Determination of stability constants of charge transfer complexes of iodine monochloride and certain ethers in solution at 303 K by ultrasonic method

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/f 2 ), internal pressure (π i ) 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 binary 1,2 and ternary 3,4 liquid mixtures. These studies can also be used to identify complexation and to calculate the stability constants of complexes 5,6, 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 equation 7. 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/f 2 ), free length (L f ), internal pressure (π i ), 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 equations 7-11. 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/f 2 ) 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 (π i ) is a measure of cohesive forces between the constituent molecules in liquids. In order to assess the cohesive forces in the ternary liquids investigated, π i 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. Diphenylehter M CH 2 Cl 2 CHCl 3 CCl 4 C 6 H 14 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 Anisole 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 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 99 Table 2 Density (kgm 3 ) values of ICl - ether systems in different solvents at 303 K Table 3 Viscosity (cp) values of ICl - ether systems in different solvents at 303 K Conc. Diphenylehter Conc. Diphenylehter M CH 2 Cl 2 CHCl 3 CCl 4 C 6 H 14 0.001 1261.4 1411.2 1515.0 625.2 0.002 1258.1 1410.8 1514.2 625.4 0.003 1259.2 1412.2 1513.5 626.0 0.004 1257.6 1410.6 1514.4 626.3 0.005 1256.8 1411.8 1513.2 625.8 0.006 1261.0 1412.2 1514.6 627.0 0.007 1258.9 1413.5 1514.2 626.6 0.008 1260.3 1412.5 1514.2 626.8 0.009 1258.1 1413.2 1514.9 626.8 0.010 1259.7 1413.1 1515.1 627.2 4-Chloroanisole 0.001 1261.4 1411.7 1516.7 631.2 0.002 1262.9 1412.8 1516.9 631.9 0.003 1262.6 1412.4 1517.7 632.0 0.004 1261.1 1413.2 1516.2 632.4 0.005 1262.6 1413.5 1515.9 632.5 0.006 1263.2 1412.4 1515.8 632.7 0.007 1264.1 1412.1 1516.7 632.7 0.008 1264.7 1412.6 1518.2 632.9 0.009 1263.8 1411.8 1516.0 633.7 0.010 1265.3 1412.2 1517.3 633.8 Anisole 0.001 1256.6 1412.1 1513.0 634.1 0.002 1257.9 1414.3 1513.4 634.7 0.003 1257.8 1412.2 1513.5 634.9 0.004 1256.7 1413.0 1514.0 634.8 0.005 1258.6 1412.9 1514.4 634.1 0.006 1259.1 1412.8 1513.8 635.0 0.007 1257.7 1413.0 1514.0 634.9 0.008 1258.5 1413.7 1512.5 635.7 0.009 1256.9 1414.1 1513.4 635.4 0.010 1258.4 1414.0 1512.5 635.1 1,4-Dioxane 0.001 1257.1 1411.3 1522.1 636.1 0.002 1258.2 1411.2 1522.1 635.8 0.003 1258.1 1410.6 1521.2 636.4 0.004 1258.0 1412.8 1521.7 637.0 0.005 1257.5 1411.4 1521.5 635.9 0.006 1260.6 1411.6 1522.3 635.6 0.007 1261.0 1410.9 1521.2 636.3 0.008 1259.0 1410.8 1522.2 635.9 0.009 1257.8 1410.8 1521.4 636.7 0.010 1258.2 1411.9 1522.3 636.8 M CH 2 Cl 2 CHCl 3 CCl 4 C 6 H 14 0.001 0.561 0.670 0.878 0.327 0.002 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 0.005 0.555 0.671 0.880 0.326 0.006 0.563 0.673 0.883 0.327 0.007 0.560 0.678 0.884 0.326 0.008 0.561 0.674 0.886 0.326 0.009 0.565 0.673 0.879 0.326 0.010 0.561 0.680 0.884 0.327 4-Chloroanisole 0.001 0.570 0.671 0.891 0.334 0.002 0.560 0.672 0.885 0.333 0.003 0.567 0.673 0.889 0.334 0.004 0.564 0.677 0.890 0.333 0.005 0.567 0.677 0.894 0.333 0.006 0.568 0.673 0.884 0.334 0.007 0.569 0.670 0.887 0.336 0.008 0.568 0.673 0.890 0.333 0.009 0.566 0.673 0.892 0.336 0.010 0.562 0.673 0.895 0.334 Anisole 0.001 0.561 0.664 0.873 0.329 0.002 0.562 0.672 0.869 0.331 0.003 0.561 0.675 0.874 0.331 0.004 0.558 0.670 0.872 0.332 0.005 0.559 0.667 0.877 0.332 0.006 0.558 0.672 0.881 0.330 0.007 0.543 0.672 0.876 0.331 0.008 0.556 0.673 0.875 0.332 0.009 0.554 0.667 0.869 0.332 0.010 0.557 0.672 0.872 0.333 1,4-Dioxane 0.001 0.561 0.675 0.905 0.337 0.002 0.567 0.675 0.903 0.334 0.003 0.566 0.673 0.900 0.333 0.004 0.566 0.680 0.902 0.335 0.005 0.568 0.682 0.899 0.334 0.006 0.571 0.681 0.903 0.333 0.007 0.566 0.678 0.902 0.335 0.008 0.564 0.680 0.902 0.335 0.009 0.565 0.677 0.902 0.336 0.010 0.564 0.675 0.903 0.334

100 INDIAN J PURE & APPL PHYS, VOL 47, FEBRUARY 2009 Fig. 3 Adiabatic coefficient versus concentration of iodine Fig. 4 Internal pressure vs concentration of iodine Fig. 1 (a-d) Ultrasonic velocity versus concentration of iodine Fig. 2 Adiabatic compressibility versus concentration of iodine Fig. 5 Cohesive energy versus concentration of iodine of complexation with increase in concentration. The trend in cohesive energy (CE) is similar to that of internal pressure (π i ). Plots of CE versus concentration are shown in Fig. 5. The stability constants are calculated from measured ultrasonic velocities using modified Bhat equation 7. 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 (dm 3 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 CH 2 Cl 2 CHCl 3 CCl 4 C 6 H 14 K K K K Diphenyl ether 312.7 197.8 57.9 47.6 4-Chloroanisole 72.2 53.8 48.6 40.0 Anisole 69.6 51.2 38.5 35.4 1,4-Dioxane 52.1 42.6 36.1 32.0 G G G G Table 5 Polarizability dipole moment and dielectric strength for ethers and solvents α µ ε CH 2 Cl 2 6.8 1.6 9.08 CHCl 3 8.5 1.01 4.806 CCl 4 10.5 0 2.238 C 6 H 14 11.6 0.08 1.89 Diphenylether 20.859 1.031 2.68 4-Chloroanisole 14.749 1.619 7.84 Anisole 13.071 1.084 4.3 1,4-Dioxane 10 0 2.22 Diphenyl ether -14.13-13.27-9.98-9.35 4-Chloroanisole -10.62-9.71-9.69-9.24 Anisole -10.52-9.73-9.16-8.91 1,4-Dioxane -9.71-9.00-9.01-8.56 G # G # G # G # Diphenyl ether 3.39 4.05 4.94 3.82 4-Chloroanisole 3.39 4.04 4.97 3.83 Anisole 3.37 4.02 4.93 3.81 1,4-Dioxane 3.41 4.06 4.97 3.82 τ τ τ τ Diphenyl ether 5.40 6.86 9.46 6.32 4-Chloroanisole 5.40 6.84 9.56 6.33 Anisole 5.37 6.80 9.45 6.28 1,4-Dioxane 5.45 6.89 9.57 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. 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 molecules 12-14. 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 References 1 Kannappan V & Jaya Santhi R, Indian J Pure & Appl Phys, 43 (2005) 750. 2 Kannappan V, Xavier Jesu Raja S & Jaya Santhi R, Indian J Pure & Appl Phys, 41 (2003) 690. 3 Jayakumar S, Karunanithi N, Kannappan V & Gunasekaran S, Asian Chem Lett, 3 (1999) 224. 4 Neuman M S & Blum, J Am Soc, 86 (1964) 5600. 5 Kannappan V, Jaya Santhi R & Xavier Jesu Raja S, Phys Chem Liq, 14(2) (2003) 133. 6 Kannappan V & Kothai S, J Acous Soc India, 30 (2002) 76. 7 Kannappan, V, Jaya Santhi R & Malar E J P, Phys Chem Liq, 40 (2002) 507. 8 Tabhane V A, Sangeeta Agarwal & Revetkar K G, J Acous Soc Ind, 8 (2000) 369. 9 Anwar Ali & Nain Anil Kumar, Acoustics Lett, 19 (1996) 181. 10 Bhatt S C, Harikrishnan Semwal & Vijendra Lingwal, J Acous Soc Ind, 28 (2000) 275. 11 Nikam P S & Hiray, Indian J Pure & Appl Phys, 29 (1991) 601. 12 McClellan A L, Tables of experimental dipole moments (W.H. Freeman & Company, San Francisco and London), 1963. 13 Timmerniam, J Physico-chemical constants of pure organic compounds, Elsevier, 1950. 14 Kothai S, Ph D Thesis, University of Madras, 2003.