Comprehensive Conformational and Rotational Analyses of the Butyl

Article pubs.acs.org/JPCB

Comprehensive Conformational and Rotational Analyses of the Butyl Group in Cyclic Cations: DFT Calculations for Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium Takatsugu Endo,*,† Shinpei Hoshino,‡ Yuichi Shimizu,‡ Kozo Fujii,‡ and Keiko Nishikawa*,‡ †

Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan

‡

S Supporting Information *

ABSTRACT: The flexibility and conformational variety of the butyl group in cations of ionic liquids (ILs) play an important role in dictating the macroscopic and microscopic properties of ILs. Here we calculate potential energy surfaces for the dihedral angles of the butyl group in four different types of cyclic cations, imidazolium, pyridinium, pyrrolidinium, and piperidinium, using the density functional theory method. The calculation results highlight the role of the butyl group in these cations by comparison of five-membered and six-membered rings, and of aromatic and alicyclic rings, in terms of stable conformations and rotational barriers. A striking result is that the butyl group rotation in pyrrolidinium induces pseudorotation of the ring whereas such a phenomenon does not occur in piperidinium. This difference is thought to be because of the relationship in rotational activation energy between the butyl group (10−40 kJ mol−1) and the ring ( gt, g′t > tg, tg′ > gg, g′g′ > g′g, gg′. These findings are very similar to those for [C4mim]+. The difference from [C4mim]+ is seen in the conformers with ϕ2 = g′ (i.e., g′t, g′g, and g′g′) that have higher energy and in the higher rotational barrier in ϕ1. These are well explained by the C

DOI: 10.1021/acs.jpcb.6b07166 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 3. (a) PES for the pyridinium cations. Numbers in red are the stable conformers while those in blue turned out to be unstable (giving imaginary frequency). Gray area is the mirror symmetry region. (b) ΔE for the stable conformers. (c) Stable conformers with ΔE < 5 kJ mol−1.

is four times higher than those in previous reports;27,28 nevertheless, most conformers have high energy. When limited to the conformers with ΔE < 5 kJ mol−1, only one conformer (tw-g′tt) is added to the previous results. It should be noted that the ring conformers that are not considered in this paper can exist as stable conformers in the gas phase, which potentially increases the total number to more than 100. Comparing the stable conformers among the different ring conformations, the axial form has 2−6 kJ mol−1 higher energy than the equatorial form. The energetic trend for the twist form is dependent on the state of ϕ1. When ϕ1 = g, the energy of the twist conformers is in between those of the equatorial and axial forms. The energy of the form with ϕ1 = g′ does not differ from that of the axial form. Although with ϕ1 = t and ϕ2 = g, the twist form has higher energy than the equatorial form, with ϕ1 = t and ϕ2 = t, the trend is reversed. As seen in Figure 4a, the

PES behaviors are dependent on both the ring conformation and the dihedral angle. For ϕ1, the rotational behavior of the twist conformers is remarkably different from those of the other ring conformers. There are discontinuous regions in the PES where the twist form is forced to convert to the other ring conformations during the structure optimization (see Tables S6−8). The PES shape of ϕ2 differs from that of ϕ1 and ϕ3. The basins at ϕ = ± 60° are shallow, and the conformers with ϕ2 = g and g′ have higher energy relative to that with ϕ2 = t. The flatness in the PES is consistent with the previous results for the N-methyl-N-propylpyrrolidinium cation.28 The missing data points in ϕ2 again indicate the conversion of the corresponding ring conformer to the other ones. The discontinuity in the PES is also observed in ϕ3 where not only the ring conversions but also the butyl group conversions exist. D


Article


Figure 4. (a) PES for [C4mpyrr]+. Numbers in red are the stable conformers while that in blue turned out to be unstable (giving imaginary frequency). Gray area is the mirror symmetry region. (b) ΔE for the stable conformers. (c) Stable conformers with ΔE < 5 kJ mol−1.

case for [C4mpyrr]+ even though the order of conformer stability is different. Comparing with [C4mpyrr]+, the equatorial form with ϕ1 = t and the axial form with ϕ1 = g are more stable while the axial form with ϕ1 = t has higher energy relative to the most stable conformer. A characteristic feature is observed in the behaviors in the discontinuous regions of the PES. In contrast to [C4mpyrr]+, ring inversion is absent and only the butyl group conversion occurs in the region (Tables S9−10), even though there is no significant difference in the rotational barriers of the butyl group between these two alicyclic cations. This implies that the activation energy of pseudorotation in [C4mpip]+ is higher than that in [C4mpyrr]+. The inversion between the equatorial and axial forms does not proceed in a direct manner; it proceeds through intermediate twist-boat states.36,37 Figure 6a,b shows one representative pathway of the ring inversion of [C4mpip]+ for eq-gtt and ax-gtt, respectively (see Figure S3 for other

These results clearly indicate that rotation of the butyl group induces pseudorotation of the ring. This would originate from the lower activation energy for the pseudorotation than that for the butyl group rotation. The former for the envelope forms28 was reported to be < ∼6 kJ mol−1 while the latter has been estimated to be 10−40 kJ mol−1, depending on ϕ. The full butyl group rotation of ϕ1 for the twist form is not allowed (Figure 4a top right), which suggests that pseudorotation of the twist form has lower activation energy than that of the envelope forms. 3.1.4. Piperidinium. There are few articles reporting the conformational behavior of the butyl group in [C4mpip]+. In 2010, two different conformers (eq-ttt, eq-gtt) were reported,49 and recently we added six stable conformers (eq-gtg, eq-gtg′, ax-gtt, ax-gtg, and ax-gtg′).50 Figure 5 shows the PES of [C4mpip]+. The equatorial and axial forms showed 11 stable conformers each. The most stable one is eq-gtt. This is also the E


Article


Figure 5. (a) PES for [C4mpip]+. Numbers in red are the stable conformers. Gray area is the mirror symmetry region. (b) ΔE for the stable conformers. (c) Stable conformers with ΔE < 5 kJ mol−1.

not only from the ring structure but also from the ring conformation. The ϕ2 PES clearly offers two categories, aromatic and alicyclic, independent of the number of atoms in ring (except one PES for the [C4mpip]+ axial form). The aromatic cations have three minima, corresponding to t, g, and g′, whereas the energy of the g and g′ conformers in the alicyclic cations is higher and their existences are vaguer. This result is in line with the previous MD simulation that indicated the absence of conformers with ϕ2 = g or g′ for [C4mpyrr]+.28 A high rotational barrier is observed at ϕ2 = 0° for the alicyclic cations, which indicates that the transformation between g and g′ does not occur directly and goes through the t conformer as an intermediate. The difference between the aromatic and alicyclic rings is barely observed in ϕ3, particularly when ϕ2 = t. This ring structure independence is also seen in ϕ3 when ϕ2 = g or g′, except for the discontinuity in the alicyclic cations in the ϕ3 PES. It would be intriguing to discuss a variety of conformations in the different cations from the view of conformational entropy (Sc), which plays an important role in determining the melting

examples), using the intrinsic reaction coordination (IRC) calculations. The activation energy of the inversion is 40−50 kJ mol−1, which is clearly higher than that of the butyl group rotation. This is a robust evidence for the absence of the ring inversion induced by the butyl group in [C4mpip]+. A schematic of the association between ring inversion and butyl group rotation for [C4mpyrr]+ and [C4mpip]+ is illustrated in Figure 6c. 3.1.5. Comparison Among the Cations. Figure 7 summarizes the PES results for detailed comparisons between five-membered ring and six-membered ring cations and between aromatic and alicyclic ring cations. Please note that in contrast to Figures 2−5, ΔE of the most stable conformer in each PES is set to zero to highlight the difference among the cations. In the ϕ1 PES, the rotational barrier of [C4mim]+ is the lowest and those of the pyridinium cations except for [oC4mpyri]+ are the next lowest. In general, alicyclic cations have higher barrier than aromatic cations. [C4 mpyrr] + and [C4mpip]+ have similar PES in the region of ϕ1 = g and g′ whereas in ϕ1 = t, they are different. The difference originates F


Article


3.2. Comparison with Cambridge Structural Database (CSD). We now examine how the calculated stable conformers correspond to the experimentally observed ones recorded in CSD. Three cations, [C4mim]+, [C4pyri]+, and [C4mpyrr]+, were considered here because the number of reported crystal structures for the other cations is currently too small. Figure 8a,b shows the normalized population of conformers observed in CSD against the calculated Gibbs free energy difference (ΔG) with respect to the most stable conformer. Although the data has some scatter, the more stable conformers show higher population, roughly independent of the cation. This demonstrates the validity of the calculation to predict experimental results. Since the figure corresponds to the density of states, the relationship should follow an exponential form as

⎛ ΔG ⎞ ⎟ P = A exp⎜ − ⎝ RT ⎠

(1)

where P is the population in CSD, ΔG is the calculated Gibbs free energy difference with respect to the most stable conformer (see Tables S11−S20), A is the constant, and R is the universal gas constant. Fitting with eq 1, expressed as a solid line in Figure 8b, was performed using the data with P > 0.01, providing T of 506 K. Some conformers, although they are stable, show very small or even zero population, e.g., pl-tt (P = 0.006) and pl-tg (P = 0.009) of [C4mim]+, ax-gtt (P = 0.036), ax-gtg (P = 0), tw-gtt (P = 0), and tw-g′tt (P = 0) of [C4mpyrr]+. The common feature for these conformers is that they have low (pseudo)rotational barrier, in other words, very short lifetime. This finding would be beneficial when assigning experimentally observed conformers using the calculation results; namely, not only low energy but also deep basin in the PES is crucial for a conformer to exist. This idea can be utilized to discuss the conformer in the crystal structure of [C4mpip]+. Only two studies have performed X-ray single crystal analysis of C4mpipbased ILs,49,51 and both have reported the axial form as the observed ring conformer. We previously found two crystalline phases of the C4mpip-based IL paired with bis(fluorosulfonyl)amide (FSA) anion, and one of them was assigned to the axial form with Raman spectroscopy.50 These observations are in contrast with the results for [C4mpyrr]+ where the population of the axial forms obtained from CSD is only 0.036. One reason could be the smaller difference in energy between the equatorial and axial forms for [C4mpip]+ compared with that for [C4mpyrr]+. However, the main cause would be the high rotational barrier of ring inversion in [C4mpip]+ (Figures 6, S3). 3.3. Raman Spectra. In the previous section, we examined the relationship between the calculated conformer in the gas phase and the observed ones from CSD. Single crystal X-ray analysis is recognized as the most powerful technique to determine molecular structure. However, obtaining single crystal is frequently difficult, particularly for ILs due to their low melting point. Although Raman spectroscopy cannot determine the complete structural configuration of a molecule of interest, its advantages, namely, sensitivity to conformational differences of molecules and applicability to a wide range of states (crystal, liquid, and even gas states), are significant. Therefore, Raman spectroscopy has been playing a prominent role in the determination of conformers of ILs, using a fingerprint region of 200−1000 cm−1 (vide infra). In this section, the applicability and limitation of Raman spectroscopy for conformer determination of each cation will be discussed.

Figure 6. Ring inversion pathway of [C4mpip]+ from (a) eq-gtt to a twist form and from (b) ax-gtt to a twist form. (c) Schematic of the ring inversion induced/not induced by the butyl group rotations for [C4mpyrr]+ and [C4mpip]+.

point of ILs, as listed in Table S21. The Sc value of [C4mim]+ was calculated to be 19.3 J K−1 mol−1. The value for the pyridinium cations is significantly dependent on the presence and position of the methyl group. The values for [C4pyri]+ and [p-C4mpyri]+, which have high structural symmetry, are 13−14 J K−1 mol−1 while the values for [o-C4mpyri]+ (17.6 J K−1 mol−1) and [m-C4mpyri]+ (19.1 J K−1 mol−1) are close to that for [C4mim]+. Many comparative studies of a wide range of properties between imidazolium and pyridinium have been made thus far; nevertheless, there is still no consensus on the adoption and the position of the methyl group for the pyridinium cations. In terms of the conformational variety and rotational behavior of the butyl group, our results indicate that [m-C4mpyri]+ has the closest properties to [C4mim]+. The Sc of the alicyclic cations are higher than those of the aromatic cations. It should be noted that particularly for [C4mpyrr]+, the actual Sc could be higher when considering the other ring conformers. As seen for the aromatic cations, the fivemembered alicyclic cation has higher Sc (25.7 J K−1 mol−1 for [C4mpyrr]+ and 20.0 J K−1 mol−1 for [C4mpip]+). This is because the number of low energy conformers is rather high in the five-membered cation; furthermore, the contribution from the twist form is notable for [C4mpyrr]+. However, considering the total number of stable conformers, the difference in Sc is not significant. This is reflected by the stable conformers with relatively high energy that are dominant in the alicyclic cations. G


Article


Figure 7. PES of [C4mim]+ (red), the four pyridinium cations (orange), [C4mpyrr]+ (blue), and [C4mpip]+ (pale blue). A plot with the lowest energy in each PES is set to be ΔE = 0.

Figure 8. (a) ΔG of the conformers of [C4mim]+ (red), [C4pyri]+ (orange), and [C4mpyrr]+ (blue) versus normalized population of the conformer observed in crystal structure from CSD. Blue filled circles, open circles, and open triangles represent the equatorial, axial, and twist forms of [C4mpyrr]+, respectively. CSD surveys provide 331 conformations from 192 crystal structure data for [C4mim]+, 25 conformations from 19 crystal structure data for [C4pyri]+, and 56 conformations from 37 crystal structure data for [C4mpyrr]+. (b) Single logarithmic plot of (a). The theoretical fitting curve (black line) of eq 1 was obtained using data with population >0.01.

3.3.1. Imidazolium. Conformation determination of [C4mim]+ has been widely investigated with Raman spectroscopy with the aid of quantum chemical calculations. The initial work was done by Hamaguchi et al. for the halide salts,11,38 and they discovered that two peaks that represent gt and tt appear in 600−630 cm−1. Since these peaks are strong and well separated, they have been utilized to characterize the gt and tt conformers under various conditions.11,52−55 In 2008, Holomb et al. revealed that the gg and tg conformers are also distinguishable by observing their Raman spectra in the fingerprint region.52 Later we found that examining the combination of the regions 600−630 cm−1 and 310−350 cm−1 highlights the g′t conformer.39 As a result, five conformers

are currently known to be distinguishable. By revisiting the region of 200−1000 cm−1 with the results calculated in this study, we have achieved further assignments of the conformers. It should be noted that the basic strategy for the assignments is similar to that by Holomb et al.52 Figure 9a compares the observed and calculated Raman patterns, and Figure 9b displays the assignment flow. By taking a look at 600−630 cm−1, the 11 conformers are divided into two groups. The conformers with a peak at 624 cm−1 have tt, tg, tg′, pl-tt, and pl-tg while those with a peak at 601 cm−1 have gt, gg, gg′, g′t, g′g, and g′g′. Then each group is further separated into three groups by using the 310−350 cm−1 region. Two Raman bands in 800−830 cm−1 distinguish g′t and g′g, H


Article


This is also the case for tt/pl-tt; nevertheless, the low rotational activation energy of pl-tt makes it less probable to be found in the liquid and crystal states. The energetic sequence of the last group is g′g′ < gg < gg′, but the difference between the first two is slight. It should be noted that not all observed conformers correspond to one of the 11 stable conformers, and the conformers of the transition state in the gas phase calculation can be present in the crystal.56 These characteristic Raman bands are important not only in the crystalline state but also in the liquid state where the conformers coexist.11,38,39,57 Temperature-dependent Raman spectroscopic measurements in the liquid state can provide the enthalpy difference between the conformers. Thus, far, the peaks at 601 and 624 cm−1 have been used to obtain the enthalpy difference between gt and tt, which is, based on our assignments, clearly an oversimplification. The use of the Raman bands at 907 and 825 cm−1 would be more appropriate. It may be possible to derive enthalpy differences for all seven groups if curve fitting analyses are conducted successfully. In that case, the key band is the one at 338 cm−1 assigned to gt, which could merge with neighboring peaks (351 cm−1 for tg/ tg′ and 327 cm−1 form tt and g′t) and could consequently emerge as one peak. 3.3.2. Pyridinium. In contrast to [C4mim]+, a little surprisingly, no research has been performed on the conformational investigation of the butyl group in pyridinium cations using Raman spectroscopy. Figure 10a shows the calculated Raman bands for the stable conformers of [C4pyri]+. Since there are only five stable conformers and the band patterns are relatively simple, all the conformers would be distinguishable in Raman spectra. The two calculated bands at 915.9 and 884.5 cm−1 only appear for tt and tg, respectively. Among the other three, gt has two rather small bands at 281.4 and 426.7 cm−1 while gg and gg′ have characteristic bands at 947.2 and 743.5 cm−1, respectively. The regions of 480−510 cm−1 and 740−820 cm−1 would provide different Raman shape for different conformers. To validate the above-mentioned conjecture, experimental Raman data for the different conformers of this cation are needed. However, such data are currently unavailable. Therefore, here we use one Raman spectrum of [C4pyri]+ paired with Cl anion58 (Figure 10b) for a rough validation. First, in the observed spectrum, there is no strong peak around 900 cm−1, which could exclude the possibility of tt and tg. The band shape in 700−800 cm−1 is reminiscent of gt or gg, rather than gg′. This is also supported by the fact that gg′ has higher energy than the others. Although the differentiation of gt and gg is a little challenging, the existence of small bands at 300 and 420 cm−1 may indicate the presence of the gt conformer, which is in line with the crystal structure data.59 For the butyl- and methyl-substituted pyridinium cations, there is no appropriate observed Raman spectrum to compare with the calculated Raman bands. However, the calculated bands, as shown in Figure 11, clearly reflect the difference in conformation, indicating that Raman spectroscopic measurements are also effective in assigning conformers of these cations. Some conformers of [o-C4mpyri]+ and [m-C4mpyri]+ exhibit very similar Raman band shape (e.g., tg/tg′ for each pyridinium), which disturbs complete conformation assignments. On the other hand, for [p-C4mpyri]+ with five stable conformers, a complete separation could be performed. 3.3.3. Pyrrolidinium. Umebayashi et al. investigated conformational equilibrium of the C4mpyrr-based ILs paired

Figure 9. (a) The observed Raman spectra of [C4mim]+ paired with hexafluorophosphate (PF6) anion is displayed at the top in different states. Please see ref 39 for the experimental details. Asterisk indicates the PF6 anion bands. Conformational assignments are linked to the characteristic bands. Please note that four conformers, gg′, g′g, pl-tt, and pl-tg, which have relatively high energy or low rotational barrier for the butyl group, are excluded from the figure for simplicity. The conformers tg/tg′ and gg/g′g′ were assumed to provide the same Raman spectral pattern. At the bottom, the calculated Raman bands for the stable conformers are shown with ΔE/ΔG (in kJ mol−1). Blue lines indicate potential marker bands for the assignments. (b) A representative flow for the conformer assignments of [C4mim]+ using characteristic Raman bands.

which is used consequently to categorize seven conformer groups. It should be noted that this flow is not necessarily followed, and the bands in 460−510 cm−1, 680−770 cm−1, and 870−920 cm−1 are also used to characterize the conformers. Judging from the calculated Raman bands, some groups are barely separated into individual conformers, as tg/tg′, tt/pl-tt, and gg/gg′/g′g′. The energy difference of tg/tg′ is fairly small. I


Article


Figure 10. (a) Calculated Raman bands of the stable conformers in [C4pyri]+ with ΔE/ΔG (in kJ mol−1). Blue lines indicate potential marker bands for the assignments. (b) Observed Raman spectra of C4pyri-based ILs. UF6 (green) and Cl salts (black). Reprinted with permission from ref 58, Copyright 2009 The Chemical Society of Japan.

with Br ([C4mpyrr]Br) and bis(trifluoromethanesulfonyl)imide ([C4mpyrr]TFSI) with the aid of DFT calculations.27 They calculated eight stable conformers (in our definition, eq-ttt, eqgtt, eq-gtg, ax-ggt, ax-gtt, tw-ggt, tw-gtt, and tw-ttt) and ascribed ax-gtt to the conformer for the [C4mpyrr]Br crystal. For [C4mpyrr]TFSI in the liquid state, they assumed the mixture of eq-gtt and ax-gtt where eq-gtt was revealed to be more stable by temperature-dependent Raman spectroscopic measurements, even though other conformers might also contribute to the conformational equilibrium. The conformational assignment of [C4mpyrr]+ is challenging because (1) a huge number of stable conformers exist (more than 50), (2) high flexibility of the pyrrolidinium cation ring induces plastic crystal phases,29,30 and (3) the number of vibrational modes is large compared with those for the aromatic cations. Figure 12 displays the calculated Raman bands for the conformers with ΔE < 5 kJ mol−1 and the ones obtained from CSD, which are more likely to be observed than the others. As can be seen in the figure, the conformational difference, both the butyl group and the ring, clearly affects the band pattern, which would enable one to

conduct conformational assignments. A characteristic conformational feature for [C4mpyrr]+ is that eq-gtt constitute 60% of the observed conformers while eq-ttt, eq-gtg′, eq-ggt, and axgtt constitute 20%. Conformers with different ring configurations constitute the remaining 20%. The Raman band located in 715−730 cm−1, which was not discussed in previous article,27,28 can be an important marker band. Even though the difference in Raman shift among the conformers is somewhat small, its vibrational mode is very similar to that of [C4mpip]+ located in 695−710 cm−1 (Figure S5), which is sensitive to the conformational difference (vide infra). 3.3.4. Piperidinium. In our recent article,50 conformational analyses with the combination of Raman spectroscopy and DFT calculations were performed for [C4mpip]FSA, which possesses two crystalline phases (α and β). Considering six stable conformers, eq-gtt, eq-gtg, eq-gtg′, ax-gtt, ax-gtg, and axgtg′, our investigation of the Raman peaks in 700−750 cm−1 revealed that the major ring conformations of the cation ring in the α and β crystals were axial and equatorial, respectively. However, the butyl group conformations remained unclear. J


Article


Figure 11. Calculated Raman bands of the stable conformers in (a) [o-C4mpyri]+, (b) [m-C4mpyri]+, and (c) [p-C4mpyri]+ with ΔE/ΔG (in kJ mol−1). Blue lines indicates potential marker bands for the assignments.

higher energy than eq-gtt. This finding strongly implies that the shoulder component comes from the eq-ttt conformer.

Figure 13 shows the calculated Raman bands of the [C4mpip]+ conformers with ΔE < 5 kJ mol−1. The figure includes the conformation eq-ttt in addition to the six previously reported conformers. The conformational difference of the butyl group could be distinguished with the Raman band in 270−280 cm−1; in the previous article, the bands from the TFSI anion overlapped the cation bands in this region.50 In both the equatorial and axial forms, gtt has relatively strong Raman intensity compared to gtg and gtg′ in this region. The eq-ttt conformer has intermediate intensity for this band, which would make assignment of this conformer difficult. However, by revisiting the band in 695−710 cm−1, we have found that eqttt is distinguishable. Figure 13a (right) shows that the band of eq-ttt is located at a lower frequency than the others. Taking a closer look at the observed Raman peak at 722 cm−1 (Figure 13b), there is clearly a shoulder component (∼715 cm−1) in this peak. Actually, the other 15 stable conformers that are not displayed in the figure possess the corresponding vibrational peak at a frequency lower than 700 cm−1 (Figure S7); nevertheless, these conformers have more than 10 kJ mol−1

4. CONCLUSIONS We performed conformational analyses with DFT calculations for seven cyclic cations that are representative constituent ions for ILs. The comprehensive PES and single point energy calculations highlight the difference between five-membered and six-membered rings and between aromatic and alicyclic rings. In the ϕ1 PES, even though rough similarities in aromatic and alicyclic groups were observed, a strong dependence on the cation structure existed. On the other hand, a clear categorization of aromatic and alicyclic rings was feasible with the ϕ2 PES. The ϕ3 PES was almost independent of the cation structure. A string finding of this study is the association between ring inversion and butyl group rotation for the alicyclic cations. The butyl group rotation of [C4mpyrr]+ induces ring inversion, whereas the induced ring inversion does not occur in [C4mpip]+. This is believed to be because ring inversion in [C4mpyrr]+ has lower activation energy whereas ring inversion K


Article


Figure 12. Calculated Raman bands of the stable conformers in [C4mpyrr]+ with ΔE/ΔG (in kJ mol−1). The population of the conformer (%) is also displayed. Frequency ranges of 200−1000 cm−1 and 700−740 cm−1 are shown at left and right, respectively.

Figure 13. (a) Calculated Raman bands of the stable conformers in [C4mpip]+ with ΔE/ΔG (in kJ mol−1). Frequency ranges of 200−1000 cm−1 and 680−720 cm−1 are shown at left and right, respectively. (b) Observed Raman spectra of the IL (please see ref 50 for the experimental details) during phase changes from liquid to crystal α (top, red), and from crystal α to crystal β (middle, blue).

in [C4mpip]+ has higher activation energy compared to the rotational barrier of the butyl group. We compared the energetic stability of the calculated conformers with the population of the conformers observed in the crystal structures recorded in CSD. Although a rough exponential relationship independent of the cation structure was observed between them for most conformers, conformers with low rotational activation energy, in other words, short lifetime, did not show the same trend. This finding will help predict conformations in real systems from calculations. In the last part of the aarticle, we discussed the applicability of Raman spectroscopy for conformational assignments and revisited the assignments of [C4mim]+, [C4mpyrr]+, and [C4mpip]+.

It should be noted that the results obtained here are based on the single-ion calculations, and in real systems, effect of a paired anion on the conformational behavior of the cation is definitely non-negligible. Even so, it is still believed that our results will contribute to a deeper understanding of the alkyl group behavior in IL cations, which play a prominent role in dictating the macroscopic and microscopic properties of ILs.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b07166. L


Article


■

of the melting of two polymorphic crystals by Raman spectroscopy and calorimetry. Chem. Phys. Lett. 2013, 584, 79−82. (13) Endo, T.; Kato, T.; Nishikawa, K. Effects of methylation at the 2 position of the cation ring on phase behaviors and conformational structures of imidazolium-based ionic liquids. J. Phys. Chem. B 2010, 114, 9201−9208. (14) Kawahata, M.; Endo, T.; Seki, H.; Nishikawa, K.; Yamaguchi, K. Polymorphic Properties of Ionic Liquid of 1-Isopropyl-3-methylimidazolium Bromide. Chem. Lett. 2009, 38, 1136−1137. (15) Yoshimura, Y.; Takekiyo, T.; Abe, H.; Hamaya, N. Highpressure phase behavior of the room temperature ionic liquid 1-ethyl3-methylimidazolium nitrate. J. Mol. Liq. 2015, 206, 89−94. (16) Bäcker, T.; Breunig, O.; Valldor, M.; Merz, K.; Vasylyeva, V.; Mudring, A.-V. In-Situ Crystal Growth and Properties of the Magnetic Ionic Liquid [C2mim][FeCl4]. Cryst. Growth Des. 2011, 11, 2564− 2571. (17) Li, H.; Wang, Z.; Chen, L.; Wu, J.; Huang, H.; Yang, K.; Wang, Y.; Su, L.; Yang, G. Kinetic Effect on Pressure-Induced Phase Transitions of Room Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate. J. Phys. Chem. B 2015, 119, 14245−14251. (18) Tsuzuki, S.; Matsumoto, H.; Shinoda, W.; Mikami, M. Effects of conformational flexibility of alkyl chains of cations on diffusion of ions in ionic liquids. Phys. Chem. Chem. Phys. 2011, 13, 5987−5993. (19) Araque, J. C.; Yadav, S. K.; Shadeck, M.; Maroncelli, M.; Margulis, C. J. How is diffusion of neutral and charged tracers related to the structure and dynamics of a room-temperature ionic Liquid? Large deviations from Stokes-Einstein behavior explained. J. Phys. Chem. B 2015, 119, 7015−7029. (20) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of ab initio calculations toward the rational design of room temperature ionic liquids. J. Phys. Chem. A 2003, 107, 2277−2288. (21) Tsuzuki, S.; Arai, A. A.; Nishikawa, K. Conformational analysis of 1-butyl-3-methylimidazolium by CCSD(T) level ab initio calculations: effects of neighboring anions. J. Phys. Chem. B 2008, 112, 7739−7747. (22) Paulechka, Y. U.; Kabo, G. J.; Emel’Yanenko, V. N. Structure, conformations, vibrations, and ideal-gas properties of 1-alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ionic pairs and constituent ions. J. Phys. Chem. B 2008, 112, 15708−15717. (23) Canongia Lopes, J. N. a.; Padua, A. a. H. Using spectroscopic data on imidazolium cation conformations to test a molecular force field for ionic liquids. J. Phys. Chem. B 2006, 110, 7485−7489. (24) Blokhin, A. V.; Paulechka, Y. U.; Strechan, A. A.; Kabo, G. J. Physicochemical properties, structure, and conformations of 1-Butyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide [C4mim]NTf2 ionic liquid. J. Phys. Chem. B 2008, 112, 4357−4364. (25) Marekha, B. A.; Kalugin, O. N.; Idrissi, A. Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis. Phys. Chem. Chem. Phys. 2015, 17, 16846−16857. (26) Endo, T.; Imanari, M.; Hidaka, Y.; Seki, H.; Nishikawa, K.; Sen, S. Structure and dynamics of room temperature ionic liquids with bromide anion: Results from 81Br NMR spectroscopy. Magn. Reson. Chem. 2015, 53, 369−378. (27) Fujimori, T.; Fujii, K.; Kanzaki, R.; Chiba, K.; Yamamoto, H.; Umebayashi, Y.; Ishiguro, S.-i. Conformational structure of room temperature ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide  Raman spectroscopic study and DFT calculations. J. Mol. Liq. 2007, 131−132, 216−224. (28) Umebayashi, Y.; Mitsugi, T.; Fujii, K.; Seki, S.; Chiba, K.; Yamamoto, H.; Canongia Lopes, J. N.; Pádua, A. a. H.; Takeuchi, M.; Kanzaki, R.; et al. Raman spectroscopic study, DFT calculations and MD simulations on the conformational isomerism of N-alkyl-Nmethylpyrrolidinium bis-(trifluoromethanesulfonyl) amide ionic liquids. J. Phys. Chem. B 2009, 113, 4338−4346. (29) Macfarlane, D. R.; Huang, J.; Forsyth, M. Lithium-doped plastic crystal electrolytes exhibiting fast ion conduction for secondary batteries. Nature 1999, 402, 792−794.

Energy values in PES (Tables S1−S10); energy difference, population, and thermodynamic properties of stable conformers (Tables S11−S20); conformational entropy (Table S21); calculated vibrational bands (Tables S22−S28); PES for [C4mpip]+ in the twist forms (Figure S1); PES figure includes all cations (Figure S2); some IRC pathways for the ring inversion of [C4mpip]+ (Figure S3); calculated ΔE of the conformers versus population in crystals (Figure S4); a vibrational mode for the alicyclic cations (Figure S5); calculated Raman bands for the alicyclic cations (Figure S6−S7) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: tkendo@staff.kanazawa-u.ac.jp.; Telephone: +81-76234-4807. *E-mail: [email protected].; Telephone: +43-2903951. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The present study was supported by JSPS Postdoctoral Fellowships for Research Abroad (T.E.). REFERENCES

(1) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; VCH-Wiley: Weinheim, 2003. (2) Electrochemical Aspects of Ionic Liquids; Ohno, H., Ed.; WileyInterscience: Hoboken, 2005. (3) Ionic Liquids: Applications and Perspectives; Kokorin, A., Ed.; InTech, 2011. (4) Holbrey, J. D.; Seddon, K. R. The phase behaviour of 1-alkyl-3methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J. Chem. Soc., Dalton Trans. 1999, 2133−2139. (5) Shimizu, Y.; Ohte, Y.; Yamamura, Y.; Saito, K. Is the liquid or the solid phase responsible for the low melting points of ionic liquids? Alkyl-chain-length dependence of thermodynamic properties of [Cnmim][Tf2N]. Chem. Phys. Lett. 2009, 470, 295−299. (6) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical properties and structures of room temperature ionic liquids. 2. Variation of alkyl chain length in imidazolium cation. J. Phys. Chem. B 2005, 109, 6103−6110. (7) Endo, T.; Masu, H.; Fujii, K.; Morita, T.; Seki, H.; Sen, S.; Nishikawa, K. Determination of missing crystal structures in the 1alkyl-3-methylimidazolium hexafluorophosphate series: Implications on structure − property relationships. Cryst. Growth Des. 2013, 13, 5383−5390. (8) Tariq, M.; Carvalho, P. J.; Coutinho, J. A. P.; Marrucho, I. M.; Lopes, J. N. C.; Rebelo, L. P. N. Viscosity of (C2-C14) 1-alkyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids in an extended temperature range. Fluid Phase Equilib. 2011, 301, 22−32. (9) Seddon, K. R.; Stark, A.; Torres, M. J. Viscosity and density of 1alkyl-3-methylimidazolium ionic liquids. ACS Symp. Ser. 2002, 819, 34−49. (10) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnson, S.; Seddon, K. R.; Rogers, R. D. Crystal polymorphism in 1-butyl-3methylimidazolium halides: Supporting ionic liquid formation by inhibition of crystallization. Chem. Commun. 2003, 1636−1637. (11) Hayashi, S.; Ozawa, R.; Hamaguchi, H.-o. Raman spectra, crystal polymorphism, and structure of a prototype ionic-liquid [bmim]Cl. Chem. Lett. 2003, 32, 498−499. (12) Endo, T.; Nishikawa, K. Thermal phase behavior of 1-butyl-3methylimidazolium hexafluorophosphate: Simultaneous measurements M


Article

The Journal of Physical Chemistry B (30) Pringle, J. M.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. Organic ionic plastic crystals: recent advances. J. Mater. Chem. 2010, 20, 2056−2062. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09; Gaussian, Inc.: Wallingford CT, 2010. (32) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (33) 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: Condens. Matter Mater. Phys. 1988, 37, 785−789. (34) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results obtained with the correlation energy density functionals of becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200−206. (35) Canongia Lopes, J. N.; Shimizu, K.; Pádua, A. a. H.; Umebayashi, Y.; Fukuda, S.; Fujii, K.; Ishiguro, S.-i. A tale of two ions: the conformational landscapes of bis(trifluoromethanesulfonyl)amide and N,N-dialkylpyrrolidinium. J. Phys. Chem. B 2008, 112, 1465−1472. (36) Cremer, D.; Pople, J. A. A general definition of ring puckering coordinates. J. Am. Chem. Soc. 1975, 97, 1354−1358. (37) Nelson, D. J.; Brammer, C. N. Toward consistent terminology for cyclohexane conformers in introductory organic chemistry. J. Chem. Educ. 2011, 88, 292−294. (38) Ozawa, R.; Hayashi, S.; Saha, S.; Kobayashi, A.; Hamaguchi, H.o. Rotational isomerism and structure of the 1-butyl-3-methylimidazolium cation in the ionic liquid state. Chem. Lett. 2003, 32, 948−949. (39) Endo, T.; Kato, T.; Tozaki, K.-i.; Nishikawa, K. Phase behaviors of room temperature ionic liquid linked with cation conformational changes: 1-butyl-3-methylimidazolium hexafluorophosphate. J. Phys. Chem. B 2010, 114, 407−411. (40) Endo, T.; Nishikawa, K. Springer Ser. Mater. Sci. 2015, 209, 379−401. (41) Dötterl, M.; Haas, I.; Alt, H. G. Solubility behaviour of TiCl4, ZrCl4, and HfCl4 in chloroaluminate ionic liquids. Z. Anorg. Allg. Chem. 2011, 637, 1502−1506. (42) Gao, X.; An, T.; Li, J.; Zhao, F.; Fan, X.; Li, X.; Zhang, G.; Gao, Z. Ferrocenyl ionic compounds based on 5-ferrocenyl-1H-tetrazole. Synthesis, characterization, migration, and catalytic effects during combustion. Z. Anorg. Allg. Chem. 2016, 642, 155−162. (43) Zhang, B.; Li, S.; Pöthig, A.; Cokoja, M.; Zang, S.-L.; Herrmann, W. A.; Kühn, F. E. Oxidation reactions catalyzed by polyoxomolybdate salts. Z. Naturforsch., B: J. Chem. Sci. 2013, 68, 587−597. (44) Lin, Y.; Massa, W.; Dehnen, S. Zeoball” [Sn36Ge24Se132]24−: A molecular anion with zeolite-related composition and spherical shape. J. Am. Chem. Soc. 2012, 134, 4497−4500. (45) Preiss, U.; Beichel, W.; Steinfeld, G.; Quiñones, G. S.; Krossing, I. CSD Communication 2014. (46) Sambasivarao, S. V.; Acevedo, O. Development of OPLS-AA force field parameters for 68 unique ionic liquids. J. Chem. Theory Comput. 2009, 5, 1038−1050. (47) Belotserkovets, N. I.; Nikol’Skii, V. M. Non-empirical quantum chemical calculations of the structure of a homologous series of Nalkylpyridinium cations. J. Struct. Chem. 2014, 55, 210−216. (48) Sashina, E. S.; Kashirskii, D. A.; Martynova, E. V. Features of the molecular structure of pyridinium salts and their dissolving power with respect to cellulose. Russ. J. Gen. Chem. 2012, 82, 729−735. (49) Reichert, W. M.; Henderson, W. A.; Trulove, P. C.; Urban, J. J.; De Long, H. C. Effects of crystal packing on the thermal behavior of N,N′- alkylpiperidinium and N,N′-alkylmorpholinium iodide salts. ECS Trans. 2010, 33, 667−677. (50) Shimizu, Y.; Fujii, K.; Imanari, M.; Nishikawa, K. Phase behavior of a piperidinium-based room-temperature ionic liquid exhibiting scanning rate dependence. J. Phys. Chem. B 2015, 119, 12552−12560. (51) Matsumoto, K.; Hagiwara, R. A new series of ionic liquids based on the difluorophosphate anion. Inorg. Chem. 2009, 48, 7350−7358. (52) Holomb, R.; Martinelli, A.; Albinsson, I.; Lassègues, J. C.; Johansson, P.; Jacobsson, P. Ionic liquid structure: The conformational

isomerism in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]). J. Raman Spectrosc. 2008, 39, 793−805. (53) Chang, H.-c.; Chang, C.-y.; Su, J.-c.; Chu, W.-c.; Jiang, J.-c.; Lin, S. H. Conformations of 1-butyl-3-methylimidazolium chloride probed by high pressure raman spectroscopy. Int. J. Mol. Sci. 2006, 7, 417− 424. (54) Russina, O.; Fazio, B.; Schmidt, C.; Triolo, A. Structural organization and phase behaviour of 1-butyl-3-methylimidazolium hexafluorophosphate: an high pressure Raman spectroscopy study. Phys. Chem. Chem. Phys. 2011, 13, 12067−12074. (55) Takekiyo, T.; Hatano, N.; Imai, Y.; Abe, H.; Yoshimura, Y. Pressure-induced phase transition of 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]. High Pressure Res. 2011, 31, 35− 38. (56) Saouane, S.; Norman, S. E.; Hardacre, C.; Fabbiani, F. P. a. Pinning down the solid-state polymorphism of the ionic liquid [bmim][PF6]. Chem. Sci. 2013, 4, 1270−1280. (57) Umebayashi, Y.; Hamano, H.; Tsuzuki, S.; Canongia Lopes, J. N.; Pádua, A. a. H.; Kameda, Y.; Kohara, S.; Yamaguchi, T.; Fujii, K.; Ishiguro, S.-i. Dependence of the conformational isomerism in 1-nbutyl-3-methylimidazolium ionic liquids on the nature of the halide anion. J. Phys. Chem. B 2010, 114, 11715−11724. (58) Kanatani, T.; Matsumoto, K.; Hagiwara, R. Syntheses and physicochemical properties of new ionic liquids based on the hexafluorouranate anion. Chem. Lett. 2009, 38, 714−715. (59) Ward, D. L.; Rhinebarger, R. R.; Popov, A. I. N-nButylpyridinium chloride. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 1771−1773.

N


Comprehensive Conformational and Rotational Analyses of the Butyl

Recommend Documents