Vaporization of protic ionic liquids studied by matrix-isolation fourier transform infrared spectroscopy

Article abstract of DOI:10.1021/jp501784w

Several protic ionic liquids (PILs) with a wide range of pK_a differences (ΔpK_a) between the parent acid and base molecules were thermally evaporated in vacuum, trapped on a CsI plate by a cryogenic neon matrix-isolation method, and studied by Fourier transform infrared spectroscopy and density functional theory calculations. The parent neutral molecules and proton-transferred cation-anion pair species were identified as chemical components evaporated from the PILs with lower and higher ΔpK_a values, respectively. The ΔpK_a-dependent vaporization mechanism is discussed in terms of thermodynamic equilibrium between acid-base and anion-cation systems in the liquid phase.

Full text of DOI:10.1021/jp501784w

Article
pubs.acs.org/JPCA
Vaporization of Protic Ionic Liquids Studied by Matrix-Isolation
Fourier Transform Infrared Spectroscopy
Mami Horikawa,‡ Nobuyuki Akai,*,† Akio Kawai,‡ and Kazuhiko Shibuya‡
†Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16,
Naka-cho, Koganei, Tokyo, 184-8588, Japan
‡Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, Ohokayama,
Meguro-ku, Tokyo, 152-8551 Japan
S
* Supporting Information
ABSTRACT: Several protic ionic liquids (PILs) with a wide range of pKa differences
(ΔpKa) between the parent acid and base molecules were thermally evaporated in
vacuum, trapped on a CsI plate by a cryogenic neon matrix-isolation method, and
studied by Fourier transform infrared spectroscopy and density functional theory
calculations. The parent neutral molecules and proton-transferred cation−anion pair
species were identified as chemical components evaporated from the PILs with lower
and higher ΔpKa values, respectively. The ΔpKa-dependent vaporization mechanism is
discussed in terms of thermodynamic equilibrium between acid−base and anion−
cation systems in the liquid phase.
independent molecules of acetic acid and methylimidazole.3
INTRODUCTION
■
The identification of the two parent molecules was also
Ionic liquids (ILs) made of anions and cations have unique
physicochemical properties different from those of ordinal
confirmed using Raman spectroscopy by Berg et al.26 They
reported on the results of 1,1,3,3-tetramethylguanidinium
chloride and bromide, or [H-TMG][Cl] and [H-TMG][Br],
and concluded that “ion pairs” probably do not exist in the
gaseous species,27,28 which accords with the results of the FT-
ICR-MS study.28,29 A theoretical work on 1-methylimidazolium
nitrate, [H-MIM][NO3], also predicts that the PIL vaporizes as
two “parent molecules”, methylimidazole and HNO3.30 As
pointed out by previous reports,31,32 the vaporization
mechanism of PILs would depend on the ionicity related
with the acidity and basicity of parent molecules. The ionicity is
estimated by using a difference in the aqueous pKa of the acid
(HA) and the protonated base (H−B+), which is defined as
ΔpKa = pKa(H−B+) − pKa(HA). Although the pKa difference
(ΔpKa) is derived from pKa determined in aqueous systems,
clear correlations between ΔpKa and physicochemical proper-
ties of pure PILs, like viscosity and ionic conductivity, are well-
known.33−35
molecular liquids and have received extensive attention from
researchers in a wide range of fields from basic science to many
practical applications. For a long time, nonvolatility had been
believed to be distinctive property of ILs , but some evaporable
aprotic ILs were discovered by Earle et al. in 2006.1 After the
landmark report on the evaporation, gaseous chemical
components evaporated from ILs have been investigated
cooperatively by spectroscopic experimentalists2−15 and theo-
reticians.14−19 Nowadays, one generally accepts a vaporization
mechanism in which the chemical components thermally
evaporated from aprotic ILs are of cation−anion pairs and
electrically neutral as a whole. The vaporization enthalpies of
many aprotic ILs have been reported,20−22 and recently some
aprotic ILs have been found to be vaporized as gaseous
chemical species made of one dication and two anions.23−25
On the other hand, the vaporization processes of protic ionic
liquids (PILs) have not been understood well. PILs are
prepared by proton transfer reactions from Brønsted acid (HA)
to Brønsted base (B): HA + B →A− + H−B+. In this paper,
[H−B][A] denotes PIL in the liquid phase, and the gaseous
chemical species evaporated from [H−B][A] are described by
“ion-pair” and/or “parent molecules” for the cases of A−−H−
B+ and HA + B, respectively. Leal et al. reported Fourier
transform ion cyclotron resonance mass spectrometry (FT-
ICR-MS) of the gaseous component evaporated from 1-
methylimidazolium ethanoate, [H-MIM][AcO], and concluded
the evaporated species to be “parent molecules”, that is, two
In the present study, we focused on the relationship between
ΔpKa and chemical species vaporized from PILs, where nine
PILs were synthesized from different combinations of three
acids and three bases. The chemical components vaporized
from each PIL were identified by matrix-isolation IR spectros-
Received: February 19, 2014
Revised: April 8, 2014
Published: April 11, 2014
© 2014 American Chemical Society
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copy aided by quantum chemical calculations by reference to
our previous studies of aprotic ILs.8,12
neon gas in a glass reservoir at room temperature. The gas
mixture was deposited on a CsI plate cooled to ca. 6 K by a
closed-cycle helium refrigerator (Iwatani, Cryo Mini). The
matrix-isolated sample was measured with an FTIR spectro-
photometer (JEOL, SPX200ST) equipped with an MCT
detector (spectral resolution, 0.5 cm−1; accumulation number,
100). The IR spectra of neat PILs were obtained with 4 cm−1
resolution by an ATR method using a ZnSe crystal or a
transmission of the sample sandwiched between two KBr
plates.
EXPERIMENTAL METHODS
■
Materials. All PILs were synthesized by mixing equimolar
quantities of acid and base molecules. Three parent bases and
acids employed are shown in Scheme 1. Parent acids of acetic
Scheme 1. Parent Base and Acid Molecules Employed for the
Synthesis of PILs
Quantum Chemical Calculation. The energies, geo-
metries, and vibrational spectra of PILs and their parent
molecules were calculated by a density functional theory
(DFT) method on the Gaussian09 program package.36 The
B3LYP as a typical hybrid-functional of DFT was used with a
basis set of 6-311++G(3df,3pd). All calculations were executed
by a TSUBAME supercomputer of Tokyo Institute of
Technology.
RESULTS AND DISCUSSION
Protic ILs Synthesized from HAcO Acid. Figure 1 shows
the IR spectra relevant to a PIL of [H-MIM][AcO], which was
■
acid (HAcO), bis(trifluoromethanesulfonyl)amide (HTf2N)
and trifluoromethylsulfoate (HOTf), were obtained from
Kanto Chemical. The three parent bases were 1-methyl-
imidazole (MIM, from Acros Organics), triethylamine (TEA,
from Wako Pure Chemical Industries), and 1,1,3,3-tetramethyl-
guanidine (TMG, from Tokyo Chemical Industry) and were
used without further purification. The PILs were synthesized on
a Teflon-coated stainless-steel heating unit and purified in a
vacuum chamber under <10−5 Pa for more than 1−10 h to
remove excess parent molecule (acid or base) and impurity
water. The synthesized PILs are listed in Table 1, together with
their ΔpKa values.
Figure 1. IR spectra of [H-MIM][AcO]. Spectrum a was measured for
neat [H-MIM][AcO] liquid at room temperature. Spectra b, c, and d
were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-MIM][AcO], HAcO, and
MIM, respectively.
Matrix-Isolation FTIR Spectra. The purified sample was
moderately evaporated and mixed with much excess neon gas
in the heating unit attached on the head of a deposition nozzle.
Each gaseous parent molecule was premixed with 1000-fold
Table 1. Gaseous Components Evacuated from Synthesized
Protic ILs with ΔpKa Values of Parent Acid and Base
Molecules
synthesized by a combination of HAcO and MIM (the weakest
acid and base molecules, respectively, listed in Scheme 1). The
IR spectrum of neat [H-MIM][AcO] liquid measured at room
temperature is shown in Figure 1a. Figure 1b shows the matrix
isolation spectrum of chemical components vaporized at room
temperature from neat [H-MIM][AcO] liquid, which is
apparently different from that of the neat PIL spectrum. The
spectrum of Figure 1b is the superposition of two spectra
shown in Figure 1c,d, which correspond to the matrix-isolation
spectra of two authentic chemical species, HAcO and MIM,
respectively. This observation means that the parent Brønsted
acid and base molecules vaporize from neat [H-MIM][AcO]
liquid, which agrees with previous reports.25,26
vaporized
components
vaporization
temperature (K)
a
protic ionic liquid
ΔpKa
[H-MIM][AcO]
[H-TEA][AcO]
[H-Tmg][Aco]
[H-Mim][Otf]
[H-Tea][Otf]
2.6
5.8
parent molecules
parent molecules
298
298
b
8.8
not detected
b
14.4
17.6
20.6
17.4
20.6
23.6
not detected
ion-pair
363
b
[H-Tmg][Otf]
[H-Mim][Tf2N]
[H-TEA][Tf2N]
[H-TMG][Tf2N]
not detected
parent molecules
393
388
428
c
ion-pair
Similar experimental results were obtained for the IR spectra
relevant to the other PIL of [H-TEA][AcO] shown in Figure 2.
The matrix-isolation spectrum of [H-TEA][AcO] (Figure 2b)
is different from the IR spectrum of neat [H-MIM][AcO]
liquid (Figure 2a) and also accords with the superposition of
ion-pair
a
The values determined in aqueous solutions are obtained from refs
b
c
31 and 35. See text. A trace of the corresponding acid was
additionally detected for the vaporization at 433 K. See text.
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Figure 2. IR spectra of [H-TEA][AcO]. Spectrum a was measured for
neat [H-TEA][AcO] liquid at room temperature. Spectra b, c, and d
were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-TEA][AcO], HAcO, and
TEA, respectively. The bands marked with an asterisk were assigned to
impurity water, which was hardly removed from samples through a
standard evacuation procedure in vacuum.
Figure 4. IR spectra of [H-MIM][Tf2N]. Spectrum a was measured
for neat [H-MIM][Tf2N] liquid at room temperature. Spectra b, c, and
d were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-MIM][Tf2N], HTf2N, and
MIM, respectively. The synthesized [H-MIM][Tf2N] sample has
negligible vapor pressure at room temperature and was evaporated at
393 K.
Figure 3. IR spectra of [H-TEA][OTf]. Spectrum a was measured for
neat [H-TEA][OTf] liquid at room temperature. Spectra b, c, and d
were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-TEA][OTf], HOTf, and
TEA, respectively. Spectrum e was simulated for the most stable ion-
pair structure shown in Figure 7.
Figure 5. IR spectra of [H-TMG][Tf2N]. Spectrum a was measured
for neat [H-TMG][Tf2N] liquid at room temperature. Spectra b, c,
and d were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-TMG][Tf2N], HTf2N, and
TMG, respectively. The synthesized [H-TMG][Tf2N] sample has
negligible vapor pressure at room temperature, and was evaporated at
428 K. Spectra e and f were simulated for structure A and structure B,
respectively, shown in Figure 8.
the matrix-isolation spectra of HAcO and TEA (spectra c and d
of Figure 2, respectively).
We could not measure the matrix-isolation spectrum of [H-
TMG][AcO] in the present study because the vapor pressure
of [H-TMG][AcO] on the heating unit at room temperature
was too high for preparing the matrix-isolated sample. This
indicates that the parent TMG and HAcO molecules vaporize
from [H-TMG][AcO] at room temperature.
Protic ILs Synthesized from HOTf Acid. Spectra a and b
of Figure 3 show the IR spectra of neat [H-TEA][OTf] liquid
and the matrix-isolated gaseous sample, respectively. In contrast
to the results observed for protic ILs synthesized from HAcO
acid, the matrix spectrum (Figure 3b) seems to be more similar
to the neat spectrum (Figure 3a), but quite different from the
superposition of two matrix-isolation IR spectra of the parent
acid and base molecules (spectra c and d of Figure 3,
respectively). The IR bands due to parent HOTf and TEA are
hardly recognized in Figure 3b. For example, the 857 and 1073
cm−1 bands characteristic of HOTf and TEA, respectively, are
not recognized in Figure 3b, which means that both the parent
molecules do not exist in the vapor evaporated from neat [H-
TEA][OTf] liquid. Additionally, the bandwidths of peaks in
Figure 3b are broader than those observed in Figures 1b and
2b, which suggests that molecular complexes like ion pairs are
isolated in the cryogenic matrix. The broad spectral feature was
also recognized in other matrix-isolation spectra of evaporable
aprotic ILs, where the gas phase formation of ion pairs was
confirmed by matrix isolation spectroscopy of the deposited
vapor.8,12 Therefore, we tentatively conclude that liquid [H-
TEA][OTf] vaporizes as not the parent acid and base
molecules but the proton-transferred ion-pair complex. We
will discuss this point later by considering remarkable
differences of the neat and matrix-isolation spectra aided by
quantum chemical calculations, especially in the 1100−1400
cm−1 region.
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Table 2. Observed and Calculated Wavenumbers of Neat
[H-TEA][OTf] Liquid with Band Assignments
b
obs.
)
calc.
int. (km mol−1
v (cm−1
abs.
9.1
v (cm−1
)
)
assignment
̃
̃
1404
1395
1339
1318
1241
1237
1210
1193
1180
1164
1159
1026
6.4
59.3
20.3
20.2
78.0
1331
100
ν(SO3)
1224
1215
1189
1163
22.9
32.8
89.5
7.21
ν(CF3) + ν(SO3)
ν(CF3) + ν(SO3)
ν(CF3) + ν(SO3)
ν(CF3) + ν(SO3)
100
9.7
12.1
33.8
20.7
26.4
1136
57.9
ν(CF3)
1046
1044
1008
6.1
6.0
ν(NCC)
ν(NCC)
νs(SO3)
Figure 6. IR spectra of [H-TEA][Tf2N]. Spectrum a was measured for
neat [H-TEA][Tf2N] liquid at room temperature. Spectra b and c
were recorded for the cryogenic neon matrix-isolated samples of
gaseous components evaporated from [H-TEA][Tf2N] at 388 and 433
K, respectively. Spectra d and e correspond to the matrix-isolation
spectra of authentic samples of HTf2N and TEA. Spectra f and g were
simulated for structure C and structure D, respectively, shown in
Figure 9.
1019
1013
847
64.4
45.3
5.6
74.4
852
636
6.3
deformation
838
5.1
762
6.6
a
642
−
44.1
σ(SO3)
a
Accurate absorbance could not be estimated because the sensitivity of
the MCT detector was low in the wavenumber region below 655 cm−1.
b
Intensities are represented as relative intensities, and those with
relative intensity under 5 are omitted. All calculated wavenumbers are
shown in Supporting Information (Table S4). The symbols ν an σ
represent stretching and bending vibrational modes, respectively.
Figure 7. Most stable ion-pair structure of [H-TEA][OTf] calculated
at the B3LYP/6-311++G(3df,3pd) level. The hydrogen bonds are
indicated by the distances in angstroms.
We also synthesized two acid−base combinations of [H-
MIM][OTf] and [H-TMG][OTf]. However, neat [H-MIM]-
[OTf] existed as a solid at room temperature and thermally
decomposed without vaporization. Though we could not
vaporize neat [H-TMG][OTf] by heat, Zhu et al. successfully
vaporizied the sample by injecting a chloroform solution of [H-
TMG][OTf] into a conventional GC/MS apparatus and
concluded that the gaseous components are composed of the
parent molecules and ionic aggregates.31
Protic ILs Synthesized from HTf2N Acid. Figures 4, 5,
and 6 show the IR spectra relevant to three synthesized PILs of
[H-MIM][Tf2N], [H-TMG][Tf2N], and [H-TEA][Tf2N],
respectively. The matrix-isolation spectrum of [H-MIM][Tf2N]
(Figure 4b) is quite different from that of neat [H-
MIM][Tf2N] (Figure 4a) and corresponds with the super-
position of two matrix-isolation IR spectra of the parent acid
and base molecules shown in spectra c and d of Figure 4,
respectively.
On the other hand, the matrix-isolation spectra of [H-
TMG][Tf2N] (Figure 5b) and [H-TEA][Tf2N] (Figure 6b)
are similar to those of the corresponding neat PILs of [H-
TMG][Tf2N] (Figure 5a) and [H-TEA][Tf2N] (Figure 6a),
respectively. The superposition of two matrix-isolation IR
Figure 8. Most stable ion-pair structures of [H-TMG][Tf2N]
calculated at the B3LYP/6-311++G(3df,3pd) level. The most and
second-most stable structures are denoted by structure A and structure
B, respectively. The hydrogen bonds are indicated by the distances in
angstroms.
spectra of the parent acid and base molecules does not agree
with the matrix-isolation spectrum of the vaporized PIL in the
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Table 3. Observed and Calculated Wavenumbers of Neat
[H-TMG][Tf2N] Liquid with Band Assignments
b
obs.
)
calc. of structure A
int. (km mol−1
v (cm−1
abs.
v (cm−1
)
)
assignment
̃
̃
1651
1630
1565
1461
1415
1360
1350
1328
1238
16.5
33.4
9.5
1693
1638
1597
38.0
90.4
33.4
σ(HNH) + ν(CN)
νas(NCN)
νs(NCN)
6.2
18.3
17.3
19.4
9.6
1451
1330
1321
15.2
84.0
49.5
σ(CH3)
νas(SO2)
νas(SO2)
15.3
1223
1222
1218
12.4
22.0
24.9
ν(CF3) + νs(SO2)
ν(CF3) + νas(SNS)
ν(CF3) + νas(SNS)
1215
1210
38.8
100
1195
1189
1179
1179
1136
1135
1116
1106
62.0
22.8
ν(CF3)
ν(CF3)
1202
1146
1140
88.3
31.4
47.9
100
ν(CF3) + νs(SO2)
ν(CF3) + νs(SO2)
νs(SO2)
72.8
14.7
28.5
54.3
31.2
νs(SO2)
Figure 9. Most stable ion-pair structures of [H-TEA][Tf2N] calculated
at the B3LYP/6-311++G(3df,3pd) level; the most and second-most
stable structures are denoted by structure C and structure D,
respectively. The hydrogen bonds are indicated by the distances in
angstroms.
νs(SO2)
νs(SO2)
1073
1064
1056
1042
880
9.0
29.4
15.3
4.3
1057
82.6
νas(SNS)
recalculated to estimate the optimized structures and
theoretical IR spectra at the B3LYP/6-311++G(3df,3pd)
level. For [H-TEA][OTf], the most stable ion-pair structure
is estimated to lie ca. 46.5 kJ mol−1 energetically below the
second-most stable one, which is graphically shown in Figure 7.
The moved proton is located 1.07 and 1.60 Å from the nitrogen
atom of TEA and the oxygen atom on OTf, respectively, and all
numerical values of the optimized nuclear coordinates are
summarized in Supporting Information (Table S1). The ion
pair is calculated to be located at 110.7 kJ mol−1 below the
parent molecules of TEA and OTf. Such energy difference
implies that the ion pair does not dissociate into two parent
molecules at the experimental vaporization temperature of 368
K. The simulated IR spectrum shown in Figure 3e satisfactorily
reproduces the experimental matrix-isolation spectrum of
Figure 3b, which suggests that neat [H-TEA][OTf] liquid is
vaporized in a form of the optimized ion-pair structure
illustrated in Figure 7. The spectral analysis based on the
DFT calculation (Figure 3e) indicates that almost all the bands
in Figure 3b originate from the vibrational modes of the [OTf]
anion part. The measured and calculated wavenumbers,
together with their vibrational assignments, are summarized
in Table 2. As mentioned above, a discrepancy between the
neat and matrix spectra is noticed in the asymmetrical SO3
stretching region in the 1100−1400 cm−1 region. The lower-
wavenumber shift in the neat spectrum is explained by the free
SO bond in [OTf] shown in Figure 7 interacting with other
ion pairs in the neat condition.
4.1
884
815
783
659
3.1
17.2
10.9
29.5
deformation
σ(NH2)
795
9.2
744
4.5
σ(NH2) + νs(SNS)
σ(SNS)
a
654
−
a
Accurate absorbance could not be estimated because the sensitivity of
the MCT detector was low in the wavenumber region below 655 cm−1.
b
Intensities are represented as relative intensities, and those with
relative intensity under 3 are omitted. All calculated wavenumbers are
shown in Supporting Information (Table S5).
cases of both [H-TMG][Tf2N] and [H-TEA][Tf2N]. For
example, the intense 857 cm−1 band characteristic for HTf2N
does not appear in Figures 5b and 6b. It should be mentioned
here that the basicity is in increasing order of MIM < TEA <
TMG as listed in Scheme 1. Additionally, we observed the
temperature dependence of the matrix-isolation spectrum of
[H-TEA][Tf2N]. The 857 cm−1 band characteristic for HTf2N
does not exist in Figure 6b recorded for the [H-TEA][Tf2N]
matrix prepared at the vaporization temperature of 388 K, but it
appears in Figure 6c where the matrix sample was prepared at
433 K. This observation means that at the elevated vaporization
temperature of 433 K, neat [H-TEA][Tf2N] liquid vaporizes as
the parent molecules in addition to the ion-pair form. We will
discuss the temperature-dependent vaporization mechanism
later.
DFT Calculations. We observed that three neat PILs, [H-
TEA][OTf], [H-TMG][Tf2N] and [H-TEA][Tf2N], vaporize
as the proton-transferred cation−anion form. We further
assumed that the gaseous chemical species are of cation−
anion 1:1 pairs in analogy with the gaseous components
evaporated from aprotic ILs and then attempted to determine
the ion-pair structures of PILs using DFT calculations. First,
several stable ion-pair structures were explored at the B3LYP/
6-31G* level, and then the candidate structures were
Figure 8 shows two ion-pair structures for [H-TMG][Tf2N],
and their optimized nuclear coordinates are summarized in
Supporting Information (Table S2). In the most stable
structure A, the complex is formed through two stronger
hydrogen bonds between the oxygen atom of Tf2N anion and
the hydrogen atom of H-TMG cation, whereas in the second-
most stable structure B, it is formed through three weaker
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Table 4. Observed and Calculated Wavenumbers of Neat [H-TEA][Tf2N] Liquid with Band Assignments
b
obs.
calc. of structure D
int. (km mol−1
v (cm−1
)
abs.
3.6
v (cm−1
)
)
assignment
̃
̃
1604
1479
1462
5.1
5.1
1438
1383
7.4
5.7
σ(NH) + σ(CH2) + σ(CH3)
σ(CH2)
1388
1362
19.0
49.2
νas(SO2) for structure C
νas(SO2)
1343
1334
1314
1308
99.4
9.2
σ(CH2)
1343
1332
1298
1279
1243
1233
19.0
9.5
28.2
29.3
deformation + νas(SO2)
deformation + νas(SO2)
9.1
4.2
23.6
12.6
1225
1219
1198
1196
1193
14.5
51.3
7.1
ν(CF3) + νs(SO2)
ν(CF3) + νas(SNS)
CH3 rocking + σ(CH2) + ν(CF3)
1223
1218
1213
1195
29.5
25.1
ν(CF3)
ν(CF3)
100
100
75.2
67.6
1177
1175
1166
1120
1117
1079
1053
1039
72.4
12.4
47.6
96.8
33.0
5.3
ν(CF3)
ν(CF3) + CH3 rocking
ν(CF3)
1179
1140
15.6
48.8
νs(SO2) + νas(SNS)
νs(SO2) + νas(SNS)
CH3 rocking
νas(SNS)
1073
1047
1039
794
28.7
9.8
67.4
5.5
ν(CN)
16.5
7.0
νas(SNS) for structure C
νs(SNS)
792
5.6
747
4.8
658
12.1
665
604
27.8
9.7
σ(SNS)
a
625
−
σ(SO2N) + σ(CCN)
a
Accurate absorbance could not be estimated because the sensitivity of the MCT detector was low in the wavenumber region below 655 cm−1.
Intensities are represented as relative intensities, and those with relative intensity under 5 are omitted. All calculated wavenumbers are shown in
b
Supporting Information (Table S6).
hydrogen bonds with an additional bond between the nitrogen
atom of the anion and the hydrogen atom of the cation.
Structure A is estimated to be 11.0 kJ mol−1 more stable than
structure B, and the estimated energy gap indicates that the
gaseous ion pair evaporated at 428 K is dominantly in a form of
structure A. The simulated IR spectrum shown in Figure 5e
satisfactorily reproduces the experimental matrix-isolation
spectrum of Figure 5b, which suggests that the PIL of [H-
TMG][Tf2N] is vaporized mainly in a form of the most stable
ion pair. Considering the S−N−S symmetrical stretching
vibration region of 1100 and 900 cm−1, the band simulation
of structure A having a free nitrogen atom of Tf2N part seems
to reproduce the experiment better than that of structure B
having a hydrogen-bonded nitrogen atom of the Tf2N part.
Both of the energetics and spectral patterns indicate that the
gaseous species of [H-TMG][Tf2N] is of the ion pair of
structure A. The measured and calculated wavenumbers are
summarized in Table 3.
spectra are also shown in Figure 6f,g, which supports an
assignment of the experimental matrix-isolation spectrum
(Figure 6b) to structure D (Figure 6g) rather than structure
C (Figure 6f). For instance, the signature 1073 and 1362 cm−1
bands of the spectrum in Figure 6b correspond with the 1053
and 1343 bands calculated for structure D (Figure 6g) rather
than the 1013 and 1387 bands for structure C (Figure 6f). Most
of the observed intense bands are consistent with the bands of
structure D, but the weak bands around 1040 and 1390 cm−1
may be those due to structure C. We tentatively conclude that
[H-TEA][Tf2N] evaporates in ion pairs mainly of structure D,
possibly with the minor contribution of structure C. The
mismatch between observation and simulation remains unclear
at the present stage, though the DFT calculation accuracy at the
B3LYP/6-311++G(3df,3pd) level might not be sufficient. The
measured and calculated wavenumbers are summarized in
Table 4.
Vaporization Mechanism. Table 1 summarizes the
detected gaseous species evaporated from protic ILs studied
in the present study. Parent acid and base molecules tend to be
released from the PILs of lower ΔpKa values, whereas ion pairs
are detected as vaporization components obtained from the
PILs of higher ΔpKa values. The phenomena will be explained
by thermodynamic equilibrium between acid−base and anion−
cation systems in the liquid phase. Lower ΔpKa PILs are
Figure 9 shows two candidates for gaseous species of [H-
TEA][Tf2N], where structure C is calculated to be slightly
more stable than structure D (by 2.0 kJ mol−1), and their
optimized geometries are summarized in Supporting Informa-
tion (Table S3). The estimated energy difference indicates that
both structures coexist in the gas phase at the vaporization
temperature of 408 K (RT ∼ 3 kJ mol−1). Their theoretical
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synthesized by combining a weaker acid and a weaker base,
where a certain concentration of parent acid and base molecules
are expected to coexist in neat protic ILs at room temperature.
Then, the acid and base molecules are evaporable without
heating neat PIL, which corresponds to examples of [H-
MIM][AcO] and [H-TEA][AcO]. Higher ΔpKa PILs prepared
by a stronger acid and a stronger base and the equilibrium
between acid−base and anion−cation pairs shifts to the anion−
cation pair side in the liquid, where less concentration of parent
molecules exist in the liquid at room temperature. With
increasing temperature, the acid and base concentration
increases in the liquid and then they are vaporized at a
temperature lower than that of ion-pair vaporization, which
corresponds to an example of [H-MIM][Tf2N]. Much higher
ΔpKa PILs are prepared by combining a strong acid and a
strong base, where the concentration of deprotonated acid and
protonated base dominates in the liquid. The parent acid and
base do not exist in the liquid at the temperature of the PIL
vaporization. As a result, the corresponding acid and base
vanishingly exist in the gaseous component, and then the PILs
with high ΔpKa behave like aprotic ILs vaporizing to be ion
pairs. This hypothesis may be confirmed by the temperature-
dependent experimental result for [H-TEA][Tf2N] as shown in
Figure 6, where the parent molecules were observed at higher
temperature, 433 K. This indicates that the population of the
parent molecules may be negligible in the liquid at 388 K but
increases with temperature to the extent that the detectable acid
and base molecules are evaporated from the neat PIL liquid at
433 K.
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Several kinds of PILs with various ΔpKa values synthesized by
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ASSOCIATED CONTENT
* Supporting Information
Full description of calculated results. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
*.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported in part by Grant-in-Aid for
Scientific Research 24750013 and 24350004 from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
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