Anomalous Conformational Change in 1-Butyl-3-methylimidazolium

Jan 4, 2012 - Y. Yoshimura , N. Hatano , T. Takekiyo , H. Abe ... Yukihiro Yoshimura , Takahiro Takekiyo , Chikara Okamoto , Naohiro Hatano , Hiroshi ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCA

Anomalous Conformational Change in 1-Butyl-3-methylimidazolium Tetrafluoroborate−D2O Mixtures Naohiro Hatano,† Mayuko Watanabe,† Takahiro Takekiyo,† Hiroshi Abe,‡ and Yukihiro Yoshimura*,† †

Department of Applied Chemistry and ‡Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan ABSTRACT: We have investigated the effect of deuterated water on the conformational equilibrium between the gauche and trans conformers of the [bmim] cation in mixtures of water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), an ionic liquid, at room temperature. A comparison of the results obtained from solutions made with H2O and with D2O highlights an anomalous conformational change in the D2O solution showing an extended N-shaped behavior. The gauche conformer of the [bmim] cation in D2O increased up to x = ∼50 (D2O mol %); however, it decreased up to higher water concentrations of x = ∼85 before again increasing drastically toward x = ∼100. We provide spectroscopic evidence that the anomalous conformational dynamics of the [bmim] cation in D2O is directly related to the H/D exchange reaction of the C−H group at position 2 of the imidazolium ring.

■

INTRODUCTION The properties of room temperature ionic liquids (RTILs) can be varied by interchanging their cations and anions.1 Recently, there has been a growing interest in the study of RTIL−water solutions.2−5 This is partly because the presence of a small amount of water can affect the dynamics of the RTILs in mixtures.6−8 In general, water is closely associated with anions, as most cations of RTILs exhibit a hydrophobic nature. However, it is interesting to mention that Nishikawa et al.9 proposed that 1-butyl-3-methylimidazolium ([bmim]) is an amphiphile because its cation has both hydrophobic and hydrophilic moieties in its constituent components. They suggested that the hydrophilic and hydrophobic effects are additive in the limited region of water concentration. Two reported facts on the unique properties of RTIL−water mixtures have been presented. First, RTILs possessing an imidazolium cation such as [bmim] show a conformational equilibrium in the liquid state.10 From the Raman spectral variations of the [bmim] cation, a rotational isomerism between the gauche and trans forms of the butyl group is well interpreted.10−12 A gauche conformation and a trans conformation with regard to the C7−C8 bond of the butyl group of the [bmim] cation coexist in the liquid state. The respective chemical structures of the conformers, along with the numbering of each carbon in the imidazolium ring, are shown schematically in Figure 1. Second, an H/D exchange reaction occurs in which HDO is generated by the exchange of D between D2O and the C−H group at position 2 of the imidazolium ring,13,14 as shown in Figure 2. In a former study, we have investigated the H/D exchange reaction of D2O in [bmim][BF4] at room temperature by NMR.15 Remarkably, we found that the H/D exchange reaction does not proceed linearly with the content of © 2012 American Chemical Society

deuterated water in [bmim][BF4]; the reaction hardly occurs at a specific concentration of ∼80 mol % D2O.15 On the basis of the facts mentioned above, the isotope effects of H2O and D2O relating to the conformational equilibrium between the gauche and trans rotational isomerism in the [bmim] cation are intriguing. We expected that the effect of deuterated water would be significant and that the results may be understood in terms of a change in the dynamics in the vicinity of the [bmim] cation, depending on the water concentration. This is the motivation for the current work. In this study, we have investigated Raman spectral changes as a function of water concentration in [bmim][BF4]−water mixtures at room temperature. Here, we show an anomalous conformational change in the D2O solution.

■

EXPERIMENTAL METHODS Raman spectroscopy is known to be highly sensitive to interand intramolecular interactions; thus, it is a suitable probe for the present study. Raman spectra were measured at room temperature (298 K) using a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a CCD detector. The 514.5-nm line from a Lexel Ar+ ion laser was used as an excitation source with a power of 250 mW. Density functional theory (DFT) calculations were carried out using the Gaussian 03 program.16 For the calculation of the [bmim] cation, Becke’s three-parameter (B3) exchange function was used.17 The B3 exchange function was combined with the Lee−Yang−Parr correlation function (B3LYP).18 All calculations for both geometry optimizations and normal Received: October 11, 2011 Revised: December 24, 2011 Published: January 4, 2012 1208

dx.doi.org/10.1021/jp2097873 | J. Phys. Chem. A 2012, 116, 1208−1212

The Journal of Physical Chemistry A

Article

Figure 1. Chemical structures of the two conformers of 1-butyl-3-methylimidazolium ([bmim]) cation.

Figure 2. Schematic representation of the H/D exchange at the C2−H group of the [bmim] cation.

frequency analyses were performed for the gas phase using the 6-311G(d) basis set. As an ionic liquid, 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%) was used. 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 Fischer titration method. All the mixtures of different concentrations (x = 0−99.5 mol % water) were prepared by mixing the required amounts of the ionic liquid and distilled H2O (Wako Pure Chemical Co.) or D2O (99.9%, Cambridge Co.). Sample preparations were carried out in a drybox to avoid atmospheric H2O and CO2.

Figure 3. Raman active bands for the gauche and trans conformers with (a) C2−D and (b) C2−H of [bmim] calculated with DFT, respectively. (c) Observed Raman spectral change with varying the D2O concentration x. The peaks at ∼600 and ∼620 cm−1 are due from gauche and trans forms of the C7−C8 bond in the butyl chain. The data after 42 days from the sample preparations are displayed.

■

RESULTS AND DISCUSSION We first investigated the conformational equilibrium of the [bmim] cation, as shown in Figure 3. In the combination of the ring deformation and the CH2 rocking bands,11 the gauche form was reported to have a lower frequency (∼600 cm−1) than the trans form (∼620 cm−1). The corresponding Raman bands for the [bmim] cation, from our DFT calculations, are also shown in Figure 3a,b. From the DFT calculations, we confirmed that the Raman bands at the higher frequency and the lower frequency are attributed to the trans and gauche forms, respectively. Thus, we can use these bands as a probe for the conformation around the C7−C8 bond of the [bmim] cation. Here, we can reasonably assume that polarizability does not change when the concentration of the solution changes. Therefore, an intensity ratio of the peaks could be considered as a number-density ratio of the respective peaks; that is, the relative intensity of the 600 cm−1 band to that of the 620 cm−1 band is proportional to the gauche/trans population ratio (Igauche/Itrans). It is necessary to point out that the values in the D2O mixtures changed with time evolution, indicating that there was a change in the equilibrium state during the time period. We followed the conformational change up to ∼3

months. A typical example of the results is shown in Figure 4a. We found that the time required to reach the apparent equilibrium state was approximately 1 month. We note that this is a surprisingly long time period for a simple isomerization reaction around a single bond in a liquid phase. Thus, hereafter, we used the values for the mixed solutions after 42 days from the sample preparation. The conformational equilibria of the [bmim] cation in H2O and D2O for a wide range of water concentrations are shown in Figure 5. Interestingly, the change in Igauche/Itrans with x in D2O solutions showed an extended N-shaped behavior, showing a maximum at x = ∼50 and a minimum at x = ∼85. The value of Igauche/Itrans increased linearly up to x = ∼50; however, it began to decrease at higher water concentrations up to x = ∼85, following which it again increased steeply toward x = ∼100. On the other hand, the value of Igauche/Itrans in H2O remained almost constant with x, though we could see a trend of a slight increase above x = ∼95. Thus, the behavior of the conformational change in the H2O solution is completely different from that in the D2O solution. 1209

dx.doi.org/10.1021/jp2097873 | J. Phys. Chem. A 2012, 116, 1208−1212

The Journal of Physical Chemistry A

Article

For more details on the D2O effect on [bmim][BF4], we next observed the H/D exchange reaction. We can estimate the extent of the change from the ratio of the peaks that belong to the combination band of the in-plane ring deformation and CH3(N) deformation (νring ip+CH3(N)) in the wavenumber region of 980−1045 cm−1.11 In the case of the H2O solution, where the H/D exchange reaction does not occur, only the peak around 1020 cm−1 appears (denoted as C2−H). However, if the D exchange between D2O and the C2−H group at position 2 of the [bmim] ring proceeds, the peak around 1007 cm−1 appears (C2−D), as shown in Figure 6e. This is also substantiated by the DFT study, as shown in Figure 6a−d; the νring ip+CH3(N) vibrational bands of C2−D appeared at a lower

Figure 4. Time evolution of (a) the area ratio (Igauche/Itrans) between the gauche and trans forms and (b) the area ratio {IC2−D/(IC2−H + IC2−D)}[100 (%)] of two peaks for C2−H and C2−D in the combination band arising from the in-plane ring deformation and CH3(N) deformation (νring ip+CH3(N)) vibrations of the D2O mixed solution at x = 40, respectively.

Figure 6. (a−d) Raman active bands for νring ip+CH3(N) vibrations calculated with DFT. (e) Observed Raman spectral change of νring ip+CH3(N) vibrations with varying D2O concentration x.

frequency than the bands of the C2−H, irrespective of their conformational forms. The results of the H/D exchange ratio, {IC2−D/(IC2−H + IC2−D)}[100%], are shown in Figure 7. Here, we measured the identical sample prepared for the conformational change used in Figure 5. We note that the H/D exchange reaction was also time dependent, as shown in Figure 4b. Interestingly, the time evolution of the H/D reaction in Figure 4b seemed to coincide with the change in the area ratio (Igauche/Itrans) in Figure 4a. Remarkably, the change in the ratio {IC2−D /(IC2−H + IC2−D)}[100%] with x also showed an extended N-shaped behavior that was consistent with our previous results obtained by NMR.15 It is noteworthy that the behavior of the changes in the H/D exchange ratio and Igauche/Itrans with x was very similar. The first and second crossover concentrations (x = 50 and 85, respectively) were in surprising agreement with the concentrations at which the changes in the conformational equilibrium

Figure 5. Variations in area ratios (Igauche/Itrans) between the gauche and trans forms of the butyl chain in [bmim][BF4]−water mixed solutions as a function of water concentration x. The open circles show the results in the H2O−[bmim][BF4] solutions by Gaussian− Lorentzian fittings for the respective peaks of the gauche and trans forms. The filled circles correspond to the results in the D2O− [bmim][BF4] solutions. 1210

dx.doi.org/10.1021/jp2097873 | J. Phys. Chem. A 2012, 116, 1208−1212

The Journal of Physical Chemistry A

Article

H2O mixed solutions (data not shown). We suppose that, beyond a certain threshold value (ca. x = 90), the ratio {IC2−D/ (IC2−H + IC2−D)}[100%] approaches the expected value quickly, probably owing to a large D2O content in contrast to [bmim][BF4] in the system. Jeon et al.22 studied the structures of pure [bmim][BF4], [bmim][I], and their aqueous mixtures (from 0.005 to 0.37 mole fraction RTIL) by attenuated total reflection (ATR) infrared absorption, Raman, and NMR techniques. They proposed that the bulk structural difference due to the type of anion is reflected in the conformation of the alkyl chain attached to the [bmim] cation: the BF4− anion is symmetrically positioned on top of the imidazolium ring, whereas the I− anion lies closer to the C2−H of the imidazolium cation and interacts more specifically as compared to BF4−. From their Raman data, they suggested two important points as follows. (1) The butyl chain would be more likely to develop a gauche conformer when the anion is over the imidazolium ring plane, as in [bmim][BF4]. (2) The slow H/D exchange rate for the [bmim][BF4]−D2O mixture (nearly intact for 0.01 mole fraction of [bmim][BF4]−D2O, even 48 h after the mixture was made) was also attributed to the relative position of the anion. Therefore, the present results are reasonably explained by their results mentioned above. In summary, the important conclusions obtained from this study on the effect of deuterated water on the conformational equilibrium between gauche and trans conformers of the [bmim] cation in [bmim][BF4]−water mixtures are as follows. (1) There is no linear concentration dependence of the conformational change in the D2O solution showing an extended N-shaped behavior. (2) A direct connection between the anomalous conformational dynamics of the [bmim] cation and the H/D exchange reaction of the C2−H group in the imidazolium ring was observed. (3) The H/D isotopic exchange induced the increase in a gauche conformer of the [bmim] cation in the D2O mixtures. We believe that the present findings contribute to the fundamental understanding of this material. A detailed understanding of the behavior of ionic liquids is of high importance to further extend the range of applications for these materials.

Figure 7. Area ratio {IC2−D/(IC2−H + IC2−D)}[100 (%)] of two peaks for C2−H and C2−D in νring ip+CH3(N) vibrational band against D2O concentration x. The filled circles correspond to the results by Gaussian−Lorentzian fittings for the respective peaks. We show the data after 42 days from the sample preparations.

ratio were observed. This fairly good correlation, together with the time evolutions shown in Figure 4, indicates that the H/D exchange reaction is directly related to the anomalous conformational dynamics of the [bmim] cation in D2O. In a water-poor composition, D2O water molecules typically lie in the vicinity of the C2−H group of the [bmim] ring. This picture, however, is different from that of the H2O solution system because of the lack of an H/D exchange reaction at the C2−H group, and we assume that the H2O molecules weakly interact with the anions, based on our previous study.5 The obtained results indicate that the occurrence of the H/D exchange promotes the gauche form; we believe that the geometric factor (i.e., a type of steric hindrance), and not the hydrogen bonding or some other peculiar property of D2O, is the key to the change in the equilibrium. Later, the correlation of the conformational equilibrium with the H/D exchange reaction will be mentioned from other point of view. Our DFT calculations also proved that the gauche (C2−D) form of [bmim] is more energetically favorable than the trans (C2−D) form (ΔEtrans→gauche = −2.34 kJ/mol). The question is, why is the exchange reaction becoming very slow in the water-rich concentration region? Plausible explanations have been provided in our previous study,15 where we assumed that this nonlinear concentration dependence in D2O correlates with the pD dependence of the solutions. There are some reports19−22 that hydrolysis of the BF4 anion in RTIL−water solutions sometimes occurs to form HF. We showed that, in the waterpoor region, the 19F NMR spectra do not have any peaks corresponding to decomposition products of the anion, indicating the absence of significant hydrolysis in this phase. However, in the water-rich phase after x = ∼75, the decomposition of the BF4 anion proceeds with an increase in x. For example, the 19F NMR measurements indicated that approximately 4% of the BF4 anions hydrolyzed at x = 85 in the mixed solution (after 42 days from the sample preparation). Thus, the hydrolysis effect causes the deviation from a linear concentration dependence of the H/D exchange reaction in the water-rich region, because the C2-position of the [bmim] cation is relatively acidic. As supporting evidence, it is important to mention that no significant acid and base catalysis pH effects on the conformational changes were observed in the

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-468-41-3810, ext 3583. Fax:+81-468-44-5901.

■

ACKNOWLEDGMENTS We appreciate Prof. A. Shimizu of Soka University for fruitful discussion.

■

REFERENCES

(1) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2002. (2) Cammarata, L.; Kazarian, S. G.; Salter, P. A..; Welton, T. Molecular States of Water in Room Temperature Ionic Liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192−5200. (3) Saha, S.; Hamaguchi, H. Effect of Water on the Molecular Structure and Arrangement of Nitrile-Functionalized Ionic Liquids. J. Phys. Chem. B 2006, 110, 2777−2781. (4) Jeon, Y.; Sung, J.; Kim, D.; Seo, C.; Cheong, H.; Ouchi, Y.; Ozawa, R.; Hamaguchi, H. Structural Change of 1-Butyl-3methylimidazolium Tetrafluoroborate + Water Mixtures Studied by

1211

dx.doi.org/10.1021/jp2097873 | J. Phys. Chem. A 2012, 116, 1208−1212

The Journal of Physical Chemistry A

Article

Infrared Vibrational Spectroscopy. J. Phys. Chem. B 2008, 112, 923− 928. (5) Yoshimura, Y.; Goto, T.; Abe, H.; Imai, Y. Existence of NearlyFree Hydrogen Bonds in an Ionic Liquid, N,N-Diethyl-N-Methyl-N(2-methoxyethyl) Ammonium Tetrafluoroborate-Water at 77 K. J. Phys. Chem. B 2009, 113, 8091−8095. (6) Adhikari, A.; Sahu, K.; Dey, S.; Ghosh, S.; Mandal., U.; Bhattacharyya, K. Femtosecond Solvation Dynamics in a Neat Ionic Liquid and Ionic Liquid Microemulsion: Excitation Wavelength Dependence. J. Phys. Chem. B 2007, 111, 12809−12816. (7) Jiang, W.; Wang, Y.; Voth, G. A. Molecular Dynamics Simulation of Nanostructural Organization in Ionic Liquid/Water Mixtures. J. Phys. Chem. B 2007, 111, 4812−4818. (8) Moreno, M.; Castiglione, F.; Mele, A.; Pasqui, C.; Raos, G. Interaction of Water with the Model Ionic Liquid [bmim][BF4]: Molecular Dynamics Simulations and Comparison with NMR Data. J. Phys. Chem. B 2008, 112, 7826−7836. (9) Kato, H.; Miki, K.; Mukai, T.; Nishikawa, K.; Koga, Y. Hydrophobicity/Hydrophilicity of 1-Butyl-2,3-dimethyl and 1-Ethyl3-methylimidazolium Ions: Toward Characterization of Room Temperature Ionic Liquids. J. Phys. Chem. B 2009, 113, 14754−14760. (10) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H. Rotational Isomerism and Structure of the 1-Butyl-3-methylimidazolium Cation in the Ionic Liquid State. Chem. Lett. 2003, 32, 948−949. (11) Berg, R. W.; Deetlefs, M.; Seddon, K. R.; Shim, I.; Thompson, J. M. Raman and ab Initio Studies of Simple and Binary 1-Alkyl-3methylimidazolium Ionic Liquids. J. Phys. Chem. B 2005, 109, 19018− 19025. (12) Berg, R. W. Raman Spectroscopy and Ab-initio Model Calculations on Ionic Liquids. Monatsh. Chem. 2007, 138, 1045−1075. (13) Nakakoshi, M.; Ishihara, S.; Utsumi, H.; Seki, H.; Koga, Y.; Nishikawa, K. Anomalous Dynamic Behavior of Ions and Water Molecules in Dilute Aqueous Solution of 1-Butyl-3-methylimidazolium Bromide Studied by NMR. Chem. Phys. Lett. 2006, 427, 87−90. (14) Yasaka, Y.; Wakai, C.; Matsubayashi, N.; Nakahara, M. 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-methylimidazolium Chloride. J. Phys. Chem. A 2007, 111, 541−543. (15) Ohta, S.; Shimizu, A.; Imai, Y.; Abe, H.; Hatano, N.; Yoshimura, Y. Peculiar Concentration Dependence of H/D Exchange Reaction in 1-Butyl-3-methyl-imidazolium Tetrafluoroborate-D2O Mixtures. Open J. Phys. Chem. 2011, 1, 70−76. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J. W.; Petersson, G. A.; Ayala, P. W.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavechari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayaakkara, A.; Gonzalez, C.; Challacombe, M. P.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asmptotic Behavior. Phys. Rev. A 1988, 38, 3098−3100. (18) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1998, 37, 785. (19) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Characterization and Comparison of Hydrophilic and Hydrophobic Room Temperature Ionic Liquids Incorporating the Imidazolium Cation. Green Chem. 2001, 3, 156− 164. (20) Tseng, M.; Liang, Y.; Chu, Y. Synthesis of Fused Tetrahydro-βcarbolinequinoxalinones in 1-n-Butyl-2,3-dimethylimidazolium Bis-

(trifluoromethylsulfonyl)imide ([bdmim][Tf2N]) and 1-n-Butyl-2,3dimethylimidazolium Perfluorobutylsulfonate ([bdmim][PFBuSO3]) Ionic Liquids. Tetrahedron Lett. 2005, 46, 6131−6136. (21) Freire, M. G.; Neves, C. M. S. S.; Marrucho, I. M.; Coutinho, J. A. P.; Fernandes, A. M. Hydrolysis of Tetrafluoroborate and Hexafluorophosphate Counter Ions in Imidazolium-based Ionic Liquids. J. Phys. Chem. A 2010, 114, 3744−3749. (22) Jeon, Y.; Sung, J.; Seo, C.; Lim, H.; Cheong, H.; Kang, M.; Moon, B.; Ouchi, Y.; Kim, D. H. Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy. J. Phys. Chem. B 2008, 112, 4735−4740.

1212

dx.doi.org/10.1021/jp2097873 | J. Phys. Chem. A 2012, 116, 1208−1212