Phase Transitions of Triflate-Based Ionic Liquids under High Pressure

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Phase Transitions of Triflate-Based Ionic Liquids under High Pressure Luiz Felipe O. Faria, and Mauro Carlos Costa Ribeiro J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b08242 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 17, 2015

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The Journal of Physical Chemistry

Phase Transitions of Triflate-Based Ionic Liquids under High Pressure Luiz F. O .Faria1 and Mauro C. C. Ribeiro1* 1

Laboratório de Espectroscopia Molecular, Instituto de Química, Universidade de São

Paulo, CP 26077, CEP 05513-970, São Paulo, SP, Brazil. ABSTRACT. Raman spectroscopy has been used to study phase transitions of ionic liquids based on the triflate anion, [TfO]-, as a function of pressure or temperature. Raman spectra of ionic liquids containing the cations 1-butyl-3-methylimidazolium, [C4C1Im]+, 1-octyl-3-methylimidazolium, [C8C1Im]+, 1-butyl-2,3-dimethylimidazolium, [C4C1C1Im]+,

and

1-butyl-1-methylpyrrolidinium,

[C4C1Pyr]+,

were

compared.

Vibrational frequencies and binding energy of ionic pairs were calculated by Quantum Chemistry methods. The ionic liquids [C4C1Im][TfO] and [C4C1Pyr][TfO] crystallize at 1.0 GPa when the pressure is increased in steps of ~ 0.2 GPa from the atmospheric pressure, whereas [C8C1Im][TfO] and [C4C1C1Im][TfO] do not crystallize up to 2.3 GPa of applied pressure. The low-frequency range of the Raman spectrum of [C4C1Im][TfO] indicates that the system undergoes glass transition, rather than crystallization, when the pressure applied on the liquid has been increased above 2.0 GPa in a single step. It has been found strong hysteresis of spectral features (frequency shift and bandwidth) of the high pressure crystalline phase when the pressure was released stepwise back to the atmospheric pressure.

KEYWORDS: ionic liquids, Raman spectroscopy, high pressure, phase transition. 1 ACS Paragon Plus Environment


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1. INTRODUCTION The interplay between forces of distinct nature in ionic liquids creates a rich phenomenology of phase transitions.1-4 Depending on the cooling rate, many ionic liquids exhibit glass transition rather than crystallization, and in the context of the dynamics of glass transition they are usually fragile glass-forming liquids.2,5-8 Cold crystallization when heating the glass or solid-solid transition of crystalline phases might be observed.2,8,9 Crystallization or glass transition has been also reported when increasing pressure at room temperature.10-18 It would be interesting to understand the structural motifs resulting from the balance of different intermolecular forces in ionic liquids. Although Coulomb interaction is the dominant force, hydrogen bonding plays an important role in structure and dynamic of ionic liquids.4,19-21 Other characteristic feature of ionic liquids is the occurrence of nanodomains in the bulk when increasing the alkyl-chain in the cation structure.22-26 These heterogeneities arise from aggregation of alkyl-chains in nonpolar domains because of van der Waals interactions, and polar domains of the charged head of cations and anions. Formation of another nanostructured domain is also possible in fluorinated ionic liquids.27 In this work we are concerned with ionic liquids based on the trifluoromethanesulfonate anion, [CF3SO3]-, or triflate, [TfO]-. Several vibrational spectroscopy studies of triflate anion have been reported mainly because [TfO]- salts are commonly used in polymer electrolytes.28-33 The vibrational modes of the CF3 and SO3 groups are good probes to unravel the coordination environment, whether the [TfO]- is “free”, encaged in ionic pair or forming aggregates. In the crystalline phases of 1-ethyl3-methylimidazolium

triflate,

[C2C1Im][TfO],

and

1-butyl-3-methylimidazolium

triflate, [C4C1Im][TfO], it has been shown that oxygen atoms of the SO3 group are

2 ACS Paragon Plus Environment

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coordinated to the most acidic hydrogen atom, which is in between the nitrogen atoms in the imidazolium ring (the C(2)-H hydrogen atom).34-40 We used Raman spectroscopy to study phase transitions of [C4C1Im][TfO] at low temperature at atmospheric pressure, and under high pressure at room temperature. For comparison purposes, related ionic liquids with the cations 1-octyl-3methylimidazolium, [C8C1Im]+, 1-butyl-2,3-dimethylimidazolium, [C4C1C1Im]+, and 1butyl-1-methylpyrrolidinium, [C4C1Pyr]+, were also investigated. (Molecular structures of the species investigated in this work are shown in Figure 1). It is known that methylation of the C2 carbon atom, resulting in the [C4C1C1Im]+ cation, implies higher viscosity and melting point,34-36,41-49 even though an important site for hydrogen bonding is eliminated.34,41-50 On the other hand, increase of the alkyl-chain in [C8C1Im]+ allows for more defined segregation of non-polar and polar domains, and [C4C1Pyr]+ is an example of a non-aromatic cation. We used the SO3 stretching and the CF3 bending mode as probes of the local anion environment, and other Raman bands as signature of conformation of the alkyl chain of the cation. In order to help interpretation of experimental results, Quantum Chemistry calculations were performed to obtain optimized structures and vibrational frequencies of ionic pairs. We found that [C4C1Im][TfO] undergoes crystallization at 1.0 GPa or glass transition in the diamond anvil cell depending on the way of increasing the pressure. [C4C1Im][TfO] crystallizes when pressure increases stepwise from atmospheric pressure to 1.0 GPa, but it vitrifies when pressure increases in a single step from the liquid phase range to far above the crystallization pressure. An interesting finding of the Raman measurements under high pressure is that once the crystalline phase of [C4C1Im][TfO] is formed at 1.0 GPa, the spectra exhibit strong hysteresis as pressure is released back to the atmospheric pressure.



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CH3 H3C

N

N

N CH3

1-butyl-3-methylimidazolium [C4C1Im]+ CH3 CH3 H3C

N

CH3

N

1-butyl-1-methylpyrrolidinium [C4C1Pyr]+

1-butyl-2,3-dimethylimidazolium [C4C1C1Im]+

F

O

C

S

F

O

CH3 H3C

N

F

N

O

1-octyl-3-methylimidazolium

trifluoromethanesulfonate

[C8C1Im]+

triflate, [TfO]-

Figure 1. Ionic structures and notation of the species investigated in this work.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS All ionic liquids used in this work were purchased from Iolitec and used without further purification. They were dried under high vacuum (below 10-5 mbar) for several days. Raman spectra as a function of temperature at atmospheric pressure were recorded with a Jobin-Yvon T64000 triple monochromator spectrometer equipped with CCD. The spectra were excited with the 647.1 nm line of a mixed argon-krypton Coherent laser, and the spectral resolution was 2.0 cm-1. Raman spectra were obtained in 180o scattering geometry with no selection of polarization of the scattered radiation. Temperature control was achieved with an OptistatDN cryostat (Oxford Instruments). The ionic liquids were cooled at a typical rate of 20 Kmin-1, waiting about 30 minutes for equilibration before acquisition of the spectrum at each temperature.


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Raman spectra as a function of pressure at room temperature were obtained in a Renishaw Raman imaging microscope (inVia) with a Leica microscope and CCD detector using a laser line at 785 nm (Renishaw diodo laser) focused into the sample by a 20x Leica objective. The spectral resolution was kept at 2.0 cm-1. In the low-frequency region (ω < 200 cm-1), Raman spectra were obtained with the above mentioned JobinYvon T64000 spectrometer having a coupled Olympus BX41 microscope. High pressure was achieved with a diamond anvil cell from Almax easyLab, model Diacell® LeverDAC-Maxi, having a diamond culet size of 500 µm. The Boehler microDriller (Almax easyLab) was used to drill a 250 µm hole in a stainless steel gasket (10 mm diameter, 250 µm thick) preindented to ∼150 µm. Pressure calibration has been done by the usual method of measuring the shift of the fluorescence line of ruby.51 Pressure was increased stepwise, typically at steps of ~ 0.1 GPa, taking about 30 minutes at each pressure for equilibration and data acquisition. Raman bands fitting was performed with Fityk program (version 0.9.8) using Voigt functions.52 Quantum Chemistry calculations to obtain ionic pair structures and the corresponding vibrational frequencies were performed using the Gaussian03 package.53 Calculations were performed with Density Functional Theory (DFT) using the Becke’s three-parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP)54,55 with 6-311++G(d,p) basis set. No imaginary vibrational frequencies were obtained indicating that the vacuum ion-pair geometry was at the minimum of the potential surface. Binding energies were also calculated as the difference between optimized ion-pair and isolated ion energies. The base-set superposition error (BSSE) and zero point energies (ZPE) were not considered as showed to be negligible compared with binding energies.4



3. RESULTS AND DISCUSSION Signatures of crystallization at low temperature or at high pressure (LT and HP crystal, respectively) in the Raman spectra of [C4C1Im][TfO] are shown in Figure 2 . Noteworthy when HP crystal is formed at 1.0 GPa is that the vibrational frequency of the intense ν(SO3) band at ∼ 1035 cm-1 is lower than the frequency of the LT crystal at atmospheric pressure. The δ(CF3) mode at 757 cm-1 shown in the inset of Figure 2 (and other vibrational frequencies not shown) shifts to higher values as usually observed with increasing pressure. The 580 – 650 cm-1 spectral range shown in the inset of Figure 2 exhibits vibrational modes that can be used to assign different alkyl-chain conformers of the [C4C1Im]+ cation. Bands at 600 and 623 cm-1 in the liquid phase are assigned to gauche and trans conformers, respectively.6,56 The lower inset of Fig. 2 shows that there is a mixture of conformers in liquid [C4C1Im][TfO] (two bands in the black line spectrum), which is an entropic effect that contributes for the low melting point of ionic liquids. In contrast, the Raman spectra indicate gauche conformation in both LT and HP crystals (single band in the blue and red lines spectra in the lower inset of Fig. 2).

Raman Intensity

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Liquid Room conditions Crystal LT T = 230 K Crystal HP P = 1.3 GPa

740

750

600

980

1000

1020

1040

760

770

620

1060

1080

780

640

1100

wavenumber / cm-1 6 ACS Paragon Plus Environment

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Figure 2. Raman spectra of [C4C1Im][TfO] in different conditions of pressure and temperature. The intense band in the main figure corresponds to the ν(SO3) stretching mode. The band at 1023 cm-1 is assigned to an imidazolium ring deformation mode.45,56 The upper inset shows the Raman band corresponding to the δ(CF3) bending mode, and the lower inset covers the spectral range giving signatures of alkyl-chain conformers. Intensities have been normalized by the most intense band in each region.

Figure 3 shows pressure and temperature dependences of ν(SO3) and δ(CF3) vibrational frequencies of [C4C1Im][TfO]. Phase transition is clearly indicated by discontinuity of the ν(SO3) vibrational frequency in this plot. In the case of the δ(CF3) vibrational frequency, the discontinuity at the liquid–crystal transition is less evident. Nevertheless, the plots of full width at half-maximum (FWHM) of both the modes shown in Figure 3 clearly indicate that the bands sharpen when the ordered crystalline structure develops. The ν(SO3) frequency and FWHM show an evident hysteresis when releasing the pressure back to atmospheric pressure (empty symbols in Figure 3). This finding was not reported for other ionic liquids when releasing the pressure from crystalline phases obtained under high pressure.12,13,57 It is worth stressing that within the hysteresis range of pressure, the [C4C1Im][TfO] remained in the crystalline phase when the sample was left for about one week inside the DAC.



10

1038

Crystal HP P ∼ 1 GPa

765

FW HM / cm-1

ν / cm-1

νs (SO3)

1034 768

6 4 8

δ s (CF 3 )

6

762 759 756

ν s (SO 3)

8

1036

4

δs (CF3) 0.0

0.4

0.8

1.2

1.6

2

2.0

0.0

0.4

P / GPa 6

1036

Crystal LT T ~ 254 K

1034

1.2

1.6

2.0

ν s (SO 3 )

4

νs (SO 3) FWHM / cm-1

1038

0.8

P / GPa

1040

ν / cm-1

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758

2 0 6

δ s (CF 3 )

4

δs (CF3) 2

757

100

150

200

250

300

100

150

T/K

200

250

300

T/K

Figure 3. Pressure (top panels) and temperature (bottom panels) dependence of the νs(SO3) and δs(CF3) vibrational frequencies of [C4C1Im][TfO] (left panels), and the corresponding band width (right panels). Full symbols indicate increasing pressure or decreasing temperature, while empty symbols indicate releasing pressure or increasing temperature. The vertical dashed lines indicate pressure and temperature of crystallization. The indicated crystallization temperature is the reported value obtained by calorimetry.58 Red arrows help visualization of hysteresis.


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The results shown in Fig. 3 were obtained by stepwise increase of pressure, followed by stepwise release, within the pressure range shown in the figure. In another experiment, being [C4C1Im][TfO] still in the liquid phase at 0.6 GPa, we increased the pressure in a single step far above 1.0 GPa. Figure 4 shows the pressure dependence of the ν(SO3) vibrational frequency of [C4C1Im][TfO] in this experiment. In this case, [C4C1Im][TfO] did not crystallize, instead it went to a glassy state, so that the ν(SO3) vibrational frequency of glassy phase is a linear extrapolation of frequency values of liquid phase. Waiting for enough time once the glassy phase of [C4C1Im][TfO] was obtained, no crystallization of the sample was observed after several hours. An evidence that the system has achieved a glassy state is provided by the comparison of band shapes of the low-frequency Raman spectrum of [C4C1Im][TfO] at 2.4 GPa and the HP crystal spectrum at 2.5 GPa (see insets of Figure 4). The lowfrequency spectrum of the high-pressure glassy phase of [C4C1Im][TfO] is analogous to Raman spectra of low-temperature glassy phases of several ionic liquids reported in the literature.59,60 Another experimental evidence that [C4C1Im][TfO] undergoes glass transition is provided by the broad fluorescence line of the ruby once the glassy phase is formed. We have obtained the glass transition pressure of glass forming ionic liquids by plotting the bandwidth of the ruby fluorescence line as a function of pressure, Γ(P).16 The Γ(P) curve changes slope at the glass transition. However, in the case of [C4C1Im][TfO], it is not possible to obtain the glass transition pressure from the full plot

Γ(P) because [C4C1Im][TfO] crystallizes if pressure is increased stepwise. Nevertheless, it is clear from Figure 5 that Γ at 2.4 GPa, corresponding to glassy phase of [C4C1Im][TfO] after sudden increase of pressure, is higher than the bandwidth of the

Γ(P) curve obtained when [C4C1Im][TfO] crystallizes by stepwise increase of pressure.



1040

Boson peak imidazolium ring libration

2.4 GPa 60

120

180

240

wavenumber / cm-1

Raman Intensity

ν / cm

-1

1042

Raman Intensity

1044

1038 1036

imidazolium ring libration

2.5 GPa 60

120

180

240

wavenumber / cm-1

1034

0.0

0.5

1.0

1.5

2.0

2.5

P / GPa

Figure 4. Pressure dependence of the ν(SO3) vibrational frequency when pressure was increased in a single step from 0.6 to 2.4 GPa leading [C4C1Im][TfO] to a glassy state. The dashed line is a guide to the eye. The top inset shows the low-frequency Raman spectrum of the glass at 2.4 GPa, and the bottom inset shows the low-frequency Raman spectrum of the HP crystal at 2.5 GPa. 1.5

Γ(P) − Γ(0) / cm

-1

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1.0

0.5

0.0 0.0

0.5

1.0

1.5

2.0

2.5

P / GPa

Figure 5. The small white triangles show the pressure dependence of the bandwidth of the R1 fluorescence line of ruby, Γ(P), along stepwise increase of applied pressure on [C4C1Im][TfO]. Bandwidth at each P is given as difference from the atmospheric pressure value, Γ(P) – Γ(0). The vertical dashed line at 1.0 GPa indicates the crystallization pressure. The large black circles show Γ(P) when pressure was increased in a single step from 0.6 GPa leading [C4C1Im][TfO] to a glassy state at 2.4 GPa. 10 ACS Paragon Plus Environment

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It is interesting to compare the high-pressure behavior of [C4C1Im][TfO] with other triflate-based ionic liquids with related cations. We did not observe crystallization of [C8C1Im][TfO] when the pressure was increased up to 2.3 GPa. [C4C1C1Im][TfO] is a solid at room condition, but it promptly melts few degrees above room temperature. We also found that [C4C1C1Im][TfO] does not crystallize under high pressure. Accordingly, Figure 6 shows that the vibrational frequency and the FWHM of the ν(SO3) Raman band of [C8C1Im][TfO] and [C4C1C1Im][TfO] increase smoothly with pressure. In contrast, Figure 6 shows that [C4C1Pyr][TfO] crystallizes at ∼1.0 GPa, and clear hysteresis is observed when releasing the pressure. It is worth mentioning that all of these four ionic liquids crystallize at low temperature as already known from

A

1038 1035 1032

Crystal

1029

ν / cm-1

1042 1040

B

1038 1036

1038

C

Crystal HP P ~ 1 GPa

1036 1034 1032 1030

0.0

0.6

1.2

12

10

FWHM / cm-1

1034

FWHM / cm-1

ν / cm-1

1041

FWHM / cm-1

differential scanning calorimetry.34,58,61,62

ν / cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8

2.4

10 8 6 Crystal

4

9 8 7 8 7 6 5 4

0.0

P / GPa

0.6

1.2

1.8

2.4

P / GPa



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Figure 6. Pressure dependence of the vibrational frequency and FWHM of the ν(SO3) Raman band of [C4C1C1Im][TfO] (A), [C8C1Im][TfO] (B), and [C4C1Pyr][TfO] (C). Full symbols indicate increasing pressure, while empty symbols indicate releasing pressure. [C4C1C1Im][TfO] crystallizes during the decompression process when approaching atmospheric pressure. The vertical dashed line indicate the pressure of crystallization in [C4C1Pyr][TfO], and the red arrows indicate the hysteresis.

Strong anion–cation interaction in the HP crystal phase of [C4C1Im][TfO] is suggested by the downward shift of the ν(SO3) vibrational frequency. Two different DFT optimized structures are shown in Figure 7 for each ionic pair of [C4C1Im][TfO], [C4C1C1Im][TfO], and [C4C1Pyr][TfO]. Table 1 gives the corresponding binding energy (BEB3LYP) and calculated ν(SO3) and δ(CF3) frequencies. Similar energy differences between structures of types I and II were found for analogous structures of [C4C1Im]Cl,41 [C3C1Im]I,46 and [C2C1Im][Tf2N].35 The BEB3LYP obtained for AI and BI, respectively, -342 and -318 kJ mol-1, are close to the value found for the less coordinating anion bis(trifluoromethanesulfonyl)imide, [Tf2N]-, in [C2C1Im][Tf2N] and [C2C1C1Im][Tf2N], respectively, -313 and -299 kJ mol-1.35 Furthermore, when we optimized [C4C1Im][TfO] ionic pairs with [C4C1Im]+ in the gauche conformation, instead of the trans conformation shown in Fig. 7, we found BEB3LYP -343 and -291 kJ.mol-1 for structures of types I and II, that is essentially the same values as for the

trans conformer of AI and AII structures. Table 1 indicates that the lowest ν(SO3) frequency corresponds to the AI structure, which is the one with strong C(2)-H...O interaction and highest binding energy between cation and anion. Quantum Chemistry calculations and vibrational spectroscopy of Li[TfO] in different solvents showed that coordination of lithium by the oxygen atoms of [TfO]- causes electronic redistribution on the anion that increases the vibrational frequency of the -CF3 group.63 In line with


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these previous results, the δ(CF3) frequency calculated for the AI structure is higher than the isolated anion.

Figure 7. Optimized DFT structures of [C4C1Im][TfO], [C4C1C1Im][TfO], and [C4C1Pyr][TfO] (from left to right). The low-energy structures are labeled AI, BI, and CI in the top. The [C4C1Pyr]+ conformation in the CI structure is axial-envelope-trans, and is equatorial-envelope-trans in the CII structure.



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Table 1. Binding energies (BEB3LYP, kJ mol-1) and calculated vibrational frequencies (cm-1) of the optimized ion-pair structures shown in Figure 7.

AI AII BI BII CI CII isolated [TfO]-

BEB3LYP

νs(SO3)

δs(CF3)

-342 -290 -318 -295 -325 -319

967 972 973 973 970 970 975

742 741 738 739 742 743 729

The finding that the spectral features of [C4C1Pyr][TfO] under high pressure (Figure 6C) is similar to [C4C1Im][TfO] indicates that the strong hysteresis observed in Figure 3 should not be assigned only to specific anion interaction with the C(2)-H site of the imidazolium ring. In contrast, [C4C1C1Im][TfO] and [C8C1Im][TfO] do not crystallizes up to 2.3 GPa of applied pressure. The glass transition of [C8C1Im][TfO] under high pressure is understood by the fact that the long alkyl-chain assume different conformations being difficult to rearrange at very high density and frustrates crystal packing. On the other hand, the enhanced glass-forming ability under high pressure of [C4C1C1Im][TfO] in comparison to [C4C1Im][TfO] is hardly understood on the basis of conformational flexibility given by the CH3 group added to the C(2) atom of the imidazolium ring. The microscopic origin of increase viscosity of ionic liquids due to methylation of the most acidic hydrogen of imidazolium ring is an unsettled issue.35,41,46,49 Hunt et al.41 proposed that loss of variation of anion–cation arrangements in [C4C1C1Im]+ based ionic liquids increases the melting point and viscosity. Izgorodina et al.46 proposed that a higher barrier of potential energy restricts ionic mobility in C(2) methylated ionic liquid. In this way, the enhanced glass forming ability of


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[C4C1C1Im][TfO] under high pressure should be assigned to difficulty of crossing potential barriers in such a high density condition as it would be necessary for structural rearrangements that lead to crystal packing. Ludwig et at.35 proposed that hydrogen bonding of anion to the C(2)-H site of the imidazolium ring causes defects in the characteristic charge ordering arrangement of an ionic system, so that a C(2) methylated ionic liquid would have a more ordered Coulomb network. This picture seems less consistent with the finding that [C4C1Im][TfO] easily crystallizes under high pressure, whereas [C4C1C1Im][TfO] does not. A recent free volume study of [Tf2N]- based ionic liquids with [CnC1Im]+ and [CnC1C1Im]+ cations (n = 3 – 8) showed that free volume decreases by C2 methylation.49 Thus, in comparison to [C4C1Im][TfO], less free volume in [C4C1C1Im][TfO] could also contribute to the enhanced glass-forming ability under high pressure.

4. CONCLUSIONS Liquid, crystalline, and glassy states of ionic liquids based on the triflate anion were investigated by Raman spectroscopy as a function of temperature or pressure. The calculated ν(SO3) frequency correlates with binding energy, reinforcing the appropriateness of the ν(SO3) mode as a good probe to study cation-anion interaction. It is worth mentioning that a low-frequency mode assigned to intermolecular hydrogen bonding has been observed by infrared spectroscopy.64,65 This low-frequency mode also correlates with calculated binding energy of many ionic liquids, including [TfO]- based liquids. Even though Coulomb interaction is the main intermolecular force in ionic liquids, hydrogen bonding, dispersion forces, and entropic effects results in a rich phenomenology of phase transitions. In the case of [C4C1Im][TfO], an important



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structural motif is the C(2)-H…O cation-anion interaction driving crystal packing at low-temperature or high pressure. However, X-ray diffraction data showed that F…F interaction also plays a role in crystalline packing.39,40 A longer alkyl chain in [C8C1Im][TfO] and methylation of the most acidic hydrogen atom of the imidazolium ring in [C4C1C1Im][TfO] inhibited crystallization under high pressure. Both [C4C1C1Im][TfO] and [C8C1Im][TfO] undergo glass transition under high pressure. Evidence of strong anion–cation interaction resulting in a significant downward shift of ν(SO3) vibrational frequency was observed when crystallization of [C4C1Im][TfO] and [C4C1Pyr][TfO] take place under high pressure. Plots of vibrational frequency vs. pressure exhibits hysteresis within a large range of pressure when releasing the applied pressure on the crystalline phases of [C4C1Im][TfO] and [C4C1Pyr][TfO]. X-ray diffraction experiments as a function of pressure covering the hysteresis range deserve future studies.

AUTHOR INFORMATION Corresponding Author * Dr. Mauro C. C. Ribeiro. E-mail: [email protected].

ACKNOWLEDGMENT The authors are indebted to FAPESP and CNPq for financial support.


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REFERENCES (1) Weingärtner, H., Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies. Angew. Chem. Int. Edit. 2008, 47, 654-670. (2) Mudring, A. V., Solidification of Ionic Liquids: Theory and Techniques. Aust. J. Chem. 2010, 63, 544-564. (3) Castner, E. W.; Margulis, C. J.; Maroncelli, M.; Wishart, J. F., Ionic Liquids: Structure and Photochemical Reactions. Annu. Rev. Phys. Chem. 2011, 62, 85105. (4) Tsuzuki, S.; Shinoda, W.; Miran, M. S.; Kinoshita, H.; Yasuda, T.; Watanabe, M., Interactions in Ion Pairs of Protic Ionic Liquids: Comparison with Aprotic Ionic Liquids. J. Chem. Phys. 2013, 139, 174504. (5) Xu, W.; Cooper, E. I.; Angell, C. A., Ionic Liquids: Ion Mobilities, Glass Temperatures, and Fragilities. J. Phy. Chem. B 2003, 107, 6170-6178. (6) Hamaguchi, H.-O.; Ozawa, R., Structure of Ionic Liquids and Ionic Liquid Compounds: Are Ionic Liquids Genuine Liquids in the Conventional Sense? In Adv. Chem. Phys.; Rice, S. A., Ed., 2005, 131, 85-104. (7) Yamamuro, O.; Inamura, Y.; Hayashi, S.; Hamaguchi, H., Glass Transitions and Low-Frequency Dynamics of Room-Temperature Ionic Liquids. In Flow Dynamics; Tokuyama, M., Maruyama, S., Eds., 2006, 832, 73-80. (8) Faria, L. F. O.; Matos, J. R.; Ribeiro, M. C. C., Thermal Analysis and Raman Spectra of Different Phases of the Ionic Liquid Butyltrimethylammonium Bis(trifluoromethylsulfonyl)imide. J. Phys. Chem. B 2012, 116, 9238−9245. (9) Erdmenger, T.; Vitz, J.; Wiesbrock, F.; Schubert, U. S., Influence of Different Branched Alkyl Side Chains on the Properties of Imidazolium-Based Ionic Liquids. J. Mater. Chem. 2008, 18, 5267-5273. (10) Su, L.; Li, M.; Zhu, X.; Wang, Z.; Chen, Z.; Li, F.; Zhou, Q.; Hong, S., In Situ Crystallization of Low-Melting Ionic Liquid BMIM PF(6) under High Pressure up to 2 GPa. J Phys. Chem. B 2010, 114, 5061-5065. (11) Russina, O.; Fazio, B.; Schmidt, C.; Triolo, A., Structural Organization and Phase Behaviour of 1-Butyl-3-methylimidazolium Hexafluorophosphate: An High Pressure Raman Spectroscopy Study. Phys. Chem. Chem. Phys. 2011, 13, 12067-12074. (12) Yoshimura, Y.; Abe, H.; Takekiyo, T.; Shigemi, M.; Hamaya, N.; Wada, R.; Kato, M., Superpressing of a Room Temperature Ionic Liquid, 1-Ethyl-3methylimidazolium Tetrafluoroborate. J. Phys. Chem. B 2013, 117, 12296−12302. (13) Yoshimura, Y.; Abe, H.; Imai, Y.; Takekiyo, T.; Hamaya, N., Decompression-Induced Crystal Polymorphism in a Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate. J. Phys. Chem. B 2013, 117, 3264-3269. (14) Saouane, S.; Norman, S. E.; Hardacre, C.; Fabbiani, F. P. A., Pinning Down the Solid-State Polymorphism of the Ionic Liquid [bmim][PF6]. Chem. Sci. 2013, 4, 1270-1280.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

(15) Faria, L. F. O.; Penna, T. C.; Ribeiro, M. C. C., Raman Spectroscopic Study of Temperature and Pressure Effects on the Ionic Liquid Propylammonium Nitrate. J. Phys. Chem. B 2013, 117, 10905-10912. (16) Ribeiro, M. C. C.; Pádua, A. A. H.; Gomes, M. F. C., Glass Transition of Ionic Liquids under High Pressure. J. Chem. Phys. 2014, 140, 244514. (17) Li, H.; Su, L.; Zhu, X.; Cheng, X.; Yang, K.; Yang, G., In Situ Crystallization of Ionic Liquid [Emim][PF6] from Methanol Solution under High Pressure. J. Phys. Chem. B 2014, 118, 8684-8690. (18) Yoshimura, Y.; Shigemi, M.; Takaku, M.; Yamamura, M.; Takekiyo, T.; Abe, H.; Hamaya, N.; Wakabayashi, D.; Nishida, K.; Funamori, N.; et al, Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature. J. Phys. Chem. B 2015, 119, 8146-8153. (19) Thar, J.; Brehm, M.; Seitsonen, A. P.; Kirchner, B., Unexpected Hydrogen Bond Dynamics in Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2009, 113, 15129-15132. (20) Matthews, R. P.; Welton, T.; Hunt, P. A., Competitive Pi Interactions and Hydrogen Bonding within Imidazolium Ionic Liquids. Phys. Chem. Chem. Phys. 2014, 16, 3238-3253. (21) Skarmoutsos, I.; Dellis, D.; Matthews, R. P.; Welton, T.; Hunt, P. A., Hydrogen Bonding in 1-Butyl- and 1-Ethyl-3-methylimidazolium Chloride Ionic Liquids. J. Phys. Chem. B 2012, 116, 4921-4933. (22) Triolo, A.; Russina, O.; Bleif, H.-J.; Di Cola, E., Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phys. Chem. B 2007, 111, 4641-4644. (23) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R., Mesoscopic Structural Heterogeneities in Room-Temperature Ionic Liquids. J. Phys. Chem. Lett. 2012, 3, 2733. (24) Kashyap, H. K.; Santos, C. S.; Murthy, N. S.; Hettige, J. J.; Kerr, K.; Ramati, S.; Gwon, J.; Gohdo, M.; Lall-Ramnarine, S. I.; Wishart, J. F.; et al, Structure of 1-Alkyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide Ionic Liquids with Linear, Branched, and Cyclic Alkyl Groups. J. Phy. Chem. B 2013, 117, 15328-15337. (25) Kashyap, H. K.; Santos, C. S.; Annapureddy, H. V. R.; Murthy, N. S.; Margulis, C. J.; Castner, E. W., Temperature-Dependent Structure of Ionic Liquids: Xray Scattering and Simulations. Farad. Discuss. 2012, 154, 133-143. (26) Hardacre, C.; Holbrey, J. D.; Mullan, C. L.; Youngs, T. G. A.; Bowron, D. T., Small Angle Neutron Scattering from 1-Alkyl-3-methylimidazolium Hexafluorophosphate Ionic Liquids ( C(n)mim PF6 , n=4, 6, and 8). J. Chem. Phys. 2010, 133, 074510. (27) Pereiro, A. B.; Pastoriza-Gallego, M. J.; Shimizu, K.; Marrucho, I. M.; Lopes, J. N. C.; Piñeiro, M. M.; Rebelo, L. P. N., On the Formation of a Third, Nanostructured Domain in Ionic Liquids. J. Phys. Chem. B 2013, 117, 10826-10833. (28) Bernson, A.; Lindgren, J., Free Ions and Ion-pairing Clustering in the System LiCF3SO3-PPON. Solid State Ion. 1993, 60, 37-41.


Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Manning, J.; Frech, R., The Structure of Associated Ionic Species in Poly(propylene oxide) Alkali-Metal Trifluoromethanesulfonate Complexes. Polymer 1992, 33, 3487-3494. (30) Jacobsson, P.; Albinsson, I.; Mellander, B. E.; Stevens, J. R., Ion Association Effects and Phase-Separation in Poly(propylene oxide) Modified Poly(dimethylsiloxane) Complexed with Triflate Salts. Polymer 1992, 33, 2778-2783. (31) Stevens, J. R.; Jacobsson, P., A Comparison of Acetone and Poly(propylene glycol) as Solvents for Lithium Triflate and Lithium Perchlorate. Can. J. Chem. 1991, 69, 1980-1984. (32) Schantz, S.; Torell, L. M.; Stevens, J. R., Ion-Pairing Effects in Poly(propylene glycol)-Salt Complexes as a Function of Molecular-Weight and Temperature: A Raman-Scattering Study Using NaCF3SO3 and LiClO4. J. Chem. Phy. 1991, 94, 6862-6867. (33) Kakihana, M.; Schantz, S.; Torell, L. M., Raman-Spectroscopic Study of Ion-Ion Interaction and Its Temperature-Dependence in a Poly(propylene oxide)-Based NaCF3SO3-Polymer Electrolyte. J. Chem. Phys. 1990, 92, 6271-6277. (34) Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M., Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inor. Chem. 1996, 35, 1168-1178. (35) Fumino, K.; Wulf, A.; Ludwig, R., Strong, Localized, and Directional Hydrogen Bonds Fluidize Ionic Liquids. Angew. Chem. Int. Edit. 2008, 47, 8731-8734. (36) Fumino, K.; Wulf, A.; Ludwig, R., The Potential Role of Hydrogen Bonding in Aprotic and Protic Ionic Liquids. Phys. Chem. Chem. Phys. 2009, 11, 87908794. (37) Cremer, T.; Kolbeck, C.; Lovelock, K. R. J.; Paape, N.; Wölfel, R.; Schulz, P. S.; Wasserscheid, P.; Weber, H.; Thar, J.; Kirchner, B.; et al, Towards a Molecular Understanding of Cation–Anion Interactions—Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectroscopy, X-ray Photoelectron Spectroscopy and Theoretical Calculations. Chem. Eur. J. 2010, 16, 9018-9033. (38) Weingärtner, H., NMR Studies of Ionic Liquids: Structure and Dynamics. Curr. Opin. Colloid Interface Sci. 2013, 18, 183-189. (39) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A., In situ Crystallization of Ionic Liquids with Melting Points below -25 °C. Crystengcomm 2006, 8, 742-745. (40) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A., In situ Crystallization of Low-Melting Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 16792-16793. (41) Hunt, P. A., Why Does a Reduction in Hydrogen Bonding Lead to an Increase in Viscosity for the 1-Butyl-2,3-dimethyl-imidazolium-Based Ionic Liquids? J. Phys. Chem. B 2007, 111, 4844-4853. (42) Zahn, S.; Bruns, G.; Thar, J.; Kirchner, B., What Keeps Ionic Liquids in Flow? Phys. Chem. Chem. Phys. 2008, 10, 6921-6924.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

(43) Wulf, A.; Fumino, K.; Ludwig, R., Spectroscopic Evidence for an Enhanced Anion-Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids. Angew. Chem. Int. Edit. 2010, 49, 449-453. (44) Endo, T.; Kato, T.; Nishikawa, K., Effects of Methylation at the 2 Position of the Cation Ring on Phase Behaviors and Conformational Structures of Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2010, 114, 9201-9208. (45) Noack, K.; Schulz, P. S.; Paape, N.; Kiefer, J.; Wasserscheid, P.; Leipertz, A., The Role of the C2 Position in Interionic Interactions of Imidazolium Based Ionic Liquids: A Vibrational and NMR Spectroscopic Study. Phys. Chem. Chem. Phys. 2010, 12, 14153-14161. (46) Izgorodina, E. I.; Maganti, R.; Armel, V.; Dean, P. M.; Pringle, J. M.; Seddon, K. R.; MacFarlane, D. R., Understanding the Effect of the C2 Proton in Promoting Low Viscosities and High Conductivities in Imidazolium-Based Ionic Liquids: Part I. Weakly Coordinating Anions. J. Phys. Chem. B 2011, 115, 1468814697. (47) Fumino, K.; Peppel, T.; Geppert-Rybczynska, M.; Zaitsau, D. H.; Lehmann, J. K.; Verevkin, S. P.; Koeckerling, M.; Ludwig, R., The Influence of Hydrogen Bonding on the Physical Properties of Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13, 14064-14075. (48) Zhang, Y.; Maginn, E. J., The Effect of C2 Substitution on Melting Point and Liquid Phase Dynamics of Imidazolium Based-Ionic Liquids: Insights from Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2012, 14, 12157-12164. (49) Chen, Z. J.; Lee, J.-M., Free Volume Model for the Unexpected Effect of C2-Methylation on the Properties of Imidazolium Ionic Liquids. J. Phys. Chem. B 2014, 118, 2712-2718. (50) Peppel, T.; Roth, C.; Fumino, K.; Paschek, D.; Koeckerling, M.; Ludwig, R.; The Influence of Hydrogen-Bond Defects on the Properties of Ionic Liquids. Angew. Chem. Int. Edit. 2011, 50, 6661-6665. (51) Bassett, W. A., Diamond anvil cell, 50th birthday. High Pressure Res. 2009, 29, CP5-186. (52) Wojdyr, M., Fityk: A General-Purpose Peak Fitting Program. J. Appl. Crystallogr. 2010, 43, 1126-1128. (53) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al, Gaussian03W, Wallingford CT, 2004. (54) Becke, A. D., A New Mixing of Hatree-Fock and Local DensityFunctional Theories. J. Chem. Phys. 1993, 98, 1372-1377. (55) Lee, C. T.; Yang, W. T.; Parr, R. G., Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 785-789. (56) Berg, R. W., Raman Spectroscopy and ab-initio Model Calculations on Ionic Liquids. Monatsh. Chem. 2007, 138, 1045-1075. (57) The Structure of Ionic Liquids. In Soft and Biological Matter; Caminiti, R., Gontrani, L., Eds.; Springer, 2014. 20 ACS Paragon Plus Environment

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(58) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S.; Brennecke, J. F., Thermophysical Properties of Imidazolium-Based Ionic Liquids. J. Chem. Eng. Data 2004, 49, 954-964. (59) Penna, T. C.; Faria, L. F. O.; Matos, J. R.; Ribeiro, M. C. C., Pressure and Temperature Effects on Intermolecular Vibrational Dynamics of Ionic Liquids. J. Chem. Phys. 2013, 138, 104503. (60) Ribeiro, M. C. C., Low-Frequency Raman Spectra and Fragility of Imidazolium Ionic Liquids. J. Chem. Phys. 2010, 133, 024503. (61) Papaiconomou, N.; Yakelis, N.; Salminen, J.; Bergman, R.; Prausnitz, J. M., Synthesis and Properties of Seven Ionic Liquids Containing 1-Methyl-3octylimidazolium or 1-Butyl-4-methylpyridinium Cations. J. Chem. Eng. Data 2006, 51, 1389-1393. (62) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M., Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species. J. Phys. Chem. B 2004, 108, 16593-16600. (63) Huang, W. W.; Frech, R.; Wheeler, R. A., Molecular-Structures and Normal Vibrations of CF3SO3 and Its Lithium Ion-Pairs and Aggregates. J. Phys. Chem. 1994, 98, 100-110. (64) Fumino, K.; Wulf, A.; Ludwig, R., The Cation-Anion Interaction in Ionic Liquids Probed by Far-Infrared Spectroscopy. Angew. Chem. Int. Edit. 2008, 47, 38303834. (65) Fumino, K.; Fossog, V.; Wittler, K.; Hempelmann, R.; Ludwig, R., Dissecting Anion-Cation Interaction Energies in Protic Ionic Liquids. Angew. Chem. Int. Edit. 2013, 52, 2368-2372.



Table of Contents

1044

ass Gl

stepwise increase of pressure sudden increase of pressure decompression

1040

l sta y r C

ν / cm-1

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qu Li

1036

1032

0.0

id

0.5

1.0

1.5

2.0

2.5

P / GPa


Phase Transitions of Triflate-Based Ionic Liquids under High Pressure

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