Open Journal of Physical Chemistry, 2011, 1, 70-76
doi:10.4236/ojpc.2011.13010 Published Online November 2011 (
Copyright © 2011 SciRes. OJPC
Peculiar Concentration Dependence of H/D Exchange
Reaction in 1-Butyl-3-methylimidazolium
Tetrafluoroborate-D2O Mixtures
Souichi Ohta1, Akio Shimizu1, Yusuke Imai2, Hiroshi Abe2, Naohiro Hatano3, Yukihiro Yoshimura3*
1Department of Environmental Engineering for Symbiosis, Soka University, Tokyo, Japan
2Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Japan
3Department of Applied Chemistry, National Defense Academy, Yokosuka, Japan
Received June 7, 2011; revised August 4, 2011; accepted September 5, 2011
We have investigated the H/D exchange reaction between heavy water and an ionic liquid, 1-butyl-3-methyl-
imidazolium tetrafluoroborate ([bmim][BF4]), throughout the whole concentration region as a function of
D2O mol% at room temperature. We expected that the extent of the H/D reaction would increase linearly
with increasing content of D2O, but the results show an extended N-shaped behavior having a small maxi-
mum at around 40 mol% and the reaction becomes very slow at a specific concentration around 80 mol%.
We found that this non-linear concentration dependence correlates with the pD dependence of the solutions.
Keywords: H/D Exchange Reaction, pD, NMR, Ionic Liquid
1. Introduction
Room temperature Ionic liquids (RTILs) shows versatil-
ity of their properties by interchanging cations and ani-
ons [1,2]. Typical ionic liquids are composed of a large
bulky, asymmetric organic cation containing nitrogen
(e.g. imidazole, pyrrole, piperidine and pyridine) and a
wide variety of anions ranging from simple halides to
more complex organic species. The liquid structure of
ionic liquids results from a balance between geometric
factors and long-range electrostatic forces is a key to
prevent crystallization.
When mixing water into RTILs, the following unique
features were so far reported to occur in the RTILs-water
mixtures. Firstly, a nearly-free hydrogen bonded band
(NFHB) of water molecules which are not associated
into the hydrogen-bonded water network structure is ob-
served up to the water-rich region (80 mol% H2O) in
the Raman spectra of 1-butyl-3-methylimidazolium tetra-
fluoroborate, [bmim][BF4]-water mixtures [3,4]. The
NFHB waters are probably very weakly interacting via
H-bonding with the BF4 anions. In previous studies, we
found that the NFHB is preserved even at 77 K and the
NFHB water survives, not forming the hydrogen bond
network among themselves as in pure H2O liquid, when
making the glassy water-ionic liquid solutions [5,6].
More surprisingly, even at water-rich conditions, H2O ice
crystals and the solitary water co-exist in the solutions.
These facts imply that the nearly-free hydrogen bonded
state of water molecule is fairly stable once formed in the
Another notable feature is the unique spatial hetero-
geneity resulting from their inherent polar/nonpolar
phase separation [7-9]. The pure 1-alkyl-3-methylimida-
zolium based RTILs with various alkyl chain lengths
show nano-phase separation (a nano-structural organiza-
tion) [8]. There is also a nano-structuring of the mixtures
of [bmim][BF4] [10] and/or 1-octyl-3-methylimidazo-
lium nitrate [omim][NO3] [11] with water, but no mac-
roscopic phase separation occurs. We presume that the
nano-phase separation existing in the RTIL may be en-
hanced by the presence of NFHB water molecules [5].
The final one is that the H/D exchange reaction in
which HDO is generated by the rapid exchange of D
between D2O and the C-H at position 2 of the 1-butyl-
3-methylimidazolium (denoted as [bmim]) ring occurs
[12,13]. Now a protein H/D exchange method in deute-
rium solvents using NMR spectroscopy can provide the
direct information on the detailed structure, dynamics,
folding and local fluctuations [14,15]. Actually, whether
the hydrogens are buried or at the solvent-exposed sur-
face is quantitatively predictable from the exchange rates
for hydrogens [16]. By applying this useful method to
the RTIL-D2O mixture, the H/D exchange reaction
rate in hydrophilic 1-butyl-3-methylimidazolium chlo-
ride ([bmim][Cl]) at 50˚C has been studied in detail by
Nakahara et al. [13]. They found a slowdown of H/D
exchange reaction rate in the water-poor conditions at
D2O below 7 M (30 mol% D2O), reflecting that soli-
tary water as a single molecule without self-associated
state is deactivated in the region where the solitary water
is bound strongly by the solvent ions (Cl) as the water-
water contact is negligible compared to water in water-
rich conditions. Thus the results can be understood in
terms of change in the solvation dynamics in the system
depending on the water concentration.
As stated above, the properties of RTILs vary de-
pending on the combination of the cation and the anion.
Importantly, the solubilities of water in RTILs highly
depend on the nature of the anion. If we use the RTIL
containing hydrophilic anion, the contamination of water
from the atmosphere might be inevitable more or less [4].
Therefore, a detailed understanding of the behavior of
RTILs-water mixtures is important, e.g., for the applica-
tion uses of these substances. We point out that there is a
limitation of the solubility of [bmim][Cl] in water (about
48 mol% at room temperature), but [bmim][BF4] is solu-
ble in water at any concentration. In this situation, the
H/D exchange reaction between heavy water and [bmim]
[BF4] is intriguing. We expect that the deuterated water
effect might be different and this is the motivation for the
current work. We have investigated the NMR spectral
changes as a function of water concentration in [bmim]
[BF4]-D2O mixtures at room temperature. Here we show
a peculiar concentration dependence of the H/D exchan-
ge reaction in the mixed solution.
2. Experimental
As an ionic liquid, we used 1-butyl-3-methylimidazolium
tetrafluoroborate [bmim][BF4] (Kanto Chemical Co., Cl
< 0.005%, Br< 0.005%, F< 0.01%, Na+ < 0.002%, Li+
< 0.002%, H2O < 0.02%). The concentration of water
contained in the [bmim][BF4] as-received sample was
doubly checked to be 130 - 150 ppm on the basis of the
Karl-Fisher titration method. All mixtures of different
concentration (x = 10, 20, 30, 40, 50, 55, 60, 65, 70, 75,
80, 85, 90, 95, 98, 99 and 99.5 mol% D2O) were pre-
pared by mixing out the required amounts of [bmim][BF4]
and D2O (99.9% D, Cambridge Isotope Laboratories).
The sample preparations were done in a dry box to avoid
atmospheric H2O and CO2. The purity of the ionic liquid
was doubly confirmed on the basis of 1H and 19F NMR
As NMR is highly sensitive to the inter- and intra-
molecular interactions, it can be used as a probe for the
present purpose. 1H and 19F NMR spectra of samples of
[bmim][BF4] mixed with D2O were measured using a
JEOL ECA500 (operating at 500 and 470 MHz, respec-
tively) at room temperature (23.3˚C). Spectra widths of
1H and 19F were 9384 and 83333 Hz respectively. The
digital resolutions were 0.14 Hz for 1H and 1.27 Hz for
19F spectra. To avoid a mixing of the sample solution and
the NMR lock solvent, we used double tubes for the H/D
measurements. The NMR lock solvent was kept in a 4
mm diameter inner glass tube and this tube was inserted
in a 5 mm diameter glass tube of the sample solution.
The mixture of CDCl3 containing 1 v/v% tetramethylsi-
lane (TMS) and D2O containing about 1 v/v% triflu-
oroacetic acid (TFA) was used as the NMR lock solvent
for 1H and 19F NMR spectral measurements, respectively.
The peaks of TMS (0 ppm) and TFA (0 ppm) were used
as the external chemical shift standards of 1H and 19F
NMR spectra, respectively. But when we use the double
tubes, the magnetic susceptibility corrections are neces-
sary to determine the accurate chemical shift. Thus, we
used a single tube for the chemical shift measurements. 5
or 100 L of 0.2 M 3-trimethylsilyl-propane sulfonic acid
sodium salt (DSS) was added to the each 1 mL samples,
and the peak of the DSS (0 ppm) was used as the internal
chemical shift standard of the 1H NMR spectra.
To detect the H/D exchange reaction, 1H NMR spec-
troscopy was used to monitor the deuteration ratio (iso-
tope exchange) in the [bmim][BF4]-D2O mixtures. A
hydrolysis of the BF4
ion in water is sometimes known
to occur [17,18]. We checked an extent of the hydrolysis
by the 19F NMR spectra and the pD. The pDs of each
mixed solutions after the equilibration conditions were
determined with a pH meter, model F-51 from Horiba co.
ltd. Values of pD were obtained by adding 0.40 to the
reading of the pH meter [19].
3. Results and Discussion
Firstly, we show a typical example of the NMR spec-
trum of [bmim][BF4]-D2O mixed solution (x = 90) in
Figure 1. The chemical structure of [bmim] along with
the numbering of each carbon in imidazolium ring is
shown schematically in the inset of Figure 1. All the
fundamental peaks corresponding to [bmim] cation at
room temperature were reported by Holbrey and Seddon
[20]. Our results are basically in agreement with their
results. The numbers () in Figure 1 correspond to
the skeleton atoms for the [bmim] cation in the inset of
Figure 1. A notable feature in the spectrum is the peak
from the exchange between the proton attached to
C(2) (hereafter denoted as C(2)-H) and D2O [12]. The
Copyright © 2011 SciRes. OJPC
Che mic a l sh ift / ppm
Figure 1. Representative 1H NMR spectrum of [bmim]
[BF4]-D2O mixtures (x = 90 mol%) at 500 MHz obtained
after a deuterium exchange for the C(2)-H proton at 23.3˚C.
1H NMR chemical shifts
relative to TMS internal stan-
dard are shown. The exchange for deuterium of the C(2)-H
of imidazolium cation in D2O were determined by monitor-
ing the decay of the C(2)-H proton () and the deuterium
exchange results in the disappearance of the signal due to
the C(2)-H proton and the appearance of that due to HDO.
The inset shows a chemical structure and the numbering of
the skeleton atoms for the [bmim] cation.
intensity increased along with the deuterium exchange
after the mixture was made. An extent of the H/D reac-
tion with time evolution (0 - 42 days) covering a whole
range of the water concentration x is shown using a rela-
tive intensity ratio (IHDO/(IC(2)-H + IHDO)) of the 1H signals
of HDO and of C(2)-H in [bmim] in Figure 2. We fol-
lowed the H/D reaction up to 42 days and found that the
time require for reaching the apparent equilibrium state
is at most 1 month.
At the equilibrium state, we expected that the extent of
the H/D reaction would increase linearly with increas-
ing content of D2O. However, the change in the IHDO/
(IC(2)-H + IHDO) with x shows an extended N-shaped be-
haviour having a small maximum at about x = 40. It is
intriguing that at a specific concentration region of
around 80 mol%, the H/D exchange reaction hardly
proceeds even if the samples left for 42 days. Notably,
we found that even if the temperature of the mixture at
x = 80.0 was raised to 75˚C, the exchange reaction rate
( as shown in Figure 2) was still slow. For a com-
parison, the value at x = 40.9 is also shown in the same
figure. Then, the IHDO/(IC(2)-H + IHDO) again increases
steeply with further increase in the water concentration
toward x = 100. Thus, contrary to the case of the
[bmim][Cl]-D2O mixtures, one can say that the slow-
down of H/D exchange reaction moves to more “wa-
ter-rich” conditions.
020 40 6080 100
x / D2O mol%
Figure 2. The intensity ratio of the IHDO/(IC(2)-H + IHDO) for
[bmim][BF4]-D2O mixed solution against the D2O concen-
tration x. The extent of a deuterium exchange is examined
with time evolution from 0 - 42 days. The symbols show the
data depicted. : 0 day, : 14 days, : 23 days, : 29
days, : 42 days, : heated for 2 hours at 75˚C immedi-
ately after the sample preparations.
On the other hand, the changes in the chemical shift of
HDO with x are shown in Figure 3. The chemical shift
of HDO increases from 3.0 ppm (x = 20.0) to 4.8 ppm (x
= 100, i.e., pure D2O) with increasing water concentra-
tion. Normally, the higher-frequency shift is ascribed to
the enhancement of hydrogen bonding between water
molecules in the water structure [21]. Here we quote that
if a relatively hydrophobic co-solvent such as t-butyl
alcohol (t-BuOH) is added to water, the chemical shift of
water shows a maximum value at about 92 - 94 mol%
water in the water-rich region, which is envisaged by the
hydrophobic hydration concept [22,23] in which t-BuOH
molecules form hydrogen bonded clusters and the en-
hancement of water-water hydrogen bonding is promoted
by the dissolved t-BuOH [24]. In contrast, all of the pro-
ton signals due from the [bmim] cation () on the
deuterium exchange showed hardly any variation in the
chemical shifts up to ca. x = 90, though there were clear
even small downfield shifts above x = 90. Additionally,
the values of all of the signals for the mixed solutions
remained unchanged with time decay from the sample
preparation. The results imply that there is no indication
of the specific cluster formation in the [bmim][BF4]-D2O
mixtures, such as that proposed for the t-BuOH-water
mixtures. Therefore, the peculiar concentration depend-
ence of H/D exchange reaction together with the time
evolution is not originated from the structural change
relating to the chemical shifts.
Next, we presumed that the rate of H/D exchange re-
action might be affected by the solution pD. Then, in
order to see more about the peculiar concentration de-
pendence of H/D exchange, we performed pD measure-
ments. The results of pD are displayed in Figure 4. In-
terestingly, the change in the IHDO/(IC(2)-H + IHDO) and the
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020 40 60 80100
x / D2O mol%
020 40 60 80100
x / D2O mol%
Chemical shift / ppm
020 406080 100
8. 4 0
8. 6 0
8. 8 0
9. 0 0
9. 2 0
x / D2O mol%
020 40 60 80100
x / D2O mol%
020 40 60 80 100
x / D2O mol%
020 40 60 80100
x / D2O mol%
020 40 60 80100
x / D2O mol%
Chemical shift / ppm
020 40 60 80 100
x / D2O mol%
020 40 60 80100
x / D2O mol%
Chemical shift / ppm
Figure 3. Chemical shifts of respective protons in [bmim] cation and HDO as a function of water concentration x. The
chemical shifts correspond to the deviations from the position of the reference (DSS).
020 40 60 80100
x / D2O mol%
pD with x is well correlated. This correlation indicates
that the H/D reaction dynamics of [bmim] in D2O along
with the minimum at x = 80 is, at least partly, connected
to the solution pD.
To understand the pDs in [bmim][BF4]-D2O mixtures,
the following information is instructive. There have been
indications [25-27] that hydrolysis of BF4 anion in
RTILs sometimes occurs to form HF etc., though to our
best knowledge there seems to have been no full-detailed
study covering whole range of water concentration. The
19F NMR spectra of the RTIL-rich phase measured by us
do not show any peaks corresponding to decomposition
products of the anion, pointing to the absence of signifi-
Figure 4. Concentration dependence for the pD of [bmim]
[BF4]-D2O mixed solutions. The values for the mixed solu-
tions after 42 days from the sample preparation are plotted.
cant hydrolysis in this phase, but those of the water-rich
phase after x = 75 indicated that the decomposition of
the BF4 anion proceeds with increasing x. At apparent
equilibrium conditions (i.e., after 42 days sample at room
temperature), the 19F NMR measurements indicated that
about 4% of the BF4 anions hydrolyze at x = 85 mol% in
the mixed solution.
cation is relatively acidic. The problem here is how
acidic. Amyes et al. [28] investigated the carbon acid pKa
values of imidazolium cations in aqueous solution. The
reported pKa value for the imidazolium cation is 23.8 and
that for the 1,3-dimethylimidazoium cation is 23.0.
These are intermediate values between the pKas of ace-
tone (19.3) and ethyl acetate (25.6). This acidity some-
As already mentioned, the C(2)-position of [bmim]
times leads to undesired side reactions for the purpose of
using imidazolium ionic liquids as solvents under certain
reaction conditions [29]. Handy and Okello [30] reported
that by replacing the H with a CH3 group the exchange
became slow, though it was detectable even in the pres-
ence of a very mild base.
Considering the aforementioned results in the litera-
tures, we presumed that the rate of H/D exchange reac-
tion might be affected by the solution pD. To confirm
this expectation, we performed the additional experi-
ments using the buffered solutions. The mixed solutions
of x = 10, 20, 40, 80, 90 and 99 mol% were adjusted to
certain pDs (= 11 - 12) by NaOD. As might be expected,
the rate of exchange in the presence of strong base at
higher x regions was found to accelerate where the reac-
tion of the starting materials were complete quickly after
mixing, done in five minutes, as shown in Fi g u r e 5.
It is interesting to refer that this results are basically in
agreement with the results on the hydrolysis of imida-
zole-2-ylidenes (imidazole-derived carbenes) under
strongly alkaline solutions (pH = 12 - 13) studied by
Hollóczki, et al. [31]. On the other hand, in a lower D2O
concentration region, little effect of the base on the rate
of H/D exchange reaction could be detected. This is rea-
sonable because an extent of the H/D reaction is already
almost completed. We also confirmed that there were no
significant acid and/or base catalysis pD effects on the
chemical shifts at given concentrations (data not shown).
4. Conclusions
In summary, we have demonstrated the H/D exchange
reaction of D2O in [bmim][BF4] throughout the whole
concentration range (x = 0 - 100) at room temperature.
We found that there is no linear increase in the extent of
the H/D reaction with D2O concentration. The results
show an extended N-shaped behavior having a small
maximum at around 40 mol% and the reaction does
hardly occurs at a specific concentration region of 80
mol%. This is, at least partly, ascribed to the hydrolysis
of BF4 anion in this concentration region. We should
keep this in mind and be cautious of the H/D exchange
reaction, e.g., for the application use of [bmim][BF4] in
water. Although we find the good correlation between
the H/D exchange reaction and the solution pD, to give a
clear-cut answer for whether the peculiar concentration
dependence of H/D exchange occurred at a specific con-
centration solely depend on the pD or not is more diffi-
cult task. The interaction between D2O water and the
RTIL is probably anisotropic because of the inherent
heterogeneity existing in the RTILs in nano-scale order
that operates at a molecular level in such systems. We
suppose that it may be also connected to the existence of
020 40 60 80 100
x / D2O mol%
Figure 5. The intensity ratio (IHDO/(IC(2)-H + IHDO)) of the
H/D exchange reaction in buffered solutions at a basic con-
dition (adjusted to pD = 11 - 12) against the D2O concentra-
tion x. For a comparison, the values for the mixed solutions
without the addition of the base (open circle symbols) after
42 days left from the sample preparation are plotted in the
same figure.
solitary water molecules in [bmim][BF4] along with the
spatial heterogeneity. Further quantitative understanding
of the detailed interactions existing in the system at mo-
lecular level would require e.g., a still more precise
modelling of structures of [bmim]-[BF4], [bmim] -water,
[BF4]-water, water-water interactions. MD simulation
and small angle x-ray scattering studies would help
greatly, but this is beyond the present study. We believe
that the present findings denote for the fundamental im-
portance of the substances.
5. Acknowledgements
We are grateful to Dr. T. Takekiyo for experimental as-
sistance and Ms. K. Yamazaki for help with the manu-
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Copyright © 2011 SciRes. OJPC
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