Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study

Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study

MOLLIQ-04918; No of Pages 11 Journal of Molecular Liquids xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Molecular Liquids ...

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MOLLIQ-04918; No of Pages 11 Journal of Molecular Liquids xxx (2015) xxx–xxx

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Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study Anastasia Maria Moschovi, Vassileios Dracopoulos ⁎ Foundation for Research & Technology Hellas, Institute of Chemical Engineering Sciences, FORTH/ICE-HT, Stadiou Str., Platani, P.O. Box 1414, GR-26504 Patras, Greece

a r t i c l e

i n f o

Article history: Received 15 February 2015 Received in revised form 8 June 2015 Accepted 9 June 2015 Available online xxxx Keywords: Mesoscale organization Raman Infrared Alkyl chain

a b s t r a c t The interactions of alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic (PILs) HCnImNTf2 (n = 0–12) and aprotic ionic liquids (APILs) C1CnNTf2 (n = 1–12) were studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman). The effect of alkyl substituent length (n = 0–12) on both polar and non-polar regions is elucidated. A sponge like structure is proposed for both systems. The spectral characteristics show that there are certain structural differences between PILs and APILs. The well defined hydrogen bonding in PILs strongly affects the local structure of both the polar head (i.e., induction effect) and the side alkyl chain. The spectral findings tentatively suggest that nanosegregation occurs at shorter alkyl chains. In APILs the data correlated with the polar and non-polar regions are discussed on the basis of the proposed sponge like structure. The trans/cis ratio of the anion conformers is related to the dispersion of the average position of the anion over the cation. Also in the oily phase the local structure of the side chain shows certain differences compared to PIL systems. A tail coupling occurred (up to eight carbon atom alkyls) followed by decoupling in higher length tails in APILs indicating an increase of van der Waals forces between the chains. Moreover, we show that the enthalpy of conformational isomerism of the anion is also a good indicator in the case of APILs, regarding the relative magnitude of these interactions. © 2015 Published by Elsevier B.V.

1. Introduction Room temperature ionic liquid is one of the most active research areas in chemistry for the last two decades. Their properties (e.g., low volatility and wide electrochemical window) make them a very promising class of materials for a wide range of applications such as in catalysis [1] as green solvents or in electrochemistry [2]. All these properties are related to subtle balance of different types of interactions between the ions, such as Coulombic, van der Waals, hydrogen bonding, π–π interactions and dipoles. There is a lot of effort to understand their structure and how this is correlated with the corresponding macroscopic properties. Early simulation work based on 1-methyl-3-alkyl imidazolium (alkyl:methyl or octyl) cation showed that independently of the anion used, the increase of alkyl chain length pushes away the nearest-neighbor anions [3]. The existence of the so-called first sharp diffraction peak (pre-peak) was identified in the calculated structure factors, indicating that an intermediate range order exists in these liquids [4]. At that time, the only experimental evidence for such structural organization was the work of Bradley et al. on 1-methyl-3-alkyl imidazolium (C1CnIm+: n N 12) based ionic liquid crystals, where a lamellae like structure exists [5]. After this work, Wang and Voth simulated ionic liquids with long tails ⁎ Corresponding author. E-mail address: [email protected] (V. Dracopoulos).

and they found that these tails tend to aggregate [6,7]. At the same time as Wang and Voth, Pádua and Canongia Lopes [8] showed that this aggregation could also exist in shorter chain system (e.g., butyl and/or hexyl) that can form a nanoscale structure, having polar and non-polar domains. The increase of the alkyl chain length increases the non-polar domain, resulting to a more closely connected side alkyl chains, which in turn causes a swelling of the ionic network like in phase separated systems. Almost at the same time, the group of Triolo [9,10,4,11] showed for the first time that the First Sharp Diffraction Peak (FSDP) also appears in imidazolium ionic liquids with using lower carbons in their alkyl chain. They correlate this peak with the existence of a nanoscale heterogeneity, based on the segregated tails, forming a non-polar domain, supporting experimentally the model of Lopes and Pádua. Also, the characteristic length associated with this peak is proportional to the ~ 2l(CH2) indicating that the alkyl chains do not interdigitate like in alcohols, but tend to exclude each other or better to have a limited interdigitation (micelle like structure) [12]. Later, Hardacre et al. used small angle neutron scattering and proposed a more simplified model for 1-methyl-3-alkyl imidazolium hexafluorophosphate ionic liquids (alkyl:butyl to octyl), where the alkyl tails are just the spacers between the charged network (lamellae like structure), similar to that exists in the solid state [13]. The latter was supported, theoretically, by the group of Margulis as well as by neutron spin echo experiments [14,15]. 0167-7322/© 2015 Published by Elsevier B.V.

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),


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Those different approaches for the same systems clearly show that the mesoscale structure of ionic liquids is complex and not well understood. This becomes more complicated with the cation variation. Scattering experiments from piperidinium [16] and pyrrolidinium [17,18] ILs show that the characteristic length of the nanoscale heterogeneity area does not follow the trend of the imidazolium melts, but their values propose an interdigitate system, in the first case, and an intermediate one for the pyrrolidinium [19]. Similar behavior (the appearance of the low-Q peak in X-ray scattering measurements) is also followed in protic ionic liquids with short alkyl groups such as ethylammonium and propylammonium nitrates [20,21]. Detailed scattering studies indicate that the charged heads are associated with electrostatic and hydrogen bonding attractions while the short alkyl chains aggregate together through solvophobic interactions, suggesting a disordered smectic or “sponge” like structure [22,21,23–26]. Also, a more interesting issue rises from the disappearance of this low-Q peak, when the polarity of side alkyl chain is increased (carbon atoms exchanged by oxygen). Very recently, it was proposed that, in contrast to imidazolium liquids, like those presented above, the appearance of oxygen atoms interrupting the carbon atom sequence of the alkyl chain could lead to the wrapping of the chain around the imidazole ring (“scorpion” like structures) [27]. This mesoscale structure and, in general the organization of IL structure, could be responsible for the peculiar properties observed. For example, their ability to dissolve polar and non-polar substances would be related to the polar and non-polar domains. Also, the trend that is observed in viscosities measured by Watanabe group could be explained with the existence of such mesoscale structure [28]. Raman spectroscopy was one of the first techniques that support the view of the existence of nano-segregation in ILs despite the fact that it is used for studying the local molecular structures. Specifically, Hamaguchi supports that certain local structures having trans and gauche conformers of a length scale b100 Å exist in alkyl–methyl imidazolium halide ionic liquids [29]. This conclusion is based on the time dependent Raman spectra during melting, as well as from the negligible enthalpy difference between trans and gauche conformers. Femtosecond optical heterodynedetected Raman-induced Kerr effect spectroscopy (OHD-RIKES) can be used for studying heterogeneities in ILs [30]. In particular, the Raman low frequency area (b250 cm−1) is quite sensitive to the intermolecular, re-orientational dynamics. For instance, Russina et al. used OHD-RIKES in combination with SAXS measurements to show that the segregation in 1methyl-3-alkyl imidazolium bis(trifluoromethane)sulfonyl imide (C1CnIm-NTf2) liquids starts at n ≥ 3 [4,31]. Very recently, we presented a systematic Raman study of protic 1-H3-alkyl imidazolium triflimide (HCnImNTf2, n = 0–12, for the structures see Figure 1) ionic liquids. It was shown that for short alkyl chain the I+ induction effect of the side chain plays an important role while for longer chains (above ethyl) the intermolecular interactions are affected from π–π interactions as well [32]. We have shown that these interactions can

be reflected in the enthalpy changes ΔHeq of the conformational isomerism of the anion, supporting the view that the anion can be the structural reporter of ionic liquids, as very nicely reported by Margulis [33]. In this contribution, we expanded this approach to the corresponding set of aprotic ionic liquids C1CnImNTf2 (n = 1–12). The molecular structures of protic and aprotic ILs studied are presented in Figure 1. We calculated the enthalpies of conformational isomerism of the anion and we compared them to the corresponding values of the protic series, the acid HNTf2 and the pure Coulombic systems of CsNTf2 and KNTf2 melts. 2. Materials and methods 2.1. Synthesis of protic ionic liquids HCnImNTf2 (n = 0–12) and ANTf2 (A:K, Cs) molten salts Brønsted bases in liquid form 1-alkyl-imidazole CnIm (n = 2, 3, 6, 8, 10, 12) N98% were purchased from IoLiTec (Ionic Liquids Technologies) GmbH, Brønsted base imidazole Im N 99.5% in solid form (flakes) was purchased from Sigma-Aldrich Co., whereas H-bis(trifluoromethanolsulfonyl)imide HNTf2 N99% was purchased from Acros Organics. The synthesis of the protic ionic liquids was carried out under inert conditions in a glovebag (Atmosbag) purchased from Sigma-Aldrich Co. by mixing equimolar quantities of base and acid according to the following stoichiometric Eq. (1) HNTf 2 þ Cn Im→HCn ImNTf 2


KNTf2 of 99% purity was provided from Solvionic while CsNTf2 salt was synthesized in our lab, using carbonate cesium salt Cs2CO3 N99.99% purchased from Alfa Aesar® according to Eq. (2). 2HNTf 2 þ Cs2 CO3 →2CsNTf 2 þ H2 O þ CO2 ↑


Appropriate amount of carbonate salt Cs2CO3 and HNTf2 was dissolved in anhydrous methanol (Merck) in a glovebag under inert conditions. The solution was then exposed to atmosphere and was kept under stirring until methanol was evaporated. The crystals were then collected and dried under vacuum in quartz tubes. Aprotic ionic liquids C1CnImNTf2 (n = 1–12) were purchased from Io·Li·Tec (Ionic Liquids Technologies) GmbH of 99% purity and used without further purification. All chemicals, before and after the synthesis procedure, were stored in a desiccator at room temperature, under inert conditions. For Raman spectroscopic studies all the chemicals studied were dried under vacuum at 60 °C. Protic and aprotic ionic liquids were introduced into quartz cells (OD: 6 mm/ID: 4 mm) and sealed under vacuum in a homemade glassy vacuum line. Alkali metal salts ANTf2 (A:K, Cs) and acid HNTf2 were introduced into pyrex cells (ID: 0.5 mm/OD: 1 mm) and sealed under vacuum as well. 3. Experimental 3.1. Raman measurements

Fig. 1. Molecular structure of the cations HCnIm+ and C1CnIm+ for n = 0–12 and the conformations of the anion NTf− 2 .

Two Raman systems were utilized for the temperature measurements (Table 1) of PILs, APILs and molten salts. System 1: A FT-Raman system. The FT-Raman spectra were recorded with a BRUKER FRA106/ S module on a EQUINOX 55 spectrometer using a NIR excitation line at 1064 nm from an R510 diode pumped Nd:YAG laser (250 mW). The Raman light was collected in the 180° configuration and analyzed by an FT-interferometer equipped with a LN2 cooled CCD. The spectra were collected as a superposition over 200 scans having a resolution of 4 cm−1 and acquisition time 1.3 s per scan. A home-made optical furnace and temperature controller specially designed for the FT-Raman system was used for the spectroscopic study at elevated temperatures [34]. System 2: A UV-Raman system. The spectra of the alkali salts and the HNTf2 at room temperatures and at elevated temperatures were

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),

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Table 1 Temperatures (°C) of FT-Raman spectra measurements for PILs and APILs and UV-Raman spectra measurements for ANTf2 (A:H, K, Cs). HC0ImNTf2


HCnImNTf2 (n = 2–12)

C1CnImNTf2 (n = 1–12)




90 120 150

60 90 120 150

25 60 90 120

25 60 90 120

65 100 130 160

220 245 270 295

140 165 190 215 240

recorded with a high resolution UV-Raman Labram HR-800 spectrograph (JY, ISA Horiba group). In the case of HNTf2 and alkali molten salts, the UVRaman spectrometer was more suitable than the FT-Raman spectrometer, due to the blackbody radiation effect at elevated temperatures. As an excitation source a 441.6 nm laser line (80 mW) from an air-cooled HeCd laser from KIMMON Electric Co. Ltd was used. The laser line was focused on the sample with 50× objective lens (NA = 0.55) through a microscope. The scattered light was then collected in a backscattered geometry and analyzed by a single monochromator equipped with a LN2 cooled CCD. The spectral resolution was 2.7–4.3 cm−1 by modifying the slit between 100–150 μm. Integration time for the spectra was 2 · 150 s. The high temperature Raman spectra were measured using a heating microscope stage (THMS600/720, Linkham Scientific Instruments Ltd) under the Raman microscope objective. The temperatures were controlled with a TMS93 Linkam Ltd temperature controller. Both Raman systems were calibrated before each set of measurements. The FT-Raman system was calibrated with a sulfur standard while the UV-Raman with a silicon wafer standard. For the polarization measurements in liquid state a CCl4 sample was used for calibrating the depolarization ratio of the two set-ups. Two polarization configurations were used for the Raman spectra of the liquids and melts: vertical–vertical (VV) and vertical–horizontal (VH). The corresponding intensities IVV(ω) and IVH(ω) at frequency shift ω were used for the calculation of the isotropic and anisotropic spectra according to the equations: 4 IISO ðωÞ ¼ IVV ðωÞ  IVH ðωÞ 3

IANISO ðωÞ ¼ IVH ðωÞ :


The reduced Raman intensity Rs(ω) where s stands for VV, ISO and VH (ANISO) related to the Is(ω) according to the equation: Rs ðωÞ ¼ Is ðωÞωðω0 −ωÞ4 ½nðωÞ þ 1−1


where ω0 is the excitation frequency and [n(ω) + 1] is the Boltzmann thermal population factor. The advantages of this spectral representation are described elsewhere [34]. 3.2. FT-IR/ATR measurements FT-IR/ATR spectra of PILs (HCnImNTf2) with n = 2–12 and APILs (C1CnImNTf2) with n = 1–12 were collected at room temperature while for PILs with n = 0, 1 spectra were obtained above their corresponding melting points. All spectra were collected using a Bruker Equinox 55 spectrometer, equipped with a single reflection diamond ATR attachment (Model: MKII Golden Gate, SPECAC Ltd.). The ATR attachment had KRS-5 lenses, enabling the acquisition of spectra from 400 cm− 1. Each sample was placed on the diamond crystal and the surface was entirely covered with a thin film of the IL. For the samples HCnImNTf2 n = 0, 1 the liquid film was formed at temperatures 73 °C and 52 °C respectively. Spectra were recorded using 4 cm−1 resolution with 200 scans under N2 atmosphere. It is very important to note here that a background spectrum at each measuring temperature was collected just before measuring the spectra of each sample. The background

spectrum was collected, after the spectrometer was purged with N2 and the background spectrum did not exhibit significant changes. 4. Results 4.1. Structure of the polar region The effect of side alkyl chain length on the imidazolium ring stretching vibrational modes of PILs and APILs is presented in Fig. 2. In the case of PILs (Fig. 2a), one band at 1587 cm − 1 is observed for the nonsubstituted HIm + cation. This band is shifted to lower energies for the C1C1ImNTf2 having a maximum at 1578 cm−1 (Fig. 2b). For the HC1ImNTf2 ionic liquid, the band at 1587 cm−1 splits into two bands at 1553 and 1587 cm−1. This split is observed in all studied PILs (n = 1– 12), while upon ethyl side chain, these bands are further shifted to the red of about 6 cm−1. In the case of APILs a similar red shift in the main band is observed followed by an increase of the FWHM. The band characteristics indicate that this increase in FWHM of the band at 1573 cm−1 is caused by the appearance of a shoulder band at lower frequencies ~1566 cm−1. For both systems no further changes are observed by further increasing the size of the side alkyl chain. This spectral behavior shows that although both systems are built from similar cations, the local structure of the polar heads is different. The shift that is observed between the symmetric protic (HC0ImNTf2) and aprotic (C1C1ImNTf2) ionic liquids is not only due to the different reduced masses, but also, due to the I+ induction effect of the methyl groups on the cation charge. The substitution of the hydrogen atoms of N–H bonds with methyls introduces a negative charge to imidazole ring, destabilizing that way the ring. The positive charge is then redistributed, causing an extension of the ring (i.e., increase of C–N distances) resulting to the red shift of the ring stretching vibration from 1587 cm−1 to 1578 cm−1. Another important issue that rises from this spectral region is the effect of the local symmetry variation, which is caused by introducing different alkyl chains, on the split of these modes. In the case of APILs, the induction effect that caused onto the ring by longer chains (ethyl, propyl, etc.) at N1 atom is controlled by the induction caused by the methyl group that exists on the N3 atom of the ring. This is not the case in PILs, where N1 has the aliphatic chain, whereas the N3 forms a directional HB between cation and anion. Thus, the distribution of the electron cloud of the ring is more disturbed in PILs, resulting to a higher band splitting. The last issue deals with the abrupt shift to the red, when the methyl group is substituted by ethyl. We, already, have discussed the case of PILs and we attributed the shift to the ring extension and to the possible existence of π–π interactions, as well. Considering the APILs, the anion is generally accepted to be placed above and below the ring. This relative position may exclude possible π–π interactions and thus, the spectral behavior of the ring stretching vibration of this spectral area is strongly related with the result of the induction I+ effect of the side chain. More pronounced differences, between the PILs and APILs, have been observed in the spectral region which depicts the vibrations of the anion. Frequencies and band shapes related with the normal modes of the anion moieties (i.e., CF3, SO2, SNS) are similar indicating that both the cis and trans conformers of the anion exist in both types of ILs(Figure S.I. 1, 2). However, the effect of alkyl chain length, on the

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),


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Fig. 2. FT-IR/ATR spectra for (a) HCnImNTf2 (n = 0–12) and (b) C1CnImNTf2 (n = 1–12) in the spectral region of the νC–N stretching vibrational modes of imidazolium ring.

ratio of the IR intensities, which corresponds to the trans and cis anion conformers is different (Fig. 3) for PILs and APILs. In the case of PILs, it increases sharply up to n = 4 (butyl chain), approaching a plateau, since further increase of number of C atoms only slightly increases this ratio. In the case of APILs the trend is different, exhibiting a more linear character. A slight decrease on the slope is observed after hexyl systems. This different behavior is probably due to the differences of hydrogen bonding between anions and cations. Very recently, for the APIL imidazole systems, a sponge like structure has been proposed. More specifically, it was found that the anion degree of freedom is increased for C1CnImNTf2 (1 b n b 6) ILs and decreased when n lies between 6 and 12. According to this model, and the behavior of the ratio of the anion conformers, the anion trans population is presumably related to the degree of freedom of the anion. In the PIL case this is more complicated and we will discuss it further at the end of the next session. The above behaviors indicate that the local structure of the charged head groups shows similarities (cations), but in any case, there are certain differences (anions). The cation spectral behavior is similar to that observed in the case of aprotic imidazolium ILs studied by SAXS. For example, a peak which exists at ~ 0.9 Å−1 in C1C1ImNTf2 melt and

Fig. 3. Experimentally calculated ratios Itrans/Icis of νSO2 vibrational mode areas of the FT-IR spectra, as a function of the number of carbon atoms of the side chain, for ■ PILs and □ APILs.

disappears in APILs with higher alkyl chains was attributed to certain local structural differences, concluding that the first member of the C1CnImNTf2 liquids has different structures compared to the rest of the series. This might be analogous to that of crystalline C1C1PF6 where two anions are bonded with the acidic carbon of the ring through hydrogen bonds [11]. Deetlefs et al. propose that in the liquid structure of C1C1ImNTf2 enhancement of anion–π interactions exists [13]. Large angle X-ray scattering of C1C2ImNTf2 melts shows that the liquid structure is quite different to the one of the solid. In the solid, the anion adopts the cis conformation, while, the cation has the methyl moiety of the ethyl side group above (or below) of the plain of the ring. This structure does not exist in the liquid phase, since the anion adopts both cis and trans conformers while the cation has the ethyl group in both staggered and planar positions. Also, the coordination number of the cation is four, indicating a tetrahedral arrangement. The anion is bonded through hydrogen bonds with the carbons of the ring having a more directional bonding with the acidic carbon (C2 position). This structural motif for the polar heads is generally accepted to exist in the entire series of APILs that are studied here [35]. Based on the above, the main difference between the first member of the APIL series and the rest is probably due to the bonding ability among the ions. The high frequency FT-IR spectra, of the Ci (i: 2,4,5)–H stretching modes of the imidazole ring, are shown in Fig. 4a. Four bands are observed in the spectra of all the studied systems; two in the lower frequency region (3150–3100 cm−1) related to the C2–H stretching modes, and two in the higher frequency (3180–3150 cm−1) related to the C4,5–H stretching modes. As the alkyl side chain of the cation increases, from methyl to ethyl, the spectral area of C2–H stretching mode shifts to lower frequencies (~7 cm−1). Furthermore, its overall intensity is decreased compared to the C4,5–H. We have to note here that the assignment of this band is a matter of debate, since it is well established that there is Fermi resonance with the ring stretching vibrational mode. This make the interpretation of this spectral area quite complicated, as we already mentioned in a recent publication and we will not offer any new discussion here [36–43]. No more shifts are observed with increasing n from 2 to 12. The higher stretching frequency envelope consisting of two bands having a maximum at 3173 cm−1 and a shoulder at 3157 cm−1 does not change in energy as the alkyl chain increases. However, a variation of the relative intensities of the existing bands occurs. These two bands were attributed (in a simplified way) to the co-existence of single (four coordinated) species and their aggregates [44]. According to this

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),

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Fig. 4. (a) High frequency FT-IR spectra of νCi–H modes for APILs and (b) HC2ImNTf2 and C1C2ImNTf2 Raman spectra in the region 700–550 cm−1.

notation the change of the relative intensities, as the alkyl chain increases from methyl to ethyl group, enhances the formation of single ionic species. This may happen due to the induction effect, caused by the ethyl chain, and the resulting redistribution of the π electron cloud of the imidazole ring, enhancing the hydrogen bonding through the C2–H position of the ring (red shift). This also affects the electronegativity of the C4,5–H bonds enhancing their intensity compared to that of C2–H. The above observation supports the structural view based on SAXS data which states that the structure of the first member (methyl) is different and more symmetric to that of the rest (ethyl, propyl, etc.) [44]. Furthermore, since there are no serious changes of the band structure of the lower frequency C–H stretching modes at 3130–3100 cm−1 (except of the red shift which is probably due to both interaction of this spectral area with the Fermi resonance of the ring stretching modes, as well as to the hydrogen bonding through C2 position) as a function of alkyl chain, indicates that the anion is bonded through the C2–H position in all the aprotic systems studied. In the case of PILs (HCnNTf2 (n b 2)), a continuous shift of the Ci–H stretching modes of the ring to lower frequencies was observed in the infrared spectra in the high frequency region. This shift was attributed to intermolecular effects like induction phenomena Ι+, which were caused by the electron donating alkyl chain. Also, we attributed the spectral behavior of the N–H band to the possible existence of π–π interactions. This was, also, explained by the abrupt red shift in the ring stretching modes present in Fig. 2a. A similar red shift was observed in the case of APILs. This shift, as we already discussed, is probably related mainly to I+ induction effect of the side chain. Recent detailed DFT [45] calculations on C1C1ImCl show that the ring stacking is mainly a deficient π+–π+ interaction, while, the energy differences between π+–π+, anion–π+ and cation–anion bonding are very small, indicating that dispersion forces play an important role. Furthermore, they show that the ion pairs are not constructed by the lowest energy conformers. Thus, similar DFT calculations about the structure of HCnImNTf2 (n: 0–12) and C1CnImNTf2 (n: 1–12) will be very helpful in order to understand the effect of dispersion forces to the π–π stacking in imidazolium liquids, and in general the differences between the protic and aprotic imidazole ILs. On the other hand, the observed changes in the anion indicate that there are certain differences in the local structure of the head groups. The strong hydrogen bond with the cation, in the PIL case, probably

facilitates the increased trans NTf− 2 population, compared to the corresponding APILs. Furthermore, this probably is in strong relation with the conformation of the side chain, in the first two carbon atoms (C6–C7, Fig. 1). This effect is more obvious in the case of ethyl systems. The bands observed in the 640–590 cm−1 spectral region of the Raman spectra are assigned to the GA (gauche–anti, ~600 cm−1) and AA (anti–anti, 630 cm−1) conformers of the first carbon atoms (C6–C7) of the side chain of the cation (Fig. 4(b)). So, in the case of aprotic C1C2ImNTf2 the GA conformer is the predominant cation species close to that found in the solid. In the corresponding ethyl protic system the situation is quite different. The band at 630 cm−1 is more intense compared to that at 590 cm−1 indicating that now the AA conformer is the most prominent configuration of the cation in the liquid. This is evident in the calculated FT-IR trans/cis rotamer ratio of the anion. The value of this ratio is almost twice in protic ethyl IL compared to that of aprotic indicating that the gauche cation conformers are associated with anion in cis conformation. 4.2. Structure of the non-polar regions In a recent publication we provided some first insights to the local structure of the non-polar regions [32]. Here we will attempt to expand all these spectral markers for this region to the APIL systems. The main spectral markers used for characterizing the alkane side chain structure in studied ionic liquids based on a. methyl and methylene stretching modes (3000–2700 cm− 1) (Fig. S.I. 3, 4), b. the bending modes of methyl and methylene groups (~ 1460 cm− 1), and c. β deformational mode of methyl group (900–800 cm− 1).

The reduced isotropic Raman spectra of the high frequency methylene stretching mode spectral area are shown in Fig. 5. The stretching modes are characteristic for the gauche/trans rotamer populations [46]. It is well established that the band observed ~ 2850 cm− 1 and assigned to the νs (CH2) symmetric vibrational mode of the methylene groups is correlated with the trans conformers of the alkyl chain while the band at ~2940 cm−1 assigned to the uncoupled or partially decoupled gauche groups appeared in the trans sequence of the alkane [47]. The

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),


A.M. Moschovi, V. Dracopoulos / Journal of Molecular Liquids xxx (2015) xxx–xxx

Fig. 5. Reduced isotropic Raman spectra in the region 3000–2800 cm−1 for (a) HCnImNTf2 (n = 0–12) and (b) C1CnImNTf2 (n = 1–12).

intensity of the latter band is in Fermi resonance with the bending modes at 1460 cm−1 and we will note it as νFR (CH3) [47]. Thus the ratio of the intensities of the bands I2935/I2850 = Rs is directly proportional (qualitatively) to the gauche conformers of the side alkyl chain. Another structural probe of the local defects on trans sequence of alkyl chain structure that is derived from this spectral area, is the ratio of bands at ~2870 cm−1 which is assigned to the asymmetric stretching mode of CH2 units, with the one at 2850 cm−1. This ratio is related again with the gauche structure population but recently this ratio was attributed to defects that do not have the trans geometry [48]. The structure probe that arises from (b) is the ratio of the bending modes of methylene and methyl groups that are observed at ~ 1470– 1430 cm−1. The high frequency band at ~ 1455 cm− 1 is assigned to the scissor mode of CH3 while the band at ~1445 cm−1 to the bending mode of CH2. The ratio of these two bands [I1455/I1445] = D is related to the chain coupling and is generally accepted that the increase of this ratio denotes that the scissor and/or bending motion are favored resulting from increased intramolecular motion of methylene and/or methyl groups that increase with chain decoupling [48]. It is well known that the Raman spectral area at 900–800 cm−1 is an important spectroscopic marker for the end chain conformational changes. The position of the β deformational vibrational mode of CH3 at the end of the chain is affected by the C–C skeletal vibration of the closest methylene groups depending on whether those groups have gauche or trans geometry. If the sequence of the last carbon atoms is in trans geometry (TT geometry) then the band is expected to be found at ~ 890 cm− 1. If the gauche defect is between the 15 and 16 carbon atoms then the band is expected to be placed ~ 870 cm− 1 (TG structure) while if the gauche defect is between the 14 and 15 carbon atoms then the band is ~840 cm−1 (GT geometry) [49]. In both sets of systems the spectra show that as the alkyl chain increases the intensity of the band related to the CH2 symmetric stretching mode progressively increases as well. At the same time the bands at 2870 cm−1 and 2935 cm−1 related to the gauche defects became less intense. Furthermore, the Fermi resonance band at 2935 cm− 1 is better defined in the spectra of the APILs compared to those of PILs. All these observations are reflected by the ratio of the I[νFR(CH3)]/I[νs(CH2)] = RS and I[νa(CH2)]/I[νs(CH2)] = RA versus the size of the alkyl side chain of the imidazole cation, presented in Fig. 6. In the case of APILs this ratio has higher values compared to PILs indicating that PILs have less gauche defects. On the other hand, the RA ratio is

(within experimental error) similar for both systems indicating that the non-trans conformers probably have similar population. In the PILs both markers (RS and RA) decrease as alkyl chain increases indicating that the trans sequences are predominant. In the case of APILs there is a change in RS slope above C8 chain. The latter is more pronounced in the bending mode spectral area probes. The D ratio (Fig. 7) slope of APILs has a clear change from negative to positive when n = 8. In the case of PILs this ratio follows closely the behavior of the trans/cis ratio of the anion (Fig. 3). Similar behavior is observed also for the GA/AA = G1 ratio (Fig. 8). As we discussed in the previous section this ratio represents the conformers of the first carbon









sym asym 3 2 Fig. 6. Experimentally calculated ratioI2930 =I sym 2 andI2873 =I sym 2 from Raman cm−1 2850 cm−1 cm−1 2850 cm−1 spectra, as a function of the number of carbon atoms of the side chain, for ■ PILs and □ APILs.

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),

A.M. Moschovi, V. Dracopoulos / Journal of Molecular Liquids xxx (2015) xxx–xxx

3 Fig. 7. Experimentally calculated ratio IδCH =IδCH2 from Raman spectra, as a function 1456 cm−1 1440 cm−1 of the number of carbon atoms of the side chain, for ■ PILs and □ APILs.

atoms (C6–C7) of the side chain. This shows that the mesoscale structure of these two systems has certain differences. The Raman spectra in the 900–800 cm−1 spectral area are shown in Fig. 9. Both protic and aprotic ILs show similar behavior. Up to hexyl side chains the conformers appearing in the end part of the chain are almost identical. The major difference is on the C12 systems studied where the TT configurations are more predominant in the aprotic liquid while in the case of protic all the possible rotamers are present (TT, TG, GT).

Fig. 8. IGA/IAA ratio of the start methylene groups of the alkyl chain in ■ PILs HCnImNTf2 and □ APILs C1CnImNTf2.


This indicates that there are many possible different structural arrangements of the alkyl chain rising from the combination of the conformers between the C6–C7 and the end part of the chains (Fig. 10). This qualitative visualization of the possible local structure shows in a very robust way the high complexity of the non-polar region of these systems. Despite this complexity, we will try to propose a tentative structural model for both aprotic and protic systems. In aprotic systems the behavior of RS and D versus n and thus of the non-polar region shows that the coupling between the chains increases up to octyl side chain. For higher alkyl chain lengths, a decoupling occurs leaving more space to the methyl groups to scissor. This probably results in an increase of the AA local structures (decrease in G1 ratio) placing closer the chains. Above C8 the GA configuration is favored resulting to the chain decoupling as well. The anionic structures (trans/cis) show a dispersion with increasing alkyl chain size compared to the corresponding PIL systems. The end parts of the side chain show certain configurations for the Cn alkyls with n b 6 indicating that the contacts between the tails are not many while at higher carbon atom chain the increased complexity probably increases these contacts and thus the local tail neighbor. This states that the spectral behavior in this area of both PILs and APILs does not differ much. It is well known that a short inter-digitation [9] of medium range alkyls exists in these systems. This is tentatively correlated with the blue shift of the TT and TG conformers (Table 2) in the aprotic systems. The latter is consistent with recent simulation on these aprotic systems as well as PFGE-NMR in combination with SAXS scattering [50, 51]. Both support the view of the sponge like structure for C1CnNTf2 ionic liquids. Our vibrational data support this structural model. Also, the correlation of the Rs and D structural markers, with the side alkyl chain in APILs, gives further insights on the strange behavior of the self-diffusion coefficients of cations and anions in APILs [50]. In the case of PILs, the local structure seems to be different. The main change between the two systems is the well-defined hydrogen bond between cation and anion through the nitrogen atom of the imidazole ring. Normally this bond should decrease the flexibility of the anion since it has a more defined position on the side of the cation. On the other hand, this increases the population of different conformers of the first carbon atoms of the side chain. As a result, this increases the dispersion of the positions of the anion increased in that way its trans population. This behavior is reflected in all spectroscopic structural markers indicating that the introduction of a directional hydrogen bond affects the local structure in a great manner. The increased decoupling (D increase) for low chains in PILs can probably be correlated with the induction effect caused by the alkyl chain [32]. Another proposition for these behaviors is that these structural markers are correlated with nanosegregation in PILs that started from shorter alkyl chains, like in EAN systems. At longer alkyl chains imidazole PILs π–π stacking is favored as we already discussed [32]. This behavior of the different structural markers strongly supports a recent proposition about the nature of hydrogen bonding in PILs where the ion arrangements are balanced between intermolecular forces and physical dimensions of the ions in order to produce a bicontinuous sponge like structure [24]. Of course scattering experiments in combination with detailed simulations will be very helpful for analyzing the local structure. We also calculated the enthalpy ΔHeq of conformational isomerism of the anion in APILs and compared it to the corresponding values of the PILs (Table 3). The calculation steps are described elsewhere [32, 34]. The Arrhenius type plots of the temperature dependence Icis/Itrans intensity ratios as they derived from reduced isotropic RISO(ω) spectra values of the APILs are presented in Fig. S.I. 5. The calculated values are listed in Table 3 and also presented in Fig. 11. We have shown that this property of the anion is closely related with the local interactions in ionic liquids and pure Coulombic melts (alkaliNTf2). The calculating enthalpy values for the APIL systems are in the upper part of this diagram. As we propose this is something that we expected for systems that except for electrostatic interactions, hydrogen bonding and dispersion forces are present [32].

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),


A.M. Moschovi, V. Dracopoulos / Journal of Molecular Liquids xxx (2015) xxx–xxx

Fig. 9. Raman spectra of the HCnImNTf2 (a) and C1CnImNTf2 (b) in the 900–800 cm−1 spectral region where the end chain gauche defects are.

This reaction equilibrium also reflects the inter- and intra-molecular effects between induction forces and π–π interactions in the protic systems. From a first point of view, the values of aprotic system are lower compared to the protic ones. This is expected since the strong hydrogen bond between the ions in the protic system increases the enthalpy of conformational isomerism since the flexibility of the anion is reduced. In the case of APILs this flexibility is further increased across the chains since G1 ratio has higher values compared to protic systems indicating that there is also more free space for the anion to rotate. Also, the change in slope after the propyl side chain systems in both PILs and APILs maybe resulted from a delicate competition between intramolecular and inter-ionic interactions i.e., I+ induction effect (APILs) and π–π interactions (PILs).

Similar results were shown by Rocha et al. [52] and Verevkin et al. [53] who calculated the enthalpy and entropy changes for vaporization (ΔΗvap and ΔSvap) of the aprotic ionic liquids C1CnImNTf2 as a function of the alkyl chain length. Both found changes in the slope of the quantities as a function of alkyl chain length and attributed to the decrease of the electrostatic interactions and increase of van der Waals interactions. 5. Conclusions A systematic vibrational spectroscopic study (Raman and FTIR) of both protic HCnImNTf2 and aprotic C1CnImNTf2 ionic liquids as a function of carbon atoms of the side alkyl chain of the imidazole ring was presented here for the first time. The spectral findings support the

Fig. 10. Conformation of the C6–C7 and the CN–CN − 1 bonds of the aliphatic chain of the aprotic C1CnImNTf2 and protic HCnImNTf2 ionic liquids according to the spectral region 980–800 cm−1 and 680–550 cm−1. Solid/dotted like arrows represent the more/less prominent configurations of the end part of the side chains as those derived from the spectral characteristics of Figure 9.

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),

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Table 2 Observed Raman frequencies of various structure markers of imidazole side alkyl chains. Start chain (C6–C7) conformations

Protic ILs

Aprotic ILs

Protic ILs

Aprotic ILs

HC2 HC3 HC4 HC6 HC8 HC10 HC12 C1C2 C1C3 C1C4 C1C6 C1C8 C1C10 C1C12

HC2 HC3 HC4 HC6 HC8 HC10 HC12 C1C2 C1C3 C1C4 C1C6 C1C8 C1C10 C1C12

End chain conformations






589 590 590 591 590 590 591 596 599 597 594 599 599 598

627 626 625 628 627 625 629 – 623 623 623 623 623 623

– – – (854) 844 847 846 – – – (854) 846 854 840

– 866 – 868 870 869 873 – 866 – 868 866 874 (866)

– – 882 892 892 895 888 – – 885 893 891 895 888

Chain coupling δ(CH2)


Gauche/trans population νs (CH2)

νa (CH2)

νFR (CH3)

1449 1447 1445 1444 1444 1442

(1456) (1457) (1456) (1457) (1454) (1456)

(2869) (2867) 2860 2856 2854

2885 2879 2875 2872 2870 2870

2944 2943 2938 2938 2936 2933

1451 1450 1445 1443 1441 1439

1457 1463 1460 (1458) (1458) 1459

(2868) (2867) 2859 2857 2854

2884 2877 2875 2873 2871 2871

2942 2946 2939 2937 2937 2933

view that both systems have a sponge like structure. Although these two systems consist of ions with very similar structure (replacement of methyl group by hydrogen atom), the corresponding ionic liquid structure is quite different. The directional NH hydrogen bond in PILs affects more the local structure of the polar region compared to the APILs. The I+ induction effect that is caused by the introduction of ethyl side chains in both systems produces more charge delocalization of the ring in the case of PILs resulting to high band splitting of the ring stretching vibrational modes. In APILs this band splitting is less since methyl group controls the charge delocalization. The trans/cis conformer ratio is correlated with the dispersion of the average position of the anion in the polar head and indirectly with the segregation of the non-polar phase. In PILs this dispersion is more intense compared to APILs reaching its highest value at shorter alkyl chains. This tentatively suggests that in PILs the nanosegregation exists also in the shorter alkyl systems.

The spectral data in the aprotic systems show that in the oily phase they have more gauche conformers compared to the protic ones. A coupling of the chain tails occurred up to C8 substitutes followed by a

Table 3 Calculated ΔΗeq values for the protic HCnImNTf2 (n = 0–12), aprotic C1CnImNTf2(n = 1–12) and the alkali molten salts KNTf2 and CsNTf2. Protic ionic liquids HImNTf2 HC1ImNTf2 HC2ImNTf2 HC3ImNTf2 HC4ImNTf2 HC6ImNTf2 HC8ImNTf2 HC10ImNTf2 HC12ImNTf2 Alkali molten salts ANTf2 KNTf2 CsNTf2

ΔΗtrans ↔ cis (kJ mol−1) [32]

Aprotic ionic liquids

ΔΗtrans ↔ cis (kJ mol−1)

14.29 ± 1.29 12.59 ± 1.99 10.73 ± 0.94 10.44 ± 1.52 9.72 ± 0.46 8.49 ± 1.08 7.13 ± 0.42 4.9 ± 0.67 4.31 ± 0.2

C1C1ImNTf2 C1C2ImNTf2 C1C3ImNTf2 C1C4ImNTf2 C1C6ImNTf2 C1C8ImNTf2 C1C10ImNTf2 C1C12ImNTf2

9.7 ± 1.03 8.13 ± 0.4 4.91 ± 0.5 4.83 ± 0.29 3.86 ± 0.66 3.01 ± 0.3 2.63 ± 0.4 2.48 ± 0.41

−13.86 ± 2.6 −20.73 ± 2.4

Fig. 11. Energy diagram of the ΔHeq values of the protic ■ HCnImNTf2 (n = 0–12) and aprotic □ C1CnImNTf2 (n = 1–12) ionic liquids, alkali metal salts ● ANTf2 (A:Cs & K) and the strong acid ● HNTf2 calculated from the analysis of experimental data at different temperatures.

Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),


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decoupling for longer chain lengths. Also, the end chain spectral markers indicate a possible increase in tail neighbors. All these structural changes, resulting from subtle changes between intermolecular to intra-ionic, are elucidated in the ΔHeq of cis/trans conformers of the anion. This quantity probably follows all these interactions as shown by slope changes across the alkyl chain length studied here. The latter support the view of recent theoretical work where the anions are the structure “reporters” [33]. Acknowledgments The authors want to thank Dr. V. Nikolakis (Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, DE, USA,) for reading the manuscript and the valuable discussion about the liquid structure of IL. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. References [1] H. Olivier-Bourbigou, L. Magna, D. Morvan, Ionic liquids and catalysis: recent progress from knowledge to applications, Appl. Catal. A Gen. 373 (2010) 1–56. [2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Ionic-liquid materials for the electrochemical challenges of the future, Nat. Mater. 8 (2009) 621–629. [3] S.M. Urahata, M.C.C. Ribeiro, Structure of ionic liquids of 1-alkyl-3-methylimidazolium cations: a systematic computer simulation study, J. Chem. Phys. 120 (2004) 1855–1863. [4] O. Russina, A. Triolo, L. Gontrani, R. Caminiti, D. Xiao, L.G. Hines Jr., R.A. Bartsch, E.L. Quitevis, N. Pleckhova, K.R. Seddon, Morphology and intermolecular dynamics of 1alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ionic liquids: structural and dynamic evidence of nanoscale segregation, J. Phys. Condens. Matter 21 (2009) (424121-424121). [5] A.E. Bradley, C. Hardacre, J.D. Holbrey, S. Johnston, S.E.J. McMath, M. Nieuwenhuyzen, Small-angle X-ray scattering studies of liquid crystalline 1alkyl-3-methylimidazolium salts, Chem. Mater. 14 (2002) 629–635. [6] Y. Wang, G.A. Voth, Tail aggregation and domain diffusion in ionic liquids, J. Phys. Chem. B 110 (2006) 18601–18608. [7] Y. Wang, G.A. Voth, Unique spatial heterogeneity in ionic liquids, J. Am. Chem. Soc. 127 (2005) 12192–12193. [8] J.N.A. Canongia Lopes, A.A.H. Pádua, Nanostructural organization in ionic liquids, J. Phys. Chem. B 110 (2006) 3330–3335. [9] E. Bodo, L. Gontrani, R. Caminiti, N.V. Plechkova, K.R. Seddon, A. Triolo, Structural properties of 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ionic liquids: X-ray diffraction data and molecular dynamics simulations, J. Phys. Chem. B 114 (2010) 16398–16407. [10] O. Russina, A. Triolo, New experimental evidence supporting the mesoscopic segregation model in room temperature ionic liquids, Faraday Discuss. 154 (2012) 97–109. [11] A. Triolo, O. Russina, H.-J. Bleif, E. Di Cola, Nanoscale segregation in room temperature ionic liquids, J. Phys. Chem. B 111 (2007) 4641–4644. [12] M.A.A. Rocha, C.M.S.S. Neves, M.G. Freire, O. Russina, A. Triolo, J.A.P. Coutinho, L.M.N.B.F. Santos, Alkylimidazolium based ionic liquids: impact of cation symmetry on their nanoscale structural organization, J. Phys. Chem. B 117 (2013) 10889–10897. [13] M. Deetlefs, C. Hardacre, M. Nieuwenhuyzen, A.A.H. Padua, O. Sheppard, A.K. Soper, Liquid structure of the ionic liquid 1,3-dimethylimidazolium bis{(trifluoromethyl)sulfonyl}amide, J. Phys. Chem. B 110 (2006) 12055–12061. [14] K. Fujii, R. Kanzaki, T. Takamuku, Y. Kameda, S. Kohara, M. Kanakubo, M. Shibayama, S.-i. Ishiguro, Y. Umebayashi, Experimental evidences for molecular origin of low-q peak in neutron/X-ray scattering of 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids, J. Chem. Phys. 135 (2011) 244502. [15] H.K. Kashyap, C.S. Santos, H.V.R. Annapureddy, N.S. Murthy, C.J. Margulis, J.E.W. Castner, Temperature-dependent structure of ionic liquids: X-ray scattering and simulations, Faraday Discuss. 154 (2012) 133–143. [16] A. Triolo, O. Russina, B. Fazio, G.B. Appetecchi, M. Carewska, S. Passerini, Nanoscale organization in piperidinium-based room temperature ionic liquids, J. Chem. Phys. 130 (2009) 164521. [17] S. Li, J.L. Bañuelos, J. Guo, L. Anovitz, G. Rother, R.W. Shaw, P.C. Hillesheim, S. Dai, G.A. Baker, P.T. Cummings, Alkyl chain length and temperature effects on structural properties of pyrrolidinium-based ionic liquids: a combined atomistic simulation and small-angle X-ray scattering study, J. Phys. Chem. Lett. 3 (2011) 125–130. [18] C.S. Santos, N.S. Murthy, G.A. Baker, E.W. Castner, Communication: X-ray scattering from ionic liquids with pyrrolidinium cations, J. Chem. Phys. 134 (2011) 121101. [19] H.V.R. Annapureddy, H.K. Kashyap, P.M. De Biase, C.J. Margulis, What is the origin of the prepeak in the X-ray scattering of imidazolium-based room-temperature ionic liquids? J. Phys. Chem. B 114 (2010) 16838–16846.

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Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),

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Please cite this article as: A.M. Moschovi, V. Dracopoulos, Structure of protic (HCnImNTf2, n = 0–12) and aprotic (C1CnImNTf2, n = 1–12) imidazolium ionic liquids: A vibrational spectroscopic study, J. Mol. Liq. (2015),