Theoretical and experimental investigation of the interactions between [emim]Ac and water molecules

Article abstract of DOI:10.1016/j.molstruc.2012.02.020

Density functional theory (DFT) calculations, atom in molecules (AIM) theory, natural bond orbital (NBO) analysis and infrared (IR) spectroscopy were performed to investigate the interactions between water molecules and ionic liquid 1-ethyl-3-methylimidazolium acetate ([emim]Ac). It was found that [emim]Ac interacts with water molecules mainly via H-bonds, and the anionic part of [emim]Ac plays a major role in the interaction with H₂O. The energies of H-bonds were estimated from spectral shifts of hydroxy antisymmetric stretching vibration. Moreover, the experimental results also indicated that hydroxy of water mainly interacts with the COO^- of [emim]Ac. Further studies indicated that the intensity of hydroxy stretching vibrations tend to be stronger with the increase of the concentration of water. In addition, the frequency of hydroxy stretching vibrations showed clearly red-shift, and the COO^- vibrational frequency gradually shifted to the lower wavenumber region, which were indicative of extended hydrogen bonded network.

Full text of DOI:10.1016/j.molstruc.2012.02.020

Journal of Molecular Structure 1015 (2012) 147–155
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Theoretical and experimental investigation of the interactions between
[
emim]Ac and water molecules
⇑
⇑
Zhen-Dong Ding, Zhen Chi, Wen-Xiu Gu , Sheng-Ming Gu, Hai-Jun Wang
School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu 214122, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Density functional theory (DFT) calculations, atom in molecules (AIM) theory, natural bond orbital (NBO)
analysis and infrared (IR) spectroscopy were performed to investigate the interactions between water
molecules and ionic liquid 1-ethyl-3-methylimidazolium acetate ([emim]Ac). It was found that [emim]Ac
interacts with water molecules mainly via H-bonds, and the anionic part of [emim]Ac plays a major role
Received 10 October 2011
Received in revised form 5 February 2012
Accepted 6 February 2012
Available online 16 February 2012
2
in the interaction with H O. The energies of H-bonds were estimated from spectral shifts of hydroxy anti-
symmetric stretching vibration. Moreover, the experimental results also indicated that hydroxy of water
mainly interacts with the COO of [emim]Ac. Further studies indicated that the intensity of hydroxy
Keywords:
ꢀ
B3LYP calculations
IR spectrum
Interaction
stretching vibrations tend to be stronger with the increase of the concentration of water. In addition,
ꢀ
the frequency of hydroxy stretching vibrations showed clearly red-shift, and the COO vibrational fre-
quency gradually shifted to the lower wavenumber region, which were indicative of extended hydrogen
[
emim]Ac
Water
bonded network.
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Ionic liquids (ILs) are a group of organic salts that are liquid at
anhydrous conditions are required. For example, cellulose can
be dissolved in pure [emim]Ac and reconstituted from the
[emim]Ac-based cellulose solution by the addition of water.
Seddon and co-workers have also proposed that efficient drying
is necessary if ILs are used as solvents for moisture-sensitive sub-
stances [9]. In addition, the water content of ILs may also affect
the rates and selectivity of reactions [10]. However, water is the
most common solvent and plays a major role in physical and
chemical processes. The water molecules existing in ILs can inter-
act with the cations or anions of ILs, which results in the change
of the nature of ILs and relevant physical procedures. In addition,
hygroscopicity causes problems in the recovery process which
would lead to material waste and environmental pollution. There-
fore, the investigation of the interactions between water mole-
cules and ILs is highly desired for understanding the properties
and performances of ILs and designing efficient ILs.
or near room temperature and have been defined as species that
are composed entirely of ions [1]. They have been used for many
applications, ranging from media for organic synthesis and catal-
ysis to lubricants. Moreover, they have generated much academic
and industrial interest as alternatives for environmentally damag-
ing volatile organic solvents [2–6]. The popularity of ILs stems
from two main advantages over traditional solvents. First, the
prime benefit is their unique chemical and physical features,
including low melting points, negligible vapor pressure, nonvola-
tile and inflammable as well as high thermal stability, and the
fact that they can be frequently recycled. This means that they
provide an environmentally benign alternative to the volatile or-
ganic compounds normally used as solvents in industrial pro-
cesses [7]. Second, the components of cation and anion of ILs
can be fine-tuned to adjust the physicochemical properties,
according to our needs. We can design and synthesize ILs.
In recent years, some researchers have investigated the inter-
actions between water molecules and ILs. For instance, Welton
and co-workers investigated the molecular states of water in ILs
using ATR-IR spectroscopy and suggested that the anionic part
of ILs is the relative active site for the interactions with water
molecules [8]. Wang et al. applied quantum chemistry (QM) cal-
culations to investigate the interactions between water molecules
Generally, some ILs are miscible with water, being ionic com-
pounds, and this hydrophilic property would sometimes affect
the properties of ILs, including their solubility, polarity, viscosity,
and conductivity, which are changeable with the amount of ab-
sorbed water [8]. This limits the application of ILs when
ꢀ
ꢀ
and ILs based on the imidazolium cation with the anions Cl , Br ,
ꢀ
ꢀ
BF , and PF [11]. Hardacre and co-workers studied the crystal-
4
6
line 1-alkyl-3-methylimidazolium chloride ionic liquid and charac-
terized the strong H-bonds interactions between the chloride ions
and water molecules [12]. Previous studies about the interactions
⇑
Corresponding authors.
E-mail addresses: wenxiugu@gmail.com (W.-X. Gu), wanghj329@hotmail.com
(
H.-J. Wang).
0
022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.02.020

1
48
Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
A
A
A
1
2
3
A
4
A
5
A
6
ꢁ
Fig. 1. The optimized configurations for [emim]Ac (A1–6) calculated at the B3LYP/6-31+G level. The corresponding atom numbering is given.
between water molecules and ILs did really refer to the interac-
tion energy and IR spectrum of them. Nevertheless, only little
information has been known for the relationship of the results
obtained by using the two methods to study the same system.
Thus, further studies are still necessary. So far, the interactions
between water molecules and [emim]Ac have not been reported.
Furthermore, [emim]Ac is a green solvent for cellulose, and cellu-
lose could be precipitated from the [emim]Ac-cellulose solution
by the addition of water. Therefore, the study of the interactions
between water molecules and [emim]Ac will also provide more
information for [emim]Ac dissolving cellulose and design ILs as
cellulose solvents.
QM calculations are a valuable technique for giving insight at
the molecular level into the interactions between water mole-
cules and ILs. Infrared spectroscopy is also the most powerful
method to probe the molecular state of water present in various
solvents [13]. The purpose of this paper is to apply QM calcula-
tions, AIM theory, NBO analysis and IR spectroscopy to study
the interactions between water molecules and [emim]Ac. In addi-
tion, the data obtained from the two different methods for the

Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
149
B
1
2
3
B4
B
B5
B
B
6
ꢁ
Fig. 2. The optimized configurations for [emim]Ac–nH
2
O (B1–6; n = 1, 2) calculated at the B3LYP/6-31+G level. H-bonds are indicated by dotted line, and bond lengths are in
Angstrom (Å). The corresponding atom numbering is given.
same system of [emim]Ac–nH
results were obtained.
2
O (n = 1, 2) were discussed and the
distilled and then stored over molecular sieves in tightly sealed
glass bottles. Other chemicals were commercially available and
were used without further purification.
2
. Materials and methods
2.2. Preparation of [emim]Ac
2.1. Materials
[emim]Ac was prepared following the reported procedures [14,15].
N-methylimidazole was obtained from Changzhou Chemical
Factory and further purified by distillation. Bromoethane was
[emim]Br was synthesized by refluxing the N-methylimidazole with
excess bromoethane at 50 °C for 48 h. The excess bromoethane was

1
50
Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
Table 1
were performed within the framework of the Gaussian 03 program
package [16], except that AIM calculation was performed by
AIM2000 package [17]. The hybrid Becke 3-Lee-Yang-Parr (B3LYP)
The total energies E (a.u.) and interaction energies
DEBSSE+ZPE of [emim]Ac (kJ/mol).
Conformer
E
DE
BSSE+ZPE
ꢁ
exchange–correlation function with the 6-31+G basis set was em-
A
A
A
A
A
A
1
2
3
4
5
6
ꢀ573.2519
ꢀ573.2517
ꢀ573.2323
ꢀ573.2752
ꢀ573.2745
ꢀ573.2674
ꢀ418.34
ꢀ418.08
ꢀ365.87
ꢀ406.46
ꢀ406.33
ꢀ352.63
ployed to perform the geometry optimizations in this work
[18,19]. No restrictions on symmetries were imposed on the initial
structures. Therefore, the geometry optimization for the saddle
points occurred with all degrees of freedom. Each final optimized
structure was checked to be a true minimum through a frequency
calculation at the corresponding level.
Table 2
The electron donor orbitals, electron acceptor orbitals and the corresponding second-
3. Results and discussion
order interaction energies E( (kJ/mol) of [emim]Ac (A1–6).
2)
3.1. Geometries optimization
Conformer
Donor (i)
Acceptor (j)
E(2)
21.76
121.72
4.45
19.91
23.39
3.11
ꢁ
A
1
LP(1)O22
LP(2)O22
LP(1)O22
LP(1)O23
LP(2)O23
LP(1)O23
BD (1)C
4
4
7
1
1
4
–H13
–H13
–H15
–H10
–H10
–H13
ꢁ
ꢁ
The geometries were originally optimized at B3LYP/6-31+G level
BD (1)C
ꢁ
BD (1)C
from different initial guesses, vibrational frequency analysis on these
optimized structures gave no imaginary frequencies, suggesting that
they are true minimum energy structures. For the configuration opti-
mization of [emim]Ac, it was investigated in the vacuum state firstly,
and A1–3 were the representative optimized configurations. Then, the
solvent effect on the conformational change of [emim]Ac was also
investigated at the molecular level. The water environment that sur-
rounds [emim]Ac could be implicitly included with the application of
a Polarizable Continuum Model (PCM) [20]. Therefore, the A1–3 geom-
etries of [emim]Ac were re-optimized and preformed the single
points using a PCM method with a dielectric constant of 80, and
ꢁ
BD (1)C
ꢁ
BD (1)C
ꢁ
BD (1)C
ꢁ
A
2
LP(1)O22
LP(2)O22
LP(1)O23
LP(2)O23
BD (1)C
4
4
7
7
–H13
–H13
–H15
–H15
59.30
144.31
4.12
ꢁ
BD (1)C
ꢁ
BD (1)C
ꢁ
BD (1)C
7.18
ꢁ
A
A
3
4
LP(1)O23
LP(2)O23
BD (1)C
5
–H14
–H14
25.54
79.55
ꢁ
BD (1)C
5
ꢁ
LP(1)O22
LP(2)O22
LP(1)O23
LP(2)O23
BD (1)C
4
4
1
1
–H13
–H13
–H10
–H10
19.32
83.66
12.10
23.60
ꢁ
BD (1)C
ꢁ
BD (1)C
A4–6 were the optimized geometries, respectively. The representative
optimized configurations of [emim]Ac and [emim]Ac–nH O (n = 1, 2)
2
were chosen and listed in Figs. 1 and 2.
ꢁ
BD (1)C
ꢁ
A
A
5
6
LP(1)O22
LP(2)O22
LP(1)O23
LP(2)O23
LP(1)O23
LP(2)O23
BD (1)C
4
4
7
7
5
5
–H13
–H13
–H15
–H15
–H14
–H14
17.81
80.93
10.79
19.61
11.34
54.94
ꢁ
BD (1)C
ꢁ
BD (1)C
+
ꢀ
ꢁ
3.2. Intramolecular interaction and NBO analysis of [emim] –Ac
BD (1)C
ꢁ
BD (1)C
ꢁ
BD (1)C
The optimized configurations were used to characterize the pre-
ferred interaction sites. The interaction energy (
lated from:
D
E) can be calcu-
ð1Þ
removed by evaporation and the crude product was recrystallized
twice by ethyl acetate. The resulting precipitate was brownish and
was dried in vacuum for 24 h under reduced pressure. Then
D
E ðkJ=molÞ ¼ 2625:5½EABða:u:Þ ꢀ ðEAða:u:Þ þ EBða:u:ÞÞꢃ
0
.2 mol [emim]Br and 0.1 mol Pb(Ac)
solved in solutions of methanol–water (4:1 v/v). Upon mixing the
two solutions, a precipitate of PbBr was produced, which was re-
2
ꢂ3H
2
O were separately dis-
A B
where EAB is the total energy of the system and (E + E ) is the sum
of the energy of the pure compositions.
2
The interaction energy
tion error (BSSE) and the zero point energy (ZPE). Thus, the cor-
rected interaction energy ZPE+BSSE can be calculated from:
D
E is corrected by basis set superposi-
moved by filtration after cooling at ꢀ20 °C overnight. The solvent
was removed from the filtrate by rotary evaporation, and the prod-
uct [emim]Ac was gotted by drying in vacuum for 2 days at 80 °C.
D
E
The structure of [emim]Ac was confirmed by 1H NMR (400 MHz,
D
E
ZPEþBSSE
¼
D
E þ ð2Þ
D
EBSSE
þ
D
EZPE
ꢀ
D
2
O): d
= 3.89 (t, 3H; N–CH
Ring-CH), d = 7.5 (s, 1H; Ring-CH), d
H
= 1.46–1.52 (t, 3H; CH
), d = 4.18–4.26 (t, 2H; CH
= 8.7 (s, 1H; Ring-CH).
3
), d
H
= 1.89–1.94 (t, 3H; CH
3
–COO ),
where
ZPE.
D
EBSSE is the correction of BSSE, and DEZPE is the correction of
d
H
3
H
), d
2 H
= 7.4 (s, 1H;
H
H
The representative optimized configurations of [emim]Ac were
shown in Fig. 1. The energy E of every configuration of [emim]Ac
2.3. ATR-IR experiment
and interaction energies
DEZPE+BSSE corrected by BSSE and ZPE be-
tween anion and cation were also listed in Table 1. It was observed
that the geometries A1–3 are very similar to A4–6, respectively. In
addition, the regular pattern of the energies and interaction ener-
gies of A1–3 are consist with that of A4–6. Therefore, configuration
optimization was investigated in the vacuum state in the study.
The experiment was performed in order to study the molecular
states of water molecules in [emin]Ac. All measurements were
done at the ambient temperature (25 ± 2 °C). ATR-IR measure-
ments were carried out on a NEXUS-470 spectrometer equipped
with a horizontal ATR accessory (ZnSe crystal, 45°) and an MCT
detector. All spectra was taken by coadding 32 scans at a resolution
Take A1–2 geometries for example, the difference between A
is negligible. Furthermore, the values of energy and interaction
energy of A are approximately equal to that of A , and in light of
the Boltzmann distribution both conformers A and A have the
same probability. At the same time, energetic analysis has also
been considered, so conformer A was chosen to investigate the
interactions of [emim]Ac–nH O (n = 1, 2) in the study.
1
and
A
2
ꢀ1
of 2 cm . In order to eliminate the absorption of the air, the
absorption spectrum of air was measured firstly and subtracted
from those of the samples.
1
2
1
2
1
2
.4. QM calculations methods
2
In this part, the NBO analysis [21] was performed to analyze the
strength of the donor–acceptor interaction. The electron donor orbi-
tals, electron acceptor orbitals and their corresponding second-order
QM calculations and AIM theory were applied to investigate the
interactionsbetween watermolecule and [emim]Ac. All calculations

Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
151
Table 3
The interaction energies
D
E
BSSE+ZPE (kJ/mol), Wiberg bond indexes (WBI) and properties of the electron density of bond critical point for the intermolecular interaction of [emim]
2
2
Ac–nH O (n = 1, 2; a.u.).
Conformer
Bond
WBI
q
BCP
r
q
BCP
DE
BSSE+ZPE
B
1
C
O
7
–H15ꢂ ꢂ ꢂO27
0.0206
0.0719
0.0172
0.0413
0.0551
0.1430
ꢀ49.41
27–H28ꢂ ꢂ ꢂO23
B
2
3
C
5
–H14ꢂ ꢂ ꢂO27
0.0143
0.0152
0.0533
ꢀ5.24
ꢀ75.05
B
C
1
–H10ꢂ ꢂ ꢂO27
0.0298
0.1150
0.0234
0.0524
0.0751
0.1371
O
27–H29ꢂ ꢂ ꢂO23
B
4
5
C
O
O
7
–H14ꢂ ꢂ ꢂO27
30–H31ꢂ ꢂ ꢂO22
27–H28ꢂ ꢂ ꢂO23
0.0156
0.0707
0.0622
0.0149
0.0392
0.0382
0.0499
0.1262
0.1345
ꢀ94.76
B
C
C
C
1
–H10ꢂ ꢂ ꢂO27
0.0139
0.0291
0.0120
0.0828
0.1211
0.0140
0.0234
0.0138
0.0428
0.0543
0.0464
0.0790
0.0458
0.1367
0.1646
ꢀ141.54
4
–H19ꢂ ꢂ ꢂO27
–H14ꢂ ꢂ ꢂO30
7
O
O
30–H32ꢂ ꢂ ꢂO23
27–H28ꢂ ꢂ ꢂO22
B
6
C
O
1
–H10ꢂ ꢂ ꢂO30
30–H32ꢂ ꢂ ꢂO22
–H15ꢂ ꢂ ꢂO27
0.0223
0.0677
0.0037
0.0513
0.0075
0.0339
0.0182
0.0339
0.0281
0.1208
0.0589
0.1208
ꢀ86.95
C
8
O
27–H28ꢂ ꢂ ꢂO23
(
2)
+
ꢀ
interaction energies E of [emim] –Ac were listed in Table 2. In the
than the C–Hꢂ ꢂ ꢂO bond. Therefore, the interaction energies of B
and B are bigger than that of B . This also implies that the anion
plays a major role in the interaction between [emim]Ac and H O,
1
+
ꢀ
+
[
emim] –Ac system, the [emim] moieties are the electron accep-
tors, and the Ac moieties are the electron donors. Clearly, the E
3
2
ꢀ
(2)
2
values of the donor–acceptor orbitals which involve the acceptor
while the cation probably plays a secondary role [24]. The results
are agreement with the previous reported that the strengths of
ꢁ
orbital BD (1)C
nor–acceptor orbitals. For example, the E of A
4
–H13 are relatively larger than that of the other do-
(
2)
1
with the donor–
–H13 is 121.72 kJ/mol, while
the interactions follow the trend anion-H
The optimized configurations (B4–6) and interaction energies of
[emim]Ac–2H O were listed in Fig. 2 and Table 3, respectively. It
2 2
O > cation-H O [11].
ꢁ
acceptor orbitals LP(2)O22 ? BD (1)C
4
the E( of the LP(2)O23 ? BD (1)C
2)
ꢁ
–H10 is only 23.39 kJ/mol, which
1
2
ꢁ
means the interaction between LP(2)O22 (donor) and BD (1)C
4
–H13
can be noted from the optimized conformations shown in Fig. 2
that two water molecules with [emim]Ac form a symmetrical
structure. It was clear that the configuration B has the biggest
5
absolute value of interaction energy (141.54 kJ/mol), which mani-
fests that [emim]Ac will be more favorable to interact with water
molecules in this way. It was found that with the increase of the
concentration of water, the interaction energies between [emim]Ac
and water molecules tend to increase, and the number of the H-
bonds tends to increase.
ꢁ
2)
(
acceptor) is stronger than LP(2)O23 ? BD (1)C
1
–H10. Furthermore,
(
among the configurations A1–6, the E
values for LP(2)O22
?
ꢁ
BD (1)C
4
–H13 in A1–2 and A4–5 are relatively larger than that of the
other donor–acceptor orbitals. Therefore, it can conclude that it is
one of the reasons that A1–2 and A4–5 have the larger values of inter-
action energies than the other configurations.
3.3. Intermolecular interactions, AIM and NBO analysis of [emim]Ac–
nH O (n = 1, 2)
2
In order to obtain more information about the optimized config-
urations, AIM theory was used to analyze the bonding characteris-
Most of the ILs can absorb water from the atmosphere [22],
tic [25], which based on a topological analysis of electron density
2
such as [emim]Ac. Water molecules tend to interact with the ILs,
this hydrophilic property would sometimes affect their physical
properties, the rates and selectivity of reactions [23,10]. The QM
calculations are valuable technique for giving insight at the molec-
ular level into the interactions between water molecules and ILs.
For example, Li et al. applied QM calculations to investigate the
interaction between water molecules and ionic liquids based on
(
q
c
) and Laplacian (
r
c
q ) [16], providing a universally applicable
tool for the classification of the bonding interactions that take
place in any molecular system, even inside a supermolecule. The
q
c
is used to describe the intensity of a bond, and the larger
q
c
va-
describes the char-
< 0, it is defined as the covalent
> 0, it refers to a closed-shell interaction and charac-
teristic of ionic bond, H-bond, or van der Waals interaction. In this
2
lue corresponds to the stronger bond. The
acteristic of the bond. As
bond. As
r
q
c
2
r
q
c
2
r
q
c
ꢀ
ꢀ
ꢀ
ꢀ
the imidazolium cation with the anions Cl , Br , BF , and PF
4
6
2
[
11]. In the study, a set of possible conformations of [emim]Ac–
paper, we are only concerned with the values of
intermolecular H-bonds. There are a set of criteria for
q
c
and
r
q
c
for
and
c
q proposed at bond critical points (BCPs) for the conventional
nH O (n = 1, 2) have been examined by DFT calculation to charac-
2
q
c
2
terize the interactions between [emim]Ac and water molecules.
The representative optimized configurations (B1–3) and the interac-
r
H-bonds. Both parameters for closed-shell interactions as H-bond
are positive within the following ranges: 0.002–0.035 a.u. for the
electron density and 0.024–0.139 a.u. for its Laplacian [26].
The obtained AIM analysis results, Wiberg bond indexes (WBI)
and the conformations with bond critical points of [emim]Ac–
tion energies of [emim]Ac–H
and Table 3, respectively. It was shown that the absolute values
of the interaction energies of [emim]Ac–H O are in the order of
> B > B . It was clear that one water molecule mainly interacts
with [emim]Ac through the cationic part, using the B as reference,
2
O were obtained as shown in Fig. 2
2
B
3
1
2
2
nH
most of the H-bonds considered here, the
within the relative proposed ranges. Therefore, for the observed
2
O (n = 1, 2) were shown in Table 3 and Fig 3, respectively. For
2
the absolute value of the interaction energy is 5.24 kJ/mol, while
water molecule mainly interacts with [emim]Ac through the anion
q
c
and
r
c
q values were
2
part, using the B
energy is 75.05 kJ/mol. In addition, B
O–Hꢂ ꢂ ꢂO and C–Hꢂ ꢂ ꢂO types, whereas B
relatively well known that O–Hꢂ ꢂ ꢂO bond is far stronger interaction
3
as reference, the absolute value of the interaction
and B have H-bonds include
has only C–Hꢂ ꢂ ꢂO type. It is
[emim]Ac–nH
2
O (n = 1, 2) system,
q
c
and
r
c
q at BCPs of H-bonds
3
1
fall within 0.002–0.055 a.u. and 0.024–0.165 a.u., respectively. It
can be concluded that the interactions are all close shell systems
2
3
(H-bond interaction). Using the AIM results of the B as reference,

1
52
Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
B
1
B
4
B
2
B
5
B
3
B
6
Fig. 3. AIM analysis of the conformations of [emim]Ac–nH
2
O (B1–6; n = 1, 2) with bond critical points (color codes: O, red; N, blue; C, black; H, gray). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
the
fore, the intensities of the H-bonds of the B
of the conformations B1–2. This may be due to the fact that the
used to describe the intensity of H-bond, and the larger value
corresponds to the stronger bond. The intensities of the H-bonds
are also in the order of B > B > B . So, the results are consistent
q
c
values: 0.0234 and 0.0524 are larger than that of B1–2. There-
are stronger than that
is
of B
values of B
tions. Therefore, the intensities of the H-bonds are also in the order
of B > B > B , the results are also agreement with the results of
5
is more than that of the other conformations. In addition, the
3
q
c
5
are relatively larger than that of the other conforma-
q
c
q
c
5
6
4
interaction energies well.
3
1
2
The electron donor orbitals, electron acceptor orbitals and
their corresponding second-order interaction energies E( of
2)
with the results of interaction energies well. For the AIM results
of the conformation B4–6, it was show that the number of H-bonds
2
[emim]Ac–nH O (n = 1, 2) were listed in Table 4. The existence

Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
153
Table 4
The electron donor orbitals, electron acceptor orbitals and the corresponding second-
Table 5
DFT-calculated O–H vibration frequencies (cmꢀ1) of [emim]Ac–H
2
O.
order interaction energies E( (kJ/mol) of [emim]Ac–nH
2)
O (B1–6; n = 1, 2).
2
Conformera
Calc. frequenciesb
Assignment
Conformer
Donor (i)
Acceptor (j)
E(2)
27.30
76.08
11.20
B
1
B
2
B
3
1645
3187
3647
O–H deformation
O–H (bonded to Ac )
O–H asym
ꢁ
ꢀ
B
1
LP(1)O27
LP(1)O23
LP(2)O23
BD (1)C
7
–H15
ꢁ
BD (1)O27–H28
ꢁ
BD (1)O27–H28
1602
3567
3679
O–H deformation
O–H sym
O–H asym
ꢁ
B
2
3
LP(1)O27
BD (1)C
5
–H14
25.41
ꢁ
B
LP(1)O27
LP(1)O23
LP(2)O23
BD (1)C
1
–H10
43.89
29.55
125.15
ꢁ
BD (1)O27–H29
1678
3054
3641
O–H deformation
O–H (bonded to Ac )
O–H asym
ꢁ
ꢀ
BD (1)O27–H29
ꢁ
B
4
LP(1)O23
LP(2)O23
LP(3)O23
LP(1)O22
LP(2)O22
LP(1)O27
BD (1)O27–H28
64.12
5.77
10.83
30.76
48.57
20.31
ꢁ
BD (1)O27–H28
a
Structures illustrated in Fig. 2.
Frequencies scaled by 0.955.
ꢁ
BD (1)O27–H28
b
ꢁ
BD (1)O30–H31
ꢁ
BD (1)O30–H31
ꢁ
BD (1)C
7
–H14
successfully applied to study the protonated ammonia–water clus-
ters [29] and the neutral benzene–water clusters [30]. Thus, we ex-
ꢁ
B
5
LP(1)O22
LP(2)O22
LP(1)O23
LP(2)O23
LP(1)O27
LP(2)O27
LP(1)O27
LP(1)O30
BD (1)O27–H28
31.56
134.01
40.59
73.82
15.80
3.51
ꢁ
BD (1)O27–H28
ꢁ
BD (1)O30–H32
pect to describe the interactions of [emin]Ac–H
means. Table 5 showed the predicted O–H vibration frequencies
of [emim]Ac–H O and the scaling factor for the calculated frequen-
cies is 0.955 [31–35]. The and bands in water vapor absorb at
1588, 3687 cm , respectively. Conformations B1–3 displayed lo-
cated at 3647, 3679 and 3641 cm , respectively. It was clear that
shifts to the lower wave numbers. Moreover, B and B displayed
larger shifts than B when water interacts with [emim]Ac. This
2
O with the same
ꢁ
BD (1)O30–H32
ꢁ
BD (1)C
1
–H10
1
–H10
4
–H19
7
–H14
2
ꢁ
BD (1)C
ꢁ
v
1
v
3
BD (1)C
38.92
14.84
ꢀ1
ꢁ
v
BD (1)C
3
ꢀ
1
ꢁ
B
6
LP(1)O23
LP(2)O23
LP(1)O22
LP(2)O22
LP(1)O27
LP(1)O30
BD (1)O27–H28
48.82
11.91
68.47
15.59
4.43
ꢁ
v
3
BD (1)O27–H28
3
1
ꢁ
BD (1)O30–H32
v
3
2
ꢁ
BD (1)O30–H32
important phenomenon certificated that [emim]Ac does interact
with water via H-bonds and the O–Hꢂ ꢂ ꢂO bond is far stronger inter-
action than the C–Hꢂ ꢂ ꢂO bond. The intensities of the H-bonds be-
tween [emim]Ac and water molecules can be estimated from the
ꢁ
BD (1)C
8
–H15
–H10
ꢁ
BD (1)C
1
30.72
wavenumber shift (
phase and v3 of the water dissolved in [emim]Ac. The intensities
of the H-bonds can be calculated by the use of [36]:
D
v
3
) between
3
v of the water in the vapor
of donor–acceptor interactions in conformer is evident from the
(2)
E
values. The donor–acceptor interactions in these conformers
involve the interaction of the electron donor oxygen atom (in
emim]Ac or H O) and O–H or C–H (in H O or [emim]Ac) anti-
bonding orbital of electron acceptor. It was observed that the
À
Á
[
2
2
vapor
IL
3
vapor
D
H ¼ ꢀ80 v3
ꢀ
v
=
v3
ð3Þ
(
2)
ꢀ1
vapor
3
IL
3
E
values of the donor–acceptor orbitals which involve the O–
where
D
H is the enthalpy of H-bond in kcal mol
,
v
and
v
are
H antibonding orbital of electron acceptor are relatively larger
than that of the other donor–acceptor orbitals. For example, the
the wavenumber of the
respectively.
The corresponding energies of the H-bonds between water and
[emim]Ac were summarized in Table 6. The calculated energies of
3
v of water in vapor and water in [emim]Ac,
(
2)
E
value of B
5
with the donor–acceptor orbital LP(2)O22
?
ꢁ
(2)
BD (1)O27–H28 is 134.01 kJ/mol, while the E
LP(2)O27 ? BD (1)C
interaction between LP(2)O22 (donor) and BD (1)O27–H28 (accep-
tor) is stronger than LP(2)O27 ? BD (1)C
5
of B with the
ꢁ
7
–H19 is only 38.92 kJ/mol, which means the
the H-bonds based on the spectral shifts of the
with the intensities of the H-bonds based on the AIM theory in the
order: B > B > B . Further support for this correlation can be
obtained from the investigation of the position of the (Table
5). It was known that the IR band of the of water shifts to higher
v
3
band agree well
ꢁ
ꢁ
7
–H19. This implied that
3
1
2
the anionic part of [emim]Ac plays a major role in the interaction
with H O, while the cation probably plays a secondary role. In
addition, the introduction of H O in the [emim]Ac, the donor–
acceptor orbitals which involve the O–H antibonding orbital of
v
1
v
1
2
ꢀ1
wavenumber compared with free water (1588 cm ) in the gas-
eous state as a consequence of the formation of H-bonds [32]. In
fact, the data presented in Table 5 showed that the frequency of
2
(
2)
electron acceptor tend to increase, the E values also increase
which enhances the overall stability of [emim]Ac–nH O (n = 1,
), and the interaction between [emim]Ac and H O become
strong.
the
> B
H-bonds become stronger. Therefore, it can be concluded that the
intensities of the H-bonds are also in the order of B > B > B . In
addition, when water mainly interact with [emim]Ac through the
v
1
shifts to higher wavenumber according to the order of
2
2
2
B
3
1
> B . The shifts to higher wavenumber means that the
2
v
1
3
1
2
3
.4. IR spectrum of [emim]Ac–H
2
O
anionic part, using the configuration B
3
as reference, the hydroxy
ꢀ
of water interacts with the COO of [emim]Ac, the hydroxyl will
The IR spectrum of water is one of the hottest topics that has
been studied among the spectra of the polyatomic molecules.
The IR band corresponding to the symmetric stretching vibration
ꢀ
bind with the COO and form H-bonds (Table 3). However, when
water mainly interacts with [emim]Ac through the cation part,
(v
2
) of water is seldom used to elucidate the molecular state of
water. By contrast, deformation ( ) and antisymmetric stretching
) vibration of water have been extensively used to study the
molecular state of water dissolved in various solvents and ab-
sorbed in many polymers [9,27,28].
v
1
Table 6
The shifts of the
(v
3
ꢀ1
v
3
(cm ) of different conformers of [emim]Ac–H
2
O compared with
the corresponding water bands in vapor, and the energies of H-bonds.
IL
Conformer
v
D
v
3
ꢀ
DH (kJ/mol)
3
In this work, density functional theory (DFT) calculations using
the Gaussian program package and the B3LYP functional together
B
1
B
2
B
3
3647
3679
3641
40
8
46
3.6
0.7
4.2
ꢁ
with a standard 6-31+G basis set were performed. This combina-
ꢁ
tion of the methods and basis set (B3LYP/6-31+G ) have been

1
54
Z.-D. Ding et al. / Journal of Molecular Structure 1015 (2012) 147–155
to note that dry [emim]Ac does not have absorptions in this region.
The water molecules produce a broad band in the region around
ꢀ1
3
450 cm . It was found that with the introduction of H
2
O in the
[
emim]Ac, the shape of bands changed clearly. For instance, the
ꢀ1
line representing ‘‘IL + 2%H
2
O’’ has its maximum at 3450 cm
,
while the line representing ‘‘IL + 30%H
2
O’’ is clearly red-shift,
which is indicative of extended H-bonds network. Furthermore,
the strengthening of O–H stretching vibration band is due to the
interactions between [emim]Ac and water become stronger, with
the amount of water in [emim]Ac increases. This is not dramati-
cally surprising, as it would be expected that water would easily
be absorbed due to forming strong H-bonds with the anion of
[
emim]Ac. The further evidence for the strong interactions came
ꢀ
from the observation of the red-shift of the COO band of
[
emim]Ac in the presence of water (Fig. 5). This red-shift further
ꢀ
proved that water molecules interact with the COO of [emim]Ac.
Moreover, it was shown that the intensity of
v
ðCOO Þ
band tends to
ꢀ
weaken. This also implies that the interactions between [emim]Ac
and water tend to be stronger with the increase of water concen-
tration. The results agree with the theoretical results well.
Fig. 4. The ATR-IR spectra in the
with different concentrations.
v
ꢀ
ðOH Þ
band region of [emim] Ac containing water
4
. Conclusions
In summary, DFT calculations, AIM theory, NBO analysis and IR
using the configuration B
appear. The absolute values of the interaction energies of configu-
rations B and B is in the order of B > B . This also improved that
the anion plays a major role in the interaction of [emim]Ac with
water, while the cation probably plays a secondary role.
2
as reference, the interaction would dis-
spectroscopy are proven to be the efficient methods for the charac-
terization of the interactions between [emim]Ac and water mole-
cules. The anionic part of [emim]Ac plays a major role in the
3
2
3
2
interaction with water molecules. The shift of the
3
v vibrational
frequency has been used to correlate the relative strength of H-
bonds. The order of the interaction energies is correlated to the
strength of the H-bonds between water molecules and anion of
3.5. The state of water in [emim]Ac
[
emim]Ac and also to the shift of
v
ðCOO Þ
vibrational frequency.
ꢀ
It is well-known that
800–3000 cm , it has been proven that more information about
v
3
of water is present in the region of
The theoretical results agree with the experimental results well.
ꢀ
1
3
H-bonds can be obtained from the O–H stretching vibration [37].
Thus, the discussion will be focused on the range of 3800–
Acknowledgements
ꢀ1
3
000 cm . ATR-IR was used to further investigate the interactions
between H O and [emim]Ac. Here, we are interested in the state of
water present in [emim]Ac when water can be dissolved in
The authors are grateful to the Fundamental Research Funds for
the Central Universities (JUSRP211A08) and the National Univer-
sity Student Investigation Program (101029520) for financial
support.
2
ꢀ1
[
emim]Ac. ATR-IR spectra in the range of 3600–3200 cm
for
[
emim]Ac with dissolved water showed in Fig. 4. It was important
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