Kinetic and thermodynamic studies on molecular interaction of antipyrine donor and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone as an electron acceptor in different solvents

Article abstract of DOI:10.1002/kin.20745

The charge-transfer (CT) complex of donor antipyrine with I-acceptor 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) has been investigated spectrophotometrically in different halocarbon and acetonitrile solvents. The results indicated immediate formation

Full text of DOI:10.1002/kin.20745

Kinetic and Thermodynamic
Studies on Molecular
Interaction of Antipyrine
Donor and 2,3-Dichloro-5,6-
dicyano-1,4-benzoquinone
as an Electron Acceptor in
Different Solvents
MASOUMEH HASANI, SHELER NIKOEE
Faculty of Chemistry, Bu-Ali Sina University, Hamedan. 65174, Iran
Received 5 November 2010; revised 11 April 2012, 30 April 2012; accepted 30 April 2012
DOI 10.1002/kin.20745
Published online 18 December 2012 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The charge–transfer (CT) complex of donor antipyrine with ꢀ-acceptor 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) has been investigated spectrophotometrically in different
halocarbon and acetonitrile solvents. The results indicated immediate formation of an electron
donor–acceptor complex (DA), which is followed by two relatively slow consecutive reactions.
The pseudo–first-order rate constants for the formation of the ionic intermediate and the final
product at various temperatures were evaluated from the absorbance–time data. The activa-
tion parameters, viz. activation energy, enthalpy, entropy, and free energy of activation, were
computed from temperature dependence of rate constants. The stoichiometry of the complex
was found to be 1:1 by Job’s method of continuous variation. The formation constants of the
resulting DA complexes were determined by the Benesi–Hildebrand equation at four different
temperatures. The enthalpies and entropies of the complex formation reactions have been
obtained by temperature dependence of the formation constants using Van’t Hoff equation.
The results indicate that DDQ complexes of antipyrine in all solvents are enthalpy stabilized
but entropy destabilized. Both the kinetics of the interaction and the formation constants of
the complexes are dependent upon the polarity of the solvents. ꢀ 2012 Wiley Periodicals, Inc.
C
Int J Chem Kinet 45: 81–91, 2013
INTRODUCTION
Correspondence to: Masoumeh Hasani; e-mail: hasani@basu
.ac.ir.
2012 Wiley Periodicals, Inc.
Intermolecular charge–transfer (CT) complexes are
formed when electron donors (D) and electron
c
ꢀ

82
HASANI AND NIKOEE
CH3
N
acceptors (A) interact, a general phenomenon in or-
ganic chemistry [1]. Mulliken [2] considered such
complexes to arise from a Lewis acid–Lewis base type
of interaction, the bond between the components of
the complex being postulated to arise from the partial
transfer of an electron from the base (D) to orbital of
the acid (A). One characteristic feature of a donor–
acceptor (DA) complex is the appearance of a new
absorption band in the spectrum of the complex, com-
monly attributed to an intermolecular CT transition,
involving electron transfer from the donor to the ac-
ceptor.
Because of their wide application and use (rang-
ing from chemistry, materials science, and medicine to
biology), CT complexes have attracted considerable re-
search interest, and over the years a very large number
of CT complexes have been prepared and experimen-
tally studied [3]. CT complexes are known to take part
in many chemical reactions such as addition, substi-
tution, and condensation. Molecular complexation and
structural recognition are important process in biolog-
ical systems. For example, drug action, enzyme catal-
ysis, and ion transfer through lipophilic membranes all
involves complexation.
CH3
N
O
Figure 1 Structure of antipyrine.
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is
a quinone, which does not occur in natural systems,
but its behavior may be considered indicative of that of
other quinines, such as ubiquinone and menadion (2-
methyl-1,4-naphthoquinone) which are of biological
interest [10]. In view of biological significance of elec-
tron donor–acceptor (EDA) complexes and antipyrine
as a drug on the one hand and usefulness of DDQ as
a suitable model compound on the other hand, it is
useful to obtain deeper insight into the CT interactions
of antipyrine and DDQ. For this purpose, a detailed
spectroscopic and thermodynamic study of CT com-
plexation of the antipyrine with DDQ has been carried
out in the present work.
Over the past few years, antipyrine and its deriva-
tives have been widely used in different fields of
medicine. They have been widely used as an ef-
fective analgesic and anti-inflammatory drug. An-
tipyrine is used to relieve pain, swelling, and
congestion in ears and eyes associated with some in-
fections [4]. New antipyrine derivatives such as amides
of 4-aminoantipyrine and 9,10-substituted stearinic
acids are effective pharmaceuticals for prophylactic
and treatment of viral diseases.
The mechanism of action of drug is largely de-
termined by its physicochemical properties in solu-
tion. Thus spectroscopic and thermodynamic studies
on drug molecules are of great relevance in phar-
macokinetics [5] and in developing analytical meth-
ods for detection and estimation of small quantities
of specific drugs. The primary event in the action of
many drugs is often a reversible association between
a drug molecule and a receptor to form a complex.
Spectroscopic and thermodynamic investigations lead
to a measure of the strength of binding of the drug
molecules to other substances present in living sys-
tems. DA complexes possibly have some role in bind-
ing [6]. An important parameter in this respect is the
electron donor ability of a drug molecule to form CT
complexes by electron transfer from an occupied or-
bital to an empty orbital of an acceptor molecule. The
formation constant of a drug–protein complex is an
important parameter in pharmaceutical science [7–9],
particularly in the context of targeted drug delivery.
EXPERIMENTAL
Antipyrine (from Fluka; Fig. 1) and DDQ (Merck)
were of the highest purity available and used with-
out any further purification. Spectroscopic-grade chlo-
roform, dichloromethane (DCM), 1,2-dichloroethane
(1,2-DCE), and acetonitrile (AN) (all from Merck)
were used as received. The selection of solvents is
based on the solubility of the components and so as
to have a wide range of relative permittivity. For pur-
poses of UV–vis spectral determination of the forma-
tion constants (KDA) and molar absorption coefficient
(εDA) of the CT complexes, stock solutions of the donor
and acceptor in proper solvents were freshly prepared
prior to use by dissolving precisely weighed amounts
of the components in the appropriate volume of sol-
vent and spectra were collected after mixing solutions
of the donor and acceptor. Computations for such KDA
and εDA determination were performed by using the
Benesi–Hildbrand method with the aid of program
based on a nonlinear least-squares fit. Van’t Hoff plots
were used for the determination of thermodynamic pa-
rameters of the CT complexes.
All UV–vis spectra were in each solvent recorded on
a Perkin Elmer Lambda 45 spectrophotometer within
the wavelength range 300–700 nm using the same
solvent in the examined solution as a blank. Ab-
sorbance measurements as a function of time, at fixed
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MOLECULAR INTERACTION OF ANTIPYRINE DONOR AND DDQ AS AN ACCEPTOR
83
Figure 2 (A) Electronic absorption spectra in the reaction of DDQ (3.0 × 10−4 M) and antipyrine (2.0 × 10−2 M) in DCM
solution at 25◦C: DDQ alone (1), 1 min after mixing (2), and at time intervals 2 min (3), 3 min (4), 4 min (5), 5 min (6), 9 min
(7), 119 min (8), and 10 h (9). (B) Electronic absorption spectra in the reaction of DDQ (3.0 × 10−4 M) and antipyrine (2.0 ×
10−2 M) in 1,2 DCE: DDQ alone (1), 1 min after mixing (2), and at time intervals 2 min (3), 3 min (4), 5 min (5), 10 min (6),
and 16 min (7).
wavelengths, and thermodynamic studies were made
with a Shimadzu UV-265 spectrometer. The appara-
tus was equipped with a temperature-controlled cell
holder. Both sample and blank compartments were
kept at constant temperature using a Shimadzu CPS-
260 thermostat, which allowed the temperature to be
maintained constant to 0.1◦C. Conductance mea-
surements were carried out with a Metrohm 712 con-
ductivity meter. A dip-type conductivity cell made of
platinum black was used. Donor solution was added
by a Hamilton microsyringe.
electronic spectrum. The electronic absorption spec-
tra of DDQ (3.0 × 10−4 mol dm−3) in the presence
of a large excess of the antipyrine (i.e., [antipyrine]/
[DDQ] = 67) were obtained as a function of time at
25◦C in different solvents. Representative spectra in
DCM and 1,2-DCE are shown in Fig. 2. Other systems
studied show a similar spectral behavior with time. As
seen, addition of the antipyrine to DDQ solution results
in some absorption bands in the 500–700 nm spectral
regions, presumably due to the formation of a CT com-
plex, which causes the appearance of some new bands
in this region. A careful examination of the absorption
spectra of the antipyrine–DDQ systems studied reveals
that the spectra are characterized by maximum absorp-
tion at wavelengths 350, 380, 543 (sh), and 585 nm.
Such spectral features are close to those reported for a
DDQ•− radical ion [11–14] and indicate that the pre-
dominant chromogen with DDQ is the radical anion
DDQ•−, which was probably formed by the dissoci-
ation of an original DA complex with a solvent. The
RESULTS AND DISCUSSION
Spectral Characteristics of the CT Bands
DDQ in halocarbon solvents reacts with antipyrine and
gives characteristic long wavelength absorption bands,
frequently with numerous vibrational maxima in an
International Journal of Chemical Kinetics DOI 10.1002/kin.20745

84
HASANI AND NIKOEE
DA complex, and it is assumed that [D•+A•−] = [P] =
0 when t = 0.
D+A
D.A
very fast
slow
(1)
(2)
π-π complex
[D.A] = [D.A]0 e−k t
(4)
(5)
1
k1
D.A
D•+ A•-
[D.A]0k1
[D•+A•−] =
(e−k t − e−k t
)
1
2
k2 − k1
Radical ions
[D.A]0
k1 − k2
(k2e−k t − k1e−k t
)
(6)
1
2
[P] = [D.A]0 +
k2
D•+ A•-
P
slow
)
(3
Final product
The absorbance of the reaction solution at time t is
given by Eq. (7):
Scheme 1
•+A•−
At = εD.A[D.A] + εD
[D•+A•−] + εP[P] (7)
dissociation of the complex was promoted by ionizing
power of the solvent.
•+A•−
where εD.A, εD
, and εP are the molar absorptivities
The observed increase in the absorption band inten-
sities of the antipyrine–DDQ complexes with lapse of
time supports the supposition that the EDA complex
formed is of the dative-type structure, which conse-
quently converts to an ionic intermediate possessing
the spectral characteristics of the DDQ•− radical ion
[15]. However, the observed gradual decrease in the
intensity of the EDA bands in the 500–650 nm spectral
regions could be due to the consumption of the ionic
intermediate through an irreversible chemical reaction,
whereas the continuous increase in the 350-nm band
with lapse of time is indicative of the formation of the
final reaction product. It is interesting to note that a
shoulder appearing around 350 nm, in the spectrum of
pure DDQ, is still present in the spectra of the DDQ–
antipyrine system, revealing the fact that the quinine
chromophore is probably not disturbed by the reaction
of DDQ with a drug [16].
of species D.A, D•+A•−, and product, respectively.
Based on our observation, the monitored absorbance
at 480 nm decreases with lapse of time and finally
reaches near to zero. So, the product P was set as a
nonabsorbing species (εP = 0).
The substitution of Eqs. (4) and (5) in Eq. (7) and
rearrangement results in
ꢀ
At = [D.A]0 εD.Ae−k t
1
ꢀ
ꢁꢁ
e−k t − e−k t
k1 − k2
2
1
•+A•−
+ k1εD
×
(8)
The pseudo–first-order rate constants k1 and k2 at
various temperatures were then evaluated by fitting the
corresponding absorbance–time data to Eq. (8) using a
nonlinear least-squares curve-fitting program KINFIT
[22]. The program is based on the iterative adjustment
of calculated to observed absorbance values by us-
ing either the Wentworth matrix [23] technique or the
Powel procedure [24]. The adjustable parameters are
The above experimental observations seem to be in
accord with the mechanism illustrated in Scheme 1, as
proposed before [19,20].
•+A•−
k1, k2, εDA, and εD
. The output of the KINFIT
Kinetic Study of Antipyrine–DDQ
Complexes
program comprises the refined parameters, the sum of
squares, and the standard deviation of the data. Mea-
sured and calculated absorbances as a function of time
for 3.0 × 10−4 M of DDQ and 2.0 × 10−2 M antipyrine
in DCM are shown in Fig. 4. A very good agreement
between the observed and calculated absorbances fur-
ther supports the occurrence of a reaction between an-
tipyrine and DDQ via the proposed mechanism.
All the values obtained for k1 and k2 at various tem-
peratures are summarized in Table I. The activation pa-
rameter, Ea, and transition state parameters, ꢀH# and
ꢀS#, were then calculated by using the corresponding
Arrhenius plots and the Eyring transition-state theory
[25], respectively; the results are given in Table II. The
To investigate the kinetics of production and consump-
tion of DDQ•− radical ions (i.e., k1 and k2), the ab-
sorbance at 480 nm was monitored as a function of
time in solutions containing reactants at an antipyrine
to DDQ mole ratio of 67:1 at various temperatures.
Absorbance–time plots for the antipyrine–DDQ sys-
tem in DCM and 1,2-DCE solutions at different tem-
peratures are shown in Fig. 3. For the pair of consec-
utive reactions given in Scheme 1, the concentrations
of species involved as a function of time, under the
pseudo–first-order condition, are given by Eqs. (4)–(6)
[21]. where [DA]0 is the initial concentration of the
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MOLECULAR INTERACTION OF ANTIPYRINE DONOR AND DDQ AS AN ACCEPTOR
85
0.3
0.25
0.2
A
10οC
15οC
25οC
35οC
0.15
0.1
0.05
0
0
10
20
30
40
50
Time (min)
0.25
0.2
0.15
0.1
0.05
0
B
10οC
15οC
25οC
35οC
0
10
20
30
40
50
Time (min)
Figure 3 Absorbance–time plots for a mixture of DDQ (3.0 × 10−4 M) and antipyrine (2.0 ×10−2 M) in (A) 1,2-DCE and
(B) DCM solutions at different temperatures at 480 nm.
0.400
0.300
0.200
0.100
0.000
0
10
20
30
40
50
Time (min)
Figure 4 Measured and calculated absorbance as a function of time for 3.0 × 10−4 M of DDQ and 2.0 × 10−2 M antipyrine
in DCM solution. The nonlinear least-square fit according to Eq. (8) is shown as a solid line at 480 nm.
data given in Table I indicate that, in all cases studied,
the pseudo–first-order rate constants increase with in-
creasing temperature. It is also obvious that k1 values
in more polar solvents are larger than those in less po-
lar ones. The order of k1 is AN > 1,2-DCE > DCM.
While an opposite solvent effect is observed on the
rate of the second steps of the reactions, k2. The k2
values are in order AN < 1,2-DCE < DCM. A similar
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HASANI AND NIKOEE
Table I Rate Constants for CT Complex Formation between DDQ and Antipyrine at Different Temperaturesa
Solvent
AN
Temperature (oC)
Rate Constant k1 (min−1
)
Rate Constant k2 (min−1
)
10
15
25
35
10
15
25
1.93 0.01
2.06 0.01
2.24 0.01
2.45 0.02
0.51 0.01
0.67 0.01
0.90 0.01
1.10 0.03b
1.22 0.03
0.24 0.01
0.32 0.01
0.52 0.01
0.55 0.02b
0.68 0.02
(5.93 0.34) × 10−4
(6.84 0.40) × 10−4
(8.55 0.39) × 10−4
(1.10 0.07) × 10−3
(8.14 1.27) × 10−4
(1.01 0.18) × 10−3
(2.10 0.15) × 10−3
(3.00 0.32) × 10−3b
(3.45 0.20) × 10−3
(1.24 0.10) × 10−3
(1.84 0.07) × 10−3
(3.2 4 0.10) × 10−3
(3.80 0.20) × 10−3b
(4.67 0.25) × 10−3
1,2- DCE
DCM
35
10
15
25
35
aListed uncertainties are one standard deviation.
bValues obtained by conductometric measurements.
Table II Activation Parameters for CT Complex Formation between DDQ and Antipyrine in Different Solventsa,b
ꢀH1o#
ꢀH2o#
ꢀS1o#
ꢀS2o#
ꢀGo1#
ꢀGo2#
Solvent
Ea1
Ea2
(kJ/mol)
(kJ/mol)
(J/mol K)
(J/mol K)
(kJ/mol)
(kJ/mol)
AN
1,2- DCE
DCM
6.6 0.2
24.3 1.1
30.32 2.0
17.4 0.3
4.2 0.2 14.9 0.2 –256.0 0.1 –287.0 0.1 81.1 0.2 100.4 0.2
39.6 7.5 20.2 0.5 42.4 1.6 –211.4 0.1 –187.9 0.1 83.2 0.5
38.2 2.4 28.0 2.0 35.8 2.4 –191.1 0.1 –207.2 0.1 85.0 2.0
98.5 1.6
97.6 2.4
aListed uncertainties are one standard deviation.
bꢀG values calculated at 25oC.
Table III Conductivity (μS/cm) of 3.0 × 10−4 M of DDQ before and after Addition of Antipyrine in Different Solvents
(DDQ/Antipyrine = 67)
Solvent
Solution
AN
1,2-DCE
DCM
CHCl3
DDQ alone
1.85
12.78
23.12
23.0
0.05
1.09
1.68
1.53
0.05
1.45
2.3
0.05
0.07
0.10
0.05
Immediately after addition of antipyrine
5 min after addition of antipyrine
30 min after addition of antipyrine
1.7
behavior was observed in the case of azacrown–DDQ
systems [14,20]. It should be noted that the observed
solvent effect on the k1 and k2 values is in support of the
proposed two-step mechanism. The resulting DDQ•−
radical ions from the first step are expected to become
more stabilized in AN as a solvent of higher solvating
ability and dielectric constant than DCM [26]. Conse-
quently, the rate of production of D+A− is expected to
increase in AN solution, while its consumption to the
final product should be decreased in this solvent.
For supporting of the formation of ionic species
in solution (i.e., D.+ and DDQ.−), the conductivity of
solution was measured before and after addition of
antipyrine to DDQ (Table III). The increase in the con-
ductivity of solution indicates the formation of ionic
species in the solution. The conductivity of a solu-
tion containing reactants at an antipyrine to DDQ mole
ratio of 67 in 1,2-DCE, DCM, and chloroform sol-
vents was also monitored as a function of time at 25oC.
The conductivity–time plots show patterns similar to
absorbance–time plots (Fig. 5) and further supports
the occurrence of reactions between the antipyrine
and DDQ via the two-step mechanism suggested. In
the case of conductivity measurements, the observed
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MOLECULAR INTERACTION OF ANTIPYRINE DONOR AND DDQ AS AN ACCEPTOR
87
Figure 5 Conductivity (μS/cm)–time plots for a mixture of DDQ (3.0 × 10−4 M) and antipyrine (2.0 × 10−2 M ) in (a)
chloroform, (b) 1,2-DCE, and (c) DCM solutions at 25oC.
conductance–time data are also fitted to the proper
equation using the KINFIT program. As seen from
Table I, the k1 values obtained by the conductometric
method at 25oC for the DDQ–antipyrine system in 1,2-
DCE and DCM solution are very similar to the those
obtained with the spectrophotometric method. The k2
values are also in fair agreement with each other. In
chloroform with a low dielectric constant, the forma-
tion of ionic species was very negligible. Because of
the little change in the conductivity of the solution in
this case, the corresponding data were not fitted well.
From Table II, it is obvious that, in all solvents stud-
ied, the values of Ea1 and ꢀH1# are in agreement with
the rate constants, i.e., Ea1 is less than Ea2. Moreover,
Ea1 in more polar solvent AN is less than that in DCM
as a solvent of lower polarity. As stated earlier, the rates
of the first step, k1 in more polar solvent, are higher
than those in the solvents of lower polarity. But the
value of Ea2 in AN is lower than DCM although the
rate of the second step in AN solution is lower. The
second step in AN is associated with a highly negative
ꢀS2#. The compensatory effect of ꢀH# and ꢀS# val-
ues would lead to ꢀG# values that is consistent with
the rate of the two processes. Thus the more negative
ꢀS2# for the consumption of DDQ•− radical ions in AN
as compared with DCM may result in the lower rate
constants.
ratio of concentrations is varied. The absorbance due
to the formation of the DA complex in these mixtures
was determined at 480 nm after applying a correction
for absorption by DDQ at this wavelength. The maxi-
mum absorbance, as is well known, corresponds to the
stoichiometric DA ratio in the complex. In the present
case, the symmetrical curves with maximum at 0.5 mol
fraction indicate the formation of 1:1 CT complex in
all the solvents studied. The corresponding plots are
shown in Fig. 6.
As seen in Fig. 7 during the first few seconds af-
ter the addition of antipyrine to the DDQ solution, a
rather sharp step-functional increase in the absorbance
at 480 nm was observed, as was previously reported
[28–31]. This initial change in the absorbance (ꢀA)
upon the mixing of antipyrine and DDQ was found
to increase along with a rise in the mole ratio of an-
tipyrine to DDQ. Such a dependence of ꢀA on the an-
tipyrine/DDQ mole ratio indicates the formation of the
EDA complex between the DDQ and the donor used.
The absorbance of the complex formed was measured
using a constant acceptor concentration (in a given
solvent) and varying concentrations of the donor de-
pending on the solvent but always [D]0 > [A]0.
The equilibrium constant (KDA) and molar ab-
sorbance (εDA) of the CT complex between DDQ
and antipyrine are determined by using the Benesi–
Hildebrand equation [32].
Spectrophotometric Study of Formation
Equilibrium of the Complex of Antipyrine
with DDQ
[A]o
Abs
1
1
1
εDA
=
+
(9)
KDAεDA [D]o
Job’s method of continuous variation [27] was em-
ployed to determine the stoichiometry of the complex,
in three different solvents. In this method, mixtures
are prepared in which the sum of the molar concentra-
tions of antipyrine and DDQ is kept fixed, while the
where [D]0 and [A]0 are the concentrations of the
antipyrine donor and iodine acceptor, respectively.
KDA is the association equilibrium constant, and εDA
is the molar extinction coefficient of the complex.
Figure 8 shows the Benesi–Hildebrand plots of
International Journal of Chemical Kinetics DOI 10.1002/kin.20745

88
HASANI AND NIKOEE
0.4
0.3
0.2
0.1
0
3
2
1
0
0.2
0.4
0.6
0.8
1
X
antipyrine
Figure 6 Continuous variation plots for DDQ–antipyrine system in (1) chloroform, (2) DCM, and (3) 1,2-DCE at 25oC.
Varying volumes of equimolar solutions of antipyrine and DDQ were mixed in a total volume of 2.5 mL; λ = 480 nm.
0.3
A
200
180
160
0.2
0.1
140
120
100
80
60
40
0
Time
0.3
B
200
180
160
140
0.2
0.1
0
120
100
80
60
40
Time
Figure 7 Absorbance–time plots for a 3.0 × 10−4 M solution of DDQ in 1,2-DCE in the presence of different [an-
tipyrine]/[DDQ] mole ratios at (A) 25oC and (B) 35oC. The corresponding molar ratio value is given on each plot.
[A0]/Abs against 1/[D]0 from the data obtained from
Fig. 7. A quite satisfactory straight relationship was ob-
tained, showing a 1:1 CT complex. The εDA and KDA
values were evaluated from the intercept and slope of
the linear plot between [A0]/Abs and 1/[D]0. Linear re-
gression analysis was carried out in the final stages of
computation of K in each case. The values evaluated
for K at all solvents are given in Table IV. The data
given in Table IV revealed that the stability of the re-
sulting complexes increases with increasing polarity of
the solvent from chloroform to AN. A similar solvent
effect on the stability and charge transition energies of
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MOLECULAR INTERACTION OF ANTIPYRINE DONOR AND DDQ AS AN ACCEPTOR
89
0.009
3
0.006
0.003
0
4
2
1
0
20
40
1/[D]0
60
80
100
Figure 8 Benesi–Hildebrand plots of DDQ with antipyrine in 1,2-DCE (1), DCM (2), AN (3), and chloroform (4) solutions.
Table IV Calculated Formation Constants, Molar Absorptivities at Different Temperatures and Thermodynamic
Parameters for the Antipyrine–DDQ System in Different Solutionsa
Temperature
(oC)
Formation Constant
(KDA
Solvent
AN
)
εDA
ꢀHo (kJ/mol)
−28.5 2.5
ꢀSo (J/ mol K)
−61.3 8.5
ꢀGo (kJ/mol)
10
15
25
35
10
15
25
35
10
15
25
35
10
15
25
35
123.2 8.7
83.9 8.2
59.6 5.7
44.2 6.6
61.5 2.0
51.7 3.4
39.6 1.6
25.7 2.2
45.5 3.1
35.6 1.9
29.3 2.0
20.9 3.2
14.1 1.2
12.1 1.1
6.3 0.6
350 12
452 25
568 35
582 61
−11.1
−10.8
−10.2
−9.6
−9.7
−9.5
−8.9
−8.4
−9.1
−8.9
−8.4
−8.0
−6.5
−5.8
−4.2
−2.7
1,2- DCE
DCM
563
8
−24.6 1.5
−21.3 1.5
−50.2 4.5
−52.6 5.2
−43.0 5.1
−154.4 15.3
664 20
781 17
980 48
565 19
528 14
582 22
730 58
789 48
883 62
1185 22
2541 893
Chloroform
2.5 0.9
aListed uncertainties are one standard deviation.
different CT complexes has been reported in the liter-
ature [33–35]. It has been suggested that the observed
trend in the stability of the CT complexes could be
due to the high stabilization of the excited states in
which the charge is probably more separated than in
the ground states.
perature if the process is endothermic but decreases if
it is exothermic, may be expressed as follows:
ln K = −ꢀH◦/RT + ꢀS◦/R
where ꢀHo and ꢀSo are changes in enthalpy and en-
tropy of the reaction, respectively. The experimental
stability constants in the three solvents determined at
283–308 K were plotted according to this equation.
For the studied systems, there is no evidence for the
deviation from the linearity of the plot of ln KDA ver-
sus 1000/T over the investigated temperature range
Thermodynamic Parameters of the CT
Complex
The Van’t Hoff equation, which shows that the equilib-
rium constant of one reaction increases with the tem-
International Journal of Chemical Kinetics DOI 10.1002/kin.20745

90
HASANI AND NIKOEE
2.5
2
that the DDQ forms the CT complex of 1:1 stoichiom-
etry with antipyrine in all solvents studied. The spectro-
scopic and thermodynamic parameters of the complex
were found to be highly solvent dependent.
1
2
3
1.5
1
4
The authors would like to thank Dr. Saeid Azizian for helpful
discussions.
0.5
0
BIBLIOGRAPHY
3.2
3.3
3.4
3.5
3.6
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