Peculiar Concentration Dependence of H/D Exchange Reaction in 1-Butyl-3-methylimidazolium Tetrafluoroborate-D<sub>2</sub>O Mixtures

doi:10.4236/ojpc.2011.13010

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Open Journal of Physical Chemistry, 2011, 1, 70-76

doi:10.4236/ojpc.2011.13010 Published Online November 2011 (http://www.SciRP.org/journal/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

*E-mail: [email protected]

Received June 7, 2011; revised August 4, 2011; accepted September 5, 2011

Abstract

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

RTIL.

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

S. OHTA ET AL.

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

spectra.

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 (①～) in Fig⑧ure 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

② from the exchange between the proton attached to

C(2) (hereafter denoted as C(2)-H) and D2O [12]. The

S. OHTA ET AL.

0.02.04.06.08.0

②

③④

①

⑤

⑥⑦

⑧

Che mic a l sh ift / ppm

HDO

③ ④

⑥ ⑧

⑤ ⑦

②

①

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 (②) 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 sti◎ll 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

0.0

0.2

0.4

0.6

0.8

1.0

x / D2O mol%

IHDO / (IC(2)-H+IHDO)

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 (①～) on the ⑧

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

S. OHTA ET AL.

020 40 60 80100

2.00

3.00

4.00

5.00

x / D2O mol%

HDO

020 40 60 80100

3.60

3.80

4.00

4.20

4.40

x / D2O mol%

①

Chemical shift / ppm

020 406080 100

8. 4 0

8. 6 0

8. 8 0

9. 0 0

9. 2 0

x / D2O mol%

②

020 40 60 80100

7.20

7.40

7.60

7.80

8.00

x / D2O mol%

③

020 40 60 80 100

3.80

4.00

4.20

4.40

4.60

4.80

x / D2O mol%

⑤

020 40 60 80100

1.60

1.80

2.00

2.20

2.40

x / D2O mol%

⑥

020 40 60 80100

1.00

1.20

1.40

1.60

1.80

x / D2O mol%

⑦

Chemical shift / ppm

020 40 60 80 100

0.60

0.80

1.00

1.20

1.40

x / D2O mol%

⑧

020 40 60 80100

7.20

7.40

7.60

7.80

8.00

x / D2O mol%

④

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

0.0

2.0

4.0

6.0

8.0

10.0

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]

S. OHTA ET AL.

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

0.0

0.2

0.4

0.6

0.8

1.0

x / D2O mol%

IHDO / (IC(2)-H+IHDO)

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-

script.

6. References

[1] T. Welton, “Ionic Liquids in Synthesis,” Wiley-VCH,

Weinheim, 2002.

[2] R. D. Rogers, K. R. Seddon and S. Volkov, “Green In-

dustrial Applications of Ionic Liquids,” NATO Science

Series, Springer, New York, 2002.

[3] Y. Jeon, J. Sung, D. Kim, C. Seo, H. Cheong, Y. Ouchi,

R. Ozawa and H. Hamaguchi, “Structural Change of

1-Butyl-3-methylimidazolium Tetrafluoroborate + Water

Mixtures Studied by Infrared Vibrational Spectroscopy,”

The Journal of Physical Chemistry B, Vol. 112, No. 3,

2008, pp. 923-928. doi:10.1021/jp0746650

[4] L. Cammarata, S. G. Kazarian, P. A. Salter and T. Welton,

“Molecular States of Water in Room Temperature Ionic

Liquids,” Physical Chemistry Chemical Physics, Vol. 3,

2001, pp. 5192-5200. doi:10.1039/b106900d

[5] Y. Yoshimura, T. Goto, H. Abe and Y. Imai, “Existence

of Nearly-Free Hydrogen Bonds in an Ionic Liquid,

S. OHTA ET AL.

N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium

Tetrafluoroborate-Water at 77 K,” The Journal of Physi-

cal Chemistry B, Vol. 113, No. 23, 2009, pp. 8091-8095.

doi:10.1021/jp902125f

[6] Y. Yoshimura, H. Kimura, C. Okamoto, T. Miyashita, Y.

Imai and H. Abe, “Glass Transition Behaviour of Ionic

Liquid, 1-Butyl-3-methylimidazolium Tetrafluoroborate-

H2O Mixed Solutions,” The Journal of Chemical Ther-

modynamics, Vol. 43, No. 3, 2011, pp. 410-412.

doi:10.1016/j.jct.2010.10.010

[7] H. Jin, X. Li and M. Maroncelli, “Heterogeneous Solute

Dynamics in Room Temperature Ionic Liquids,” The

Journal of Physical Chemistry B, Vol. 111, 2007, pp.

13473-13478. doi:10.1021/jp077226+

[8] A. Paul, P. Kumar and A. Samanta, “How Transparent

Are the Imidazolium Ionic Liquids? A Case Study with

1-Methyl-3-butylimidazolium Hexafluorophosphate, [b-

mim][PF6],” Chemical Physics Letters, Vol. 402, No. 4-6,

2005, pp. 375-379. doi:10.1016/j.cplett.2004.12.060

[9] Z. Hu and C. Margulis, “Heterogeneity in a Room-

Temperature Ionic Liquid: Persistent Local Environments

and the Red-Edge Effect,” Proceedings of the National

Academy of Sciences of the United States of America, Vol.

103, No. 4, 2006, pp. 831-836.

doi:10.1073/pnas.0507364103

[10] M. Moreno, F. Castiglione, A. Mele, C. Pasqui and G.

Raos, “Interaction of Water with the Model Ionic Liquid

[bmim][BF4]: Molecular Dynamics Simulations and

Comparison with NMR Data,” The Journal of Physical

Chemistry B, Vol. 112, No. 26, 2008, pp. 7826-7836.

doi:10.1021/jp800383g

[11] W. Jiang, Y. Wang and G. A. Voth, “Molecular Dynam-

ics Simulation of Nanostructural Organization in Ionic

Liquid/Water Mixtures,” The Journal of Physical Chem-

istry B, Vol. 111, No. 18, 2007, pp. 4812-4818.

doi:10.1021/jp067142l

[12] M. Nakakoshi, S. Ishihara, H. Utsumi, H. Seki, Y. Koga

and K. Nishikawa, “Anomalous Dynamic Behavior of

Ions and Water Molecules in Dilute Aqueous Solution of

1-Butyl-3-methylimidazolium Bromide Studied by

NMR,” Chemical Physics Letters, Vol. 427, No. 1-3,

2006, pp. 87-90. doi:10.1016/j.cplett.2006.06.052

[13] Y. Yasaka, C. Wakai, N. Matubayashi and M. Nakahara,

“Slowdown of H/D Exchange Reaction Rate and Water

Dynamics in Ionic Liquids: Deactivation of Solitary

Water Solvated by Small Anions in 1-Butyl-3-methyl-

imidazolium Chloride,” The Journal of Physical Chemis-

try A, Vol. 111, No. 4, 2007, pp. 541-543.

doi:10.1021/jp0673720

[14] H. Maity, W. Kilim, J. N. Rumbley and S. W. Englander,

“Protein Hydrogen Exchange Mechanism: Local Fluctua-

tions,” Protein Science, Vol. 12, No. 1, 2003, pp. 153-160.

doi:10.1110/ps.0225803

[15] C. K. Woodward and B. D. Hilton, “Hydrogen Exchange

Kinetics and Internal Motions in Proteins and Nucleic

Acids,” Annual Review of Biophysics & Bioengineering,

Vol. 8, 1979, pp. 99-127.

[16] Y. Bai, J. S. Milne, L. Mayne and S. W. Englander,

“Primary Structure Effects on Peptide Group Hydrogen

Exchange,” Proteins: Structure, Function, and Bioinfor-

matics, Vol. 17, No. 1, 1993, pp. 75-86.

doi:10.1002/prot.340170110

[17] C. A. Wamser, “Equilibria in the System Boron Trifluo-

ride-Water at 25˚,” Journal of the American Chemical

Society, Vol. 73, No. 1, 1951, pp. 409-416.

doi:10.1021/ja01145a134

[18] M. Anbar and S. Guttmann, “The Isotopic Exchange of

Fluoroboric Acid with Hydrofluoric Acid,” The Journal

of Physical Chemistry, Vol. 64, No. 12, 1960, pp. 1896-

1899. doi:10.1021/j100841a021

[19] P. K. Glasoe and F. A. Long, “Use of Glass Electrodes to

Measure Acidities in Deuterium Oxide,” The Journal of

Physical Chemistry, Vol. 64, No. 1, 1960, pp. 188-190.

doi:10.1021/j100830a521

[20] J. D. Holbrey and K. R. Seddon, “The Phase Behaviour

of 1-Alkyl-3-methylimidazolium Tetrafluoroborates; Io-

nic Liquids and Ionic Liquid Crystals,” Journal of the

Chemical Society, Dalton Transactions, No. 13, 1999, pp.

2133-2140. doi:10.1039/a902818h

[21] J. A. Pople, W. G. Scheneider and H. J. Bernstein, “High-

Resolution Nuclear Magnetic Resonance,” McGraw-Hill,

New York, 1959.

[22] F. Franks, “Water, A Comprehensive Treatise,” Plenum

Press, New York, 1972.

[23] J. M. Harvey, S. E. Jackson and M. C. R. Symons, “In-

teractions in Water-Alcohol Mixtures Studied by NMR

and Infrared Spectroscopy,” Chemical Physics Letters,

1977, Vol. 47, No. 3, pp. 440-441.

doi:10.1016/0009-2614(77)85011-2

[24] P. K. Kipkemboi, P. C. Kiprono and A. J. Easteal, “Pro-

ton Nuclear Magnetic Resonance Study of Water +

t-Butyl Alcohol, Water + t-Butylamine and Water +

t-Butyl Alcohol + t-Butylamine Mixtures,” Bulletin of the

Chemical Society of Ethiopia, Vol. 16, 2002, pp. 187-

198.

[25] J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D.

Willauer, G. A. Broker and R. D. Rogers, “Charac-

terization and Comparison of Hydrophilic and Hydro-

phobic Room Temperature Ionic Liquids Incorporating

the Imidazolium Cation,” Green Chemistry, Vol. 3, No. 4,

2001, pp. 156-164. doi:10.1039/b103275p

[26] M. Tseng, Y. Liang and Y. Chu, “Synthesis of Fused

Tetrahydro-β-carbolinequinoxalinones in 1-n-Butyl-2,3-

dimethylimidazolium Bis(trifluoromethylsulfonyl)imide

([bdmim][Tf2N]) and 1-n-Butyl-2,3-dimethylimidazolium

Perfluorobutylsulfonate ([bdmim][PFBuSO3]) Ionic Liq-

uids,” Tetrahedron Letters, Vol. 46, No. 36, 2005, pp.

6131-6136. doi:10.1016/j.tetlet.2005.06.153

[27] M. G. Freire, C. M. S. S. Neves, I. M. Marrucho, J. A. P.

Coutinho and A. M. Fernandes, “Hydrolysis of Tetra-

fluoroborate and Hexafluorophosphate Counter Ions in

Imidazolium-Based Ionic Liquids,” The Journal of Phy-

sical Chemistry A, Vol. 114, No. 11, 2010, pp. 3744-3749.

doi:10.1021/jp903292n

S. OHTA ET AL.

[28] T. L. Amyes, S. T. Diver, J. P. Richard, F. M. Rivas and

K. Toth, “Formation and Stability of N-Heterocyclic

Carbenes in Water: The Carbon Acid pKa of Imida-

zolium Cations in Aqueous Solution,” Journal of the

American Chemical Society, Vol. 126, No. 13, 2004, pp.

4366-4374. doi:10.1021/ja039890j

[30] S. T. Handy and M. Okello, “The 2-Position of Imida-

zolium Ionic Liquids: Substitution and Exchange,” The

Journal of Organic Chemistry, Vol. 70, No. 5, 2005, pp.

1915-1918. doi:10.1021/jo0480850

[31] O. Hollóczki, P. Terleczky, D. Szieberth, G. Mourgas, D.

Gudat and L. Nyulászi, “Hydrolysis of Imidazole-2-

ylidenes,” Journal of the American Chemical Society, Vol.

133, No. 4, 2011, pp. 780-789.

doi:10.1021/ja103578y

[29] J. Dupont and J. Spencer, “On the Noninnocent Nature of

1,3-Dial kylimidazolium Ionic Liquids,” Angewandte

Chemie International Edition, Vol. 43, No. 40, 2004, pp.

5296-5297. doi:10.1002/anie.200460431