity, Ired(ω) = χ″(ω)/ω.2706Since the factor [n(ω)+1]−1 depends linearly with ω at low-2707frequencies, Ired(ω) is similar to the experimental I(ω), whereas2708the low-frequency bands are slightly shifted to higher2709frequencies in the χ″(ω) representation.306 In order to2710highlight that the low-frequency bands are not artifact of the2711χ″(ω) representation, the bottom panel of Figure 36 shows the2712raw Raman spectra, I(ω), of [C4C1im][NTf2] as a function of2713temperature. Low-frequency Raman band assigned to inter-2714molecular vibration is clearly seen in the raw spectrum of2715[C4C1im][NTf2] as the glass transition temperature, Tg = 1812716K, is achieved from above. The quasi-elastic component is due2717to anharmonicity and fast relaxation processes, so that its2718intensity decreases significantly with decreasing temper-2719ature.290−292,298,300,307−310 In fact, the plot of the quasi-elastic2720intensity versus temperature exhibits break of slope at the glass-2721transition temperature for several ionic liquids.302,311,312

2722The optical Kerr effect (OKE) spectrum is equivalent to the2723depolarized Raman spectrum, the results of these techniques2724differing by the population factor.313 It has been found that the2725depolarization ratio is essentially constant at 0.75 in the low-2726frequency range of the Raman spectrum of [C4C1im][PF6].

302

2727In other words, essentially the same band shape is obtained in2728the low-frequency range for polarized, depolarized, or without2729polarization selection of the scattered radiation. It is worth2730noting that the frequency-dependent depolarization ratio has2731been found in Raman spectra of high temperature molten salts,2732especially when complex molecular-like structures occur in the2733melt.298,299,314 In the case of ionic liquids, the intermolecular2734vibrational dynamics has been much more investigated by2735OKE315−320 than low-frequency Raman spectroscopy. Both2736OKE and Raman spectroscopies probe polarizability fluctua-

Figure 36. Low-frequency Raman spectra of [C4C1im][NTf2]. Thetop panel shows the raw spectrum, I(ω), and the inset shows thesusceptibility representation, χ″(ω) (eq 8), at room temperature. Thebottom panel shows I(ω) as a function of temperature. Note differentintensity scales between top and bottom panels.



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2737 tions resulting from the molecular dynamics of the liquid, so2738 that the spectral pattern of χ″(ω) is the same as the OKE2739 spectra of ionic liquids. The OKE spectrum encompasses a2740 temperature-dependent relaxation contribution within the GHz2741 range (the structural or α-relaxation) not accessible in the2742 Raman spectrum and the THz range of intermolecular2743 vibrations common to Raman spectroscopy (1 THz = 33.32744 cm−1).321 OKE spectroscopy is a time-resolved spectroscopy2745 technique, which is beyond the scope of this review, and the2746 reader is recommended previous review focusing on OKE2747 spectroscopy of ionic liquids.322

2748 Low-frequency χ″(ω) and OKE spectra, like FIR spectra of2749 ionic liquids, demand curve fit in order to grasp any trend of2750 the components when changing ionic species or thermody-2751 namic state. Different numbers and functional forms of bands2752 (e.g., damped harmonic oscillator) antisymmetrized Gaussian2753 and log-normal functions have been used in a curve fitting the2754 low-frequency range of OKE and Raman spectra.315−320,322

2755 Iwata et al.8 considered two components in the fit of low-2756 frequency Raman spectra of imidazolium ionic liquids, but most2757 probably the broad spectral pattern encompasses three2758 components around 20, 60, and 90 cm−1 assigned to2759 intermolecular vibrations.312,322 (An example of this curve fit2760 is shown below in Figure 43 for the χ″(ω) spectrum of2761 [C2C1im][NTf2].) In line with assignments in aromatic2762 molecular liquids,321 the 90 cm−1 component has been assigned2763 to librational motion (i.e., hindered rotation) of the

f37 2764 imidazolium ring. The left panel of Figure 37 illustrates this2765 point by comparing χ″(ω) of [C4C1im][NTf2] and2766 [C4C1C1C1N][NTf2] at room temperature. When the aromatic2767 cation is replaced by a nonaromatic one, the χ″(ω) spectrum is2768 less broad because of the missing 90 cm−1 component, and the2769 intramolecular [NTf2]

− mode at 120 cm−1 becomes a well-2770 defined band in the [C4C1C1C1N][NTf2] spectrum. It is also2771 clear from the asymmetric band shape of the χ″(ω) spectrum of2772 [C4C1C1C1N][NTf2] that there is at least two components at2773 ∼20 and ∼60 cm−1, and these seem to be common spectral2774 features whatever the combination of anion and cation. In FIR2775 spectra, Buffeteau et al.287 found the same intensity of the2776 intermolecular band at ∼84 cm−1 for [C4C1im][NTf2] and2777 [C4C1C1C1N][NTf2]. In contrast, when Raman intensities are2778 normalized by the high-frequency bands of [NTf2]

−, rather2779 than normalization by the low-frequency band as in Figure 37,2780 low-frequency Raman spectra are more intense for imidazolium2781 than ammonium ionic liquids,312 as one expects from larger2782 fluctuation of polarizability in a system containing an aromatic2783 ring.

2784The anion−cation hydrogen bond vibrations are not2785expected to give as intense signals in Raman as in FIR spectra.2786The distinctive features of low-frequency Raman spectra are2787made clearer in the right panel of Figure 37 showing χ″(ω)2788spectra for [C4C1im][BF4] and [C3NH3][NO3], which are two2789systems corresponding to the extreme cases of FIR spectra2790shown in Figure 35. The significant frequency shift in the FIR2791spectrum when moving from the nonprotic [C4C1im][BF4] to2792the protic [C3NH3][NO3] (see Figure 35) is not manifest in2793the low-frequency Raman spectra. The χ″(ω) spectra of2794[C4C1im][NTf2] and [C4C1im][BF4] (black lines in the right2795and left panels of Figure 37) are similar, except for the [NTf2]

−

2796normal mode at 120 cm−1, which is evidently absent in the2797spectrum of the latter. On the other hand, the spectral patterns2798of χ″(ω) for the two ammonium ionic liquids shown by red2799lines in the right and left panels of Figure 37 differ in relative2800intensities of the components encompassing the band shape.2801The intense component with maximum at ∼70 cm−1 in the2802χ″(ω) spectrum of [C3NH3][NO3] has also been found by2803Sonnleitner et al.323 in OKE spectra of ethyl- and2804propylammonium nitrate. This spectral feature is close to the2805FIR band at ∼60 cm−1, which Fumino et al.276 assigned to2806bending of the anion−cation hydrogen bond. Kruger et al.324

2807followed the Fumino et al.276 assignment for the corresponding2808feature observed at ∼40 cm−1 in the THz spectrum of2809ethylammonium nitrate. However, Sonnleitner et al.323 assigned2810the band at ∼70 cm−1 to [NO3]

− libration (i.e., restricted out-2811of-plane rotation about the C2 axes of [NO3]

−) because OKE2812spectroscopy is primarily sensitive to rotational motions and to2813the large polarizability anisotropy of the [NO3]

− anion. It is2814worth mentioning that the additional band assigned to libration2815of aromatic ring is also found at 60 cm−1 in the OKE spectrum2816when a phenyl group is attached to the longer chain of the2817imidazolium cation.322 The long tail extending up to 250 cm−1

2818in the χ″(ω) spectrum of [C3NH3][NO3] is absent in the2819[C4C1C1C1N][NTf2] spectrum (compare red lines in right and2820left panels of Figure 37). On the basis of curve fitting,2821Sonnleitner et al.323 proposed that the high-frequency tail in2822OKE spectra of [C2NH3][NO3] and [C3NH3][NO3] includes2823unresolved bands due to hydrogen bond stretching and cation2824librational motions. It is worth mentioning, however, early2825Raman spectroscopic studies on molten alkali nitrates in which2826broad bands assigned to [NO3]

− libration have been found at2827relatively high wavenumber (maxima at ∼90, ∼115, and ∼1402828cm−1 in molten KNO3, NaNO3, and LiNO3, respectively).

325

2829Thus, the high-frequency tail in the χ″(ω) spectrum of2830[C3NH3][NO3] might also manifest an inhomogeneous

Figure 37. Low-frequency Raman spectra in the susceptibility representation of ionic liquids at room temperature. Left panel: [C4C1im][NTf2](black) and [C4C1C1C1N][NTf2] (red); right panel: [C4C1im][BF4] (black) and [C3NH3][NO3] (red). Intensities of χ″(ω) spectra have beennormalized.



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2831 distribution of librations in this protic ionic liquid, being the2832 low-frequency counterpart of the previous discussion (see2833 Figure 34) concerning the Raman band shape of νs(NO3).2834 Despite similarities in positions of the components after2835 curve fit of these featureless bands, low-frequency Raman and2836 IR active intermolecular modes are not necessarily the same.2837 Within the context of large literature about intermolecular2838 vibrational dynamics of glass-forming liquids, the band at ∼202839 cm−1 seen in low-frequency Raman spectra of the glassy phase2840 of ionic liquids (see Figure 36) is the so-called boson2841 peak.291−293,298−300,306,308−310 It has been pointed out that2842 the boson peak vibrations remain in the liquid state even2843 though the peak is hidden under the intense quasi-elastic2844 scattering of the low-frequency Raman spectrum.326−329 In an2845 OKE spectroscopy study of 1-ethyl-3-methylimidazolium2846 tosylate, Li et al.330 assigned to the boson peak dynamics the2847 oscillatory component with period of ∼2.0 ps (i.e., ∼17 cm−1)2848 seen in the time domain data a few degrees above the glass2849 transition temperature (Tg = 201 K). The partial nature of2850 longitudinal and transverse acoustic modes of the intermo-2851 lecular dynamics in the spatial range of wavevectors 0.1 < k <2852 1.0 Å−1 and frequency range 0 < ω < 100 cm−1 in viscous glass-2853 forming liquids has been established in many studies by2854 inelastic X-ray scattering spectroscopy and MD simula-2855 tions.331−334 Although the exact origin of the boson peak is2856 yet a lively debated issue, several studies have been pointing to2857 the nature of transverse acoustic vibrations of high wave-vectors2858 (i.e., soundlike modes of short wavelength).335−338 It is worth2859 stressing that projecting into soundlike modes recovers only2860 part of the intermolecular vibrations of liquids because the2861 other part is random phase motion66,67,339,340 and eventually2862 molecular-like normal modes in high temperature molten salts2863 in which complexes survive for a relatively long time.1,2

2864 The assignment of FIR spectra by Fumino et al.275 focused2865 on local motions related to stretching and bending modes of2866 ion pairs, whereas assignment of low-frequency Raman and2867 OKE spectra emphasized librational and cage-rattling motions2868 or the many body nature of the intermolecular dynamics.2869 Shirota322 considered the average frequency (i.e., the first2870 moment M1 = ∫ωI(ω)dω/∫ I(ω) dω) as a single quantitative2871 parameter for the whole low-frequency range of the OKE2872 spectrum. It has been found that M1 obtained from OKE2873 spectrum of nonaromatic ionic liquids correlates with (γ/ρ)1/2,2874 where γ is the surface tension and ρ is the density.322 The2875 correlation between M1 and (γ/ρ)1/2 is analogous to a simple2876 harmonic oscillator result for the dependence of vibrational2877 frequency with force constant and reduced mass (k/μ)1/2,2878 however, involving the surface tension as a bulk property2879 related to the strength of intermolecular forces.2880 Comparison between Figures 35 and 37 indicates that the2881 overall spectral pattern is different for FIR and low-frequency2882 Raman (and OKE) spectra for a given ionic liquid. Molecular2883 dynamics simulations have shown that the density of vibrational2884 states resembles low-frequency Raman and OKE spectra of2885 ionic liquids.341,342 The group of Balasubramanian attempted a2886 theoretical approach for understanding the low-frequency2887 vibrations of nonprotic imidazolium88 and protic ammo-2888 nium89,343 ionic liquids. These authors used quantum chemistry2889 calculations of ion pairs and computer simulations of liquid and2890 crystalline phases by classical (force field based) and ab initio2891 MD simulations. The vibrational analysis by MD simulations2892 has been performed by a normal-mode analysis after2893 diagonalization of the Hessian matrix obtained from quenched

2894configurations and also by Fourier transforming the time2895correlation function of atomic velocities. The density of2896vibrational states resulting from MD simulations exhibits the2897trend of higher frequency shift with increasing strength of2898anion−cation interactions in imidazolium88 or the number of2899hydrogen-bonding sites in ammonium89 ionic liquids. In the2900case of protic trialkylammonium triflate ionic liquids, N−H···O2901stretching modes calculated around 160 cm−1 agree with FIR2902spectra.343 However, the peaks in the theoretical density of2903vibrational states are found at significantly lower frequencies2904than in the experimental FIR spectra.88,343 The density of states2905calculated by MD simulation is more like the low-frequency2906Raman than the FIR spectrum. In other words, even though2907polarizability fluctuations are not being taken into account in2908the density of states, it resembles the susceptibility2909representation of low-frequency Raman spectra of ionic liquids.2910Nevertheless, the calculations88,89,343 point out that the2911vibrational frequency of intermolecular dynamics is dominated2912by short-range interactions, and the peak position is indeed2913modulated by the strength of anion−cation hydrogen bond.275

2914However, the actual mode composition involves a large number2915of atoms rather than being localized only in the C−H···A2916moiety.88 Furthermore, the density of states calculated by MD2917simulation of the liquid phase of [C4C1im][PF6] resembles an2918envelope over the corresponding density of states of the2919crystalline phase,88 a finding in line with the previous2920proposition of Buffeteau et al.287 in their FIR study of ionic2921liquids. At this point we reach the application of vibrational2922spectroscopy in studying phase transitions of ionic liquids, an2923issue to be discussed in the next section.2924The MD simulations of Balasubramanian et al.88,89,343 aimed2925the analysis of atomic displacements of low-frequency2926vibrations by the calculation of time correlation function of2927velocities, rather than time correlation functions of dipole2928moment or polarizability fluctuation. In contrast, Hu et al.344

2929calculated time correlation function of polarizability by MD2930simulation of 1-methoxyethylpyridinium dicyanamide aiming a2931direct comparison with a previous OKE spectroscopy study by2932Shirota and Castner.345 The calculations of Hu et al.344

2933considered polarizability fluctuations arising from reorienta-2934tional dynamics and interaction-induced mechanism according2935to the dipole−induced dipole (DID) model in analogy with the2936large body of literature on MD simulations of properties of2937spectroscopic interest in molecular liquids.86,103,346 It is worth2938noting that depolarization ratio as low as 0.1 in Raman spectra2939of high temperature molten salts1,347 prompted Madden et2940al.100,101 to propose other interaction-induced mechanisms2941resulting in polarizability fluctuation (e.g., short-range overlap,2942field, and field gradient). In the case of ionic liquids, including2943reorientation and DID mechanism is consistent with the2944depolarization ratio of 0.75 found in low-frequency Raman2945spectra.302 Reorientational and DID mechanisms in low-viscous2946molecular liquids account for long- and short-time parts,2947respectively, of the time correlation function of polarizability2948fluctuation.86 In other words, DID mechanism is expected to2949manifest higher-frequency dynamics of molecules rattling in the2950cage of neighbors. It is known that a clear time or frequency2951separation between reorientational and interaction-induced2952dynamics might not be strictly valid even in molecular liquids2953(e.g., CS2).

86,103 Accordingly, Hu et al.344 found that2954interaction-induced contribution extends to a relatively long-2955time of a hundred picoseconds because the cage effect and2956nondiffusive dynamics take longer in ionic liquids. On the other



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2957 hand, cross-correlation between different mechanisms contri-2958 buting to dipole and polarizability fluctuations and strong2959 coupling between reorientational and translational motions lead2960 to the warning put forward by some authors323,320 that,2961 irrespective of number and functional forms chosen in curve2962 fitting of FIR, low-frequency Raman and OKE spectra of ionic2963 liquids, each component might not necessarily be related in a2964 one-to-one basis to specific intermolecular dynamics.

6. VIBRATIONAL SPECTROSCOPY FOR STUDYING2965 IONIC LIQUID PHASE TRANSITIONS2966 Ionic liquids exhibit a rich phenomenology of different phases2967 (e.g., crystalline, glassy, and supercooled or superpressed2968 liquid) which the same system may achieve depending on the2969 rate that the thermodynamic state is changed. Vibrational2970 spectroscopy has been instrumental in revealing structural2971 modifications accompanying phase transitions. Most of these2972 studies were performed with variable temperature at atmos-2973 pheric pressure or variable pressure at room temperature. The2974 temperature and pressure dependence of vibrational frequen-2975 cies, band shapes, and relative intensities in Raman and IR2976 spectra provide insights on ion conformation or the local2977 environment experienced by the ions in different phases.2978 Early vibrational spectroscopic studies took advantage of the2979 crystalline structure to infer about liquid phase structure before2980 X-ray and neutron scattering experiments were used to study2981 directly ionic liquids in the normal liquid phase.348−351 In this2982 context, Dupont352 considered the C−H stretching modes in2983 IR spectra of [C4C1im]+ based ionic liquids with different2984 anions and emphasized structural similarities between solid and2985 liquid states. The group of Hamaguchi181,183−185 compared2986 Raman spectra of different crystal polymorphs and liquid phase2987 of [C4C1im]Cl and [C4C1im]Br, concluding that the local2988 structure in the liquid is similar to the one in the solid despite2989 an ionic conformation being eventually selected in the

f38 2990 crystalline phase. We illustrate this point in Figure 38 with

2991 Raman spectra of [C4C1im][CF3SO3] in liquid and low2992 temperature crystalline phases. There is indeed reasonable2993 match between crystal and liquid spectra, the gauche2994 conformation of [C4C1im]+ being selected in the crystal2995 according to the occurrence of a single Raman band at 5992996 cm−1 (see inset of Figure 38).

2997Ionic liquid mesophases (i.e., liquid crystal) are formed with2998increasing length of the alkyl chain of cation or anion.350,353,354

2999Among the first vibrational spectroscopy studies concerning3000ionic liquid phase transitions, De Roche et al.355 used Raman3001spectroscopy to analyze the crystal at room temperature and3002the smectic phase at 353 K of [C16C1im][PF6]. By using as3003vibrational probes the Raman bands at 1055, 1065, and 11193004cm−1, related to νs(CC) and νas(CC) modes, they concluded3005that the hexadecyl chain is in the anti conformation for all of3006the dihedrals in the crystalline phase and the crystal melts with3007formation of the mesophase and occurrence of gauche3008conformers. There are interesting questions concerning ionic3009liquid crystals,356 but we will focus in this review on phase3010transitions of medium size alkyl-chain ionic liquids, which3011usually have low melting points.3012A recurrent issue addressed by vibrational spectroscopy3013studies of ionic liquid phase transitions is eventual conforma-3014tional changes indicated by the characteristic bands of different3015conformers. The possibility for the ions acquiring different3016conformations can lead to crystal polymorphism and solid−3017solid transitions due to interchange between conformers.3018Hayashi et al.181 characterized two crystal polymorphs of3019[C4C1im]Cl by a combined analysis of X-ray powder patterns3020and Raman spectra. Soon after this work, Ozawa et al.184 used3021Raman spectroscopy and DFT calculations to show that3022rotational isomerism of the butyl chain of [C4C1im]

+ implies3023crystal polymorphism, one phase composed by anti−anti and3024the other one composed by gauche−anti conformers (see3025Figure 20 for the characteristic Raman bands of [CnC1im]

+

3026conformers). In a chapter of a recently published book,4

3027Hamaguchi and coauthors reviewed their Raman spectroscopic3028studies on structure and phase transitions of imidazolium-based3029ionic liquids. Endo et al.198,357−359 extended the analysis of3030phase transitions to [C4C1im]+ based ionic liquids with3031different anions using Raman spectroscopy along other3032experimental techniques, such as NMR and differential3033scanning calorimetry (DSC). They reported the formation of3034three crystalline phases with [C4C1im]

+ having anti−anti,3035gauche−anti, and gauche′−anti conformations. Raman spectra3036and DFT calculations reported by Endo et al.191 indicated the3037occurrence of two conformers, asymmetric and symmetric, for30381-isopropyl-3-methylimidazolium, [i-C3C1im]

+, in the liquid3039phase. However, crystalline phases of [i-C3C1im]Br and [i-3040C3C1im]I contain only the asymmetric conformer.198 Endo et3041al.191 also found similar behavior of solid−solid transitions and3042cation conformational changes when methylation in the C(2)3043atom of the imidazolium ring lead [C4C1im]+ to3044[C4C1C1im]

+based ionic liquids. Other systems for which3045temperature-dependent Raman spectroscopy was used to reveal3046conformational changes accompanying crystal polymorphism3047and solid−solid transition included piperidinium132 and3048ammonium230,232,235,360,123,128 ionic liquids.3049Turning now to conformational changes of the anion, Raman3050and IR spectra have been used to characterize the [NTf2]

−

3051conformation along phase transitions of [NTf2]− based ionic

3052liquids. It is worth mentioning that before ionic liquids3053becoming a broadly investigated class of compounds, vibra-3054tional spectroscopy has been extensively used for studying3055[NTf2]

− conformation in polymer electrolytes.113,361 Crystal-3056line phases of typical ionic liquids have been found with3057[NTf2]

− in cisoid or transoid conformations (see Figure 12 for3058the characteristic Raman bands of [NTf2]

− conformers).3059Castriota et al.114 reported one of the first vibrational studies

Figure 38. Raman spectra of [C4C1im][CF3SO3] in liquid and lowtemperature crystalline phases. Raman intensities have beennormalized by the band marked with the asterisk. The inset highlightsthe range of 585−635 cm−1.



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3060 on temperature-dependent phase transition of a [NTf2]− based

3061 ionic liquid, namely [Pyr1,4][NTf2], and its mixture with3062 Li[NTf2]. Raman spectroscopy provided insight about3063 interactions between [NTf2]

− and [Pyr1,4]+ and Li+ cations in

3064 crystal and liquid phases, but [NTf2]− conformation was not an

3065 issue. Herstedt et al.360 performed a study of cation and anion3066 conformations in different solid phases of [C2C2C2C2N]-3067 [NTf2], whose melting temperature is 377 K. The relative3068 amount of [NTf2]

− conformers in three different solid phases3069 were evaluated on the basis of characteristic [NTf2]

− Raman3070 bands. The analysis helped understanding the disorder caused3071 by [NTf2]

− anion in the solid, thus explaining the formation of3072 plastic crystals and, in part, accounting for the low melting3073 point of [NTf2]

− ionic liquids.362 Martinelli et al.116,363 used3074 Raman spectroscopy to follow the evolution of [NTf2]

−

3075 conformation as a function of temperature in bulk and3076 nanoconfined ionic liquids with protic and aprotic cations.3077 Some studies also considered IR spectra as a function of3078 temperature to follow [NTf2]

− conformational changes during3079 phase transitions.118,122,259,364 Vitucci et al.122 suggested that3080 the longer alkyl chain in ammonium ionic liquids stabilizes the3081 lower energy cisoid [NTf2]

− conformer in solid phase because3082 of stronger interactions between [NTf2]

− and the polar head of3083 the cation. It became apparent from vibrational spectroscopy3084 studies that structural rearrangements in polar and nonpolar3085 domains during phase transitions may be independent of each3086 other.235,123,128,365 The [N(SO2F)2]

− is another anion for3087 which conformational analysis in low temperature phase3088 behavior was investigated by Raman spectroscopy. Shimizu et3089 al.132 found two different crystalline phases for [Pip1,4][N-3090 (SO2F)2] with [N(SO2F)2]

− either in the cisoid or transoid3091 conformation (see Figure 14 for the characteristic Raman bands3092 of [N(SO2F)2]

− conformers).3093 Kinetic effects play a central role in determining the complex3094 phase behavior of ionic liquids. Lassegues et al.125 reported3095 Raman spectra of [C2C1im][NTf2] in the crystal phase at 113 K3096 obtained after slow cooling (∼1 K min−1) and glassy phase at3097 113 K obtained after fast cooling (∼20 K min−1). These3098 authors found that the crystal phase contained the cisoid3099 [NTf2]

− conformer, whereas the glassy phase contained mainly3100 the transoid conformer. The cooling-rate dependence of3101 [NTf2]

− conformation illustrates how the thermal history3102 determines the ionic liquid phase behavior. In fact, Raman3103 spectroscopy indicates that [NTf2]

− achieves different con-3104 formations when partial crystallization of [C4C1C1C1N][NTf2]3105 is obtained by slow cooling or by cold crystallization (i.e.,3106 crystallization by heating the glassy phase).123,128 The Raman

f39 3107 spectra of [C4C1C1C1N][NTf2] shown in Figure 39 indicate3108 there is mixture of [NTf2]

− conformers in the normal liquid3109 phase, whereas the cisoid or transoid conformation is obtained3110 in the crystal formed by slow cooling or cold crystallization,3111 respectively.3112 Many ionic liquids are easily supercooled, exhibiting glass3113 transition typically around 190−210 K, so that it might be3114 difficult to obtain the low-temperature crystal.366,367 Crystalline3115 phase has never been obtained for some ionic liquids along3116 cooling, or it has been obtained only by the cold crystallization3117 process. The ionic liquid can also become partially crystallized3118 forming the so-called glacial state (i.e., microcrystallites3119 immersed in a matrix of supercooled liquid), for instance,3120 [C4C1C1C1N][NTf2].

123,128 A large number of studies using3121 DSC, NMR, and Raman spectroscopy have investigated the3122 slow dynamics during the melting process of ionic

3123liquids.185,191,368−373 Hamaguchi and Ozawa185 followed the3124melting of [C4C1im]Cl crystal by the time dependence of3125Raman bands which characterize different conformations of the3126butyl chain. They observed an unusually long time for3127equilibration of anti/gauche conformers in the liquid phase,3128concluding that conformational interconversion happens along3129slow collective transformation of ensembles of [C4C1im]

+

3130cations. In a subsequent paper, the melting process of3131[C4C1im]Cl was investigated by Okajima and Hamaguchi372

3132using simultaneously the lattice vibration bands in the low-3133frequency range and the Raman bands of butyl chain3134 f40conformers. Figure 40 taken from their work shows Raman3135spectra of [C4C1im]Cl during the melting process and Raman3136intensities as a function of time for the lattice vibration at 1113137cm−1 and the characteristic bands of butyl chain conformation.3138In this experiment, a small piece of [C4C1im]Cl single crystal3139was rapidly heated to 363 K, that is 30 K higher than the3140melting point.372 The low-frequency Raman spectrum of liquid3141[C4C1im]Cl has the typical quasi-elastic scattering (see section31425), whereas the crystal phase spectrum has peaks characteristic3143of lattice modes. Lattice modes gradually disappear during3144melting, while the quasi-elastic scattering intensity increases.3145However, the bottom panel of Figure 40 indicates time lag3146between the disappearance of lattice modes and the3147equilibration of anti/gauche population in the liquid phase.3148Thus, conformers remain in local structures, and only after3149conversion of those arrangements as a whole is the melting3150process completed.185,372 Endo and Nishikawa191 have also3151observed that conformational changes are linked to the melting3152process of [i-C3C1im]I, concluding that the premelting region3153observed in DSC as broad peaks corresponds to an equilibrium3154state distinct from liquid and crystalline states. Altogether, the3155kinetics of ionic liquids during crystallization or melting3156processes has been assigned to complex conformational3157changes depending on cooperative rearrangements and a3158sluggish collective dynamics.3159Vibrational spectroscopy has been used more recently to3160investigate crystallization, solid−solid, and glass transition of3161ionic liquids under high pressure. In analogy with temperature-3162dependent studies, Raman and IR spectra provide insights on3163conformational changes accompanying high pressure phase3164transitions. These studies concerned mainly anti/gauche3165conformations of 1-alkyl-3-methylimidazolium cati-3166ons,365,374−390 but some studies also analyzed conformations

Figure 39. Raman spectra of [C4C1C1C1N][NTf2] in liquid (298 K,black line) and crystalline phases after slow cooling (240 K, blue line)and cold crystallization (230 K, red line).



AI


3167 of ammonium cations128,235,391,392 and the [NTf2]−

3168 anion.128,364,385,393,394 Many of these papers report the glass3169 transition pressure at room temperature, Pg, according to the3170 method of measuring the pressure dependence of the3171 bandwidth of ruby fluorescence spectrum.395 Stress relaxation3172 is no longer achieved when the glass transition takes place, so3173 that the ruby acts like a microscopic probe of stress3174 heterogeneity in the sample. Thus, the plot of the bandwidth3175 of ruby fluorescence spectrum as a function of pressure exhibits3176 a noticeable change in slope at Pg. The Pg of 1-alkyl-3-3177 methylimidazolium-based ionic liquids depends on the alkyl3178 chain length. For instance, Pg ranges from ca. 3.0 to 2.0 GPa3179 along the series [CnC1im][BF4], as n increases from 2 to 8.386

3180 The anion also plays a role in determining the glass transition3181 under high pressure, for instance, Pg is 1.6 and 2.1 GPa for3182 [C8C1im][PF6] and [C8C1im][BF4], respectively.

396 Interest-3183 ingly, plots of the ruby emission bandwidth eventually suggests3184 that other transitions take place for pressures higher than3185 Pg.

379,386 Yoshimura et al.386 proposed the formation of other3186 densified structures most probably related to dynamic3187 heterogeneity with the intrinsic hierarchy of structures relaxing3188 at different time scales. However, this is an open issue which3189 deserves further studies.3190 Some ionic liquids in glassy phase at high pressure experience3191 crystallization when decompressed, this finding being the3192 pressure counterpart of cold crystallization when the low-3193 temperature glass is heated.386,397 Kinetic of high pressure3194 phase transition also exhibits similarities with low-temperature3195 transition. It has been found that [C2C1im][CF3SO3] and3196 [C4C1im][CF3SO3] may become a glass or a crystal depending

3197on the compression rate.387,388 Moreover, large hysteresis of3198vibrational frequency and bandwidth of the high-pressure3199[C4C1im][CF3SO3] crystal was found when the pressure was3200released stepwise back to the atmospheric pressure.387

3201Raman spectroscopy has been used to show that high3202pressure crystallization of the protic ionic liquid [C3NH3]-3203[NO3] may result in a microscopically heterogeneous3204 f41sample.235,392 Figure 41A shows two spectral patterns recorded

3205by focusing the laser beam in different regions of the same3206sample inside the DAC after a quick increase of pressure to ca.32071.5 GPa. (Compare with the νs(NO3) Raman band of3208[C3NH3][NO3] in the normal liquid phase shown in Figure320934.) A photograph of the sample chamber with the [C3NH3]-3210[NO3] crystal is shown in Figure 41B. The distorted3211arrangement of hydrogen-bonded ions resulting in a distribu-3212tion of νs(NO3) vibrational frequencies in liquid [C3NH3]-3213[NO3] was reproduced by DFT calculation of a cluster of four3214ionic pairs.235 Thus, distinct local structures can be arrested in3215isles of microscopic heterogeneity under high pressure,3216resulting in the anomalous crystallization of [C3NH3][NO3].3217The micro-Raman imaging in Figure 41C illustrates the spatial3218distribution of microscopic heterogeneity of the high-pressure3219[C3NH3][NO3] crystal. It is worth mentioning that stepwise3220increase of pressure to 1.5 GPa also generates the microscopic3221heterogeneity, and the actual νs(NO3) band shape depends on3222the rate of increasing pressure.235,392 Concerning crystal growth3223under high pressure, it is usually verified heterogeneous3224nucleation starting up from the gasket wall in the DAC sample3225chamber. We provide as a movie showing the crystallization3226process of [C2C1im][NTf2] inside the DAC at ca. 1.0 GPa.3227Magnetic ionic liquids are composed of metal-containing3228anions with magnetic behavior, [FeCl4]

− being the most3229common. Garcia-Saiz et al.398 performed magnetization and3230Raman spectroscopy studies of [C2C1im][FeCl4] under high3231pressure. They found that the relatively low applied pressure of32320.34 GPa is enough to modify magnetic interactions and to3233induce transition from antiferromagnetic to ferromagnetic3234ordering. Furthermore, [C2C1im][FeCl4] exhibits magnetic3235hysteresis linked to liquid−solid phase transition due to the3236alignment of [FeCl4]

− anions.

Figure 40. Top panel: Raman spectra from the low-frequency regionup to ∼700 cm−1 of a [C4C1im]Cl crystal during its melting process.The lattice vibration at 111 cm−1 and anti/gauche conformer bands(625 and 603 cm−1) are indicated by dashed lines. Time startscounting when melting begins. Bottom panel: time-dependence ofintensity of the band at 111 cm−1 and the intensity ratio of anti/gauchebands during the melting process. Arrows indicate 48, 54, and 57 s.Reproduced with permission from ref 372. Copyright 2011 TheChemical Society of Japan.

Figure 41. (A) Spectral patterns in the range of the νs(NO3) Ramanband in two different regions of the crystalline sample of [C3NH3]-[NO3] inside the DAC at ca. 1.5 GPa. Raman intensities have beennormalized. (B) Photograph of the DAC sample chamber showing the[C3NH3][NO3] crystal. (C) Micro-Raman imaging using the spectralregion indicated by the blue square in (A). Spectra in red and green of(A) correspond to different regions of the mapping in (C). Adaptedfrom ref 235. Copyright 2013 American Chemical Society.



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3237 Vibrational spectroscopy under high pressure allows one to3238 obtain the volume of conformational change, ΔV, in analogy to3239 the temperature-dependent studies discussed in section 4.3.3240 One obtains ΔV from the pressure dependence of intensities of3241 bands that characterize each conformer, Iconf1 and Iconf2. If one3242 assumes that Raman scattering cross sections for conformers 13243 and 2 do not depend on pressure then ΔV is given by

Δ = −∂

∂⎡⎣⎢

⎤⎦⎥V RT

I IP

ln( / )

T

conf2 conf1

3244 (11)

3245 where R, T, and P are the gas constant, temperature, and3246 pressure, respectively. Takekiyo et al.378 determined3247 ΔV(planar→nonplanar) = +1.6 cm3/mol for [C2C1im][BF4] and3248 ΔV(anti→gauche) = −0.7 cm3/mol for [C4C1im][BF4]. Capitani et3249 al.364,394 obtained ΔV(transoid→cisoid) for the [NTf2]

− anion in3250 [Pyr41][NTf2] and [N1116][NTf2], respectively, − 0.34 and3251 −0.41 cm3/mol. In the case of [N1116][NTf2], Capitani et al.

364

3252 obtained ΔV(transoid→cisoid) from IR spectra, and they also3253 obtained ΔV(transoid→cisoid) = +0.7 cm3/mol in a linear regime3254 for pressures higher than Pg (i.e., above 2 GPa). These authors3255 also obtained ΔH for [N1116][NTf2] by temperature-dependent3256 spectra, and they discussed the competition between anion−3257 cation interactions, relative energy of conformers, and the anion3258 volume.3259 The low-frequency range probing lattice dynamics is of3260 course expected to be very sensitive to ionic liquid phase3261 transitions. As discussed in section 5, the low-frequency Raman3262 spectra of amorphous phases exhibit the intense quasi-elastic3263 scattering and the intermolecular vibrations appear as a broad3264 band, the so-called boson peak, in contrast to the sharp peaks of

f42 3265 crystal lattice modes. Figure 42 illustrates low-frequency Raman3266 spectra at different temperature and pressure conditions for3267 [C4C1im][CF3SO3] as it undergoes glass transition or3268 crystallization. The quasi-elastic scattering dominates the3269 Raman spectrum for the normal liquid phase at 0.1 MPa and

3270298 K. The Raman spectrum of the glassy phase obtained by3271sudden increase of pressure (2.4 GPa, 298 K) exhibits the3272boson peak at ∼21 cm−1 and the imidazolium ring librational3273mode at ∼140 cm−1. The high-pressure crystal (1.3 GPa, 2983274K), with consequent sharp bands in the Raman spectrum, is3275obtained when pressure is stepwise increased. [C4C1im]-3276[CF3SO3] always crystallizes at low temperature under usual3277cooling rates,387 and the spectral pattern of the crystal at 0.13278MPa and 230 K does not resemble the pattern of the high-3279pressure crystal. However, a direct qualitative interpretation of3280different crystal structures based only on the low-frequency3281vibrational spectrum has to be done with care. Chen et al.399

3282obtained single-crystal X-ray diffraction data for a series of3283nitrile functionalized ionic liquids and analyzed low-frequency3284Raman spectra taking into account the crystal structures. They3285found that ionic liquids having the same space group exhibit3286indeed similar low-frequency Raman spectra, but they also3287warned that the analysis based only on the similarity of Raman3288spectra might lead to erroneous conclusions and it should be3289complemented by X-ray diffraction measurements.399

3290Few works concerning ionic liquid phase transitions have3291been published using the low-frequency range of vibrational3292spectra. Roth et al.283 discussed the effects of hydrogen bonds3293in FIR spectra of [NTf2]

− based systems with a series of3294imidazolium cations with the ring hydrogen atoms substituted3295by methyl groups, including the solid phase of 1,2,3,4,5-3296pentamethylimidazolium derivative, whose melting point is 3913297K. Low-frequency Raman spectroscopy has been used by3298Okajima and Hamaguchi372 in order to follow the melting3299process of [C4C1im]Cl (see Figure 40) and by Faria et al.235 to3300distinguish crystalline phases of [C3NH3][NO3] at different3301conditions of temperature and pressure. The low-frequency3302range of the Raman spectrum of [C4C1C1C1N][NTf2] also3303indicated the glacial state123 (i.e., mixture of microcrystals and3304supercooled liquid), according to the occurrence of sharp bands3305of lattice modes on top of the broad band characteristic of3306amorphous phase. The ionic liquids [C4C1C1C1N][NTf2] and3307[C1C4C4C4N][NTf2] do not crystallize under high-pressure at3308room temperature, instead they undergo glass transition at 1.13309and 1.3 GPa, respectively.128 Accordingly, the low-frequency3310Raman spectrum under high-pressure exhibits low intensity of3311quasi-elastic scattering and the characteristic boson peak due to3312intermolecular dynamics as expected for a glassy phase.128

3313Penna et al.400 have found a correspondence between the3314position of intermolecular vibrational modes in the liquid state3315and the spectral features observed after partial crystallization of3316samples at low temperature or high pressure. This point is3317 f43illustrated in Figure 43 with the susceptibility representation of3318the Raman spectra of [C2C1im][NTf2] in supercooled liquid3319(250 K, ●) and partially crystallized (230 K, black line)3320phases.400 The components (blue lines) used in the curve fit3321(red line) of the liquid spectrum seems to correspond to3322broadening of sharp bands of the crystal spectrum. This finding3323strongly suggests that there is some kind of mesoscopic order in3324the supercooled ionic liquid beyond the well-known nanoscale3325heterogeneity of polar/nonpolar domains.400 However, it is3326worth noting that in many cases, e.g. [C4C1im]Cl, the broad3327bands in the low-frequency Raman spectrum of the glassy phase3328barely indicate any direct correspondence to the sharp peaks in3329the crystal parent spectrum.3330It is natural that there are several open questions related to3331phase behavior of ionic liquids proper to the complexity of3332structural and kinetic aspects of the transitions under low

Figure 42. Low-frequency Raman spectra of liquid, glassy, andcrystalline phases of [C4C1im][CF3SO3] obtained under differentconditions of temperature and pressure as indicated in the figure.Raman spectra have been shifted vertically to aid in visualization.



AK


3333 temperature or high pressure. Studies simultaneously changing3334 temperature and pressure are on demand for covering a wider3335 region of the phase diagram of ionic liquids. These3336 investigations would go beyond the scope of ionic liquids by3337 shedding light on more general issues, such as the interplay3338 between glass transition and crystallization, nucleation and3339 crystal growth processes, amorphous−amorphous transitions,3340 and the melting process. Moreover, the mixture of ionic liquid3341 with other compounds can produce hybrid materials with3342 interesting properties and phase transitions. For example, Abe3343 et al.401 used Raman spectroscopy in studying confinement of3344 water in an ionic liquid and the concentration dependence of3345 phase behavior. Furthermore, crystallization of ionic liquid from3346 different solvents might be an efficient method for purification3347 and to generate new crystalline phases, as shown by Li et al.383

3348 for an ionic liquid-methanol solution under high pressure. IR3349 and Raman spectroscopies are powerful tools for addressing3350 many of these issues.

7. VIBRATIONAL SPECTROSCOPY OF IONIC LIQUID3351 SOLUTIONS3352 An appealing feature of the ionic liquid chemistry is the3353 possibility of fine-tuning the properties by proper combination3354 of cations and anions or mixing different ionic liquids.402−404

3355 From the point of view of vibrational spectroscopy, mixtures of3356 ionic liquids with other ionic liquids, molecular solvents, ions,3357 polymers, etc. can serve as model systems for understanding the3358 complex balance of intermolecular interactions determining3359 solvent properties and the liquid structure. A large number of3360 systems has been studied using vibrational spectroscopy, but in3361 this section we limit our discussion to mixtures of ionic liquids3362 with other ionic liquids, water, or other molecular solvents,3363 gases, and salts. The examples given in this section show that3364 attempts to get insights on intermolecular interactions from3365 vibrational spectroscopy of ionic liquids rely on frequency shift,3366 band broadening, intensities variation, etc. take place in the3367 mixtures. The approach of assigning physically meaningful3368 interpretation to spectral changes in Raman excess spectrosco-3369 py usually demands a reference spectrum. In Raman excess3370 spectroscopy, or other excess spectroscopies, the deviation of3371 the experimental spectrum of the mixture from the ideal3372 counterpart, which is the weighted average of pure samples

3373spectra, may provide chemical information about interactions in3374the mixture.405,406 However, this approach is undermined when3375the reference spectrum cannot be obtained. In this context,3376Koch et al.407 discussed a multivariate approach for quaternary3377mixtures of [C2C1im][C2SO4], water, and D- and L-glucose. A3378word of caution is in order since not all multivariate analysis3379methods will provide meaningful information in the first3380approach, a careful choice being fundamental for the employed3381method and proper data pre and post processing.408

3382Hydrogen bonding in equimolar mixtures of ionic liquids has3383been studied by Fumino et al.409 using FIR spectroscopy. These3384authors performed an investigation of imidazolium-based ionic3385liquids with systematic methylation in the imidazolium ring3386while keeping the same [NTf2]

− anion409 and also the mixture3387of ionic liquids with the same triethylammonium cation and3388methylsulfate and triflate anions.410 The underlying assumption3389in this approach is the additivity of spectra of different ionic3390liquids, an issue which has been discussed mainly using optical3391heterodyne-detected Raman-induced Kerr effect spectrosco-3392py,411,319,412 and also using far409,413 and mid414 infrared3393spectroscopies. Cha and Kim414 considered the IR bands3394belonging to γ(CH), ν(C(2)−H), and νas(C(4,5)−H) modes to3395study hydrogen-bonding effects due to anion coordination in3396mixtures of regular or C(2)-deuterated [C4C1im]

+ cation with3397different anions, Cl−, I−, [BF4]

−, and [NTf2]−. If additivity were

3398valid, the spectrum of the mixture would be the sum of spectra3399of the neat liquids weighted by the concentration. The3400agreement between the actual spectrum of the mixture and3401the concentration-simulated spectrum for [C4C1im]I/[C4C1im]3402Cl and [C4C1im]Cl/[C4C1im]BF4 indicated that cation−anion3403interactions were not significantly changed upon mixture. The3404same conclusion is valid for [C4C1im][BF4]/[C8C1im][BF4]3405mixture.414 Aparicio and Atilhan415 also concluded for3406additivity of IR spectra in mixtures of 1-butyl-3-methylpyr-3407idinium, [Py1,4]

+, and 1-octyl-3-methylpiridinium, [Py1,8]+, with

3408the [BF4]− anion. In contrast, in deuterated [C4C1im]+

3409mixtures of [NTf2]− with either Cl− or I−, the hydrogen

3410bond imposed by the more coordinating halide anion caused3411red shift of vibrational frequency of the ν(C(2)-D) mode and3412change in dipole moment derivative as inferred by the3413dependence of band area with concentration.414 Analogous3414effect of mixing anions with different coordination strength has3415been found by Fumino et al.410 in mixtures of protic3416imidazolium ionic liquids. Aparicio and Atilhan415 found3417nonlinear concentration dependence of vibrational frequencies3418of C−H stretching modes in the range of 2800−3200 cm−1 for3419[Py1,4][BF4]/[Py1,4][N(CN)2] mixtures. This finding has been3420attributed to structural changes and dominance of [N(CN)2]

−

3421interaction, a conclusion being corroborated by molecular3422dynamics simulations. Miran et al.416 found anion effects in IR3423spectra of mixtures of protic trimethylammonium ionic liquids3424with [NTf2]

− and [HSO4]−. Therefore, one finds significant

3425spectral signatures in mixtures of ionic liquids, in particular3426involving different anions, when there is large difference in3427coordinating and hydrogen bonding abilities of the ions.3428Furthermore, the different coordination capacity of anions3429results in different chemical environments,416 as also suggested3430by the NMR measurements of the work of Cha and Kim.414

3431Even though mixtures of cations with different lengths of the3432alkyl chain (e.g., 1-alkyl-3-methylimidazolium cations), but the3433same anion, implies only small effects on vibrational spectra in3434comparison with spectra of the parent neat liquids, the3435transport coefficients, excess thermodynamic properties, and

Figure 43. Low-frequency Raman spectra in the susceptibilityrepresentation of [C2C1im][NTf2] in supercooled liquid phase (250K, ●) and after crystallization (230 K, black line). The curve fit (redline) and the individual components of fit (blue lines) of thesupercooled liquid spectrum are shown. Reproduced with permissionfrom ref 400. Copyright 2013 American Institute of Physics.



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3436 phase transitions may be greatly altered.417−420 In order to3437 record Raman spectra of glassy [C2C1im][NTf2] at low3438 temperature, Penna et al.400 added a small amount (10%3439 mol) of [C6C1im][NTf2] to prevent crystallization of3440 [C2C1im][NTf2]. This allowed studying the characteristic3441 boson peak in the low-frequency Raman spectra of glassy3442 [C2C1im][NTf2], [C4C1im][NTf2], and [C6C1im][NTf2]. The3443 boson peak frequency does not depend on the chain length,3444 instead the frequency depends on the strength of the anion−3445 cation interaction when the anion is changed while keeping the3446 same cation.400

3447 Mixtures of ionic liquids with molecular solvents have been3448 used to probe intermolecular interactions and structure,3449 transport properties, phase transitions, and excess thermody-3450 namic properties.401,421−424 Vibrational spectroscopy may3451 address structural features such as nanoscale segrega-3452 tion,422,425,426 solvent properties and ionic liquid polar-3453 ity,427−429 and intermolecular interactions (e.g., the role played3454 by hydrogen bonding).401,30,430,406,431,432 Methanol, ethanol,3455 ethylene glycol, dimethyl sulfoxide, and water are among the3456 most commonly investigated molecular solvents in ionic liquids3457 solutions.3458 As pointed out in section 2, water might be a problem while3459 handling ionic liquids for spectroscopic studies, but the3460 absorption of water may provide interesting information3461 about structure and intermolecular interactions in ionic liquids.3462 Andanson433 used water as a molecular probe of the effects on3463 the liquid structure when mixing different anions in ternary3464 mixtures of [C4C1im]

+ based ionic liquids with [PF6]−, Cl−,

3465 Br−, and [NTf2]−. These authors performed ab initio

3466 calculations of vibrational frequency of water’s symmetric and3467 antisymmetric stretching modes when the water molecule is3468 bound to one or two anions in the presence of a cation. The3469 relative population of water molecules in different environ-3470 ments is then obtained by adjusting the calculation as a3471 weighted sum of Gaussian band shapes to the experimental3472 data. They found there was no significant segregation in the3473 mixtures and essentially the same affinity of both the anions for3474 water molecules in concentrations as high as 1% mol of3475 water.433 In contrast, Tran et al.434 showed the anion effect on3476 the amount of water absorbed by the ionic liquid by studying3477 the near-infrared region 1400−2000 nm (ca. 5000−71423478 cm−1), the absorption coefficient of water at ca. 1419 nm3479 being the most appropriate one to quantify water content. They3480 found that [C4C1im][BF4] absorbs more water than [C4C1im]-3481 [PF6] or [C4C1im][NTf2] because of stronger hydrogen3482 bonding between water and [BF4]

−. The view of stronger3483 [BF4]

−−water hydrogen bonding, at least in the comparison3484 between [BF4]

− and [PF6]−, was shared by Dominguez-Vidal et

3485 al.435 along a FIR spectroscopy study of [C4C1im]+ based ionic3486 liquids. It is worth mentioning that Fadeeva et al.28 did not find3487 different molar absorptivities of water in near-infrared spectra of3488 [Pyr1,4]

+ ionic liquids with [CF3SO3]− or [NTf2]

−. Further-3489 more, hydrogen bond strength can be assessed by the shift of3490 vibrational frequencies of ν(C(2)−H) and ν(C(4,5)−H) modes3491 of imidazolium cations in the mid-infrared region with3492 increasing water content.176,423,406,436−439

3493 There are other spectral changes when ionic liquids are3494 mixed with water besides those modes involving the hydrogen3495 atoms of the imidazolium ring. Saha and Hamaguchi440 found3496 all anti conformation of the butyl chains in [CN-C4C1im]Cl3497 and [CN-C4C1im]I crystals, but the intensities of Raman bands3498 belonging to the gauche conformer increase when the crystals

3499were exposed to water. Proper to the appearance of a Raman3500band at 3300 cm−1 assigned to OH stretching mode, they also3501suggested that water molecules make a tight hydrogen bond3502network with the anions displacing them from their original3503equilibrium position.440 Raman spectroscopy indeed indicates3504that the population of gauche conformer increases upon the3505anti conformer when a [C4C1im]

+ based ionic liquid is diluted3506in water.190 Jeon et al.30 also observed that the population of3507gauche increases with respect to the anti conformer as the water3508content increases, but the conformers population ratio3509decreases again beyond ca. 45 mol L−1. These authors3510attributed the increase in vibrational frequencies of νs(CH3)3511and νas(CH3) modes of the alkyl chain to interactions, which3512are mainly repulsive in nature, with the oxygen atoms of water.3513In line with this finding, Bodo et al.441 also observed a slight3514blue shift of vibrational modes related to the alkyl chain and the3515polar head in the Raman spectra of [C4NH3][NO3] protic ionic3516liquid in mixtures with water.3517Danten et al.442 carried out a systematic study of [CnC1im]

+

3518with increasing length of the alkyl chain (n = 1, 2, 4, 8), with3519[BF4]

− and [PF6]−, for water content below the limit of

3520solubility in these ionic liquids. These authors used IR and3521Raman spectroscopies in combination with ab initio3522calculations for a nearly symmetrical complex made of one3523water molecule and two anions. The experimental difference of3524ca. 70−80 cm−1 between symmetric and antisymmetric OH3525stretching modes was reproduced by the calculations of such3526complexes, but they did not find any significant effect of the3527alkyl chain length on the vibrational spectra of the mixtures.3528Cammarata et al.29 studied the effect of imidazolium ring3529methylation, [C4C1im]

+, [C4C1C1im]+, and 1-butyl-2,3,4,5-

3530tetramethylimidazolium, and the anions, [PF6]−, [SbF6]

−,3531[BF4]

−, [NTf2]−, [ClO4]

−, [CF3SO3]−, [NO3]

−, and3532[CF3CO2]

−, on IR spectra in ATR and transmission modes3533with the amount of water ranging from 2540 to 33090 ppm.3534The enthalpy of vaporization of water molecules from the bulk3535of the ionic liquid was estimated from the vibrational frequency3536shift of the antisymmetric stretching mode and resulted in the3537following order for the water−anion interaction strength:3538[PF6]

− < [SbF6]− < [BF4]

− < [NTf2]− < [ClO4]

− < [CF3SO3]−

3539< [NO3]− < [CF3CO2]

−.3540The enhancement of the asymmetric stretching mode3541intensity of water molecule in ionic liquids and in some3542electrolytic solutions has been found.442,443 Danten et al.443

3543evaluated the anion dependence of this spectral feature in3544[C4C1im]

+ ionic liquids, keeping the water content below the3545respective solubility. Using IR, Raman, and ab initio3546calculations, they showed that the “interaction hierarchy” for3547anions with water is [PF6]

− < [BF4]− < [NTf2]

− < [CF3SO3]−.

3548The trend in interaction energy has the correspondence in3549distances between water and the fluorine atoms on the3550calculated clusters, 1.84 and 1.72 Å in [PF6]

− and [BF4]−,

3551respectively, or the oxygen atoms of the anions, 2.0 and 1.9 Å in3552[NTf2]

− and [CF3SO3]−, respectively.442,443 Dahi et al.444

3553shared analogous conclusions concerning the anion effect on3554water uptake and miscibility, resulting in a “hierarchy” similar to3555one proposed by Danten et al.443 and Tran et al.434 while3556keeping the same cation and water activity (ca. 0.80). Dahi et3557al.444 studied the water sorption isotherms and ATR spectra of3558several ionic liquids with the protic cations [C2NH3]

+, [C2im]+,

3559and [C4im]+ and the nonprotic [C4C1im]

+ and [C6C1im]+ with

3560anions [PF6]−, [BF4]

−, [CF3SO3]−, dibutylphosphate, and

3561bis(2-ethylhexyl)phosphate. These authors found a small effect



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3562 of the alkyl chain length of the cation for the same anion and3563 whether the cation is protic or not. They also show that3564 characteristic IR bands of free water do not appear in the3565 spectrum of [C2NH3][CF3SO3], suggesting that water3566 molecules are part of a hydrogen bond network even at low3567 concentrations so that vibrational frequencies of symmetric and3568 asymmetric modes are the same as in liquid water.444 There is3569 strong dependence of the νas(SO3) mode of [CF3SO3]

− with3570 the water activity, in contrast to [PF6]

−, [BF4]−, and [NTf2]

−,3571 whose vibrational frequencies are only weakly concentration-3572 dependent. Strong dependence of anion vibrational frequencies3573 with the amount of water has also been reported for3574 [CH3COO]−,34 [CF3COO]−,430 [NO3]

−,441 and [C-3575 (CN)3]

−.445

3576 The ab initio calculations at the DFT/B3LYP level of theory3577 performed by Danten et al.443 for the water-[C4C1im][NTf2]3578 system considered a 1:2 complex with the [NTf2]

− in the cisoid3579 conformation. The anion−water conformation and stoichiom-3580 etry they used, however, were contrary to previous results.3581 Yaghini et al.438 found that the anion conformation does not3582 change upon addition of water (recall that in pure [C2C1im]-3583 [NTf2] and [C4C1im][NTf2] the transoid conformation is3584 preferred, see section 4.3). Moreover, Wulf et al.429 pointed out3585 that [NTf2]

− is unlikely to form 1:2 complex with water due to3586 steric reasons. When the 1:2 water−anion complexes are3587 considered in order to represent solutions of low-water3588 concentration,429,433,442,443,386 the calculations should also3589 include two cations to take into account five body nonadditive3590 interactions.442,443

3591 High concentration of water seems to disrupt the nanoscale3592 segregation of polar/apolar domains in ionic liquids.441 In the3593 case of intermediate water concentration, some authors claim3594 that the system is homogeneous without phase segregation, but3595 others argue in favor of formation of water clusters. If the water3596 content in N,N-diethyl-N-methyl-N-2-methoxyethylammonium3597 tetrafluoroborate is below 80% (mol), it has been proposed that3598 water is mostly confined since Raman frequencies of symmetric3599 and asymmetric modes are close to the values of free water3600 molecules.401,446 The limit value of 80% is related to the ability3601 of forming 1:4 [BF4]

−−water complexes, and beyond that,3602 water starts to interact with the more electronegative part of the3603 cation. The OH stretching modes appear in the spectral range3604 characteristic of bulk water when the amount of water in the3605 ionic liquid is above 90% (mol).401,446 These three regimes of3606 concentration (up to 80%, 80−90%, and above 90%) has been3607 also identified by Yoshimura et al.424 in the plot of vibrational3608 frequency of the νs(BF4) mode versus water content in3609 [C4C1im][BF4]. On the other hand, Fumino et al.276 proposed3610 that the pure protic ionic liquid [C2NH3][NO3] has a three-3611 dimensional hydrogen-bonded network, and Bodo et al.441

3612 pointed out that in [C4NH3][NO3] the H2O molecules will3613 take part of the extended hydrogen bond network at any3614 concentration of water.3615 Vibrational spectroscopy has been used to investigate several3616 others molecular solvent−ionic liquid mixtures. Fumino et3617 al.447 combined FIR spectroscopy and ab initio calculations to3618 study the equilibrium between contact and solvent-shared ionic3619 pairs of [C3C3C3NH][CF3SO3] dissolved in different molecular3620 solvents over a wide range of concentration. In the case of3621 solvents of a low dielectric constant (chloroform and3622 tetrahydrofuran), there is larger concentration of contact3623 ionic pairs over solvent-separated ionic pairs, whereas for3624 high dielectric constant (DMSO), solvent-separated ion pairs

3625dominate. FIR spectra of the protic system [C2C2C2NH]I3626reveal that the IR band at 106 cm−1, which characterizes the3627contact ion pair of hydrogen bonded cation−anion, remains as3628the protic ionic liquid is diluted in molecular solvents. The IR3629band assigned to solvent-separated ion pair is found at a slightly3630higher wavenumber, ∼150 cm−1, and the intensity ratio3631between these two bands as a function of temperature allows3632for a van’t Hoff analysis of the equilibrium between contact and3633solvent-separated ion pairs.448 Jiang et al.449 considered IR3634spectra of deuterated DMSO−[C4C1im][BF4] mixtures and3635found that the occurrence of polar domains is particularly3636affected by increasing of pressure. Rodrigues and Santos450

3637used the CO stretching mode of dimethylformamide as a probe3638of the alkyl chain length effect in 1-alkyl-3-methylimidazolium3639bromide ionic liquids. These authors found frequency shift and3640change in band shape of the CO mode proportional to the3641length of the alkyl chain. Wang et al.451 used IR spectroscopy3642and ab initio calculations to study hydrogen bonding of 1-3643butylpiridinium tetrafluoroborate with D2O and deuterated3644DMSO. The C−H stretching modes of the cation alkyl chain3645exhibit a blue shift in D2O, but a red shift in DMSO, while the3646C−D stretching modes of deuterated DMSO exhibit a blue3647shift in the mixture. These findings were assigned to the3648behavior of the C−D bonds of deuterated DMSO as electronic3649density acceptors and the alkyl chain atoms of the cations as3650electronic density donors.3651Other molecular solvents carrying the OH group may be3652used as a probe for anion−cation interactions, hydrogen bonds,3653and structural features of ionic liquids. Noack et al.423

3654compared the extent of conventional and unconventional3655hydrogen bonds in vibrational frequencies of ν(C(2)−H) and3656νas(C(4,5)−H) modes in the Raman spectra of [C2C1im]-3657[C2SO4] in mixtures of methanol and ethanol as cosolvents for3658water. Conventional and unconventional hydrogen bonds lead3659to red and blue shifts in vibrational frequencies of modes3660involving the hydrogen atom, respectively, this behavior being3661related to changes in both the C−H bond length and the3662electronic density.423,452 The authors correlated the effects on3663frequency shift with the competition between dispersion and3664electrostatic interactions. They also linked the balance between3665these interactions with several macroscopic properties, such as3666excess thermodynamic properties and excess transport3667coefficients.423 Abe et al.422 used Raman spectroscopy, among3668several other techniques, to probe the relevance of length scales3669associated with both the alkyl chain size of dialkylimidazolium3670cations and the one imposed when adding different alcohols.3671The authors studied 1-alkyl-3-methylimidazolium-based ionic3672liquids, ranging from ethyl to decyl, with primary (propanol, n-3673butanol), secondary (2-butanol), and tertiary alcohols (2-3674methyl-2-propanol). Besides consequences on liquid structure3675and phase transitions, Abe et al.422 found that the cisoid to3676transoid ratio of [NTf2]

− conformers, as revealed by the3677fingerprint region of the Raman spectrum, exhibits instability3678close to a critical length of the cation alkyl chain in mixtures3679with n-butanol, therefore, being a signature of effects of liquid−3680liquid equilibrium because of interplay between two length3681scales.3682Singh et al.427 studied the effects of different cations and3683anions in IR spectra of [C4C1im][BF4], [C8C1im][BF4], and3684[C8C1im][C8SO4] in mixtures with ethylene glycol. They3685assigned the splitting into three bands of the O−H mode of3686ethylene glycol to different chemical environments, and they3687also found frequency shifts of ν(C−H) modes of the



AN


3688 imidazolium ring, specifically the νas(C(4,5)−H), in line with3689 similar results obtained by Pal et al.452 The νas(S−O) and3690 νs(C(2)−H) modes also exhibit frequency shifts in the3691 [C8C1im][C8SO4]−ethylene glycol mixture.427 In the case of3692 ionic liquids containing [BF4]

−, it has been proposed that3693 regions rich in either ethylene glycol or ionic liquid are formed.3694 Furthermore, Singh et al.427 used the blue shift of ν(OH) in3695 order to make qualitative inference about the extent of3696 disturbance of the hydrogen bond network of ethylene glycol.3697 Comparatively, the effect is larger for [C8C1im][C8SO4], and in3698 the case of [BF4]

− based ionic liquids, the disturbance of the3699 hydrogen bond network of ethylene glycol is larger as the alkyl3700 chain is longer. Pal et al.452 suggested that the interaction that3701 causes frequency shift of νas(C(4,5)−H) is of electrostatic nature,3702 rather than hydrogen bonding between hydrogen atoms of the3703 imidazolium ring and ethylene glycol. On the other hand, the3704 fact that aggregation is not observed in the alkylsulfate-based3705 ionic liquid might be assigned to the strong anion-ethylene3706 glycol hydrogen bond. This finding is in line with conclusions3707 from the IR study by Pal et al.,425 who used 1,2-propanediol as3708 a probe in mixtures with [C4C1im]+ ionic liquids with [NTf2]

−,3709 [C1SO4]

−, or [BF4]−. They pointed out from ATR measure-

3710 ments that [C1SO4]− disturbs the most the hydrogen bond

3711 network of the alcohol, followed by [NTf2]− and [BF4]

−. It is3712 worth noting that such order of “strength” of disturbance is3713 somewhat similar to the interaction strength series proposed by3714 Cammarata et al.29 and the polarity series obtained by Wulf et3715 al.429

3716 Shirota et al.453 and Shimomura et al.454 used IR spectros-3717 copy, among other techniques, to study mixtures of benzene in3718 imidazolium ionic liquids by following the effect of the ions on3719 the vibrational frequency of the out-of-plane C−H bending3720 mode, δop(CH), of benzene. They found a significant blue shift3721 of δop(CH), and supported by structural data and quantum3722 chemistry calculations, they suggested that the interaction3723 between benzene and the imidazolium ring is stronger than3724 interactions between benzene rings (π···π interactions) or3725 between benzene and hydrogen atoms (C−H···π interactions).3726 Along the comparison between [C8C1im][BF4] and [C8C1im]-3727 [NTf2], Shirota et al.453 assigned the larger frequency shift in3728 the latter to weaker interaction between [NTf2]

− and the3729 cation, allowing for stronger interaction between the probe3730 molecule (benzene) and the cation. One could argue from the3731 results of Shimomura et al.454 that the alkyl chain length plays a3732 less important role because the vibrational frequency shift of3733 δop(CH) is almost the same in the [C12C1im][NTf2]−benzene3734 mixture. It is worth mentioning that benzene−ionic liquid3735 mixtures have been studied more often by OKE spectrosco-3736 py.205,453−457

3737 Vibrational spectra of different solutes have been used to3738 probe the solvent ability of ionic liquids. Wulf et al.429 used3739 water as a probe of solvent polarity of imidazolium ionic liquids3740 with the anions [SCN]−, [N(CN)2]

−, [C2SO4]−, and [NTf2]

−.3741 Their method relies on measuring the redshift of vibrational3742 frequencies of stretching modes of water, whose magnitude is3743 then related to dielectric constant and different polarity3744 parameters (e.g., the Kamlet−Taft solvatochromic parame-3745 ters).458,459 The anion increases the ionic liquid polarity in the3746 order: [NTf2]

− < [C2SO4]− < [N(CN)2]

− < [SCN]−, while the3747 length of the alkyl chain of imidazolium cations has negligible3748 effect on polarity of the investigated ionic liquids.3749 Other molecules besides water have been used to probe ionic3750 liquid polarity using vibrational spectroscopy. Tao et al.460

3751considered the (CO) mode of acetone, N,N-dimethylforma-3752mide, and Fe(CO)5 as polarity probes. The authors did not3753provide parameters to quantify the polarity, but they showed3754qualitatively the correlation between the polarity and the3755vibrational frequency shift of (CO). Using acetone as a probe in3756ionic liquids with different anions, they showed that polarity of3757[NTf2]

− is smaller than [SCN]− based ionic liquids, in line with3758the conclusions drawn by Wulf et al.429 using water as a probe.3759The ν(CO) frequency of Fe(CO)5 allowed the authors to3760propose a series of decreasing polarity as the alkyl chain length3761increases in [CnC1im][BF4], 3 ≤ n ≤ 10, and [CnC1im][PF6], 33762≤ n ≤ 8. Garcia et al.461 used frequency shifts of ν(CN) and3763νs(CD3) modes of normal and deuterated acetonitrile to3764estimate acceptor (AN) and donor (DN) numbers458 of ionic3765liquids on the basis of extensive data available for acetonitrile in3766common molecular solvents. It has been found consistent AN3767and DN values estimated from either ν(CN) or νs(CD3) modes3768of acetonitrile. The observed trend is that fluorination of anions3769implies more acidic local environments (higher AN), while3770nonfluorinated anions implies more basic environments (higher3771DN).461 Summing up, the results of vibrational spectroscopy3772investigations of ionic liquids polarity indicate that the anion3773plays the dominant role, whereas the effect of changing the3774length of the alkyl chain of dialkylimidazolium cations is less3775important.3776A well-known application of ionic liquids concerns their3777capacity to absorb gases. This can be a selective process aiming3778sample purification or gas capture to remove greenhouse gases3779from the atmosphere.403,404 Vibrational spectroscopy has been3780used as an analytical tool for quantifying the amount of3781absorbed gas462 or for studying the mechanism and eventual3782reactions between the gas molecules and ionic species.32,463

3783Among the gas−ionic liquid solutions most widely investigated3784by vibrational spectroscopy, SO2 and CO2 are the gases which3785draw more attention proper to their impact on the environ-3786ment.3787Zeng et al.464 carried out a systematic IR spectroscopy study3788of SO2 absorption by pyridinium-based ionic liquids with3789different lengths of the alkyl chain, while keeping the same3790[BF4]

− anion, or different anions, [SCN]−, [BF4]−, and

3791[NTf2]−, while keeping the same [Py4]

+ cation. In the case of3792[Py4][NTf2], the authors did not observe the SO2 bands after3793gas absorption because of overlapping with the [NTf2]

− bands3794at 1139 and 1352 cm−1. The symmetric and antisymmetric S−3795O stretching modes of the SO2 molecule, νs(SO) and νa(SO),3796are observed at 1149 and 1332 cm−1 in [Py4][BF4], and 11243797and 1299 cm−1 in [Py4][SCN], respectively. The vibrational3798frequencies of νs(SO) and νas(SO) in pure liquid SO2 are 11443799and 1336 cm−1, respectively.42 Although Zeng et al.464 did not3800address the issue of vibrational frequency shift between pure3801liquid SO2 and SO2-ionic liquid solutions, it is worth noting3802that νs(SO) and νa(SO) exhibit shifts to opposite directions3803when SO2 is dissolved in [Py4][BF4] and a much larger red shift3804of both νs(SO) and νa(SO) in [Py4][SCN]. Variation of the3805alkyl chain length of the pyridinium cation, while keeping the3806same [BF4]

− anion, has no effect on vibrational frequencies of3807SO2 modes.3808Shang et al.465 and Huang et al.466 studied by IR3809spectroscopy the SO2 absorption in ionic liquids containing3810tetramethylguanidinium cations and different anions. Huang et3811al.446 claimed there is no chemical absorption but only physical3812absorption when SO2 is absorbed by [TMGH][BF4] or3813[TMGH][NTf2], as the only spectral feature is occurrence of



AO


3814 two νa(SO) modes, 1376 and 1360 cm−1, observed in both3815 ionic liquids. Shang et al.,465 dealing with the same [TMGH]+

3816 cation, but the imidazolate, phenolate, and 2,2,2-trifluoroeta-3817 noate anions, argued that new bands at ca. 1410 and 954 cm−1

3818 are due to S−O and S−O−H groups, respectively. The latter3819 band was also observed when SO2 was dissolved in another3820 imidazolate ionic liquid with the 1-(N,N-diethylaminoethyl)-3-3821 methylimidazolium cation,467 so that the authors claimed that3822 the mechanism of SO2 uptake in these ionic liquids involves3823 chemical absorption. Ando et al.468 and Siqueira et al.469

3824 studied both the high- and the low-frequency spectral features3825 of Raman spectra of [C4C1im]Br after SO2 absorption. Solid3826 [C4C1im]Br at room temperature melts upon absorption of3827 SO2. When SO2 is absorbed by [C4C1im]Br, the relative3828 intensities of Raman bands at ca. 600 and 620 cm−1, which3829 characterize gauche and anti conformers (see Figure 20),3830 become more similar to that found in [C4C1im]I. Furthermore,3831 vibrational frequency shifts of νs(SO) and νa(SO) modes were3832 attributed to specific charge transfer interactions between SO23833 and Br−. The proposed physical picture of shielding the3834 cation−anion interactions after uptake of SO2 was supported by3835 molecular dynamics simulations.468,469 The authors found good3836 agreement between the density of states calculated by the3837 Fourier transform of the autocorrelation function of velocity3838 (see section 3) and the low-frequency Raman spectra at 100 K.3839 Kazarian et al.470 obtained ATR spectra of subcritical CO23840 dissolved in [C4C1im][BF4] and [C4C1im][PF6] at 40 °C and3841 6.8 MPa. Splitting of the IR band belonging to the bending3842 mode of CO2 has been found. Such splitting of the band is due3843 to the lifting of degeneracy, with the expectation that the3844 splitting would be larger for solvents with more pronounced3845 Lewis base character. Since the splitting was more pronounced3846 in [C4C1im][BF4] than [C4C1im][PF6], Kazarian et al.470

3847 concluded that [BF4]− is a stronger Lewis base than [PF6]

−.3848 Andanson et al.471 characterized CO2 solutions in [C4C1im]-3849 [PF6] under different CO2 pressure at 40 °C using ATR3850 spectroscopy. The authors were able to estimate the swelling of3851 the ionic liquid with increasing CO2 pressure, the CO2 diffusion3852 within the ionic liquid, and solubility in [C4C1im][PF6]. The3853 vibrational spectrum of [C4C1im][PF6] suggested only minor3854 structural modifications after CO2 uptake. The most significant3855 effects of CO2 absorption on the IR spectrum were the change3856 in relative intensities of bands at 600 and 625 cm−1, which3857 characterize the relative proportion of gauche and anti3858 [C4C1im]+ conformers and frequency shift of the νs(PF)3859 mode of [PF6]

−.471 Seki et al.472 studied supercritical CO23860 dissolved in [C4C1im]

+ based ionic liquids with [PF6]−, [BF4]

−,3861 and [NTf2]

−, and [Py4][BF4], using ATR spectroscopy at 503862 °C and 12 MPa. In contrast to Kazarian et al.,470 Seki et al.472

3863 have not found splitting of the CO2 bending mode in the CO23864 solution in [C4C1im][BF4], arguing that the doublet convolutes3865 into a single band under higher CO2 pressure. In the case of3866 CO2 solution in [Py4][BF4], no frequency shift of [Py4]

+ modes3867 was observed, whereas in [C4C1im][BF4] there were frequency3868 shifts of the cation νs(C(4,5)−H) mode and the anion νs(BF)3869 mode. Seki et al.472 claimed that CO2 interacts mainly with the3870 [BF4]

− and that the new species formed CO2-[BF4]− is a

3871 stronger base than the single [BF4]−.

3872 Vibrational spectroscopy has been extensively used to study3873 CO2 absorption in imidazolium ionic liquids with the3874 [CH3COO]

− anion.32,162,473,474,462 The mechanism of chemical3875 absorption of CO2 in [C4C1im][CH3COO] based on the3876 reaction between [CH3COO]− and the C(2)−H acidic

3877hydrogen atom of the imidazolium ring, leading to the3878formation of a carbene complex with CO2, 1-butyl-3-3879methylimidazolium-2-carboxylate, was verified through IR and3880Raman spectroscopies.32,162 Formation of such carboxylate3881species leads to new bands in both the IR (ca. 792, 1323, and38821665 cm−1) and Raman (ca. 794, 1323, and 1672 cm−1)3883 f44spectra. Figure 44 taken from the work of Cabaco et al.162

3884indicates by arrows these IR and Raman appearing in the3885spectra of [C4C1im][CH3COO] containing CO2 or 13CO2.3886Cabaco et al.162 draws attention to the fact that, at molar3887fractions smaller than 0.3 (or CO2 pressures of the order of 63888MPa), the spectra do not exhibit the Fermi dyad [i.e., the3889overtone of the CO2 bending mode in Fermi resonance with3890the νs(CO)].

475 This indicates that the CO2 symmetry has been3891altered in [C4C1im][CH3COO], whereas the Fermi dyad3892appears when CO2 is dissolved in ionic liquids containing3893anions other than [CH3COO]

−. Besides [CH3COO]− based

3894ionic liquids, there are reports of other anions with3895carboxylate474,476 or phenolate477 groups which also favor3896mechanism of chemical absorption of CO2. Furthermore, the3897vibrational dynamics of CO2 in ionic liquids has also been3898studied by time-resolved IR spectroscopy.478

3899Other important contexts for ionic liquid applications include3900electrochemistry and catalysis, in which they can be used as3901solvents for electrodeposition (or electroplating) of materials,3902electrolytes for batteries, etc. Dissolution of precursors or3903catalysts (e.g., TaCl5, AlCl3, and NbCl5) or ionic species (e.g.,3904Li+ and Na+) may be studied using IR and Raman3905spectroscopies for a deeper understanding of solvation and3906structural modification of the ionic liquid upon dissolution of3907small ions. In analogy to the previously discussed AlCl33908mixtures with imidazolium- and pyrrolidinium-based ionic3909liquids (see section 4.1), in which speciation of aluminum as3910[AlCl4]

− or [Al2Cl7]− was inferred by IR and Raman

3911spectroscopies,96,217,479 spectroscopic studies have been carried3912out for other metals, for instance, tantalum and niobium, whose3913deposition process cannot be done in water.480,481 Babushki-3914na480 studied TaCl5-[Py1,4]Cl mixtures using IR spectroscopy3915over a wide range of compositions at room temperature. It has3916been found that all of the vibrational modes of [Py1,4]

+ are

Figure 44. Comparison of Raman (top) and IR (bottom) spectra of[C4C1im][CH3COO] and its mixtures with CO2 (red) and 13CO2(blue) (mole fraction less than ca. 0.3). The arrows pinpoint the threenew bands assigned to 1-butyl-3-methylimidazolium-2-carboxylate.Reproduced with permission from ref 162. Copyright 2012 AmericanChemical Society.



AP


3917 disturbed upon dissolution of the tantalum salt, the ring3918 breathing vibrations within 890−930 cm−1 being the most3919 affected modes exhibiting change on relative intensities and3920 frequency shift depending on the TaCl5 concentration.480

3921 Using Raman spectroscopy, Alves et al.481 studied NbCl5-3922 [C4C1im]Cl and ZnCl2-[C4C1im]Cl over a wide range of3923 compositions at room temperature. Besides speciation of3924 niobium ([NbCl6]

− and Nb2Cl10) and zinc ([ZnCl4]2−,

3925 [Zn3Cl8]2−, and [Zn4Cl10]

2−), these authors reported several3926 changes in the Raman spectra of the mixtures in comparison3927 with the neat ionic liquid spectrum, in particular, in the C−H3928 modes of the imidazolium481 as it has also been found by3929 Goujon et al.482 in water-ZnCl2 or water-MgCl2 mixtures with3930 [C8C1im]Cl. Andriyko et al.483 studied TiF4-[C4C1C1im][BF4]3931 using IR spectroscopy over a wide range of concentrations. The3932 authors were able to observe spectral features, indicating the3933 formation of [TiF6]

2− and a heteronuclear complex between3934 TiF4 and the anion, [TiBF8]

−. They also found decreasing3935 intensity of anion bands because of a side reaction leading to3936 BF3 evolution and formation of [TiF6]

2−.3937 Arellano et al.484 studied Zn[NTf2]2-[C2C1im][NTf2]3938 mixtures using IR spectroscopy with the support of quantum3939 chemical calculations. These authors claim that a new anionic3940 species, [Zn(NTf2)3]

−, is formed upon the addition of a3941 stoichiometric amount of the solute. Formation of [Zn-3942 (NTf2)3]

− results in frequency shift of [NTf2]− and

3943 [C2C1im]+ modes, in particular the ν(C(2)−H) mode. Curve3944 fit of spectra showed significant changes, such as the number of3945 components in the spectral range of 700−820 cm−1, between3946 spectra of mixture and the neat ionic liquid, but the authors did3947 not address this issue.484 In line with the zinc coordination3948 compound, there are similar reports of stable compounds3949 formed between ytterbium485 and europium486 with [NTf2]

−.3950 Liu et al.487 studied 0.2 mol L−1 solutions of Zn[CF3SO3]2 in3951 [C1im][CF3SO3], [C2C1im][CF3SO3], and [C2C1C1im]-3952 [CF3SO3] at 393 K, using Raman spectroscopy. The authors3953 considered the δs(CF3) in order to infer about the coordination3954 of Zn2+ ions and the changes on local environment upon3955 addition of the solute. The authors found that addition of Zn2+

3956 causes a higher wavenumber shoulder in the δs(CF3) Raman3957 band due to those anions coordinating the Zn2+, while the3958 lower wavenumber band is assigned to uncoordinated (“free”)3959 anions.487 Methylation of the imidazolium ring (i.e., from3960 [C1im]+ to [C2C1C1im]+), implies larger splitting of vibrational3961 frequencies between free and bounded anions. The authors3962 estimated the coordination number of anions around Zn2+ from3963 the ratio between the band areas of free and bounded anions. In3964 the case of ionic liquid based on [C1im]+, [C2C1im]+, and3965 [C2C1C1im]+, the most favored species are [Zn(CF3SO3)3]

−,3966 [Zn(CF3SO3)4]

2−, and [Zn(CF3SO3)5]3−, respectively.487

3967 Oliveira et al.488 using Raman spectroscopy studied the3968 dissolution of [NH4]

+, with [CH3COO]−, Cl−, [SCN]−, and

3969 [C2SO3]− as counterions, and Na+, with [CH3COO]

− and3970 [SCN]− as counterions, in [C2C1im][CH3COO]. The authors3971 showed that the [CH3COO]

− mode at 910 cm−1 is a good3972 probe of the local environment as the corresponding Raman3973 band is sensitive to the dissolved species. Chimdi et al.489

3974 studied Na[N(CN)2]−[Pyr1,1][N(CN)2] mixtures using3975 Raman spectroscopy, whereas Carstens et al.490 studied3976 Na[N(SO2F)2]−[Py1,4][N(SO2F)2] and Forsyth et al.491

3977 studied Na[NTf2]−[Pyr1,2][NTf2] mixtures using Raman and3978 ATR spectroscopies. Chimdi et al.489 and Carstens et al.490

3979 reported only frequency and intensity changes of Raman bands

3980related to anion modes, but Forsyth et al.491 observed also the3981occurrence of shoulders at 534, 576, 598 651, and 1150 cm−1 in3982IR bands, the latter being assigned to the twisting mode of the3983pyrrolidinium ring.3984Lithium cation solutions in ionic liquids have been widely3985studied because of the application as electrolyte in lithium3986batteries. Ionic liquids based on the [NTf2]

− anion show3987remarkable structural and dynamical changes when mixed with3988the lithium salt. For example, it was shown by NMR3989measurements492 that in Li[NTf2]−[C4C1im][NTf2] mixtures3990the Li+ self-diffusion coefficient drops by almost a third when3991the concentration ranges from 2% to 22% of Li+. Nicolau et3992al.493 using OKE spectroscopy estimated that the viscosity of3993[C4C1im][NTf2] increases by almost ten times when3994approximately 40% mole fraction of Li+ is added. The overall3995observed trend for different systems which have been studied is3996that increasing Li+ content results in increase of viscosity and3997density and decrease of self-diffusion coefficient and con-3998ductivity.494−497 These effects on transport coefficients upon3999addition of Li+ are accompanied by conformational changes of4000the [NTf2]

− anion as revealed by vibrational spectroscopy.4001Lassegues et al.498,499 and Duluard et al.500 used Raman4002spectroscopy to study mixtures of [C2C1im][NTf2] and4003[C4C1im][NTf2] with Li[NTf2] over a wide range of4004concentration. The authors deconvoluted the anion Raman4005band with the maximum at ∼745 cm−1 into two components,4006the lower wavenumber component being assigned to4007uncoordinated (“free”) [NTf2]

− and the higher wavenumber4008component to coordinated (“bounded”) [NTf2]

− to Li+. These4009authors500,498,499 evaluated the coordination number of Li+

4010equal to two and then inferring for the formation of the4011[Li(NTf2)2]

− species in line with the proposition of4012Umebayashi et al.501−503 According to Lassegues et al.498 and4013Hardwick et al.,504 such complexes are weakly bounded, being4014promptly destabilized in the presence of other highly4015coordinating solvents (e.g., diglyme, tetraglyme, and ethylene4016carbonate). As pointed out by Martins et al.,505 the formation4017of [Li(NTf2)2]

− is also influenced by water content. The4018splitting of the 745 cm−1 band depends on the Li+ molar4019fraction, and a pseudoisosbestic point is observed in the Raman4020spectra.501 Furthermore, Umebayashi et al.506 showed that in4021the series of alkali cations, from Li+ to Cs+, the coordination4022number grows from two to four. Monteiro et al.496 argued that4023the formation of [Li(NTf2)2]

− is the main reason for the4024decrease of Li+ ionic mobility and conductivity in Li[NTf2]−4025[C4C1C1im][NTf2] mixtures.4026Duluard et al.492 studying [C4C1im][NTf2] mixtures with Li

+

4027pointed out that the conformation of the butyl chain remains4028essentially unchanged upon dissolution of Li+. Umebayashi et4029al.503 and Lassegues et al.499 addressed the issue of [NTf2]

−

4030conformation around the Li+ ion, analyzing the fingerprint4031range of the Raman spectrum of the [NTf2]

− anion with the aid4032of quantum chemistry calculations at the DFT/B3LYP level of4033theory. In both of these papers, the authors concluded that the4034cisoid conformation is favored in the 1:1 (Li+:[NTf2]

−)4035complex, in contrast to the situation in the neat ionic liquid,4036in which the transoid conformation dominates. In the case of40371:2 complex, the authors498,503 suggest that the complex4038structures involves the two anions in cisoid or transoid, or a mix4039of cisoid−transoid conformations. Lassegues et al.499 claim that4040the mix of [NTf2]

− conformers gives better agreement to the4041fingerprint region of the Raman spectra.



AQ


4042 The above-mentioned effects of Li+ dissolution on thef45 4043 Raman spectrum of [C4C1im][NTf2] is illustrated in Figure 45.

4044 This figure shows Raman spectra of pure [C4C1im][NTf2],4045 solid Li[NTf2], and the mixture, in the range of 250−450 cm−1,4046 which characterizes the [NTf2]

− conformation and the 7414047 cm−1 band, whose frequency shift and split characterizes4048 coordinated and uncoordinated [NTf2]

−. Despite identifying4049 more than one possible arrangement for the [Li(NTf2)2]

−

4050 complex,499,503 no more than two components were assumed4051 under the band at ∼745 cm−1. In contrast, Pitawala et al.495,507

4052 and Watkins et al.,508 the latter studying Mg[NTf2]2, accounted4053 for three or more components while fitting this band with the4054 support of previously reported quantum chemistry calcula-4055 tions.500,499,501−503 Pitawala et al.507 considered three bands in4056 the fit of the 741 cm−1 Raman band in Li[NTf2]−[Pyr1,4]-4057 [NTf2] mixtures and reached the same coordination number as4058 previous works for high molar fractions of Li+. However, for4059 low concentrations of Li+ (equal or below 0.05), the authors4060 obtained a coordination number as high as four. Change in4061 coordination number might have consequences in transport4062 coefficients, glass transition, and melting temperatures of the4063 mixtures.114,495−497,507

4064 It is worth stressing that only part of the literature4065 concerning ionic liquid solutions was reviewed in this section4066 proper to the broadness of the theme and the large number of4067 examples in which vibrational spectroscopy served only as a4068 complementary tool to better understand some process (e.g.,4069 cellulose and carbohydrates dissolution) or for the character-4070 ization of new materials. It is also worth mentioning solvate4071 ionic liquids,509−516 which are systems composed from weakly4072 coordinating anions such as [NTf2]

−, small cations (e.g., Li+)4073 and a chelating molecular solvent such as tetraglyme,512 not4074 discussed in this review. Such systems are of increasing interest4075 as their properties might extend the range of applications of4076 ionic liquids.

8. CONCLUDING REMARKS4077 In this review, we discussed the application of vibrational4078 spectroscopy for getting information on structure and4079 intermolecular interactions in ionic liquids and the related4080 experimental and theoretical issues. Quantum chemistry4081 calculations of vibrational frequencies beyond the harmonic

4082approximation may be needed for proper assignment and for4083including important anharmonicity effects on vibrational4084spectra of ionic liquids (e.g., Fermi resonance). Strong ionic4085interactions may imply that an ab initio calculation of4086vibrational frequencies for an isolated ion eventually only4087estimates the frequencies when compared with the actual4088spectra of the liquid phase. Fortunately, the computational4089resources available today allow for quantum chemistry4090calculations of vibrational frequencies for clusters made of a4091few ion pairs. However, the approach of cluster calculation still4092relies on an optimized geometry of minimum energy. Thus, ab4093initio molecular dynamics simulations of ionic liquids open the4094perspective for vibrational frequency calculations with the4095proper account of liquid dynamics. On the experimental point4096of view, linear IR and Raman spectroscopies of ionic liquids,4097which were the focus of this review, are being recently extended4098for time-resolved vibrational spectroscopy. Structural fluctua-4099tions of the local environment experienced by a probe oscillator4100cause vibrational dephasing due to energy relaxation and loss of4101phase (pure dephasing). The traditional approach using linear4102IR and Raman spectroscopies by Fourier transforming the band4103shape in order to get time correlation functions of vibrational4104dephasing and reorientational dynamics74,83,85,92 has been4105applied for the CN stretching mode of ionic liquids containing4106cyano-anions since the corresponding Raman band is relatively4107free of overlapping bands.106 However, this methodology is not4108fully appropriate when processes of multiple time ranges4109simultaneously contribute with homogeneous and inhomoge-4110neous mechanisms for the band shape. Thus, perspectives for4111vibrational spectroscopy of ionic liquids include ongoing4112studies by time-resolved techniques, such as time-resolved IR4113spectroscopy and coherent anti-Stokes Raman scattering4114(CARS),6−11 providing insights on the short-time molecular4115dynamics. For instance, time-resolved CARS measurements4116give dephasing times in the subpicosecond range for C−H4117stretching modes of 1-alkyl-3-methylimidazolium cations that4118can be related to flexibility of molecular structure and the4119strength of different sites for hydrogen bonding to the anion.10

4120Time-resolved IR spectroscopy has also been used to follow the4121time evolution of modes belonging to molecule probes4122dissolved in ionic liquids. This allows addressing the molecular4123dynamics at the different time scales of fluctuations of the4124hydrogen bond structure and reorientational motions and the4125relative contributions of homogeneous and inhomogeneous4126broadening to the band shape.517,518 Thus, time-resolved4127vibrational spectroscopy is expected to provide experimental4128data related to short-time molecular dynamics allowing for4129direct comparison to results of computer simulations of ionic4130liquids. Nevertheless, there is plenty of room for applications of4131linear IR and Raman spectroscopies in many areas related to4132the ones discussed in this review. Besides the application on4133ionic liquid solutions discussed in this review, vibrational4134spectroscopy is a complementary technique within the broad4135field of research on hybrid materials. Vibrational spectroscopy4136has been used in combination with other techniques to4137characterize ionic liquid interactions with nanotube and4138graphene,519−524 clays,525−529 polymers,530,492,531−539 carbohy-4139drates,540−544 etc. Furthermore, surface-enhanced Raman4140scattering (SERS) studies have been reported for ionic liquids4141with different metals as substrates.545−549 Within the context of4142studies on phase transitions, vibrational spectra of ionic liquids4143have been reported as a function of temperature, at atmospheric4144pressure, or as a function of pressure, at room temperature.

Figure 45. Comparison between Raman spectra of pure [C4C1im]-[NTf2] (red line), solid Li[NTf2] (black line), and the mixtureLi[NTf2]−[C4C1im][NTf2]) at molar fraction of Li[NTf2] equal to0.37 (blue line). Spectra have been normalized by the most intenseband at each spectral window.



AR


4145 Simultaneous variation of temperature and pressure aiming4146 more complete mapping of the phase diagrams most probably4147 will be the subject of further vibrational spectroscopy studies of4148 ionic liquids.

4149 ASSOCIATED CONTENT4150 *S Supporting Information

4151 The Supporting Information is available free of charge on the4152 ACS Publications website at DOI: 10.1021/acs.chem-4153 rev.6b00461.

4154 IR and Raman spectra recorded in this work are available4155 as TXT files. Each file is identified by the number of the4156 figure, the ionic liquid name, and whether it corresponds4157 to Raman or IR spectrum. The files give Raman and IR4158 spectra covering the same spectral window as shown in4159 the corresponding figure of the paper. The file4160 DAC_Crystallization.wmv is a movie showing the real4161 time crystallization process of [C2C1im][NTf2] inside4162 the diamond anvil cell at 0.7 GPa and room temperature4163 (ZIP)

4164 AUTHOR INFORMATION4165 Corresponding Author

4166 *E-mail: [email protected] ORCID

4168 Vitor H. Paschoal: 0000-0002-0935-37724169 Luiz F. O. Faria: 0000-0002-0711-46044170 Mauro C. C. Ribeiro: 0000-0002-4301-50214171 Notes

4172 The authors declare no competing financial interest.

4173 Biographies

4174 Vitor Hugo Paschoal received his degree in chemistry from the State4175 University of Londrina (Brazil) in 2013 starting his Ph.D. studies4176 under the supervision of Prof. Mauro C. C. Ribeiro in early 2014. His4177 research interests are molecular dynamics simulations and spectros-4178 copy of high-frequency collective dynamics of liquids and their glassy4179 phases (especially ionic liquids and solutions).

4180 Luiz Felipe de Oliveira Faria received his degree in chemistry from4181 Federal University of Juiz de Fora in 2010 and his Ph.D. degree from4182 University of Sao Paulo (Brazil) in 2015. He completed his Ph.D.4183 under the supervision of Prof. Dr. Mauro C. C. Ribeiro on the4184 structure and phase transitions of ionic liquids in different conditions4185 of temperature and pressure. He is currently a postdoc in Prof. Mauro4186 C. C. Ribeiro group, and his research has been focused on the4187 structure and phase transitions of ionic liquids under high pressure4188 using Raman spectroscopy and X-ray scattering techniques.

4189 Mauro Carlos Costa Ribeiro is an Associate Professor at the Chemistry4190 Institute of the Universidade de Sao Paulo, IQ-USP. Mauro obtained4191 his Bachelor degree in Chemistry in 1989 from Universidade Santa4192 Cecilia (Santos-SP). He obtained his Master’s degree in 1992 and his4193 Ph.D. degree in 1995, both from Universidade de Sao Paulo under4194 supervision of Prof. Paulo S. Santos. His Master’s studies concerned4195 calculations of resonant Raman spectra and his Ph.D. thesis on Raman4196 spectroscopy and molecular dynamics of liquids. He started teaching at4197 IQ-USP in 1996. Mauro spent one and a half years (1996−1998) as a4198 postdoc in the group of Prof. Paul A. Madden at Oxford University,4199 U.K., working with molecular dynamics of high-temperature molten4200 salts. After returning to Sao Paulo, he started working with vibrational4201 spectroscopy and molecular dynamics simulation of room-temperature

4202ionic liquids. He also spent shorter periods working in the groups of4203Prof. Giancarlo Ruocco in 2007 (Universita di Roma La Sapienza) and4204Prof. Agilio A. H. Padua in 2013 (Universite Blaise Pascal, Clermont-4205Ferrand, France). His research concerns structure and dynamics of4206ionic liquids with particular focus on changes observed along the glass4207transition and crystallization taking place under low temperature or4208high pressure.

4209ACKNOWLEDGMENTS

4210The authors acknowledge the Brazilian agencies CNPq and4211FAPESP (Grant nos. 2015/07516-8, 2015/05803-0, and 2012/421213119-3) for fellowships and financial support.

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