Indian Journal of Pure & Applied Physics
Vol. 47, February 2009, pp. 97-102
Determination of stability constants of charge transfer complexes of iodine
monochloride and certain ethers in solution at 303 K by ultrasonic method
V Kannappan*, S J Askar Ali & P A Abdul Mahaboob
Postgraduate and Research Department of Chemistry, Presidency College (Autonomous), Chennai 600 005
Received 13 July 2007; revised 13 August 2008; accepted 6 November 2008
Ultrasonic velocities (U), densities (ρ), and coefficient of viscosities (η) have been measured for solutions containing
iodine monochloride (ICl) and one of the following ethers in the equimolar concentration in the range 0.001-0.01M at
303 K. Diphenyl ether, 4-chloroanisole, anisole and 1,4-dioxane are used as donors. Dichloromethane, chloroform, carbon
tetrachloride and n-hexane have been used as solvents. Acoustical parameters such as adiabatic compressibility (β),
absorption coefficient (α/f2), internal pressure (πi) and cohesive energy (CE) values are calculated from the measured values
of U, ρ and η. The trend in the acoustical parameters establishes the formation of charge transfer complexes between iodine
monochloride (acceptor) and ethers (donors). The stability constants (K) are calculated for these complexes. The free energy
changes (∆G) for the formation of these complexes are also calculated from K values. Attempt has been made to correlate
the formation constants with polarizability, dielectric strength and dipole moment of the donor and solvent molecules. The
free energy of activation (∆G#) and viscous relaxation time (τ) are found to be almost constant for these complexes
indicating the formation of similar charge transfer complexes in these systems.
Keywords: Ultrasonic velocity, Formation constants, Donor-acceptor complexes, Iodine monochloride, Ethers
1 Introduction
Ultrasonic velocity measurements have been
successfully employed to detect and assess weak and
strong molecular interactions, present in binary1,2 and
ternary3,4 liquid mixtures. These studies can also be
used to identify complexation and to calculate the
stability constants of complexes5,6, in particular the
charge transfer complexes formed between organic
compounds containing electron rich centers and
electron deficient compounds. Ethers are Lewis bases
as they contain electron rich ethereal oxygen and thus
they can function as donors. Iodine monochloride is a
polar diatomic molecule containing positive iodine
and this end of the dipole can be attracted by the
electron rich ethereal oxygen. In the present work,
ultrasonic studies have been carried out to detect the
formation of charge transfer complexes between
ethers and iodine monochloride and the stability
constants of these complexes were calculated using
modified Bhat equation7. In this paper, we report the
results obtained in the study of molecular interaction
between iodine monochloride and four ethers namely,
diphenyl ether, 4-chloroanisole, anisole and 1,4-dioxane
in dichloromethane, chloroform, carbon tetrachloride
and n-hexane at 303K.
These studies are made mainly to investigate the
effect of structure of donor molecules and polarity of
medium on the stability of this type of complexes and
the factor which plays significant role in the
complexation.
2 Experimental Details
Iodine monochloride (Merck-AR) is used as such.
The solvents dichlromethane, chloroform, carbon
tetrachloride and n-hexane are distilled before use.
The four ethers diphenyl ether, 4-chloroanisole,
anisole and 1,4-dioxane used as donors are of AnalaR
grade (SDS). Accurately weighed amount of samples
were dissolved in suitable solvent to obtain solution in
the concentration range 1×10−3-1×10−2 M. Ultrasonic
velocities have been measured in ultrasonic
interferometer (model F81) supplied by Mittal
Enterprises, New Delhi operating at a frequency of
2 MHz. It has an accuracy of ± 0.1%. Viscosities of
pure compounds and their mixtures were determined
using Oswald’s viscometer calibrated with double
distilled water. The densities of pure compounds and
their solutions were measured accurately using 10 ml
specific gravity bottles in an electronic balance
precisely and the accuracy in weighing is ± 0.1 mg.
The temperature of the test solutions and their
mixtures were maintained at 303 ± 0.1 K using a
thermostat. Acoustical parameters such as adiabatic
compressibility (β), absorption coefficient (α/f2), free
length (Lf), internal pressure (πi), cohesive energy
(CE), relaxation time (τ), stability constant (K) and
98
INDIAN J PURE & APPL PHYS, VOL 47, FEBRUARY 2009
the thermodynamic parameter, free energy change
(∆G) were calculated using standard equations7-11.
3 Results and Discussion
The measured ultrasonic velocities, densities and
viscosities at various equimolar concentrations of
diphenyl ether, 4-chloroanisole, anisole and 1,4-dioxane
with iodine monochloride in dichloromethane,
chloroform, carbon tetrachloride and n-hexane at 303
K are given in Tables 1-3. The plots of ultrasonic
velocity versus concentration of four ethers in four
different solvents were presented in Figs 1(a-d). From
the plots, it is seen that the ultrasonic velocity
decreases with increase in concentration for all the
sixteen systems. At a characteristic concentration, the
ultrasound velocity of solution is less than that of
ideal mixing indicating the formation of charge
transfer complex between the donor and acceptor.
Further, the decrease in velocity with increase in
concentration (Table 1) suggests that the extent of
complexation increases with concentration in all the
systems investigated. There is decrease in density
(Table 2) at a specific concentration (3×10−3-6×10−3 M)
and this suggests that the complexation is
concentration dependant. This is also suggested by the
trend in the viscosity values (Table 3).
The adiabatic compressibility is a measure of
intermolecular association between the donor and
acceptor. A typical plot of adiabatic compressibility
versus concentration of iodine monochloride and
ethers in dichloromethane is shown in Fig. 2. It is
observed that there is a drop in the adiabatic
compressibility values in the concentration range
3×10−3-5×10−3 M indicating significant formation of
donor-acceptor complex in this concentration range
for all the above systems.
The absorption coefficient (α/f2) generally increases
with increase in concentration (Fig. 2) and this trend
suggests that the extent of complexity increases with
increase in concentration. The internal pressure (πi) is
a measure of cohesive forces between the constituent
molecules in liquids. In order to assess the cohesive
forces in the ternary liquids investigated, πi values are
calculated for all the systems. It is found that the
internal pressure in ternary solution is generally
greater than that of pure solvent which shows that the
intermolecular attractive forces between the
molecules of complexes are strong in solution.
Further, the internal pressure for a given system
increases with increase in concentration (Fig. 4)
which suggests that there is an increase in the extent
Table 1 — Ultrasonic velocity (ms−1) values of ICl - ether systems
in different solvents at 303 K
Conc.
M
CH2Cl2
Diphenylehter
CHCl3
CCl4
C6H14
0.001
1052.0
964.8
908.8
1052.8
0.002
1050.0
964.4
908.0
1051.6
0.003
1049.6
964.0
907.2
1050.4
0.004
1049.0
963.9
907.2
1049.2
0.005
1048.6
963.6
906.4
1048.0
0.006
1048.0
962.8
905.6
1047.8
0.007
1047.2
962.4
904.8
1047.6
0.008
1046.4
961.6
904.4
1047.2
0.009
1046.0
960.8
904.0
1046.8
0.010
1045.2
960.0
903.2
1046.4
4-Chloroanisole
0.001
1054.8
966.4
906.8
1050.8
0.002
1054.4
966.0
906.4
1051.6
0.003
1054.0
965.2
906.2
1053.2
0.004
1053.2
964.8
905.6
1054.0
0.005
1052.8
964.4
905.2
1054.4
0.006
1052.0
964.0
904.0
1055.6
0.007
1051.6
963.2
903.2
1056.0
0.008
1050.8
962.4
902.4
1056.4
0.009
1050.4
962.0
901.6
1057.2
0.010
1049.6
961.6
900.8
1057.6
0.001
1052.8
967.6
899.6
1048.8
0.002
1050.8
966.8
900.4
1050.2
0.003
1050.6
966.4
901.2
1051.0
0.004
1048.8
965.2
902.0
1051.6
0.005
1048.0
964.8
902.4
1052.0
0.006
1047.4
964.0
903.4
1052.4
0.007
1047.2
963.2
903.6
1052.8
0.008
1047.0
962.4
904.0
1054.0
0.009
1046.6
962.0
904.4
1054.4
0.010
1044.8
961.2
905.0
1055.2
Anisole
1,4-Dioxane
0.001
1052.9
967.6
907.6
1056.4
0.002
1051.8
966.0
907.2
1055.6
0.003
1052.4
965.6
906.8
1055.2
0.004
1050.2
965.2
905.6
1054.4
0.005
1048.0
964.0
906.0
1054.0
0.006
1048.2
963.6
907.6
1052.8
0.007
1046.6
962.4
909.6
1054.4
0.008
1046.0
961.6
912.0
1055.2
0.009
1045.6
960.8
912.4
1054.4
0.010
1045.2
960.4
912.8
1053.6
KANNAPPAN et al.: STABILITY CONSTANTS OF CHARGE TRANSFER COMPLEXES OF IODINE MONOCHLORIDE
Table 2 — Density (kgm−3) values of ICl - ether systems in
different solvents at 303 K
Conc.
M
Table 3 — Viscosity (cP) values of ICl - ether systems in
different solvents at 303 K
CH2Cl2
Diphenylehter
CHCl3
CCl4
C6H14
Conc.
M
CH2Cl2
0.001
1261.4
1411.2
1515.0
625.2
0.001
0.002
1258.1
1410.8
1514.2
625.4
0.002
0.003
1259.2
1412.2
1513.5
626.0
0.004
1257.6
1410.6
1514.4
626.3
Diphenylehter
CHCl3
CCl4
C6H14
0.561
0.670
0.878
0.327
0.559
0.675
0.881
0.328
0.003
0.560
0.672
0.879
0.326
0.004
0.559
0.670
0.883
0.327
0.555
0.671
0.880
0.326
0.005
1256.8
1411.8
1513.2
625.8
0.005
0.006
1261.0
1412.2
1514.6
627.0
0.006
0.563
0.673
0.883
0.327
0.007
1258.9
1413.5
1514.2
626.6
0.007
0.560
0.678
0.884
0.326
0.008
1260.3
1412.5
1514.2
626.8
0.008
0.561
0.674
0.886
0.326
0.009
1258.1
1413.2
1514.9
626.8
0.009
0.565
0.673
0.879
0.326
627.2
0.010
0.561
0.680
0.884
0.327
0.570
0.671
0.891
0.334
0.010
1259.7
1413.1
1515.1
4-Chloroanisole
4-Chloroanisole
0.001
1261.4
1411.7
1516.7
631.2
0.001
0.002
1262.9
1412.8
1516.9
631.9
0.002
0.560
0.672
0.885
0.333
0.003
1262.6
1412.4
1517.7
632.0
0.003
0.567
0.673
0.889
0.334
0.004
1261.1
1413.2
1516.2
632.4
0.004
0.564
0.677
0.890
0.333
0.005
1262.6
1413.5
1515.9
632.5
0.005
0.567
0.677
0.894
0.333
0.568
0.673
0.884
0.334
0.006
1263.2
1412.4
1515.8
632.7
0.006
0.007
1264.1
1412.1
1516.7
632.7
0.007
0.569
0.670
0.887
0.336
0.568
0.673
0.890
0.333
0.008
1264.7
1412.6
1518.2
632.9
0.008
0.009
1263.8
1411.8
1516.0
633.7
0.009
0.566
0.673
0.892
0.336
0.010
1265.3
1412.2
1517.3
633.8
0.010
0.562
0.673
0.895
0.334
0.001
1256.6
1412.1
634.1
0.001
0.561
0.664
0.873
0.329
0.562
0.672
0.869
0.331
Anisole
Anisole
1513.0
0.002
1257.9
1414.3
1513.4
634.7
0.002
0.003
1257.8
1412.2
1513.5
634.9
0.003
0.561
0.675
0.874
0.331
0.558
0.670
0.872
0.332
0.004
1256.7
1413.0
1514.0
634.8
0.004
0.005
1258.6
1412.9
1514.4
634.1
0.005
0.559
0.667
0.877
0.332
0.006
1259.1
1412.8
1513.8
635.0
0.006
0.558
0.672
0.881
0.330
0.007
1257.7
1413.0
1514.0
634.9
0.007
0.543
0.672
0.876
0.331
0.008
1258.5
1413.7
1512.5
635.7
0.008
0.556
0.673
0.875
0.332
0.554
0.667
0.869
0.332
0.557
0.672
0.872
0.333
0.009
1256.9
1414.1
1513.4
635.4
0.009
0.010
1258.4
1414.0
1512.5
635.1
0.010
1,4-Dioxane
1,4-Dioxane
0.001
1257.1
1411.3
1522.1
636.1
0.001
0.561
0.675
0.905
0.337
0.002
1258.2
1411.2
1522.1
635.8
0.002
0.567
0.675
0.903
0.334
0.003
1258.1
1410.6
1521.2
636.4
0.003
0.566
0.673
0.900
0.333
0.004
1258.0
1412.8
1521.7
637.0
0.004
0.566
0.680
0.902
0.335
0.568
0.682
0.899
0.334
0.005
1257.5
1411.4
1521.5
635.9
0.005
0.006
1260.6
1411.6
1522.3
635.6
0.006
0.571
0.681
0.903
0.333
0.007
1261.0
1410.9
1521.2
636.3
0.007
0.566
0.678
0.902
0.335
0.008
1259.0
1410.8
1522.2
635.9
0.008
0.564
0.680
0.902
0.335
0.009
1257.8
1410.8
1521.4
636.7
0.009
0.565
0.677
0.902
0.336
636.8
0.010
0.564
0.675
0.903
0.334
0.010
1258.2
1411.9
1522.3
99
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INDIAN J PURE & APPL PHYS, VOL 47, FEBRUARY 2009
Fig. 3 — Adiabatic coefficient versus concentration of iodine
monochloride and ethers in dichloromethane at 303 K
Fig. 4 — Internal pressure vs concentration of iodine
monochloride and ethers in dichloromethane at 303 K
Fig. 5 — Cohesive energy versus concentration of iodine
monochloride and ethers in dichloromethane at 303 K
Fig. 1 — (a-d) Ultrasonic velocity versus concentration of iodine
monochloride and ethers in dichloromethane at 303 K
Fig. 2 — Adiabatic compressibility versus concentration of iodine
monochloride and ethers in dichloromethane at 303 K
of complexation with increase in concentration. The
trend in cohesive energy (CE) is similar to that of
internal pressure (πi). Plots of CE versus
concentration are shown in Fig. 5.
The stability constants are calculated from
measured ultrasonic velocities using modified Bhat
equation7. These values for all the donor-acceptor
complexes are given in Table 4. It may be noted that
the stability constant values are almost constant for a
given system at a given temperature indicating that
the equilibrium constant depends on the structure of
ethers. By comparing the values of equilibrium
constant of four ethers in one solvent, the ease of
complexation with iodine monochloride and ethers is
KANNAPPAN et al.: STABILITY CONSTANTS OF CHARGE TRANSFER COMPLEXES OF IODINE MONOCHLORIDE 101
Table 4 — Stability constant (dm3 mol−1), free energy (kJmol−1),
free energy of activation (kJmol−1) and relaxation time (×10−13 s)
values of certain charge transfer complexes of ICl - ether systems
at 303 K
Donor
Diphenyl ether
4-Chloroanisole
Anisole
1,4-Dioxane
Diphenyl ether
4-Chloroanisole
Anisole
1,4-Dioxane
Diphenyl ether
4-Chloroanisole
Anisole
1,4-Dioxane
Diphenyl ether
4-Chloroanisole
Anisole
1,4-Dioxane
CH2Cl2
K
CHCl3
K
CCl4
K
C6H14
K
312.7
72.2
69.6
52.1
197.8
53.8
51.2
42.6
57.9
48.6
38.5
36.1
47.6
40.0
35.4
32.0
∆G
∆G
∆G
∆G
-14.13
-10.62
-10.52
-9.71
-13.27
-9.71
-9.73
-9.00
-9.98
-9.69
-9.16
-9.01
-9.35
-9.24
-8.91
-8.56
∆G#
∆G#
∆G#
∆G#
3.39
3.39
3.37
3.41
4.05
4.04
4.02
4.06
4.94
4.97
4.93
4.97
3.82
3.83
3.81
3.82
τ
τ
τ
τ
5.40
5.40
5.37
5.45
6.86
6.84
6.80
6.89
9.46
9.56
9.45
9.57
6.32
6.33
6.28
6.31
found to be in the order; diphenyl ether > 4-chloroanisole ≈ anisole > 1,4-dioxane. In complexation
between ether and ICl, ethereal oxygen donates
electron which is attracted by positive end of dipole in
ICl. Diphenyl ether contains electron releasing phenyl
groups on either side of the donor atom and similarly
in 4-chloroanisole, chlorine atom in the para position
to methoxy group releases electron by mesomeric
effect. But in anisole molecule, the resonance effect is
limited although methoxy group which is directly
attached to a phenyl ring. In dioxane molecule, no
mesomeric effect is possible and hence, the stability
constant value is the least in the case of dioxane.
From the stability constants obtained for the above
systems, the free energy of formation (∆G) and free
energy of activation (∆G#) are calculated at 303 K and
presented in Table 4. For all the systems, ∆G values
are negative indicating that the charge transfer
complexes are thermodynamically stable. The free
energy of activation (∆G#) and relaxation time (τ) are
intrinsic properties of a charge transfer complex.
These two properties are almost constant in the four
different systems.
Table 5 — Polarizability dipole moment and dielectric strength
for ethers and solvents
CH2Cl2
CHCl3
CCl4
C6H14
Diphenylether
4-Chloroanisole
Anisole
1,4-Dioxane
α
6.8
8.5
10.5
11.6
20.859
14.749
13.071
10
µ
1.6
1.01
0
0.08
1.031
1.619
1.084
0
ε
9.08
4.806
2.238
1.89
2.68
7.84
4.3
2.22
Fig. 6 — logK versus dielectric strength of solvents
3.1 Correlation of stability constant with molecular properties
The complex formation is also influenced by the
molecular properties such as polarizability (α), dipole
moment (µ) and dielectric constant (ε) of donor
molecules12-14. These parameters for the four ethers
are listed in Table 5.
The formation constant (K) increases with increase
in polarizability of donor molecules. Therefore,
increase in polarizability of donor increases the ease
of complexation. Further, stability constant K
increases with dipole moment in three systems. But in
the case of diphenyl ether, the stability constant is
abnormally high eventhough it has a low dipole
moment. This may be due to rich π electrons in
diphenyl ether. It is also found that the stability
constant value increases with increase in dielectric
constant of all donor molecules except diphenyl ether.
The greater stability constant of diphenyl ether may
be due to increase in π electron density by the two
neighbouring phenyl rings.
These correlations of K with molecular properties
indicate that it is the polarizability factor which
mainly determines the ease of complexation. Thus ,
the acceptor molecule first polarizes the donor
molecule during the formation of a charge transfer
complex.
The stability of charge transfer complex is also
influenced by the polarity of the medium (Table 5).
The plots of logK versus dielectric strength of the
102
INDIAN J PURE & APPL PHYS, VOL 47, FEBRUARY 2009
solvents are shown in Fig. 6. As the dielectric
constant of the medium increases, the stability
constant of the charge transfer complex also
increases.
4 Conclusion
Iodine monochloride forms thermodynamically
stable charge transfer complexes with ethers. The
formation constants correlate with the molecular
properties of donor molecules and the correlation is
better with polarizability. The stability of such
complexes is also influenced by the dielectric constant
of the medium.
Acknowledgement
Two of the authors (S. J. Askar Ali and P. A. Abdul
Mahaboob) are thankful to the University Grants
Commission (UGC), New Delhi for the award of
teacher fellowship
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