Journal of Molecular Structure 1079 (2015) 120–129

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Journal of Molecular Structure

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Quantifying the hydrogen-bonding interaction between cation and anionof pure [EMIM][Ac] and evidencing the ion pairs existence in itsextremely diluted water solution: Via 13C, 1H, 15N and 2D NMR

http://dx.doi.org/10.1016/j.molstruc.2014.09.0230022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +86 10 62514925; fax: +86 10 62516444.E-mail addresses: lishehong@chem.ruc.edu.cn (S. Li), tcmu@chem.ruc.edu.cn (T. Mu).

Yu Chen a, Shehong Li a,⇑, Zhimin Xue b, Mingyang Hao a, Tiancheng Mu a,⇑a Department of Chemistry, Renmin University of China, Beijing 100872, Chinab College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China

h i g h l i g h t s

� The proportion of hydrogen bonds ofpure [EMIM][Ac] is quantified by 1D-and 2D-NMR.� The existence of ion pairs of

[EMIM][Ac] was corroborated by 1D-and 2D-NMR.� The hydrogen-bonding strength of

H6/anion of [EMIM][Ac] determinesthe association of ion pairs.� Ion pairs of [EMIM][Ac] exist via the

through-space weak van der Waalsforce.

g r a p h i c a l a b s t r a c t

The proportion of hydrogen-bonding interaction between hydrogens in the cation and anion for pure[EMIM][Ac] (100%) is approximately quantified in descending order as follows: H2 (42%), H4 (24%), H5(22%), H6 (6%), H7 (5%), H8 (1%). Ion pairs exist in the extremely-water-diluted [EMIM][Ac] (0.5 mol%fra. of IL) via the through-space weak van der Waals force between H6 of the cation and Hb of the anion.

a r t i c l e i n f o

Article history:Received 15 June 2014Received in revised form 6 September 2014Accepted 6 September 2014Available online 16 September 2014

Keywords:Ionic liquidWaterHydrogen bondingNMRQuantification

a b s t r a c t

The acetate-based ionic liquid (AcIL) [EMIM][Ac] does not fully dissociated into isolated ions in extre-mely diluted water solution (0.5 mol% of IL). Still, ion pairs exist via the through-space weak van derWaals force between H6 of the cation and Hb of the anion. In this ion pairs, except for H6 and Hb,all other hydrogen atoms (i.e., H2, H4, H5, H7, H8) are totally hydrated by water; the acetate anion suf-fers from a more extent of hydration due to its higher hydrophilicity. One dimension (1D) nuclear mag-netic resonance (NMR) (1H, 13C, 15N,) and two dimensions (2D) NMR are used in this study. 2D NMRused includes through-space 1H–1H NOSEY (nuclear Overhauser effect spectroscopy), through-bond1H–13C HSQC COSY (heteronuclear single-quantum correlation spectroscopy), and HMBC COSY (heter-onuclear multiple-bond correlation spectroscopy). The much stronger (H245/anion) or weaker (H78/anion) hydrogen-bonding interaction in the pure [EMIM][Ac] disfavors the association of ions in thediluted state due to a better hydrogen-bonding donor or a weaker hydrogen-bonding strength, respec-tively. However, H6/anion with the moderate hydrogen-bonding strength and the moderate hydrogen-bonding donating ability existed in the pure [EMIM][Ac] plays the role in determining the associating

NN3

21

5 4

67

8

Scheme 1. Chemical structure and notation n

Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129 121

ion pairs. The proportion of hydrogen-bonding interaction between hydrogens in the cation with anion(100%) is approximately quantified in descending order as follows: H2 (42%), H4 (24%), H5 (22%), H6(6%), H7 (5%), and H8 (1%).

� 2014 Elsevier B.V. All rights reserved.

Introduction

Acetate-based ionic liquids (AcILs) have been paid particularattention recently. They possess not only the general feature of ILs(e.g., low volatility and high tunability), but also other favorablephysical properties (e.g., low viscosity, low melting point, low toxic-ity and high biodegradability), which denotes their potential appli-cations for biomass (e.g., cellulose, [1–3] chitin, [4–6] and chitosan[4,6–8]) dissolution, sour gases capture, [9,10] solvents or catalystsfor chemical reaction, [11] drying materials, [12] and so on.

Nevertheless, the presence of water would play an importantrole in the utilization of AcILs. Water could be used as antisolventto regenerate biomass from AcILs, or prevent AcIL from dissolvingbiomass [5,6,13–15]. Water also has significant influence on thesolubility of CO2 in AcILs [16]. Mixing with water would dramati-cally alter the property and structure of AcILs [17]. Particularly,the hygroscopicity of AcILs is higher than other ILs due to its higherbasic anion [12,18,19], hence they are easy to absorb water fromthe ubiquitous moisture air.

Thus, it is necessary to understand the interactions between AcILsand water. Molecular dynamics simulation, density functional theoryand various experimental methods have been employed to investi-gate the intermolecular interactions of AcILs [20–22], and the inter-actions between AcILs and water [23–28]. The results showed thatno new compounds existed in the mixture of water and AcILs, andone AcIL molecule tended to be surrounded by three water molecules[24]. The anion–water interaction by hydrogen bonding dominatedthe AcILs/water interaction [26]. Brehm et al. [25] concluded thatwater would disturb the hydrogen bonds networks and alter the for-mation of carbenes and dipole moments of AcILs.

However, most of the investigations were focused on interac-tions (e.g., hydrogen bonds, van der Waals force, and electrostaticforce) between AcILs and water; while the association of AcILsand dissociation of AcILs in water had seldom been reported. Pre-viously, we proposed a two-times explosion mechanism to inter-pret the dynamic solvation process of AcILs in water; the firstand second time explosion referred to from network, sub-net workto cluster, and from cluster in the moderate concentration of AcILs(ca. 80 mol%), sub-cluster to ion pairs in the low concentration ofAcILs (ca. 20 mol%), respectively [29]. The existence of ion pairsafter the second explosion was only a hypothesis based on infraredspectroscopy (IR) and nuclear magnetic resonance (NMR) of thewater/AcILs mixture.

Hydrogen bonds plays a very important role in many chemicaland biological system (e.g., enzymatic reaction, supramolecularself-assembly) [30], and the structure, property even applicationof ILs (including AcILs) could be enormously affected by hydrogenbonds [31,32]. Quantifying hydrogen bonds of ILs seems veryimportant to understand and explain their physical property,

C

O

Oab

umber of [EMIM][Ac].

industrial application, and design new task-specific ILs for a spe-cific purpose. Furthermore, dissecting or quantifying intramolecu-lar hydrogen bonds of ILs has seen in Ludwig’s reports [33,34], butin his paper only a discrimination of interaction among Coulombforces, hydrogen bonding, and dispersion forces is described for aconventional ILs rather than AcILs. Here, it is the first time forthe quantification of hydrogen bonds between difference H atomsof the cation and the acetate anion. However, it should be notedthat there are many other kinds of interaction among ILs (e.g.,[EMIM][Ac]) and their mixture with water, such as van der Wallforce, Coulomb force, aggregation, halogen bonds etc [14,35–40].Our main interest is the hydrogen bonds, its quantification, andthe association evidence in the extremely diluted water solution.

In this study, we proved the existence of ion pairs of AcILs at lowconcentration in water via 1D-NMR (i.e., 1H, 13C, 15N) and 2D-NMR(i.e., 1H–1H nuclear Overhauser effect spectroscopy (NOSEY) and1H–13C correlation spectroscopy (COSY)) technique. Also, quantifi-cation of cation–anion hydrogen bonds for [EMIM][Ac], seeScheme 1 could be determined by NMR, which might be more effi-cient and direct than the overlapped IR spectra and gaseous-stateapplicable DFT. 1D- and 2D-NMR are powerful tools to analyze theinteractions (including hydrogen bonds) among ILs and betweenILs and other solvents [36,41–50]. However, it should be noted thatthere exist some arguments for the application of NOSEY in neat oraqueous ILs. [51,52] Steinhauser and Weingärtner et al. demon-strated that NOE-based methods were not suitable for probingnext-neighbor interactions in neat ILs [51], while Halle much earlierclaimed the same to be true for diluted aqueous systems [52]. Here,we still assume that the short range (4–5 Å) of intermolecular NOEcould provide information of the cation/anion interaction for [EMI-M][Ac], thus the mobility of water molecules interacting with [EMI-M][Ac] could be detected by NOE. Note again that we do not quantifyhydrogen bonds directly by NOE, rather, we use the results derivedfrom NOE as the premise for the deduction of hydrogen-bonds quan-tification in combination with the 1D-NMR shift.

[EMIM][Ac] was selected as the reprehensive AcILs because it hasbeen used to the industrial application for biomass dissolution (e.g.,cellulose) by BASF. A very low concentration of [EMIM][Ac] in water(0.5 mol%) was used to analyze because if ion pairs exist in thisdilute aqueous solution, it would exist in a higher concentration of[EMIM][Ac] (i.e., 20 mol% reported in our previous report [29]). Deu-terated water (D2O) was used to represent normal water (H2O), for aconsistency with previous selection [29]. Note that dissociation orassociation of other kinds of ILs (i.e., excluding AcILs, e.g.,[BMIM][Cl], [BMIM][BF4]) in common solvents (e.g., H2O, CH3OH)have been reported [53–60]. The similarity and difference of themwith [EMIM][Ac], investigated in this paper, would be illustrated.

Experimental section

Materials

[EMIM][Ac] (purity >98.5%) was purchased from LanzhouGreenchem ILs, LICP, CAS (Lanzhou, China). It was dried in a vac-uum oven (at 60 �C for 80 h) and then analyzed its impurity beforeusage for NMR spectra. The content of water was less than740 ppm measured by Karl Fisher titration (ZDJ-400S Multifunc-tional titrator, Beijing Xianqu Weifeng Company, Beijing, China).

Fig. 2. Relative change of 1H NMR spectra for 0.5 mol% [EMIM] [Ac] in D2O solutionreferred with the pure [EMIM][Ac] with the order of original chemical shift (a) andwith the order of absolute chemical shift change (b).

122 Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129

The presence of halogen ion could be approximately excluded withthe method of AgNO3 precipitation. The possible thermal degra-dants during the drying process were negligible because theNMR and IR spectra did not change before and after drying.

1D and 2D NMR

The sample, i.e., 0.5 mol% of [EMIM][Ac] in D2O, was preparedby weighing methods. Afterwards, the 1H and 13C NMR spectra ofpure [EMIM][Ac] and [EMIM][Ac]-D2O solutions were measuredusing Bruker DMX 300 NMR spectrometer (300 MHz) at 298 K withTMS as the internal standard, as suggested by Yu et al. [43,61].Namely, the NMR spectra were calibrated by TMS, which was dis-solved in CCl4 (the volume fraction of TMS is about 5%). Then, thisTMS/CCl4 solution was loaded into a glass capillary with its oneend already sealed by heating. In order to prevent the evaporationof TMS from its CCl4 solution, another end of the glass capillary wassealed by melting white candle (due to the high evaporation abilityof TMS). Once the melting white candle was condensed, anotherend of the glass capillary could be assumed to be sealed. Note thatduring the above process, the hand should better refrain fromholding the TMS/CCl4-loading part of the capillary for the purposeof preventing the TMS extruding the condensed candle and cameout, it was because that the heat (i.e., close to the human bodytemperature, ca. 36 �C) generated from the hand would be trans-ferred into the TMS part and then lead to a easy spurt of the highlyvolatile TMS (with the boiling point as low as ca. 27 �C). Finally,this glass capillary was loaded into the NMR tube before all the1D- and 2D-NMR measurement. 15N NMR spectrums were con-ducted in the same condition with an NMR spectrometer(600 MHz). 1H–1H NOESY and 1H–13C COSY (containing HSQCand HMBC) were carried out at 298 K on a Bruker AVANCE 600NMR spectrometer (600 MHz).

Results and discussion

The absolute value and the relative change of 1H and 13C NMR for[EMIM][Ac] with and without dilution are displayed in Figs. 1–4,respectively. Fig. 5 shows the 2D-1H–1H NOSEY NMR spectra withor without diluted [EMIM][Ac], detecting the through-space inter-action of nearby hydrogen atoms within about 5 Å regardless ofwhether there is a bond between them. Fig. 6 presents the 2D

Fig. 1. 1H NMR spectrum of pure [EMIM][Ac] (up) an

1H–13C HSQC-COSY NMR spectrum for the purpose of detecting het-eronuclear correlations separated by one bond. Fig. 7 shows the 2D1H–13C HMBC-COSY NMR spectrum to detect heteronuclear correla-tions over longer ranges of about 2–4 bonds. Fig. 8 is the comparisonof 15N NMR before and after being highly diluted by water.

Figs. 1–4 show that the relative change of chemical shift ofextremely diluted water solution referred to the pure [EMIM][Ac]

d 0.5 mol% [EMIM] [Ac] in D2O solutions (down).

Fig. 3. 13C NMR spectrum of pure [EMIM][Ac] (up) and 0.5 mol% [EMIM] [Ac] in D2O solutions (down).

Fig. 4. Relative change of 13C NMR spectra for 0.5 mol% [EMIM] [Ac] in D2O solutionreferred with the pure [EMIM][Ac] with the order of original chemical shift (a) andwith the order of absolute chemical shift change (b).

Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129 123

in 1H and 13C NMR has three patterns: decrease in both 1H and 13CNMR (position 2, 4, 5); decrease in 1H NMR but increase in 13C NMR(position 6, 7), increase in 1H NMR but decrease in 13C NMR (posi-tion 8, b). The first pattern related to ring atoms changes in thesame direction but the last two patterns show an opposite direc-tion in alternation.

First, we discuss the first pattern for positions in the cation ring(i.e., 2, 4, 5) of [EMIM][Ac]. The decrease in chemical shift in thispattern suggests that the electro-density of H2, H4, H5, C2, C4,C5 of pure [EMIM][Ac] increases after dilution (i.e., 0.5 mol% of[EMIM][Ac]). Thus, the overall hydrogen bonding interactionwould reduce, i.e., the increase in hydrogen bonding interactionof water/C245H after introducing water for [EMIM][Ac] is over-whelmed by the decrease in hydrogen bonding interaction ofanion/C245H in the pure [EMIM][Ac] because hydrogen bondinginteraction of anion/C245H (decrease part) is stronger than thatof water/C245H (increase part).

Then, we focus on the positions in the alkyl adjacent to nitrogenatom of the cation ring (i.e., 6, 7). The relative chemical shiftdecreases in 1H NMR while increases in 13C NMR, which suggeststhat an increasing electro-density of H6, H7 and a decreasing elec-tro-density of C6, C7 of pure [EMIM][Ac] after dilution (i.e.,0.5 mol% of [EMIM][Ac]). In this pattern, the overall hydrogenbonding interaction would also reduce, i.e., the increase in hydro-gen bonding interaction of water/C67H after introducing water for[EMIM][Ac] is overwhelmed by the decrease in hydrogen bondinginteraction of anion/C67H in pure [EMIM][Ac]. It is because thathydrogen-bonding interaction of anion/C67H is stronger than thatof water/C67H. Note that in the first pattern related to ring atoms,this difference would be more drastic due to much stronger anion/C245H hydrogen bonding interaction than anion/C67H.

Finally, the longest positions from the cation ring or anion (i.e.,8, b) were investigated. The relative chemical shift increases in 1HNMR while decreases in 13C NMR suggests a decreasing electro-density of H8, Hb and an increasing electro-density of C8, Cb ofpure [EMIM][Ac] after being diluted by water (i.e., 0.5 mol% of[EMIM][Ac]). Hence, the overall hydrogen bonding interactionwould abnormally reduce, i.e., the increase in hydrogen bondinginteraction of water/C8bH after introducing water for [EMIM][Ac]dominates over the decrease in hydrogen bonding interaction ofanion/C8bH in pure [EMIM][Ac] since the hydrogen bonding inter-action of anion/C8bH is weaker than that of water/C8bH.

It is found in Fig. S1 that 4 mol% is the turning point concentra-tion for the change of 1H NMR almost for all the hydrogen atoms(including H2, H4, H5, H6, H7, H8, Hb). Namely, low than 4 mol%the NMR chemicals shift becomes bigger while it turns less whenthe concentration is high than 4 mol%. It is consistent with our pre-vious report [28,29]. It indicates that when the concentration of

Fig. 5. 2D 1H–1H NOESY NMR spectrum of pure [EMIM][Ac] (a) and 0.5 mol% [EMIM][Ac] in D2O solutions (b).

124 Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129

[EMIM][Ac] is less than 4 mol%, [EMIM][Ac] exists in the form ofion pairs. It also corroborates our right choose for selecting0.5 mol% as the concentration to investigate the state of ionic asso-ciation for [EMIM][Ac] in water solution. Also, it suggests thatwhen the concentration is low enough (e.g., less than 0.5 mol%),the hydrogen bonds and pi–pi stacking for [EMIM][Ac]–D2O sys-tem might be negligible. The possible existence form for the cationand anion of [EMIM][Ac] might be the ion pairs via weak van derWalls force.

Note that the H–D exchange is also very obvious for the ILs(including [EMIM][Ac] investigated here)-D2O system [62,63].First, H2 signal is dramatically shifted and reduced in intensity(Fig. 1 and Fig. S1), it might indicate that there is a H–D exchangein the C2 and other position. By looking at the lower 13C NMR spec-tra (Fig. 3), one can see that C2 carbon looks as triplet. A triplet

peak indicates coupling to deuterium, which further strengthensthat there is a H–D exchange in position 2 and possible other posi-tion. In quite old studies, the hydrogen shift in systems of aceticacid/water, the hydrogen shift has been determined to be due tothe hydrogen exchange between the two molecules. To further cor-roborate the H–D exchange, we conducted several more experi-ments. The higher concentration (6 mol% of [EMIM][Ac] in D2O)presents a obvious partial H–D exchange (Fig. S1). After ca. 3 h,almost all the H in [EMIM][Ac] shifts to some extent in the repre-sentative concentration of 2 mol% (Fig. S2), 4 mol% (Fig. S3), 6 mol%(Fig. S4), indicating that all the H in [EMIM][Ac] has a H–Dexchange with D2O. Particularly, the largest shift of C2H(Figs. S2–S4) indicates that the easiest H–D exchange occurs. Amore interesting finding on the H–D exchange process after ca.3 h later is that the change direction of chemical shift of [EMI-

Fig. 6. 2D 1H–13C HSQC-COSEY NMR spectrum of pure [EMIM][Ac] (a) and 0.5 mol% [EMIM][Ac] in D2O solutions (b).

Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129 125

M][Ac] with 4 mol% concentration alters oppositely to that of2 mol% and 6 mol% concentration. It might also be due to the turn-ing point at the concentration of 4 mol%.

The 1H–1H NOSEY spectra of pure and diluted [EMIM][Ac] pres-ent more information. It can be seen from Fig. 5 that only H6–Hbcorrelation could produce a detectable through-space NOSEY sig-nal. It means that [EMIM][Ac] would not fully dissociate in thediluted water solution (0.5 mol%); so there exists ion pairs in thehighly water-diluted [EMIM][Ac], as shown in Scheme 2. It wasconsistent with the assumption of existing ion pairs after being

much diluted in our previous report [29]. Yee reported that hydro-phobic Tf2N-based ILs (i.e., [CnMIM][Tf2N], n = 2, 4, 6) were pre-dominately associative; hydrophilic [EMIM][Cl] was almost fullydissociative; and moderate hydrophilic [EMIM][ESO4] showed theintermediate hydration behavior [54]. [EMIM][Ac] is hydrophilic,thus might be almost fully dissociative according to Yee’s conclu-sion. However, we obtained a more accurate and specific result,i.e., weak association via H6 and Hb. It was similar to the conclu-sion drawn by Bešter-Rogac et al. [58] with another hydrophilicIL [BMIM][Cl]: weak association in water. Another reason to

Fig. 7. 2D 1H–13C HMBC-COSEY NMR spectrum of pure [EMIM][Ac] (a) and 0.5 mol% [EMIM][Ac] in D2O solutions (b).

126 Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129

explain why acetate-based ILs does not fully dissociate in dilutewater is the higher cation–anion interaction. There exist onlyone-hydrogen-bonded ion-pairs in ILs [EMIM][X] (X = Br, Cl), one-and two-hydrogen-bonded ion-pairs in ILs [EMIM][X] (X = BF4,PF6) [64]. Nevertheless, the ILs with the acetate anion [EMIM][Ac]own the probable structure of one anion surrounded by three imi-dazolium rings [21], indicating a stronger intermolecular hydro-gen-bonding interaction in [EMIM][Ac] than other kinds of ILs. Inaddition, the higher basic acetate anion than other types of anionalso suggests a higher tendency of cation–anion binding ability in[EMIM][Ac] than other ILs (e.g., [BMIM][BF4]) [19]. This higher cat-ion–anion interaction of [EMIM][Ac] would prevent the totally dis-sociation of cation and anion in spite of being exposed to theextremely diluted water solution. The cation is also the probable

candidate for dissociation in diluted water due to the reasons listedas below: the optimized structure of one anion surrounded bythree cations investigated by molecular dynamics and neutron dif-fraction disfavors the formation of cation–cation ring stack [21];two positively-charged imidazolium cation rings tend to repulsewith each other, which also hinders the aggregation of ring–ringpack; our previous investigation on the ring–ring pack for a similaracetate-tethered IL [BMIM][Ac] also suggests a negligible ring–ringpiling [65]. All the above reasons contribute to an easier nearlytotal dissociation of cation–cation ring stack when perturbed bymuch water.

A comparison between a similar systems by Stark et al. [50] andour work would also be interesting. By investigating the binarysystem of water and the ionic liquid 1-ethyl-3-methylimidazolium

Fig. 8. 15N NMR spectrum of pure [EMIM][Ac] (up) and 0.5 mol% [EMIM] [Ac] in D2O solutions (down).

Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129 127

methanesulfonate (i.e., [emim][MeSO3]) via density, viscosity, heatcapacity, conductivity, heats of dissolution, as well as NMR mea-surements, Stark et al. concluded that the IL [EMIM][MeSO3]framework keeping was so highly structured that made the watermolecules physically separated [50]. Rather here for [BMIM][Ac],we propose a weak association between the cation and anion of[BMIM][Ac] via H6 and Hb after the introduction of water. The pos-sible reason might be the similar binding energy between the cat-ion and anion for [BMIM][Ac] and [EMIM][MeSO3], and the higherhydrophilicity of acetate anion (Ac, in [BMIM][Ac]) than methane-sulfonate anion (MeSO3, in [EMIM][MeSO3]). The comparable bind-ing energy between the two ILs would make them tend not to fullydissociate; the higher hydrophilicity of Ac than MeSO3 would ren-der a highly hydration of Ac by water while a physically separatedwater by [EMIM][MeSO3].

Specifically, this ion pairs is composed by the through-spaceinteraction between H6 and Hb (Scheme 2). In this ion pairs, theacetate anion with Hb tends to get close to H6 of the cation inthe highly diluted [EMIM][Ac] aqueous solution (Scheme 2). Fur-thermore, the H of the cation did not fully dissociate, as shownfrom HSQC (Fig. 6) and HMBC COSY NMR (Fig. 7). Note that

Scheme 2. Proposed ion pairs of [EMIM][Ac] in the extremely diluted water solutioninteraction between water and the corresponding atoms.

C2AH2 distance could be very long due to slight signal of HSQCand HMBC after zooming in the figures. It corroborates the ideathat the most acidic hydrogen or the best hydrogen bonding donor(i.e., H2) prefers to be hydrated than other hydrogens (i.e., H4, H5,H6, H7, H8, and Hb). It is displayed with the biggest area of hydra-tion and with the most expanding bond length compared to otherhydrogens (other than acetate anion) in Scheme 2.

On the other hand, the undetectability NOSEY (Fig. 5) betweenHb and other hydrogen in the cation (i.e., H2, H4, H5, H7, H8 exceptfor H6) suggests that ion pairs is not composed by these methods,such as H6–H2. It is because these hydrogens (i.e., H2, H4, H5, H7,H8) are totally hydrated by water due to the high concentration(99.5 mol%) of water in [EMIM][Ac] (Scheme 2). The total hydra-tion of H2, H4, H5, H7 and H8 in the highly diluted state impliesthat their hydrogen bonding interaction with acetate anion isnegligible.

Therefore, the overall change in hydrogen bonding interactionbetween H of the cation and acetate anion could be deemed asthe difference of hydrogen bonding interaction between water/Hand anion/H. Assuming that hydrogen bonding interaction ofwater/H is proportional to that of anion/H, the overall change in

(0.5 mol%). Bigger area of water hydration means a stronger hydrogen-bonding

128 Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129

hydrogen bonding (reflected by the relative change in chemicalshift in 1H NMR) is proportional to that of anion/H. Interactionbetween H6 and Hb in the highly diluted water solution mightbe the weak van der Waals force because the highly hydration ofthe anion could be supported by the much lower electro-density(i.e., much higher increase in chemical shift in Fig. 4) of Ca aftermixing with water. For the comparison with hydrogen bondinginteraction of other hydrogens, this weak van der Waals force isalso negligible.

In this way, the proportion of hydrogen bonding interactionbetween H of the cation and the acetate anion could be derivedby their corresponding relative change in 1H NMR chemical shift.As shown in Scheme 3, the proportion of hydrogen bonding inter-action of H of the cation with anion in the overall cation–anionhydrogen bonding interaction (100%) could be approximated esti-mated as below: H2 (42%), H4 (24%), H5 (22%), H6 (6%), H7 (5%), H8(1%). The difference between H4 and H5, and between H6 and H7,might be ascribed to the difference electron environment betweenN1 (adjacent to H5 and H6) with high electro-density and N3 (adja-cent to H4 and H7) with low electro-density, as shown in Fig. 8related to 15N NMR. Note that 15N NMR spectra of the highlydiluted [EMIM][Ac] are undetectable (Fig. 8).

Another interesting finding could be derived. In pure [EMI-M][Ac], the hydrogen bonding interaction between H of the cationand the acetate anion is ordered as: H245 > H6 > H78. However, inthe highly diluted [EMIM][Ac], ion pair occurs only in the form ofassociation between H6 and Hb of the anion. It means that a muchhigher anion/H (i.e., H245) hydrogen bonding interaction woulddisfavor their association in the highly diluted state because higheranion/H implies a better hydrogen bonding donor of the corre-sponding H (i.e., H245), which would also be easily attacked orhydrated by water if much water is added. Thus, in the highlydiluted water solution of [EMIM][Ac], H2-anion would fully disso-ciate after the full hydration of H245 by water. Previous report byLi [66] and Wu [67] also suggested that H245/anion hydrogen-bonding interaction was enormously weakened after the introduc-tion of much water. The pure AcILs such as pyrrolidinium acetate[Pyrrol][Ac] could also be found a weakness of intramolecularhydrogen bonds under a elevated temperature [39].

Also, it means that a much lower anion/H (i.e., H78) hydrogenbonding interaction would disfavor their association in the extre-mely diluted state because lower anion/H implies a weaker hydro-gen bonding donor of the corresponding H (i.e., H78). Although, inthis case, this weak hydrogen bonding donor could not be easilyattacked or hydrated by water, the hydrogen bonding interactionis so weak that it could be easily broken by water if much wateris added. Thus, in the highly diluted water solution of [EMIM][Ac],

Scheme 3. Quantification of hydrogen-bonding interaction between hydrogenatom of imidazolium cation and oxygen atom of acetate anion for pure [EMIM][Ac].

H78-anion would also fully dissociate. The previous arrow-shoot-ing-ball mechanism also suggested that amino-acid ILs (ball) withlower interaction between cation and anion would be easily dis-turbed by water (arrow) [68]. Brennecke et al. [69] also concludedthat a weaker intramolecular hydrogen-bonding interactionbetween the cation and anion of 1-ethyl-3-methylimidazolium tri-fluoroacetate [EMIM][TFA] than 1-(2-hydroxyethyl)-3-methylimi-dazolium trifluoroacetate [OH-EMIM][TFA] could be more easilyattacked by the water molecules, hence a higher water/[EMIM][TFA] hydrogen-bonding interaction than water/[OH-EMIM][TFA].

Only the moderately hydrogen bonded H6/anion could stillexist in the highly diluted water solution of [EMIM][Ac]. The mod-erate strength of hydrogen bonding interaction and the moderatehydrogen bonding donor of H6 results in an association of H6 withanion. Unlike H245, H6 would not be totally hydrated by water dueto its lower hydrogen-bonding donating ability than H245 whenbeing extremely diluted. Unlike H78, H6 would not be easily dis-rupted because of a relative higher strength of H6/anion thanH78/anion. H6 possess both the advantages of H245 (moderatehydrogen-bonding donating ability, thus not be totally hydratedby water) and H78 (moderate hydrogen-bonding strength withanion, thus not be totally disrupted), favoring the association of[EMIM][Ac] in its extremely diluted water solution. After beingextremely diluted for [EMIM][Ac], H6 would interact with Hb ofthe anion with weak van der Waals force, while the C@O of theanion is highly hydrated by water. Our previous investigation ofAcIL (e.g., [Pyrrol][Ac]) under a high temperature also could alsoform ion pair [39].

Conclusion

The presence of water would influence the chemical structure(hydrogen bonds, van der Waals force) of AcILs. [EMIM][Ac] wouldnot fully dissociate into isolated ion after being extremely dilutedby water (0.5 mol% of IL), but still exist ion pairs via the through-space van der Waals force between H6 of the cation and Hb ofthe anion. The ion pair also has other two characteristics: all thehydrogen atoms except for H6 and Hb (i.e., H2, H4, H5, H7, andH8) are totally hydrated by water; the acetate anion suffers froma more extent of hydration due to its higher hydrophilicity, henceunable to from hydrogen bonding with the H of the cation. Themuch stronger (H245/anion) and weaker (H78/anion) hydrogenbonding interaction in pure [EMIM][Ac] disfavors the associationof diluted state due to a better hydrogen bonding donor or aweaker strength of hydrogen bonds, respectively. However, themoderate hydrogen bonding strength of H6/anion in pure [EMI-M][Ac] determines the associating ion pairs by transforming intoweak van der Waals force after being highly diluted. Quantificationof hydrogen bonding interaction between hydrogens in the cationwith anion (100%) is approximately estimated and listed indescending order as follows: H2 (42%), H4 (24%), H5 (22%), H6(6%), H7 (5%), H8 (1%).

Acknowledgement

This work was supported by the National Natural ScienceFoundation of China (21173267 and 21473252).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2014.09.023.

Y. Chen et al. / Journal of Molecular Structure 1079 (2015) 120–129 129

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