CHARGE-TRANSFER STUDIES.THIOAMIDE-IODINE SYSTEM
2405
Intermolecular Charge-Transfer Studies. Thioamide-Iodine System by R. Abu-Eittah* and A. El-Kourashy Chemistry Department, Faculty of Science, Cairo UnCersity, Giza, Egypt, U.A . R.
(Received October 12, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The intermolecular charge-transfer spectra of 1,3-diphenylthiourea and ethylenethiourea (as donors) and iodine (as an acceptor) were studied in the visible-ultraviolet region. The numerical values of the equilibrium constants of thioamide-iodine complexation were computed at different temperatures. Some thermodynamic properties of the complexes were investigated. Ionization potentials of the donors were calculated from the energies of the charge-transfer transitions.
Introduction Benzene and its derivatives, when mixed with some sorts of Lewis acids, form a large group of relatively unstable molecular complexes. These complexes may The most extensively either be n or n studied group of these complexes is probably the benzene (and alky1benzene)-iodine c o m p l e x e ~ . ~ - ~ The donor properties of sulfur-containing organic compounds are not yet extensively studied. Drago, et al.,9 investigated the donor properties of some sulfoxides, sulfones, and sulfites. The effect of ring size on the donor properties of cyclic ethers and sulfides has been investigated. lo The charge-transfer complexes of some five-membered heteroaromatics with iodine, chloranil, tetracyanoethylene, and maleic anhydride have been studied.ll-l4 Drago, et al.,16 studied the charge-transfer interaction of tetramethylthiourea with iodine. Complexes of some derivatives of thiourea with transition metals have becn investigated. The donor-acceptor interactions of alkylthioureas, thiocarbanilides, and thioacetamides with halogens have been reported. 17a-c In this work an attempt is made to study the complexes of some thioamides with halogens in view of the limited information available on these systems in the literature. The donating properties of sulfur are expected to be pronounced since its ionization potential is relatively small. The charge-transfer interaction of lJ3-diphenylthiourea and ethylenethiourea with iodine was studied by electronic spectroscopy. Equilibrium constants, standard enthalpies, and entropies of formation are computed. Finally an estimate of the ionization potentials of the donors was obtained from the charge-transfer studics. The donor site in the studied molecules was identified.
Experimental Section A . Solvents. Chloroform and carbon tetrachloride are analar grade rehgents and were used without further purification. B. Compounds. 1,3-Diphenylthiourea was prepared by the conventional methods found in 1iterat~re.l~
Ethylenethiourea was provided by Aldrich Chemical Co. Samples were purified by repetitive crystallization. C. Apparatus. Measurement of absorption spectra were carried out on a Beckman DK-1 spectrophotometer using 1.0-cm fused silica cells. Solutions were thermostated in a Metrimbex Neo-thermostat.
Results and Discussion A . 1 ,S-Diphenylthiourea-Iodine System. 1. Eguilibrium Studies. When 1,3-diphenylthiourea (DPT) (A) solution is mixed with that of iodine (B), using carbon tetrachloride as a solvent, the following equilibrium exists assuming that only one molecular complex (I : 1)is formed
A+B=AB (1) I. Omura, H. Baba, K. Higasi, and Y. Kanaoka, Bull. Chem. SOC.Jap., 30, b33 (1957). (2) K. Higasi, I. Omura, and H. Baba, J . Chem. Phys., 24, 263 (1956). (3) M. A. El-Sayed, M. Kasha, and Y . Tanaka, ibid., 34, 334 (1961). (4) R. S. Mulliken, J . Amer. Chem. SOC.,74, 811 (1952). (5) H. A. Benesi and J. H. Hildebrand, ibid., 71, 2703 (1949). (6) L. J. Andrews, Chem. Res., 51, 713 (1954). (7) N. 6. Ham, H . R. Platt, and H. McConnell, J . Chem. Phys., 19, 1301 (1951). (8) R. M. Keefer and L. J. Andrews, J . Amer. Chem. SOC.,77, 2164 (1955). (9) R. S. Drago, B. Wayland, and R. L. Carlson, ibid., 86, 388 (1964). (10) M. Tamres and S. Searles, Jr., J . Phys. Chem., 66, 1099 (1962). (11) R. P. Lang, J . Amer. Chem. SOC.,84, 4438 (1962). (12) R. Zahradnik and C. Parkanyi, Coll. Czech. Chem. Commun., 30, 195 (1965). (13) A. R. Cooper, C . W. Crowne, and P. G. Farrell, Trans. Faraday SOC.,62, 18 (1966). (14) 2. Yoshida and T . Kobayashi, Tetrahedron, 26, 267 (1970). (15) R. J. Niedzieiski, R. S. Drago, and R. L. Middaugh, J . Amer. Chem. SOC.,86, 1694 (1964). (16) T. J. Lane, A. Yamaguchi, J. V. Quagliano, J. A. Ryan, and S. Mizushima, ibid., 81, 3824 (1959). (17) (a) K. R. Bhaskar, R. K. Gosavi, and C. N. R. Rao, Trans. Faraday Soc., 62, 29 (1966); (b) A. F. Grand and M.Tamres, J . Phys. Chem., 74, 208 (1970); (e) A. F. Grand and M. Tamres, Inorg. Chem., 8, 2495 (1969). (18) “Practical Organic Chemistry,” A. I. Vogel, 3rd ed, Longmans and Green, London, 1967.
The Journal of Physical Chemistry, Vol. 76, No. 17, 1972
R. ABU-EITTAHAND A. EL-KOURASHY
2406
where C A B is the concentration of the complex DPT.12, C A O is the original concentration of DPT, and CA is the concentration of noncomplexed DPT; similar designations are applied to iodine (component B). For small concentrations of A and B CAOCBO
CAB
-
1
+
KAB
+
(CAO
~ B O )
- KABCAOCBO (2)
If the absorbance D, a t a specific wavelength (410 nm in case of DPT.12 complex), is only due to the charge-transfer complex AB, then
D
=
~ABCAB
(3)
Substituting by (3) in (2), one gets CAOCBO
D
-
CA' + + KABEAB
If C A o and reduced to
1
CBO
- KABCAOCBO €AB
€AB
CBo
are small and
CBO -C A=O =
D
>>
CAO
CB', eq 4 is
+KABBAB 1
(4)
CA €AB
(5)
Equation 5 is the form of Benesi-Hildebrand6 relation. If C A O and C B o are equal (a usual procedure followed in the charge-transfer study) or are of the same order of magnitude and both are small, then the last term in eq 4 can be neglected. Hence, CB") will not a plot C A O C B O I D against ( C A O be completely linear. According to Scott, the best straight line is drawn through the experimental points. From the slope and intercept €AB and KABare computed. Figure 1 shows the absorption spectra of solutions, in carbon tetrachloride, of pure iodine (5.500 X M ) , pure D P T (4.665 X 10-4 M ) and mixtures of both. Concentration of iodine is constant in all mixtures but that of D P T varied between 4.665 X lod4 and 1.166 X IM. Iodine solution in carbon tetrachloride has an absorption maximum at 512 nm and a very low absorbance in the 440-350-nm region. Absorption of D P T is also negligibly small in this spectral region. For mixed solutions of iodine-DPT, there is a definite at 410 nm. The and discrete absorption band with A,, pink color of iodine solution changes to pale orange on addition of the colorless solution of DPT. Figure 1 indicates clearly the following experimental results: (i) a blue shift of the iodine 512-nm maximum which increases with the increase of the D P T concentration; (ii) a decrease in the absorbance of the 512 nm of iodine, which also increases with the increase of the D P T concentration; (iii) a new absorption band appears at -410 nm; intensity of this band increases with the concentration of DPT; (iv) the presence of an isosbestic point at 482 nm is quite clear. These
+
The Journal of Physical Chemistry, Vol. 76, No. 17, 197.9
Figure 1. Absorption spectra of pure iodine (A) (5.500 X M ) , pure diphenylthiourea (B) (4.665 X lom4M) and their mixed solutions a t 29". The concentrations of DPT are (1) 4.665 x 10-4 M , (2) 4.082 x 10-4 M,(3) 3.499 x 10-4 M , (4) 2.915 x 10-4 M , (5) 2.332 x 10-4 M,(6) 1.749 X M, (7) 1.166 X M.
results clearly indicate the formation of a chargetransfer complex on mixing iodine and D P T solution, which has an absorption band of its own with Amax at -410 nm. The blue shift of the iodine visible band, on mixing with an electron donor, has been interpreted by Mullikenz0to be due to repulsive interaction between the two components of the charge-transfer complex. In our case, the charge-transfer attraction originates from interaction between the uu MO of iodine and, most probably, the nonbonding orbital of sulfur atom of DPT. As a result, the Q, MO of iodine will be raised while the energy of the nonbonding orbital of the donor will be lowered. This leads to the observed blue shift of the iodine visible band. To begin with, we tried to apply the Benesi-Hildebrand relation to evaluate the equilibrium constant for However, such a procedure was not applicable. When the concentration of the donor was increased much more than that of iodine, the pink color of iodine turned to pale yellow. This indicates a very strong interaction between the two partners of the chargetransfer complex. This led us to use small concentrations of the donor and, as shown, this procedure was successful. (19) R. L.Scott, Red. Trau. Chim. Pays-Bas, 75, 787 (1956). (20) R.S. Mulliken, ibid., 75, 845 (195G).
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CHARGE-TRANSFER STUDIES. THIOAMIDE-IODINE SYSTEM
:I1 '5
2
I
'
4
6
I
,
I
b
10
1
i4
C'A + c.,
Figure 3. Variation of absorbance of the diphenylthiourea-iodine charge-transfer complex with temperature ([Is] = 5.500X 10-4 M , [DPT] = 3.499 x 10-4 M ) .
+ CB' at
Figure 2. Relation between CA'CB'/D and CA' 29' : -, diphenylthiourea-iodine system; ----, ethylenethiourea-iodine system.
Equation 4 was used to evaluate the equilibrium constant and the molar extinction coefficient of the chargetransfer complex. Plots of CA'CB'/D against CA' CBO were drawn at different wavelengths in the chargetransfer absorption region, one of which is shown in Figure 2. An average value of 622 is computed for the stability constant of "DPT. I," charge-transfer complex at 29' (Table I).
+
Table I : Equilibrium Constants and Molar Extinction Coefficients of DPT-Iodine Charge-Transfer Complex WaveTemp,
OC
25
29
32
length, nm
KAB
e
9,700 10,262 10,140
Av
888 898 875 885
10,501 10,807 10,770
Av
622 620 624 622
11,111 11,904 11,764
Av
494 484 467 481
416 410 404 416 410 404 416 410 404
Wave -Length, nm
have indicated Auterly, et ~ 1 . and ~ 2 ~Buckles, et that the Is- ion has an absorption maximum a t 365 and 296 nm. The slight variation of KABwith wavelength is attributed to the presence of a small concentration of 13-. 2 . Thermodynamic Properties. To get an estimate of the enthalpy and entropy of formation of the DPT.12 charge-transfer complex, the equilibrium constants were determined at different temperatures. Absorption spectra of mixtures of "iodine-DPT" solutions were studied at 25 and 32' (in addition to those at 29'). Features of the spectra were very much similar to those presented in Figure 1. Equilibrium constants were also calculated and results are presented in Table I. In Figure 3 the spectra (in charge-transfer region) of one of the studied DPT-iodine solutions, a t different temperatures, are presented. The relation between R In KABvs. 1 / T is plotted, and a straight line was drawn to fit the points as well as possible (Figure 4). The heats of formation (AH') and the entropy of formation of the complex (As') are obtained from the slope and intercept with abscissa, respectively, and are presented in Table 11. (21) A. D. Auterly and R. E. Gonnick, J . Amer. Chem. Soe., 73, 1842 (1951). (22) R. E. Buckles, J. P. Yuk, and I. Popov, ibid., 74, 4379 (1952). The Journal of Physical Chemistry, Vol. 76, N o . 17, 1073
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R. ABU-EIWAHAND A. EL-KOURASHY EOT= ID - 5.2
i’
T
Energy of the charge-transfer transition for DPTiodine complex is 3.02 eV. Using this value one gets 7.59 eV as the ionization potential for DPT. The first ionization potential of tetramethylthiourea has been found to be 8.12 eV by electron impact method.” This value is quite comparable with the value obtained for D P T by charge-transfer studies. B. Ethylenethiourea-Iodine System. To get a fairly complete idea about the nature of the charge-transfer complex between “thioamides” as donors and “iodine” as the acceptor, we chose ethylenethiorea (ETU) as the second donor in this study. The absorption spectra of free iodine (7.930 X lo-‘ M ) , free ETU (3.102 X 10-4 M ) , and mixtures of both were investigated. All spectra were scanned using chloroform as the solvent and blank since the solubility of ETU in carbon tetrachloride is very low. Features of the spectra are very much similar to those of the DPT-iodine solutions. That is, one isosbestic point, a characteristic charge-transfer band whose intensity is proportional a
Figure 4. The relation between R In KABand 1/T: diphenylthiourea-iodine system; --, ethylenethiourea-iodine system.
-,
*lo 13
Donor
at 2Q0
-AHo
- ASo
Diphenylthiourea Ethylenethiourea Di-tert-butylthiourea“ Tetramethylt hiourea”
622 558
15.2 f 0.4 8.1rt0.3 18.6
37.5i 0.9 14.4i0.5 39.8
9.5
12.3
a
-
la-
Table 11: Comparison of the Physical Constants of Some Thio Compounds-Iodine Charge-Transfer Complexes KAB
+ 1.5/(I~- 5.2)
Reference 17.
11
10
-
*w 8 -
1-
6-
The values obtained for K A Bare too large for a A charge-transfer complex. The equilibrium constants of some ?r complexes of benzene, toluene, and hexamethylbenzene vary between 0.13 and 1.35.2a,24 Hence we conclude that the obtained charge-transfer complex is an “n” one. The donor site in D P T is expected to be the sulfur atom. This has been confirmed by ir studies on thiourea-iodine system.” 3. Position of the Charge-Transfer Absorption Band of DPT-Iodine System and Ionization Potential of the Donor. Figure 5 shows the charge-transfer absorption band of DPT-iodine complex at 29’. The Breigleb26 ‘equation is, usually, used to get an estimate of the ionization potential of the donor, when the acceptor is iodine, as The Journal of Physical Ch.emistry, Vol. 76,No. 17, 1072
S-
c. aw
I
I
I
a10
aw
410
8
~ao
1
4so
4
I
(23) L.J. Andrews and R. M. Keefer, J . AmeT. Chem. Soc., 74, 4500 (1952). (24) N.W. Blake, H. Winston, and J. A. Patterson, ibid., 73, 4437 (1951). (25) G. Breigleb, “Electrom DonatorAcceptor Komplexe,” Springer-Verlag, West Berlin, 1961.
MECHANICAL PROPERTIES OF A NEMATIC LIQUIDCRYSTAL to the donor concentrations and the blue shift of iodine visible maximum on mixing with the donor were the experimental results for the ETU-iodine system. Proceeding as before, values of K A B ,AH", and AS" were computed. The average K A Bis found to be 558 a t 29" (less than that of DPT-iodine complex). Table I1 compares the values of AH" and AS" for complexation of some thioamides and iodine. Enthalpy as well as entropy of formation of ETU-iodine complex
2409
are much less than those of DPT-iodine complex. This goes along with the low enthalpy and entropy of formation of tetramethylthiourea-iodine complex as compared with those of di-tert-butylthiourea-iodine ~omplex.'~In fact we expect ETU to be a weaker donor than tetramethylthiourea. The Breigleb equation was used to get an estimate of the ionization potential of ETU. A value of 7.71 eV was obtained for its first ionization potential.
An Ultrasonic Shear Wave Study of the Mechanical Properties
of a Nematic Liquid Crystal by Y. S. Lee, Sherman L. Golub, and Glenn H. Brown* Liquid Crystal Institute, Department of Chemistry, and Department of Physics, Kent State University, Kent, Ohio 44248 (Received November 16, 1971) Publication costs assisted by the Air Force Ofice of Scientific Research
The dynamic shear properties of p-rnethoxybenzylidene-p-n-butylaniline(MBBA) in its nematic and isotropic states are studied in the temperature range 25 to 65" using a shear reflectance technique. Shear wave frequencies from 2.75 to 10 MHz are used, and three orientations of the preferred direction of the molecular axis with respect to the direction of polarization of the shear wave are considered. The anisotropy ratio for the dynamic viscosity is about 2; the ratio for the shear modulus is highly frequency dependent, being as large as 3.5. Approximations are made which allow the Leslie-Ericksen coefficients to be calculated.
Introduction Liquid crystals differ from isotropic liquids and crystalline solids in that they have special structural characteristics. The intermolecular relationship in liquid crystals is not yet completely known. To understand the structural characteristics of the liquid crystal on a molecular scale, the application of wave techniques with ultrasonic vibrations seems to be most appropriate. It is the purpose of this work to use the ultrasonic shear wave technique to study the mechanical properties of a nematogenic compound p-methoxybenzylidene-p-n-butylaniline (MBBA) in both its nematic and isotropic states. The compound as used had a nematic-isotropic transition temperature of 44.0 f 0.5". We report the calculated values of the anisotropic dynamic viscosity and shear storage modulus of the compound. There have been several previous studies of the viscosity of nematic liquid crystals. 1--3 These involved macroscopic flow techniques. Because flow has an orienting effect on the molecules of an anisotropic fluid, these techniques yield viscosity for definite molecular
orientations only with extreme difficulty. The classic measurements of Miesowicz on p-azoxyanisole were, of course, for three definite orientations. Ultrasonic shear techniques overcome this difficulty. The first application of this latter technique to nematic liquid crystals has been recently reported by Candau and Mar tinoty
.
Theoretical Background Earlier attempts at the application of the ultrasonic technique to liquid crystal studies involved mostly the use of longitudinal waves. Some of the interesting o b ~ e r v a t i o n s are ~ ~ ~the anomalous variation in the velocity and absorption of ultrasound in p-azoxyanisole (1) M . Miesowicz, Nature (London), 136, 261 (1935); 158, 27 (1946). (2) R. S. Porter, J. F. Johnson, and E. M. Barrall, J . Chem. Phys., 45, 1452 (1966). (3) J. Fisher and A. G. Frederickson, Mol. Cryst., Liquid Cryst., 8 , 267 (1969). (4) P. Martinoty and S. Candau, ibid., 14, 243 (1971). (5) W.H. Hoyer and A. W. Nolle, J. Chem. Phys., 24, 803 (1956). (6) I. Gabrielli and L. Verdini, Nuovo Cimento, 2, 426 (1955). The Journal of Physical Chemiatry, Vol. 76,N o . 17, lO7d
