Journal of Molecular Liquids 190 (2014) 151–158

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Journal of Molecular Liquids

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Hydrogen bonding interaction between acetate-based ionic liquid1-ethyl-3-methylimidazolium acetate and common solvents

Yu Chen, Yuanyuan Cao, Xiaofu Sun, Tiancheng Mu ⁎Department of Chemistry, Renmin University of China, Beijing 100872, PR China

⁎ Corresponding author. Tel.: +86 10 62514925; fax: +E-mail address: tcmu@chem.ruc.edu.cn (T. Mu).

0167-7322/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.molliq.2013.11.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 September 2013Received in revised form 7 November 2013Accepted 7 November 2013Available online 21 November 2013

Keywords:Hydrogen bond1-Ethyl-3-methylimidazolium acetateWaterOrganic solvent

The hydrogen bonding interaction between acetate-based ionic liquids (AcILs) 1-ethyl-3-methylimidazolium ac-etate [EMIM][Ac] and seven solvents (deuterated water D2O, deuterated methanol CD3OD, deuterated acetoni-trile CD3CN, deuterated dimethylsulfoxide d6-DMSO, deuterated acetone CD3COCD3, deuterated benzene C6D6,and deuterated chloroformCDCl3)was investigated in theirwhole concentrations byATR-IR and 1HNMR. Resultsshow that both [EMIM][Ac] and solvent chemical shifts δ present a different change in the 1H NMR in differentsolvents. ATR-IRwavenumber shifts ν of [EMIM][Ac] and solvents also present corresponding variation tendency.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids (ILs) are organic salts but showing liquid state aroundthe room temperature [1]. Up to now, ILs have attractedmuch attentionboth in the laboratory research and industrial application. They aredeemed as “green” solvents for their negligible vapor pressure andhigh thermal stability in the normal operating conditions [2–4]. ILscould also be “designed” or “tuned” by selecting the appropriate cation,anion, or both; Thus ILs have been widely applied in organic synthesisand chemical reaction [5,6], electrochemistry [7,8], emulsions andnanocrystal [9,10], extraction and separation processes [11,12], and soon.

Among all kinds of ILs studied, acetate-based ILs (AcILs) were paidparticular attention, mainly in the field of cellulose dissolution. AcILs usu-ally have higher cellulose dissolution than organic solvents or other ILs.Cellulose is the most abundant biorenewable material, thus theirderivitized products are widely used in the industry of fiber, membrane,paper, polymer, and paints [13]. Unfortunately, most common organicsolvents (e.g., water) dissolve negligible cellulose. The current celluloseprocessing solvents might also suffer from many drawbacks, such as therequirement of low melting points (e.g., by N-ethylpyridinium chloride),the threat of toxicity (e.g., by N-methylmorpholine-N-oxide NMNO,phosphoric acid), and the low regeneration efficiency (e.g., bydimethylacetamide (DMAC)/LiCl) [14,15]. ILs were thus proposed to dis-solve cellulose due to their good physical properties and high tunability.Cellulose is nearly insoluble in the conventional ILs (e.g. [BMIM][BF4])[16], but very soluble in the chloride-based (e.g., [BMIM][Cl] [16]), allyl-based (e.g., [AMIM][Cl] [17]), and acetate-based (AcILs, e.g., [EMIM][Ac]

86 10 62516444.

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[18,19]) ILs. The first two kinds of ILs might be corrosive, toxic, and envi-ronmentally unfriendly due to the presence of halogen elements [20,21].AcILs are thus considered the relatively favorable candidates to dissolvecellulose because of thewonderful properties (e.g., low toxicity, lowmelt-ing point, low corrosiveness, lowviscosity and goodbiodegradability) andthe favorable solubility (e.g., solubility of cellulose in [EMIM][Ac] wasabout two times than that of [BMIM][Cl]) [19]. The industrial applicationfor the cellulose dissolutionwith [EMIM][Ac] has already been conductedby BASF [22].

The above discussion suggests that AcILs are so important in the ap-plication of cellulose dissolution. However, most ILs (either hydropho-bic or hydrophilic) had suffered from the disadvantages of absorbingwater from the moisture environment [23–30]. Also, water is ubiqui-tous, meaning that contacting with water is almost inevitable for ILs.More importantly, the presence of water affects the microscopicstructure [31–33], physical property [24,34,35], and chemical reactivity,hence the application of ILs [36,37]. For example, cal. 1 wt.%water couldmake the cellulose from soluble to insoluble in ILs [16]. It might be se-verer for AcILs due to their higher hydrophilic anion [26]. Likewise,the presence of organic solvent also alters the structure, property, andthe corresponding application of ILs [34,38–42].

Therefore, investigation of their interaction mechanism with com-mon solvents (i.e., water and organic solvent) is necessary. There areonly several reports related to the interaction mechanism betweenAcILs and water, to the best our knowledge. One report by Shi et al.[43] focused on the interaction mechanism between AcILs (i.e.,[EMIM][Ac], [HMIM][Ac], [P4444][Ac]) and water investigated by abinitio simulation methods and pulse field gradient NMR spectroscopy;the results were that water–anion interaction predominates over thewater–water and water–cation interaction at most water concentra-tions. Instead, an earlier study investigated the interaction between

152 Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

[EMIM][Ac] and water by DFT and IR spectroscopies [44]; the resultsshowed that [EMIM][Ac] interacted with water mainly by hydrogenbonding. Hall et al. [45] investigated [EMIM][Ac]/water interaction bymacroscopic and microscopic methods; they found that the maximaldeviations were all found at about 0.74 mol% of water, correspondingto approximately three water molecules per [EMIM][Ac]. Brehm et al.[46] concluded that hydrogen bonds exist in the pure [EMIM][Ac] andthe presence of water modifies the hydrogen bond network, the forma-tion of carbenes, and the dipole moment of the ions of [EMIM][Ac]. Itcould be easily seen that the investigations on interaction betweenAcILs and water are limited. Nevertheless, the interaction betweenAcILs and organic solvent has not been reported, as we know.

So,we systematically investigate theAcILs/solvent interaction in thisstudy. [EMIM][Ac] (Scheme 1) was selected as the representative AcILs.Deuterated water (D2O), deuterated methanol (CD3OD), deuteratedacetonitrile (CD3CN), deuterated dimethylsulfoxide (d6-DMSO), deu-terated acetone (CD3COCD3), deuterated benzene (C6D6), and deuterat-ed chloroform (CDCl3) were selected as the representative commonsolvents. The whole concentration range for [EMIM][Ac]/solvents mix-tures by 1H NMR and attenuated total reflection infrared (ATR-IR) wasinvestigated. This whole concentration range was studied for the pur-pose of explaining the dynamic solvation process.

2. Experimental section

2.1. Materials

[EMIM][Ac] (Scheme 1, purity N 98.5%) was purchased from Lan-zhou Greenchem ILs, LICP, CAS (Lanzhou, China). [EMIM][Ac] wasdried in a vacuum oven at 50 °C for 96 h. After the drying, water con-tents (less than 498 ppm, measured by Karl Fisher titration, Karl FisherZDJ-400S, Multifunctional titrator, Beijing Xianqu Weifeng Company,Beijing, China), halogen ion (undetectable, measured by AgNO3 precip-itation) and other impurities (undetectable, measured by NMR, BrukerAM 400 MHz spectrometer) were determined. The seven solventsD2O, CD3OD, CD3CN, d6-DMSO, CD3COCD3, C6D6 and CDCl3 (all 99.9atom % D) were all purchased from J & K Chemical Limited.

2.2. ATR-IR spectra

Multiple attenuated total reflection (ATR) cell (ATR-8200H) wasmade of ZnSe crystal with an incident angle of 45°. The mixtures of[EMIM][Ac]–solvent were put on the top of the crystal for the purposeof covering the 584 mm2 surface. The ATR-IR spectra were recordedfrom 400 to 4600 cm−1 at room temperature (~25 °C) using aPrestige-21 FTIR spectrometer (Shimadzu, Japan). The spectrometerwas equipped with a DTGS detector. Specifically, 40 scans werecomplemented with a resolution of 4 cm−1 for each sample.

Scheme 1. Structure and notation of 1-ethyl-3-methylimidazolium acetate [EMIM][Ac].

2.3. 1H NMR spectra

A certain concentration interval of [EMIM][Ac]–solvents (i.e., D2O,CD3OD, CD3CN, d-DMSO, CD3OCD3, C6D6, and CDCl3) binary mixtureswas prepared by weighing methods. The mole fractions of solvent in[EMIM][Ac]–solvent mixtures vary from 0 (i.e., pure AcIL), then cal.0.01, and then from cal. 0.1 to cal. 0.9 at an interval of cal. 0.1, andthen cal. 0.98 and 1 mol fractions (i.e., pure solvents) at last. Note thatall the [EMIM][Ac]–solvent mixtures were homogenous except forCD3CN in the concentration cal. 0.9-1, for C6D6 cal. 0.8-1, and for CDCl3cal. 0.5–0.9. Afterwards, each sample was measured using ATR-IR spec-tra (as above experimental method mentioned) and 1H NMR spectra.The 1H NMR spectra were obtained on a Bruker DMX 300 NMR spec-trometer (300 MHz) at 298 Kwith TMS as the internal standard, as sug-gested by Yu et al. [31,47].

3. Results and discussion

The chemical structure and atom notation of [EMIM][Ac] was de-scribed in Scheme 1. The NMR spectra of [EMIM][Ac]–solvents werelisted in Figs. 1, 2, 3 and 4; the ATR-IR spectra of [EMIM][Ac]–solventswere listed in Fig. 5.

3.1. [EMIM][Ac]–D2O interaction

The absolute change of 1H NMR chemical shifts δ for [EMIM][Ac] and the solvent D2O was displayed in Fig. 1. Results showthat the δH of [EMIM][Ac] in D2O are ordered as: δH2 N δH4 ≈δH5 N δD2O N δH7 N δH6 N δHb N δH8 (Fig. 1). Note that thereis only minor difference in chemical shift δ between H4 and H5of [EMIM][Ac] in D2O (also in other six solvents investigated),so they were neglected in the discussion because of their highsimilarity.

The relative change (compared with that of the pure [EMIM][Ac],i.e., referred chemical shift δ) of 1H NMR chemical shifts Δδ for [EMIM][Ac] as a function of solvent D2O was displayed in Fig. 2. The order ofΔδ is as below: ΔδHb N ΔδH8 N ΔδH7 N ΔδH6 N ΔδH5 N ΔδH4 N ΔδH2.In most cases, the values of Δδ are minus except ΔδHb and ΔδH8 in theextremely high concentration of D2O. δH6, δH7, δH8 and δHbhave amin-imal value while ΔδH5, ΔδH4, and ΔδH2 are always decreasing. Specifi-cally, δH2, δH4 and δH5 move upfield in the whole concentration ofD2O (the below is the same), while δH6, δH7, δH8 and δHbmove upfieldin the low concentration but move downfield in the high concentration.Particularly, only the values of δH8 and δHb are bigger than that of thereferred pure [EMIM][Ac] in the extremely high concentration of D2O.It indicates that the decrease of hydrogen bond between H (i.e., H2, H4and H5) and acetate anion outweighs the increase of hydrogen bond be-tween H (i.e., H2, H4 and H5) andwater when increasing the concentra-tion of D2O. Yu et al. also draw a similar conclusion, i.e., the presence ofwater weakens cation/anion hydrogen bonding by introducing a lessstronghydrogen bondingwith cation [31,47]. However, for the hydrogenbond related to H6, H7, H8, and Hb, when increasing the concentrationof D2O, the decrease (H–anion hydrogen bond) overtakes the increase(H-water hydrogen bond) in the low concentration of D2O, but falls be-hind in the high concentration of D2O. It indicates that the weakening ofcation/anion hydrogen bonding is dominating in the low concentrationof D2O, while the forming of cation/water hydrogen bonding is dominat-ing in the high concentration of D2O. In the high concentration of solvents(cal. 0.8 mol. fra.), theremight be a nearly complete solvation around theH6, H7, H8 and Hb and a nearly complete dissociation of H–anion.

The ATR-IR spectra of [EMIM][Ac] as a function of D2O were alsoinvestigated (Fig. 5). The blue shift of νC2\H, νC4\H and νalkyl-H(including νC6\H, νC7\H, νC8\H) also suggests a dominating de-crease of H–anion hydrogen bond compared to that of an increase ofH-water hydrogen bond (Fig. 5). The red shifts of νC_N mean a cat-ion–water–cation sandwiched shape, which is formed by the disruption

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Fig. 1. Absolute change of 1H NMR spectra for [EMIM][Ac] and solvents as a function of the corresponding solvents.

153Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

of cation ring pile after introducing the water molecule with a highwater–cation hydrogen-bonding interaction. Andrea et al. [48] investi-gated [BMIM][BF4]–water interaction by NMR spectroscopy throughintermolecular nuclearOverhauser enhancements (NOEs) and also con-cluded that increasing water content would lead to a lower degree ofthe imidazolium–imidazolium stacking. Li et al. [49] attributed it tothe change from IL (with the model IL [EMIM][BF4] as a function ofwater content) network to ionic cluster, then to ionic pairs. The redshifts of νO\D when increasing the concentration of D2O indicate anoverall increase of hydrogen bond interaction among water, i.e., alongwith increasing water concentration, the hydrogen bond betweenwater and [EMIM][Ac] is disrupted while the hydrogen bond amongwater is established. However, the effect of the former is less than thelatter thus leading to the overall increase of hydrogen bond interactionamong water. Reflection in IR spectra is the red shifts of νO\D. Notethat the IR spectra could not easily obtain themessage for the hydrogenbonding interaction at the extremely high concentration of D2O, but 1HNMR could.

3.2. [EMIM][Ac]–CD3OD interaction

The absolute change of 1HNMR chemical shifts δ for [EMIM][Ac] andthe solvent CD3OD was displayed in Fig. 1. Fig. 1 shows that δH of[EMIM][Ac] in CD3OD is similar to that of [EMIM][Ac] in D2O (exceptsolvent peak), i.e., δH2 N δH4 ≈ δH5 N δH7 N δH6 N δHb N δH8 N

δCD3OD. The order of Δδ of [EMIM][Ac] in CD3OD is the same withthat of [EMIM][Ac] in D2O (Fig. 2). All the ΔδHs have a minimal value

except for ΔδH2. The change type of Δδ is described as below: H2moves upfield in the whole concentration, while H4, H5, H6, H7, H8,and Hb move upfield first then move downfield. Specifically, H4 and H5moveupfield drastically in themost of concentration butmove downfieldonly a little in the extremely high concentration; H6, H7, H8 and Hb areconverse (move downfield drastically in the most of concentration butmove upfield only a little in the extremely low concentration).

The relative change of 1H NMR chemical shifts for H in [EMIM][Ac]could be used to interpret the difference in hydrogen bonding betweenIL–D2O and IL–CD3OD (Fig. 3). The value of Δδ for each H of [EMIM][Ac]in CD3OD is bigger than that in D2O (Fig. 3). Specifically, when increas-ing the concentration of solvents, the tendency of ΔδH2 is similar forthem; the tendencies of ΔδH4 and ΔδH5 are similar in the low concen-tration but CD3OD shows an increase in the extremely high concentra-tion; the tendencies of δH6, H7, H8 and Hb all have a minimum valuewhile CD3OD shows a stepper transition. It indicates that theweakeningofH–anionhydrogenbonding in CD3OD is less strong,while the formingof H–solvent hydrogen bonding in CD3OD is stronger than that in D2O.Yu et al. [47,50] also witnessed a similar result (except in the highconcentration of solvents) and ascribed it to the methyl group inCD3OD (compared with D2O) which had a positive contribution on theformation of hydrogen bond. In the high concentration of solvents(cal. 0.8 mol. fra.), there might be a nearly complete solvation aroundthe H4, H5, H6, H7, H8 and Hb and a nearly complete dissociationof H–anion.

Fig. 4 presents the 1H NMR spectra of solvents as a function of[EMIM][Ac]. The value of Δδ for D2O (positive) is bigger than that for

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154 Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

CD3OD (negative). It means that hydrogen bonding among CD3OD iseasier to break than among D2O when increasing [EMIM][Ac]. It givesan indirect evidence that the fragile CD3OD (due to a more breakablehydrogen bonding among them upon AcILs) could form hydrogenbond easier with [EMIM][Ac] than the hard D2O (due to a less breakablehydrogen-bonding among them upon AcILs).

The ATR-IR blue shift of νC\D and νO\D in increasing concentra-tion of CD3OD (Fig. 5) might be the result of its positive charge effectafter exposing to the AcILs [47,50]. Comparing with the red shift ofνO\D in increasing concentration of D2O, it could be concluded that hy-drogen bond among D2O is easier to be established in [EMIM][Ac] thanthat among CD3OD when increasing the concentration of solvent.

3.3. [EMIM][Ac]–CD3CN interaction

The absolute change of 1H NMR of [EMIM][Ac] in CD3CN is orderedas: δH2 N δH4 ≈ δH5 N δH7 N δH6 N δCD3CN N δHb N δH8 (Fig. 1).Only the order of δCD3CN is different with that of δCD3OD and δD2O.Fig. 2 gives a more obvious difference. ΔδH2, ΔδH4 and Δδ H5 presenta more accelerating upfield shift in the extremely high concentrationof CD3CN, while CD3OD and D2O do not. It indicates that H2–anion,H4–anion and H5–anion pairs would dissociate more completely inCD3CN than in CD3OD and D2O; H2, H4 and H5 would be totally

surrounded by CD3CN after replacing the position of water in the ex-tremely high concentration of CD3CN.

Comparing the Δδ of each H, we found that all the types of H in D2Ohave the least value, while the order of H in CD3OD and CD3CN is com-plicated (Fig. 3). Specifically, the values of ΔδH2, ΔδH6, ΔδH7 and ΔδH8in CD3CNare bigger than that in CD3OD; the values ofΔδH4andΔδH5 inCD3CN are bigger than that in CD3OD except in the extremely high con-centration of solvent; the value of ΔδHb in CD3CN is similar to that inCD3OD. It indicates that in most cases, CD3CN has stronger ability toform hydrogen bond with H2, H4, H5, H6, H7 and H8 than CD3OD andD2O. CD3CN could possess the same ability to form hydrogen bondwith Hb of the anion when compared with CD3OD.

The hydrogen bonds among solvents are shown in Fig. 4. D2O tendsto increase the overall hydrogen bond with AcIL when increasing theAcIL content, i.e., the disruption of hydrogen bond among D2O is lessprominent than the establishment of hydrogen bond between D2Oand AcIL with the addition of AcIL. However, the patterns for CD3CNand CD3OD are opposite. More specifically, CD3CN tends to increasemore overall hydrogen bond than CD3OD as a function of AcIL.

ATR-IR of CD3CN, as well as D2O and CD3OD, is also pictured (Fig. 5).The obvious feature is that νC\N in the imidazolium cation shows ablue shift, which is opposite to that in D2O. It might be due to the higherability of CD3CN to form hydrogen bond with the imidazolium cationthan that of D2O; the formation of CD3CN-cation hydrogen bond

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155Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

interaction could thus be easier to outweigh the disruption of cation–cation piling. Another feature is the blue shift of νC`N in CD3CN. Itmeans a decrease of overall hydrogen bond related to the solvent(solvent–solvent and solvent–IL) as a function of CD3CN. Namely, the

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overall hydrogen bond of CD3CN increases as a function of [EMIM][Ac], which is consistent with the result obtained from NMR in Fig. 4.

3.4. [EMIM][Ac]–d6-DMSO interaction

The absolute change of 1H NMR for [EMIM][Ac] in d6-DMSO is thesame with that in CD3CN (Fig. 1). The change types of 1H NMR andATR-IR for [EMIM][Ac] are also similar for them (Figs. 2 and 5). Itmight be due to the similar chemical structure CD3ROCD3 (R = C orS) of those two solvents.

Another interesting finding is thatΔδH2,ΔδH4 andΔδH5 tend to co-incide in the two above solvents in the extremely high concentration ofsolvents (Fig. 2). It might be due to the similar change of H2, H4 and H5upon high dilution. Chang et al. [51] found that all the H in theimidazolium ring (for [BMIM][BF4] and [BMMIM][BF4]) play non-negligible roles to interact with d6-DMSO. Instead, we noticed two dif-ferences. First, ΔδH8 is the most positive H in d6-DMSO while inCD3CN ΔδHb has the biggest value in the extremely high concentrationof solvents. It implies that H8 is more affiliatedwith d6-DMSOwhile Hbis more affiliated with CD3CN. Second, ΔδH2 moves upfield more thanΔδH4 andΔδH5 in d6-DMSO in the extremely high concentration of sol-vent, while between ΔδH4 and ΔδH5 in CD3CN. One possible explana-tion is that the H2–anion hydrogen bond suffers from more drasticdisruption in d6-DMSO than in CD3CN when extremely diluted bysolvent.

The Δδ of solvents upon the addition of AcIL is both negative for d6-DMSO and CD3CN (Fig. 4), indicating an overall disruption of hydrogen

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1.6

2.0

2290 2280 2270 2260 2250

sC-Das

C-D

C-D

C=N

Alkyl-C-H

C2-H

C4/5-H

C6D

6

Abs

orba

nce

0.0

0.4

0.8

1.2

1.6

2.0

2300 2250 2200 2150 2100

C-D

C=O

Alkyl-C-H

C2-H

C4/5-H

CD3COCD

3

Abs

orba

nce

sC-Das

C-D

0.0

0.4

0.8

1.2

1.6

2.0

2300 2250 2200 2150 2100

C-D

C=N

Alkyl-C-H

C2-H

C4/5-H

d6-DMSO

Abs

orba

nce

sC-D

asC-D

0.0

0.4

0.8

1.2

1.6

2.0

2300 2250 2200 2150 2100

C-D

C-D

C=N

Alkyl-C-HC2-H

C4/5-H

CDCl3

Abs

orba

nce

0.0

0.4

0.8

1.2

1.6

2.0

2270 2260 2250 2240 2230 2220

C-D

C-D

C = N C=N

C = N

Alkyl-C-H

C2-H

C4/5-H

CD3CN

Abs

orba

nce

3200 2800 2400 1600 1500

0.0

0.4

0.8

1.2

1.6

2.0

O-DC=N

D2O

Alkyl-C-HC2-H

C4/5-H

wavenumber / cm-1

Abs

orba

nce

3200 2800 2400 1600 1500wavenumber / cm-1

3200 2800 2400 1600 1500wavenumber / cm-1

3200 2800 2400 1600 1500wavenumber / cm-1

3200 2800 2400 1760 1680wavenumber / cm-1

3200 2800 2400 1600 1500wavenumber / cm-1

3200 2800 2400 1600 1500wavenumber / cm-1

Fig. 5. ATR-IR spectra of [EMIM][Ac] (compared with the pure [EMIM][Ac]) as a function of solvents. The green arrow represents the change of absorbance; the red and blue arrows rep-resent the red and blue shift compared with the pure AcIL, respectively.

156 Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

bond. Specifically, the value of Δδd6-DMSO is less negative thanΔδCD3CN. It suggests that hydrogen bond among d6-DMSO is easierto be broken by the added [EMIM][Ac] than among CD3CN. Namely,d6-DMSO is easier to formhydrogenbondwith [EMIM][Ac] thanCD3CN.

3.5. [EMIM][Ac]–CD3COCD3 interaction

The order of absolute change of 1H NMR for [EMIM][Ac] inCD3COCD3 is similar to that in CD3CN and d6-DMSO (Fig. 1). A notabledifference is the abrupt upfield shift of H2 in the extremely high concen-tration of CD3COCD3. This difference would be more obvious in therelative change of 1H NMR for [EMIM][Ac] (Fig. 2).

Fig. 2 also shows a distinct feature that all the values of ΔδH are pos-itive except for ΔδH2 in the high concentration of CD3COCD3 about 0.99(mol. fra.). It indicates that all kinds of H in [EMIM][Ac] are very easy toform stronger hydrogen bondwith CD3COCD3 than thedisruption of hy-drogen bond between H and anion; only H2–anion is totally dissociatedin the extremely high concentration of CD3COCD3, which is similar tothat of CD3CN and d6-DMSO.

Moreover, there is one turning point for ΔδH4, ΔδH5, ΔδH6 andΔδH7 in CD3COCD3 (Figs. 2 and 3). The reason might be that before theturning point, the formation of hydrogen bond is dominating, but dis-ruption of hydrogen bond is overwhelming after the turning point. How-ever, there are two turning points for ΔδHb andΔδH8 (Figs. 2 and 3).

Explanation for the first turning point is the same. The appearance ofthe second turning point might be due to the high ability of Hb and H8(good donor) to form hydrogen bond with CD3COCD3 (good acceptor)even when CD3COCD3 is very concentrated. The two turning points aredistinctive in CD3COCD3 because CD3COCD3 is a better hydrogen bondacceptor than other solvents due to the presence of C_O group.The red shifts of νC\D and νC_O in CD3COCD3 when increasing thesolvent concentration also means the good hydrogen bonding ability ofCD3COCD3 (Fig. 5).

3.6. [EMIM][Ac]–CDCl3 interaction

One distinct feature for the [EMIM][Ac]–CDCl3 interaction is that thevalue of ΔδH2 is greater than that of ΔδH4 and ΔδH5 in the whole con-centration of CDCl3;ΔδH2 slightlymoves upfield in the concentration ofCDCl3 below 0.8, while drasticallymoves downfield to themost positivevalue in the higher concentration (Figs. 1 and 2). It means that in thelow concentration of CDCl3 the dissociation of H2–anion hydrogenbond is a little stronger than the establishment of H2–CDCl3 hydrogenbond, while CDCl3 could form hydrogen bond with H2 very strongly inthe high concentration of CDCl3. Actually, in the extremely high concen-tration there aremore CDCl3 to formhydrogen bondwithH2 than othertypes of H.

157Y. Chen et al. / Journal of Molecular Liquids 190 (2014) 151–158

Another distinct feature is that the value of ΔδCDCl3 is the greatestcompared with other solvents when increasing the concentration of[EMIM][Ac] (Fig. 4). One possible explanation is that hydrogen bond be-tween CDCl3 and AcIL is stronger than the hydrogen bond among CDCl3.It also gives an indirect evidence for the abrupt downfield shift of ΔδH2in the high concentration.

When increasing the concentration of CDCl3, νC\D shows blue shift(Fig. 5). It also indicates that hydrogen bond between CDCl3 and AcIL isstronger than the hydrogen bond among CDCl3. However, the stackingof imidazolium ring is less affected after introducing the CDCl3, takingthe cue from the nearly no shift of νC_N. It might be due to thatCDCl3 is more favorable to interact with the imidazolium ring fromthe flank of ring rather than above the ring.

3.7. [EMIM][Ac]–C6D6 interaction

The order of absolute change of 1H NMR for [EMIM][Ac] in C6D6 issimilar to that in D2O and CDCl3, i.e., δH2 N δH4 ≈ δH5 N δsolvent NδH7 N δH6 N δHb N δH8 (Fig. 1). Moreover, the abrupt upfield shift ofH2 in the extremely high concentration of C6D6 is the same as that ofd6-DMSO, CD3COCD3 and CD3CN (Figs. 1 and 2).

However, the relative change of 1H NMR for [EMIM][Ac] in C6D6

shows an interesting oscillating up (for ΔδHb), down (for ΔδH4, ΔδH5,ΔδH6, ΔδH7 and ΔδH8), and up first then down (for ΔδH2) modes(Figs. 2 and 3). This oscillation phenomenon might be due to heteroge-neity of AcIL–C6D6 in the mole fraction of C6D6 ranging from cal. 0.4 tocal. 0.9. The notable downfield shift of ΔδH2 in the extremely highconcentration of C6D6 might be ascribed to the benzene–imidazolium–

benzene sandwich shape by a strong benzene–imidazolium hydrogenbond. Takamuku et al. [52] also found a similar sandwich structure inthe mixture of [C12MIM][Tf2N] and benzene. This heterogeneity ofAcIL–C6D6 mixture in the corresponding concentration could be ex-plained by the formation of a crystalline compound with the benzenetrapped in ILs [53].

The blue shift of νC_N (Fig. 5) is caused by the decrease of overallhydrogen bond, i.e., the decrease of cation–anion hydrogen bond ismore than the increase of cation–benzene hydrogen bond. It could beused to explain the disruption of imidazolium–imidazolium packingby the introduction of benzene. The negligible shift of νC\D (Fig. 5) sig-nifies that the environment of C\D in benzene and [EMIM][Ac] is sim-ilar. The low shift ofΔδC6D6 in [EMIM][Ac] (Fig. 4) also corroborates thisconjecture.

4. Conclusions

The hydrogen-bonding interaction between [EMIM][Ac] and sevensolvents (deuteratedwater D2O, deuteratedmethanol CD3OD, deuterat-ed acetonitrile CD3CN, deuterated dimethylsulfoxide d6-DMSO, deuter-ated acetone CD3COCD3, deuterated benzene C6D6, and deuteratedchloroform CDCl3) were investigated in their whole concentrationsby ATR-IR and 1H NMR. Results show that H of [EMIM][Ac] and solventspresents a different change both in the chemical shifts δ by 1H NMRand wavenumber shifts ν by ATR-IR. Namely, the hydrogen-bondinginteractions of solvent–solvent, IL–IL, and solvent–IL, are differentat different concentrations. Specifically, along with the increasing ofsolvent concentration, the hydrogen bonding interaction among[EMIM][Ac] decreases, while the hydrogen bonding interactionbetween [EMIM][Ac] and solvent increases. The interaction at extremelydiluted solvents (i.e., 0.99 mol fraction) depends on the dominatingaspects. This dominating force is different in different solvents.For example, the decrease of hydrogen bond among [EMIM][Ac] out-weighs the increase of hydrogen bond between H (i.e., H2, H4 andH5) and D2O, CD3OD while this is opposite in CD3CN, d6-DMSO andCD3COCD3.

Competing interest

The authors declare no competing financial interest.

Acknowledgment

This work was supported by the National Natural Science Founda-tion of China (21173267) and the Fundamental Research Funds forthe Central Universities and the Research Funds of Renmin Universityof China (12XNLL05).

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