Effects of methylation at the 2 position of the cation ring on phase behaviors and conformational structures of imidazolium-based ionic liquids

Article abstract of DOI:10.1021/jp104123v

The proton at the 2 position of the cation ring in imidazolium-based ionic liquids (ILs) strongly interacts with anions; therefore, the methylation at this position (C(2) methylation) causes significant changes in the physicochemical properties of these liquids. We investigated the C(2) methylation effects on the phase behaviors and cation conformations of ILs by calorimetric and Raman spectroscopic measurements, focusing on the pairs of 1-butyl-3-methylimidazolium salt ([C₄mim]X) and 1-butyl-2,3-dimethylimidazolium salt ([C ₄C₁mim]X), where X^- is Cl^-, Br ^-, I^-, BF₄^-, and PF₆ ^-. The melting and freezing points of all pairs increased after the C(2) methylation, as reported previously, and the reason for the increase was the overcompensation of the ??S_trans decrease for the ??H_trans decrease. The C(2) methylation also affected the phase behaviors of the ILs. With Raman spectroscopic measurements, all cation conformations in crystalline phases were assigned to trans-trans (TT), gauche-trans (GT), or gauchea?2-trans (Ga?2T) conformers of the butyl group. Except in [C₄C₁mim]BF₄, all crystal-crystal phase transitions of the present samples occurred accompanied by conformational changes among TT, GT, and Ga?2T. For the gas states of [C₄mim]⁺ and [C₄C₁mim]⁺, DFT calculations showed that there were hardly any differences in the structures of the butyl group for each set of paired conformers or in the energetic orders among the conformers. On the other hand, the conformer adopted in the crystalline phase differed between [C₄mim]X and [C₄C ₁mim]X. In addition, the population of the conformers in the liquid state also differed in each pair. The data from higher frequency Raman spectra suggested that the difference in cation conformation in each pair, for the crystalline and liquid states, was due to the shift in the position of the anion relative to that of the cation. By C(2) methylation, the relative distance between the anion and cation decreased for Cl^-, Br^-, and I^- salts, but it increased for BF₄^- and PF ₆^- salts. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp104123v

J. Phys. Chem. B 2010, 114, 9201–9208
9201
Effects of Methylation at the 2 Position of the Cation Ring on Phase Behaviors and
Conformational Structures of Imidazolium-Based Ionic Liquids
Takatsugu Endo, Tatsuya Kato, and Keiko Nishikawa*
Graduate School of AdVanced Integration Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
ReceiVed: May 6, 2010; ReVised Manuscript ReceiVed: June 7, 2010
The proton at the 2 position of the cation ring in imidazolium-based ionic liquids (ILs) strongly interacts with
anions; therefore, the methylation at this position (C(2) methylation) causes significant changes in the
physicochemical properties of these liquids. We investigated the C(2) methylation effects on the phase behaviors
and cation conformations of ILs by calorimetric and Raman spectroscopic measurements, focusing on the
pairs of 1-butyl-3-methylimidazolium salt ([C
where X is Cl , Br , I , BF , and PF
4 6
4
mim]X) and 1-butyl-2,3-dimethylimidazolium salt ([C
4
C
1
mim]X),
. The melting and freezing points of all pairs increased after the
C(2) methylation, as reported previously, and the reason for the increase was the overcompensation of the
trans decrease for the ∆Htrans decrease. The C(2) methylation also affected the phase behaviors of the ILs.
-
-
-
-
-
-
∆
S
With Raman spectroscopic measurements, all cation conformations in crystalline phases were assigned to
trans-trans (TT), gauche-trans (GT), or gauche′-trans (G′T) conformers of the butyl group. Except in
4 1 4
[C C mim]BF , all crystal-crystal phase transitions of the present samples occurred accompanied by
+
+
4 4 1
conformational changes among TT, GT, and G′T. For the gas states of [C mim] and [C C mim] , DFT
calculations showed that there were hardly any differences in the structures of the butyl group for each set
of paired conformers or in the energetic orders among the conformers. On the other hand, the conformer
4 4 1
adopted in the crystalline phase differed between [C mim]X and [C C mim]X. In addition, the population of
the conformers in the liquid state also differed in each pair. The data from higher frequency Raman spectra
suggested that the difference in cation conformation in each pair, for the crystalline and liquid states, was due
to the shift in the position of the anion relative to that of the cation. By C(2) methylation, the relative distance
-
-
-
-
-
between the anion and cation decreased for Cl , Br , and I salts, but it increased for BF
4 6
and PF salts.
in density,1
1,17,18
conductivity,11,13,16,17,21 polarity,13,27 and liquid
Introduction
28
crystallinity. It is also reported that the C(2) methylation affects
Ionic liquids (ILs), which are composed solely of ions, are
salts with low melting points. ILs are well-known to have certain
outstanding properties as solvents, such as extremely low
volatility and flammability, unique solubility, and wide elec-
trochemical window. Therefore, ILs have attracted much at-
tention in recent years and are considered as potential replace-
29
chemical reactivity in the methylated ILs and improves
tribological properties. On the other hand, methylation at the
or 5 position of the imidazolium ring does not seem to affect
the physicochemical properties of the ILs greatly. Therefore,
detailed investigations of the C(2) methylation effects at the
molecular level are required. Recently, some reports on the C(2)
30
4
11
1
-5
ment for traditional volatile organic solvents.
methylation effects of ion structures and interactions have been
published.3
1-36
For example, Hunt predicted by quantum chemi-
Imidazolium-based ILs are the most typical and widely
studied ones. It is well-known that a proton at the 2 position of
an imidazolium ring interacts strongly with anions. Therefore,
the proton at this position plays an important role in the
physicochemical properties, the structure, and the dynamics of
the IL. It is noted that the interaction between the proton and
cal calculations that an increase in the melting points and
viscosities from the C(2) methylation originated from the
reduced entropy.31 However, the C(2) methylation effects are
still unclear because few systematic and comprehensive studies
exist.
In this paper, we report the C(2) methylation effects of
imidazolium-based ILs on their phase behaviors and cation
conformations by investigating with calorimetry and Raman
spectroscopy, comparing five pairs of ILs, each comprising a
nonmethylated and a methylated IL. The ILs used in this study
6-9
anions is generally regarded as hydrogen bonding. However,
whether the interactions are hydrogen bonding in the conven-
1
0
tional sense is still an open question. Methylation at the 2
position of an imidazolium ring (C(2) methylation) significantly
alters the physicochemical properties of ILs because of the
drastic change in the interactions, structures, and dynamics of
the liquid. For example, the C(2) methylation generally induces
are simple and typical; namely, cations of 1-butyl-3-methylimi-
+
dazolium ([C
4
mim] ) and 1-butyl-2,3-dimethylimidazolium
+
-
-
-
-
-
1
1-16
13,15,17-19
(
4 1 4 6
[C C mim] ) and anions of Cl , Br , I , BF , and PF .
an increase in the melting point,
thermal stability,
chemical stability,
vaporization enthalpy, heat
viscosity,1
1,17,20-22
surface tension,
13,23
17,24
Structures and numbering of the atoms for these samples are
shown in Chart 1.
1
1,17
25
electrochemical stability,
capacity,1 and amphiphilicity and also induces a decrease
2,18
26
Experimental Section
-
-
1
. Samples. [C
4
mim]X and [C
4
C
1
mim]X (where X is Cl ,
*
To whom correspondence should be addressed. Fax: +81-43-290-3939.
-
-
-
-
E-mail: k.nishikawa@faculty.chiba-u.jp.
Br , I , BF , and PF ) were prepared by the following
4 6
1
0.1021/jp104123v  2010 American Chemical Society
Published on Web 06/24/2010

9
202 J. Phys. Chem. B, Vol. 114, No. 28, 2010
Endo et al.
CHART 1: Structures of [C
4
mim]X and [C
4 6
, and PF ) and
4
C
1
mim]X
-
-
-
-
-
-
(
where X is Cl , Br , I , BF
Numbering of Atoms
procedures. The ILs including halide anions were synthesized
from 1-methylimidazole or 1,2-dimethylimidazole with halo-
genobutane. These ILs, except [C
several times to obtain colorless ILs. The ILs containing BF
or PF anions were prepared by metathesis of the corresponding
halides using NaBF or NaPF . Then they were washed with
distilled water several times until halide ions were not detected
by AgNO aqueous solution.
All the samples were characterized by H NMR (JEOL JNM-
LA500). In addition, the ILs containing BF and PF anions
were also characterized by elemental analysis (Perkin-Elmer
400), the results of which are summarized in the Supporting
Information. All samples were dried at ∼333 K under vacuum
4
mim]I, were recrystallized
4
6
4
6
3
1
4
6
2
-3
(
10 Pa) over 24 h before use. All sample handling was
performed in a N atmosphere glovebox to avoid absorption of
atmospheric moisture. The water content after completion of
all preparations was below 150 ppm, except for [C mim]I (360
2
4
ppm), as measured by Karl Fischer titration (Mettler-Toledo
model DL39 coulometer).
2
. Measurements. Previously, we constructed an apparatus
combining a commercially available Raman spectrometer and
3
7
a laboratory-made calorimeter. This is applicable as a Raman
spectrometer equipped with a temperature controller with high
stability as well as an apparatus for simultaneous measurement
of Raman spectroscopy and calorimetry. We adopted a Raman
spectrometer with an optical fiber (HoloLab 5000, Kaiser Optical
Systems) and a GaAlAs diode laser (wavelength 785 nm). A
Figure 1. Calorimetric curves of [C
4
mim]X (black curves) and
mim]X (red curves). (a) Cl salts, (b) Br salts, (c) I- salts, (d)
-
-
[
BF
C
4
C
4
1
-
-
salts, and (e) PF
6
salts. Dotted lines indicate crystal-crystal
phase transitions. Temperature range is from 183 K to the temperature
overmelting point, and scanning rate is 5 mK/s. The sequence of cooling
and heating is demonstrated by the black arrows in each panel. Except
4
-1
for [C mim]Cl, all measurements started with sample cooling from the
spectrum in the range of 100-3450 cm can be measured at
-1
liquid state down to 183 K, followed by heating. [C
first with heating from the crystalline state, and then followed by the
same procedure as the other samples. The red arrow in [C mim]Br
indicates a crystal-crystal phase transition in the premelting region.
The small peaks observed at ∼190 K are experimental artifacts. The
blue arrows demonstrate the measuring points of Raman scattering (see
Figure 3).
4
mim]Cl melted
once with a spectral resolution of 4 cm . The maximum output
power of the laser was 400 mW, and the diameter of the laser
spot was ∼80 µm. Curve fittings of Raman bands were
performed by mixed Gaussian and Lorentzian functions. The
laboratory-made calorimeter used here is a simpler type of our
supersensitive DSC, the fundamental and original idea for which
4
C
1
3
8
was previously reported. These calorimeters adopt thermo
modules as both heat-flow sensor and heat pump and enable us
to perform calorimetric measurements with high sensitivity and
temperature stability. The baseline stability and temperature of
the present calorimeter were measured to be ∼5 µW and (0.001
K, respectively. Temperature can be scanned at very slow
heating and cooling rates (minimum value, 0.5 mK/s).
40
based on the Becke’s three-parameter hybrid method with the
LYP correlation (B3LYP).4 No imaginary frequencies were
produced by the optimized structures; this ensured the presence
of a minimum. A scaling factor was not applied to the calculated
frequency or to the thermal properties in the calculations.
1,42
3
. DFT Calculations. DFT calculations were carried out
3
9
Results and Discussion
using the Gaussian 03 program package. Both full geometry
optimizations and normal frequency analyses for the ions in
the gas phase were performed using 6-311+G(d, p) basis sets
1. Phase Behaviors Concerned with Melting and Crystal-
lization. Figure 1 shows calorimetric curves of the ILs studied

Imidazolium-Based Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 28, 2010 9203
TABLE 1: Thermodynamic Properties of [C
the ILsa
4
mim]X and [C
4
C
1
mim]X and the Number of Crystal Phases Observed Here for
∆
H
fusion
∆Sfusion
-1
∆Hfreezing
∆Sfreezing
crystal phase
number
(kJ mol-1)
(J mol K-1
)
fp (K)
(kJ mol-1)
(J mol K-1
)
-1
mp (K)
[C
[C
[C
[C
[C
[C
[C
[C
[C
[C
4
4
4
4
4
4
4
4
4
4
mim]Cl
mim]Cl
mim]Br
mim]Br
mim]I
mim]I
mim]BF
mim]BF
mim]PF
mim]PF
342.3
372.5
353
21
61.3
49.4
75.6
74.4
1
1
1
2
C
1
18.4
26.7
27.5
301.8
-268.5
334
13.4
18.3
17.9
44.4
68.2
53.6
C
1
369.8
C
1
370.8
21.6
58.3
317.9
19
59.8
2
4
C
1
4
313.1
285.3
293.6
17.4
13.3
9
55.6
46.6
30.7
-238.1
-226.4
-247.9
7.2
7.1
5.2
30.2
31.4
21
2
3
1
6
C
1
6
a
The melting and freezing points are defined as the peak-top values of the calorimetric curves.
TABLE 2: Reported Melting Point, ∆Hfusion and ∆Sfusiona
mp (K)/∆Hfusion (kJ mol-1)/
4 4 1
here: left panels for [C mim]X and right panels for [C C mim]X.
The measured temperature range was from 183 K to the
temperature exceeding the individual melting point. To divide
overlaid peaks, the scanning rate was set to 5 mK/s, which is
much lower than the conventional rate. The broken lines indicate
crystal-crystal transitions.
-1
-1
∆S
fusion (J mol
K )
1
2
77
[
C
4
mim]Cl
326.57/10.31/-, 341.95/14.057/-,
43 78
330/14.5/-, 341/25.86/77.1
15
[C
4
4
4
4
4
C
1
mim]Cl
mim]Br
mim]Br
mim]I
mim]I
365.89/14.41/-, 370.0/18.5/-, 373.0/14.53/-79
1
2
80 43
81
[
[
[
C
C
C
347.5/16.3/-, 347.5/23.6/-, 351.35/22.88/65.12
12 15
C
1
349.66/15.62/-, 365.9/20.2/-
55 82
Investigating the calorimetric curves, it is noted that there
270/-/-, 201/-/-
were significant differences in the melting and crystallization
[C
C
1
8
2
+
[C mim]BF
4
192/-/-
267.3/10.0/-
behaviors between [C
4
mim]X and [C
4
C
1
mim]X. All [C
4
C
1
mim] -
4
1
5
[
[
C
C
4
4
C
1
mim]BF
4
based ILs showed freezing and melting points, whereas those
for [C mim]I and [C mim]BF were not observed. As for
mim]Cl, although crystallization from the supercooled liquid
15
46
mim]PF
6
281.0/13.2/-, 280.03/19.91/-,
282/12/-, 276.43/9.21/-, 283.51/19.60/-84
79
47 83
4
4
4
[C
4
C
1
mim]PF
6
314.9/17.09/-
[C
4
43
was reported, the freezing point was not observed here. These
results indicate that the [C
to crystallize as compared with the [C
a
Only melting points were reported for [C
BF4.
4
mim]I and [C
4
mim]-
+
4
mim] -based ILs are generally hard
+
4
1
C mim] -based ILs.
-
-
-
All freezing and melting points of Cl , Br , and PF
except the freezing point of [C mim]Cl, because this freezing
point was not observed) increased following the C(2) methy-
6
salts
-
Bis((trifluoromethyl)sulfonyl)amide anion (Tf
2
N ) is one of
(
4
the most typical ions in ILs, and it is noted that the trend of
-
melting points for Tf
2
N is different from that obtained here;
2
mim]Tf N and [C mim]Tf N
1
1-16
lation, which is consistent with previous reports.
phase-transition temperatures are described by
These
namely, the melting points of [C
4
2
C
4 1
45
17
are ∼270 and 260 K, respectively. This seems to be due to
-
the flexibility of the Tf
2
N anion. The flexibility can increase
Ttrans ) ∆Htrans/∆Strans
(1)
∆Sfusion; hence, the difference in ∆Sfusion between [C
4 2
mim]Tf N
and [C mim]Tf N will become smaller as compared with that
C
4 1
2
of the ILs studied here. Then, a decrease in ∆Hfusion from the
where ∆Htrans and ∆Strans are transition enthalpy and entropy,
respectively. A summary of the thermodynamic properties of
the present ILs, including ∆Strans calculated from eq 1, are listed
in Table 1. By the C(2) methylation, all of the fusion and
C(2) methylation leads to a lower melting point. The relatively
-
1
-1 45-49
high ∆Sfusion values (77.3-91.4 J mol K )
Tf N support this interpretation (no ∆Sfusion values of
[C mim]Tf N were reported).
The thermodynamic properties obtained here were compared
4
of [C mim]-
2
4
C
1
2
freezing enthalpies, ∆Hfusion and ∆Hfreezing, decreased, except
-
∆
H
fusion of Br salts. This decrease was considered to be due to
with those previously reported. There are large differences in
the values due to the differences in purities or experimental and
analytical methods. Table 2 summarizes the reported thermo-
dynamic values of the ILs as a reference.
2. Cation Conformation. As shown in Figure 1, some ILs
studied here exhibited crystal-crystal phase transitions. These
phenomena will be discussed in this section because they are
related to the conformation of the cations.
a decrease in either the interaction energy between a cation and
an anion or the lattice energy by the C(2) methylation, as
31,33,44
reported earlier.
in Br- salts was due to a crystal-crystal transition of
mim]Br during premelting (see the red arrow in the inset
The exceptional slight increase of ∆Hfusion
4 1
[C C
of Figure 1b). This small endothermic peak observed during
the heating process corresponds to the broad exothermic peak
observed in the cooling process just after crystallization. These
traces suggest that sluggish phase transitions occurred; we will
be investigating this in detail. On the other hand, all fusion and
freezing entropies, ∆Sfusion and ∆Sfreezing, decrease followed by
the C(2) methylation. This entropy decrease causes increasing
melting and freezing points, that is, the decrease in ∆Strans
overcompensates for the decrease in ∆Htrans. This finding is
consistent with the results obtained from the quantum chemical
2.1. Gas States. First, we performed the DFT calculations
+
to reveal structural differences between [C
4
mim] and
+
4 1
[C C mim] . The structures and interaction energies of both
cations in the gas phase were previously reported in detail by
Hunt.31 Here, we focus on the cation conformations for the two
ions. The number of stable cation conformations was found to
+
+
4 4 1
be 11 for [C mim] and 9 for [C C mim] at the B3LYP/6-
311+G(d,p) level. The difference in these conformations in each
IL was caused by the rotation of the butyl group. Although these
numbers depend on the computational level, the fact that the
3
1
calculations by Hunt, who reported that the ∆Strans decrease
was caused by the inhibition of the butyl group rotation and
the decrease of stable ion pairs by the C(2) methylation.31
+
+
4 4 1
number in [C mim] is greater than that in [C C mim] is

9
204 J. Phys. Chem. B, Vol. 114, No. 28, 2010
Endo et al.
Figure 2. Structures of TT, GT, and G′T conformers for both [C
4
mim]+
+
(
4 1
upper) and [C C mim] (lower).
consistent with the ∆Strans values obtained from the calorimetric
measurements. For each conformer, the cation-conformational
+
+
4 1 4
structure of [C C mim] is almost the same as that of [C mim] ,
except for the small difference in the dihedral angle of the base
of the butyl group (see Supporting Information Table 1S). The
trend of the difference in energy (∆E), enthalpy (∆H), and Gibbs
+
4 1
free energy (∆G) in the cation conformations for [C C mim]
is similar to that for [C
+
4
mim] (Supporting Information Table
2
S). These findings indicate that neither the structure nor the
energy would be affected by the C(2) methylation in the gas
phase.
2
.2. Crystalline States. Crystal-crystal phase transitions
were observed in some ILs, as shown in Figure 1, and their
transition behaviors changed after the C(2) methylation. We
measured Raman spectra to identify the cation-conformational
structure for each crystal phase. As mentioned above, there are
Figure 3. Observed Raman spectra of [C
each crystalline phase and calculated Raman bands of TT (red), GT
4
C
1
mim]X (black lines) for
(
blue), and G′T (green). Asterisks indicate anion bands. The measuring
points of these Raman spectra are demonstrated by blue arrows in the
calorimetric curves in Figure 1.
9
or more possible conformations in each cation. Nevertheless,
+
we focused on only three conformations for each [C
4
mim] and
mim] in the crystal and liquid states by considering both
the results from the single crystal structure analyses for the
+
TABLE 3: Cation Conformations Observed Here in the
Crystal Phases and Previously Reported in Single-Crystal
Structure-Analysis Data
4 1
[C C
16,50-57
IL
S). The three conformers were the trans-trans (TT), gauche-
trans (GT), and gauche′-trans (G′T), which are shown in Figure
. It should be noted that other conformations may exist, especially
in the liquid statessfor example, a gauche-gauche confor-
and the energy differences in the conformations (Table
observed cation
conformation
reported cation
conformation
2
5
0,51
2
4
[C mim]Cl
TT
TT, GT
TT, GT
5
2,54
[C
4
C
1
mim]Cl
TT
5
5
5
1
7
5
58-60
59,60
4
[C mim]Br
GT
GT
GT
GT
mer
sbut they can be regarded as minor components.
[
[
[
[
[
C
C
C
C
C
4
4
4
4
4
4
4
C
1
mim]Br
mim]I
mim]I
mim]BF
mim]BF
mim]PF
mim]PF
TT T GT
The cation conformations in the crystalline states for the
+
[
C
4
mim] -based ILs except [C
4
mim]BF
4
were revealed previ-
57
C
1
TT T GT
TT
5
0,51,53,55,56
ously with X-ray single crystal analyses
or Raman
The results for the ILs observed here agreed
4
spectroscopy.6
1-63
C
1
4
TT
TT16
5
3,56
with previous Raman spectroscopic results. Therefore, assign-
ments of cation conformation in the crystalline state were
[C
[C
6
GT f TT f G′T
TT
G′T
1
6
C
1
6
GT
4 1
performed with Raman spectroscopy only for [C C mim]X in
this section. Figure 3 shows both obtained Raman spectra in
each crystalline phase and calculated Raman bands of TT, GT,
and G′T. The measuring points of the Raman spectra are denoted
by blue arrows in Figure 1. Remarkable differences in the
obtained spectra were observed in the range of 690-780 cm .
By comparing observed and calculated Raman spectra, the
differences originated from the conformational isomerism due
Almost all crystal-crystal phase transitions for [C
4 1
C mim]X
occurred accompanied by conformational changes, the trends
63
of which were the same as that of [C
4 6
mim]PF . However, it
-
1
is noted that the phase transition in [C C mim]BF occurred
4
1
4
without an accompanying conformational change. We named
the two crystalline phases crystal R and crystal ꢀ, in order of
increasing temperature. Figure 4a shows Raman spectra of
[C C mim]BF before and after the transition, that is, crystal R
6
1-63
to the butyl rotation, as in the case of [C
4
mim]X.
The crystal
-1
phase with a peak at ∼725 cm was assigned to the crystal
with the TT conformer, and the phase with two peaks at ∼708
and 725 cm- was assigned to the crystal with the GT or G′T
conformer. The differences between GT and G′T appear in the
4
1
4
and crystal ꢀ, respectively. Figure 4b shows the calorimetric
curve of [C mim]BF around the transition. To highlight the
1
4
C
1
4
crystal-crystal phase transition in the calorimetric measurement,
heating began from the crystalline state at 207 K, obtained by
cooling immediately after crystallization at 238 K, that is, by
cooling of crystal R. In this heating process, the large peak due
to the crystallization from supercooled liquid to crystal R did
not appear. Although a broad peak was observed near 253 K in
the calorimetric curve, which agrees with the peak as shown in
Figure 1, the Raman spectrum did not change after the phase
-1
intensity ratio of the Raman peak at 708 cm against the peak
at 725 cm- and in the spectral range of 450-550 cm . The
1
-1
summary of conformation assignments of the cations for both
[
C
4
mim]X and [C
4 1
C mim]X are listed in Table 3 with the results
+
from single-crystal analyses. All [C
4
1
C mim] -based ILs have
the crystalline phase with the TT conformer, but no crystalline
phase with the G′T conformer was observed.

Imidazolium-Based Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 28, 2010 9205
figure qualitatively shows the relative stability of the TT
conformer in the liquid state. It is noted that the quantitative
analysis was difficult because there were many conformers in
the liquid states, and the marker Raman bands of some
conformers overlapped with others.
The results indicate that the population ratios of most ILs
increase with increasing temperature, which means that the TT
conformer is not generally the most stable conformer in the
4 4 1
liquid state for both [C mim]X and [C C mim]X. This is not
consistent with the results from the DFT calculations. However,
Tsuzuki et al. reported that the GT conformer becomes the most
stable conformer through ion pair formation,65 which supports
the findings obtained here in the liquid states. The relative ratio
in Figure 6 shows anion dependence. In [C
4
mim]X, the trend
-
-
of relative population of the TT conformer is in BF
4
≈ PF
6
-
-
-
≈
Cl < Br < I , which agrees with the results reported by
66
4 1
Hamaguchi et al. On the other hand, the trend in [C C mim]X
-
-
-
is different from that in [C
4
mim]X; that is, PF
≈ Cl ≈ I . This difference between [C
4 1
[C C mim]X seems to be caused by the presence of anions and
6
< BF
4
4
≈ Br
-
-
mim]X and
may relate to the differences in the phase transitions and cation
conformations in the crystalline states.
3. Anion Position. As mentioned above, the different trends
in adoption of the conformers in [C mim]X and [C mim]X
4
4 1
C
were observed in both crystalline and liquid states, and DFT
calculation, for gas phases of the cations, did not reveal distinct
differences in the structures of the butyl group for the paired
conformers or in the energetic order of the conformers. This
was due to the influence of the anions. Therefore, we investi-
gated the anion position relative to the cation by analyses of
higher-frequency Raman spectra. The bands of C-H stretching
modes were observed in a frequency region higher than 2900
Figure 4. (a) Raman spectra of [C
crystal-crystal transition; (b) a calorimetric curve of [C
for the crystal-crystal transition. Scanning rate was set to 20 mK/s.
Dotted lines indicate temperatures where the Raman spectra were
measured.
4
C
1
mim]BF
4
before and after a
mim]BF
4
4
C
1
-
1
cm , and it is known that these bands shift depending on the
position of the anion relative to that of the cation of imidazo-
lium-based ILs.8
,33,58,60,67-74
+
Figure 7 shows calculated Raman spectra of [C
4
mim] and
+
[
2
4 1
C C mim] in the high-frequency region. In the range of
950-3200 cm , which includes the C-H stretching modes
of butyl and methyl groups, the spectra were different for two
-
1
cations because of the presence of the methyl group at the 2
+
position of [C
4
C
1
mim] . On the other hand, the spectra were
-1
similar in the region 3240-3320 cm , which includes the C-H
stretching modes of the imidazolium ring. We note that our
results of DFT calculations showed that spectra in this region
were scarcely affected by the conformational change of the
cations (data are not shown). The bands at the highest frequency,
3290 cm , were assigned to the symmetric stretching modes
of C-H at the 4 and 5 positions of the imidazolium ring
C(4)-H/C(5)-H). Therefore, anion positions relative to a
Figure 5. Observed Raman spectra of [C
mim]X (red lines) in the liquid state at 373 K. Calculated Raman
bands are shown as reference. Asterisks indicate anion bands.
4
mim]X (black lines) and
4 1
[C C
-
1
∼
transition. Such a transition was also observed in Tf
2
N-based
ILs, and this unique transition is under investigation.
.3. Liquid States. Figure 5 shows Raman spectra of both
mim]X and [C mim]X in the liquid states at 373 K in the
6
4
(
proton at the 4 or 5 position of the imidazolium ring can be
estimated by comparing these bands. It should be noted that
the higher-frequency region of vibrational spectra would include
2
[
C
4
4 1
C
spectral range where marker bands for TT, GT, and G′T were
observed. This figure indicates that the liquid state consists of
a mixture of conformers, and the population ratio of the
conformers seemed to vary depending on anion species,
73,75
the bands originating from Fermi resonance
or ion pair
formation.68
Figure 8 shows higher-frequency Raman spectra of [C
4
mim]X
especially in [C
dependence of the intensity ratios of the Raman bands that were
assigned as the marker bands for the conformational isomers.
4
mim]X. Figure 6 shows the temperature
and [C mim]X in the crystalline states. Here, we focused on
4 1
C
-
-
-
Cl , Br , and PF
6
4 4 4
salts because [C mim]I and [C mim]BF
did not crystallize in our experiments. Changing the anion
caused the Raman spectra for each series to differ. It is important
to mention here that Raman spectra in the region 3000-3200
I
4
TT and IGT+G′T of [C mim]X are the intensities for the TT
-1
conformer band at ∼622 cm and for the mixed GT and G′T
-1
-1
conformers band at ∼601 cm , respectively. ITT+GT+G′T and
cm , which includes the C-H stretching modes of the
I
GT+G′T of [C
4
C
1
mim]X are the intensities for the mixed TT,
imidazolium ring, significantly change due to C(2) methylation.
This suggests that the anion position relative to the cation
-1
GT, and G′T conformers band at ∼726 cm and for the mixed
-1
-
GT and G′T conformers band at ∼705 cm , respectively. This
changes with the C(2) methylation. For Cl salts, the band of

9
206 J. Phys. Chem. B, Vol. 114, No. 28, 2010
Endo et al.
Figure 6. Temperature dependences of intensity ratios of Raman bands for [C
4
mim]X (black lines) and [C
4
C
1
mim]X (red lines). ITT and IGT+G′T
C mim]X are the integrated band
4 1
-
1
of [C
4
mim]X are the integrated band intensities at ∼622 and 601 cm , respectively. ITT+GT+G′T and IGT+G′T of-[C
-
1
-
-
intensities at ∼726 and 705 cm , respectively. Solid circle, square, and triangle represent Cl , Br , and I salts, respectively. Open circle and
-
-
square represent BF
4
and PF
6
salts, respectively.
Figure 7. Calculated Raman spectra of [C
4
mim]+ (black line) and
+
+
[
C
4
C
1
mim] (red line) in the higher-frequency region. In [C
4
mim] ,
-
1
the bands at 3275, 3278, and 3292 cm are assigned to C(4)-H/
C(5)-H asymmetric stretching, C(2)-H stretching, and C(4)-H/
+
C(5)-H symmetric stretching modes, respectively. In [C
4 1
C
mim] , the
-
1
bands at 3277 and 3294 cm are assigned to C(4)-H/C(5)-H
Figure 8. Observed higher-frequency Raman spectra of [C
4
mim]X
mim]X (red lines) in the crystalline states.
at 263 K, [C mim]I at 353 K, [C mim]PF at 273 K,
asymmetric and symmetric stretching modes, respectively.
(
[
black lines) and [C
mim]PF
4 1
C
C
4
6
4
C
1
4
C
1
6
-
1
[
C
4
mim]Cl at 3153.2 cm , marked by a black broken line, was
assigned to the C(4)-H/C(5)-H symmetric stretching mode,
and the band for [C mim]Cl shifted to a lower frequency of
112.5 cm- (the band marked by a red broken line). This
and all the other ILs at 293 K. Dotted lines indicate the bands assigned
to the C(4)-H/C(5)-H symmetric stretching mode.
4
C
1
1
3
the C(2) methylation caused the Raman band to shift higher
-
1
frequency shift corresponds to the results of single crystal
(3180.0 to 3194.2 cm ), and the average distance of C(4)-H/
56
structure analyses showing that the C(4)-H or the C(5)-
C(5)-H· · ·F increased (2.434 and 2.563 Å or 2.448, 2.590,
53 16
-
H· · ·Cl distance becomes shorter due to the C(2) methylation
and 2.669 Å for [C
for [C mim]PF ). In the case of PF
that different conformational structures of the cation were
compared here: specifically, the GT conformer for [C mim]-
Raman spectra
in this region were similar
mim]Cl) and [C mim]PF
4 6
mim]PF to 2.479, 2.649, and 2.669 Å
reduction of 2.8825 or 2.918 Å for [C
0
51
mim]Cl to 2.663 or
C
1
-
(
2
4
4
6
6
salts, it should be noted
5
2
4 1
.793 Å for [C C mim]Cl ). As a result, the origin of the lower
frequency shift due to the C(2) methylation will be the closer
position of Cl to the protons at the 4 and 5 positions of the
cation ring. Strong interaction between Cl and the protons at
the 4 and 5 positions of the ring was also reported for 1-ethyl-
4 1
C
-
16
53,56
PF
of [C
6
and the G′T conformer for [C
mim]I and [C mim]BF
mim]Br (or [C
4 6
mim]PF .
-
4
C
1
4
C
1
4
to those of [C
respectively.
C
4 1
4
C
1
C
4 1
6
,
76
2
,3-dimethylimidazolium chloride. The same trend was found
-
in the case of Br salts; namely, the C(2) methylation caused
the peak shift to the lower-frequency peak (from 3124.5 or
Figure 9 shows the experimental results for the liquid states.
-
1
The trends of spectra in the region 2800-3250 cm were
135.1 cm- to 3115.5 cm ), and the distance became shorter
1
-1
divided into two groups: Cl , Br , and I salts and BF
PF
6
-
-
-
-
and
3
(
4
-
longer distances than the sum of van der Waals radii (3.050
mim]Br to 2.733 Å and 2.821 Å for [C
However, the opposite was true in the case of PF
salts. With the C(2) methylation, all spectra changed in
51
57
-1
Å) for [C
4
4
C
1
mim]Br ).
the region 3000-3250 cm , where C-H stretching modes of
-
6
salts; namely,
the imidazolium ring were included. For the Raman band

Imidazolium-Based Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 28, 2010 9207
consistent with the previous results obtained from quantum
chemical calculations. Different phase behaviors were observed
between [C C mim]X and [C mim]X. Some ILs showed
4 1 4
crystal-crystal phase transitions, and the cation conformations
in each crystalline phase were determined by Raman spectros-
copy. The cation conformations in crystalline phases were
limited to trans-trans, gauche-trans, or gauche′-trans con-
formers of the butyl group. All crystal-crystal phase transitions
occurred accompanied by cation-conformational changes, except
4 1 4
in the case of [C C mim]BF . In both the crystalline and liquid
states, the conformational difference was observed with the C(2)
methylation, although for the gas state, there were no distinct
differences in the structures of the butyl group for the paired
conformers or in the energetic order of the conformers. The
results from higher-frequency Raman spectra suggest that the
differences in those states may be attributed to the change of
the anion position relative to the cation following the C(2)
-
-
-
+
methylation. As for Cl , Br , and I salts of [C
4 1
C mim] , these
anions were situated closer to the proton at the 4 or 5 positions
+
4
of the ring than in the salts of [C mim] , whereas the opposite
-
-
was true in the cases of BF
4
and PF
6
salts.
Acknowledgment. The present study was supported in part
by the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of Japan (No. 17073002, Grant-in-Aid for
Scientific Research in Priority Area “Science of Ionic Liquids”
(
(
K.N.); No. 21245003, Grant-in-Aid for Scientific Research (A)
K.N.)). This work was also supported in part by the Global
Center-of-Excellence Program “Advanced School for Organic
Electronics” supported by MEXT (T.E.). We thank the Institute
of Media and Information Technology of Chiba University for
the provision of the computational facilities.
4
Figure 9. Observed higher-frequency Raman spectra of [C mim]X
(
4 1
black lines) and [C C mim]X (red lines) in the liquid states at 373 K.
Dotted lines indicate the bands assigned to the C(4)-H/C(5)-H
symmetric stretching mode, and asterisks indicate the small bands that
are not assigned.
Supporting Information Available: Elemental analysis
results and tables listing detailed computational results and
experimental Raman band positions at the high-frequency
region. This material is available free of charge via the Internet
at http://pubs.acs.org.
assigned to the C(4)-H/C(5)-H symmetric stretching mode,
the bands of [C
4
mim]Cl, [C
4
mim]Br, and [C
4
mim]I shifted lower
mim]BF
shifted higher. This finding indicates that anion
mim]Cl, -Br, and -I are closer to the proton
at the 4 or 5 position than those in [C mim]Cl, -Br, and -I, but
due to the C(2) methylation, whereas the bands of [C
and [C mim]PF
positions in [C
4
4
4
6
4
C
1
References and Notes
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