Theoretical and experimental studies on proton transfer in acetate-based protic ionic liquids

Article abstract of DOI:10.1016/j.molliq.2016.05.080

In this study, the proton transfer of three protic ionic liquids (PILs) pyrrolidinium acetate ([Pyrrol]OAc), diethylammonium acetate ([DEA]OAc) and bis-(2-methoxyethyl)-ammonium acetate ([BMOEA]OAc) were investigated. At first, the structures of the ion-pairs and molecular pairs of these PILs were optimized at B3LYP/6-311 ++G(d,p) level. The interaction energy between anions and cations was also obtained. The proton transfer processes were verified by intrinsic reaction coordinate (IRC) pathways tracing to the energy profiles connecting the transition state (TS) to the two desired minima, i.e. ion pair and molecular pair. The experimental attenuated total reflection (ATR) FTIR spectra of these PILs at room temperature were determined and compared with the results calculated at B3LYP/6-311 ++G(d,p) level. Vibrational mode analyses (VMA) for [Pyrrol]OAc found that δ(NH) has an imaginary frequency (- 147.3 cm^{- 1}), which is accounted for proton transfer from [NH₂]⁺ to OAc^-. Natural bond orbital (NBO) analyses pointed out that second order perturbation stabilization energy of (E(2)) of LP(N1) → σ?(O2-H5) was much larger than that of other orbitals, and should be the symmetrical matching with the maximum overlap and the minimum gap (0.73 au). The hybridized index of N atom is varied from sp^3.65 in ionic pair to sp^4.49 in TS. The constituent of s orbital decreases 3.3% and the length of N1-H5 increases from 1.02 ? in ionic pair to 1.65 ? in TS, and the symmetric stretching vibration takes place the red shift. It could be explained that the N1-O2-H5 played an important role in the stabilization of molecular pair. The electron density ρ(r) and the Laplacian of the electron density ?²ρ(r) derived from atoms in molecules (AIM) analyses were used to describe the intensity and characteristic of a bond. The results indicate that a very strong interaction of the hydrogen bonds exists in the ion-pair geometries and the bonds are the covalent bond.

Full text of DOI:10.1016/j.molliq.2016.05.080

Journal of Molecular Liquids 221 (2016) 254–261
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Theoretical and experimental studies on proton transfer in acetate-based
protic ionic liquids
b,
Xuejun Sun a, Bobo Cao a, Xinming Zhou b, Shuangyue Liu a, Xiao Zhu a, , Hu Fu
⁎
⁎
a
School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165,China
b
College of Science, Chian University of Petroleum, Shandong, Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the proton transfer of three protic ionic liquids (PILs) pyrrolidinium acetate ([Pyrrol]OAc),
diethylammonium acetate ([DEA]OAc) and bis-(2-methoxyethyl)-ammonium acetate ([BMOEA]OAc) were in-
vestigated. At first, the structures of the ion-pairs and molecular pairs of these PILs were optimized at B3LYP/6-
311++G(d,p) level. The interaction energy between anions and cations was also obtained. The proton transfer
processes were verified by intrinsic reaction coordinate (IRC) pathways tracing to the energy profiles connecting
the transition state (TS) to the two desired minima, i.e. ion pair and molecular pair. The experimental attenuated
total reflection (ATR) FTIR spectra of these PILs at room temperature were determined and compared with the
results calculated at B3LYP/6-311++G(d,p) level. Vibrational mode analyses (VMA) for [Pyrrol]OAc found
that δ(NH) has an imaginary frequency (−147.3 cm−1), which is accounted for proton transfer from [NH2]+
to OAc−. Natural bond orbital (NBO) analyses pointed out that second order perturbation stabilization energy
of (E(2)) of LP(N1) → σ*(O2-H5) was much larger than that of other orbitals, and should be the symmetrical
matching with the maximum overlap and the minimum gap (0.73 au). The hybridized index of N atom is varied
from sp3.65 in ionic pair to sp4.49 in TS. The constituent of s orbital decreases 3.3% and the length of N1-H5 in-
creases from 1.02 Å in ionic pair to 1.65 Å in TS, and the symmetric stretching vibration takes place the red
shift. It could be explained that the N1-O2-H5 played an important role in the stabilization of molecular pair.
The electron density ρ(r) and the Laplacian of the electron density ∇2ρ(r) derived from atoms in molecules
(AIM) analyses were used to describe the intensity and characteristic of a bond. The results indicate that a very
strong interaction of the hydrogen bonds exists in the ion-pair geometries and the bonds are the covalent bond.
© 2015 Elsevier B.V. All rights reserved.
Received 5 March 2016
Received in revised form 13 May 2016
Accepted 25 May 2016
Available online 26 May 2016
Keywords:
Protic ionic liquids
Proton transfer
Interaction energy
FTIR spectra
NBO
AIM
1. Introduction
gas phase. Recently, Li et al. [15] used EI-MS spectroscopy to analyse
the compositions of gaseous ILs, and the study showed that the dynamic
As an important branch of ILs, protic ionic liquid (PIL) is mentioned
frequently in recent years [1,2]. PILs is often prepared through the ex-
change of protons between a Brønsted acid and a Brønsted base [3].
Compared to aprotic ILs (AILs), PILs possess strong acidic protons [4–
7] (acts not only as a donor, but also as an acceptor) as well as significant
vapour pressure and so on, which can be easily prepared, purified and
protonated [8]. The earliest report on PILs was in the end of 19th century
[9,10]. Rebelo et al. [11] and Ballone et al. [12] had revealed that the va-
pour phase of AILs consists of ion pairs, with no detectable concentra-
tion of either free ions or larger clusters. Vaghjiani et al. [13]
demonstrated the presence of intact ion pairs in the gas phase using
mass spectrometry. Ohno et al. [14] revealed that protons in PILs trans-
fer incompletely, the molecule pairs could coexist with ion pairs in the
balance between ions and molecules could be controlled by the acidity.
Since the EI-MS spectroscope [14–17] experiments have confirmed the
dynamic balance of the molecular pairs with the ionic pairs in the gas
phase, the proton transfer from the ionic pairs to molecular pairs should
happen. Through the investigation of the experimental results and the-
oretical calculations, we believe that a transition state (TS) exists in the
proton transfer processes.
In this paper, the proton transfer processes in three acetate-based PILs,
pyrrolidinium acetate ([Pyrrol]OAc), diethylammonium acetate ([DEA]OAc)
and bis-(2-methoxyethyl)-ammonium acetate ([BMOEA]OAc) were in-
vestigated. The stable geometries and interaction modes of the cation,
anion, ionic pair, TS and molecular pair have been obtained and charac-
terized by density functional theory (DFT) calculations. The proton
transfer from cation to anion can be observed and testified by the TS
and intrinsic reaction coordinate (IRC) calculations. The lower energy
barriers indicate that dynamic balance exists between the ionic pairs
and molecular pairs. Attenuated total reflection Fourier transform infrared
(ATR-FTIR) spectra are used to detect the vibrational frequencies of the
⁎
Corresponding authors.
E-mail addresses: sxsunxuejun@163.com (X. Sun), qufucaobobo@163.com (B. Cao),
zhouxinming@upc.edu.cn (X. Zhou), liushuangyue0919@163.com (S. Liu),
qfnu_zx@163.com (X. Zhu), fuhui@upc.edu.cn (H. Fu).
http://dx.doi.org/10.1016/j.molliq.2016.05.080
0167-7322/© 2015 Elsevier B.V. All rights reserved.

X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
255
ionic pairs and molecular pairs. The chemical nature of covalent bond and
hydrogen bond as well as proton transfer process were analysed by the vi-
brational modes analyses (VMA), nature bond orbital (NBO) and theory of
atoms in molecules (AIM). The results indicate that proton transfer process
plays an important role in the physicochemical property of PILs.
stereo structure and the charge density distribution. 3D plots of the
ESP surface for above ions are given in Fig. 1. It indicates that the highly
negative regions (red) of OAc− anion are found around the polar oxy-
gen atom of the carboxyl group and show high activity on the electro-
negative O atoms. In contrast, the highly positive regions (blue) in
[Pyrrol]+, [DEA]+and [BMOEA]+ are localized on the hydrogen atom
of the amine groups, which can be considered as possible sites for nucle-
ophilic attack of the oxygen atom.
2. Experimental methods
2.1. Synthesis of acetate-based PILs
The interaction energy (ΔE) is defined as the difference between the
energy of the ionic system (Eionpair) and the sum of the energies of the
purely cationic (Ecation) and anionic (Eanion) species
Equivalent mol of Brønsted acid AcOH and Brønsted base [Pyrrol],
[DEA] and [BMOEA] were added into a round-bottom flask simulta-
neously and stirred vigorously in acetone-dry ice bath to dissipate the
exothermic heat, then the mixture was dried under vacuum for 24 h
at 40 °C. The purities of the as-prepared PILs were determined by 1H
NMR and 13C NMR (Bruker AM 400 MHz spectrometer) spectra, and
no impurities were found. The water contents of these PILs were deter-
mined by Karl-Fischer titration, which should be below 100 ppm.
ΔE ¼ Eionpair−Ecation−Eanion
ð1Þ
the interaction energies of [Pyrrol]OAc, [DEA]OAC and [BMOEA]OAc are
515.05, 505.38 and 499.76 kJ/mol, respectively. It can be found that all of
the interaction energies between cation and acetate anion are around
500 kJ/mol. They are much larger than the normal hydrogen bond ener-
gy (50 kJ/mol), which indicate the existence of strong electrostatic at-
tractions between cations and acetate anion [25]. The maximum value
of the interaction energies is ion pair of [Pyrrol]OAc, the reason is that
the hydrogen atom in N1 atom possessing larger positive charge. For
[BMOEA]OAc ion pair, the interaction energies decreases along with
the increasing alkyl chain length.
2.2. ATR-FTIR spectra measurement
ATR-IR spectra were carried out at a Prestige-21 FT-IR spectrometer
(Shimadzu, Japan) using a single reflection ATR cell. The results were
obtained in the DTGS detector mode using an accumulation rate of 40
scans at a resolution of 4 cm−1 in the spectral range of 400 to
4600 cm−1 at room temperature. The FTIR spectrum measurement for
each sample was repeated for three times.
4.2. Proton transfer process
The equilibrium of proton transfer process exists in acetate-based
PILs, which can be expressed as
3. Computation methods
HBþ þ A− ¼ HA þ B
ð2Þ
All ground state and TS geometries were located by using hybrid
Becke 3-Lee-Yang-Parr (B3LYP) exchange-correlation functional with
the 6-311++G(d,p) basis sets [18,19]. The chosen basis set contains
diffuse functions that are required for the correct description of intra-
molecular hydrogen bonds [20]. No restrictions on symmetries were
imposed on the initial structures, therefore the geometry optimization
for the saddle points occurred with all degrees of freedom. All the geom-
etries were optimized at B3LYP/6-31G(d,p) level and also characterized
as minima by frequency analyses at first, then geometry optimizations
were performed at B3LYP/6-311++G(d,p) basis sets level, followed
by frequency calculations to verify the reasonability of the optimized
structures. After that, the IRC pathways were traced to verify the energy
profiles connecting the transition structure to the two desired minima
of the proposed mechanism. Furthermore, the interaction energy (ΔE)
including the basis set superposition errors (BSSE) [21] correction
using the counterpoise (CP) [22] method was estimated. Finally, NBO
and AIM analyses were performed using B3LYP/6-311++G(d,p)
basis set to confirm the existence and characteristic of the hydrogen
bond. All the DFT calculations were carried out with Gaussian 03
package [23].
where HB+ and A− are Brønsted acid of B and Brønsted base of HA, re-
spectively. For acetate-based PILs, the equilibrium constant is not very
large. The PIL contains not only ionic pair but also molecular pair to
form a complex system. The interaction energies of HB+ and A− are
very large, corresponding to Coulomb force. HA and B are neutral spe-
cies, the interaction energies among them are smaller, corresponding
to hydrogen bonding and van der Waals forces. This is why PILs have a
vapour pressure [3,16].
4. Results and discussion
4.1. Geometries and electrostatic potential analyses
Geometries of [Pyrrol]+, [DEA]+, [BMOEA]+ and OAc− were opti-
mized at B3LYP/6-311+G(d,p) level in order to give a visual under-
standing on interactions before the design of initial geometries for the
ion pairs. The structural parameters are shown in Table S1 (Electronic
Supplementary information). The electrostatic potential (ESP) is a rigor-
ously defined expectation quantity which is measured as the first-order
interaction between the charge (electrons and nuclei) distribution and a
positive unit charge at any point in space surrounding molecules and
ions or solvents, and a real physical property that can be determined ex-
perimentally by diffraction methods, as well as computationally [24].
The ESP surface at sites close to the polar group is influenced by the
Fig. 1. The 3D plots of the electrostatic potential surface for four kinds ions, respectively.

256
X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
For the proton transfer process of Eq. (2), IRC pathways are traced
1700 cm−1 and 0.958 for wave numbers greater than 1700 cm−1 [26,
27]. The calculated wave numbers for B3LYP method scale down the cal-
culated harmonic frequencies in order to improve the agreement with
the experiments. Taking [Pyrrol]OAc as an example, the infrared peaks
appearing at 3169.1 cm−1 and 3246.9 cm−1 in the experiment are de-
signed to ν(s)(−N1-H5) and ν(as)(−N1-H5) stretching vibration cor-
to verify the energy profiles connecting the TS to the two desired
minima. The energy profiles are shown in Fig. 2. For [Pyrrol]OAc,
[DEA]OAc and [BMOEA]OAc, the energy barriers are 20.3, 17.3 and
11.5 kJ/mol, respectively, therefore, the order of the proton transfer
are [Pyrrol]OAc N [DEA]OAc N [BMOEA]OAc.
Taking [Pyrrol]OAc for an example, when OAc− approaches
[Pyrrol]+, O2 atom in OAc− forms hydrogen bonding with H5 in
[Pyrrol]+, and the bonding length is 1.02 Å. Then the proton will transfer
from [Pyrrol]+ through TS to molecular pair. Main configuration chang-
es are displayed in Fig. 2. For [DEA]OAc and [BMOEA]OAc, the configura-
tion changes are similar to [Pyrrol]OAc, but bonding lengths of O2…H5
are 1.01 Å.
responding with the calculated values 3456.5 cm−1 and 3504.7 cm−1
,
respectively (Table 1). Also, the infrared band at 1821.3 cm−1 refers to
δ(−N1-H5) symmetric bending vibration which is in agreement with
the theory result 1899.5 cm−1. The experimental values 1355.9 cm−1
and 1656.2 cm−1 are due to ν(s)(O2-C1-O3) and ν(as)(O2-C1-O3)
stretching vibration in consistent with 1344.9 cm−1 and 1648.8 cm−1
too. Table 1 shows that the experimental results are in better agreement
with the scaled calculated values. The difference between the observed
and calculated frequencies may be due to the fact that the calculations
have been actually done on a single ion pair in the gaseous state con-
trary to the experimental values recorded in the presence of intermolec-
ular interactions. It can be found that the experiment values of the
frequencies are limited by some experimental conditions. For example,
there is no way to get the vibration spectrum in TS from the experiment,
so the calculated frequency values are helpful to analyses the vibration
spectrum of the ILs.
4.3. Vibrational mode analyses
Tables 1–3 list the wave numbers of the bands observed in ATR-FTIR
spectra for the three PILs at room temperature. The vibration spectrum
variation of ion pair, TS and molecular pair calculated at B3LYP/6-
311++G(d, p) level also shown for comparison. The experimental IR
spectra are given in Figs. S1–S3 (Electronic Supplementary informa-
tion). We used the scaling factor values 0.983 for wave numbers up to
Fig. 2. The energy profiles of [Pyrrol]OAc (a), [DEA]OAc (b) and [BMOEA]OAc (c).

X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
257
Table 1
Table 3
The vibrational frequencies observed in ATR-FTIR spectra and theoretical calculation for
the ion-pair of acetate-based PILs at room temperature.
The vibrational frequencies observed in ATR-FTIR spectra and theoretical calculation for
the Mol-pair of acetate-based PILs at room temperature.
Bonds
Assignments
Experiments/ (cm−1
)
DFT calculations/ (cm−1
)
Bonds
Assignments
IR spectrum/ (cm−1
)
DFT calculations/ (cm−1
)
[Pyrrol]OAc
N-H
[Pyrrol]
N-H
δ
1821.3
3169.1,3246.9
–
931.6
978.8, 1014.3
–
–
1899.5
3456.5, 3504.7
–
822.3, 904.7
893.8, 1007.9
616.1
613.8, 1028.8
1344.9
δ
1378.9
3258.5
–
976.1
1076.5
1425.4
3541.8
–
655.7, 892.7
917.8, 1075.5
ν(s), ν(a)
ν(s), ν(as)
δ
ν(s)
C-N-C
O-C-O
δ
C-N-C
ν(s)
ν(as)
δ
ν(as)
[DEA]
N-H
ω
ν(s)
ν(as)
δ
1454.7
3279.6
–
1474.9
3474.0
420.6
1355.9
1656.2
ν(s), ν(as)
δ
1648.8
C-N-C
[DEA]OAc
N-H
ν(s)
ν(as)
1039.9
1131.9
1050.1
1094.7, 1120.0
δ
–
1661.3
ν(s), ν(as)
δ
ν(s)
ν(as)
δ
3224.4, 3261.3
–
820.7
911.3, 1007.4
–
3409.3, 3456.7
389.4
840.5
898.6, 1005.0
616.1
[BMOEA]
N-H
C-N-C
O-C-O
δ
1454.7
3241.8
–
1094.2
1198.0
1503.6
3479.6
242.4, 365.8, 550.5
948.1, 1011.3
1133.3, 1150.5
ν(s), ν(as)
δ
ν(s)
C-N-C
ω
ν(s)
ν(as)
1034.2
1332.8
1631.4
613.8, 1028.8
1344.9
1648.8
ν(as)
HAc
O-C-O
δ
ω
–
621.9
604.6, 1067.0
1302.6
1759.0
2724.5
[BMOEA]OAc
1052.6
1226.1
1711.4
2715.2
N-H
δ
–
1441.5
ν(s)
ν(as)
ν(s), ν(as)
ν(s), ν(as)
3235.8,3278.7
–
923.3, 1010.3
953.9
3408.1, 3455.0
244.2, 306.0, 508.7
928.9, 1001.5
955.5, 977.0
616.1
C-N-C
δ
O-H
ν(s)
ν(as)
δ
O-C-O
–
ω
ν(s)
ν(as)
1047.6
1328.1
1707.8
613.8, 1028.8
1344.9
1648.8
(−N1-H5) to (−C1-O2-H5), and the C_O double bond of carbonyl
stretched to C\\O single bond, and further demonstrates the above pro-
ton transfer. In addition to these modes, the bending vibration δ(−NH2)
(ion pair) in 1899.5 cm−1 has a red shift towards 1424.1 cm−1 (TS), and
asymmetric stretching vibration ν(as)(−NH2) in 3504.7 cm−1 (ionic
pair) has a blue shift to 3544.9 cm−1 (TS). At the same time the carbonyl
stretching vibration ν(s)(O-C-O) has a red shift from 1344.9 cm−1
(ionic pair) to 1238.5 cm−1 (TS), and ν(s)(O-C-O) is increased from
1648.8 cm−1 (ionic pair) to 1783.3 cm−1 (TS). It is a strong indication
that a carbonic acid species is produced from the interaction between
the amino group and CO2. The appearance of the new peak at
2724.5 cm−1 corresponding to the ν-(OH) frequency of –COOH is also
indicative of carbonic acid formation [28,29].
For [Pyrrol]OAc, it can be found from Tables 1–3 that there are some
differences in proton transfer process. The symmetric stretching vibra-
tion ν(s)(NH2) in 3456.5 cm−1 and asymmetric stretching vibration
ν(as)(NH2) in 3504.7 cm−1 of ion pair disappear and the symmetric
bending vibration ν(s)(OH) appears an imaginary frequency
−147.3 cm−1 (TS). It can account for proton of NH2 transfers from
Table 2
The vibrational frequencies of theoretical calculation for the transition state of acetate-
based PILs at room temperature.
PILs
Bonds
N-H
Assignments
DFT calculations/ (cm−1
)
4.4. NBO analyses
[Pyrrol]OAc
δ
−147.3
ν(s), ν(a)
δ
1424.1, 3544.9
–
C-N-C
O-C-O
Natural bond orbital (NBO) theories are carried out to obtain the
charge distribution and intrinsic property of the interactions between
the most stable acetate based PILs [30]. The main charge distribution,
donor-acceptor interactions, and their second order perturbation stabi-
lization energies E(2) calculated at the B3LYP/6-311++G(d,p) level are
given in Tables 4–5.
ν(s)
ν(as)
δ
ω
ν(s)
ν(as)
δ
ν(s), ν(as)
δ
ν(s)
630.7, 893.0
927.4, 1075.1
603.7
1051.0, 590.2
1238.5
1783.3
−141.3
1473.2, 3473.2
421.7
1048.3
[DEA]OAc
N-H
The complete results are shown in Tables S4–S5 (Electronic Supple-
mentary information). Table 4 shows that most of the positive charge is
C-N-C
focused on the peripheral hydrogen atoms of N atoms in [Pyrrol]+
,
[DEA]+ and [BMOEA]+ cations, while OAc− preferentially approaches
the positively charged groups, indicating that the electrostatic interac-
tion between cations and OAc− is dominative for the formation of ion
pair. Also, H5 on N1 atom transfers to the oxygen atom, and forms
N1…H5…O2 hydrogen bond. For [Pyrrol]OAc, the charge density of
N1 atom changes from −0.55010 (ion pair) to −0.71618 (TS), then
to 0.71420 (molecular pair); the charge density of H5 atom changes
from 0.43241 (ion pair) to 0.50400 (TS), then to 0.50291 (molecular
pair). The charge density of O2 atom changes from −0.80228 (ion
pair) to −0.79482 (TS), then to −0.73013 (molecular pair). For
[DEA]OAc and [BMOEA]OAc, the change of the charge density is similar
as that of [Pyrrol]OAc.
ν(as)
δ
ω
ν(s)
ν(as)
δ
ν(s), ν(as)
δ
ν(s)
ν(as)
δ
ω
ν(s)
ν(as)
1121.3, 1125.9
604.2
589.7, 1050.8
1238.0
1784.9
−139.9
1488.3, 3471.6
256.6, 324.2, 529.9
959.8, 1046.6
1123.3, 1147.6
603.6
590.4, 1049.5
1234.8
1787.3
O-C-O
[BMOEA]OAc
N-H
C-N-C
O-C-O

258
X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
Table 4
pathways from the TS to the corresponding ionic pair and molecular
The main charge distribution in acetate-based PILs.
pair. As shown in Table 5, for ionic pair, there are seven symmetrical
matching orbital in OAc− and four symmetrical matching orbital in
[Pyrrol]+, that is (LP1(O3) → σ*(C1-O2), (LP2(O3) → σ*(C1-C6),
(LP2(O3) → σ*(C1-O2), (LP1(O2) → σ*(C1-O3), (LP2(O2) → σ*(C1-
C6), (LP2(O2) → σ*(C1-O3) and (LP3(O2) → σ*(C1-O3) in OAc−, and
σ(N1-H4) → σ*(C2-H7), σ(N1-H4) → σ*(C3-H9), σ(N1-H5) → σ*(C2-
H6), and σ(N1-H5) → σ*(C3-H8) in [Pyrrol]+. The schematic graphs of
the electron transfer occurring from the lone pairs of N atoms to the an-
tibonding orbital of the proton donor based on the NBO analyses are
shown in Fig. S4. The largest value of E(2) is LP3(O2) → σ*(C1-O3) inter-
action orbital, it means the strongest orbital interactions between elec-
tron donor of the long pair LP3(O2) and the sigma antibond orbital of
the electron acceptor σ*(C1-O3). Fig. 3A shows that it should be the
best overlap symmetrical matching orbital, and the smaller energy gap
(0.25 au) also play an important role in proton transfer reaction. For
[Pyrrol]+, all of E(2) values of orbital interaction are smaller and almost
identical (3.1768 kJ/mol). It may be due to the symmetrical matching
orbital and the larger energy gap (1.11 au). The strongest interaction
of LP1(N1) → σ*(O2-H5) exists in transition state, the E(2) value
(160.9718 kJ/mol) is much larger than those of other orbital, as shown
in Fig. 3B. It should be the symmetrical matching with the maximum
overlap and the minimum gap (0.73 au). What is more, the hybridized
index of N atom varies from sp3.65 in ionic pair to sp4.49 in TS, the constit-
uent of s orbital decreases 3.3% and the length of N1-H5 increases from
1.02 Å in ionic pair to 1.65 Å in TS, and the symmetric stretching vibra-
tion takes place the red shift. The hybridized index of O atom in OAc−
changes from sp1.63 in ionic pair to sp3.35 in TS. The component of s or-
bital decreases 14.99%, which indicates that the proton has transferred
to O atom, and the covalent bond of O\\H has been formed, thus it is
concerted reaction. For molecular pair the strongest interaction orbital
is LP1(N1) → σ*(H5-O2), the E(2) value (156.6246 kJ/mol) is much larg-
er than those of other orbital, and is slightly less than that (160.9718 kJ/
mol) in transition state, the schematic graph is given in Fig. 3C1. The
long pair orbital of N1 in sp4.44 overlaps with H5-O2 orbital by head to
head way, and with the minimum gap (0.71 au), which leads to the for-
mation of molecular pair (product). In contrast, the interaction orbital of
Atom
Ion-pair
TS
Mol-pair
[Pyrrol]OAc
[DEA]OAc
N1
O2
O3
H4
H5
N1
O2
O3
H4
H5
N1
O2
O3
H4
H5
−0.55010
−0.80228
−0.79391
0.43241
−0.71618
−0.79482
−0.58049
0.37009
−0.71420
−0.73013
−0.64265
0.36666
0.43241
0.50400
0.50291
−0.53022
−0.80228
−0.79391
0.41900
−0.69987
−0.79113
−0.57717
0.35454
−0.70071
−0.73135
−0.63743
0.34872
0.41900
0.50175
0.50108
[BMOEA]OAc
−0.53451
−0.80228
−0.79391
0.42181
−0.69419
−0.79271
−0.57501
0.35627
−0.70715
−0.72179
−0.58691
0.35630
0.42181
0.50068
0.49692
Second order perturbation stabilization energy E(2) denotes the in-
tensity of the donor-acceptor interactions. The larger of E(2) is, the
stronger of the donor-acceptor capacity will be. Taking [Pyrrol]OAc as
an example, it can be mainly considered that the proton transfer takes
part in isomerization reaction. Since TS has only single imaginary fre-
quency, the IRC pathways are traced to verify the minimal energetic
Table 5
The main donor-acceptor interactions and their second order perturbation stabilization
energies, E(2) calculated at the B3LYP/6–311++G(d,p) levels.
Donor
Acceptor
E(2) (kJ/mol)
E(j)-E(i) (a.u.)
Ion pairs
[Pyrrol]+
σ(N1-H5)
σ(N1-H5)
σ(N1-H4)
σ(N1-H4)
σ*(C2-H6)
σ*(C3-H8)
σ*(C2-H7)
σ*(C3-H9)
3.1768
3.1768
3.1768
3.1768
1.11
1.11
1.11
1.11
[DEA]+
σ(N1-C4)
σ(N1-H4)
σ(N1-C2)
σ(C2-C4)
σ*(C2-H6)
σ*(C3-H8)
σ*(C4-H10)
σ*(C2-H6)
5.1414
5.1414
3.2604
2.2154
1.09
1.09
1.18
1.03
the asymmetrical matching LP1(O3)
→
σ*(H8-C3) and
LP1(O3) → σ*(H10-C4) are given in Fig. 3C2 and C3, respectively. It
can been found that the overlaps are less from Fig. 3C2 and C3, while
the energy gaps are more than 1.10 au, so E(2) values are only 0.209
and 0.7106 kJ/mol, respectively.
The proton transfer reaction of [DEA]OAc and [BMOEA]OAc, is simi-
lar to that of [Pyrrol]OAc, the differences are only in the values of the in-
teraction orbital. For the same anion in ionic pair, the interaction orbital
of σ(N1-H4) → σ*(C2-H7) in [Pyrrol]+ is corresponding to that of σ(N1-
H4) → σ*(C2-H6) in [DEA]+ and σ(N1-H4) → σ*(C2-H6) in [BMOEA]+,
and the E(2) values in [Pyrrol]+, [DEA]+ and [BMOEA]+ are 3.18, 5.14
and 5.35 kJ/mol, respectively. For TS, the largest interaction orbital of
[BMOEA]+
σ(N1-C2)
σ(N1-C3)
σ(N1-H4)
σ(C3-C5)
σ*(N1-C3)
σ*(N1-C2)
σ*(C2-H6)
σ*(C3-H9)
2.3408
2.3408
5.3504
2.1318
1.00
1.00
1.10
1.03
OAc−
LP(O2)
LP(O2)
LP(O3)
LP(O3)
LP(O3)
LP(O2)
LP(O2)
σ*(C1-O3)
σ*(C1-C6)
σ*(C1-C6)
σ*(C1-O2)
σ*(C1-O2)
σ*(C1-O3)
σ*(C1-O3)
451.482
7.8166
0.25
1.01
0.56
0.81
1.26
0.81
1.26
78.9602
77.2464
13.6686
76.9120
12.9580
LP1(N1)
→
σ*(O2-H5) in [Pyrrol]OAc is corresponding to
LP1(N1) → σ*(H5-O2) in [DEA]OAc and LP1(N1) → σ*(H5-O2) in
[BMOEA]OAc, and E(2) values are 160.97, 136.64 and 130.08 kJ/mol, re-
spectively. For molecular pair, the largest E(2) values are 156.62, 132.96
and 88.62 kJ/mol, respectively. The results are in agreement with the
above relative energy analyses.
TS
[Pyrrol]
σ(O2-H5)
LP (O2)
σ(H4-N1)
LP (N1)
σ*(H4-N1)
σ*(H4-N1)
σ*(O2-H5)
σ*(O2-H5)
0.2926
0.3344
2.299
1.08
0.72
1.03
0.73
160.972
4.5. AIM analyses
[DEA]
σ(N1-H4)
σ(N1-C2)
σ(C2-H7)
LP (N1)
σ*(H5-O2)
σ*(H5-O2)
σ*(H5-O2)
σ*(H5-O2)
2.8006
1.5884
0.418
1.02
1.12
0.91
0.73
Atoms in molecules theory has been applied widely to study the
bond properties between each pair of atoms [31]. Within the framework
of this theory, a chemical bond can be characterized by topological anal-
yses based on the electron density ρ(r) and the Laplacian of the electron
density ∇2ρ(r). ρ(r) is used to describe the intensity of a bond, and a
larger ρ(r) value corresponds to a stronger bond. ∇2ρ(r) describes the
characteristic of the bond, and ∇2ρ(r) b 0 corresponds to covalent
bond. While ∇2ρ(r) N 0 corresponds to closed-shell interactions like
136.644
[BMOEA]
σ(N1-H4)
σ(N1-C2)
σ(C2-H7)
LP (N1)
σ*(H5–O2)
σ*(H5–O2)
σ*(H5–O2)
σ*(H5–O2)
2.6752
1.2958
0.4598
1.02
1.13
0.91
0.74
130.082

X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
259
Fig. 3. The schematic graphs of the electron transfer occurring from the lone pairs of N atoms to the antibonding orbital of the proton donor based on the NBO analyses.
van der Waals interactions, hydrogen and ionic bonds. ∇2ρ(r) =
λ1 + λ2 + λ3, where λi is an eigenvalue of the Hessian matrix of ρ(r).
A Bond Critical Point (BCP) is defined that two of three eigenvalues
are negative and the other positive.
imidazolium ILs, for example ρ(r) of [emim]+Cl− [34] and imidazolium
amino acid ILs, ([emim]+[AA]− [35] are in the range of 0.0242–
0.0436 a.u. and 0.0486–0.0571, respectively. It indicates that very strong
interaction of the hydrogen bonds exists in the ionic pair of [Pyrrol]OAc,
[DEA]OAc and [BMOEA]OAc. It is why the interaction energies of those
ILs are very large. From Table 7 it can be found that Laplacian values of
the electron density, ∇2ρ(r) of O2-H5 in TS and molecular pair are
more than zero, which indicates that the bonds are the covalent bond
and a carbonic acid, –COOH [36,37] forms. In other word, the more neg-
ative values are, the stronger the covalent bonds are, and the easier the
trend of the proton transfer will be.
The ρ(r), ∇2ρ(r), and λi of ion pair, TS and molecular pair are given in
Tables 6 and S6. For [Pyrrol]OAc, [DEA]OAc and [BMOEA]OAc, the bonds
of N1-H5 in TS and molecular pair as well as O2-H5 in ionic pair are in a
BCP, i.e. two of three eigenvalues λi are negative and the other is posi-
tive. All the ρ(r) values are in the range of 0.033–0.070. The results are
not only more than the criteria values proposed by Popelier [32,33],
i.e. 0.002–0.035 a.u., but also more than the electron density of

260
X. Sun et al. / Journal of Molecular Liquids 221 (2016) 254–261
Table 6
The main electron density, ρI and the Laplacian of the electron density, ∇2ρI as well as are the eigenvalue (λi) of Hessian matrix of ion pair, TS and molecular pair.
Bond
ρ
∇2ρ
λ1
λ2
λ3
[Pyrrol]OAc
[DEA]OAc
Mol-pair
TS
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
N1-H5
O2-H5
0.060253
0.29885
0.063998
0.29232
0.33842
0.06002
0.055188
0.30359
0.05824
0.29782
0.33604
0.04893
0.041642
0.32785
0.055446
0.30144
0.33606
0.05218
0.09395
−1.917
0.09576
−1.818
−1.707
0.09921
0.09275
−1.979
0.09538
−1.8648
−1.663
0.09319
0.09283
−2.230
0.09505
−1.933
−1.674
0.09137
−0.1088
−1.425
−0.1187
−1.371
−0.1083
−1.404
−0.1166
−1.342
0.3111
0.9113
0.3310
0.8948
0.8697
0.3213
0.2840
0.9227
0.3179
0.9049
0.8711
0.2318
0.2205
0.9835
0.2860
0.9140
0.8679
0.2934
Ion-pair
Mol-pair
TS
−1.289
−1.288
−0.09248
−0.09595
−1.461
−0.1076
−1.4120
−1.268
−0.08313
−0.06415
−1.619
−0.09607
−1.439
−0.1334
−0.09535
−1.440
−0.0846
−1.3912
−1.266
−0.07159
−0.06355
−1.594
−0.0493
−1.408
Ion-pair
Mol-pair
TS
[BMOEA]OAc
Ion-pair
−1.271
−0.09013
−1.271
−0.09059
5. Conclusions
interaction of hydrogen bonds in the ion-pair. ∇2ρ(r) of O2-H5 in TS and
Mol-pair ˃ 0 indicates that the bonds are covalent bond.
In this paper, the proton transfer process of three acetated based PILs
[DEA]OAc, [Pyrrol]OAc and [BMOEA]OAc are investigated by DFT calcu-
Acknowledgements
lations and ATR-FTIR experiments. At first, the geometries of [DEA]+
,
[BMOEA]+
,
[Pyrrol]+ and OAc− are optimized by B3LYP/6-
This work was supported by the National Natural Science Founda-
tion of China (No. 21206085 and 21203250).
311++G(d,p) level in order to give a visual understanding on the inter-
actions before the design of initial geometries for the ion-pairs. Then,
the interaction energies of [DEA]OAc, [Pyrrol]OAc and [BMOEA]OAc
are calculated, which indicate that there exist strong electrostatic attrac-
tions between cations and acetate anion. After that, the proton transfer
process of PILs is verified by tracing IRC pathways, and the transition
state is obtained. Also, the vibration spectrum variation of ion pair, tran-
sition state and molecular pair are calculated at B3LYP/6-311++G(d,p)
level and compared with the experimental ATR-FTIR spectra. Vibration-
al mode analyses results indicate the existence of an imaginary frequen-
cy, thus, δ(s)(OH) of transition state can account for H proton of NH2
transfers from (−N1-H5) to (−C1-O2-H5). NBO analyses give that
E(2) of LP(N1) → σ*(O2-H5) are ten fold larger than other hydrogen
bonds, which means that it plays an important role in the stabilization
of molecular pair. AIM analyses afford the information of the topological
electron density ρ(r) and Laplacian of the electron density ∇2ρ(r), which
describing the intensity and the characteristic of the bond. All the ρ(r)
values are in the range of 0.033–0.070 which are much larger than nor-
mal hydrogen bond energy, which indicates the existence of very strong
Appendix A. Supplementary data
This material is available via the Internet at the website of Journal of
Molecular Liquids for free.
We are gonging to a color printing when this manuscript being ac-
cepted by Journal of Molecular Liquids. Supplementary data associated
with this article can be found in the online version, at http://dx.doi.
org/10.1016/j.molliq.2016.05.080.
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