ARTICLE pubs.acs.org/JPCA
Effects of Methylation at Position 2 of Cation Ring on Rotational Dynamics of Imidazolium-Based Ionic Liquids Investigated by NMR Spectroscopy: [C4mim]Br vs [C4C1mim]Br Takatsugu Endo,† Mamoru Imanari,‡ Hiroko Seki,‡ and Keiko Nishikawa*,† † ‡
Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan Chemical Analysis Center, Chiba University, Yayoi, Inage-ku, Chiba 263-8522, Japan ABSTRACT: We investigated the rotational dynamics of two imidazolium-based ionic liquids, 1-butyl-3-methylimidazolium bromide ([C4mim]Br) and 1-butyl-2,3-dimethylimidazolium bromide ([C4C1mim]Br), to reveal the effects of methylation at position 2 of the imidazolium ring (C(2) methylation). The rotational correlation time (τlocal) for each carbon in the cations is derived from the spinlattice relaxation time of 13C nuclear magnetic resonance. The τlocal results obtained here provide three principle insights into the rotational dynamics of ionic liquids. First, all τlocal values for [C4C1mim]Br are greater than those for [C4mim]Br owing to a viscosity increase due to C(2) methylation. Second, the rate of change in τlocal on C(2) methylation differs among the carbons in the cation, which indicates that each carbon has a different microviscosity. Third, the τlocal increase in the 13C at the root of the butyl group on C(2) methylation is very small compared to both intuitive prediction and the results from quantum chemical calculations. This indicates that the motion of the butyl group root in [C4C1mim]Br is not significantly inhibited by the methyl group at the position 2 of the imidazolium ring. The finding provides conclusive information on the origin of the increases in the melting point on C(2) methylation. Hunt previously found through calculation that decreases in entropy are caused by two factors, namely, reductions in the rotational mobility of the butyl group and in the number of stable anion interaction sites with C(2) methylation, resulting in an increase in melting point and viscosity. Our finding experimentally illustrates that the origin of the increases in melting point is not the inhibition of butyl group motion and that the reduction in stable anion interaction sites plays a major role in the increases. Additionally, it is suggested that the viscosity increase on C(2) methylation can be interpreted in the same manner.
’ INTRODUCTION Room temperature ionic liquids (ILs) are salts that are liquid at or near room temperature. Since ILs are composed solely of ions, they have many of the characteristic properties of solvents, for example, low melting point, extremely low vapor pressure and flammability, wide electrochemical window, and unique solubility. Recently, ILs have been widely recognized as potential replacements for traditional volatile organic solvents.15 Imidazolium-based ILs have been widely studied and are considered the most typical ILs. Methylation at position 2 of the imidazolium cation ring (C(2) methylation) drastically changes the physicochemical properties of ILs. For example, C(2) methylation generally increases the melting point,612 thermal stability,8,10,1215 viscosity,6,12,1518 surface tension,10,19 chemical stability,15,20 electrochemical stability,6,15 vaporization enthalpy,21 heat capacity,11,14 and amphiphilicity22 and also decreases the density,6,12,14,15 conductivity,6,9,10,15,18 polarity,10,23 and liquid crystallinity.24 C(2) methylation is also reported to affect the chemical reactivity of methylated ILs25 and improve their tribological properties.26 In particular, as indicated by Hunt,27 the increases in melting point and viscosity are unexpected because r 2011 American Chemical Society
C(2) methylation decreases the interaction between cations and anions or the lattice energy.2729 It is important to determine the origin of the changes in the physicochemical properties of imidazolium-based ILs on C(2) methylation for a basic understanding of ILs and also for the molecular design of ILs having desirable properties. Some reports on C(2) methylation at the molecular level have been recently published.12,27,2934 Hunt found through quantum chemical calculations that reduction in entropy contributes to the increases in melting point and viscosity.27 Recently, our group experimentally observed through calorimetric measurements that there is a decrease in fusion and crystallization entropies on C(2) methylation.34 However, there is a lack of empirical evidence regarding the details of the decrease in entropy, especially in terms of ion dynamics. In this paper, we investigated the rotational dynamics of two imidazolium-based ILs in the liquid state to reveal the effects of Received: January 20, 2011 Revised: March 1, 2011 Published: March 21, 2011 2999
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Chart 1. Structure of [C4Rmim]þ Where R Is H or Me and Abbreviations and Symbols for All Carbons
C(2) methylation. The rotational dynamics of the ILs were estimated from 13C spinlattice relaxation times (T1) of nuclear magnetic resonance (NMR). The T1 values of some ILs have already been measured by various groups,31,3546 and they have provided useful insight into the rotational dynamics. For the samples used here, we selected 1-butyl-3-methylimidazolium [C4mim]þ and 1-butyl-2,3-dimethylimidazolium [C4C1mim]þ as typical cations, and a bromide anion, which has the simplest structure. The structures and abbreviations for all the carbons of [C4Rmim]Br, where R is H or a methyl group, are shown in Chart 1.
’ EXPERIMENTAL SECTION [C4mim]Br and [C4C1mim]Br were prepared as described previouly.34 The ILs were characterized by 1H NMR (JEOL JNM-LA500). The samples were dried at ca. 333 K under vacuum (103 Pa) for 24 h and then handled in an N2 atmosphere glovebox to avoid absorption of atmospheric moisture. The samples were sealed in 4-mm NMR tubes under vacuum and then the sealed tubes were inserted into 5-mm NMR tubes with dimethyl sulfoxide-d6 as a deuterated solvent. The water content of the ILs was less than 100 ppm as measured by Karl Fischer titration (Mettler-Toledo model DL39 coulometer). NMR measurements were conducted using JEOL JNMLA400 with a 9.4 T magnet without sample spinning to prevent perturbations from the spinning. The measured temperature was stabilized for more than 10 min to ensure thermal equilibrium of the ILs. T1 values were determined using the inversion recovery method. Density functional theory (DFT) calculations were performed using the Gaussian 03 program package.47 Full geometry optimization analyses for the ions in the gas phase were performed using 6-311þG(d, p) basis sets based on Becke’s three-parameter hybrid method48 with the LYP correlation (B3LYP).49,50 No imaginary frequencies were produced by the optimized structures; this ensured the presence of a minimum. ’ RESULTS NMR Relaxation Time. The T1 results of the ILs are shown in Figure 1, which is divided into three panels: imidazolium rings [green, Figure 1(a)], methyl groups [orange, Figure 1 (b)], and butyl groups [purple, Figure 1 (c)]. Since T1 is mainly influenced by the rotational dynamics, which is also true of other ILs,43 the difference between the T1 values of [C4mim]Br (filled symbols) and [C4C1mim]Br (open symbols) represents differences in rotational dynamics due to C(2) methylation. In addition, almost every minimum in the T1 plots of [C4C1mim]Br is located at a temperature higher than that in the plots of [C4mim]Br. This
Figure 1. Spinlattice relaxation times T1 of [C4mim]Br (filled symbols) and [C4C1mim]Br (open symbols). (a) Green represents imidazolium rings; circles are Im 2, squares are Im 4, and triangles are Im 5. (b) Orange represents methyl groups; circles are 3 Me and squares are 2 Me. (c) Purple represents butyl groups; circles are Bu 1, squares are Bu 2, triangles are Bu 3, and reversed triangles are Bu 4. These colors and symbols have the same meanings in Figures 25.
indicates that most 13C rotational motion becomes slower on C(2) methylation. To quantitatively discuss rotational dynamics, the rotational correlation time (τlocal) of each carbon is derived from the T1 3000
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spin rotation, scalar, and quadrupole interactions, respectively. For the samples used here, 13C T1DD is expected to dominate owing to the existence of 1H directly attached to 13C atoms, which have strong magnetic dipoles. The contribution of T1DD to the measured T1 is estimated from nuclear Overhauser effect (NOE) measurements, as described in eq 2 1 NOE 1 ¼ T1DD NOEmax T1
ð2Þ
where NOE and NOEmax are the measured and theoretically maximum NOE factors, respectively. Contribution of T1DD to Measured T1. The NOE results are shown in Figure 2. The NOE factor of 13C nuclei is represented by eq 3 NOE ¼
γH ½6JðωC þ ωH Þ JðωC ωH Þ γC ½JðωC ωH Þ þ 3JðωC Þ þ 6JðωC þ ωH Þ
ð3Þ
where γH and γC are the magnetogyric ratios of 1H and 13C, respectively, and J(ω) is the spectral density. The value of the NOE factor depends on τlocal, and when the contribution of T1DD to the measured T1 is 100%, the value reaches a maximum of 1.988. In [C4mim]Br, the NOE factors of 13C in the imidazolium ring and butyl group almost reach 1.988, and thus the contribution of T1DD to T1 for these 13C atoms can be regarded as 100%. Although T1CSA of 13C in the imidazolium ring might occur, its contribution would be small.36,38,51 The NOE factors of 13C in the methyl group are saturated at ca. 1.6, and the T1DD contribution is estimated to be 80%. The residual 20% is considered to be due to T1SR.36,38 In [C4C1mim]Br, except for 13C at the position 2 of the imidazolium ring, which has no directly attached 1 H, the T1DD contributions are the same as in [C4mim]Br owing to the similarity of the cation structure. The contribution of T1DD of 13C at the position 2 is estimated to be 50% from Figure 2 (a), assuming that the NOE plot reaches a maximum at the highest measured temperature. Note that the residual 50% is considered to be T1CSA. Estimation of Rotational Correlation Time. τlocal is derived from the following equations 1 NH ð2πDCH Þ2 ½JðωC ωH Þ þ 3JðωC Þ þ 6JðωC þ ωH Þ ¼ T1DD 10 ð4Þ DCH ¼
μ0 p γC γH 3 4π 2π rCH
ð5Þ
τ 1 þ ðωτÞ2
ð6Þ
Figure 2. NOE factors of [C4mim]Br (filled symbols) and [C4C1mim]Br (open symbols). Gray dotted lines show the theoretically maximum NOE factor, 1.988.
JðωÞ ¼
plots as described below. Five relaxation mechanisms are represented in T1, as shown by eq 1
τ ¼ τ0 exp
1 1 1 1 1 1 ¼ DD þ CSA þ SR þ Sc þ Q T1 T1 T1 T1 T1 T1
ð1Þ
where T1DD, T1CSA, T1SR, T1Sc, and T1Q are the relaxation times caused by magnetic dipoledipole, chemical shift anisotropy,
Ea RT
1 1 1 ¼ þ τ τoverall τlocal
ð7Þ
ð8Þ
where NH is the number of protons directly attached to 13C atoms, DCH is the dipolar coupling constant, μ0 is the 3001
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Figure 3. T1DD plots of the methyl groups in [C4mim]Br (filled symbols) and [C4C1mim]Br (open symbols) and the fitting curves (black lines).
permeability of vacuum, rCH is the average distance between a proton and 13C, τ is the apparent rotational correlation time, τ0 is the correlation time at infinite temperature, Ea is the activation energy, R is the gas constant, T is the temperature, and τoverall is the overall rotational correlation time of cations. The rCH value is obtained from DFT calculations, and τ0, Ea, and τlocal are obtained by fitting the T1 plots. The fit was performed by fixing DCH in the temperature range from 373.15 K for [C4mim]Br and 383.15 K for [C4C1mim]Br to 422.15 K, which is higher than the melting points of the ILs, which are 353.0 K ([C4mim]Br) and 369.8 K ([C4C1mim]Br).34 Figure 3 shows the fitting curves for the methyl groups of the ILs. The theoretical curves were well fitted to the measured T1 plots in this temperature range; this is also true for the other 13C T1 plots. However, the curves deviate from the T1 plots at lower temperatures. This is believed to be due to a deviation from the simple theoretical model that represents isotropic rotation of 13C and the Arrhenius temperature dependence of τlocal. Below this temperature range, in the supercooled liquid region, a more complex theoretical model would be required for fitting. Figure 4 and Table 1 show the temperature dependence of τlocal in the range of 383.15 to 422.15 K and the fitting parameters DCH, Ea, and τ0, respectively. The τlocal values and the change in τlocal with C(2) methylation at 393.15 K are summarized in Table 2 for further discussion.
’ DISCUSSION τlocal Value of Each
13
C. The rotational correlation times τlocal of some imidazolium-based ILs were reported previously.31,3545 Since the absolute values of τlocal depend on the theoretical model used and/or the analytical method, it is difficult to compare the values obtained here with those reported previously. Thus, we compare the trend in τlocal values in each IL. In [C4mim]Br, the trend of τlocal is Bu 1 > Im 5 ≈ Im 4 ≈ Im 2 > Bu 2 > Bu 3 > 3 Me > Bu 4, and in [C4C1mim]Br it is Im 2 ≈ Bu 1≈ Im 5 ≈ Im 4 > Bu 2 > Bu 3 > 3 Me > 2 Me > Bu 4. The trend in [C4mim]Br is similar to that in [C4C1mim]Br. More flexible parts, namely, the methyl groups or the end of butyl group, have higher mobility, and this trend is also reported in other ILs.31,36,3840,43,44,52 Note that the mobility of the 13C at the root of butyl group Bu 1 is comparable to those in the imidazolium ring, which was also observed in previous reports.39,44 Unlike the case of [C4C1mim]
Figure 4. Rotational correlation time τlocal of [C4mim]Br (filled symbols) and [C4C1mim]Br (open symbols) in the temperature range of 383.15 to 422.15 K.
Br, the τlocal of [C4mim]Br was already reported by Imanari et al.44 Although the absolute τlocal values here differ from that reported by them owing to a difference in analytical method, the trend of τlocal is the same as their result. Effect of C(2) Methylation on Rotational Dynamics. In this section, the change in τlocal with C(2) methylation is discussed. As already shown in Figure 4 and Table 2, all τlocal values increase on C(2) methylation, due to an increase in viscosity. The viscosity of ILs is known to increase with C(2) methylation,6,12,1518 and the relationship between viscosity 3002
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Table 1. Magnetic Dipole Coupling Constant DCH, Activation Energy Ea, and Correlation Time at Infinite Temperature τ0 for All 13 C Both in [C4mim]Br and [C4C1mim]Bra Im 2
Im 5
2 Me
3 Me 23.38
23.25
22.96
22.96
23.14
23.38 22.9
23.25 26.3
22.96 26.2
22.96 25.9
23.14 20.5
DCH/kHz
[C4mim]Br
24.21
24.21
24.21
9.77 30.2
24.21 31.0
24.21 31.1
23.23
Ea/kJ mol1
[C4C1mim]Br [C4mim]Br [C4C1mim]Br
47.9
34.4
34.9
20.9
[C4mim]Br
7.3
6.0
6.1
[C4C1mim]Br
0.1
5.7
5.1
τ0/fs a
Im 4
19.2
Bu 1
Bu 2
Bu 3
Bu 4
19.9
22.6
27.8
28.7
24.0
8.5
32.7
11.2
5.5
6.9
34.3
226.6
13.9
4.6
4.0
DCH are obtained from rCH estimated by the DFT calculation, and Ea and τ0 are obtained from the curve fits of T1.
Table 2. Rotational Correlation Time τlocal for All 13C Both of [C4mim]Br and [C4C1mim]Br and Rate of τlocal Change on C(2) Methylation at 393.15 K Im 2 τlocal/ps
Im 4
Im 5
[C4mim]Br
75.2
79.2
81.7
[C4C1mim]Br
231.0
213.3
218.3
307
269
267
τlocal change/%
2 Me
3 Me
Bu 1
Bu 2
Bu 3
Bu 4
9.2
103.4
33.3
15.4
3.7
11.6
15.2
226.4
68.3
30.5
6.1
164
219
205
198
165
Figure 5. Potential energies as a function of all dihedral angles for the butyl group in [C4mim]þ (filled symbols) and [C4C1mim]þ (open symbols). (a) Bu 1, (b) Bu 2, (c) Bu 3, (d) Bu 4. All calculations were performed using an all-trans conformation.
and τlocal, given in eq 9, is known as the StokesEinstein Debye equation τlocal ¼
Vη kB T
ð9Þ
where V is the effective molecular volume, η is the viscosity, and kB is the Boltzmann constant. The viscosity increase is the reason for τlocal increase. On the other hand, the rate of increase in τlocal differs among the carbons (Table 2, bottom). Whereas 13C at the imidazolium ring show a large increase of ca. 300%, 13C atoms with higher mobility show a lower increase; for example, the increase in values for Bu 4 and 3 Me are estimated to be 160%170%. The trend of τlocal increase is Im 2 > Im 4 ≈ Im 5 > Bu 1 > Bu 2 > Bu 3 > Bu 4 ≈ 3 Me. The difference in τlocal increase on C(2) methylation cannot be explained by eq 9. The nonuniform τlocal increase would indicate that each 13C has a different microviscosity. The different microviscosities can be assumed to originate in the anion position. Hunt reported that one reason for the viscosity increase on C(2) methylation is the elimination of some stable anion interaction sites,27 which decreases anion mobility. The decrease in anion mobility more strongly influences the 13C atoms closer to anions such as imidazolium carbons. This indicates that the mobility of 13C atoms that interact more strongly with anions becomes slower, and thus 13C atoms in Bu 4 or 3 Me,
which do not interact with anions strongly, would show a lower τlocal increase. Hunt’s calculations predicted that Bu 1 in [C4C1mim]þ has a higher rotational barrier owing to the presence of 2 Me.27 The calculations were carried out for each ion pair of [C4mim]Cl and [C4C1mim]Cl. On the other hand, our calculations covered only the cations and all dihedral angles of the butyl group, as shown in Figure 5. The results also indicate a higher rotational barrier for Bu 1 in [C4C1mim]þ, whereas the 13C rotation in the other butyl group shows almost the same energy barrier as that of [C4mim]þ. However, unlike the calculated results, the τlocal values obtained in this paper indicate that Bu 1 rotation does not seem to be inhibited by 2 Me. Although the τlocal increase of Bu 1, 219%, is actually larger than that of 13C in other butyl group, smaller than that of 13C in the imidazolium ring. Considering that Bu 1 rotation would be completely restricted by 2 Me, the τlocal increase was very small. The reason for the relatively smaller τlocal increase with C(2) methylation is considered to be that Bu 1 rotation occurs mainly at small rotation angles (namely, librational rotation) in the liquid state, and thus perfect rotation is not experienced owing to inhibition by 2 Me. Ions in ionic melts are known to rotate by small angles.43,53,54 Hayamizu et al. determined that the rotation angle of 1,2-dimethyl-3-propylimidazolium cations with bis(trifluoromethylsulfonyl)amide anions is ca. 20° at 283.15 K and increases with increasing temperature.43 In addition, the fitting parameters summarized in Table 1 also 3003
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Figure 6. Potential energies as a function of dihedral angle for the ethyl group of [C2mim]þ (black filled circles), [C2C1mim]þ (red open circles), and [C25C1mim]þ (blue open squares).
support this hypothesis. The activation energies (Ea) for rotation of Bu 1 are 26.3 kJ mol1 for [C4mim]Br and 22.6 kJ mol1 for [C4C1mim]Br; on the other hand, τ0 is 32.7 fs for [C4mim]Br and 226.6 fs for [C4C1mim]Br. Rotational inhibition of Bu 1 by 2 Me does not appear in the activation energy but in τ0, which can be regarded as the correlation time at infinite temperature. Note that there is negligible or no difference between the τ0 values of other 13C atoms of [C4mim]Br and [C4C1mim]Br. This might indicate that when the rotation angle of Bu 1 increases at higher temperatures, the rotational mobility is decreased by inhibition by 2 Me. Similar results are obtained using the parameters of 3 Me, which is located near 2 Me; that is, Ea is 22.9 kJ mol1for [C4mim]Br and 19.9 kJ mol1 for [C4C1mim]Br, whereas τ0 is 8.5 fs for [C4mim]Br and 34.3 fs for [C4C1mim]Br. Origin of Melting Point and Viscosity Increases with C(2) Methylation. The findings obtained above provide a fundamental understanding of the origin of the melting point and viscosity increases on C(2) methylation. As mentioned in the Introduction, the melting point and viscosity of imidazolium-based ILs increase on C(2) methylation, which is contrary to intuitive expectation. Hunt hypothesized, on the basis of quantum chemical calculations, that this was due to a decrease in entropy on C(2) methylation.27 There could be several reasons of the decrease in entropy on C(2) methylation. For example, C(2) methylation would change the curvature of the potential energy surfaces for both cation (Figure 5 (a), see later) and ion pair conformations.55 However, the author indicated that the entropy decreased for two main reasons: inhibition of butyl group rotation in an imidazolium cation and reduction of stable anion interaction sites. Our previous calorimetric results experimentally demonstrated reductions in the entropies of fusion and crystallization on C(2) methylation.34 The NMR findings we report here indicate that the increases in melting point do not originate because of the inhibition of the butyl group rotation but by a reduction in stable anion interaction sites. We consider that the origin of the viscosity increase on C(2) methylation would be the same as that of the melting point increase even though there is no experimental evidence that the viscosity increase is entropy driven. Our hypothesis is also supported by the results reported by Bonh^ote et al.6 They measured the melting points and viscosities of 1-ethyl-3-methylimidazolium([C2mim]þ)-based ILs. In that paper, the melting points and viscosities of [C2mim]X were smaller than those of 1-ethyl-2,3-dimethylimidazolium ILs [C2C1mim]X, but they were almost the same as those of 1-ethyl-3,5-dimethylimidazolium ILs [C25C1mim]X. However,
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our DFT calculations, the results of which are shown in Figure 6, indicate almost no difference in the rotational barrier of the ethyl group between [C2C1mim]þ and [C25C1mim]þ. This demonstrates that rotational inhibition of the alkyl group in imidazolium-based ILs by C(2) or C(5) methylation does not affect the melting point and viscosity. Moreover, it is suggested that the number of stable anion interaction sites remains almost unchanged on C(5) methylation. This is a reasonable consideration because the interaction energy between a cation and an anion located close to C(5) is calculated to be much lower than the energy in the case of an anion located close to C(2).27,28,5661 For example, the difference in the interaction energy is 32.02 kJ mol1 in [C2mim]Cl57 and 35.82 kJ mol1 in [C2mim]Br.59 The low interaction energy for anions located close to C(5) leads to small population in the liquid states.
’ CONCLUSION We examined the effect of C(2) methylation on rotational dynamics by comparing [C4mim]Br and [C4C1mim]Br. The rotational correlation time τlocal is estimated from the spinlattice relaxation time T1 of 13C NMR. The τlocal results obtained here provide three fruitful insights into the rotational dynamics of ILs. First, all τlocal values of [C4C1mim]Br are larger than those of [C4mim]Br owing to a viscosity increase on C(2) methylation. Second, the rate of change in τlocal on C(2) methylation differs among the carbons in the cation. This would indicate that each carbon has a different microviscosity, and we consider that the positioning of the anion near the imidazolium ring leads to the different rate of change in τlocal. Third and we believe the most meaningful finding in this study, the τlocal increase in Bu 1 with C(2) methylation is not as significant as we expected. The finding also differs from the results of calculation and indicates that the mobility of the butyl group in [C4C1mim]Br is not inhibited by 2 Me. It is believed that not rotational but librational motions are still active even for [C4C1mim]Br. This finding gives us conclusive information on the origin of the increases in melting point with C(2) methylation. Hunt previously indicated through calculations that the melting point and viscosity increase because of decreases in entropy caused by reductions in the rotational mobility of the butyl group and the number of stable anion interaction sites on C(2) methylation. Our finding experimentally illustrates that the origin of the increases in the melting point is not the inhibition of butyl group rotation and that the reduction in stable anion interaction sites plays a major role in the increasing. Additionally, it is suggested that the viscosity increase on C(2) methylation can be interpreted in the same manner. ’ AUTHOR INFORMATION Corresponding Author
*Fax: þ81-43-290-3939. E-mail:
[email protected].
’ ACKNOWLEDGMENT The present study was supported in part by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (No. 17073002, Grant-in-Aid for Scientific Research in Priority Area “Science of Ionic Liquids” (K.N.); No. 21245003, Grant-in-Aid for Scientific Research (A) (K.N.)). This work was also supported in part by the Global Center-of-Excellence 3004
dx.doi.org/10.1021/jp200635h |J. Phys. Chem. A 2011, 115, 2999–3005
The Journal of Physical Chemistry A Program “Advanced School for Organic Electronics” supported by MEXT (T.E.). We thank the Institute of Media and Information Technology of Chiba University for the provision of the computational facilities.
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