Vaporization and formation enthalpies of 1-alkyl-3-methylimidazolium tricyanomethanides

Article abstract of DOI:10.1021/jp207335m

Thermochemical studies of the ionic liquids 1-ethyl-3-methylimidazolium tricyanomethanide [C₂MIM][C(CN)₃] and 1-butyl-3- methylimidazolium tricyanomethanide [C₄MIM][C(CN)₃] have been performed in this work. Vaporization enthalpies have been obtained using a recently developed quartz crystal microbalance (QCM) technique. The molar enthalpies of formation of these ionic liquids in the liquid state were measured by means of combustion calorimetry. A combination of the results obtained from QCM and combustion calorimetry lead to values of gaseous molar enthalpies of formation of [C_nMIM][C(CN)₃]. First-principles calculations of the enthalpies of formation in the gaseous phase for the ionic liquids [C_nMIM][C(CN)₃] have been performed using the CBS-QB3 and G3MP2 theory and have been compared with the experimental data. Furthermore, experimental results of enthalpies of formation of imidazolium-based ionic liquids with the cation [C_nMIM] (where n = 2 and 4) and anions [N(CN)₂], [NO₃], and [C(CN)₃] available in the literature have been collected and checked for consistency using a group additivity procedure. It has been found that the enthalpies of formation of these ionic liquids roughly obey group additivity rules.

Full text of DOI:10.1021/jp207335m

ARTICLE
pubs.acs.org/JPCB
Vaporization and Formation Enthalpies of 1-Alkyl-3-methylimidazolium
Tricyanomethanides
Vladimir N. Emel’yanenko, Dzmitry H. Zaitsau, Sergey P. Verevkin,* and Andreas Heintz*
Department of Physical Chemistry, University of Rostock, Dr-Lorenz-Weg-1, 18059 Rostock, Germany
Karsten Voß and Axel Schulz
Department of Inorganic Chemistry, University of Rostock, Albert-Einstein Str. 3a, 18059 Rostock, Germany
S Supporting Information
b
ABSTRACT: Thermochemical studies of the ionic liquids 1-ethyl-3-methylimidazolium
tricyanomethanide [C2MIM][C(CN)3] and 1-butyl-3-methylimidazolium tricyano-
methanide [C4MIM][C(CN)3] have been performed in this work. Vaporization
enthalpies have been obtained using a recently developed quartz crystal microbalance
(QCM) technique. The molar enthalpies of formation of these ionic liquids in the liquid
state were measured by means of combustion calorimetry. A combination of the results
obtained from QCM and combustion calorimetry lead to values of gaseous molar
enthalpies of formation of [CnMIM][C(CN)3]. First-principles calculations of the
enthalpies of formation in the gaseous phase for the ionic liquids [CnMIM][C(CN)3]
have been performed using the CBS-QB3 and G3MP2 theory and have been compared
with the experimental data. Furthermore, experimental results of enthalpies of formation of imidazolium-based ionic liquids with the
cation [CnMIM] (where n = 2 and 4) and anions [N(CN)2], [NO3], and [C(CN)3] available in the literature have been collected and
checked for consistency using a group additivity procedure. It has been found that the enthalpies of formation of these ionic liquids
roughly obey group additivity rules.
it has become possible to measure vaporization enthalpies at
T < 400 K. In this work, we have measured the enthalpies of
vaporization enthalpies for 1-butyl-3-methylimidazolium tricyano-
methanide [C2MIM][C(CN)3] and 1-ethyl-3-methylimidazolium
tricyanomethanide [C4MIM][C(CN)3] using the QCM method.
The combination of vaporization enthalpies obtained with the results
from combustion calorimetry has allowed us to determine gaseous
molar enthalpies of formation ΔfH°m(g) of [CnMIM][C(CN)3].
Additionally, first-principles calculations for these compounds have
been performed to test the consistency of the experimental data with
calculated ones. Moreover, a set of thermochemical properties of ILs
of the general formula [CnMIM][Anion] have been collected and
analyzed in terms of group additivity rules.10,11
1. INTRODUCTION
The almost negligible vapor pressure of ionic liquids (ILs) at
ambient temperatures is one of the most remarkable features of
these neoteric solvents. It is of importance for many practical
applications, where ILs are used as a valuable alternative for com-
mon volatile solvents.1 The structure of ionic liquids directly
impacts their properties. ILs are well-known as the “designer
solvents” with easily tunable properties. Experimental and the-
oretical study of the relationship between structure and proper-
ties of ionic liquids may provide an important basis for designing
new ionic liquids with desired properties. Ionic liquids with the
tricyanomethanide anion [C(CN)3] are low melting, thermally
stable (Tdec ≈ 300 °C) compounds which have a considerable
potential as a reaction medium.2 The tricyanomethanide ionic
liquids have electric conductivities, thermal stabilities, and elec-
trochemical properties similar to the dicyanoamide ILs, but they
are less hygroscopic.2 Studies of vaporization enthalpies and
vapor pressures of ILs are scarce3 because of obvious problems
with measuring the extremely low vapor pressures and their
temperature dependencies. We have contributed to the solution
of this problem by using an improved version of the Knudsen
method4 and the transpiration method5 as well as by combina-
tion of quantum-chemical calculations with the combustion
calorimetry.4ꢀ8 Just recently a new method for the determination
of vaporization enthalpies of extremely low volatile ionic liquids
has been developed using the quartz crystal microbalance (QCM)
tecnique.9 Due to the very high sensitivity of the quartz sensor,
2. EXPERIMENTAL PROCEDURE AND METHODS OF
FIRST-PRINCIPLES CALCULATIONS
2.1. Materials. 1-Ethyl-3-methylimidazolium tricyanometha-
nide [C2MIM][C(CN)3], C10H11N5 (CAS 666823-18-3), was
synthesized. The purity of the sample for the thermochemical
studies was g99.0% (NMR). The sample of 1-butyl-3-methyli-
midazolium tricyanomethanide [C4MIM][C(CN)3], C12H15N5
(CAS 878027-73-7), was of commercial origin (Merck, 4.90330)
Received: August 1, 2011
Revised:
September 2, 2011
Published: September 06, 2011
r
2011 American Chemical Society
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ARTICLE
with a purity of g98.0% (HPLC). It contains <0.1% of halides
according to specifications stated by the suppliers. Such an amount
of halide impurity does not affect vapor pressure measurements
and combustion experiments. Prior to experiments, all IL sam-
ples were subjected to vacuum at 333 K for more than 24 h
to remove possible traces of solvents and moisture. The water
concentration of 712 ppm in [C2MIM][C(CN)3] and of
660 ppm in [C4MIM][C(CN)3] was determined by Karl Fischer
titration before starting the experiments, and appropriate correc-
tions have been made for combustion results. We used a Mettler
DL35 Karl Fischer Titrator with Hydranal Composite 2, Hydra-
nal Methanol Dry, and Hydranal Eichstandard 5.0 (Riedel-de
Haen). Samples of the ILs were kept and handled under a
nitrogen stream in a special glass device furnished with a septum
for the sample extraction using a syringe.
2.2. Synthesis of [C2MIM][C(CN)3]. 1-Ethyl-3-methylimidazo-
lium tricyanomethanide [C2MIM][C(CN)3] was not commer-
cially available, and it was synthesized according to the procedure
developed by Brand et al.2 The first synthetic step includes the
formation of the nearly insoluble silver salts in water (Ag[C(CN)3])
from AgNO3 and Na[C(CN)3]. Reaction of Ag[C(CN)3] with
[C2MIM][Br] results in the formation of a water-soluble [C2MIM]-
[C(CN)3] which can be separated from the AgBr precipitate by
filtration. A specific drying procedure involving stepwise adding and
removing dried methanol, tetrahydrofuran, and dichloromethane
followed by removing traces of water or solvent molecules in high
vacuum is necessary to obtain pure ionic liquids.
2.3. Determination of the Vaporization Enthalpy Using
the Quartz Crystal Microbalance Technique. Enthalpies of
vaporization of of [CnMIM][C(CN)3] have been measured using
the QCM technique. The experimental setup and the measuring
procedure have been reported recently.9 In short, a sample of IL
filled into an open stainless steel (316Ti) crucible is placed in a
vacuum chamber (at 10ꢀ5 Pa). In contrast to the Knudsen
technique, the total surface of the sample in the crucible is
exposed to vacuum. The QCM is positioned directly over the
crucible with the IL. The change of the vibrational frequency of
the crystal Δf (is a measure of an amount of IL deposited on the
cold QCM) was registered at constant temperatures. The quartz
crystal is part of a commercially available sensor (BSH-150 by
Inficon). The distance between the surfaces of the quartz crystal
and IL was kept constant at ca. 25 mm. The temperature of the
QCM and its holder is kept at (30.0 ( 0.1) °C by using a Julabo
F12-MC thermostat. The change of the resonance frequency Δf
was measured by a commercial device Q-pod from Inficon. The
value of Δf is directly related to the mass deposition on the crystal
according to the Sauerbrey equation12
Table 1. Enthalpies of Vaporization of [CnMIM][C(CN)3]
(in kJ molꢀ1
)
3
T-range,
Tav,
Δgl Hom at
Δgl H°m at
298.15 Ka
ILs
K
K
Tav
[C2MIM][C(CN)3]
[C4MIM][C(CN)3]
400ꢀ448
405ꢀ453
423.2
428.3
126.0 ( 1.0
129.8 ( 1.4
138.8 ( 5.0
143.2 ( 5.0
a Experimental enthalpies of vaporization for ILs are obtained at 298 K
with ΔlgCpo,m = ꢀ100 J Kꢀ1 molꢀ1
.
3
3
with a constant A0 which is essentially unknown including all empirical
parameters which are specific for the apparatus and the substance
under study. T0 appearing in eq 2 is an arbitrarily chosen
reference temperature. In our study T0 was set equal to 298 K.
The value Δgl Copm = Cpom(g) ꢀ Cpom(l) is the difference of the
molar heat capacities of the gaseous Copm(g) and the liquid phase
Copm(l), respectively. The temperature-dependent vaporization
enthalpy Δgl H°m (T) is given by
Δgl Hm° ðTÞ ¼ ΔlgHm° ðT0Þ þ Δgl Cpom 3 ðT ꢀ T0Þ
ð3Þ
To detect and avoid any possible effect of the impurities on the
measured frequency loss rate df/dt, a typical experiment has
been performed in several series with increasing and decreas-
ing temperature steps. Every step consisted of 7 to 11 constant
temperature points of mass loss rate determination. The study
was finished when the enthalpy of vaporization obtained in
sequential runs agreed within the assessed uncertainty of deter-
mination of (1 kJ molꢀ1. To confirm the absence of decom-
3
position of IL under the experimental conditions, the residual IL
in the crucible and the IL deposit on QCM were analyzed by
ATR-IR. Experimental results are given in Table 1, and the
primary experimental data are listed in Tables S1 and S2 (see
Supporting Information).
Due to the very high sensitivity of the quartz crystal micro-
balance, it is possible to study vaporization processes already at
400 K, reducing the temperature of measuring vaporization
enthalpies by approximately 100 K in comparison to other conven-
tional techniques. Test measurements with [C2mim][NTf2] gave
results very close to the available experimental values indicating
thermodynamic consistency of the procedure.9 In the current work,
we have obtained vaporization enthalpies of [CnMIM][C(CN)3]
using the QCM method.
Our primary experimental results obtained by the QCM
measurements on [CnMIM][C(CN)3] are given in Tables S1
and S2 (Supporting Information). We measured vaporization
enthalpies for [C2MIM][C(CN)3] in the range 400ꢀ448 K and
for [C4MIM][C(CN)3] in the range 405ꢀ453 K. To apply eqs 2
and 3 for the data treatment, values of Δgl Cpom are required.
We have already discussed recently9 that the value of ΔlgCopm = ꢀ
100 J Kꢀ1 molꢀ1 commonly used for all ILs regardless of their
Δf ¼ ꢀ C f2 Δm SC
ð1Þ
ꢀ1
3
3
3
where f is the fundamental frequency of the crystal (6 MHz in this
case); Δm is the mass increase (in g); SC is the area of the crystal
(in cm2); and C is a constant (2.26 ꢁ 10ꢀ6 cm2 gꢀ1 Hzꢀ1).12
3
3
structure is apparently a doubtful choice due to the lack of
experimental data and questionable quantum-chemical estima-
tions of Copm(g). To assess the impact of the Δgl Cp on ΔlgH°m(T),
we have calculated enthalpies of vaporization using arbitrary fixed
Δgl Cp values of ꢀ40, ꢀ100, and ꢀ200 J Kꢀ1 molꢀ1 which cover
3
3
The measured frequency loss rate df/dt is related to the molar
enthalpy of vaporization by eq 29
ꢀ
ꢁ
ꢀ
ꢁ
Δgl Hmo ðT0Þ ꢀ ΔlgCopmT0
pffiffiffi
df
dt
1
1
3
3
¼ A0 ꢀ
ꢀ
the range of Δgl Cpom values expected (see Tables S1 and S2 in
ln
T
R
T
T0
Supporting Information). It turned out that changes of the Δgl H°m-
(T) values were about 2.5 kJ molꢀ1 within the range of these
ꢀ
ꢁ
Δgl Copm
3
T
Δgl Copm values. This increases the uncertainties of the vaporization
enthalpy derived by all experimental methods due to ill-defined
Δgl Cp values.
þ
ln
R
T0
ð2Þ
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ARTICLE
Table 2. Thermochemical Data at T = 298.15 K (p° = 0.1 MPa) for [CnMIM][N(CN)3], kJ molꢀ1
3
ILs
ΔcH°m(l)
ΔfH°m(l)
Δgl H°m
ΔfH°m(g) exp
ΔfH°m(g) CBS-QB3
ΔfH°m(g) G3MP2
[C2MIM][C(CN)3]
[C4MIM][C(CN)3]
ꢀ5849.4 ( 2.2
ꢀ7145.1 ( 2.1
342.2 ( 2.5
279.2 ( 2.6
138.8 ( 5.0
143.2 ( 5.0
481.0 ( 5.6
422.4 ( 5.6
473.3 ( 5.0
429.7 ( 5.0
486.0 ( 5.0
439.8 ( 5.0
Figure 1. Optimized with G3MP2 structures of [C2MIM][C(CN)3] and [C4MIM][C(CN)3].
From adjusting eq 2 to the QCM data with T0 = 298 K and
Δgl Copm = ꢀ100 J Kꢀ1 molꢀ1, enthalpies of vaporization at the
according to the guidelines presented by Olofsson.15 The uncer-
tainty assigned to ΔfH°m(l) is twice the overall standard deviation
and includes the uncertainties from calibration, from the combus-
tion energies of the auxiliary materials, and of the enthalpies of
formation of the reaction products H2O and CO2.
3
3
reference temperature 298 K have also been obtained (see
Table 1). For [C2mim][C(CN)3], the values of Δgl Hmo (298 K)
of 131.3, 138.8, and 151.3 kJ molꢀ1, as well as for [C4mim]-
3
[C(CN)3], the values of Δgl Hom(298 K) of 135.4, 143.2, and
Results of combustion experiments on [CnMIM][C(CN)3]
are summarized in the Supporting Information. The standard
156.2 kJ molꢀ1, have been obtained using Δgl Cpom of ꢀ40, ꢀ100,
3
and ꢀ200 J Kꢀ1 molꢀ1, respectively (Table S3 in Supporting
energies of combustion Δcu° = ꢀ(29 065.6 ( 4.4) J gꢀ1 for
3
3
3
3
Information). Obviously the uncertainty of Δgl Cpom affects dis-
tinctly the values of Δgl H°m at 298 K. As a result, no reliable
standard data for Δgl Hmo (298 K) can be obtained. For the sake of
comparison with our earlier work, we calculated Δgl Hmo (298 K)
using the previously acknowledged5 for ILs value ΔlgCopm = ꢀ
100 J Kꢀ1 molꢀ1. Values of ΔlgHom(298 K) are given in Table 1
[C2MIM][C(CN)3] and Δcu° = ꢀ(31149.5 ( 3.3) J gꢀ1 for
[C4MIM][C(CN)3] have been measured and used to derive
standard molar enthalpies of combustion and standard molar
enthalpies of formation in the liquid state, ΔfH°m(l), based on
the chemical reactions
3
3
C10H11N5 þ 12:75O2 ¼ 10CO2 þ 5:5H2O þ 2:5N2
ð4Þ
together with the data referred to the average temperatures
Δgl H°m(Tav). To account for the uncertainty of the vaporization
enthalpy due to use of the ill-defined for ILs Δgl Cpom values, we
suggest to assign an uncertainty of 5.0 kJ molꢀ1 for ΔlgHmo (298 K).
C12H15N5 þ 15:75O2 ¼ 12CO2 þ 7:5H2O þ 2:5N2
ð5Þ
Since the molarenthalpies offormation of H2O(l) and CO2(g) are
known,14 values for ΔfH°m(l) of [CnMIM][C(CN)3] have been
obtained from the enthalpic balance according to eqs 3 and 5 (see
Table 2).
2.5. First-Principles Calculations. Standard ab initio calcula-
tions were performed using the Gaussian 03 Rev.04 program
package.16 Rotational conformers of the [CnMIM] cation were
studied at the level HF/6-31G* at 0 K. Molecular structures and
relative energies of all conformations for the cation formed by
rotation of the alkyl group around the NꢀC bond by 360° have
been studied with 10° steps starting from the coplanar confor-
mation. Energies and frequencies of normal modes were calcu-
lated for all the stable conformers using the B3LYP/6-31+G(d,p)
basis set. Corresponding calculations have been performed for
the molecular ionic pair [CnMIM][C(CN)3] at the HF/3-21G*,
HF/6-31G(d,p) level and fully optimized at the B3LYP/6-31
+G(d,p) level. Starting from 20 to 30 initial geometries, the
energy of formation of ion pairs from the separated ions was
calculated using the supermolecule method at the B3LYP level.
The optimized structures of the [CnMIM][C(CN)3] ion pairs
are presented in Figure 1.
3
2.4. Thermochemical Measurements: Combustion Calo-
rimetry. An isoperibol bomb calorimeter was used for measuring
the energy of combustion of [CnMIM][C(CN)3]. At least six
successful experiments were carried out for each compound (see
Table S4 and S5 in Supporting Information). The detailed
procedure has been described previously.5 The combustion pro-
ducts were analyzed for carbon monoxide (Dr€ager tube) and
unburned carbon, but none was detected. The energy equivalent
of the calorimeter εcalor was determined with a standard reference
sample of benzoic acid (sample SRM 39j, N.I.S.T.). Correction for
nitric acidformationwas basedonthe titration with0.1 mol dmꢀ3
3
NaOH (aq). For converting the energy of the actual bomb process
to that of the isothermal process, and reducing to standard states,
the conventional procedure was applied.13 Values of the standard
specific energies of combustion Δcu°, together with the necessary
auxiliary quantities, are given in Table S6 (Supporting In-
formation). To derive the molar enthalpy of formation in the
liquid state ΔfH°m(l) from the molar standard enthalpy of
combustion ΔcH°m, molar enthalpies of formation of H2O(l)
and CO2(g) were taken from the literature, as assigned by
CODATA.14 Table 2 contains the derived standard molar en-
thalpy of combustion and standard molar enthalpy of formation of
the [CnMIM][C(CN)3]. The total uncertainty was calculated
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ARTICLE
Figure 2. Most stable conformer of [C2MIM][C(CN)3] calculated using G3MP2 and CBS-QB3. The difference in the enthalpies is 3.4 kJ molꢀ1
.
3
Optimized structures and energies of the ion pair were also
obtained using the G3MP2 and CBS-QB3 methods. The G3
theory is a procedure for calculating energies of molecules
containing atoms of the first and second row of the periodic
table based on ab initio molecular orbital theory. Its modification,
the G3MP2 theory, reduces orders of MøllerꢀPlesset perturbation
theory.17 This method saves considerable computational time
compared to the G3 theory with limited loss in accuracy.
G3MP2 uses geometries from second-order perturbation theory
and scaled zero-point energies from the HartreeꢀFock 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 the theory (for details see ref 17). The
CBS-QB3 theory uses geometries from B3LYP/6-311G(2d,d,p)
calculation and 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.18 Calcu-
lated values of the enthalpy of formation in the gaseous phases
are based on the electronic energy calculations using standard
procedures19 of statistical thermodynamics for calculating tem-
perature-dependent contributions to the enthalpy based on all
frequencies and moments of inertia.
CBS-QB3 methods with the help of the following reactions
½C2MIMꢂ½CðCNÞ3ꢂ þ 34CH4 ¼ 22C2H6 þ 5NH3
ð6Þ
ð7Þ
½C4MIMꢂ½CðCNÞ3ꢂ þ 36CH4 ¼ 24C2H6 þ 5NH3
Using enthalpies of the chemical reactions (according to eqs 6
and 7) calculated by G3MP2 or CBS-QB3 and experimental
enthalpies of formation ΔfH°m(g, 298 K) for CH4, C2H6, and
NH3 as recommended by Pedley et al.20 (see Table S7 and S8 in
Supporting Information), the enthalpies of formation of [CnMIM]-
[C(CN)3] have been calculated (see Table 2). Figure 1 shows the
structures of the ILs optimized using G3MP2. In contrast to our
previous experiences5ꢀ8 with the composite methods G3MP2
and CBS-QB3, the optimized conformations of ionic liquids
[CnMIM][C(CN)3] obtained from both methods have turned
out to be slightly different (see Figure 2). The difference in the
enthalpies between the most stable conformer of [C2MIM]
[C(CN)3] calculated using G3MP2 and CBS-QB3 is 3.4 kJ molꢀ1.
3
The difference in the enthalpies between the most stable con-
former of [C4MIM][C(CN)3] calculated using G3MP2 and
CBS-QB3 is 8.8 kJ molꢀ1. As a consequence, values of ΔfH°m(g,
3
298 K) derived for [C2MIM][C(CN)3] and [C4MIM]-
[C(CN)3] by using G3MP2 and CBS-QB3 are noticeably different.
The results from the CBS-QB3 method are in agreement with the
experimental values. The result obtained by the G3MP2 method
is in good agreement with the experimental value for [C2MIM]-
[C(CN)3] and is overestimated for [C4MIM][C(CN)3]. The
result from the CBS-QB3 method is in good agreement with the
experimental value for [C4MIM][C(CN)3] and is underesti-
mated for [C4MIM][C(CN)3]. Both theoretical methods,
G3MP2 and CBS-QB3, are supposed to be not reliable enough
to predict the experimental results for ILs containing the
tricyanometanide anion. These composite methods should be
applied with precautions when calculating gaseous enthalpies of
formation of ionic liquids containing the [C(CN)3] anion.
3.3. Application of Group-Additivity Rules for ILs. Group-
additivity (GA) methods are well-acknowledged approaches for
testing the consistency of the experimental data of thermoche-
mical properties of molecular compounds.8ꢀ11 The most suc-
cessful GA method for estimating thermodynamic properties was
suggested by Benson.10 This method serves as a valuable tool in
3. DISCUSSION
3.1. Enthalpies of Formation of [CnMIM][C(CN)3] in the
Gaseous State. The enthalpies of formation, ΔfH°m(l), of
[CnMIM][C(CN)3], derived from the combustion experiments
(Table 2, column 3) together with the vaporization enthalpies,
derived from the QCM method (Table 2, column 4), are referred
to the reference temperature T = 298.15 K. Using the equation
ΔfH°m(g) = ΔfH°m(l) + Δgl H°m, we calculated the experimental
values of the standard molar enthalpies of formation ΔfH°m(g)
for (Table 2, column 5). These values have been used to check
the validity of first-principle methods as follows.
3.2. Quantum Chemical Calculations for [CnMIM][N(CN)3].
In recent papers,5ꢀ7 we have shown that 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. We have calculated the enthalpies of formation,
ΔfH°m(g, 298 K), of [CnMIM][C(CN)3] using G3MP2 and
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Table 3. Compilation of the Enthalpies of Vaporization,
Δgl H°m and Enthalpies of Formation in the Liquid, ΔfH°m(l),
and in the Gaseous Phase, ΔfH°m(g), for Some Alkanes,
Alkanols, And Imidazolium-Based ILs at 298.15 K (in
corresponding difference for the [C4MIM][Anion] and [C2MIM]-
[Anion] (Table 3, column 3) is only ꢀ(40ꢀ45) kJ molꢀ1, which is
3
distinctly lower than 2(CH2) = 2(ꢀ25.5) = ꢀ51.0 kJ molꢀ1
3
expected9 from the molecular compounds. Our new results
obtained from combustion calorimetry for [CnMIM][C(CN)3]
with the difference between C4 and C2 derivatives of Δ = ꢀ(63.5 (
kJ molꢀ1
)
3
fragment
Δgl Hom
ΔfH°m(l)
ΔfH°m(g)
3.6) kJ molꢀ1 do not correspond to our expectation for simi-
3
larity in comparison with the data available for molecular and
ionic compounds collected in Table 3.
CH3ꢀ[CH2]2ꢀCH3
CH3ꢀ[CH2]4ꢀCH3
Δa
22.4 ( 0.1
31.7 ( 0.1
9.3 ( 0.2
ꢀ146.6 ( 0.7
ꢀ198.7 ( 0.8
ꢀ52.1 ( 1.1
ꢀ302.6 ( 0.5
ꢀ351.6 ( 0.4
ꢀ49.0 ( 0.6
235.3 ( 3.1
ꢀ125.6 ( 0.7
ꢀ167.1 ( 0.8
ꢀ41.5 ( 1.1
ꢀ255.1 ( 0.5
ꢀ294.7 ( 0.5
ꢀ39.6 ( 0.7
392.3 ( 3.7
Experimental data on enthalpies of formation, ΔfH°m(g), of
two n-alkanes and n-alcohols are presented in column 4
of Table 3. The differences between CH3ꢀ[CH2]4ꢀR and
CH3ꢀ[CH2]2ꢀR as well as for [C4MIM][Anion] and
[C2MIM][Anion] (Table 3, column 4) seem to be remarkably
CH3ꢀ[CH2]2ꢀOH
CH3ꢀ[CH2]4ꢀOH
Δa
[C2MIM][N(CN)2]5
[C4MIM][N(CN)2]5
Δa
[C2MIM][NO3]7
[C4MIM][NO3]7
Δa
52.3 ( 0.1
61.7 ( 0.1
9.4 ( 0.2
consistent. On average, the contribution of ꢀ(40 ꢀ 45) kJ molꢀ1
157.0 ( 2.1
157.2 ( 1.1
0.2 ( 2.4
3
195.0 ( 2.7
352.2 ( 2.9
for two CH2 groups is common to molecular species and ionic
liquid molecules. Such a similarity supposes that gaseous en-
thalpies of formation, ΔfH°m(g), of ILs follow the group
additivity rules established for molecular compounds. However,
our new ΔfH°m(g) results for [CnMIM][C(CN)3] with the differ-
ꢀ40.0 ( 4.1
ꢀ216.9 ( 2.0
ꢀ261.4 ( 2.9
ꢀ44.5 ( 3.5
342.2 ( 2.5
ꢀ40.1 ( 4.7
ꢀ53.2 ( 4.9
ꢀ99.0 ( 4.9
ꢀ45.8 ( 6.9
481.0 ( 5.6
163.7 ( 5.3
162.4 ( 5.7
ꢀ1.3 ( 7.8
138.8 ( 5.0
143.2 ( 5.0
4.4 ( 7.0
ence between C4 and C2 derivatives of Δ = ꢀ(58.6 ( 7.6) kJ molꢀ1
[C2MIM][C(CN)3]
[C4MIM][C(CN)3]
Δa
a Difference between C4 and C2 derivatives.
3
are somewhat higher in comparison with the data available for
molecular and ionic compounds collected in Table 3.
We conclude that application of group additivity rules for the
enthalpies of vaporization and enthalpies of formation of the
series of [CnMIM][Anion] ionic liquids is still questionable.
Apparently, more experimental information is required for
detecting similarities or disparities of additivity parameters of
molecular and ionic compounds.
279.2 ( 2.6
422.4 ( 5.6
ꢀ63.5 ( 3.6
ꢀ58.6 ( 7.9
chemical engineering. In our recent study, we have shown the
general capability to apply a group contribution method to ionic
liquids.8 We have extended now these studies using the new data
obtained in the current work. In Table 3 the thermochemical data
on imidazolium-based ionic liquids [CnMIM][Anion] with n = 2
and 4 are collected. Due to the scarcity of the available data, only
simple structural patterns could be considered before starting the
development of any kind of GA method. As a first step in this
direction, differences in enthalpies (vaporization or formation)
of structurally parent molecules have to be considered
’ ASSOCIATED CONTENT
S
Supporting Information. Primary experimental results
b
from OCM techniques; results for combustion experiments;
auxiliary quantities for combustion calorimetry; CBS-QB3 and
G3MP2 energies and enthalpies of compounds under study; and
spectra of [CnMIM][C(CN)3] used in QCM investigations. This
material is available free of charge via the Internet at http://pubs.
acs.org.
CH3 ꢀ ½CH2ꢂ2 ꢀ CH3 and CH3 ꢀ ½CH2ꢂ ꢀ CH3
CH3 ꢀ ½CH2ꢂ ꢀ OH and CH3 ꢀ ½CH2ꢂ4 ꢀ OH
2
4
½C2MIMꢂ½Anionꢂ and ½C4MIMꢂ½Anionꢂ
’ AUTHOR INFORMATION
These differences could serve as a simple indication whether the
molecules obey additivity rules or not.
Corresponding Author
*E-mail: sergey.verevkin@uni-rostock.de and andreas.heintz@
uni-rostock.de.
Experimental data on vaporization enthalpies of two n-alkanes
and n-alcohols are presented in column 2 of Table 3. It is evident
from this column that the differences between CH3ꢀ[CH2]4ꢀR
and CH3ꢀ[CH2]2ꢀR are identical (9.3 kJ molꢀ1) and are within
’ ACKNOWLEDGMENT
3
the boundaries of the experimental uncertainties for alkanes and
alcohols. We should expect a similar tendency for ILs. Surprisingly,
this is not the case for ionic liquids. The corresponding differences
between [C4MIM][Anion] and [C2MIM][Anion] (Table 3, col-
umn 2) are close to zero (within the boundaries of the experi-
mental uncertainties), and no additivity rules for this property
seem to exist for ILs. However, our new results from QCM for
[CnMIM][C(CN)3] with the difference between C4 and C2
This work has been supported by the German Science
Foundation (DFG) priority program SPP 1191.
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