Inter- and intramolecular interactions in imidazolium protic ionic liquids

Article abstract of DOI:10.1021/jp412352k

The interactions of alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HC _nImNTf₂ (n = 0-12) were studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman) and differential scanning calorimetry (DSC). The effect of alkyl substituent length (n = 0-12) and temperature on the relative magnitude of the different interactions is elucidated. For short carbon alkyl chains (n < 3), the PIL structure is affected from intramolecular interaction caused from the induction effect (+I) due to the chain substituent of the imidazolium ring, while for PILs with n > 3 the van der Waals forces between the chains and ??-?? interaction between neighboring imidazolium rings become important. The tendency of reducing the melting point and increasing glass transition values with the lengthening of the alkyl chain was also noticed as a result the increasing contribution of the van der Waals forces to the overall interactions. Finally, we also show that the conformational isomerism of the anion (expressed by ??H_eq) is a good indicator of the relative magnitude of the interactions. When Coulombic interactions are predominant, the trans conformer is the most probable, while when other type of interactions (HB, vdW, etc.) become important the cis conformer is favored. ? 2014 American Chemical Society.

Full text of DOI:10.1021/jp412352k

Article
pubs.acs.org/JPCB
Inter- and Intramolecular Interactions in Imidazolium Protic Ionic
Liquids
Anastasia Maria Moschovi,†,‡ Vassileios Dracopoulos,*,‡ and Vladimiros Nikolakis*,§
†Department of Chemical Engineering, University of Patras, Karatheodori 1, University Campus, GR-26500 Patras, Greece
‡Foundation for Research & Technology Hellas, Institute of Chemical Engineering Sciences, FORTH/ICE-HT, Stadiou Str., Platani,
P.O. Box 1414, GR-26504 Patras, Greece
§Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark,
Delaware 19716, United States
S
* Supporting Information
ABSTRACT: The interactions of alkyl substituted imidazolium bis-
(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HCnImNTf2 (n = 0−12) were
studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman) and differential scanning
calorimetry (DSC). The effect of alkyl substituent length (n = 0−12) and temperature on the
relative magnitude of the different interactions is elucidated. For short carbon alkyl chains (n <
3), the PIL structure is affected from intramolecular interaction caused from the induction
effect (+I) due to the chain substituent of the imidazolium ring, while for PILs with n > 3 the
van der Waals forces between the chains and π−π interaction between neighboring
imidazolium rings become important. The tendency of reducing the melting point and
increasing glass transition values with the lengthening of the alkyl chain was also noticed as a
result the increasing contribution of the van der Waals forces to the overall interactions. Finally,
we also show that the conformational isomerism of the anion (expressed by ΔHeq) is a good
indicator of the relative magnitude of the interactions. When Coulombic interactions are
predominant, the trans conformer is the most probable, while when other type of interactions
(HB, vdW, etc.) become important the cis conformer is favored.
This is in contrast to their high temperature analogue halide
molten salts which mainly form more well-defined coordinated
species.11
INTRODUCTION
■
The properties of ionic liquids (ILs) (e.g., low volatility and
melting point, wide electrochemical windows, etc.) render them
as a promising class of materials for a wide range of applications
in catalysis,1 electrochemistry,2,3 photovoltaics,4 or as green
solvents.5 Their structure and physicochemical properties
depend on the balance of several types of interactions between
the ions such as Coulombic, various dipole, electron pair
donor−acceptor, and hydrogen bonds.6 The contribution of
each one to the total interaction energy has recently been
estimated using quantum chemical computations. Even though
the dispersion forces contribute ∼10%, they have a profound
impact on the ability of the anions and the cations to adopt
different configurations.6−8 Furthermore, in order to select the
most suitable IL from the almost countless numbers of
structures available, it is necessary to link their properties to
their structure, which in turn depends on the inter- and
intramolecular interactions.9
ILs with imidazolium based cations comprise a subcategory
that has been extensively used in previous studies (Figure 1).
Most of them focused on aprotic ionic liquids (AILs), in which
the substitutions in positions 1 and 3 were alkyl groups, while
the substitutions in positions 2, 4, and 5 were either hydrogen
atoms or alkyl groups. The ions of AILs interact via Coulombic,
van der Waals (vdW), and hydrogen bond (HB) forces, thus
forming directional bonding between the constituent ions.10
Imidazolium AILs with bis(trifluoromethanolsulfonyl)imide
−
(NTf2 ) anion are ideal systems for studying interactions,
−
because NTf2 has several hydrogen bond acceptor sites. In
particular, hydrogen bonds can be formed between the H
atoms in sites 2, 4, and 5 of the imidazolium ring and the N or
−
O atoms of NTf2 . As a result, substitution of alkyl groups in
these sites affects their physical properties (e.g., melting point,
viscosity, etc.).12−15
In addition, the balance between the electrostatic and van der
Waals interactions changes with increasing alkyl chain length
on site 1. After a certain length, vdW interactions are
responsible for the creation of polar and nonpolar nanophase
regions in the liquid phase.11 Polar regions consist of polar
heads of the cation and anions, whereas nonpolar regions
consist of alkyl chain aggregates.13,16
All previous studies reveal that the properties of ILs and their
local molecular environment are in close relation; hence, in this
work, we studied the interactions of protic ionic liquids
(PILs).17−19 PILs are a different subset of ILs which are formed
Received: December 17, 2013
Revised: June 27, 2014
© XXXX American Chemical Society
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−
Figure 1. Molecular structure of the cation HCnIm+ for n = 0−12 and the conformations of the anion NTf2 .
by the reaction between a Brønsted base and a Brønsted acid.
The main characteristic of the imidazolium based PILs is the
presence of a proton on site 1 of the imidazolium ring (H1). As
a result, PILs can form an extensive network of hydrogen
bonds. The systems that we chose for this study are alkyl
substituted imidazolium bis(trifluoromethanolsulfonyl)imide
chain to the local structure and ion interactions of imidazolium
PILs. Raman and FT-IR/ATR spectra from HCnImNTf2 (n =
0−12) PILs at room and elevated temperatures were recorded
in order to reveal the local structure as well as the effect of local
interactions due to alkyl chain on the conformational isomerism
−
of the anion. To elucidate the role of H bonding on the NTf2
−
(HCnIm+NTf2 (n: 0−12)) PILs. Schematics of all the cations
isomerism, we also carried out similar experiments using alkali
metal salts that have only Coulombic interactions. Also, the
basic thermodynamic macroscopic parameters like ΔHm, Tm,
and Tg were measured by using differential scanning
calorimetry (DSC) and the results will be discussed together
with the findings from the vibrational spectroscopic study.
HCnIm+ (n = 0−12) and the isomers of the anion NTf2− of all
−
the ILs studied are shown in Figure 1. The anion NTf2 can
exist in two conformational isomers, cis and trans.20,21 In the cis
conformation, CF3 groups lay on the same side of the S−N−S
bond, whereas, in the trans conformation, the groups lay on the
opposite side of the bond. The conformation of the anion
−
NTf2 shows a significant dependence on the HB strength,
EXPERIMENTAL SECTION
■
since the ions of the PILs studied interact with a well-defined
Preparation of the 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) >98%
were purchased from IoLiTec (Ionic Liquids Technologies)
GmbH, Brønsted base imidazole Im >99.5% in solid form
(flakes) was purchased from Sigma-Aldrich Co., whereas H-
bis(trifluoromethanolsulfonyl)imide HNTf2 >99% was pur-
chased from Acros Organics.
hydrogen bond between the cation and the proton acceptor
−
sites of NTf2 .
On the other hand, the aliphatic chain can also exist in
different conformers. An alkyl group, attached to the N3 atom
of the ring, rotating around the C6−N3 or any Ci−Ci+1 (i = 6−
16) bond, exhibits two isomers, gauche (G) and trans (T). The
two isomers differ in the rotation of the alkyl chain around the
C6−N bond approximately by 106°, as shown in Figure 2.22
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 reaction
−
HNTf2 + CnIm → HCnIm+ NTf2
(1)
KNTf2 of 99% purity was provided from Solvionic, while
CsNTf2 salt was synthesized in our lab, using carbonate cesium
salt Cs2CO3 >99.99% purchased from Alfa Aesar according to
reaction 2.
2HNTf2 + Cs2CO3 → 2CsNTf2 + H2O + CO2↑
(2)
Figure 2. Conformation of the alkyl chain in the cation HC4Im+.
Appropriate amounts of carbonate salt Cs2CO3 and HNTf2
were dissolved in anhydrous methanol (Merck) in a glovebag
under inert conditions. The solution was then exposed to the
atmosphere and was kept under stirring until methanol was
evaporated. The crystals were then collected and dried under a
vacuum in quartz tubes.
All chemicals, before and after the synthesis procedure, were
stored in a desiccator at room temperature, under inert
conditions. For Raman spectroscopic studies, all of the
chemicals (except HNTf2) studied were dried under a vacuum
at 60 °C. Protic ionic liquids were introduced into quartz cells
The structure of longer alkyl chains is described by the
isomerization of Ci−Ci+1 bonds in the raw, GTTT, TTTT, etc.
Since the conformation of the side alkyl chain does not involve
any of the H-bonding donors of the imidazole ring, it is not
expected to play a significant role on the HB donor
capabilities.7
Vibrational spectroscopy was used to study the local
structure and thus the local interactions between the ions.
We focused on the systematic study of the effect of the alkyl
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Table 1. Temperatures (°C) of FT-Raman Spectra Recorded for PILs and ANTf2 (A: H, K, Cs)
HC0ImNTf2
HC1ImNTf2
HCnImNTf2 (n = 2−12)
HNTf2
KNTf2
CsNTf2
60
90
25
60
65
100
130
160
220
245
270
295
140
165
190
215
240
90
120
150
120
150
90
120
(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.
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
The water content of the PILs was determined after the
acquisition of all spectra using Karl Fisher titration. The ionic
liquids were dissolved in anhydrous methanol (Merck), and
each solution was titrated with an automatic Metrohm 633
titrator (Merck Karl Fischer reagent 5, single component with
pyridine 1 mL = 5 mg of H2O). The water uptake for the PILs
tested was estimated between 0.00013 and 0.005% w/w. Details
about the water content of each sample are listed in Table S.I.1
of the Supporting Information.
4
3
IISO(ω) = IVV(ω) −
IVH(ω);
IANISO(ω) = IVH(ω)
The reduced Raman intensity Rs(ω), where s stands for VV,
ISO, and VH (ANISO), related to the Is(ω) according to the
relation
Rs(ω) = Is(ω)ω(ω0 − ω)4[n(ω) + 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 expressed elsewhere.23
FT-Raman Measurements. Two Raman systems were
utilized for the temperature measurements (Table 1) of ILs and
molten salts. System 1: A FT-Raman system. The FT-Raman
spectra were recorded with a BRUKER FRA106/S module on
an 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 an acquisition
time of ∼1.3 s per scan. A homemade optical furnace and
temperature controller specially designed for the FT-Raman
system was used for the spectroscopic study at elevated
temperatures.23 System 2: A UV-Raman system. The spectra of
the alkali salts and the HNTf2 at room temperatures and at
elevated temperatures were recorded with a high resolution
UV-Raman Labram HR-800 spectrograph (JY, ISA Horiba
group). In the case of HNTf2 and alkali molten salts, UV-
Raman setup was more suitable than a FT-Raman system, due
to the presence of 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 a 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 cm−1,
and the 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 Instru-
ments Ltd.) under the Raman microscope objective. The
temperatures were controlled with a TMS93 Linkam Ltd.
temperature controller.
FT-IR/ATR Measurements. FT-IR/ATR spectra of PILs
(HCnImNTf2) with n between 2 and 12 were collected at room
temperature, while, for PILs with n = 0 and 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 HCnImNTf2. For the samples with n = 0 and 1, the
liquid film was formed at temperatures of 73 and 52 °C,
respectively. Spectra were recorded using a 4 cm−1 resolution
with 200 scans under a 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.
DSC Measurements. Differential scanning calorimetry
thermograms were obtained using a Q100 calorimeter
equipped with a LN2 cooling accessory from TA Instruments
SA. The DSC device was calibrated with an indium standard.
The cooling rate was 10 °C/min, while the heating was 5 °C/
min and covered the range from −120 to 120 °C. The melting
points were calculated on the onset of the endothermic peaks
while glass transition temperatures on the midpoint of the glass
transition step, as shown in Figure S.I.1 (Supporting
Information).
RESULTS AND DISCUSSION
■
Thermal Properties. The melting points and glass
transition temperatures as a function of alkyl chain are
presented in Figure 3 (the thermograms of all the PILs studied
are shown in Figure S.I.2 as Supporting Information). Melting
points were observed for the two ends of alkyl chain length,
below four and above eight C atoms in the chain. Glass
transition temperatures were measured for all of the PILs
except for the two first members of the sequence. The melting
points of PILs studied here are also compared with reported
values of the corresponding APILs (Table 2).24 It is clear that
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 the liquid state,
a CCl4 sample was used for calibrating the depolarization ratio
of the two setups.
Two polarization configurations were used for the Raman
spectra of the liquids and melts: vertical−vertical (VV) and
C
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the stiffness of PIL structural units compared to the APILs and
could explain the observed behavior.
The glass transition temperature increases as a function of
alkyl chain length, indicating that there are segments in the
structure that became less mobile. One possible explanation for
this effect is the strengthening of the interactions between the
side alkyl chains. This implies that the local structure is
probably similar to the corresponding APILs where the long
alkyl chains form nonpolar domains due to vdW interactions,
resulting in less mobile areas and thus the increase of the glass
transition temperatures. A local minimum in Tg, observed for n
= 8 and 10, is probably caused by the formation of metastable
phases, due to conformational or orientation disordering of the
alkyl chain.26 Moreover, the addition of −CH2 groups results in
the free volume increase which probably is the reason for the
glass transition temperature depression.
The melting point shows a depression up to n = 2, while
upon further increase of n its value slightly increases. The fast
depression is probably correlated with the number and
magnitude of the hydrogen bonds, together with a reduction
of the Coulombic interactions between the ions due to the
increase of the cation size.27 This cannot explain the high end
of the alkyl chain length studied. If we adopt the explanation
proposed in the previous paragraph, then the slight increase of
the melting point can be attributed to the increased stiffness of
the side alkyl chain. The above discussion indicates that certain
structural changes occurred in these PILs with increasing alkyl
chain length, which strongly affect their thermal properties.
Previous WAXS studies indicated the structural organization of
the polar and nonpolar regions to nanostructures, indicating
that the separation of the high charged heads from the low
charged aliphatic chains results in the formation of low and
high density regions.28,29 This density fluctuation can be
reflected in the thermal properties of the ILs.30 However, unlike
vibrational spectroscopy, the thermodynamic measurements
give only indications, which can be considered as speculative,
about the interactions and local structure. To address this issue,
we used vibrational spectroscopy.
25
■
▲
Figure 3. Experimental melting Tm of ( ) PILs and ( ) AILs and
●
( ) glass transition Tg temperatures of PILs as a function of the
number of C atoms in the alkyl chain.
Table 2. Melting Tm and Glass Transition Points Tg for
Protic HCnImNTf2 and Aprotic C1CnImNTf2 (n = 0−12)
ILs24
Tm for HCnImNTf2
(°C)
Tg for HCnImNTf2
(°C)
Tm for C1CnImNTf2
(°C)
n
0
72.76
48.15
2.35
1
2
22.15
−84.65
−81.15
−78.87
−77.14
−82.47
−81.21
−69.93
−14.85
3
7.49
4
−3.85
6
8
5.78
3.45
10
12
−28.85
17.74
16.85
the melting points of PILs are higher compared to those of the
APILs, probably due to the additional hydrogen bond between
the nitrogen atom of the cation and NTf2 , which increases
25
−
Figure 4. (a) Isotropic Raman spectra for HCnImNTf2 (n = 0−12) in the region 3000−2800 cm−1. (b) Raman spectra of the 3000−2800 cm−1
spectral region of the HC12ImNTf2 salt in the solid and liquid phases.
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Effect of Alkyl Chain Length on the Structure of the
Cation. The Raman spectra of the alkyl group CH2 stretching
region of all PILs studied are shown in Figure 4a. The intensity
of the band assigned to the CH2 symmetric stretching mode (T
band) increases compared to that of asymmetric stretching (G
band). These two bands are correlated to the trans (T band)
and gauche (G band) conformations of the alkyl chain. The
deconvoluted spectra of this region are shown in Figure S.I.3
(Supporting Information).
Furthermore, the frequencies of both bands continuously
shift to lower wavenumbers, reaching the frequencies observed
in molten polymethylene31 and liquid n-alkanes,32 indicating
that the alkyl side chain of the imidazole ring probably has a
local structure similar to that observed in these melts. This is
further supported by the spectra in the solid and liquid
HC12ImNTf2 in Figure 4b.
The spectra of the solid HC12ImNTf2 are similar to those
observed in solid alkanes and polymethylene in which the alkyl
chains are packed in an all trans geometry.31 Due to the
similarities of both systems, we assigned the observed bands to
the existence of the alkyl chain in the crystal to an all trans
chain. Upon melting, the symmetric stretching vibration shifts
to higher energies of about 8 cm−1. A similar observation in
polymethylene was attributed to the appearance of gauche
defects on the trans structure of the alkyl chain. The red shift of
the CH2 symmetric and antisymmetric stretching modes as the
alkyl chain size increases is probably due to the reduction of the
reorientation ability and flexibility of the side chain. This is
consistent with what has been observed in aprotic ILs where
the chains are oriented due to vdW interactions introducing
nonpolar regions into a polar network.13,16 Thus, we propose a
similar structure model for the protic imidazolium ILs.
Apart from this general observation, a closer view to the local
gauche/trans chain conformations can be given by the external
deformation β of the CH3 at the end of the chain. This mode is
localized in the 900−820 cm−1 spectral region.33 This spectral
region for the crystalline and liquid phases for HC12ImNTf2 is
shown in Figure 5. If the alkyl chain of the solid HC12ImNTf2 is
packed in the all trans geometry (as hypothesized above), then
only the ∼900 cm−1 band which corresponds to the TT
conformers of the end carbon atoms of the chain should be
observed. This is indeed the case in Figure 5.
Figure 5. Raman spectra of the 940−800 cm−1 spectral region of the
solid and liquid phases of HC12ImNTf2.
It is well established that the β deformation vibrational mode
of the methyl group at the end of the alkyl chain is strongly
affected by the gauche defects caused by the carbon atoms of
the methylene group close to them. From the pioneer study of
the melting of nonadecane hydrocarbons, we expected that, if
we have a gauche structure between the carbon atoms of the
second and third positions (C16−C17 TG end chain
conformation) of the chain, then a band at ∼870 cm−1 should
be observed.34
If the gauche defects lay between the third (C16) and fourth
(C15) carbon atoms, then the band is expected to be observed
at ∼847 cm−1 (GT conformer). In the spectra of the liquid
HC12ImNTf2, we observe all the possible configurations of the
TT, TG, and GT structures for the end of the chain of the 12
carbon atoms. This shows that the structure of the side chain is
quite distorted compared to that of the crystal. The presence of
many gauche conformers makes the orientational motion of the
chains more complicated and of course more difficult. This
spectral region for all the PILs measured is presented in Figure
6.
Figure 6. Spectra of the HCnImNTf2 in the 900−800 cm−1 spectral
region where the end chain gauche and trans defects are.
It is clear that the picture of the gauche conformers is more
complicated for the longer chains compared to the short ones.
It seems that when n is higher than 6 all possible conformers
exist (in different populations), while when n is less than 6 the
spectra reveal a more defined local structure. Thus, for the butyl
system, the chain ends with a TT conformation, while, for the
propyl system, a TG structure is dominant.
Effect of Alkyl Chain Length on Hydrogen Bonding.
The observed red shifts of the N−H stretching mode (3270−
3250 cm−1)23 and the C−H stretching mode (3170−3150
cm−1) IR frequencies as a function of the alkyl chain length are
shown in Figure 7 (the deconvoluted FT-IR spectra of these
regions are provided as Supporting Information, Figure S.I.4).
The spectra have been smoothed by a FFT filter and 5 points of
window using Origin 7.5 software. Considering that the water
content of the samples determined after these measurements
E
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Figure 7. (a) FT-IR/ATR spectra of the HCnImNTf2 PILs in the liquid phase at 90 °C (n = 0), 60 °C (n = 1), and 25 °C (n ≥ 2). (b) Observed FT-
■
●
IR/ATR frequencies of the ( ) N−H modes and ( ) Caromatic−H as a function of the number of alkyl chain C atoms.
was less than 0.002 wt % (see Table S.I.1 in the Supporting
Information), we do not expect to have interference of H2O
vibrational modes in the high frequency spectral region of the
N−H···NTf2 bonds.
The spectral charachteristics in this area are quite different
spectrum of C2OHImPF6 to the C2−H while the lower one to
the C4,5−H, showing again the difficulties in the assignment of
these bands.42 To address the assignment uncertainties, they
adopted the Caromatic−H notation for describing these modes.
They also suggested that if weak hydrogen bonds are formed
then a blue shift can be expected due to secondary effects
caused, e.g., by a certain arrangement between the anion and
cations (the anion on the top or bottom of the imidazolium
ring).
−
than those of the corresponding APILs. In addition to the
−
presence of the N−H···NTf2 mode, the shape and band
characteristics of Ci−H···O (i: 2, 4, or 5) modes are also
different. Furthermore, there has been a controversy regarding
the assignment of the νCi−H···O stretching modes. Normally,
the FT-IR bands at 3170 15 and 3120 15 cm−1 observed in
imidazolium APILs are assigned to the C4,5−H and C2−H
stretching modes.35,36 In 2009, Lassegues and co-workers
revisited these assignments by taking into account the
possibility of a Fermi resonance caused by the overtones/
combinations of the ring vibrations at 1500−1600 cm−1 with
the stretching fundamentals at 3100−3200 cm−1.35 Their main
conclusion was that the high frequency mode has also a C2−H
contribution (in addition to the C4,5−H) while the low
frequency mode is mainly due to the Fermi resonance. Indeed,
the overtone of the ring stretching mode at 1587 cm−1 (see
paragraphs that follow) overlaps with the stretching vibration of
All of the above discussion shows clearly that precise
assignment of this spectral area is difficult and extreme care
must be taken to avoid misnterpretations. The shape of the
3170−3150 cm−1 band shows that probably all of the Ci−H (i:
2, 4, 5) stretching modes of the ring are included in this
envelope (this area can be fitted quite well by two Gaussians:
one for the N−H···O and one for the Caromatic−H···O band).
One possible interpretation is that the arrangement between
the anion and the cation is in that way resulting in a further
blue shift of the C2−H mode and overlapped by the C4,5−H
modes.42 Another approach for these band shape characteristics
could be the possible vibrational energy transfer through
hydrogen bonds between the C2−H and C4,5−H modes
resulting in the enhancement of the lifetime of the receiving
mode and reduction of the lifetime of the donating mode.8,43 In
order to avoid any misinterpretation, we will follow the
Caromatic−H···NTf2− notation for this band.42 If the contibution
of the Fermi resonance is important, then the weak absorbance
of the 2vRS will be significantly enhanced and probably explain
the observed asymmetry on the low frequency part of the
−
the Caromatic−H··· NTf2 mode and could result in a Fermi
resonance interaction of these two modes. This interpretation
sparked a debate between the spectroscopists.37,38 In a recent
publication, Ludwig et al.39 used nonlinear Raman spectroscopy
(CARS: coherent anti-Stokes Raman spectroscopy) to show
that the high frequency area is attributed to the C4,5−H in-
phase and out-of-phase modes while the lower one had
contributions from the C2−H mode and the corresponding
Fermi resonance of this mode with the ring modes at 1575
cm−1. These assignments were further supported by recent IR
spectra of vapor phase jet-cooled EMImNTf2.40,41 This study
also showed that an assignment of the low frequency band only
to a Fermi resonance cannot explain the spectral behavior of
the C2−H band as a function of jet temperature and contribute
probably to a further broadening of the observed band.
Katsyuba et al. assigned the high frequency mode in the FT-IR
−
Caromatic−H···NTf2 stretching mode. Theoretical calculations
in combination with nonlinear vibrational spectroscopy studies
are two possible methods that can be used to shed further light
in our interpretation.
In any case, the frequency of the N−H or Caromatic−H
stretching bands depends on the HB strength between the H
−
atom of the imidazolium ring and NTf2 and it can be
correlated to the existence of directional interactions between
the anions and the cations in the liquid phase. In Figure 7b, we
F
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present the shift of the N−H and Caromatic−H as a function of
the number of C atoms in the alkyl chain. Both modes show a
red shift with increasing alkyl chain length even though they do
not follow the same trend. More pronounced shifts of the
Caromatic−H mode are observed when the alkyl chain length n is
increased from 0 to 3, while the N−H shifts seem to have an
almost linear dependence on n. One hypothesis is that the shift
of the Caromatic−H mode can be attributed to the inductive
effect of the side alkyl chain to the imidazole ring which in turn
affects the hydrogen bond strength of the ring hydrogen atoms
−
with NTf2 . The induction effect is also expected to be more
pronounced on the Caromatic−H bonds, since they are closer to
N3 than the N1−H bond. Furthermore, its magnitude should
depend on the alkyl chain length only for small values of n,
whereas when n > 3 the effect of n should be negligible. This
seems to be in qualitative agreement with the observed
Caromatic−H bond shifts shown in Figure 7b, indicating that this
vibration is primarily affected by the side chain substituent
inductive effect on the ring. However, since the alkyl chains are
electron donating groups, the overall positive charge of the
imidazole ring should decrease with increasing n. Moreover, the
group of Kirchner showed that, with increasing alkyl chain
length, the distance between the anion and the cation increases
as well.44 As a result, we should expect to see hydrogen bond
weakening. Usually, weakening of an X−H···Y hydrogen bond
is associated with a blue shift of the X−H vibration instead of
the observed red shift with increasing n. However, there are
numerous examples in the literature demonstrating that it is
possible, under certain circumstances, to observe a blue shift of
the X−H vibration when an X−H···Y hydrogen bond forms
(such hydrogen bonds are often called improper hydrogen
bonds).8,45,46 Thus, if the observed red shift with increasing n is
attributed to the hydrogen bond weakening due to the
inductive effect, then the Caromatic−H bonds of the imidazolium
ring probably form improper hydrogen bonds (blue-shifted)
Figure 8. FT-IR/ATR spectra for HCnImNTf2 (n = 0−12) in the
region 1625−1525 cm−1.
of π−π interactions between monosubstituted benzene rings
and imidazole rings.47 They found that the hydrogen bonding
ability of the imidazole ring increases when it is in parallel
stacking with benzene groups substituted with electron
donating groups (e.g., alkyl chains), while the opposite is
observed in the case of T-shaped stacking.
In a similar manner, the observed redshift of the N−H bond
which is formed between the proton of the imidazolium ring
−
and NTf2 with increasing alkyl chain length reflects the
hydrogen bond’s strength changes, which can be attributed to a
larger fraction of imidazolium rings in parallel stacking.
The latter is also supported by the spectral changes of the
C−N stretching modes (Figure 8) which show that there is a
ring extension (destabilization) for chains with n > 2. Thus, the
relative contribution of the intercationic π−π interactions
becomes more important than the intramolecular (+I effect)
ones.6,17
−
with NTf2 . However, as discussed earlier, this area probably
has contributions due to Fermi resonance of the ring stretching
vibrations and the Caromatic−H stretching. As a result, the effect
of the Caromatic−H shifts on the existence and contribution of
Fermi resonance bands has also to be considered, and is an area
that insights from molecular simulations will be extremely
valuable.
The FT-IR C−N ring stretching vibrational mode region of
the protonated imidazolium ring for the PILs HCnIMNTf2, n =
0−12, is shown in Figure 8. The bands at ∼1553 and ∼1587
cm−1 correspond to R2 and R1 stretching vibrational modes of
the protonated cation HIm+, respectively. The splitting of these
bands is higher compared to the corresponding APILs,
indicating that the directional HB, between the ions, distorts
more the local symmetry of the ring. An abrupt shift to lower
energies (∼6 cm−1) is observed when the alkyl chain changes
from methyl to ethyl, whereas there is no further shift as the
alkyl chain length increases from 2 to 12 (ethyl to dodecyl).
The observed frequency jump indicates an increase of the C−N
bond distances, which results in the imidazole ring extension.
This ring extension does not further change as the alkyl chain
increases.
An important question that still has to be answered is why
the dependence of the N−H shift on alkyl chain length is
different than that of the Caromatic−H shift. This observation can
be explained if the N−H mode is affected primarily from
intercationic interactions like the π−π interactions between
neighboring imidazolium rings. Mignon et al. studied the effect
The Anion Conformation Isomerism as a Monitor of
Local Interactions. The effect of the intra- and intermolecular
−
forces in the relative populations of the NTf2 trans and cis
conformations was also studied. The frequency of the N−H
and Caromatic−H stretching as a function of the ratio of the
populations of the two NTf2− conformers (Itrans/Icis) is shown in
Figure 9. This ratio was calculated from FT-IR normalized
spectra of the region of vasymSO2 modes (Figure S.I.5,
Supporting Information). The normalized FT-IR spectra of
symmetric and asymmetric vibrational modes show that the
increase of the −CH2− groups in the alkyl chain favors the
trans conformation of the anion.
The Caromatic−H frequency decreases linearly with Itrans/Icis.
This, in turn, shows that the population of these two
conformers is affected primarily by the intramolecular
interactions and to a lesser extent by the interionic π−π
interactions between the rings.
−
Therefore, it seems that the equilibrium of the NTf2
conformers depends on the PIL local interactions. We also
investigated the effect of temperature on this equilibrium from
the corresponding Raman spectra collected at different
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■
●
Figure 9. FT-IR/ATR frequencies of the ( ) N−H modes and ( )
Caromatic−H as a function of the ratio of the relative intensities Itrans/Icis
of the vasymSO2 from the normalized FT-IR/ATR spectra of the
HCnImNTf2 (n = 0−12).
Figure 10. Energy diagram of the ΔHeq values of the protic ionic
liquids HCnImNTf2 (n = 0−12), alkali metal salts ANTf2 (A: Cs and
K), and the strong acid HNTf2 calculated from the analysis of
experimental data at different temperatures. The lines are guides for
the eye.
temperatures. In the case of HCnImNTf2, the cis conformer
becomes more stable at elevated temperatures. On the other
hand, in the case of molten alkali-NTf2− salts, we found that the
trans conformer is favored at elevated temperatures. Finally, in
the case of the strong acid HNTf2, no spectral changes were
observed, indicating that no changes in conformational
equilibrium with temperature can be observed with vibrational
Table 3. Experimentally Determined ΔHeq Values for the
Protic Ionic Liquids HCnImNTf2 (n = 0−12) and the
Alkaline Molten Salts KNTf2 and CsNTf2
ΔHtrans↔cis (kJ mol−1
)
Ionic Liquids
−
HC0ImNTf2
HC1ImNTf2
HC2ImNTf2
HC3ImNTf2
HC4ImNTf2
HC6ImNTf2
HC8ImNTf2
HC10ImNTf2
HC12ImNTf2
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
spectroscopy. Thus, how does the equilibrium of NTf2
conformers “reflect” the local interactions in ILs?
To answer this question, we calculated the ΔHeq of the anion
conformational isomerism from the vSO2 symmetric stretching
vibrational mode area (1120−1150 cm−1) of the reduced
isotropic Raman spectra. The deconvoluted spectral regions for
different temperatures which were used for the ΔHeq
calculation for PILs, ANTf2 molten salts, and Brønsted acid
HNTf2 are shown in Figure S.I.6−8 (Supporting Information).
The methodology of the calculations has been analyzed
thoroughly elsewhere.23 The Arrhenius plots of the van’t Hoff
equation for the measured PILs are provided as Supporting
Information in Figure S.I.9. The ΔHeq values in the case of PILs
as a function of C atoms in the alkyl chain and as a function of
alkali metal ionic radius are shown in Figure 10 and listed in
Table 3.
The most evident observation is the opposite sign of ΔHeq
depending on the cation (alkali vs imidazolium). In the case of
alkali-NTf2 molten salts, the trans conformation of the anion is
favored at elevated temperatures; thus, ΔHeq is negative.
On the contrary, in the case of imidazolium based NTf2 ILs,
the cis conformation population increases with temperature,
resulting in positive ΔHeq values. However, in both cases, ΔHeq
decreases, either with ionic radius or with the alkyl group
number of carbon atoms following the same trend as the Tm of
the PILs. The decrease with the alkyl chain length is probably
due to the reduction of the electrostatic force contribution to
the overall interaction between the ions, since the contributions
4.31 0.2
Alkali Molten Salts ANTf2
−13.86 2.6
−20.73 2.4
KNTf2
CsNTf2
of the other forces (HBs, vdW, dispersion due to π−π
interactions) to the total energy of the pair of ions become
more pronounced. Furthermore, in the case of PILs, there is a
change of the slope at n = 3. On the high carbon atom chain,
the ΔHeq shifts to even lower values, showing a gap and
becoming closer to the value predicted by theory for the free
rotamer. This probably happens due to the increased distance
between the polar heads that leaves space for the anion to
rotate more freely.48 The same trend of the ΔHeq decrease was
also observed from calculations from the spectral region of
ρSO2 vibrational modes which is presented in Figure S.I.10
(Supporting Information). For the PILs HCnImNTf2, n = 4, 8,
10, it was not possible to calculate the ΔHeq from this low
frequency spectral region, due to vibrational modes over-
lapping.
H
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Article
Similar results were shown by Rocha et al.18 who calculated
the enthalpy and entropy change for vaporization (ΔHvap and
ΔSvap) of the aprotic ionic liquids C1CnImNTf2 as a function of
the alkyl chain length (n = 2−12).18 The changes of the ΔHvap
and ΔSvap with alkyl chain length were attributed to the
decrease of the electrostatic interactions and the increase of the
van der Waals ones with n. They found that for small alkyl
chains (2 < n < 6) the slope of the calculated values with n is
higher than the same slope when n is between 6 and 12.
Furthermore, the change of slope at n = 6 was attributed to the
formation of distinctive polar and nonpolar regions at n > 6
instead of isolated islands of nonpolar chains in the continuous
polar phase when n < 6. On the other hand, according to a
more recent study of the aprotic ionic liquids C1CnImNTf2 (n =
1−18), the calculated values of the ΔHvap and ΔSvap had a linear
dependence on n for 2 < n < 18, while for n = 1 and n = 2 both
quantities were relatively constant.19 Furthermore, they showed
that the contribution of the electrostatic interactions in the
thermodynamic properties reduces more sharply when n is
changed from 1 to 2 rather than in the region 2 < n < 18. Thus,
the change in the slope in our experimental ΔHeq values might
reflect the change of the most pronounced interaction from
intramolecular to interionic. Moreover, the melting points of
the PILs, for n < 3, show a similar decrease to the values of the
ΔHeq, indicating that the +I effect contributes to the depression
of the melting points, whereas, for n > 3, the dominance of the
intermolecular interactions shows no significant effect on the
macroscopic properties of the PILs.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis procedure and structure of aprotic ionic liquids
HCnImNTf2 (n = 0−12) and alkali molten salts ANTf2 (K and
Cs), vibrational spectroscopy measurements, and calculation
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: indy@iceht.forth.gr.
*E-mail: vlad@udel.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.N. acknowledges support from the Catalysis Center for
Energy Innovation, an Energy Frontier Research Center funded
by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under award number DE-SC0001004.
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