Thermochemistry of the pyridinium- and pyrrolidinium-based ionic liquids

Article abstract of DOI:10.1007/s10973-012-2725-4

We applied DSC for the determination of enthalpies of synthesis reactions of pyridinium- and pyrrolidinium-based ionic liquids (ILs) from pyridine (or N-methyl-pyrrolidine) and n-alkyl bromides (with n = 4, 5, 6, 7, and 8). The combination of reaction enthalpy measurements by DSC with modern high-level first-principles calculations opens valuable indirect thermochemical options to obtain values of enthalpies of the formation and vaporization enthalpies of ILs.

Full text of DOI:10.1007/s10973-012-2725-4

J Therm Anal Calorim
DOI 10.1007/s10973-012-2725-4
Thermochemistry of the pyridinium- and pyrrolidinium-based
ionic liquids
•
Sergey P. Verevkin Ricardas V. Ralys
•
•
•
Vladimir N. Emel’yanenko Dzmitry H. Zaitsau
Christoph Schick
Received: 17 July 2012 / Accepted: 21 September 2012
´
´
Ó Akademiai Kiado, Budapest, Hungary 2012
Abstract We applied DSC for the determination of
enthalpies of synthesis reactions of pyridinium- and
pyrrolidinium-based ionic liquids (ILs) from pyridine
(or N-methyl-pyrrolidine) and n-alkyl bromides (with
n = 4, 5, 6, 7, and 8). The combination of reaction
enthalpy measurements by DSC with modern high-level
first-principles calculations opens valuable indirect ther-
mochemical options to obtain values of enthalpies of the
formation and vaporization enthalpies of ILs.
friendly solvents due to their molecularly ‘‘tunable’’
properties and lack of volatility. The synthesis of ILs
consists generally of two major steps. In the first step,
desired cation (imidazolium, pyridinium, pyrrolidinium,
etc.) has to be generated, usually by direct alkylation of a
nitrogen atom with a suitable alkyl halide. In the second
step, the halide-anion resulting from the alkylation reaction
can be exchanged for a different one by metathesis reac-
tion. The first step is usually very exothermic. In order to
avoid side reactions a careful temperature control during
the synthesis reaction is required.
Keywords Ionic liquids ꢀ Differential scanning
calorimetry ꢀ First-principles calculations ꢀ Enthalpy of
reaction ꢀ Enthalpy of formation ꢀ Enthalpy of vaporization
Differential scanning calorimetry (DSC) is broadly used
for kinetic and thermodynamic studies of chemical reac-
tions. Just recently, we reported that DSC can be a suitable
experimental tool to measure thermodynamic properties of
ILs such as enthalpies of formation, enthalpies of fusion,
and even it can be possible to derive vaporization enthal-
pies indirectly [1]. In this study, we have used DSC to
study the thermochemistry of the following reactions:
Introduction
Ionic liquids (ILs) are novel advanced materials and they
are often considered as a new class of environmentally
Pyridine (liq:Þ þ CnBr ðliq:Þ ¼ ½CnPyꢁ½Brꢁ ðliq:Þ
ð1Þ
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-012-2725-4) contains supplementary
material, which is available to authorized users.
N-Me-pyrrolidine (liq:Þ þ CnBr (liq:Þ
¼ ½Cn;1Pyrrꢁ½Brꢁ ðliq:Þ:
ð2Þ
According to the Hess’s Law, reaction enthalpy DrHmꢂ is
defined as the difference between enthalpies of formation
(DfHmꢂ ) of the products and reactants. For example for the
alkylation of pyridine with butyl bromide CnBr:
S. P. Verevkin (&) ꢀ R. V. Ralys ꢀ V. N. Emel’yanenko ꢀ
D. H. Zaitsau
Department of Physical Chemistry, University of Rostock,
18059 Rostock, Germany
e-mail: sergey.verevkin@uni-rostock.de
DrHmꢂ ðliq.Þ ¼ DfHmꢂ ð½C4Pyꢁ½Brꢁ; liq:Þ
S. P. Verevkin ꢀ C. Schick
ꢃ DfHmꢂ ðPyridine; liq:Þ ꢃ DfHmꢂ ðC4Br; liq:Þ:
Department ‘‘Life, Light and Matter’’,
Faculty of Interdisciplinary Research, University of Rostock,
Rostock, Germany
ð3Þ
With enthalpies of reactions (1 or 2) measured by DSC,
the enthalpies of formation of the ILs DfHmꢂ ð½CnPyꢁ½Brꢁ;
liq.Þ and ½Cn;1Pyrrꢁ½Brꢁ can be derived (provided that the
C. Schick
Department of Physics, University of Rostock,
18057 Rostock, Germany
123

S. P. Verevkin et al.
enthalpies of formation of the precursors are known) and
used for the estimation of vaporization enthalpies, Dgl Hmꢂ ,
of the ILs indirectly. Enthalpies of reactions, enthalpies of
vaporization, and enthalpies of formations of ILs are key
properties required for temperature management and
optimization of the industrial synthesis of ILs.
the liquid heat capacity, Clp, of the IL and precursors
(amines and alkyl bromides) is required
DrHmꢂ ð298 KÞ=J molꢃ1
ð4Þ
¼ DrHmꢂ ðTmaxÞ ꢃ DrCp;mðTmax ꢃ 298 KÞ:
Unfortunately, the experimental liquid heat capacities
for the ILs [CnPy][Br] and [Cn,1Pyrr][Br] are not known.
For this reason, we have tried to asses the value of the
temperature correction from Tmax to 298 K with the help of
available data for the synthesis of imidazolium-based ILs
from alkyl halides
Experimental
Materials and chemicals
1-Me-imidazole ðliq:Þ þ C4Hal ðliq:Þ
¼ ½C4mimꢁ½Halꢁ ðliq:Þ
Pyridine and alkyl bromide were commercially available
with purity C99 %. Prior to use they were additionally
distilled under reduced pressure and stored under dry
nitrogen. No impurities (greater than 0.1 mass%) could be
detected in the samples using a gas chromatograph equip-
ped with a flame ionization detector. Handling with the
samples occurred in a glove box.
collected in Table 2. It is apparent from this table that the
values of DrCp,m for all three reactions are slightly nega-
tive. Using the results for DrCp,m for the synthesis of
[C4mim][Br] and [C4mim][I], the average value DrCp,m
=
-(1.4 3.3) J K-1 mol-1 was estimated. The data for
[C4mim][Cl] are in agreement with estimates within the
boundaries of experimental uncertainties, however, we
excluded this data from calculation of the average DrCp,m
because the heat capacity of [C4mim][Cl] reported in
Ref. [3] has uncertainties of 25 % according to the recent
evaluation by Paulechka et al. [4]. Application of the
average value DrCp,m = -(1.4 3.3) J K-1 mol-1 esti-
mated in this study for adjustment of the measured
enthalpies to the reference temperature according to Eq. 4
should provide generally a very small correction at the
level of about 0.3–0.4 kJ mol-1 and this correction is well
within the boundaries of the DSC experimental uncertain-
ties of 1–3 kJ mol-1. For this reason, we ascribed in this
study the enthalpies of reactions measured by DSC to the
reference temperature without any temperature correction.
DSC: measurement of reaction enthalpies
Enthalpies of reaction according to Eq. (1 or 2) were
measured using the calibrated computer controlled Mettler-
Toledo 822 DSC. The area of the DSC peak occurring
during the chemical reaction was a measure of the reaction
enthalpy. The experimental procedure has been elaborated
in our lab just recently and it was reported elsewhere [1].
Reaction enthalpies of the reactions (1 and 2), DrHmꢂ ðliq:Þ,
were determined according to Eq. 3 in a series of three
DSC runs under optimized conditions recommended in [2]
in excess of alkyl bromide, dilution with the solvent IL
[C4mim][NTf2] (1-butyl-3-methylimidazolium bis-(trifluo-
romethanesulfonyl)-imide) about 70–95 wt%; temperature
scanning with 50 K/min in the range of 298–523 K. In
these conditions, a sharp reaction peak with well-defined
baseline was achieved for each reaction under study pro-
viding reproducible results. Experimental results are given
in Table 1. Using the experimental molar enthalpies of
formation of the precursors of reactions (1 and 2) pyridine,
pyrrolidine, and alkyl halides (see Tables S1 and S2 in
ESI) the enthalpies of formation of the pyridinium- and
pyrrolidinium-based ILs, DfHmꢂ ([IL], liq), in the liquid state
were calculated by using Eq. 3.
Quantum chemical calculations
Standard first-principles molecular orbital calculations were
performed using the Gaussian 09 program package [5]. Con-
formation analysis of the IL was performed using B3LYP/6-
31?G(d,p) with the help of the procedure developed in our
previous work [6]. Optimized structure and energyof the most
stable conformer of the ionic pair were further obtained using
the CBS-QB3 and G3MP2 composite methods. CBS-
QB3 theory uses geometries from B3LYP/6-311G(2d,d,p)
calculation, scaled zero-point energies from B3LYP/
6-311G(2d,d,p) calculation followed by a series of single-
point energy calculations at the MP2/6-311G(3df,2df,2p),
MP4(SDQ)/6-31G(d(f),p), and CCSD(T)/6-31G levels of
theory [7]. G3MP2 theory uses geometries from second-order
perturbation theory and scaled zero-point energies from
As a matter of fact, enthalpies of reaction, DrHmꢂ ,
derived by integration of the DSC peak (see Table 1) are
referred to the temperature between the on-set and end-set
of the peak. In most cases, we measured symmetric reac-
tion peaks and we could assign the DrHmꢂ to the temperature
of the peak maximum, Tmax (see Table 1). In order to
calculate the enthalpy of reaction DrHmꢂ at the reference
temperature T = 298.15 K the difference, DrCp,m, between
123

Thermochemistry of the pyridinium and pyrrolidinium
compounds. Calculated values of the enthalpies of reaction
and enthalpies of formation are therefore based on the elec-
tronic energy calculations obtained using standard procedures
of statistical thermodynamics [9].
Table 1 Experimental results for measurements of reaction enthal-
pies, DrHmꢂ ðliq:Þ, using DSC
a
System (1) ? (2)
Molar Solvent
ratio
DrHmꢂ ðliq:Þ /kJ mol-1
Tmax/°C
content/
wt%
Pyridinium
Pyridine ? C4Br
Pyridine ? C5Br
Pyridine ? C6Br
Pyridine ? C7Br
Pyridine ? C8Br
Pyridine ? C4Cl
Pyridine ? CnCl
Pyrrolidinium
0.43
0.30
0.28
0.50
0.38
0.52
85
80
85
90
90
70
-86.7 0.8
-86.2 1.0
-86.8 1.2
-86.3 1.8
-86.4 1.2
-76.5 1.4 [10]
-76.5b
166
165
166
166
165
171
1
2
5
3
3
2
Results and discussion
Molar enthalpies of formation of ILs from
DSC-measurements
Enthalpies of reactions of amines with alkyl bromides
measured by DSC are collected in Table 1. All synthesis
reactions of the ILs are strongly exothermic, as it can be
seen from this table. Formation of the pyrrolidinium-based
ILs is about 10 kJ mol-1 less negative than the formation
of pyridinium-based ILs. It has turned out that the reaction
enthalpies of both reactions 1 and 2 are practically inde-
pendent on the chain length for the alkyl substituents with
number of the C-atoms n = 4–8. Similar trends were also
observed for [Cnmim][Br] in our previous work [1].
We were able to suggest another useful assumption by
analyzing the data given in Table 1. Unfortunately, it has
turned out that reactivity of alkyl chlorides with pyridine
and N-Me-pyrrolidine was significantly lower in contrast to
alkyl bromides. For this reason, the spread of experimental
enthalpies for the reactions
N-MePyrr ? C4Br 0.43
N-MePyrr ? C5Br 0.43
N-MePyrr ? C6Br 0.53
N-MePyrr ? C7Br 1.10
N-MePyrr ? C8Br 1.51
N-MePyrr ? CnCl
95
95
95
90
95
-79.2 2.8
-79.1 2.4
-81.5 2.8
-77.4 0.6
-76.9 0.3
-65.4c
117
120
118
117
118
3
4
2
3
4
a
Uncertainties are twice the standard deviations of the mean value
b
Average enthalpy of reaction pyridine ? CnCl was suggested to be the same
as those for pyridine ? C4Cl (see text)
c
Average enthalpy of reaction N-MePyrr ? CnCl was assessed from analogy
between Pyridine ? CnCl and N-MePyrr ? C4Br (see text)
Pyridine ðliq:Þ þ CnCl ðliq:Þ ¼ ½CnPyꢁ½Clꢁ ðliq:Þ
ð5Þ
Table 2 Heat capacities, C1p of precursors and changes of reaction
heat capacities, DrCp,m, for the reactions of synthesis of imidazolium
halides from 1-Me-imidazole and alkyl halides available in the
literature
N-Me-pyrrolidine ðliq:Þ þ CnCl ðliq:Þ
¼ ½Cn;1Pyrrꢁ½Clꢁ ðliq:Þ
ð6Þ
was unacceptably large. We have succeeded only with
the reaction butyl chloride ? pyridine [10] (see Table 1).
However, we have already data [1] for similar systems
[Cnmim][Cl] and [Cnmim][Br], where the differences
between enthalpies of synthesis reactions of chloride and
bromide ILs were constant (within the boundaries of
experimental uncertainties) for species with the chain
length n = 4–8. Thus, in this study, we have assumed that
an average enthalpy of reaction N-MePyrr ? CnCl could be
assessed from analogous differences between (pyridine ?
CnCl) and (N-MePyrr ? C4Br): D = (-76.5) -(-87.0) =
10.5 kJ mol-1 (see Table 1). Derived in such a way
enthalpies are collected in column 5 of Tables 3 and 4.
Using the classic Hess’s law expressed in Eq. 3 and the
experimental DfHmꢂ -values of the precursors (see ESI), the
enthalpies of formation for [CnPy][Br] and [Cn,1Pyrr][Br]
were calculated (see column 6 in Tables 3, 4). These
enthalpies of formation can open an excess to a lattice
potential energy UPOT and lattice enthalpy of ILs which are
usually rationalized in different types of the Born–Fajans–
Haber cycles [11–13]. Lattice potential energy is usually
C1p/J K-1 mol-1
C4Cl
1-Me-imidazole
[C4mim][Cl]a
158.9 0.6 [18]
C4Br
147.3 0.6 [3]
1-Me-imidazole
299 7.5 [2, 4]
[C4mim][Br]b
162.3 0.6 [18]
C4I
147.3 0.6 [3]
1-Me-imidazole
147.3 0.6 [3]
308.7 1.0 [19]
[C4mim][I]c
164.5 4.9 [18]
310.0 5.1 [20]
a
DrCp,m = -7
8
b
DrCp,m = -0.9 1.3
DrCp,m = -1.8 5.1
c
Hartree–Fock theory followed by a series of single-point
energy calculations at the MP2(Full)/6-31G(d), QCISD(T)/
6-31G(d), and MP2/GTMP2Large levels of theory (for details
see Ref. [8]). However, the G3MP2 method is not able to
calculate correctly the energy of the bromine containing
123

S. P. Verevkin et al.
Table 3 Thermodynamic data for IL synthesis reaction: enthalpies H298 of ILs, reaction enthalpies, DrHmꢂ ðgÞ, and enthalpies of formation,
DfHmꢂ ðgÞ, calculated using the CBS-QB3 composite method and properties derived from this calculation: enthalpies of reaction, DrHmꢂ ðliq:Þ,
enthalpies of formation, DfHmꢂ ðliq:Þ, and enthalpies of vaporization, DlgHmꢂ , of [CnPy][Br] and [CnPy][Cl] (in kJ mol-1
)
Dgl Hmꢂ Eq. 7
Dgl Hmꢂ
Eq. 10
8
ILs
1
H298
(in Hartree)
2
DrHmꢂ ðgÞ
DfHmꢂ ðgÞ
DrHmꢂ ðliq:Þ (DSC)
DfHmꢂ ðliq:Þ Eq. 3
3
4
5
6
7
Chlorides
[C4Py][Cl]
[C5Py][Cl]
[C6Py][Cl]
[C7Py][Cl]
[C8Py][Cl]
Bromides
[C4Py][Br]
[C5Py][Br]
[C6Py][Br]
[C7Py][Br]
[C8Py][Br]
a
-865.053046
-904.277872
-943.502442
-982.726906
-1,021.95137
19.5
18.7
18.5
18.5
18.4
5.9
-15.1
-35.4
-55.4
-75.4
-76.5 [4]
-76.5a
-76.5a
-76.5a
-76.5a
-164.6
-189.7a
-215.2a
-240.8a
-267.8a
170.5
174.6
179.8
185.4
192.4
169.7
173.6
178.0
182.9
187.5
-2,978.182567
-3,017.407734
-3,056.632328
-3,095.856554
-3,135.080780
10.2
8.6
8.1
8.8
9.4
41.2
26.3
-86.7
-86.2
-86.8
-86.3
-86.4
-130.5
-156.4
-181.0
-204.7
-231.5
172.3
183.6
186.4
191.0
198.2
173.8
176.1
181.2
186.1
191.8
4.7
-14.8
-34.2
Assessed values (see text). Values in bold are recommended for further thermochemical calculations
defined as the enthalpy of formation of the ionic compound
from gaseous ions. UPOT is a measure of the strength of
bonds in that ionic compound. Lattice potential energy is a
dominant term in the thermodynamic analysis of the exis-
tence and stability of ionic solids. Experimental DfHmꢂ -
values for ILs studied in this study are referred to the liquid
state. With help of a simple fusion enthalpy measurements
the required values in the crystalline state, DfHmꢂ (cr,
298 K), can be obtained and applied for calculation of the a
lattice potential energy and lattice enthalpy of an ionic
solid material [11–13].
atomization procedure for chlorine containing compounds
does not require special corrections (see Fig. S2). We
additionally tested another composite method—G3MP2,
which is significantly less time consuming compared with
the CBS-QB3. The G3MP2 method seems to be also
suitable for calculations of DfHmꢂ (g, 298 K) of the chlorine
containing ILs (see Table S5). Results of the CBS-QB3
and the G3MP2 computations are in agreement within
a few kJ mol-1. But the G3MP2 method has no
parameterization for Br.
Vaporization enthalpy from DSC study
Quantum chemical calculations of enthalpy
of formation DfHmꢂ (g, 298 K)
Vaporization enthalpies, Dgl Hmꢂ of ILs are required for
validation of the molecular dynamic simulations, assess-
ment of solubility parameters, as well as for safety calcu-
lation of different thermal processes using ILs as
thermofluids. Values of Dgl Hmꢂ are possible to derive indi-
rectly using a combination of DSC and quantum chemical
calculations. In this study, we estimated vaporization
enthalpies of the [CnPy][Br] and [Cn,1Pyrr][Br] as the dif-
ference between gaseous enthalpies of formation, DfHmꢂ ðgÞ
calculated using the composite CBS-QB3 method and the
molar enthalpies of formation in the liquid state,
DfHmꢂ ðliq:Þ measured by DSC as follows:
We calculated the total enthalpies H298 at T = 298 K using
the CBS-QB3 (column 2, Tables 3, 4). Atomization reac-
tions (AT) were used to obtain theoretical standard
enthalpies of formation, DfHmꢂ ðgÞ, from the ab initio results
for chlorine and bromine containing compounds. However,
in our previous study [1] we have observed that the CBS-
QB3 standard atomization procedure applied for bromine
containing compounds need to be corrected (see Tables S3,
S4 and Fig. S1 in ESI) with a simple linear equation:
DfHmꢂ ðgÞðexpÞ=kJ molꢃ1
¼ 1:002 ꢄ DfHmꢂ ðgÞðATÞ þ 28:2 ðr ¼ 0:9981Þ:
Dgl Hmꢂ ¼ DfHmꢂ ðgÞ ꢃ DfHmꢂ ðliq:Þ:
ð7Þ
Using this correlation, the ‘‘corrected’’ enthalpies of
formation of the [CnPy][Br] and [Cn,1Pyrr][Br] have been
calculated (see column 4 of Tables 3, 4). The CBS-QB3
Values of DfHmꢂ ðgÞ calculated by CBS-QB3 are given
in column 3, Tables 3 and 4. Enthalpies of vaporization
123

Thermochemistry of the pyridinium and pyrrolidinium
Table 4 Thermodynamic data for IL synthesis reaction: enthalpies H298 of ILs, reaction enthalpies, DrHmꢂ ðgÞ, and enthalpies of formation,
DfHmꢂ ðgÞ, calculated using the CBS-QB3 composite method and properties derived from this calculation: enthalpies of reaction, DrHmꢂ ðliq:Þ,
enthalpies of formation, DfHmꢂ ðliq:Þ, and enthalpies of vaporization DlgHmꢂ , of [Cn,1Pyrr][Br] and [Cn,1Pyrr][Cl] (in kJ mol-1
)
Dgl Hmꢂ Eq. 7
Dgl Hmꢂ Eq. 10
8
ILs
1
H298 (in Hartree)
2
DrHmꢂ ðgÞ
DfHmꢂ ðgÞ
DrHmꢂ ðliq:Þ (DSC)
DfHmꢂ ðliq:Þ Eq. 3
3
4
5
6
7
Chlorides
[C41Pyrr][Cl]
[C51Pyrr][Cl]
[C61Pyrr][Cl]
[C71Pyrr][Cl]
[C81Pyrr][Cl]
Bromides
-868.605697
-907.830294
-947.054852
-986.2793255
-1,025.503799
-20.7
-20.9
-21.1
-21.1
-21.2
-177.2
-197.6
-217.8
-237.8
-257.9
-65.4a
-65.4a
-65.4a
-65.4a
-65.4a
-289.5a
-314.6a
-340.1a
-365.7a
-392.7a
112.3
117.0
122.3
127.9
134.8
112.4
116.9
121.3
126.2
130.8
[C41Pyrr][Br]
[C51Pyrr][Br]
[C61Pyrr][Br]
[C71Pyrr][Br]
[C81Pyrr][Br]
a
-2,981.736166
-3,020.961321
-3,060.185640
-3,099.410002
-3,138.634364
-32.5
-34.1
-33.8
-33.5
-33.2
-138.9
-160.8
-180.4
-200.2
-220.0
-79.2
-79.1
-81.5
-77.4
-76.9
-259.0
-285.3
-311.7
-331.8
-358.0
120.2
124.5
131.3
131.9
138.0
117.6
120.3
128.0
128.9
133.7
Assessed values (see text). Values in bold are recommended for further thermochemical calculations
calculated by Eq. 7 are given in column 7, Tables 3 and 4.
However, determination of the vaporization enthalpy using
Eq. 7 is not optimal, because it suffers from one of the input
requirements: the value of the gaseous enthalpyof formation,
DfHmꢂ ðgÞ used in Eq. 7 is usually calculated employing
quantum chemical methods by using atomisation or
isodesmic reactions [14]. From our experiences, there are
still some ambiguities with respect to procedures for
converting results of first-principles calculations (total
energies E0 at T = 0 K and enthalpies H298 at T = 298 K)
into the desired value of DfHmꢂ ðgÞ because enthalpies of
formation obtained from the first-principles calculations are
very sensitive to the choice of the bond separation or
isodesmic reactions used for this purpose [14]. Recently, we
suggested a solution to the aforementioned problem [1]: the
most effective way to avoid ambiguity is to calculate with the
first-principles method directly the enthalpy of the reaction
shown in Eq. 8:
of formation with the help of atomization or isodesmic
procedures. As a consequence of this maneuvre, the resulting
DrHmꢂ ðgÞ is not affected at all by the choice and the quality of
the auxiliary quantities commonly required for atomization or
bond separation reactions. Values of DrHmꢂ ðgÞ calculated for
all reactions under study according to Eq. 9 are listed in
column 3 of Tables 3 and 4.
Having established DrHmꢂ ðliq:Þexp from the experiment
and DrHmꢂ ðgÞ from the quantum chemistry, we have derived
the vaporization enthalpy of, e.g., for [C4Py][Br], as
follows:
Dgl Hmꢂ ðIL; 298 KÞ ¼ ꢃDrHmꢂ ðliq.Þexp þ DrHmꢂ ðgÞ
þ Dgl Hmꢂ ðN-Me-piperidineÞ
þ Dgl Hmꢂ ðC4BrÞꢁ ¼ ꢃðꢃ86:7Þ
þ ð10:2Þ þ ð40:2Þ þ ð36:7Þ
¼ 173:8 ꢅ 4:1 kJ molꢃ1
:
ð10Þ
Using the theoretical value DrHmꢂ ðgÞ calculated with
CBS-QB3 in the gas phase and enthalpies of vaporization
at 298 K of precursors Dgl Hmꢂ (pyridine) = 40.2 0.3
kJ mol-1 [15], DlgHmꢂ (C4H9Cl)] = 36.7 0.1 kJ mol-1
[16] and the enthalpy of reaction DrHmꢂ ðliq:Þexp measured
PyridineðgÞ þ C4BrðgÞ ¼ ½C4Pyꢁ½BrꢁðgÞ:
ð8Þ
Having calculated the enthalpies H298 at T = 298.15 K
for all reaction participants of reaction (8) using the CBS-
QB3 approach and using the Hess’ Law, the enthalpy of
reaction was calculated as follows (e.g., for [C4Py][Br]):
using the DSCinthe liquidphase, theenthalpyofvaporization
of[C4Py][Br]hasbeencalculated. Inthesameway, enthalpies
of vaporization of other ILs were calculated with the help of
Eq. 10 and listed in the last column of Tables 3 and 4.
Comparison of the vaporization enthalpies derived using two
different procedures (i.e., Eqs. 7 and 10) shows that both
methods provide values in close agreement. However, we
should recommend Eq. 10, because the DrHmꢂ ðgÞ for this
DrHmꢂ ðgÞ ¼ H298ð½C4Pyꢁ½Brꢁ; gÞ ꢃ H298ðPyridine; gÞ
ꢃ H298ðC4Br; gÞ
¼ ð10:2 ꢅ 4:0Þ kJ molꢃ1
:
ð9Þ
It is of a crucial importance that the DrHmꢂ ðgÞ was obtained
using quantum chemical methods directly from the calculated
H298 bypassing the conventional calculation of the enthalpies
123

S. P. Verevkin et al.
indirect method for determination of vaporization enthalpies.
J Phys Chem B. 2012;116:4276–85.
procedure has been calculated directly from H298 avoiding
any procedure for converting quantum chemical results into
enthalpy of formation.
2. Paulechka YU. Heat capacity of room-temperature ionic liquids:
a critical review. J Phys Chem Ref Data. 2010;39(3):1–033108.
3. Paulechka YU, Kabo AG, Blokhin AV. Calorimetric determina-
tion of the enthalpy of 1-butyl-3-methylimidazolium bromide
synthesis: a key quantity in thermodynamics of ionic liquids.
J Phys Chem B. 2009;113:14742–6.
Quantum chemical calculations for degree
of dissociation of the ion pair
4. Holbrey JD, Reichert WM, Reddy RG, Rogers RD. In: Rogers
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and prospects. ACS Symposium Series, vol 856. New York:
American Chemical Society; 2003.
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6. Emel’yanenko VN, Verevkin SP, Heintz A. The gaseous enthalpy
of formation of the ionic liquid 1-butyl-3-methylimidazolium
dicyanamide from combustion calorimetry, vapor pressure mea-
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10. Verevkin SP, Zaitsau Dz, Emel’yanenko VN, Ralys RV, Schick
The aprotic ILs such as [C4mim][N(CN)2], [Pyrr1,4][N(CN)2],
and [Cnmim][NO3] exist in the gaseous phase as contact ion
pairs and not as separated ions [6, 17, 21]. Calculations for the
free cation [C2Py]?, [C12Pyrr]? and the free anions [Cl]-,
[Br]-, as well as the ion pairs [C2Py]?[Cl]-, [C2Py]?[Br]-,
[C21Pyrr]?[Cl]-, [C21Pyrr]?[Br]-, have been performed by
CBS-QB3 (Table S6). The molar Gibbs energy, DrG°, the
molar enthalpy of formation, DrH°, and the molar entropy,
DrS°, at 298 K have been calculated using standard proce-
dures of statistical thermodynamics. The purpose of this
procedure was to obtain values DrG°, DrH°, DrS°, and the
molar reaction enthalpy for the process of dissociation of the
ion pair, e.g., [C2Py]?[Cl]-, or [C21Pyrr]?[Cl]-, into ions in
the gaseous phase according to
½C2Pyꢁþ½Clꢁꢃ ! ½C2Pyꢁþ þ ½Clꢁꢃ;
ð11Þ
ð12Þ
´
Ch, Geppert-Rybczynska M, Jayaramanb S, Maginn EJ. Bench-
mark values: thermochemistry of the ionic liquid [C4Py][Cl].
Aust J Chem. 2012, accepted.
½C21Pyrrꢁþ½Clꢁꢃ ! ½C2Pyrrꢁþ þ ½Clꢁꢃ:
11. Glasser L, Jenkins HDB. Lattice energies and unit cell volumes
of complex ionic solids. J Am Chem Soc. 2000;122:632–8.
12. Slattery J, Daguenet C, Dyson P, Krossing I, Weinga¨rtner H,
Oleinikova A. Why are ionic liquids liquid? A simple explanation
based on lattice and solvation energies. J Am Chem Soc.
2006;128:13427–34.
The chemical equilibrium constant Kp in the ideal
gaseous state has been calculated at 298 K (Table S6)
using the CBS-QB3. Calculations have revealed very small
equilibrium constants Kp at the level of 10-65 for both
pyridinim- and pyrrolidinium-based ILs (see Table S6).
These values are in agreement with Kp = 1.4 9 10-54 for
[C4mim][N(CN)2] derived in our previous work [6]. Such
extremely low values allow us to conclude with high
reliability that the degree of dissociation of the ion pair is
zero for the ILs under study, i.e., these ionic ILs exist
exclusively as ion pairs in the gaseous phase.
´
13. Glasser L, von Szentpaly L. Born–Haber–Fajans cycle general-
ized: linear energy relation between molecules, crystals, and
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14. Ohlinger WS, Klunzinger PE, Deppmeier BJ, Hehre WJH. Efficient
calculation of heats of formation. J Phys Chem A. 2009;113:
2165–75.
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and formation of pyridine and hippuric acid. J Phys Chem.
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16. Wadso I. Heats of vaporization of organic compounds II. Chlo-
rides, bromides, and iodides. Acta Chem Scand. 1968;22:
2438–44.
Conclusions
17. Emel’yanenko VN, Verevkin SP, Heintz A, Schick C. Ionic liq-
uids. Combination of combustion calorimetry with high-level
quantum chemical calculations for deriving vaporization enthal-
pies. J Phys Chem B. 2008;112:8095–8.
18. Shehatta I. Heat capacity at constant pressure of some halogen
compounds. Thermochim Acta. 1993;213:1–10.
Alliance of experimental and computational methods has
been used to derive thermochemical values for four
homologous series of pyridinium- and pyrrolidinium-based
ILs. These data can be used now for optimization of the
technologies of IL synthesis.
19. Blokhin AV, Shaplov AS, Lozinskaya EI, Vygodskii YaS.
Thermodynamic properties of 1-alkyl-3-methylimidazolium bro-
mide ionic liquids. J Chem Thermodyn. 2007;39(1):158–66.
20. Kabo GJ, Paulechka YU, Kabo AG, Blokhin AV. Experimental
determination of enthalpy of 1-butyl-3-methylimidazolium iodide
synthesis and prediction of enthalpies of formation for imidazo-
lium ionic liquids. J Chem Thermodyn. 2010;42:1292–7.
21. Emel’yanenko VN, Verevkin SP, Heintz A, Corfield JA, Deyko
A, Lovelock KRJ, Licence P, Jones RG. Pyrrolidinium-based
ionic liquids. 1-Butyl-1-methyl pyrrolidinium dicyanoamide:
thermochemical measurement, mass spectrometry, and ab initio
calculations. J Phys Chem B. 2008;112:11734–42.
Acknowledgements This study was supported by the German
Science Foundation (DFG) in the frame of the priority program SPP
1191 ‘‘Ionic Liquids’’.
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