Association structures of ionic liquid/DMSO mixtures studied by high-pressure infrared
spectroscopy
Jyh-Chiang Jiang, Kuan-Hung Lin, Sz-Chi Li, Pao-Ming Shih, Kai-Chan Hung, Sheng Hsien Lin, and Hai-Chou Chang
Citation: The Journal of Chemical Physics 134, 044506 (2011); doi: 10.1063/1.3526485 View online: http://dx.doi.org/10.1063/1.3526485
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/134/4?ver=pdfcov Published by the AIP Publishing
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Association structures of ionic liquid/DMSO mixtures studied
by high-pressure infrared spectroscopy
Jyh-Chiang Jiang,1Kuan-Hung Lin,2Sz-Chi Li,2Pao-Ming Shih,2Kai-Chan Hung,2
Sheng Hsien Lin,3,4and Hai-Chou Chang2,a)
1Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
2Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan 3Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan 4Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 106, Taiwan
(Received 1 March 2010; accepted 18 November 2010; published online 24 January 2011)
Using high-pressure infrared methods, we have investigated close interactions of charge-enhanced C– H–O type in ionic liquid/dimethyl sulfoxide (DMSO) mixtures. The solvation and association of the 1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF−4) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMM+BF−4) in DMSO-d6 were examined by analysis of C–H spectral features.
Based on our concentration-dependent results, the imidazolium C–H groups are more sensitive sites for C–H–O than the alkyl C–H groups and the dominant imidazolium C–H species in dilute ionic liquid/DMSO-d6should be assigned to the isolated (or dissociated) structures. As the dilute mixtures
were compressed by high pressures, the loss in intensity of the bands attributed to the isolated struc-tures was observed. In other words, high pressure can be used to perturb the association–dissociation equilibrium in the polar region. This result is remarkably different from what is revealed for the imi-dazolium C–H in the BMM+BF−4/D2O mixtures. DFT-calculations are in agreement with our
exper-imental results indicating that C4–H–O and C5–H–O interactions seem to play non-negligible roles
for BMM+BF−4/DMSO mixtures. © 2011 American Institute of Physics. [doi:10.1063/1.3526485]
I. INTRODUCTION
Ionic liquids show some interesting features, such as neg-ligible vapor pressure, ability of dissolving a variety of chemi-cals, and wide liquid range.1–8Their quite rapid emergence as alternative solvents has involved chemical reactions, liquid– liquid extractions, battery electrolytes, and gas storage, to mention just a few examples.1,2Research exploring the uses of ionic liquids is growing at an exponential pace. However, one of the barriers in the application of ionic liquids is the general lack of physical property data for these compounds toward fundamental understanding of the system. The physi-cal and chemiphysi-cal properties of ionic liquids can be fine-tuned through the selection of their cation and anion moieties.1,2 Consequently, ionic liquids can be made compatible with a wide range of materials. An important issue in the structures of ionic liquids is the relative geometry between the anion and cation comprising the bulk liquid, while the type of anion acts a significant role in the structural changes.2
Ionic liquids can be employed together with other sol-vents, which may affect their properties. Properties of binary liquid mixtures have been extensively studied to understand the nature and extent of various intermolecular interactions existing between different species present in the mixtures. Of particular interest are the effects of water and numerous studies on ionic liquid/water systems have been made.9–15 Compared with the intensive investigations on the ionic liq-uid/water mixtures, there have been a few reports on solvation
a)Author to whom correspondence should be addressed. Electronic mail: hc-chang@mail.ndhu.edu.tw; Fax:+886-3-8633570; Tel. +886-3-8633585.
behavior of ionic liquid in nonaqueous molecular liquids.16–20 In this contribution, we present a means of looking at physicochemical properties of ionic liquid/dimethyl sulfoxide (DMSO) mixtures, by using variable pressures as a window into the nature of charge-enhanced C–H–O interactions.
Among the common ionic liquids developed, the imidazolium-type ionic liquids derived from 1-alkyl-3-methyl imidazolium cations in association with weakly coordinating anions represent the most popular in many applications.1,2,21,22With three hydrogen atoms bound to the imidazolium ring, two or three resolved absorption bands are observed with 1-alkyl-3-methylimidazolium salts in the IR spectral region between 3000 and 3200 cm−1. These can be attributed to coupled aromatic C–H stretching vibrations. The relative acidity of the hydrogen atoms in the imidazolium ring is important for quantifying the effect of ionic liquids in the binary mixtures. The C2 proton is the most acidic proton on the imidazolium ring and may form hydrogen bonds with the anions. However, there is also experimental evidence that imi-dazolium salts will, under certain conditions, prefer to bind to transition metals at C4,5, that is the less-acidic sites.23In this article, we characterize the effect of hydrogen-atom substitu-tion by methyl group at C2 carbon of the imidazolium ring
on the physical properties of ionic liquids based on the BF−4 anion.
One of the attractive features of the 1-alkyl-3-methylimidazolium cation is its inherently amphiphilic character as a surfactant. Both experiments and simulations have found that the alkyl side-chain length has an influence on the supramolecular assemblies of ionic liquids.2,7,24Alkyl
044506-2 Jianget al. J. Chem. Phys. 134, 044506 (2011)
imidazolium cations with long enough alkyl chain length (n≥ 4) were characterized by the existence of structural orga-nization at the nanometer scale, as reported by Triolo et al.24 Studies were carried out to investigate the formation of asso-ciated species such as ion pairs and aggregates in ionic liquid mixtures.25–28The existence of a critical aggregation concen-tration (CAC) for imidazolium has been investigated in water and in organic solvents using several experimental techniques. Knowledge of the nature and strength of hydrogen bond-ing interactions is fundamental to understand the physical properties of ionic liquids. The hydrogen bonding interactions are complex for aqueous ionic liquids with varying anions, due to pronounced anion–water interactions. However, some research groups infer the non-negligible role of cation–water interactions.29,30 Miscibility of imidazolium-based ionic liq-uids with water depend on the side chain lengths at the cation.30 As this changes the solubility behavior, the cation may have some implications on the water–ionic liquid inter-actions. Various studies have been performed to elucidate the role of weak hydrogen bonds, such as C–H–O and C–H–X, in the structure of ionic liquids.29,31–33The cation–water interac-tion was deduced by Mele et al. from ROESY spectra.29One of the intriguing aspects of weak hydrogen bonds is that the C–H covalent bond tends to shorten as a result of formation of a hydrogen bond with a Lewis base. Hobza et al. suggested that the strengthened C–H bond originates from the electron density transfer from the proton acceptor to the remote part of the proton donor.34 Scheiner35 and Dannenberg,36 however, view conventional and C–H–O hydrogen bonds to be very similar in nature. The origin of both the red- and blue-shifted hydrogen bonds was concluded to be the same by the Schlegel group37 and the Hermansson group.38 One of the underlying reasons for this debate is the weakness of C–H–O interac-tions. The study of methods that enhance C–H–O interactions is crucial if we are to obtain further details of this important phenomenon. In this study, we present a means of looking at this phenomenon, by using variable pressure as a window into the nature of charge enhanced C–H–O interactions.
Changing the temperature of a chemical system at ambi-ent pressure produces a simultaneous change in thermal en-ergy and volume. To separate the thermal and volume effects, one must perform high-pressure experiments. Static pressures up to several megabars can be generated using diamond anvil cells. Nevertheless, the pressures used to investigate chemi-cal systems typichemi-cally range from ambient to several GPa. The static approach is of interest because it allows continuous tun-ing of the pressure and the possibility to employ a large num-ber of probing techniques that allow in situ measurements. In this article, we report how in situ high-pressure infrared spec-troscopy using a diamond anvil was applied to ionic liquid mixtures.
II. EXPERIMENTAL
Samples were prepared using
1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF−4 >97%, Fluka), 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMM+BF−4 >99%, Fluka), DMSO-d6 (99.9 at. % D,
Aldrich), and D2O (99.9% D, Aldrich). A diamond anvil cell
(DAC) of Merril–Bassett design, having a diamond culet size of 0.6 mm, was used for generating pressures up to ca. 2 GPa. Two type-IIa diamonds were used for mid-infrared measure-ments. The sample was contained in a 0.3-mm-diameter hole in a 0.25-mm-thick inconel gasket mounted on the diamond anvil cell. To reduce the absorbance of the samples, CaF2
crystals (prepared from a CaF2optical window) were placed
into the holes and compressed firmly prior to inserting the samples. A droplet of a sample filled the empty space of the entire hole of the gasket in the DAC, which was subsequently sealed when the opposed anvils were pushed toward one another. Infrared spectra of the samples were measured on a Perkin-Elmer Fourier transform spectrophotometer (model Spectrum RXI) equipped with a lithium tantalite (LITA) mid-infrared detector. The mid-infrared beam was condensed through a 5× beam condenser onto the sample in the diamond anvil cell. Typically, we chose a resolution of 4 cm−1 (data point res-olution of 2 cm−1). For each spectrum, typically 1000 scans were compiled. To remove the absorption of the diamond anvils, the absorption spectra of DAC were measured first and subtracted from those of the samples. Pressure calibration follows Wong’s method.39,40 Spectra of samples measured at ambient pressure were taken by filling the samples in a cell having two CaF2windows but lacking the spacers.
The Raman spectra were measured using a 100 mW diode pumped solid state laser (λ = 532 nm) and a micro-scopic based Raman spectrometer having a 300 mm spectro-graph (Acton SP308) and a side window photon counting de-tector system. Two type-Ia diamonds were used for Raman measurements
III. RESULTS AND DISCUSSION
The infrared spectrum of
pure1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF−4) exhibits five discernible peaks, i.e., 2877, 2939, 2964, 3122, and 3162 cm−1, respectively, in the 2800–3200 cm−1 region. The absorption bands at 2877, 2939, and 2964 cm−1 can be attributed to C–H stretching modes of the alkyl groups. The coupled imidazolium C–H stretching vibrations locate at 3122 and 3162 cm−1. Figure1 presents the concentration dependence of the maximum positions of the characteristic C–H bands of BMI+BF−4 upon dilution with DMSO-d6.
Looking into more detail in Fig. 1, we observe no drastic changes in the concentration dependence of the alkyl C–H band frequency. In contrast with the trend observed for alkyl C–H stretches, the imidazolium C–H bands (Fig. 1) display anomalous concentration-induced frequency shifts as the BMI+BF−4 is diluted. We observed no drastic changes in the imidazolium C–H band frequency at high concentration of BMI+BF−4, that is 0.6 < mole fraction (BMI+BF−4). This behavior may indicate a clustering of ionic liquid in polar region and a slight perturbation by the presence of DMSO-d6 at high concentration. The imidazolium C–H
absorption exhibits a decrease in frequency upon dilution at low concentration of ionic liquid, that is 0.6 > mole fraction (BMI+BF−4). A possible explanation for this effect is the C–H–O interactions between imidazolium C–H groups and DMSO-d6. The red-shifts of imidazolium C–H stretches
FIG. 1. Concentration dependence of the C–H stretching frequency of BMI+BF−4/DMSO-d6versus the mole fraction of BMI+BF−4.
FIG. 2. Pressure dependence of IR spectra of pure BMI+BF−4 under (a) am-bient pressure and at (b) 0.3 GPa, (c) 0.9 GPa, (d) 1.5 GPa, (e) 1.9 GPa, (f) 2.3 GPa, and (g) 2.5 GPa.
observed in Fig. 1 may be due to the replacement of an interaction between the imidazolium C–H and an anion with an interaction with DMSO-d6. Based on the results of
Fig.1, the imidazolium C–H groups seem to be more sensi-tive sites for C–H–O than the alkyl C–H groups. Several stud-ies have suggested that ionic liquids tend to segregate into stable non-polar regions by aggregation of the alkyl groups for C4 and longer and polar regions by charge ordering of
the anions and imidazolium rings.10,11,24 As shown in our previous reports, no appreciable changes in band frequency of the imidazolium C–H occurred as the BMI+BF−4 was di-luted by D2O.15 Nevertheless, the infrared absorption spectra
of DMSO-d6mixtures exhibit a decrease in imidazolium C–
H frequency upon dilution in Fig.1. Thus, DMSO-d6can be
added to significantly change the structural organization of BMI+BF−4. Our results in Fig.1indicate that the presence of DMSO-d6 perturbs the ionic liquid–ionic liquid associations
in the imidazolium region.
Figure 2 presents infrared spectra of pure BMI+BF−4 obtained under ambient pressure (curve a), and at 0.3 (curve b), 0.9 (curve c), 1.5 (curve d), 1.9 (curve e), 2.3 (curve f), and 2.5 GPa (curve g). As revealed in Figs. 2and S1,41 both the alkyl and imidazolium C–H stretching bands display nonmonotonic pressure dependences. They blue-shift initially [Figs. 2(a)–2(c)], then undergoes no change [Figs. 2(c) and 2(d)], and then blue-shift [Figs. 2(d)–2(g)] again as the pressure is elevated. This discontinuity in fre-quency shift in Figs. 2 and S1 is similar to the trend re-vealed in the pressure-dependent study of pure 1-butyl-3-methylimidazolium hexafluorophosphate (BMI+PF−6). The frequency shifts may originate from the combined effect of the overlap repulsion enhanced by hydrostatic pressure, the phase transition, C–H–F contacts, and so forth.
044506-4 Jianget al. J. Chem. Phys. 134, 044506 (2011)
FIG. 3. IR spectra of a BMI+BF−4/DMSO-d6 mixture (mole fraction of
BMI+BF−4: ca. 0.15) obtained under ambient pressure (curve a) and at 0.3 GPa (curve b), 0.9 GPa (curve c), 1.5 GPa (curve d), 1.9 GPa (curve e), 2.3 GPa (curve f) and 2.5 GPa (curve g).
Figure 3 displays the infrared spectra of a
BMI+BF−4/DMSO-d6 mixture having its mole fraction
of BMI+BF−4 equal to 0.15 obtained under ambient pres-sure (curve a), and at 0.3 (curve b), 0.9 (curve c), 1.5 (curve d), 1.9 (curve e), 2.3 (curve f), and 2.5 GPa (curve g). The imidazolium C–H bands are red-shifted to 3091 and 3153 cm−1 in Fig. 3(a) in comparison to the frequencies of 3122 and 3162 cm−1, respectively, in Fig.
2(a). The broad features for imidazolium C–H absorption in Fig. 3(a) suggest the presence of at least two species, i.e., associated and isolated structures.16,31,32 The associated species may be ion pairs or larger ion clusters and the isolated species may mean the dissociation into free ions or smaller ion clusters. Based on the concentration-dependent results in Fig. 1, the dominant imidazolium C–H species revealed in Fig. 3(a) should be assigned to the isolated (or dissoci-ated) structure. As the sample was compressed to 0.3 GPa [Fig. 3(b)], the imidazolium C–H stretching modes in the region 3000–3250 cm−1underwent dramatic changes in their spectral profiles with bandwidth narrowing. The spectrum in Fig. 3(b) shows the imidazolium C–H stretching locating at 3121 and 3168 cm−1, respectively. This behavior may
FIG. 4. IR spectrum of pure BMI+BF−4 (curve a) and pressure dependence of IR spectra of pure BMM+BF−4 under (b) ambient pressure and at (c) 0.3 GPa, (d) 0.9 GPa, (e) 1.5 GPa, (f) 1.9 GPa, (g) 2.3 GPa, and (h) 2.5 GPa.
be caused by the loss in intensity of the bands attributed to the isolated structures. In other words, the associated configuration is favored with increasing pressure by debiting the isolated form. The features of cation–anion interactions have attracted much attention recently, but the nature of the possible association is still under debate.16,26–28 Therefore, the investigation of pressure effect on the association– dissociation transformation may be important to provide further clues. As revealed in Figs. 3(b)–3(g), the spectral features of the C–H stretching modes show further evolution upon compression through the observation of monotonic blueshifts in frequency. The associated form, being a less stable form for BMI+BF−4/ DMSO-d6 under ambient
pres-sure [Fig. 3(a)], is switched to a more stable state under the condition of high pressures [Figs.3(b)–3(g)].
We obtained a complementary insight into the C–H stretching spectral features revealed in Figs. 4(b)–4(h) by measuring the pressure-dependent variations in the infrared spectra of pure 1-butyl-2,3-dimethylimidazolium tetrafluo-roborate (BMM+BF−4). To see the differences of the spec-tra for BMI+BF−4 and BMM+BF−4, the IR spectrum of pure BMI+BF−4 obtained at ambient pressure is also displayed in Fig. 4(a). As seen in Figs. 4(a) and 4(b), the imidazolium C–H absorptions undergo significant changes as the C2
car-bon has been methylated for BMM+BF−4. Figures4(b)–4(g)
FIG. 5. IR spectra of a BMM+BF−4/DMSO-d6mixture having mole fraction
of EMI+TFSA−equal to 0.14 obtained under ambient pressure (curve a) and at 0.3 GPa (curve b), 0.9 GPa (curve c), 1.5 GPa (curve d), 1.9 GPa (curve e), 2.3 GPa (curve f) and 2.5 GPa (curve g).
ambient pressure (curve b), and at 0.3 (curve c), 0.9 (curve d), 1.5 (curve e), 1.9 (curve f), 2.3 (curve g), and 2.5 GPa (curve h). As shown in Fig.4(b), the imidazolium C–H ab-sorptions appear at 3166 and 3198 cm−1 corresponding to coupled C4–H and C5–H stretching vibrations. In
compari-son to BMI+BF−4 [Figs.2 and4(a)], the coupled C4–H and
C5–H bands of BMM+BF−
4 display similar nonmonotonic
frequency shifts as the BMM+BF−4 was compressed. The cou-pled C4–H and C5–H bands blue-shift initially [Figs.4(b)and
4(c)], then undergoes no change [Figs. 4(c) and4(d)], and then blue-shift again [Figs.4(b)–4(h)]. On replacement of the H at C2 with CH
3 in BMM+BF−4, the hydrogen bonding
in-teraction between the cation and the anion may be reduced. However, the unexpected increases in viscosities and melting points for BMM+-based ionic liquids have been reported.42,43 This finding suggests that the C4–H and C5–H groups can play non-negligible roles in ion pairs and clusters. This behavior is in accord with the pressure-dependent results [Figs.2and4] indicating the similar trends observed for C2–H, C4–H, and
C5–H.
Figure 5 displays the infrared spectra of a
BMM+BF−4/DMSO-d6 mixture having its mole fraction
of BMM+BF−4− equal to 0.14 obtained under ambient
FIG. 6. IR spectra of a BMM+BF−4/D2O mixture having mole fraction of
BMM+BF−4 equal to 0.06 obtained under ambient pressure (curve a) and at 0.3 GPa (curve b), 0.9 GPa (curve c), 1.5 GPa (curve d), 1.9 GPa (curve e), 2.3 GPa (curve f), and 2.5 GPa (curve g).
pressure (curve a), and at 0.3 (curve b), 0.9 (curve c), 1.5 (curve d), 1.9 (curve e), 2.3 (curve f), and 2.5 GPa (curve g). As revealed in Fig. 5(a), the imidazolium C4–H and C5–H stretching modes underwent changes as DMSO-d
6was
added, and the spectrum shows a broad band centered at about 3140 cm−1with a shoulder at 3085 cm−1. The 3140 and 3085 cm−1 bands can be assigned as isolated-(or dissociated-) C–H or isolated-C–H interacting with DMSO-d6. As shown
in Fig. 5(b), compression leads to a loss of isolated C–H band intensity. This observation indicates that imidazolium C–H hydrogen-bonded networks can be modified by varying the pressure, and the sharper structure of imidazolium C–H band at ca. 3164 cm−1revealed in Fig.5(b)is in part due to the higher order and anisotropic environment in associated structures. The evolutions of the imidazolium C–H spectral features observed in Fig. 5 may arise from changes in local structures of C4–H and C5–H and the hydrogen-bond networks are likely perturbed by high pressures.
Figure6presents infrared spectra of a BMM+BF−4/ D2O
mixture having mole fraction of BMM+BF−4 equal to 0.06 ob-tained under ambient pressure (curve a), and at 0.3 (curve b), 0.9 (curve c), 1.5 (curve d), 1.9 (curve e), 2.3 (curve f), and 2.5 GPa (curve g). The C–H stretching absorptions overlap
044506-6 Jianget al. J. Chem. Phys. 134, 044506 (2011)
with the O–H stretching bands of H2O, so we measured the
infrared spectra in a solution of D2O, rather than H2O in
Fig.6. Looking into more detail in Fig.6(a), we observe no drastic change in the concentration dependence of the imi-dazolium C–H spectral features. This behavior may suggest the association or clustering of BMM+BF−4 in aqueous solu-tions and a slight perturbation by the presence of D2O. This
result is remarkably different from what is revealed for the imidazolium C–H groups in Fig.5. The results of Fig.6also suggest that the associated structures are the favorable con-figurations up to the pressure of 2.5 GPa [Fig. 6(b)]. Re-cent investigations have suggested that the structure of ionic liquids exhibits spatial heterogeneity that results from their polar/nonpolar phase separation.10,11,24 The association or aggregation of ionic liquids even in aqueous solution seems to be a general trend as revealed in Fig.6. However, our results in Fig.5indicate that the presence of DMSO-d6significantly
perturbs the ionic liquid–ionic liquid associations in the polar region.
We perform density functional theory (DFT) calcula-tions using the GAUSSIAN program package.44 Several
sta-ble complexes of BMI+/BMM+ with DMSO or ion pairs (BMM+BF−4, BMM+BF−4) with DMSO have been opti-mized. Figure S2 (Ref. 41) and Table I display some of the most stable DFT-calculated results of BMI+DMSO [Figs. S2(a) and S2(b)], BMM+DMSO [Figs. S2(c) and S2(d)], BMI+BF−4DMSO [Figs. S2(e) and S2(f)], and BMM+BF−4DMSO [Figs. S2(g) and S2(h)] complexes. We employed the B3LYP functional together with a standard 6– 31+G* basis set. The geometry optimizations were made by analytical determinations of the first and second deriva-tives of the total energy. The energetically favored approach for the DMSO molecule to interact with the BMI+ cation is through the formation of C2–H–O and CR–H–O [Figs. S2(a) and S2(b); R denotes the alkyl group], whereas the former plays more important role due to the shorter dis-tance. This fact could be related to the well-known acidity of C2–H.1,2The substitution for a methyl group at the 2-position, i.e., BMM+ cation, eliminates the hydrogen bonding inter-action at C2 [Figs. S2(c) and S2(d)], so C–H–O
interac-tions are more favorable at C5–H for Fig. S2(c) and at C4–H
for Fig. S2(d), respectively. The C–H–O distance in Fig. S2(a) (1.9386 Å) or Fig. S2(b) (1.9523Å) is a little shorter than that in Fig. S2(c) (2.0653 Å) or Fig. S2(d) (2.0658 Å), which reveals C2–H–O interaction being stronger. TableIindicates
that the interaction energy in BMI+DMSO is larger than that in BMM+DMSO by ca. 3 kcal/mol, and C4–H (total inter-action energy = 14.42 kcal/mol) is a slightly better proton donor than C5–H (total interaction energy= 13.76 kcal/mol) due to steric effects of the butyl group adjacent to the C5–H group and the positive inductive effect caused by the elec-tron pushing butyl group.45This observation is in agreement with our experimental results in Fig. 5 indicating that C4–
H–O and C5–H–O interactions seem to play non-negligible
roles. As the anion (BF−4) is included in the calculation [see Figs. S2(e)–S2(h) and TableI], the interactions in complexes become complicated and the total interaction energy signifi-cantly increases due to the existence of more C–H–O and C– H–F interactions in BMI+BF−4DMSO [Figs. S2(e) and S2(f)],
TABLE I. Calculated relative energies (hartree/mol) and total interaction energies (kcal/mol)
Speciesa,b Relative energies −E
DMSO −553.120468 BF−4 −424.553137 BMI+ −422.959462 BMM+ −462.258348 a −976.107254 16.66 b −976.108394 17.25 c −1015.401749 13.76 d −1015.402847 14.42 e −1400.779635 90.64 f −1400.779255 90.42 g −1440.075060 88.14 h −1440.072819 86.80
aStructures illustrated in Fig. S2.
bNumbering of the skeleton atoms for the BMI+cation.
N N 1 2 3 6 4 5 7 8 9 10
and BMM+BF−4DMSO [Figs. S2(g) and S2(h)] complexes. This behavior indicates that cohesion in ionic liquid is strong and the local organization between ionic species is prelimi-nary governed by electrostatic interaction. The calculations of larger clusters may be interesting, but the number of low-lying isomers increases exponentially and the structural identifica-tion is complicated. Our calculaidentifica-tions show that the total inter-actions energies in many stable optimized BMI+BF−4DMSO and BMM+BF−4DMSO complexes are close to ca. 90 and 85 kcal/mol, respectively. Figures 2(e)–2(h) illustrate that DMSO can interact with cation through the C–H–O interac-tion and interact with anion via C–H–F interacinterac-tions.
Figure 7 shows the Raman spectra in the region of C–D stretching of BMI+BF−4/DMSO-d6 mixture (curve a–
g) and pure DMSO-d6 (curve h), respectively. As revealed
in Fig. 7(a), the spectrum has symmetric and asymmetric C–D stretching modes locating at 2143 and 2260 cm−1, respectively, and the appearance of a shoulder at approxi-mately 2116 cm−1 should be attributed to the association of DMSO-d6with anions (BF−4) via C–D-F interactions. As the
mixture was compressed, Figs. 7(b)–7(g) show the loss in intensity of the band at 2143 cm−1. This observation sug-gests that the associated configuration via C–D–F is favored with increasing pressure by debiting the 2143 cm−1 band. Figure 7(h)illustrates the experimental Raman spectrum of pure DMSO-d6 obtained under the pressure of 2.5 GPa and
the asymmetric C–D stretching mode becomes two sepa-rated bands at 2260 and 2278 cm−1. In light of our previ-ous report,46we anticipate that the high frequency component at 2278 cm−1 is attributed to C–D–O interactions between neighboring DMSO-d6 molecules or adjacent unit cells.
In agreement with the prediction of our DFT-calculations, the Raman spectra suggest the non-negligible role of DMSO-anion association via C–H–F interaction. The S=O stretching frequencies at about 1000 cm−1 should be a
FIG. 7. Raman spectra of a BMI+BF−4/DMSO-d6mixture (mole fraction of
BMI+BF−4: ca. 0.5) obtained under ambient pressure (curve a) and at 0.3 GPa (curve b), 0.9 GPa (curve c), 1.5 GPa (curve d), 1.9 GPa (curve e), 2.3 GPa (curve f) and 2.5 GPa (curve g). Curve h shows the C-D stretching of pure DMSO-d6under the pressure of 2.5 GPa.
sensitive probe for the interactions between cations and DMSO. Unfortunately, the S=O stretching absorption over-laps with the absorption of BMI+BF−4 [Fig. 1(a) in Ref.18].
IV. CONCLUSION
The pressure-dependent behaviors for BMI+BF−4/ DMSO-d6 and BMM+BF−4/DMSO-d6 mixtures exhibit
transformations at high pressures including a transition from isolated configurations to associated forms. The associated forms are stable up to the pressure of 2.5 GPa. This obser-vation indicates that imidazolium C–H hydrogen-bonded networks can be modified by varying the pressure. A possible explanation for this effect is the C–H–O interactions between imidazolium C–H groups and DMSO-d6. Imidazolium
C–H groups seem to be more favorable sites for C–H–O than the alkyl groups. In contrast to DMSO-d6 mixtures,
the local organization of imidazolium C–H groups in D2O
mixtures is slightly perturbed by the presence of D2O. Our
DFT-calculations suggest that all imidazolium C–H groups, i.e., C2–H, C4–H, and C5–H, play non-negligible roles.
ACKNOWLEDGMENTS
The authors thank the National Dong Hwa University and the National Science Council (Contract No. NSC 98-2113-M-259-005-MY3) of Taiwan for financial support. The authors also thank Hou-Kuan Li and Yu-Fang Yeh for their assistance.
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