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Electronic spectral studies of solvent effects on the charge-transfer complex of ethylene thiourea-iodine system
SpecfrochlmicaActa, Vol 34A.pp. 449 m 451 0 Pergamon Press Ltd. 1978. Printed in Great
Bntain
Electronic spectral studies of solvent effects on the charge-transfer complex of ethylene thiourea-iodine system P. BALASUBRAMANIAN, K. RAMARAJ and V. P. SENTIIILNATHAN* of Chemistry, P.S.G. Arts College Coimbatore 641014, Tamil Nadu, India
Department
(Received
forpublicorion 23 September 1977)
Abstract-The study of the ethylene thiourea-iodine charge-transfer complex in various solvents like CH2C12, CHC13, CHC13 : Ccl., solvent mixtures and 1,2-dichloroethane reveals a good correlation for the equilibrium constants and other thermodynamic chlorinated methane solvents.
quantities
INTRODUCTION BHOUMIK et al. [l] studied the effect of solvents (cyclohexane, CC14, CHCls and CHZCf2) on the charge-transfer complex formed by tetracyano ethylene with benzene and mesitylene and reported that the equilibrium constants and all thermodynamic quantities were found to decrease with the increasing polarity of the solvent. This is explained on the basis of the competitive nature of the polar solvent for contact charge-transfer complex formation with the acceptor. This polar solvent-acceptor complex does not have a separate absorption band. Therefore, it is of interest to make a similar study for the ethylene thiourea-iodine complex in various solvents. Recently
should be addressed.
Table 1. Charge-transfer
Solvents
1cont. X lo5
spectral
in various
RESULTS AND DISCUSSION
Since both donor and acceptor concentrations are very small in magnitude and comparable, the following equation of ABU-ETIIAH et al. [4] has been used for the equilibrium constant determinations: CSC$ p=
All the solvents were purified employing standard methods [2]. Ethylene thiourea was prepared by refluxing a mixture of calculated quantities of ethylene diamine and carbondisulphide employing KOH solution as the medium. It is recrystallised from a 1: 1 methanol and CCIL mixture (m.p. 196°C).
coefficients
A Beckman DK-2A model UV-Vis. spectrophotometer has been used for all the spectral measurements. For quantitative studies the intensity has been checked by standard methods [3] using KZCr04 solution. At each temperature six measurements were made with different donor concentrations keeping acceptor concentration constant. To maintain the conditions the same, all the measurements in a particular solvent have been made on the same day. The equilibrium constants and other values for the ethylene thiourea-iodine system are presented in Table 1.
EXPERIMENTAL
* To whom all the correspondence
with the extinction
A
-+-.
1
C$ + C$
KAD&AD
&AD
A plot of (C$Cg)/A against Ci + C$ has been made for various concentrations of the donor. From the slope and intercept the extinction coefficient and equilibrium constants have been determined. Using van? Hoff equation, a plot of In K against l/T was made to determine the change in enthalpy of the system
data of ethylene
thiourea-iodine
in various
solvents
Range of M
CD
X 10'M
1. CHsCls
4.74
1.383-4.610
2. CHCfs
4.73
3.895-10.127
3. 50% CHCIS + 50% ccl,
4.23
2.349-6.264
4. 1,2-dichloroethane
4.94
1.110-3.700
Temp. “C
KAD
&AD
28.0 30.0 32.0 34.0 34.4 36.2 38.0 40.0 26.0 28.0 30.0 32.0 32.6 34.5 36.4 38.5
22,190 17,190 15,300 13,160 10,400 8,814 8,392 7,098 14,290 12,340 9,699 8,778 6,622 6,173 5,937 5,454
49,170 51,760 51,480 51,710 33,160 34,380 33,100 33,550 39,990 39,990 41,650 41,430 100,000 100,000 97,360 95,980
449
Lx
(nm)
1,s -1 vcr s x lo-’
297
3.178
299
3.165
300
3.162
296
3.184
450
P.
et ul.
BALASUBRAMANIAN
Table 2. Thermodynamic quantities Dielectric constant, D
Solvents 1. CH2C12
2. CHCl, 3. 50% tiHCl3 + 50% cc14 4. 1,2-dichloroethane
-AH”
-AG”
kcal/mol
cal/mol
- AS” in cu.
c*
9.080 4.806
18.00 13.89
6495 5689
38.30 26.69
51,030 33,548
3.491
15.35
5539
31.78
40,765
10.360
6.59
5378
3.97
97,780
* E is the averaged extinction coefficient.
in a particular solvent. The change in standard free energies was determined from which the change in entropy has been determined by assuming the variation of enthalpy change over the temperature ranges to be zero. All the thermodynamic quantities and the dielectric constants are given in Table 2. The dielectric constant of the solvent mixture was determined employing the relationship D = D, + W(D, - D,) where D, D1 and D, are the dielectric constants of the solvent mixture, the more polar and less polar solvent respectively and W is the weight fraction of the more polar solvent, by assuming ideal behaviour. In almost all the solvents there is a good correlation for dielectric constants with the charge-transfer transition energies. A plot of dielectric constant against vhv s-r has been made and is found to be linear. It is shown in Fig. 1. This trend has been found to be in line with the proposition of RAO et al. [5] that the shift of chargetransfer energy with the polarity of the solvent can be -compared with the blue shift of the n - R* electronic transitions in polar solvents. Therefore, it is quite likely that in the ground state of the complex, the bonding molecular orbital is more stabilised in polar solvents. Hence one may expect an enhancement of the extinction coefficient values in the more polar solvents
[6]. It is true with the values in dichloromethane, chloroform and 1: 1 chloroform-carbontetrachloride solvent mixture. However, the values in 1,2-dichloroethane deviate to a greater extent and this may be due to the steric factors operating in this solvent. It is observed from the present investigation that the chlorinated methane solvents CH2C12, CHC13 and the mixture of CHC13 and CC& form one category and 1,2-dichloroethane forms another category at least with respect to their effect on thermodynamic quantities. A plot of enthalpy change against entropy change for the ethylene thiourea-iodine system in these chlorinated methane solvents is given in Fig. 2. The linearity of the plot shows that there is a regular increase in the complex strength with entropy change of the system. The AH” values obtained in chlorinated methane solvents, i.e. - 13 to - 18 kcal/mol is even greater than that of the aliphatic amine-iodine system [7]. Even though a perfect correlation is not observed for the averaged extinction coefficient values with the dielectric constants of all the solvents, a satisfactory one has been obtained for the extinction coefficient values with the enthalpy change in various chlorinated methane solvents. A plot of H against -AH” is given in Fig. 3. The linearity is found to be very good. A similar plot for the averaged extinction coefficients
24-
t
“I ?
20-/ 16
0
12 1’
I
31X)
3160
6
3.180
36
48
-AS’_
x10-L
Fig. 1. A plot of dielectric constant vs. frequency”‘.
, 26
Fig. 2.
A
plot of -AH” us. -AS” for ethylene system in various solvents.
thiourea-12
Electronic spectral studies of solvent effects on the charge-transfer complex
61
15
17
451
t
19
-AH",
-AS’_
Fig. 3. A plot of E us. -AH for ethylene thiourea-I2 system in various solvents.
Fig. 4. A plot of B US. -AS for ethylene thiourea-Z2 system in various solvents.
with -AS” in the same solvents is also given (Fig. 4). Therefore, it may be concluded that the chlorinated methane solvents form a separate category with respect to their effect on the thermodynamic quantities of the ethylene thiourea-iodine system. Work ii being carried out for the same complex in various substituted ethane solvents to explore the steric factors and other allied effects.
SINGH of Indian Institute of Technology, Madras for his valuable suggestions.
REFERENCES
[l] B. B. BHOUMIK,Spectrochim. Actn A29,935 (1973). [2] A. WEISSBERGER, Techniques of Organic Chemistry, Vol. VIL Interscience. New York (1955). [3] G. HAUPT,J. Opt SOC. Am. 42,44i (1952). [4] R. ABU-E~AH and A. EL-KOBRASHY,J. Phys. Chem. 76,2405 (1972). [5] K. R. BHASKAR,R. K. Gos~w~ and C. N. R. RAO, Acknowledgements-The authors are grateful to the PrinciTrans. Faraday. Sot. 62,29 (1966). pal, D. K. P. VARADHARAJAN and the management of PSG [6] C. SANDORFY,Electronic Spectra and Quantum ChemArts College, Coimbatore for providing the necessary faciliistry, p. 90, Prentice Hall (1964). ties. One of us (V.P.S.) is thankful to the UGC India for a [7] H. YADA, T. TANAKAand S. NAGAKURA,Technical research grant. Thanks are also due to Mr NARAYANAN Reports of the Institute of solid state physics, Tokyo for typing the manuscript. The authors thank DR SURJIT University. Series A, No. 7 (1960).
s*
34.4-F
Bntain
Electronic spectral studies of solvent effects on the charge-transfer complex of ethylene thiourea-iodine system P. BALASUBRAMANIAN, K. RAMARAJ and V. P. SENTIIILNATHAN* of Chemistry, P.S.G. Arts College Coimbatore 641014, Tamil Nadu, India
Department
(Received
forpublicorion 23 September 1977)
Abstract-The study of the ethylene thiourea-iodine charge-transfer complex in various solvents like CH2C12, CHC13, CHC13 : Ccl., solvent mixtures and 1,2-dichloroethane reveals a good correlation for the equilibrium constants and other thermodynamic chlorinated methane solvents.
quantities
INTRODUCTION BHOUMIK et al. [l] studied the effect of solvents (cyclohexane, CC14, CHCls and CHZCf2) on the charge-transfer complex formed by tetracyano ethylene with benzene and mesitylene and reported that the equilibrium constants and all thermodynamic quantities were found to decrease with the increasing polarity of the solvent. This is explained on the basis of the competitive nature of the polar solvent for contact charge-transfer complex formation with the acceptor. This polar solvent-acceptor complex does not have a separate absorption band. Therefore, it is of interest to make a similar study for the ethylene thiourea-iodine complex in various solvents. Recently
should be addressed.
Table 1. Charge-transfer
Solvents
1cont. X lo5
spectral
in various
RESULTS AND DISCUSSION
Since both donor and acceptor concentrations are very small in magnitude and comparable, the following equation of ABU-ETIIAH et al. [4] has been used for the equilibrium constant determinations: CSC$ p=
All the solvents were purified employing standard methods [2]. Ethylene thiourea was prepared by refluxing a mixture of calculated quantities of ethylene diamine and carbondisulphide employing KOH solution as the medium. It is recrystallised from a 1: 1 methanol and CCIL mixture (m.p. 196°C).
coefficients
A Beckman DK-2A model UV-Vis. spectrophotometer has been used for all the spectral measurements. For quantitative studies the intensity has been checked by standard methods [3] using KZCr04 solution. At each temperature six measurements were made with different donor concentrations keeping acceptor concentration constant. To maintain the conditions the same, all the measurements in a particular solvent have been made on the same day. The equilibrium constants and other values for the ethylene thiourea-iodine system are presented in Table 1.
EXPERIMENTAL
* To whom all the correspondence
with the extinction
A
-+-.
1
C$ + C$
KAD&AD
&AD
A plot of (C$Cg)/A against Ci + C$ has been made for various concentrations of the donor. From the slope and intercept the extinction coefficient and equilibrium constants have been determined. Using van? Hoff equation, a plot of In K against l/T was made to determine the change in enthalpy of the system
data of ethylene
thiourea-iodine
in various
solvents
Range of M
CD
X 10'M
1. CHsCls
4.74
1.383-4.610
2. CHCfs
4.73
3.895-10.127
3. 50% CHCIS + 50% ccl,
4.23
2.349-6.264
4. 1,2-dichloroethane
4.94
1.110-3.700
Temp. “C
KAD
&AD
28.0 30.0 32.0 34.0 34.4 36.2 38.0 40.0 26.0 28.0 30.0 32.0 32.6 34.5 36.4 38.5
22,190 17,190 15,300 13,160 10,400 8,814 8,392 7,098 14,290 12,340 9,699 8,778 6,622 6,173 5,937 5,454
49,170 51,760 51,480 51,710 33,160 34,380 33,100 33,550 39,990 39,990 41,650 41,430 100,000 100,000 97,360 95,980
449
Lx
(nm)
1,s -1 vcr s x lo-’
297
3.178
299
3.165
300
3.162
296
3.184
450
P.
et ul.
BALASUBRAMANIAN
Table 2. Thermodynamic quantities Dielectric constant, D
Solvents 1. CH2C12
2. CHCl, 3. 50% tiHCl3 + 50% cc14 4. 1,2-dichloroethane
-AH”
-AG”
kcal/mol
cal/mol
- AS” in cu.
c*
9.080 4.806
18.00 13.89
6495 5689
38.30 26.69
51,030 33,548
3.491
15.35
5539
31.78
40,765
10.360
6.59
5378
3.97
97,780
* E is the averaged extinction coefficient.
in a particular solvent. The change in standard free energies was determined from which the change in entropy has been determined by assuming the variation of enthalpy change over the temperature ranges to be zero. All the thermodynamic quantities and the dielectric constants are given in Table 2. The dielectric constant of the solvent mixture was determined employing the relationship D = D, + W(D, - D,) where D, D1 and D, are the dielectric constants of the solvent mixture, the more polar and less polar solvent respectively and W is the weight fraction of the more polar solvent, by assuming ideal behaviour. In almost all the solvents there is a good correlation for dielectric constants with the charge-transfer transition energies. A plot of dielectric constant against vhv s-r has been made and is found to be linear. It is shown in Fig. 1. This trend has been found to be in line with the proposition of RAO et al. [5] that the shift of chargetransfer energy with the polarity of the solvent can be -compared with the blue shift of the n - R* electronic transitions in polar solvents. Therefore, it is quite likely that in the ground state of the complex, the bonding molecular orbital is more stabilised in polar solvents. Hence one may expect an enhancement of the extinction coefficient values in the more polar solvents
[6]. It is true with the values in dichloromethane, chloroform and 1: 1 chloroform-carbontetrachloride solvent mixture. However, the values in 1,2-dichloroethane deviate to a greater extent and this may be due to the steric factors operating in this solvent. It is observed from the present investigation that the chlorinated methane solvents CH2C12, CHC13 and the mixture of CHC13 and CC& form one category and 1,2-dichloroethane forms another category at least with respect to their effect on thermodynamic quantities. A plot of enthalpy change against entropy change for the ethylene thiourea-iodine system in these chlorinated methane solvents is given in Fig. 2. The linearity of the plot shows that there is a regular increase in the complex strength with entropy change of the system. The AH” values obtained in chlorinated methane solvents, i.e. - 13 to - 18 kcal/mol is even greater than that of the aliphatic amine-iodine system [7]. Even though a perfect correlation is not observed for the averaged extinction coefficient values with the dielectric constants of all the solvents, a satisfactory one has been obtained for the extinction coefficient values with the enthalpy change in various chlorinated methane solvents. A plot of H against -AH” is given in Fig. 3. The linearity is found to be very good. A similar plot for the averaged extinction coefficients
24-
t
“I ?
20-/ 16
0
12 1’
I
31X)
3160
6
3.180
36
48
-AS’_
x10-L
Fig. 1. A plot of dielectric constant vs. frequency”‘.
, 26
Fig. 2.
A
plot of -AH” us. -AS” for ethylene system in various solvents.
thiourea-12
Electronic spectral studies of solvent effects on the charge-transfer complex
61
15
17
451
t
19
-AH",
-AS’_
Fig. 3. A plot of E us. -AH for ethylene thiourea-I2 system in various solvents.
Fig. 4. A plot of B US. -AS for ethylene thiourea-Z2 system in various solvents.
with -AS” in the same solvents is also given (Fig. 4). Therefore, it may be concluded that the chlorinated methane solvents form a separate category with respect to their effect on the thermodynamic quantities of the ethylene thiourea-iodine system. Work ii being carried out for the same complex in various substituted ethane solvents to explore the steric factors and other allied effects.
SINGH of Indian Institute of Technology, Madras for his valuable suggestions.
REFERENCES
[l] B. B. BHOUMIK,Spectrochim. Actn A29,935 (1973). [2] A. WEISSBERGER, Techniques of Organic Chemistry, Vol. VIL Interscience. New York (1955). [3] G. HAUPT,J. Opt SOC. Am. 42,44i (1952). [4] R. ABU-E~AH and A. EL-KOBRASHY,J. Phys. Chem. 76,2405 (1972). [5] K. R. BHASKAR,R. K. Gos~w~ and C. N. R. RAO, Acknowledgements-The authors are grateful to the PrinciTrans. Faraday. Sot. 62,29 (1966). pal, D. K. P. VARADHARAJAN and the management of PSG [6] C. SANDORFY,Electronic Spectra and Quantum ChemArts College, Coimbatore for providing the necessary faciliistry, p. 90, Prentice Hall (1964). ties. One of us (V.P.S.) is thankful to the UGC India for a [7] H. YADA, T. TANAKAand S. NAGAKURA,Technical research grant. Thanks are also due to Mr NARAYANAN Reports of the Institute of solid state physics, Tokyo for typing the manuscript. The authors thank DR SURJIT University. Series A, No. 7 (1960).
s*
34.4-F