Molecular states of water in room temperature ionic liquids

Article abstract of DOI:10.1039/b106900d

ATR and transmission IR spectroscopy have been used to investigate the state of water in room temperature ionic liquids (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF₆]^-, [SbF₆]^-, [

Full text of DOI:10.1039/b106900d

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Molecular states of water in room temperature ionic liquidsy
a
a
b
b
L. Cammarata, S. G. Kazarian,* P. A. Salter and T. Welton
a
Department of Chemical Engineering and Chemical Technology,
Imperial College of Science, Technology and Medicine, L ondon, UK SW 7 2BY.
E-mail: s.kazarian@ic.ac.uk
Department of Chemistry, Imperial College of Science, Technology and Medicine,
L ondon, UK SW 7 2BY.
b
Received 31st July 2001, Accepted 3rd October 2001
First published as an Advance Article on the web
ATR and transmission IR spectroscopy have been used to investigate the state of water in room temperature
�
�
�
ionic liquids (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF
6
] , [SbF
] . It has been shown that in these RTILs water
molecules absorbed from the air are present mostly in the ``free'' (not self-associated) state, bound via
6
] , [BF
4
] ,
�
�
�
�
�
[ClO
4
] , [CF
3
SO
3
] , [(CF
3
SO
2
)
2
N] , [NO
3
]
and [CF
3
CO
2
� � � � � �
H-bonding with [PF ] , [BF ] , [SbF ] , [ClO ] , [CF SO ] , [(CF SO ) N] with the concentrations of
6 4 6 4 3 3 3 2 2
�
3
dissolved water in the range 0.2±1.0 mol dm . It has been concluded that most of the water molecules at these
concentrations exist in symmetric 1 : 2 type H-bonded complexes: anion...HOH...anion. Additional evidence
that the preferred sites of interaction with water molecules are the anions has been obtained from the
experiments with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium
cations. Water molecules can also form associated liquid-like formations in RTILs with anions of stronger
�
�
basicity such as [NO ] and [CF CO ] . When these RTILs are exposed to air the water concentrations exceed
2
3
3
�
3
.0 mol dm . The strength of H-bonding between water molecules and anions increases in the order
1
� � � � � � � �
6 6 4 3 2 2 4 3 3 3 3 2
] < [SbF ] < [BF ] < [(CF SO ) N] < [ClO ] < [CF SO ] < [NO ] < [CF CO ] . The energies of
[PF
this H-bonding were estimated from spectral shifts, with the resulting enthalpies being in the range 8±13 kJ
�
1
mol . ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.
water dissolved in RTILs is needed for further understanding of
Introduction
RTILs as media for chemical synthesis and separation. In this
study, in order to observe the behaviour under conditions in
which the RTILs would ®nd large-scale use, the RTILs were
investigated with water contents in equilibrium with the air.
Infrared spectroscopy, arguably, is the most powerful
method to probe the molecular state of water present in var-
RTILs are attracting signi®cant attention as novel solvent sys-
1
±7
tems for chemical reactions and separations. The challenge of
separation of the reaction products from RTILs can be over-
come with the use of biphasic systems, for example via com-
bining reactions in RTILs with separations using supercritical
10
¯uids. The combination of ionic liquids with supercritical CO
for example, provides a new and promising method for catalyst
2
,
ious solvents. Indeed, the molecular state of water dissolved
in a variety of organic solvents and polymers has been eluci-
5
,7±9
11±17
separation and the design of novel synthetic routes.
dated using IR spectroscopy.
The vibrational modes of
water that result in bands in the IR spectrum are very sensitive
The presence of water in RTILs may a€ect many of their
solvent properties such as polarity, viscosity and conductivity.
This may have an e€ect on reaction rates due to the coordinating
ability with a catalyst and also on the solubility of other sub-
stances in RTILs. For example, the presence of water in an ionic
18,19
to the environment and intermolecular interactions.
In
particular, the stretching modes of water have been used to
understand the type of bonding between water molecules in the
liquid, solid and vapour phases, as well as interactions between
water and many chemical substances.
liquid has resulted in the underestimation of the solubility of
7
in the ionic liquid. Water may also react with constituents
CO
of the RTIL; it has also been reported that water in RTILs based
�
2
In this work we applied ATR (attentuated total re¯ectance)
and transmission IR spectroscopy to study the molecular state
of dissolved water in several ionic liquids based on the 1-butyl-
1a,4b
on the [PF
6
]
anion leads to a reaction producing HF.
‡ �
6
3-methylimidazolium cation ([bmim] ) with the [PF ] ,
Seddon and co-workers have recently emphasized that ecient
drying is necessary if RTILs are to be used as solvents for
�
�
�
�
�
�
[SbF ] , [BF ] , [ClO ] , [CF SO ] , [NO ] and [CF CO ]
2
6
4
4
3
3
3
3
4a
moisture sensitive substances. It is also clear that the com-
monly called ``hydrophobic'' RTILs are actually hydroscopic
and can absorb signi®cant amounts of water from the atmo-
anions (I±VII). We have also investigated both 1-butyl-2,3-
‡
dimethylimidazolium ([bm im] ) and 1-butyl-2,3,4,5-tetra-
2
‡
methylimidazolium ([bm im] ) based salts.
4
4
a,b
sphere.
The water content of various RTILs may be strongly
in¯uenced by the nature of the cation and anion. However, the
molecular state of water dissolved in various RTILs and the
origins of water solubility in the RTILs have not yet been
addressed. An insight at the molecular level into the state of
Experimental
An ATR-IR accessory (``Golden Gate'', Specac, Ltd., UK)
that utilises a diamond as the ATR crystal was used to obtain
IR spectra, which were measured on an Equinox-55 FTIR
spectrometer (Bruker, Germany) with an MCT detector and a
y Electronic Supplementary Information available. See http:==
www.rsc.org=suppdata=cp=b1=b106900d=
�
1
resolution of 2 cm . A small droplet of ionic liquid was placed
5
192
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200
DOI: 10.1039/b106900d
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, BF , ClO
1
-Butyl-3-methylimidazolium and 1-butyl-2,3-
dimethylimidazolium ionic liquids (anion ˆ PF
CF CO , CF SO , (CF SO ) N, SbF , NO )
6
4
4
,
3
2
3
3
3
2 2
6
3
The respective metal salt (0.32 mol), required to give the desired
ionic liquid, was added to a solution of either [bmim]Cl or
[bmmim]Cl (0.29 mol) in dichloromethane and stirred for 24 h.
The suspension was ®ltered to remove the precipitated chloride
salt and the organic phase was repeatedly washed with small
Fig. 1 Schematic diagram of ATR-IR spectroscopic measurement
with a single re¯ection diamond ATR crystal showing formation of an
evanescent wave.
3
volumes of water (ca. 30 cm ) until no precipitation of AgCl
occurred in the aqueous phase on addition of a concentrated
AgNO solution. The organic phase was then washed twice
3
on top of the diamond crystal. Single re¯ection ATR (incident
ꢀ
more with water to ensure complete removal of the chloride
salt. The solvent was removed in vacuo and the resulting ionic
liquid stirred with activated charcoal for 12 h, after which the
ionic liquid was passed through a short alumina column(s)
(acidic and=or neutral) to give a colourless ionic liquid, which
angle 45 ) ensures that the IR bands of vibrational modes of
water dissolved in ionic liquids are not o€-scale due to very
short e€ective path length. This also has the advantage of
minimising usage of the sample since very small amounts of
2
ꢀ
RTILs were required to cover the 4 mm surface of the dia-
was dried at ca. 100 C in vacuo for ca. 24 h or until no visible
mond. The evanescent wave that forms in ATR (as shown
schematically by the exponential line in Fig. 1) probes a very
shallow layer of the sample. Thus, the depth of penetration of
the IR beam in this ATR arrangement is ca. 0.5 mm in the
region of the n(O±H) stretching modes of water. It is dicult to
achieve such a small and constant path length in transmission
spectroscopic cells. This advantage of ATR-IR spectroscopy
has been widely utilised to analyse aqueous solutions. High-
pressure in situ ATR-IR spectroscopy has recently been
2
signs of water were present in the IR spectrum (CaF plates).
Yields were generally 70 to 80%. The ionic liquids were char-
1
acterised by H-NMR and mass spectrometry.
1-Butyl-2,3,4,5-tetramethylimidazolium iodide [bm im][I]
4
To a vigorously stirred solution of 1,2,4,5-tetramethyl-
3
imidazole (25 g, 0.201 mol) in toluene (100 cm ) 1-iodobutane
3
27.5 cm , 0.242 mol) was added. The solution was heated to
(
re¯ux at ca. 110 C for 24 h, after which it was placed in a
ꢀ
applied to measure the spectra of CO dissolved in ionic
2
ꢀ
8
liquids and of high-pressure gases, supercritical ¯uids and
freezer at ca. � 20 C for 12 h. The toluene was decanted and
2
0
the remaining viscous oil=semi-solid repeatedly recrystallized
from ethyl acetate to yield an o€-white semicrystalline solid,
near-critical water. An additional advantage of the ATR-IR
approach is the ease of heating the diamond ATR element,
which allowed us to measure the temperature dependence of
IR spectra of RTILs and to monitor drying of RTILs upon
heating. Karl±Fischer titrations were conducted using a
which was dried in vacuo to give [bm
5% yield, this was stored in the dark and characterised by H-
NMR and mass spectrometry.
4
im]I in approximately
1
8
1
Metrohm 737 KF coulometer. H-NMR data was recorded on
a Jeol GSX-270 MHz spectrometer. The resonance frequency
of deuterium in the solvents, which had a purity of > 95
atom% D, was used as an internal reference.
1-Butyl-2,3,4,5-tetramethylimidazolium [bm
(anion ˆ PF , BF , (CF SO N)
4
im] salts
6
4
3
2 2
)
The respective metal salt (53.5 mmol), required to give the
desired ionic salt=liquid, was added to a solution of [bm im]I
4
Syntheses
(15g, 48.6 mmol) in dichloromethane and stirred for 24 h. The
suspension was ®ltered to remove the precipitated chloride salt
and the organic phase reduced in vacuo. This was then re-
dissolved in diethyl ether and repeatedly washed with small
volumes of water (ca. 30 cm ) until no precipitation of AgCl
occurred in the aqueous phase on addition of a concentrated
Unless otherwise stated all reactions were conducted in an
anaerobic atmosphere of nitrogen. Solvents were distilled from
the relevant drying agents prior to use. Toluene was stood over
Na and distilled (benzophenone was used as a moisture indi-
cator). Ethyl acetate and dichloromethane were distilled from
CaH . 1-Methylimidazole and 1,2-dimethylimidazole were dis-
2
tilled from KOH. 1,2,4,5-Tetramethylimidazole was sublimed
under vacuum prior to use. 1-Chlorobutane and 1-iodobutane
3
AgNO solution. The organic phase was then washed twice
3
more with water to ensure complete removal of the chloride
salt. Removal of the organic solvent in vacuo gave either solids
� � �
(
Yields were generally 70 to 80%. The ionic liquid=solids were
with [PF ] and [BF ] ) or a viscous tar (with [(CF SO ) N] ).
6 4 3 2 2
were washed with concentrated H
2 4
SO then distilled from
P
2
O
by a AgNO test. AgCl has a solubility of 1.4 mg l in water,
5
. The chloride content of the ionic liquids was determined
�
1
characterised by H-NMR and mass spectrometry.
1
3
hence all the ionic liquids synthesised will have a chloride
content below this value. Full experimental details for all ionic
liquids have been deposited as ESI.y
Results and discussions
The IR spectrum of water is one of the most studied among the
spectra of the polyatomic molecules. The water molecule has
1-Butyl-3-methylimidazolium chloridesÐ[bmim]Cl
and [bmmim]Cl
C2v symmetry with three IR active vibrations. One can, how-
ever, observe more than three bands in the IR spectrum of
water due to the appearance of combination, librational and
overtone bands. The IR band corresponding to the bending
To a vigorously stirred solution of 1-methylimidazole or 1,2-
3
dimethylimidazole (1.25 mol) in toluene (125 cm ) at 0 C 1-
ꢀ
3
chlorobutane (144 cm , 1.38 mol) was added. The solutions
mode (n ) of water (either pure or dissolved in solvents) usually
2
ꢀ
� 1
were heated to re¯ux at ca. 110 C for 24 h, after which they
ꢀ
absorbs in the region 1595±1650 cm . However, this band
were placed in a freezer at ca. � 20 C for 12 h. The toluene
alone is seldom used to elucidate the molecular state of water.
By contrast, stretching vibrational modes of water have been
extensively used to study the molecular state of water dissolved
in various solvents and absorbed in many materials, such as
was decanted from the remaining viscous oils=semi-solids and
recrystallized from acetonitrile and then repeatedly recrys-
tallized from ethyl acetate to yield white crystalline solids,
which were dried in vacuo to give [bmim]Cl and [bmmim]Cl in
11,14,21
polymers.
The IR bands corresponding to the antisym-
1
approximately 86 to 90% yield and characterised by H-NMR
and mass spectrometry.
metric (n ) and symmetric (n ) stretching modes of water
3
1
�
1
usually lie in the region 3000±3800 cm . The position and
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200 5193

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Fig. 2 ATR-IR spectra in the n(O±H) region measured with a single
re¯ection diamond ATR crystal: spectrum of liquid water (dashed-
dotted line), spectrum of RTIL III (solid line) showing the bands of
Fig. 3 ATR-IR spectra of III in the n(O±H) region showing the
uptake of water from air as a function of time: spectrum of III after it
has been placed on the diamond ATR crystal.
�
1
water dissolved in III at 3640 and 3560 cm
.
intensity of these bands are very sensitive to the water envir-
onment and to the state of water association via H-bonding.
The n and n bands in water vapour absorb at 3756 and 3657
anions of the added salt. An analogous spectral picture has
been reported for the weakly interacting water, involving both
its protons, in the case of water absorbed into polymers where
the interaction with the basic sites of the opposite polymer
3
1
�
1
22,23
cm respectively; these bands shift to lower wavenumber
when water interacts with the environment (e.g. water dis-
solved in a solvent or sorbed into a polymer). The di€erence in
12b
chains was possible.
If RTILs are left open to the atmo-
sphere they can easily absorb water, the amount depending on
the nature of the RTILs and on the relative humidity and
temperature. However, here we are interested in the molecular
state of water present in RTILs when they are open to the air
at ambient conditions, rather than the solubilities of water in
RTILs for two-phase systems. The question arises whether the
molecular state of water is strongly in¯uenced by its con-
centration. Fig. 3 shows the IR spectra of water in III as a
function of time upon water absorption from the atmosphere
into III placed on the diamond ATR crystal. It can be seen that
the positions of the bands and their shapes do not change
essentially as a function of concentration, indicating that there
is no change in the state of water in concentrations up to ca.
�
1
the maximum positions of these bands is ca. 100 cm in the
case of water bound in symmetric complexes, with both pro-
tons of water participating in H-bonding. In the IR spectrum
of liquid water the n
a broad band with a maximum at ca. 3300 cm (the dashed
line in Fig. 2). In fact, this band is comprised of the n and n
bands that broaden considerably due to the e€ect of H-
bonding. Fermi resonance between n and 2n further com-
3
1
and n bands appear to merge, producing
�
1
3
1
1
2
plicates the structure. As mentioned, it is rather dicult to
deduce information on the molecular state of water dissolved
in various solvents based on the shifts of the stretching bands
3
of water because there is a strong coupling between the n and
�
3
n1 bands. The use of HDO molecules provides a possibility to
overcome this problem and has been successfully used to study
the state of water dissolved in organic solvents. The presence
of HDO molecules is usually easy to achieve by adding small
amounts of D O into H O. However, there is a danger of
0.8 mol dm . The dashed line represents the spectrum of III
showing the presence of a small but measurable amount of
water. The growth of the water bands with time demonstrates
that the amount of water in III increases due to absorption of
water from the atmosphere. We are continuing to investigate
the nature of water in RTILs at greater concentrations and this
will be reported elsewhere. Another important observation is
2
2
isotope exchange with solvent molecules, especially when the
solvent possesses acidic protons, as in the case of the cations in
ionic liquids. Moreover, it is the presence of H O, that is
ꢀ
2
that heating of III at 80 C resulted in the decrease of the water
absorbed from the atmosphere or as a byproduct from syn-
thetic routes and a€ects the properties of RTILs, which is of
major interest. Therefore, our analysis of the molecular state of
bands up to their disappearance (not illustrated). The dis-
appearance of the IR bands of water in III upon heating
indicates that RTILs can be dried by heating and simulta-
neously monitored via spectroscopy for the presence or
absence of water.
2
water will be based mostly on H O, supported by additional
measurements of HDO in RTILs.
Fig. 2 presents ATR-IR spectra of the RTIL, III, and liquid
water in the n(O±H) region. Two distinct bands corresponding
To determine the e€ect of the anion on the interaction
between water molecules and ionic liquid a series of RTILs
with various anions was studied. In these experiments the
droplet of ionic liquid was placed on the diamond ATR crystal
to allow moisture from the air to be absorbed at ambient
conditions into the ionic liquid until saturation at a given
humidity (ca. 60±70% in our experiments). For quantitative
comparison of the solubility of water in di€erent ionic liquids
the two-phase system, water=RTIL, has to be studied. How-
ever, it is important to stress that here we investigated the
molecular state of water based on the positions of the IR bands
of water rather than by comparing their concentrations based
to the n
3
and n
1
modes of water dissolved in III are seen at 3640
�
1
and 3560 cm . The IR spectrum of liquid water is shown for
comparison. The position of these two bands of water dis-
solved in III indicates that the water molecules are not asso-
ciated into clusters or pools of water, and can be assigned as
`
[
`free'' water molecules interacting via H-bonding with the
�
BF4] anion. This assignment is consistent with the literature
data on the spectroscopic manifestation of the formation of
symmetrical complexes of water molecules with weak bases.
�
1
Two bands at 3640 and 3590 cm in the Raman spectrum of
24
4a
water with added NaPF6 have been observed previously.
However, the presence of the second band was assigned to
on the intensities of these bands. Seddon and co-workers
have measured the water content in some RTILs using the
Karl±Fischer titration method. Their results were also based
on the saturation of the ionic liquids by moisture from the
atmosphere. We have shown that the state of water in III in the
2
`electrostatic interactions'', while Scherer has suggested that
1
`
the appearance of these bands can be explained by the for-
mation of two weak H-bonds of water molecules with the two
5
194
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200

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concentration range from dry III to water-saturated when
�
3
open to air (up to 1mol dm for RTILs I±V) does not depend
on the amount of water present. The concentration range of
the water absorbed into these RTILs has been estimated using
25
the molar absorptivity of the stretching bands of water dis-
solved in CH Cl . The molar absorptivity (e) based on the
2
2
3 2 2
peak intensity for the n band of water dissolved in CH Cl is
3
� 1
� 1
ca. 100 dm mol cm . Then, following the Lambert±Beer
law (A ˆ ecl where c is concentration, and l is the path length),
the concentration of water can be calculated. The e€ective
3
path length for the n band has been calculated according to
the method of Harrick (taking the refractive index of the
1e
RTILs as 1.467 and the refractive index of diamond as 2.4),
giving l ' 1.1mm.
3
The measured ATR-IR absorbance (A) of the n band of
water in I in equilibrium with water from the air is 0.004. This
�
3
results in a concentration of water of ca. 0.36 mol dm
.
Fig. 4 ATR-IR spectra of VII in the n(O±H) region showing the
uptake of water from air as a function of time: spectrum of VII
immediately after it has been placed on the diamond ATR crystal
(dashed-dotted line), spectrum of VII after it has been exposed to at-
mosphere for 90 min (dashed line), spectrum of VII after it has been
saturated by atmospheric water (solid line).
However, the absorbance of the n band used in our calcula-
3
tion is very small and well below the typical absorbances used
in quantitative calculations. Therefore, we have also measured
the spectra of I saturated with water using a transmission cell
with CaF
2
windows and path length ca. 66 mm. This mea-
surement resulted in the absorbance of the n
3
band, A ˆ 0.3,
which is a very reasonable absorbance for quantitative calcu-
lations. The calculated concentration of water in I in this case
�
3
is ca. 0.45 mol dm , which is in good agreement with the
estimated concentration based on ATR-IR data. It should be
noted that in our calculations the assumption was made that
the molar absorptivity of the IR bands of water is similar to
the absorptivity of these bands of water dissolved in CH
2 2
Cl .
The measured absorbance of the n band of water in III in
3
equilibrium with air exceeds the absorbance of this band of
water in I by a factor of two, thus resulting in a water content
�
3
of ca. 0.9 mol dm . This result is consistent with our (see
below) measurements via Karl±Fischer titration of the water
content sorbed into I and III from the air and is in agreement
4
a
with those of Seddon and co-workers. Transmission spec-
troscopy has also been used to analyse the combination band
n ‡ n of water dissolved in I. Additional evidence for H-
3
2
bonding of water dissolved in I has been provided by obser-
vation of the combination band of water (n ‡ n ) in I using a
3
2
transmission cell with path length ca. 120 mm. The band has its
Fig. 5 ATR-IR spectra of water in six RTILs: I,thick solid line (weak
�
1
�
1
peak maximum at 5273 cm and the half-width is ca. 60 cm
� 1
bands at 3672 and 3595 cm ); III, dashed line; IV, thin solid line; V,
dashed-dotted line; VI, dashed-double dotted line; VII, dotted line.
Two vertical lines at 3756 and 3657 cm indicating the position of the
.
Its position provides additional evidence that water in I is
weakly H-bonded. This conclusion is based on comparison
with the literature data on the position of this combination
� 1
3 1
n and n bands of water vapour are given for comparison.
2
2
band of water in various solvents, which places I between
CCl and polycarbonate in the ability of these bases to H-bond
with water.
4
the bands in the n(O±H) region was detected after the samples
were cooled to room temperature. Two bands corresponding
Fig. 4 shows the ATR-IR spectra in the region 3700±3000
�
1
cm for [bmim][CF CO ], and shows that the state of water
to the n
3
1
and n can be identi®ed in the nine ionic liquids
3
2
does not change as a function of its concentration within the
range that is achieved by absorption of water from the
atmosphere at ambient temperature. The shape of the band in
the n(O±H) region is remarkably di€erent from the absorption
in this region in the case of III, nevertheless, this shape remains
essentially unchanged upon heating, indicating that although
the state of water is di€erent from that in III it does not change
as a function of water concentration (however, the appearance
of a weak band at ca. 3670 cm will be discussed below).
Additionally, heating this RTIL resulted in the total removal
of measurable amounts of water, indicated by the bottom line
in the Fig. 4.
studied and the maxima of the positions of these bands are
summarised in Table 1. By contrast, the spectra of the two
other ionic liquids (VI and VII) presented in Fig. 5 do not
show well-resolved structure in this region and are dominated
by strong broad bands. Molecules of water dissolved in the six
�
� � �
6 4 4
] , [BF ] , [ClO ] ,
ionic liquids with either [PF
6
] , [SbF
�
or [(CF SO )N] anions are ``free'' water mole-
3 2
�
[CF
SO
]
3
3
cules, interacting via H-bonds with the anions in a symmetric
complex (both protons of water bound to two discrete anions):
�
1
�
�
�
A ...H±O±H...A (where A represents the anion in RTILs).
Additional support for the assignment of the molecular state
of water in RTILs I±V, and XIII stems from the two following
Fig. 5 shows the comparison of the IR spectra of the dif-
ferent RTILs in the n(O±H) region after each of the ionic
liquids was placed on the diamond ATR crystal and exposed
to the atmosphere. It is important to note that dry ionic liquids
do not have absorptions in this region. This was con®rmed by
observations. Firstly, the di€erence in the positions of the n
and n bands of water (n ) remains almost constant within
3
1
3
� n
1
�
1
the range 70±80 cm , indicating a similar type of bonding of
water. It was found earlier that such a di€erence is char-
acteristic for 2 : 1 complexes of water.
14,21
Moreover, Ford and
ꢀ
17
heating the ionic liquids placed on the diamond to 80±100 C
to remove any residual water. The appearance and growth of
co-workers have analysed a very broad range of binary
water±solvent mixtures, and found that for 2 : 1 complexes
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200
5195

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the broad distribution of the water±anion distances resulting in
�
1
2
Table 1 Wavenumbers (cm ) of the vibrational modes of H O in the
vapour phase, dissolved in RTILs (with [bmim] cation) and organic
solvents (for comparison from ref. 16 and 21 )
the broad band. This may occur because of the planar sym-
metry of the nitrate anion. This possibility has been mentioned
in related work on the hydration of this anion in aqueous
a
Anion
n
3
n
1
n
3
� n
1
n
2
26
3 2
solutions. In the case of [bmim][CF CO ] the IR band of
water strongly resembles the band of liquid-like water. At
higher water concentrations in this RTIL it was possible to
Vapour, no anion
[PF
3756
3672
3671
3663
3640
3639
3637
3613
3612
3605
3575
3572
3520
3657
3595
3592
3580
3560
3544
3560
3540
3535
3544
3506
3499
3450
99
77
79
83
80
95
77
73
77
61
69
73
70
1595
1610
�
6
]
(I)
,2-Dichloroethane
�
1
detect a weak band at ca. 3670 cm (see Fig. 4) which can be
assigned to water bound weakly to the CF end of the anion,
in contrast to most of the water molecules that absorb at ca.
1
�
�
[SbF
6
]
(II)
(III)
3
[BF
4
]
1625
Benzaldehyde
[
� 1
3
interaction of water with anions in aqueous solutions. Thus,
400 cm . This is consistent with the previous studies on the
�
(CF
ClO
3
SO
2
)
(IV)
2
N] (XIII)
�
[
4
]
26
Bergstrom and co-workers have noted the presence of a high-
wavenumber OD stretching band when studying the hydration
Acetophenone
�
b
[(CF
CF
3
SO
2
)
2
N] (XII)
(V)
1631
1630
�
�
[
3
3
SO ]
of the [CF
that produce a broad band in the region around 3400 cm
3
COO] anion with HDO. The molecules of water
�
1
Tetrahydrofuran
�
[
[
NO
CF
3
]
CO
(VI)
1640
�
represent water interacting strongly with the COO end of the
anion and water forming aggregates around this anion. The
support for the strong interaction between water molecules
�
3
2
]
(VII)
3 1
n and n bands merge into a
broad band at ca. 3400 cm
�
1
�
a
b
Entries in bold type are results from this work. In this RTIL all the
and the COO end of the anions comes from the observation
�
1
of the shoulder with the low frequency shift (ca. 10 cm ) of
the carboxyl band of this anion in the presence of water
hydrogen atoms on the imidazolium ring have been substituted with
CH groups.
3
(Fig. 6). This shift provides direct evidence that water mole-
�
cules are bound to the COO end of the anion since the
�
n(COO ) band shifts down in wavenumber. It was also pos-
sible to bind all COO moieties with water, when water was
between solvent and water molecules the following relation-
ship takes place between the shifts of the n and the n bands of
�
3
1
deliberately added to VII, as was evidenced by the dis-
� 1
water molecules: Dn ˆ 1.17Dn � 0.66. It can be seen from
3
1
appearance of the initial band at 1680 cm . Assignment of the
resultant spectrum is somewhat complicated due to the pres-
ence of the bending mode of water in the proximity of the
Table 2 that shifts of the n
3
1
and the n bands of water molecules
in the six RTILs studied roughly follow the above relationship
for 2 : 1 complexes of water. Thus, taking into account all
spectral information we conclude that most of the water
molecules absorbed from the atmosphere into the eight RTILs
presented in Tables 1 and 2 are H-bonded via both hydrogen
atoms of water to two anions of RTILs. Table 1 also includes
data from the literature on the wavenumbers of the stretching
modes of water dissolved in some selected organic solvents for
�
shifted n(COO ) band. Therefore, these measurements were
repeated by adding D
lower wavenumbers, and thus does not interfere with the
2
O (since its bending band absorbs at
�
�
n(COO ) band), which allowed observation of the n(COO ) as
a single band at 1670 cm indicating that all anions are bound
� 1
�
to water via the COO group (Fig. 7).
The important implications of these ®ndings is that H O
2
2
: 1 complexes of water. This shows that the state of water in
�
3
dissolved in some ionic liquids with a stronger anion basicity
�
the concentration range 0.2±2.0 mol dm absorbed from the
air compares well with the state of water in these selected
organic solvents.
(such as [CF CO ] ) can form aggregates or, perhaps, even
droplets of the bulk liquid-like water in ionic liquids, as evi-
3
2
� 1
denced by a broad band at ca. 3400 cm . Strong interactions
The broad n(O±H) bands of water in the ionic liquids with
� �
3 3 2
] and [CF CO ] anions indicate the presence of water
of water with the anions and the formation of associated water
could be a precursor of the water miscibility with nitrate- and
tri¯uoroacetate-based (VI and VII) ionic liquids.
The strength of H-bonding between H O molecules and
2
anions can be estimated from the wavenumber shift (Dn)
[NO
aggregates or other types of water association. The shape of
±
3
the n(O±H) band in the [NO ] based RTIL has a visible
�
1
shoulder at ca. 3530 cm , which could represent the anti-
symmetric stretching mode of water H-bonded with an anion.
Nevertheless, the overall broad structure of the band indicates
that water clusters or water aggregates may be present. There
is also the possibility that several water molecules are inter-
�
acting with the [NO3] anions in a slightly di€erent way, with
�
1
Table 2 The shifts of the n and n (cm ) of water in di€erent ionic
3 1
liquids (with [bmim] cation) compared with the corresponding water
bands in vapour, and the enthalpies of H-bonding
�
DH=kcal mol ,
1
1
�
�
Anion
Dn
3
Dn
1
[kJ mol
]
�
[
[
[
[
[
[
[
[
PF
SbF
BF
(CF
ClO
(CF
CF
6
]
84
93
62
77
97
97
117
113
151
207
1.8 [7.5]
2.0 [8.3]
2.3 [9.6]
2.5 [10.5]
3.0 [12.5]
3.2 [13.4]
3.8 [15.9]
5.0 [20.1]
�
6
]
�
4
]
116
119
143
151
181
236
�
�
3
SO
2
)
)
2
N]
N]
�
4
]
SO
a
3
2
2
�
3
SO
3
]
�
�
NO
3
]
Fig. 6 ATR-IR spectra in the n(COO) band region for dry VII
(
solid line) and wet VII (dashed line), showing the appearance of the
at ca. 1640 cm after VII has been saturated
a
� 1
In this RTIL all of the hydrogen atoms on the imidazolium ring have
been substituted with CH groups.
bending band of water n
by atmospheric water and the appearance of the shifted n(COO) band
of VII at 1670 cm
2
�
3
�
1
.
5
196
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200

View Article Online
�
� 1
Fig. 8 ATR-IR spectra of I (top spectrum) and VIII (bottom spec-
trum) in the region of the n and n bands of water adsorbed in these
3 1
RTILs.
Fig. 7 ATR-IR spectra of VII in the n(COO) band region showing
�
2
disappearance of the n(COO) band at 1680 cm on addition of D O
�
� 1
resulting in a shift of the n(COO) band to 1670 cm
.
probably play a secondary role. Support for the last statement
also comes from the measurements of the two RTILs (I and
III), in which one of the hydrogen atoms was substituted by a
between n of the water in the vapour phase and n of the water
3
3
dissolved in RTILs (for RTILs where separated bands due to
the n and n bands are observed), by using the relationship:
16
3
1
CH
in such ionic liquids based on the [PF6] anion are almost
3
group. As Fig. 8 shows, the positions of the water bands
�
vapour
DH ˆ � 80ꢁn3
IL
3
vapour
3
� n †=n
4a
exactly the same. Seddon and co-workers have studied the
�
1
vapour
(
where DH is the enthalpy of H-bonding in kcal mol , n
3
IL
water content of several RTILs in which the absorption of
water from the atmosphere at ambient temperature was mea-
sured as a function of time. Their trend in water content is
consistent with our scale of the strength of the interaction
between water molecules and the anion in RTILs. Brennecke
and co-workers have reported results on the measurements of
is the wavenumber of the n
3
of water in vapour, and n
3
is the
3
wavenumber of the n band of water in a RTIL). The corre-
sponding enthalpies for the interaction of water with di€erent
anions in ionic liquids are summarised in Table 2. The
3
calculated energies based on the spectral shifts of the n band
30
place the anions of the corresponding IL with respect to the
�
water solubilities in the two phase system of water and I. Our
spectroscopic study provides an additional insight into the
state of water dissolved in RTILs open to the air at ambient
conditions and shows that, ``molecularly dispersed'', water H-
bonded to anions is a major state of water molecules dissolved
in RTILs via absorption from the atmosphere at concentra-
strength of the interaction with water in the order: [PF
6
] <
�
�
�
�
�
[
[
SbF6] < [BF ] < [(CF SO ) N] < [ClO ] < [CF SO ] <
3
3
NO ] . Additional support for this correlation stems from the
4
3
2 2
4
3
�
investigation of the position of the bending mode of water
) dissolved in the studied RTILs (Table 1). It is known that
(
2
n
�
3
the maximum of the IR band of the bending mode of water
shifts to higher wavenumber compared to free water in the
gaseous state as a result of hydrogen bonding. Indeed, the data
presented in Table 1 show that the frequency of the bending
tions in the range 0.2±2.0 mol dm . Nevertheless, we cannot
exclude the presence of water±water aggregates in these RTILs
at higher water concentrations when water is added to these
RTILs. Indeed, on addition of water to I to achieve saturation
conditions (ca. 2 wt.% at room temperature), and study by
transmission spectroscopy with a longer path length (60 mm),
we were able to detect a weak, broad band, with a maximum
�
�
�
mode increases in the order [PF ] < [BF ] < [CF SO ] <
3
[
6
4
3
�
NO ] , re¯ecting the same order of the strength of interaction
3
between water molecules and anions in RTILs as was revealed
from the bands corresponding to the stretching mode region.
The comparison of our spectroscopic results with the studies
�
1
at 3473 cm , along with the strong bands at 3672 and 3595
�
1
cm . The latter two bands correspond to the water interacting
with anions in RTILs, as discussed before, whilst the former
corresponds to the water aggregates, probably forming around
those molecules of water that have already H-bonded to the
anions. This demonstrates the power of IR spectroscopy in
studying the water content in RTILs, while gravimetric mea-
2
7
2 2
of water=salt=CH Cl systems and with studies of iso-
2
8
topically dilute HDO molecules of water shows that the
relative order of the strength of interaction of water with
anions in the systems containing the same anions as in the
RTILs studied here is identical. Our ®ndings are also con-
sistent with the work on the interaction between methanol and
31
surements of water in RTILs do not provide information on
the state of the water in RTILs when the water is added
deliberately. In such cases the molecular state of water can
change as a function of water concentration, thus complicating
any type of modelling of the quantitative results for water
absorption. The quantitative measurement of water=RTIL
2
9
various anions, where exactly the same trend of basicity of
the anions was shown, based on the shifts of the n(O±H) band
of methanol dissolved in solutions of salts in CH
2 2
Cl . The
n(O±H) band of methanol dissolved in I appeared at ca. 3618
cm , which coincides with the position reported for the
�
1
±
] in
This also indicates that the relative basicity of
n(O±H) band of methanol in a solution containing [PF
CH Cl .
2 2
6
two-phase systems deserves
spectroscopy.
a separate study via IR
2
9
anions in dilute solutions and in ionic liquids is similar. It is
important to note that in the latter case methanol was dis-
solved in RTILs in the absence of any other solvent. In both
cases the corresponding spectral shifts of the n(O±H) bands
We have also investigated the imidazolium n(C±H) stretch-
�
1
ing region of the RTILs' spectra (3200±3000 cm ). One would
expect a shift in the position of the n(C±H) bands, if signi®cant
hydrogen bonding between the acidic protons of the imida-
zolium ring and water were present. However, our spectra
show negligible changes in this region on going from dry to wet
ionic liquids. The n(C±H) bands are sensitive to the nature of
the anion of the dry ionic liquid. Analysis of the spectra in this
(
appeared similar. This implies that, as suggested by Seddon
for water and methanol weakly bound to the anions)
4
a
and co-workers, the anions play a major role in the solubility
and miscibility of water with RTILs, while the cations
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200
5197

View Article Online
region clearly indicate a cation±anion interaction that increa-
�
�
�
�
�
ses in the order [PF ] , [BF ] , [(CF SO ) N] , [ClO ] ,
6
4
3
2 2
4
�
�
[CF
3
SO ] , [NO
3
3
]
and [CF
3
CO
2
] . This is consistent with the
possible formation of a cation±anion hydrogen bond. We are
continuing to investigate these phenomena and will report our
results elsewhere.
In addition, the possible e€ect of the acidic ring protons of
the imidazolium ring cation on the molecular state of water has
also been studied. Although the spectroscopic evidence repor-
ted above strongly supports that water molecules preferentially
interact with anions, there is, in principal, a possibility that
water may interact via H-bonding between the oxygen atom
and acidic protons in the imidazolium ring. Therefore, it is
important to study the molecular state of water in RTILs where
all or some of the hydrogen atoms on the imidazolium ring
have been substituted with CH groups (the chemical structures
3
of such cations are presented schematically below).
Fig. 9 ATR-IR spectra of XII (thick line) and XIII (thin line) in the
region of the n and n bands of water adsorbed in these RTILs.
3
1
�
bmim][(CF SO ) N] (XIII), was compared with the ATR-IR
3 2 2
[
spectrum of (XII), where all of the ring hydrogen atoms have
been substituted by CH3 groups. Fig. 9 shows the n(O±H)
region of the IR spectra of XII and XIII. The absence of the
imidazolium ring hydrogen atoms is clearly seen in the spec-
trum of XII, by the absence of the bands in the region around
�
1
3
to water. Indeed, two overlapping bands at 3605 and 3544
cm indicate that water molecules have been absorbed into
100 cm . There is a strong doublet, which can be attributed
RTILs comprising the 1-butyl-2,3-dimethylimidazolium
�
�
�
� 1
cation with [PF
IX and X) along with 1-butyl-2,3,4,5-tetramethylimidazolium
[
6
] , [BF
4
]
and [(CF
3
SO
2
)
2
N] anions (VIII,
XII from the air. The similarity of the spectra of the water
molecules absorbed into IV and XII (based on the wave-
numbers at the maximum of the bands) suggests that water
molecules are H-bonded to the anions in XII. In the region of
the bending mode of water there is a relatively strong band at
� �
4 4 3 2 2
bm im] [BF ] and [(CF SO ) N] have been investigated and
their ATR-IR spectra recorded. Analysis of the n(O±H) region
of VIII and IX has demonstrated that there is no e€ect from
substituting a proton for a methyl group at the C2 position of
the imidazolium ring for the corresponding RTILs I and III.
Fig. 8 shows the n(O±H) bands of water in the ATR-IR spectra
of I and VIII. The positions and shapes of the n and n bands
�
1
1633 cm (not illustrated) that also supports weak H-bonding
between water molecules and XII. Most importantly, Fig. 9
demonstrates that the absence of acidic protons in the imida-
zolium ring increases the amount of absorbed water since there
were relatively weak bands due to water being absorbed into
XIII, compared with water absorbed into XII. As the hydrogen
bond donor ability of the cation decreases (XIII > XII) the
strength of the hydrogen bond between water and the anion
increases. This can be interpreted as arising from a competition
between the imidazolium ring and water for the hydrogen
bond acceptor sites of the anions. We can also tentatively
explain the di€erence in the amount of water absorbed from
the air into these analogous RTILs by the suggestion that
1
3
of water absorbed from the air into these two RTILs are very
similar, indicating that substitution of the C2 proton in the
imidazolium ring with a CH group has no e€ect on the
3
molecular state of water. Nevertheless, the substitution of one
of the hydrogen atoms in the imidazolium ring (even the most
acidic one), does not exclude the possibility of interactions
between water molecules and the remaining acidic protons of
the imidazolium ring. To this end, we have studied the IR
spectra of RTILs with all of the H atoms of the imidazolium
ring substituted by CH groups. Perhaps surprisingly, the 1-
3
butyl-2,3,4,5-tetramethylimidazolium-based salt with the
�
3
introducing CH groups into the imidazolium ring creates
[BF4] anion (XI) is a solid at room temperature. (It is sur-
more free volume in XII than in XIII, thus allowing water
molecules to reach their preferred sites of interaction more
easily. Nevertheless, this is very strong evidence that the pre-
ferred sites of interaction with water molecules are the anions
in RTILs, as has been demonstrated above for other RTILs,
discussed in this paper.
prising as one would expect that with the absence of the acidic
protons of the imidazolium ring the interaction of the cation
with the anion would be weaker, therefore favouring a liquid
state). The ATR-IR spectrum of XI has been recorded and
showed no evidence of water (up to the limit of detection via
single re¯ection ATR-IR spectroscopy). Nevertheless, a small
amount of water was deliberately added to XI and the ATR-IR
Table 3 shows the amount of water absorbed by ionic liquids
after 24 h exposure to the atmosphere at a relative humidity of
59%, using the Karl±Fischer titration method. The results for
spectra recorded. The weak bands corresponding to the n
bands of water in XI were detected. The maxima of each
band were almost identical to those of the n and n bands of
1
and
�
n
3
the tri¯uorosulfonimide [(CF
3
SO )
2
2
N] based ionic liquids are
consistent with the results obtained using ATR-IR spectro-
scopy (see e.g. Fig. 9). As the protons on the imidazolium ring
are replaced with methyl groups the amount of absorbed water
increases. This supports our proposal that the acidic protons
of the imidazolium ring do not play a direct role in water
absorption and H-bonding with water molecules. Additional
trends that can be deduced are that ionic liquids with analo-
1
3
water dissolved in III. This observation suggests that the water
molecules dissolved in XI interact similarly with their sur-
roundings to the water molecules dissolved in III.
Although III and XI have the same anions, these RTILs
exist in di€erent physical states. In order to prove the concept
that the acidic protons in the imidazolium ring have no e€ect
on the molecular state of water analogous imidazolium salts
gous cations but di€ering anions show a more pronounced
�
(with and without acidic protons) need to be studied in the
same physical state. To this end, the ATR-IR spectrum of
uptake of water for [BF
�
4
] -based ionic liquids and follow the
�
�
N] < [BF
4
general trend: [PF
] < [(CF
SO )
2
] .
6
3
2
5
198
Phys. Chem. Chem. Phys., 2001, 3, 5192±5200

View Article Online
modes of water indicated that molecules of water are present in
the ``free'' (not self-aggregated) state bound to the basic anion
via H-bonding. The shift of the antisymmetric stretching band
has been used to correlate the relative strength of the interaction
between water and RTILs with di€erent anions. The strength
Table 3 Water content of ionic liquids exposed to air at relative hu-
midity 59% (determined by Karl±Fischer titrations)
Ionic liquid
Water content=ppm
[
[
[
[
[
[
[
bmim] [PF
bm im] [PF
bmim] [BF
bm im] [BF
bmim] [(CF
6
]
2640
2540
19500
13720
3300
�
�
of H-bonding increases in the order [PF ] < [SbF ] <
6
6
2
6
]
�
�
�
�
�
[BF
4
] < [ClO
4
] < [CF
3
SO ] < [NO
3
3
] < [CF
3
CO
2
] .
The
4
]
hydration of anions in the ®rst six RTILs from this series
appears to be similar to hydration of these anions in the salts
2
4
3
]
2 2
SO ) N]
27
bm
bm
2
im] [(CF
im] [(CF
3
SO
SO
2
)
)
2
N]
N]
6380
33090
with di€erent cations reported previously. The energies of H-
bonding between water and these RTILs were estimated from
4
3
2
2
�
1
.
the spectral shifts, with enthalpies in the range 8±13 kJ mol
It has been shown that the basicity of anions in RTILs mea-
sured via their ability to H-bond with water and methanol
hardly di€ers from the corresponding basicities of these anions
in dilute solutions of the salts in organic solvents. It has also
been found that water can form liquid-like associated aggre-
gates when it is absorbed from the air into RTILs with rela-
�
1
Table 4 Wavenumbers of the stretching vibrational modes (cm ) of
O and HDO dissolved in vapour phase, some ionic liquids and dif-
ferent media
D
2
n (OH)
in HDO
n (OD)
in HDO
� �
3 2
and [CF CO ] . The
a
Anion
n
3
(D
2
O)
n
1
(D O)
2
tively strong anions such as [NO
3
]
presence of such water aggregates in these ionic liquids may
have an e€ect on the reactivity of metal catalysts and other
solutes. Water may also act as a co-solvent to dissolve sub-
stances that would otherwise not be soluble in the RTILs.
The formation of H-bonding between water and anions
in RTILs may have several other implications. Firstly, it
demonstrates that the origins of water absorption from the air
in the studied RTILs arises from the water interactions with
anions. The water content in RTILs is also in direct correlation
with the strength of these H-bonding interactions. The pre-
sence of water in ionic liquids may have important implica-
tions on the properties of RTILs as a solvent, such as
�
[
[
[
[
[
PF
6
]
2726
2694
2665
2626
2602
2592
3630
3600
3570
3540
2673
2650
2634
2600
2634
2719
�
BF
ClO
CF
ClO
4
]
�
4
]
�
3
SO
3
]
�
4
]
2665
2788
2734
2700
2750
2592
2671
2630
2600
2646
Vapour
2
3
6 6
C H
3
5
PMMA
PVDF
2650
2696
3
6
3669
a
PMMA poly(methyl methacrylate), PVDF poly(vinylidene¯uoride).
33
conductivity, viscosity and di€usivity.
In addition, the
molecular state of H-bonded water molecules may a€ect the
reactivity of some solutes dissolved in RTILs. It has been
shown, for example, that H-bonding of water molecules
enhances the basicity of the oxygen atom in the water
molecules.
Finally, experiments were carried out with small amounts of
D
2
O added into the studied RTILs that had already been
saturated by water from the atmosphere. This resulted in the
appearance of the bands of D O dissolved in these RTILs as
well as HDO, due to the isotope exchange with H O. The
results are summarised in Table 4. The positions of the n
O), n (D O), n(OH) in HDO and n(OD) in HDO in RTILs
34
2
2
3
(
D
2
1
2
I±VI con®rm the same trend of the basicity for anions in
RTILs. The positions for n(OD) in HDO in RTILs I±V are
very similar to the positions of this band for the same anions in
Acknowledgements
We thank EPSRC for ®nancial support, Specac, Ltd. and
Bruker optics, Ltd. for the loan of equipment, SACHEM for
the loan of some ionic liquids (I) and Prof. B. J. Briscoe, Dr. C.
J. Lawrence, Mr. N. M. B. Flichy, Dr. G. G. Martirosyan and
Dr. L. Lancaster for their help and advice.
2
6,28
aqueous solution,
study the hydration of anions in RTILs further. Previously it
and this provides an opportunity to
3
2
has been shown that the n(OD) in the bulk, liquid HDO
absorbs at 2509 cm while the corresponding band a€ected by
�
1
‡
� 1
the cation (Bu N ) absorbs at 2524 cm , these values are
4
much lower than the measured bands of the n(OD) for HDO in
the studied RTILs, thus supporting the assignment of these
bands as a€ected by the anions in the RTILs.
References
1
(a) A. E. Visser, R. P. Swatloski, W. M. Reichert, S. T. Grin and
R. D. Rogers, Ind. Eng. Chem. Res., 2000, 39, 3596; (b) K. R.
Seddon, J. Chem. Technol. Biotechnol., 1997, 68, 351; (c) T. Wel-
ton, Chem. Rev., 1999, 99, 2071; (d) P. Wasserscheid and W. Keim,
Angew. Chem., Int. Ed., 2000, 39, 3773; (e) P. Bonhote, A. P. Dias,
N. Papageorgiou, K. Kalyanasundaram and M. Gratzel, Inorg.
Chem., 1996, 35, 1168; ( f ) S. N. Baker, G. A. Baker, M. A. Kane
and F. V. Bright, J. Phys. Chem. B, 2001, 105, 9663.
Our ®ndings on the interactions between water and ionic
liquids are also consistent with our previous report on the
Lewis acid±base interactions between CO2 and two ionic
�
liquids, where it has been shown that [BF
�
4
]
base than [PF6] in corresponding RTILs. In both these cases,
was a stronger
2 2
H O and CO , the anions appear to play a major role in the
interactions, acting as a base towards these solutes. However,
it was dicult to quantify the strength of the anions acting as a
Lewis base towards CO molecules. Fortunately, the IR bands
2
3
4
C. E. Song, W. H. Shim, E. J. Roh, S. G. Lee and J. H. Choi,
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C. M. Gordon, C. Hilgers, M. J. Muldoon and I. R. Dunkin,
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(a) K. R. Seddon, A. Stark and M.-J. Torres, Pure Appl. Chem.,
2
corresponding to the stretching modes of water act as a sen-
sitive probe of the anion basicity in ionic liquids, and a
quantitative estimation of the anions relative basicity was
possible in this work.
2001, 72, 2275.; (b) J. G. Huddleston, A. E. Visser, W. M.
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5
F. Liu, M. B. Abrams, R. T. Baker and W. Tumas, Chem. Com-
mun., 2001, 433.
Conclusions and implications
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7
A. G. Fadeev and M. M. Meagher, Chem. Commun., 2001, 295.
L. A. Blanchard, Z. Gu and J. F. Brennecke, J. Phys. Chem. B,
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ionic liquids have been investigated. The presence of two bands
corresponding to the antisymmetric and symmetric stretching
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S. G. Kazarian, B. J. Briscoe and T. Welton, Chem. Commun.,
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8
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