J. Chem. Thermodynamics 40 (2008) 1661–1665
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J. Chem. Thermodynamics
journal homepage: www.elsevier.com/locate/jct
Thermodynamic properties of caffeine: Reconciliation of available
experimental data
Vladimir N. Emel’yanenko, Sergey P. Verevkin *
Department of Physical Chemistry, University of Rostock, Hermannstrasse 14, D-18051 Rostock, Germany
a r t i c l e
i n f o
Article history:
Received 3 April 2008
Received in revised form 2 July 2008
Accepted 3 July 2008
Available online 11 July 2008
Keywords:
Caffeine
Thermochemistry
Enthalpy of formation
Enthalpy of sublimation
Ab initio calculations
a b s t r a c t
Molar enthalpies of sublimation of two crystal forms of caffeine were obtained from the temperature
dependence of the vapour pressure measured by the transpiration method. A large number of primary
experimental results on the temperature dependences of vapour pressure and phase transitions have
been collected from the literature and have been treated in a uniform manner in order to derive sublimation enthalpies of caffeine at T = 298.15 K. This collection together with the new experimental results
reported here has helped to resolve contradictions in the available sublimation enthalpies data and to
recommend a consistent and reliable set of sublimation and formation enthalpies for both crystal forms
under study. Ab initio calculations of the gaseous molar enthalpy of formation of caffeine have been performed using the G3MP2 method and the results are in excellent agreement with the selected experimental data.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
We have commenced studies on the thermochemical properties of purine-like compounds with the aim of enlarging insight
into the energetics of nucleic acids. Caffeine is a compound of
considerable industrial and environmental significance. Caffeine
is a relatively simple model compound, which is useful in understanding the structure–energy relationships of nucleic acids. Thermodynamic properties of caffeine have been extensively studied
[1–5]. However, thermochemical data on caffeine are in apparent
disarray (see Table 1). A very careful thermochemical study (combustion calorimetry and DSC) of two anhydrous polymorphs of
caffeine has been recently published in this journal [6]. Several
months later, another thermochemical study of caffeine [7] appeared in the same journal, where the reported combustion enthalpy of the a-phase of caffeine differs by about 20 kJ mol1.
Since 1979, vapour pressure and enthalpies of sublimation of caffeine polymorphs have been measured [1] using diverse methods.
We have carefully collected the primary experimental results on
the temperature dependence of vapour pressure and phase transitions available in the literature [1–8] (see Tables 1 and 2). Analysis of the primary experimental data reveals that the sublimation
enthalpies of the caffeine spread over 11 kJ mol1 for the a-polymorph, and over 4 kJ mol1 for the b-polymorph. The purpose of
this paper is to resolve the existing disagreement between the
available values of the sublimation enthalpies of caffeine, with
* Corresponding author. Tel.: +49 381 498 6508; fax: +49 381 498 6502.
E-mail address: sergey.verevkin@uni-rostock.de (S.P. Verevkin).
0021-9614/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jct.2008.07.002
the help of additional experimental measurements. Additional
support for the measured values comes from ab initio calculations
of gaseous molar enthalpy of formation of caffeine using the
G3MP2 method.
2. Experimental
2.1. Materials
A sample of anhydrous caffeine obtained as the a-phase material (CAS registry number 58-08-2) was obtained from Sigma
(USP grade). It was further purified by fractional sublimation at
T = 383 K and at reduced pressure. The purity analyses were performed using a gas chromatograph (GC) with a flame ionisation
detector. A HP-5 capillary column (stationary phase cross-linked
5% PH ME silicone) was used in all our experiments. The column
was 30 m long, had an inside diameter of 0.32 mm , and a film
thickness of 0.25 lm. The flow rate of the carrier gas (nitrogen)
was maintained at 7.2 dm3 h1. The starting temperature for
the GC was 323 K for the first 180 s, followed by heating to
T = 523 K at the rate of 10 K min1. No impurities greater than
0.02 mass per cent were detected in the sample used in this
work.
2.2. Vapour pressure measurements of caffeine
Vapour pressure and enthalpies of sublimation, Dgcr Hm , of caffeine polymorphs were determined using the method of transpiration [9] in a saturated nitrogen stream. About 0.5 g of the sample
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V.N. Emel’yanenko, S.P. Verevkin / J. Chem. Thermodynamics 40 (2008) 1661–1665
2.3. Computations
TABLE 1
Compilation of data on enthalpies of sublimation Dgcr Hm ð298:15KÞ of caffeine
Phase
Technique
T/K
Dgcr Hm ðT av Þ/
(kJ mol1)
Dgcr Hm ð298:15KÞa/
(kJ mol1)
Reference
Crystal
(a)
Transpiration
Effusion
Static
manometer
Effusion
Transpiration
344 to 395
332 to 365
350 to 373
113.8
115.6
110.7
116.6 ± 0.3
116.9 ± 1.8
112.4 ± 0.7
[4]
[3]
[1]
349 to 356
366 to 410
105.1
105.7
106.6 ± 0.7
108.1 ± 1.1
[5]
This work
Crystal
(b)
Transpiration
Effusion
Transpiration
414 to 465
446 to 509
420 to 434
104.7
100.0
98.9
108.5 ± 0.4
107.2 ± 0.5
104.2 ± 3.6
[4]
[1]
This work
Liquid
Static
manometer
509 to 524
75.0
95.3 ± 0.7
[1]
a
Vapour pressures available in the literature were treated using equations (2) and
(3) in order to evaluate enthalpy of vaporization at T = 298.15 K, in the same way as
our own results in table 3.
TABLE 2
Compilation of the data available for the solid phase transition, DtransHm, and the
enthalpy of fusion, Dlcr Hm , of caffeine
Techniquesa
Phase
transition,
Ttrans/K
DtransHm/
(kJmol1)
DTA
MCB
DSC
DSC
DSC
DSC
SC
CC
T
414.2 ± 2.0
4.0 ± 0.1b
426
428.5 ± 1.2
413.4 ± 0.1
428.2
298.15
298.15
298.15
3.9 ± 0.3b
3.6 ± 1.2b
3.4 ± 0.1b
4.0 ± 0.1b
2.0 ± 0.3
4.5 ± 3.2
3.9 ± 3.8
Fusion,
Tfus/K
Dlcr Hm =c
1
(kJ mol )
Reference
512
20.1 ± 0.7
21.6 ± 0.7
23.4 ± 1.3
509.0 ± 0.1
509.6
19.9 ± 0.1
21.9
[1]
[1]
[2]
[8]
[7]
[6]
[6]
[6]
This work
a
Techniques: DTA, Du Pont 990 Thermal Analyzer; MCB, differential heat flux
calorimeter MCB; TE, torsion effusion method; DSC, differential scanning calorimetry; SC, solution calorimetry; CC, combustion calorimetry; T, transpiration.
b
Taken into account for the calculation of the average enthalpy of the phase
transition (3.8 ± 0.3) kJ.mol1.
c
Average enthalpy of fusion (21.4 ± 0.6) kJ mol1 was calculated from the data
presented in this table.
was mixed with glass beads and placed in a thermostated
U-shaped tube having a length of 20 cm and a diameter of
0.5 cm. Glass beads with a diameter of 1 mm provide a surface
large enough for rapid (vapour + solid) equilibration. At constant
temperature (±0.1 K), a nitrogen stream was passed through the
U-tube and the transported amount of material was collected in
a cooling trap. The flow rate of the nitrogen stream was measured
using a soap bubble flow meter and was optimised in order to
reach the saturation equilibrium of the transporting gas at each
temperature under study. The transported material was collected
in a special cold trap and the amount of condensed product was
determined by weighing (±0.0001 g). The saturation vapour presat each temperature Ti was calculated from the amount
sure psat
i
of the product collected within a definite period of time. Assuming
that Dalton’s law of partial pressures applied to the nitrogen
stream saturated with the substance i of interest is valid, values
were calculated using the equation
of psat
i
psat
¼ mi R T a =V M i ;
i
V ¼ V N2 þ V i ;
1
1
ðV N2 V i Þ;
ð1Þ
where R = 8.314472 J K mol , mi is the mass of the transported
compound, Mi is the molar mass of the compound, and Vi is its volume contribution to the gaseous phase. VN2 is the volume of the carrier gas and Ta is the temperature of the soap bubble meter. The
volume of the carrier gas, VN2, was determined from the flow rate
and the time measurement.
Standard ab initio molecular orbital calculations were performed with the Gaussian 03 Rev. 04 series of programs [10]. Energies were obtained at the G3MP2 level of theory. The G3 theory is a
procedure for calculating the energies of molecules containing
atoms of the first and second rows of the periodic chart based on
ab initio molecular orbital theory. A modification of G3 theory that
uses reduced orders of Moller–Plesset perturbation theory is the
G3(MP2) theory [11,12]. This method saves considerable computational time compared to G3 theory with some loss in accuracy, but
is much more accurate than G2MP2 theory. For all the species included in this study, full geometry optimisations were carried
out at the HF/6-31G(d) level. The corresponding harmonic vibrational frequencies were evaluated at the same level of theory to
confirm that the optimised structures found correspond to the potential energy minima and to allow the evaluation of the corresponding zero-point vibrational energies, ZPE, and the thermal
corrections at T = 298 K. The ZPE values were scaled by the empirical factor 0.8929. All the minima found at the HF/6-31G(d) level
were again fully re-optimised at the MP2(FULL)/6-31G(d) level.
The G3MP2 theory uses geometries from second-order perturbation theory and scaled zero-point energies from Hartry–Fock theory followed by a series of single-point energy calculations at the
MP2(Full), QCISD(T), and MP2/GTMP2Large levels of theory (for
details see reference [12]). The enthalpy value of the compound
studied at T = 298 K was evaluated according to the standard thermodynamic procedures [13].
3. Results and discussion
Caffeine was found to display a low-temperature a-polymorph
modification at room temperature with the trigonal crystal structure [8]. At T = 414 K, the a-polymorph of caffeine transforms into
a b-crystal phase [1] and remains in this phase until the melting
process at T = 509 K [6,7] (see Table 2).
3.1. Vapour pressure and sublimation enthalpies
Vapour pressures of caffeine polymorphs measured in this work
and those from the literature [1–5] were treated with equations (2)
and (3), respectively,
b
T
þ Dgcr C p ln
;
T
T0
g
g
Dcr Hm ðTÞ ¼ b þ Dcr C p T:
R ln psat
¼ aþ
i
Dgcr C p ,
ð2Þ
ð3Þ
The values of
required for the correction of the sublimation enthalpies, have been derived according to a procedure developed by Chickos and Acree [14]. The following values: Dgcr C p ¼
1
[6,14] for the a-phase and Dgcr C p ¼
26:8 J K1 mol
1
1
40:8 J K mol [6,14] for the b-phase have been used in this
1
work. The value of Dgl C p ¼ 95:3 J K1 mol , required for adjusting the vaporization enthalpy (see Table 1), was calculated using
the group contribution method of Chickos and Acree [14]. In order
to assess the uncertainty of the sublimation enthalpy, the experimental data were approximated with the linear equation
1
Þ using the method of least squares. The uncertainty
lnðpsat
i Þ ¼ f ðT
in the enthalpy of sublimation was assumed to be identical with
the average deviation of the experimental lnðpsat
i Þ values from this
linear correlation. The experimental results for sublimation enthalpies, and parameters a and b according to equation (2) are listed in
tables 1 and 3. In order to ensure complete phase conversion to the
b-phase during the transpiration measurements, the sample was
firstly heated to T = 434.5 K and was kept at this temperature for
about 30 min. The vapour pressure experiments were consequently done from the higher to lower temperatures.
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V.N. Emel’yanenko, S.P. Verevkin / J. Chem. Thermodynamics 40 (2008) 1661–1665
TABLE 3
Vapor pressures p and Dgcr Hm obtained by the transpiration method
T/Ka
m/mgb
V(N2)c/dm3
flow of N2/(dm3 h1)
p/Pad
1
(pexp pcalc)/Pa
Dgcr Hm =ðkJ mol
0.01
0.01
0.02
0.04
0.01
0.07
0.00
0.06
0.17
0.30
0.01
0.18
106.29
106.10
105.92
105.82
105.73
105.66
105.49
105.36
105.25
105.22
105.17
105.11
0.27
0.46
0.36
0.61
99.19
99.00
98.79
98.61
Þ
1
Caffeine (solid ); Dgcr Hm ð298:15 KÞ ¼ ð108:1 1:1Þ=ðkJ
mol Þ
T=K
116102:54
ðT=KÞ 26:8
lnðp=PaÞ ¼ 307:56
R
R
R ln 298:15
a
366.2
373.3
379.9
383.9
386.9
389.8
395.9
400.9
405.0
406.0
408.0
410.1
2.80
9.80
15.8
11.6
42.5
12.7
54.6
21.0
9.10
20.4
14.2
73.2
204.3
379.0
361.7
189.9
507.0
125.8
309.8
78.60
26.10
49.30
31.20
133.7
5.14
5.14
5.14
5.14
8.12
5.14
5.26
5.14
5.14
8.12
5.26
8.12
0.17
0.33
0.55
0.77
1.06
1.28
2.23
3.38
4.41
5.25
5.77
6.93
Caffeine (solid b); Dgcr Hm ð298:15 KÞ ¼ ð104:2 3:6Þ=ðkJ
mol
116335:98
ðT=KÞ 40:8
lnðp=PaÞ ¼ 313:26
R
R
R ln
420.2
424.9
430.1
434.5
a
b
c
d
11.6
47.2
14.4
68.5
10.2
29.0
6.40
23.5
8.12
8.12
8.12
8.12
1
Þ
T=K
298:15
14.48
20.64
28.68
36.85
Temperature of saturation.
Mass of transferred sample, condensed at T = 293 K.
Volume of nitrogen, used to transfer the mass m of the sample.
Vapour pressure at temperature T, calculated from m and the residual vapour pressure at T= 293 K.
9.0
7.0
5.0
3.0
lnP/Pa
The temperature dependence of the vapour pressure for the solid caffeine is presented in figure 1. As can be seen, our vapour
pressures are in a good agreement with the results measured by
the static method [1] and by the Knudsen effusion method [3,5].
Vapour pressures reported by Griesser et al. [4] lie systematically
above the data available, most probably due to an error in the calibration. Vapour pressures measured using the effusion method [5]
are somewhat below the available data, however, the trend seems
to be correct.
The sublimation enthalpy of the b-phase of caffeine derived in
1
this work, Dgcr Hm ð298:15 KÞ ¼ ð104:2 3:6Þ kJ mol , is definitely
g
lower than those from Griesser et al. [4] Dcr Hm ð298:15 KÞ ¼
1
and
Dgr Hm ð298:15 KÞ ¼ ð107:2 0:5Þ
ð108:5 0:4Þ kJ mol ,
1
kJ mol from Bothe and Cammenga [1], even taking into account the large uncertainty of our results. It was not possible for
us to determine the sublimation enthalpy of the b-phase of caffeine
by the transpiration method more precisely, because of the very restricted temperature range from (420 to 434) K, which was at the
upper limit of our apparatus.
The set of available sublimation enthalpies of the a-phase of
caffeine also shows a large spread in values, from (106 to
117) kJ mol1 (see Table 1). There are two values from [3] and
from [4] close to the level of 117 kJ mol1 and there are also
two values ([5] and this work]) close to the level of 106 kJ mol1.
Which value of sublimation enthalpy is preferred?
1.0
-1.0
-3.0
-5.0
-7.0
0.0019
0.0021
0.0023
0.0025
0.0027
0.0029
0.0031
1000K/T
FIGURE 1. Plot of vapour pressure measurements against reciprocal temperature
for caffeine: ‘d’ – this work; ‘s’ – a-phase [4]; ‘+’ – b-phase [4]; ‘N’ – a-phase [5];
‘h’ – a-phase [1]; ‘M’ – b-phase [1]; ‘j’ – liquid phase [1]; ‘}’ – a-phase [3]. The
dotted lines indicate the temperatures of the phase transitions.
3.2. Consistency tests of the experimental results
Since a significant discrepancy in the available experimental
sublimation enthalpy results collected in table 1 has been found,
additional arguments to support the reliability of our new measurements are required.
3.2.1. Internal consistency of sublimation enthalpies and the enthalpy
of phase transition
A valuable test of the internal consistency of the experimental
data of sublimation enthalpies for the a- and b-phase measured
in this work (see Tables 1 and 3) is the comparison with the enthal-
py of phase transition of caffeine obtained by calorimetry [1,2,6,7]
(see Table 2) according to the general equation
Dtrans Hm ða ! bÞ ¼ Dgcr Hm ða-phaseÞ Dgcr Hm ðb-phaseÞ:
ð4Þ
As can be seen from table 2, the DSC results for the phase transition
DtransHm(a ? b) from different sources are very consistent and the
average value of (3.8 ± 0.3) kJ mol1 was calculated from these
results. Comparing the latter value with the enthalpy of phase transition calculated using equation (4) from the difference of Dgcr Hm
(for the a and b phases) measured in this work (see Table 3):
1664
V.N. Emel’yanenko, S.P. Verevkin / J. Chem. Thermodynamics 40 (2008) 1661–1665
DtransHm(a ? b) = 108.1–104.2 = (3.9 ± 3.8) kJ mol1. This estimate
does not differ from the results measured by calorimetry (see Table
3). Hence, in this way, our results for the sublimation enthalpies of
the a- and b-phase seem to be consistent. Similar treatment of
the sublimations enthalpies measured by Bothe and Cammenga
[1] provide the values DtransHm (a ? b) = 112.4–107.2 =
(5.2 ± 0.9) kJ mol1 and those from Griesser et al. [4]: Dtrans
Hm(a ? b) = 116.6–108.5 = (8.1 ± 0.5) kJ mol1. Our results as well
as those from Bothe and Cammenga [1] are internally consistent;
disagreement between these data sets remains apparent.
3.2.2. Internal consistency of sublimation enthalpies adjusted to the
temperature of fusion
An additional argument to support the experimental results
measured in this work is to consider the following thermochemical
cycle:
Dgcr Hm ðT fus Þ
¼
Dgl Hm ðT fus Þ
þ
Dlcr Hm ðT fus Þ
¼ 75:7 þ 21:4
1
¼ ð97:1 0:9Þ kJ mol :
ð5Þ
The value Dgl Hm ðT fus Þ in equation (5) was obtained by adjusting the
enthalpy of vaporization measured by a static manometer [1] (see
Table 1) from Tav = 516.5 K to Tfus = 509 K. The value Dgl Hm ðT fus Þ in
equation (5) is the average of the available literature data (see Table
2). The resulting Dgcr Hm ðT fus Þ ¼ ð97:1 0:9Þ kJ mol 1 could be
compared with the similar adjustments of our new sublimation
enthalpies (table 1), according to equation (6) for the b-phase of
caffeine
Dgcr Hm ðT fus Þ ¼ Dgcr Hm ðT av Þ þ Dgcr C p ðT av T fus Þ
¼ 98:9 0:0408 ð509 427Þ
1
¼ ð95:6 3:6Þ kJ mol :
ð6Þ
For the adjustment of the sublimation enthalpy for the a-phase of
caffeine, the phase transition DtransHm(a ? b) = (3.8 ± 0.3) kJ mol1
at Ttrans = 414 K (see Table 2) should be additionally taken into account and it should also be adjusted to Tfus. The adjustment for
the a-phase of caffeine is as follows:
Dgcr Hm ðT fus Þ ¼ Dgcr Hm ðT av Þ þ Dgcr C p ðT av T fus Þ Dtrans Hm ða ! bÞ
ðT trans Þ þ Dgcr C p ðT trans T fus Þ
¼ 105:7 0:0268 ð509 388Þ 3:8 þ 0:0014 ð414 509Þ
1
¼ ð97:4 1:1Þ kJ mol :
ð7Þ
As can be seen, results which have been obtained according to
equations (5) to (7) are in excellent agreement. This fact provides
the additional evidence for the internal consistency of the experimental results determined in this work.
3.2.3. Experimental enthalpies of formation of caffeine in the
crystalline phase
The values of enthalpies of sublimation Dgcr Hm at T = 298.15 K
are required to obtain gaseous enthalpies of formation, Df Hm ðgÞ,
of organic compounds, provided that their enthalpies of formation
in the condensed phase, Df Hm (cr), are known. Standard molar
1
for
enthalpies of formation Df Hm ðcrÞ ¼ ð345:1 2:3Þ kJ mol
1
for the bthe a-phase and Df Hm ðcrÞ ¼ ð340:6 2:3Þ kJ mol
phase of caffeine were measured by Pinto and Diogo [6] by
micro-combustion calorimetry. The difference between these values was derived according to
Dtrans Hm ða ! bÞ ¼ Df Hm ; ðcrb-phaseÞ Df Hm ðcr;a-phaseÞ
1
¼ ð4:5 3:2Þ kJ mol
ð8Þ
that has been proven [6] to be consistent with the phase transition
enthalpy measured by DSC (see Table 3). The values are in close
agreement with the value DtransHm(a ? b) = (2.0 ± 0.3) kJ mol1,
measured using a solution calorimeter [6]. Surprisingly, the
1
for the
most recent value, Df Hm ðcrÞ ¼ ð322:2 4:8Þ kJ mol
a-phase of caffeine measured by Dong et al. [7] by using
macro-combustion calorimetry is about 20 kJ mol1 less negative
than the results from Pinto and Diogo [6]. We do not have any
reasonable explanation for such discordance. Even if the
sample by Dong et al. [7] was contaminated with the b-phase, the
disagreement could be only within (2 to 5) kJ mol1 as shown
above. We shall try to resolve this contradiction with the help of
our new values of sublimation enthalpies and high-level ab initio
calculations.
3.2.4. Experimental enthalpies of formation of caffeine in the gaseous
phase
Values of sublimation enthalpies of caffeine, derived in this
work, have been checked for internal consistency. These values
(see Table 3) can now be used for further calculation of the standard enthalpies of formation, Df Hm ðgÞ at T = 298.15 K, of caffeine.
For this purpose, we selected first the enthalpies of formation
Df Hm ðcrÞ of for a- and b-phase of caffeine reported by Pinto and
Diogo [6] and the resulting values of the standard molar enthalpies
1
of formation Df Hm ðgÞ ¼ ð237:0 2:5Þ kJ mol (from the ther
modynamic properties of a-phase) and Df Hm ðgÞ ¼ ð236:4 4:3Þ
1
kJ mol (from the thermodynamic properties of b-phase). The
very good agreement between these values is again further evidence of the internal consistency of the results selected in this
work. However, the absolute value of the gaseous enthalpy of formation of caffeine still remains questionable. An additional possibility to test the consistency of the selected data is the
comparison of the experimental gaseous enthalpy of formation of
caffeine with the value calculated using quantum chemical calculations. Such a test could be performed in the manner we suggested recently [15,16].
3.2.5. Enthalpy of formation of caffeine in the gaseous phase: quantum
chemical calculations
The ab initio molecular orbital methods used for the calculation
of the enthalpy of formation of caffeine have not been yet reported
in the literature. We have calculated using the G3MP2, a total energy at T = 0 K, E0 = 679.383314 Hartree and enthalpy at T =
298.15 K, H298 = 679.369094 Hartree. In standard Gaussian-n theories, theoretical standard enthalpies of formation, Df Hm ðgÞ, are
calculated through atomization reactions [12]. Using this proce1
dure we have obtained for caffeine Df Hm ðgÞ ¼ 235:5kJ mol .
Thus, the theoretical enthalpy of formation of caffeine is in excellent agreement with the experimental values derived from combustion experiments by Pinto and Diogo [6] and enthalpies of
sublimation measured in this work. Hence, with the help of this
theoretical result we are able to resolve the uncertainty in the
available thermochemical data on caffeine.
4. Conclusion
This investigation was undertaken to establish a consistent set
of vapour pressures, sublimation, and formation enthalpies of caffeine. We collected from the literature a large number of primary
experimental results and treated them uniformly in order to derive
the sublimation enthalpies at the reference temperature
T = 298.15 K. The data sets on phase transitions were checked for
internal consistency. This collection together with the new experimental results and theoretical calculations reported here has
helped to resolve contradictions in the available thermochemical
V.N. Emel’yanenko, S.P. Verevkin / J. Chem. Thermodynamics 40 (2008) 1661–1665
data and to recommend consistent and reliable sublimation and
formation enthalpies for caffeine.
Acknowledgement
This work has been supported by the Research Training Group
‘‘New Methods for Sustainability in Catalysis and Technique”
(DFG).
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JCT 08-133