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Molecular complex formation of thiosalicylic acid with ethanol
SpecfrocMmic~ Am, Vol. 4lA. Printed in Great Britain.
No. 9. pp. 1063-1067,
1985. 0
Molecular complex formation
of thiosalicylic
0584-8s39/tts s3.00 + 0.00 1985 Pergamon Press Ltd.
acid with ethanol
K. A. IDRISS, A. S. EL-SHAHAWY,M. S. ABU-BAKR, A. A. HARFOUSH and E. Y. HASHEM Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt (Received in reoisedform 29 March 1985) formation of a 1: 1 molecular complex of thiosalicylic acid (TSA) with ethanol was studied spectrophotometrically. Ethanol is postulated to form an intermolecular hydrogen bond with the carboxyl OH group of TSA, the intramolecular hydrogen bond of the SH group remaining intact. Results of SCF-CI molecular orbital calculations support the spectral interpretation. The formation constant (K,), free energy change (AC) and molar absorptivity of the TSA-ethanol molecular complex with cyclohexane, CC& or CHCla as solvents were determined. K, values vary with solvent in the order cyclohexane r Ccl, > CHCl,. The electronic absorption spectra of TSA in one- and two-component solvents are discussed. Abstract-The
which is higher than the energy of the ground state for the a& form (EG = - 505.168 eV). Accordingly, the a-form of TSA is the more stable molecular form. It is well established that molecules having an intramolecular hydrogen bond, such as TSA, undergo the so-called proton transfer in the excited singlet state, resulting in a significantly red shifted emission band. Such observations suggest that the absorption and emission spectra of TSA are dependent upon the solvent wherein the intramolecular hydrogen bond may be replaced by intermolecular hydrogen bonds with the solvent molecules. Ethanol is known as a good solvent capable of forming intermolecular hydrogen bonds with various polar groups of solute molecules [ll-131. In this paper, the effect of molecular complex formation between TSA and ethanol on intermolecular hydrogen bonding will be examined and characterized. The formation constant (K,) is determined from spectral measurements in mixed solvents.
INTRODUCTION Thiosalicylic acid (o-mercaptobenzoic acid) offers good opportunities for intramolecular association [l] and examination of the vsHregion indicates two bands both of which are at lower frequency than that expected for an unassociated thiol group. Examination of the carbonyl bands provides additional evidence for intramolecular association and indicates that the latter persists even in the fairly basic solvent acetonitrile. The very weak acidity of the thiol group in thiosalicylic acid may indicate stabilization by intramolecular hydrogen bonding, an effect which is apparent with salicylic acid [2]. The results of our preliminary molecular orbital calculation confirm the existence of the a-form of thiosalicylic acid as the predominant molecular species. H. S’
0 b
’ p"
S
Ii
K.
p - Form
/B-.Q
b
0
EXPERIMENTAL
XH
a - Form
In the present work, the interpretation of the electronic spectra of thiosalicylic acid (TSA) is supported by molecular orbital calculations. These calculations usually start with the well-known semiempirical HMO theory, then the self consistent field molecular orbital method (SCF-MO) using the PARISER-PARR-POPLE (PPP) approximation [3-6] in addition to that of NISHIMOTO and FORSTER 171. Then, as a final step, configuration interaction calculations [8-lo] were carried out to compute state functions, state energies and transition energies. The results of these calculations gave a qualitative fit to the observed transition for the neutral molecule in solution. The results of the SCF-CI calculations indicate that the n-electronic energy of the group state of the TSA molecule has the value - 498.561 eV for the @is form,
The thiosalicylic acid used in the present investigation was B.D.H. pure grade. E. Merck spectrograde Ccl4 and cyclohexane were purged with oxygen-free dry nitrogen immediately before use. E. Merck spectrograde CHCls was purifed according to recommended procedures and was then purged with nitrogen immediately before use. Spectrograde ethanol and diethvl ether were obtained from E. Merck. Various steps were taken to ensure their purity. 10s3 M stock solutions of TSA were prepared accurately. More dilute solutions of the reagent were obtained, as required, by accurate dilution. The absorption spectra were recorded on a Unicam S.P. 8ooO spectrophotometer using 1 cm matched stoppered quartz cells. The measurements were carried out at 20°C. Data processing calculations were done on a Casio FP-200 computer.
RRSULTSAND DISCUSStON Absorption spectra of TSA in single solvents The spectra of TSA in organic solvents of varying polarities indicate that both 1, and E of the absorp-
1063
1064
K. A.
IDRISS er al
tion bands are influenced by the nature of the solvent used (cf. Fig. 1). The high solvating power of ethanol leads to a high solubility of the compound and no association of the solute molecules can take place especially in dilute solutions. The linear variation of absorbance with solute concentration in ethanol confirms that solute-solute interaction is of minor importance in this solvent. The U.V. spectrum of TSA, in ethanol, has peaks at 210, 235, 275 and 345nm (cf. Fig. I). The lower intensity band at 210nm, compared with that of 235 nm band, is due to Z-Z* triplet transition in the benzoic moiety as confirmed by SCF-CI calculations. The 235 nm band corresponds to transitions within the benzenoid system. This band is shifted to red on increasing the polarity of the medium, behaviour which is in accordance with the n--s* nature of the electronic transitions leading to this absorption band. The large molar absorptivity, the red shift observed when solvent polarity is increased and the disappearance of the band in nonpolar solvents suggest that the band at 275 nm is due to a charge transfer (C.T.) transition. The results of SCF-CI calculations indicate that this band is unambiguously due to mixed Z-Z* transitions within the benzoic moiety. The effect of polar solvents is to perturb the symmetry of the excited molecular orbitals (ti7 and Ijlg) to be similar to that of the highest two occupied molecular orbitals (I+%~ and 1(/e) Hence, the transition from the higher two molecular orbitals to the lowest vacant molecular orbitals is allowed (cf. Table la, b). Therefore, the transition appears much more strongly with increasing polarity of the medium. After configuration interaction calculations, the state functions and state energies were computed and given in Table 3. The transition energies were calculated and compared with the experimental data to confirm the assignment of the electronic bands (cf. Table 4).
Table la. Molecular orbital eigenfunctions of the highest two occupied and the lowest two vacant molecular orbitals of TSA molecule (a-form) *5
Z: 46 47 ds k0
)(/7
+ 0.028229 - 0.301083 - 0.570237 - 0.276840 + 0.293074 + 0.567611 + 0.268455 + 0.035057 - 0.012546 + 0.158790
+ 0.003484 + 0.284204 + 0.485447 - 0.285063 - 0.222655 + 0.489076 - 0.223568 + 0.381116 - 0.048663 - 0.340465
*a
+ 0.005889 - 0.500237 + 0.499786 - 0.500154 + 0.499789 O.oooO O.WOO O.OOQO O.OCQO 0.0000
Table lb. Transition
moment components between the occupied MOs and vacant MOs in thiosalicylic acid
Transition
$:I$: ;:r:;:
M, - 0.292514 0.613850 - 0.606178 - 0.330928
MY 0.543c01 0.335218 - 0.397571 0.606951
MZ 0.00 0.00 0.00 0.00
The absorption band of 1, at 345 nm is tentatively assigned as an intramolecular charge transfer band. This transition occurs, according to our SCF-CI calculations, between the oxygen of the carboxyl OH and the ring n-electrons. The most interesting effects of changing the solvent are observed with the C.T. band. The neutral form of TSA is expected to predominate in nonpolar solvents while the anionic form may exist in polar solvents. Although for salicylic acid the anionic form is present only to a small extent in any solvent, the greater acidity of the mercapto group causes monovalent ions to become more prevalent for TSA in polar solvents. Hydrogen bonding solvents will also have an effect on the formation of the anionic species.
Wavelength Fig. 1. Electronic
0.048343 - 0.491721 + 0.014303 + 0.506455 + 0.491920 - 0.014907 - 0.506903 0.0000 O.OC00 - 0.004031 +
I;
1(/c
(nm)
absorption spectra of thiosalicylicacid in organic solvents ofvarying polarities. 2, cyclohexane; 3, ether; 4, chloroform and 5, carbon tetrachloride.
1, Ethanol;
Thiosalicyclic acid-ethanol complexes Table 2. Results of SCF calculations of xelectron densities for different atoms in thiosalicylic and salicylic acid molecules
(a) X =0 (b) X=S Atom
(a)
(b)
1
1.989070 0.987453 1.009780 0.996110 1.006630 0.996729 1.007670 0.765201 1.982940 1.259520
1.99989 0.996428 1.002350 0.996486 1.000190 0.997158 1.000210 0.765186 1.982930 1.259190
2 3 4 5 6 I 8 9 10
Table 3 State function
State energy (eV)
E: = -500.551
“I’, ;; = - 500.029 s = - 499.862 tiexs = - 0.553903 J& + 0.832581 +D, +=, = ‘@?9 Sas = 0.832581’8, + 0.553903 3@,
E: = - 499.862 E: = -499.808
1065
The carboxyl group can be strongly hydrogen bonded in both proton donor and proton acceptor solvents. Thus, hydrogen bonding solvents such as ethanol will tend to stabilize the monoanionic form of TSA more than the neutral molecules. Therefore, in nonpolar solvents, the neutral TSA species will predominate and as the polarity or the hydrogen bonding strength of the solvent increases, the equilibrium will be shifted toward the anionic form. In TSA, the main band not having an analogue in the benzoic acid spectrum is the band at 345 nm. This band has been identified as being a charge-transfer transition in the thiosalicylate anion. The SCF-CI calculations also indicate a large amount of charge-transfer character for this band. The rapid decrease in the absorbance of the C.T. band as the solvent polarity is decreased can be ascribed to the decrease of the concentration of the anionic form of TSA. Thus, the C.T. band becomes less intense as the equilibrium is shifted toward the neutral form of TSA by less polar or less hydrogen bonding solvents. Solute molecules exist in a unpolarized state in solvents of low polarity not capable of forming an intermolecular hydrogen bond. In ether, the carboxyl OH of TSA can be involved in a weak intermolecular hydrogen bond with the oxygen of the ether molecule. This type of bonding facilitates the intramolecular charge-transfer and thus a red shift of the C.T. band is observed in ether compared to CCL or CHCl,. TSA can readily interact with ethanol as a proton acceptor solvent. This interaction leaves a residual negative charge on the oxygen of the carboxyl OH. Accordingly, the intramolecular charge transfer is enhanced leading to a red shift of the C.T. band in the proton acceptor solvent.
E: = - 499.808
Table 4. Comparison of the calculated transition energies (SCF-CI) with the experimental data for the TSA molecule (aform) in cyclohexane
AEcai,
-Not
AE
rl
(eV)
hE!J
(efl’
(Z)
4.387 4.170
283 298
4.005 -
310 -
4.616 5.131
269 242 236 234 232 212
4.720 -
263 -
5.283 5.469
235 227
observed.
On the other hand, since the thiosalicylic anion is stabilized by hydrogen bonding, its pK value is less than that of benzoic acid but much greater than that of salicylic acid [14]. This is reasonable as the replacement of 0 by S atom reduces the strength of the hydrogen bond (cf. Table 2).
It is to be noted that the resonance energy in the carboxyl group must be partially overcome in the complexation to ethanol. As a result, it is possible that the intramolecular hydrogen bond in TSA is actually somewhat strengthened owing to stabilization of the above structure rather than weakened by conjugation of the carboxyl group to ethanol. The addition of two ethanol molecules, one to the carboxyl OH and one to the SH group, to one TSA molecule is more or less excluded on electrostatic grounds. Absorption spectra of TSA in mixed solvents and molecular complex formation The formation of the TSA-ethanol molecular complex was investigated by studying the spectral behaviour in mixed solvents. A set of absorption spectra was measured taking the three-component systems, viz. TSA, the proton-acceptor solvent ethanol and the
1066
K. A.
IDRISS er al.
nonpolar solvent. The concentration of ethanol was increased in steps from 0.35 to 13 M. In every measurement, a mixture of ethanol and a nonpolar solvent in which the concentration of ethanol was identical with that in the sample solution was taken as a reference. As shown in Figs 2 and 3, the spectra in mixed solvents show a regular decrease in the absorption of the band at 345 nm and its shift to shorter wavelengths with an increasing proportion of the nonpolar component. The decrease of absorbance can be ascribed to the decreased participation of the ionic form of TSA in the equilibrium mixture with decreasing solvent polarity. The increased proportion of ethanol causes a red shift of the CT. band. The red shift of ,I,, denotes an
0.6
300
325
'I 350
375
Fig. 2. Absorption spectra of thiosalicylic acid in mixed solvents (ethanol-CC&), 1 x 10-4M TSA. C,I,. = (a) 0.86 M; (b) 1.72 M; (c) 2.57 M; (d) 3.43 M; (e) 4.97 M; (f) 6.86 M; (g) 9.43 M; (h) 9.95 M; (m) 10.98 M and (n) 12.0 M.
375
300
increased solvation energy in the excited state. Accordingly, the molecules in the excited state are present in a cage of dipoles which are more oriented than in the pure low polarity solvent [ 15, 161. The red shift observed can, according to the work of PIMENTAL [17], be explained by the increase of the hydrogen bond strength between solute and solvent on addition of alcohol. The increase of H-bond strength would be higher for the excited than for the ground state. To investigate the effect of the dielectric constant of the medium on the band shift (Av’), the relation of GATI and SZALAY [ 181 was applied Av’ = (a-b)($$+b(g) in which a and 6 are constants and n is the refractive index of the medium. The plots of Av’ as a function of (D - l)/(D + 1) in all systems investigated are nonlinear relations indicating that the band shift is governed by other factors besides the dielectric constant of the medium. These factors include solute-solvent interaction through intermolecular hydrogen bonds which leads to the formation of a TSA-ethanol molecular complex. The intensity change of the C.T. band in two-component solvents is accompanied by the development of a new band at the longer wavelength side. A clear isosbestic point is observed which indicates the existence of an equilibrium between the bonded and free molecules. The oscillaror strength (f) for the bonded molecules is always greater than that for free molecules, which means an enhanced interaction between the lone pair electrons on the oxygen atom of the OH group in TSA molecule and the ring n-electrons presumably due to an intermolecular (O-H . 0) bond formation. The value of the formation constant (X,) of the TSA-ethanol molecular complex was determined from the spectral behaviour in mixed solvents using the relations previously reported [ 11, 19, 203. The mean
325
350
375
400
Wavelength (nm) Fig. 3. Absorption spectra of thiosalicylic acid in mixed solvents (ethanokyclohexane), 2 x 10e4 M TSA. C,I,. = (a) 1.72 M; (b) 2.57 M; (c) 3.43 M; (d) 5.15 M; (e) 6.68 M; (I) 8.57 M; (g) 9.43 M and (h) 11.14 M.
Tbiosalicyclic acid-ethanol complexes
value of K, in all systems investigated and the corresponding AG values are given in Table 3. For each binary solvent system, the data near the charge-transfer band maximum are consistent with the existence of a single 1: 1 (TSA-ethanol) complex. The constancy of the K, values for a given binary system at different wavelengths is a valid criterion for the existence of a 1: 1 complex in solution [Zl]. This condition is very well satisfied for the TSA-ethanol complex. For the donor-acceptor-solvent ternary systems under investigation a change of the nonpolar solvent effects drastic changes in the computed K, value (Table 5). Competitive donor-solvent complexes in CHCIJ , and possibly CC14,can block the donor orbital and thus reduce the apparent formation constant in these solvents. The absorptivity of the assumed 1: 1 complex near the absorption maxima is approximately constant (cf. Table 6). For the systems studied in the present investigation, the wavelength of maximum absorption for the TSA-ethanol complex varies with solvent thus: cyclo-
Table 5. Average values of K, (1mole-‘) and AG (kcalmol- ‘) for the TSA-etbanol complex at 20°C System Etbanol-cyclohexane EtbanolCHCI, Ethanol-XXI,
logK,
K,
AG
0.744 0.236 0.377
5.55 1.72 2.38
- 0.998 -0.316 - 0.505
Table 6. Maximum molar absorptivity and wave length of band maximum for the TSA-ethanol complex system
Ethanol-cyclohexane Ethanol-CHCl, Ethanol-CCl,
2.11 x 104 2.32 x 10“ 2.65 x 10’
348 350 353
1067
hexane < CHCls < Ccl*. Some authors [22,23] have reported similar behaviour for various complexes. The results cannot be explained in terms of average solvent properties alone, but they can be interpreted by the supposition that the formation of one isomeric complex is favoured by the structure of the solvent.
REFERENCES A. WAGNER,H. J. BECHERand K. KO~ENHAHN, Chem. Ber. 89, 1708 (1956). [2] C. H. ROCHESTER,% Chemistry oftheHydroxyl Group, Vol. I (edited by S. PATAI). Wiley, London (1971). c31 R. PARISERand R. C. PARR, Ji them. Phys. 21, 767 (1953). J. A. POPLE,Trans. Faraday Sot. 49, 1375 (1953). I:] R. PARISER,J. &em. Phys. 24,250 (1956). II61 R. G. PARR, Quantum Theory of Molecular Electronic Structure. Benjamin, New York (1964). c71 K. NISHIMOTOand L. S. FORSTER, Bull. Chem. Sot. Japan 41,2254 (1968). PI R. L. FLURRY, JR., Molecular Orbital Theories of Bonding in Organic Molecules. Dekker, New York (1968). iI91 C. SANDORFY, Electronic Spectra and Quantum Chemistry. Prentice-Hall, Englwood Cliffs (1964). Molecular Orbital Theory for Organic WI A. STREITWEISER, Chemists. Wiley, New York (1962). Cl11 M. S. EL-EWBY, T. M. SALEM,A. H. ZEWAILand R. M. &A, 1. them. Sot. B 1293 (1970). Cl21 I. M. WA, R. M. Iss~, K. A. IDRI~~end A. HAMMAM, Egypt. J. Chem. Special issue 67 (1973). Cl31 R. M. ISSA, M. M. GHONEIM, K. A. IDRIS~and A. A. HARFOUSH,Z. physik. Chem. Neue Folge 94,135 (1975). and S. ADINA, Z. Cl41 C. S. LAHIRI,U. C. BHAITACHARYYA physik. Chem. Leipzig 249, 49 (1972). Cl51 N. S. BAYLE~~and E. G. MACRAE, J. phys. Chem. 56, 1002 (1954). Cl61 E. G. MACRAE, J. phys. Chem. 61, 562 (1957). G. C. PIMENTAL,J. Am. them. Sot. 79,3323 (1957). [ii] L. GAn and L. SWLAY, Acta phys. them. 5, 87 (1959). Cl91 T. GANGULYand S. B. BANERJEE,Spectrochim. Acta 34A, 617 (1978). WI R. FOSTER, Molecular Association, Vol. 1. Academic Press, London (1975). and R. E. BOWEN,J. Am. them. Sot. 87, Pll G. D. JOHNSTON 1655 (1965). ml R. FOSTER,J. them. Sot. 1075 (1960). J. them. Phys. 30, 1367 (1959). c231 R. BHATTACHARYA, [l]
No. 9. pp. 1063-1067,
1985. 0
Molecular complex formation
of thiosalicylic
0584-8s39/tts s3.00 + 0.00 1985 Pergamon Press Ltd.
acid with ethanol
K. A. IDRISS, A. S. EL-SHAHAWY,M. S. ABU-BAKR, A. A. HARFOUSH and E. Y. HASHEM Department of Chemistry, Faculty of Science, Assiut University, Assiut, Egypt (Received in reoisedform 29 March 1985) formation of a 1: 1 molecular complex of thiosalicylic acid (TSA) with ethanol was studied spectrophotometrically. Ethanol is postulated to form an intermolecular hydrogen bond with the carboxyl OH group of TSA, the intramolecular hydrogen bond of the SH group remaining intact. Results of SCF-CI molecular orbital calculations support the spectral interpretation. The formation constant (K,), free energy change (AC) and molar absorptivity of the TSA-ethanol molecular complex with cyclohexane, CC& or CHCla as solvents were determined. K, values vary with solvent in the order cyclohexane r Ccl, > CHCl,. The electronic absorption spectra of TSA in one- and two-component solvents are discussed. Abstract-The
which is higher than the energy of the ground state for the a& form (EG = - 505.168 eV). Accordingly, the a-form of TSA is the more stable molecular form. It is well established that molecules having an intramolecular hydrogen bond, such as TSA, undergo the so-called proton transfer in the excited singlet state, resulting in a significantly red shifted emission band. Such observations suggest that the absorption and emission spectra of TSA are dependent upon the solvent wherein the intramolecular hydrogen bond may be replaced by intermolecular hydrogen bonds with the solvent molecules. Ethanol is known as a good solvent capable of forming intermolecular hydrogen bonds with various polar groups of solute molecules [ll-131. In this paper, the effect of molecular complex formation between TSA and ethanol on intermolecular hydrogen bonding will be examined and characterized. The formation constant (K,) is determined from spectral measurements in mixed solvents.
INTRODUCTION Thiosalicylic acid (o-mercaptobenzoic acid) offers good opportunities for intramolecular association [l] and examination of the vsHregion indicates two bands both of which are at lower frequency than that expected for an unassociated thiol group. Examination of the carbonyl bands provides additional evidence for intramolecular association and indicates that the latter persists even in the fairly basic solvent acetonitrile. The very weak acidity of the thiol group in thiosalicylic acid may indicate stabilization by intramolecular hydrogen bonding, an effect which is apparent with salicylic acid [2]. The results of our preliminary molecular orbital calculation confirm the existence of the a-form of thiosalicylic acid as the predominant molecular species. H. S’
0 b
’ p"
S
Ii
K.
p - Form
/B-.Q
b
0
EXPERIMENTAL
XH
a - Form
In the present work, the interpretation of the electronic spectra of thiosalicylic acid (TSA) is supported by molecular orbital calculations. These calculations usually start with the well-known semiempirical HMO theory, then the self consistent field molecular orbital method (SCF-MO) using the PARISER-PARR-POPLE (PPP) approximation [3-6] in addition to that of NISHIMOTO and FORSTER 171. Then, as a final step, configuration interaction calculations [8-lo] were carried out to compute state functions, state energies and transition energies. The results of these calculations gave a qualitative fit to the observed transition for the neutral molecule in solution. The results of the SCF-CI calculations indicate that the n-electronic energy of the group state of the TSA molecule has the value - 498.561 eV for the @is form,
The thiosalicylic acid used in the present investigation was B.D.H. pure grade. E. Merck spectrograde Ccl4 and cyclohexane were purged with oxygen-free dry nitrogen immediately before use. E. Merck spectrograde CHCls was purifed according to recommended procedures and was then purged with nitrogen immediately before use. Spectrograde ethanol and diethvl ether were obtained from E. Merck. Various steps were taken to ensure their purity. 10s3 M stock solutions of TSA were prepared accurately. More dilute solutions of the reagent were obtained, as required, by accurate dilution. The absorption spectra were recorded on a Unicam S.P. 8ooO spectrophotometer using 1 cm matched stoppered quartz cells. The measurements were carried out at 20°C. Data processing calculations were done on a Casio FP-200 computer.
RRSULTSAND DISCUSStON Absorption spectra of TSA in single solvents The spectra of TSA in organic solvents of varying polarities indicate that both 1, and E of the absorp-
1063
1064
K. A.
IDRISS er al
tion bands are influenced by the nature of the solvent used (cf. Fig. 1). The high solvating power of ethanol leads to a high solubility of the compound and no association of the solute molecules can take place especially in dilute solutions. The linear variation of absorbance with solute concentration in ethanol confirms that solute-solute interaction is of minor importance in this solvent. The U.V. spectrum of TSA, in ethanol, has peaks at 210, 235, 275 and 345nm (cf. Fig. I). The lower intensity band at 210nm, compared with that of 235 nm band, is due to Z-Z* triplet transition in the benzoic moiety as confirmed by SCF-CI calculations. The 235 nm band corresponds to transitions within the benzenoid system. This band is shifted to red on increasing the polarity of the medium, behaviour which is in accordance with the n--s* nature of the electronic transitions leading to this absorption band. The large molar absorptivity, the red shift observed when solvent polarity is increased and the disappearance of the band in nonpolar solvents suggest that the band at 275 nm is due to a charge transfer (C.T.) transition. The results of SCF-CI calculations indicate that this band is unambiguously due to mixed Z-Z* transitions within the benzoic moiety. The effect of polar solvents is to perturb the symmetry of the excited molecular orbitals (ti7 and Ijlg) to be similar to that of the highest two occupied molecular orbitals (I+%~ and 1(/e) Hence, the transition from the higher two molecular orbitals to the lowest vacant molecular orbitals is allowed (cf. Table la, b). Therefore, the transition appears much more strongly with increasing polarity of the medium. After configuration interaction calculations, the state functions and state energies were computed and given in Table 3. The transition energies were calculated and compared with the experimental data to confirm the assignment of the electronic bands (cf. Table 4).
Table la. Molecular orbital eigenfunctions of the highest two occupied and the lowest two vacant molecular orbitals of TSA molecule (a-form) *5
Z: 46 47 ds k0
)(/7
+ 0.028229 - 0.301083 - 0.570237 - 0.276840 + 0.293074 + 0.567611 + 0.268455 + 0.035057 - 0.012546 + 0.158790
+ 0.003484 + 0.284204 + 0.485447 - 0.285063 - 0.222655 + 0.489076 - 0.223568 + 0.381116 - 0.048663 - 0.340465
*a
+ 0.005889 - 0.500237 + 0.499786 - 0.500154 + 0.499789 O.oooO O.WOO O.OOQO O.OCQO 0.0000
Table lb. Transition
moment components between the occupied MOs and vacant MOs in thiosalicylic acid
Transition
$:I$: ;:r:;:
M, - 0.292514 0.613850 - 0.606178 - 0.330928
MY 0.543c01 0.335218 - 0.397571 0.606951
MZ 0.00 0.00 0.00 0.00
The absorption band of 1, at 345 nm is tentatively assigned as an intramolecular charge transfer band. This transition occurs, according to our SCF-CI calculations, between the oxygen of the carboxyl OH and the ring n-electrons. The most interesting effects of changing the solvent are observed with the C.T. band. The neutral form of TSA is expected to predominate in nonpolar solvents while the anionic form may exist in polar solvents. Although for salicylic acid the anionic form is present only to a small extent in any solvent, the greater acidity of the mercapto group causes monovalent ions to become more prevalent for TSA in polar solvents. Hydrogen bonding solvents will also have an effect on the formation of the anionic species.
Wavelength Fig. 1. Electronic
0.048343 - 0.491721 + 0.014303 + 0.506455 + 0.491920 - 0.014907 - 0.506903 0.0000 O.OC00 - 0.004031 +
I;
1(/c
(nm)
absorption spectra of thiosalicylicacid in organic solvents ofvarying polarities. 2, cyclohexane; 3, ether; 4, chloroform and 5, carbon tetrachloride.
1, Ethanol;
Thiosalicyclic acid-ethanol complexes Table 2. Results of SCF calculations of xelectron densities for different atoms in thiosalicylic and salicylic acid molecules
(a) X =0 (b) X=S Atom
(a)
(b)
1
1.989070 0.987453 1.009780 0.996110 1.006630 0.996729 1.007670 0.765201 1.982940 1.259520
1.99989 0.996428 1.002350 0.996486 1.000190 0.997158 1.000210 0.765186 1.982930 1.259190
2 3 4 5 6 I 8 9 10
Table 3 State function
State energy (eV)
E: = -500.551
“I’, ;; = - 500.029 s = - 499.862 tiexs = - 0.553903 J& + 0.832581 +D, +=, = ‘@?9 Sas = 0.832581’8, + 0.553903 3@,
E: = - 499.862 E: = -499.808
1065
The carboxyl group can be strongly hydrogen bonded in both proton donor and proton acceptor solvents. Thus, hydrogen bonding solvents such as ethanol will tend to stabilize the monoanionic form of TSA more than the neutral molecules. Therefore, in nonpolar solvents, the neutral TSA species will predominate and as the polarity or the hydrogen bonding strength of the solvent increases, the equilibrium will be shifted toward the anionic form. In TSA, the main band not having an analogue in the benzoic acid spectrum is the band at 345 nm. This band has been identified as being a charge-transfer transition in the thiosalicylate anion. The SCF-CI calculations also indicate a large amount of charge-transfer character for this band. The rapid decrease in the absorbance of the C.T. band as the solvent polarity is decreased can be ascribed to the decrease of the concentration of the anionic form of TSA. Thus, the C.T. band becomes less intense as the equilibrium is shifted toward the neutral form of TSA by less polar or less hydrogen bonding solvents. Solute molecules exist in a unpolarized state in solvents of low polarity not capable of forming an intermolecular hydrogen bond. In ether, the carboxyl OH of TSA can be involved in a weak intermolecular hydrogen bond with the oxygen of the ether molecule. This type of bonding facilitates the intramolecular charge-transfer and thus a red shift of the C.T. band is observed in ether compared to CCL or CHCl,. TSA can readily interact with ethanol as a proton acceptor solvent. This interaction leaves a residual negative charge on the oxygen of the carboxyl OH. Accordingly, the intramolecular charge transfer is enhanced leading to a red shift of the C.T. band in the proton acceptor solvent.
E: = - 499.808
Table 4. Comparison of the calculated transition energies (SCF-CI) with the experimental data for the TSA molecule (aform) in cyclohexane
AEcai,
-Not
AE
rl
(eV)
hE!J
(efl’
(Z)
4.387 4.170
283 298
4.005 -
310 -
4.616 5.131
269 242 236 234 232 212
4.720 -
263 -
5.283 5.469
235 227
observed.
On the other hand, since the thiosalicylic anion is stabilized by hydrogen bonding, its pK value is less than that of benzoic acid but much greater than that of salicylic acid [14]. This is reasonable as the replacement of 0 by S atom reduces the strength of the hydrogen bond (cf. Table 2).
It is to be noted that the resonance energy in the carboxyl group must be partially overcome in the complexation to ethanol. As a result, it is possible that the intramolecular hydrogen bond in TSA is actually somewhat strengthened owing to stabilization of the above structure rather than weakened by conjugation of the carboxyl group to ethanol. The addition of two ethanol molecules, one to the carboxyl OH and one to the SH group, to one TSA molecule is more or less excluded on electrostatic grounds. Absorption spectra of TSA in mixed solvents and molecular complex formation The formation of the TSA-ethanol molecular complex was investigated by studying the spectral behaviour in mixed solvents. A set of absorption spectra was measured taking the three-component systems, viz. TSA, the proton-acceptor solvent ethanol and the
1066
K. A.
IDRISS er al.
nonpolar solvent. The concentration of ethanol was increased in steps from 0.35 to 13 M. In every measurement, a mixture of ethanol and a nonpolar solvent in which the concentration of ethanol was identical with that in the sample solution was taken as a reference. As shown in Figs 2 and 3, the spectra in mixed solvents show a regular decrease in the absorption of the band at 345 nm and its shift to shorter wavelengths with an increasing proportion of the nonpolar component. The decrease of absorbance can be ascribed to the decreased participation of the ionic form of TSA in the equilibrium mixture with decreasing solvent polarity. The increased proportion of ethanol causes a red shift of the CT. band. The red shift of ,I,, denotes an
0.6
300
325
'I 350
375
Fig. 2. Absorption spectra of thiosalicylic acid in mixed solvents (ethanol-CC&), 1 x 10-4M TSA. C,I,. = (a) 0.86 M; (b) 1.72 M; (c) 2.57 M; (d) 3.43 M; (e) 4.97 M; (f) 6.86 M; (g) 9.43 M; (h) 9.95 M; (m) 10.98 M and (n) 12.0 M.
375
300
increased solvation energy in the excited state. Accordingly, the molecules in the excited state are present in a cage of dipoles which are more oriented than in the pure low polarity solvent [ 15, 161. The red shift observed can, according to the work of PIMENTAL [17], be explained by the increase of the hydrogen bond strength between solute and solvent on addition of alcohol. The increase of H-bond strength would be higher for the excited than for the ground state. To investigate the effect of the dielectric constant of the medium on the band shift (Av’), the relation of GATI and SZALAY [ 181 was applied Av’ = (a-b)($$+b(g) in which a and 6 are constants and n is the refractive index of the medium. The plots of Av’ as a function of (D - l)/(D + 1) in all systems investigated are nonlinear relations indicating that the band shift is governed by other factors besides the dielectric constant of the medium. These factors include solute-solvent interaction through intermolecular hydrogen bonds which leads to the formation of a TSA-ethanol molecular complex. The intensity change of the C.T. band in two-component solvents is accompanied by the development of a new band at the longer wavelength side. A clear isosbestic point is observed which indicates the existence of an equilibrium between the bonded and free molecules. The oscillaror strength (f) for the bonded molecules is always greater than that for free molecules, which means an enhanced interaction between the lone pair electrons on the oxygen atom of the OH group in TSA molecule and the ring n-electrons presumably due to an intermolecular (O-H . 0) bond formation. The value of the formation constant (X,) of the TSA-ethanol molecular complex was determined from the spectral behaviour in mixed solvents using the relations previously reported [ 11, 19, 203. The mean
325
350
375
400
Wavelength (nm) Fig. 3. Absorption spectra of thiosalicylic acid in mixed solvents (ethanokyclohexane), 2 x 10e4 M TSA. C,I,. = (a) 1.72 M; (b) 2.57 M; (c) 3.43 M; (d) 5.15 M; (e) 6.68 M; (I) 8.57 M; (g) 9.43 M and (h) 11.14 M.
Tbiosalicyclic acid-ethanol complexes
value of K, in all systems investigated and the corresponding AG values are given in Table 3. For each binary solvent system, the data near the charge-transfer band maximum are consistent with the existence of a single 1: 1 (TSA-ethanol) complex. The constancy of the K, values for a given binary system at different wavelengths is a valid criterion for the existence of a 1: 1 complex in solution [Zl]. This condition is very well satisfied for the TSA-ethanol complex. For the donor-acceptor-solvent ternary systems under investigation a change of the nonpolar solvent effects drastic changes in the computed K, value (Table 5). Competitive donor-solvent complexes in CHCIJ , and possibly CC14,can block the donor orbital and thus reduce the apparent formation constant in these solvents. The absorptivity of the assumed 1: 1 complex near the absorption maxima is approximately constant (cf. Table 6). For the systems studied in the present investigation, the wavelength of maximum absorption for the TSA-ethanol complex varies with solvent thus: cyclo-
Table 5. Average values of K, (1mole-‘) and AG (kcalmol- ‘) for the TSA-etbanol complex at 20°C System Etbanol-cyclohexane EtbanolCHCI, Ethanol-XXI,
logK,
K,
AG
0.744 0.236 0.377
5.55 1.72 2.38
- 0.998 -0.316 - 0.505
Table 6. Maximum molar absorptivity and wave length of band maximum for the TSA-ethanol complex system
Ethanol-cyclohexane Ethanol-CHCl, Ethanol-CCl,
2.11 x 104 2.32 x 10“ 2.65 x 10’
348 350 353
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hexane < CHCls < Ccl*. Some authors [22,23] have reported similar behaviour for various complexes. The results cannot be explained in terms of average solvent properties alone, but they can be interpreted by the supposition that the formation of one isomeric complex is favoured by the structure of the solvent.
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