Multiple and sequential charge transfer interactions occurring in situ: A redox reaction of thiazolidine-2-thione with 2,3-dichloro-5,6-dicyano-1,4- benzoquinone

Article abstract of DOI:10.1016/j.saa.2011.04.078

Interaction of thiazolidine-2-thione (T2T) as an electron donor with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an electron π-acceptor has been studied. Electronic absorption spectra of the system T2T-DDQ in several organic solvents of different p

Full text of DOI:10.1016/j.saa.2011.04.078

Spectrochimica Acta Part A 79 (2011) 1411–1417
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Multiple and sequential charge transfer interactions occurring in situ: A redox
reaction of thiazolidine-2-thione with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
Usama M. Rabie∗, Moustafa H.M. Abou-El-Wafa, Heba Nassar
Chemistry Department, Faculty of Science at Qena, South Valley University, Qena 83523, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Interaction of thiazolidine-2-thione (T2T) as an electron donor with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) as an electron ␲-acceptor has been studied. Electronic absorption spectra of the
system T2T–DDQ in several organic solvents of different polarities have been measured. A charge transfer
(CT) complexation has occurred between T2T and DDQ. This CT interaction has led to a redox reaction in
which T2T has been oxidized to the corresponding dehydrogenated T2T (T2T-2H), meanwhile DDQ has
been fully reduced to the corresponding hydroquinone (DDQH2). However, the two new species, resulting
in situ, have been interacted, whereas a CT complex having the formula (T2T-2H·DDQH2) has occurred.
IR, 1H NMR and mass spectra were used for ascertaining the structural formula of the synthesized CT
complex. Formation constants (KCT), molar absorption coefficients (εCT) and thermodynamic properties
of this CT interaction in various organic solvents were determined and discussed. The obtained KCT and
εCT values have indicated that T2T-2H is a weak CT donor, whereas the formed CT complex has a low
stability and it is classified as a contact-type CT complex.
Received 29 November 2010
Received in revised form 27 April 2011
Accepted 29 April 2011
Keywords:
Charge transfer complex
2-Thiazoline-2-thione
DDQ ␲-acceptor
Tautomerism
Anti-thyroid drug
1H NMR and mass spectra
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Thiazolidine-2-thione (T2T) is one of the heterocyclic organic
compounds which are of great importance and different applica-
tions due to the –(NH)–(C S)– grouping and the electron donor
properties of N and S atoms. Thus, T2T and its derivatives have
diverse pharmaceutical [1–11] and industrial applications [12–16].
T2T has a low molecular weight and it seems having simple
structural features, however, it has, interestingly, many structural
formulae that can be discussed in terms of isomerism, tautomerism
and ionized–unionized forms.
On the other hand, T2T has been widely used [17] as a charge
transfer donor of low strength that it is capable to form CT com-
plexes with iodine as a typical ␴-acceptor. However, according to
our knowledge, till date no previous work has been reported for
the interaction of T2T with ␲-type acceptors. The complexity of
this compound (T2T) and the willingness to continue our studies
[18–22] concerning the ␲-type acceptors, have motivated us for
this piece of work aiming to study the capability of T2T to form
n–␲ CT complexes.
2.1. Materials and solutions
The electron donor, thiazolidine-2-thione (T2T), was purchased
from Aldrich Chemical Co. The electron acceptor, 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) (Aldrich Co.), was recrystallized
from both chlorobenzene and dry methylenechloride. The sol-
vents used viz. 1,4-dioxane, benzene, ethanol (EtOH), methanol
(MeOH), acetonitrile (CH3CN)), N,N-dimethylformamide (DMF),
and dimethyl sulphoxide (DMSO), and the chlorinated solvents
(carbon tetrachloride (CCl4), chloroform (CHCl3), dichloromethane
(CH2Cl2), and 1,2-dichloroethane (C2H4Cl2)) were of pure spectral
grade (BDH or E. Merck).
2.2. Physical measurements
The electronic absorption spectra were recorded on a Perkin-
Elmer Lambda 40 spectrophotometer equipped with a Julabo FP
40 thermostat ( 0.1 ◦C) using 1.0 cm matched quartz cells. IR spec-
tra of the synthesized solid product were recorded on a Shimadzu
IR-408 spectrophotometer. 1H NMR and mass spectra measure-
ments were carried out in the microanalytical laboratory, Cairo
University. Computations were performed making use of both the
Benesi–Hildebrand [23] and Scott [24] methods with the aid of
two programs based on unweighted linear least-squares fits. Stock
∗
Corresponding author. Tel.: +20 0106755160; fax: +20 0965213383.
E-mail address: umrabie@yahoo.com (U.M. Rabie).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.04.078

1412
U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
solutions of the donor or acceptors in proper solvents were freshly
prepared prior to use. For determination of the formation constants
(KCT) and molar absorption coefficients (εCT) of the CT complexes
by applying the Benesi–Hildebrand [23] and Scott [24] methods,
spectra of the CT mixtures were measured after mixing solutions
of the donor and acceptor and elapsing the time necessary to reach
maximum absorption intensity, whereas the concentration of the
acceptor was kept constant, meanwhile, the concentration of the
donor varied; at least nine mixtures solutions were prepared for
each experiment. The different thermodynamic parameters of the
CT complexes were determined by applying Van’t Hoff plots [25].
2.3. Synthesis of the solid CT complex
The solid CT complex of the donor (T2T) with the acceptor
(DDQ) was synthesized by mixing the required amount of T2T (1.2 g,
0.01 mol) dissolved in 50 ml CH2CL2 with appropriate amount of
the DDQ (2.27 g, 0.01 mol) in 75 ml CH2CL2 in 1:1 mole ratio. The
mixture was refluxed for (1/2) h and the resulting complex solution
was left standing overnight at room temperature. The separated CT
solid complex was filtered and washed several times with min-
imum amounts of ethanol and then dried (colour, yellow; m.p.,
decomposed).
Fig. 1. Electronic absorption spectra of the C2H4Cl2 solutions of the compounds: (a)
T2T, (b) DDQ and (c) T2T–DDQ mixture at 25 ◦C. [Compound] = 1.0 × 10−4 mol dm−3
.
of the thione form of the compound T2T. In addition, it has been
demonstrated [17] that the compound T2T exists in solution in
two tautomeric forms, the thione (Structure I) and thiole (Structure
II), and each form has its unique absorption spectra. However, the
two forms do not exist in equilibrium but always one form (mostly
the thione form) predominates depending on the solvent polarity.
Moreover, it has been reported [17] that according to the MNDO cal-
culation, the difference between the enthalpies of formation of the
thione and thiol forms of the unsubstituted potential mercaptoth-
iazole is 56.82 kJ mol−1, which is evidence in favour of the thione
form.
3. Results and discussion
3.1. Spectral characteristics of thiazolidine-2-thione (T2T)
Before turning our attention to discuss the interaction of T2T
with DDQ, it is worthwhile to recall our very recent study [17] con-
cerning the spectral behaviour of this compound (T2T) intending
to use and refer to the established data to assist the discussion and
to ascertain the results, herein, for the interaction between T2T
and DDQ. It has been reported [17] from the IR and UV–vis spectra
and the X-ray studies [17] that the compound T2T exists in the
solid state as a hydrogen-bonded thioamide complex (Structure
III) (NH· · ·S) formed between the NH of the thione form (Structure
I) and the exocyclic sulfur atom. This thioamide complex partially
breaks down in solutions giving a mixture of free molecules with
dimers or higher combinations. The dimer thioamide complex is
formed and originated from the thione structure, meanwhile the
free molecules exist either as the thione form or the thiole one
(Structure II), i.e., a tautomerism of thione–thiole forms.
3.2. Spectral study of the interaction between T2T and DDQ
Electronic adsorption spectra of C2H4Cl2 solution mixture
(T2T–DDQ) were recorded in the wavelength range 250–750 nm.
A new band which does not belong to the donor (T2T) and the
acceptor (DDQ) appeared at 350 nm, cf. Fig. 1.
The compound T2T is an electron donor of n-type [17] owing to
its unshared electrons on the nitrogen and the two sulfur atoms,
meanwhile DDQ is a well known ␲-acceptor [18–22,26]. Thus the
new band located at 350 nm might be assigned as a charge trans-
fer (CT) absorption band. It has been shown [17] that the spectral
behaviours of the individual T2T in different solvents are amazing,
whereas, as it will be shown hereafter, the spectral behaviours of
the T2T–DDQ interaction are very interesting too.
S
NH
S
N
S
SH
( II )
( I )
The electronic absorption spectra of the T2T–DDQ interaction
in different organic solvents of different polarities are found to be
dependent on both the time and the solvent polarity. The electronic
absorption spectra of the T2T–DDQ mixture solution in MeOH (or
EtOH), a polar protic solvent, are characterized by developing of
a new spectral band at 350 nm which does not instantaneously
appeared on mixing the two interacting species. The intensity of
this new band is gradually increasing by lapse of time and it reaches
its maximum intensity after about 70 min from the time of mix-
ing the two reactants, cf. Fig. 2, but the position of this band (at
350 nm) does not change during this lapse of time. Meanwhile, the
electronic absorption spectra of the T2T–DDQ solution mixtures
in non-polar chlorinated solvents are characterized by the instant
observation of that new spectral band in the range of 340–355 nm,
and the intensity of this new band is also gradually increasing by
lapse of time without a considerable change in the band positions
too. Representative spectra are shown in Fig. 3. This prompted us
Thione
Thiole
Structures (I) and (II): thione–thiole tautomeres.
S
H
N
S
H
S
N
S
Structure (III): dimeric hydrogen-bonded thioamide complex.
The UV–vis absorption spectra of T2T in organic solvents have
shown two distinct main spectral bands located at 270–280 nm
and 315–343 nm. The first absorption spectral band observed at
270–280 nm has been assigned [17] as an intramolecular charge
transfer transition (␲–␲*) within the thiazoline ring, meanwhile
the second band shown at 315–343 nm has been assigned [17] as
a n–␲* intra-transition originated from the exocyclic sulfur atom

U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
1413
initial formation of CT interactions (i.e., D + A → D·A outer sphere CT
complex → D+·A− inner sphere → C product of the chemical reac-
tion). Other authors [19–21,33] have showed that the stepwise
reduction of the ␲-acceptors to the corresponding anion radicals
(A•−) or even to the reduced form (AH) is responsible for the lapse
of time. They [19–21,33] have presented suggestions that, in some
cases, the reduced forms of the ␲-acceptors can also act as new
acceptors [19,20,34], and in other cases, the oxidized forms of the
donors can also act as new donors [19], whereas both the new
acceptors and the new donors have resulted in situ during the
primary donor–acceptor interactions. Thus, the transformation to
these new species (the new donors and acceptors) causes this lapse
of time [19–21,33].
Herein, in our case, measurements of the electronic absorption
spectra of the T2T–DDQ mixtures solutions in each of studied sol-
vents did not show the characteristic [19,21,35–39] spectral bands
of the DDQ anion radicals (DDQ·−). In addition, as it will be proved
hereafter, investigation of the formed CT solid complex by IR, 1H
NMR and mass spectra has indicated that no chemical reaction that
could produce a new species occurred.
Thus, we believe that the observed lapse of time, which is nec-
essary for the CT spectral band of the T2T–DDQ interaction to reach
its maximum intensity, might be explained by: (1) dissociation of
the dimeric thioamide hydrogen-bonded solid complex (Structure
III) in the solutions to its individual molecules, whereas the result-
ing individual molecules exist in the solutions in two tautomeric
forms (thione (Structure I) and thiole (Structure II)). (2) Reduc-
tion of the DDQ to its corresponding hydroquinone, i.e., DDQH2.
(3) Simultaneous oxidation of the T2T (either in thione or thiole
form) to the corresponding dehydrogenated T2T, i.e., T2T-2H. In
turn, the observed CT band is referred to a charge transfer arising
from the highest occupied molecular orbital (HOMO) of the result-
ing dehydrogenated new donor (T2T-2H) to the lowest unoccupied
molecular orbital (LUMO) of the resulting reduced new acceptor,
DDQH2, whereas the two new interacting species (T2T-2H and
DDQH2) are resulting in situ through a period of time dependent
on the polarity of the solvent used.
Quinones, in general, and DDQ, in specific, are well known
[32,40–42] as dehydrogenating agents since the pioneering work
[43] published in 1954. Several mechanisms, as shown hereafter,
have been suggested [32,42] for these kinds of redox reactions that
caused by the action of the quinones as dehydrogenating agents.
However, it has been reported [41,42] that in many of these redox
reactions involving DDQ, CT complexations have been occurred at
least initially.
Fig. 2. Electronic absorption spectra of the (T2T–DDQ) mixture solution in MeOH
at 25 ◦C. [T2T] = 1.0 × 10−3 mol dm−3, and [DDQ] = 4.0 × 10−4 mol dm−3
.
to undertake a further investigation on this phenomenon of time-
dependency in correlation with the change of the spectral bands
appearing through the interaction of the T2T donor with the ␲-
acceptor DDQ. Moreover, this system, T2T–DDQ, was completely
tracked and evaluated for the probable occurrence of ca chemical
reaction.
In general, time-dependency in correlation with the CT interac-
tions is still controversy. Some authors [27–31] have referred that
time dependency to the transformation of the CT complex from the
outer-sphere, a nonbonding structure, to the inner-sphere dative
structure (i.e., D + A → D·A outer sphere CT complex → D+·A− inner
sphere (dative structure)). Other authors [18,32] have referred that
time dependency to chemical reactions that take place in some
cases between the reacting species, even though it has been sug-
gested [18,32] that these chemical reactions have been occurred via
Dehydrogenation of a substrate (AH2) by a quinone (Q) is gen-
erally represented by Eq. (1):
AH2 + Q → A + QH2
(1)
Meanwhile, this redox reaction might proceed following one
of the following suggested [32,42] mechanisms: (1) by hydride
ion transfer to the quinone in the rate-determining step (Eq. (2))
followed by rapid proton transfer from the conjugate acid to the
hydroquinone anion (Eq. (3)):
slow
AH2 + Q−→ AH+ + QH−
(2)
(3)
fast
AH+ + QH−−→ A + QH2
(2) By proton catalysis (Eqs. (4)–(6)) in which the protonated
quinone (QH+) acts as an efficient hydride ion acceptor:
Q + H+ → QH+
AH2 + QH+ → AH+ + QH2
AH+ → A + H+
(4)
(5)
(6)
Fig. 3. Electronic absorption spectra of the (T2T–DDQ) mixture solution in CHCl3 at
25 ◦C. [T2T] = 4.0 × 10−4 mol dm−3, and [DDQ] = 2.0 × 10−4 mol dm−3
.

1414
U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
T2T + DDQ → T2T · DDQ (first CT interaction)
T2T · DDQ → T2T-2H + DDQH2 (Redox reaction)
(17)
(18)
T2T-2H + DDQH2 → T2T-2H · DDQH2 (second CT interaction)
(19)
Though the chemistry of the DDQ as a dehydrogenating agent
is not the only manifested observation but we are the first to
show that both the oxidized and reduced form of T2T and DDQ,
respectively, resulting in situ through first CT interaction between
T2T and DDQ can also interact as a new donor (T2T-2H) and a
new unconventional ␲-acceptor (DDQH2) to yield a new CT com-
plex (T2T-2H·DDQH2) (Eq. (19)). Some evidences to confirm our
assumptions will be mentioned hereafter.
Fig. 4. Infrared spectra of free T2T donor (a), free DDQ acceptor (b) and T2T–DDQ
CT complex (c).
3.3. Characterization of the T2T–DDQ solid CT complex:
The IR spectra of the free donor T2T, the free acceptor DDQ
and the solid complex synthesized from the interaction of T2T
with DDQ were measured, cf. Fig. 4. A careful examination of the
important observed IR spectral peaks of the solid complex has indi-
cated [14,35–37,44,45] the existence of the dehydrogenated T2T
(i.e., T2T-2H; oxidized form) and the reduced form of the DDQ
(i.e., DDQH2). Thus, the IR spectra of the complex have showed
several peaks located at 1450 and 1070 cm−1 which are charac-
teristic [14,44,45] to the stretching vibrations of ꢀ(NH) and ꢀ(C S)
of the thione form, respectively, whereas these peaks have previ-
ously seen in the free T2T except for some reasonable shift due
to the CT complexation. In addition, the IR spectra of this com-
plex do not show peaks in the regions 2800–3000, 2400–2700 and
1640–1690 cm−1 which may characterize [14,44,45] the ꢀ(–CH2–)
of the thiazoline, ꢀ(SH) and ꢀ(C N) of the thiazol forms, respec-
tively, meanwhile the peak that characterizes [44] to the postulated
existence of the ꢀ(–C H–) of the thiazole (the dehydrogenated T2T)
may be overlapped with other vibration(s) within the broad peak
(3) By first formation of stable free radicals that lead to hydride
ion transfer (Eqs. (7) and (8)):
AH2 + Q → AH• + QH•
AH• + QH• → AH+ + QH−
(7)
(8)
(4) By ene-reaction between the two species, AH2 and Q, to
give an intermediate adduct (I) (Eq. (9)), followed by a slow base-
induced elimination (Eq. (10)):
AH2 + QfꢀastI
(9)
slow
I−→ AH+ + QH−
(10)
(5) By sequential electron–proton–electron transfer which most
likely occurs via a CT-complex (Eqs. (11)–(14)).
AH2 + Q ꢀ AH2· · ·Q
AH2· · ·Q ꢀ AH• + QH•
(11)
(12)
+
−
observed at 3200 cm−1
.
Also, the IR spectra of the synthesized complex indicate the
existence of the hydroquinone (DDQH2). Thus, the spectral peaks
observed at 2250, 1450 and 890 cm−1 are characteristic [35–37]
to the stretching vibrations of C≡N, C C (aromatic) and C–Cl of the
hydroquinone, respectively, cf. Fig. 4. In addition, the most remark-
able and interesting observation concerning the IR spectra of the
(T2T/DDQ) CT complex is the disappearance of the 1670 cm−1 band
slow
+
−
AH• + QH• −→ AH•QH•
AH•QH• → AH+QH−
(13)
(14)
However, it is worthy to mention that the aforementioned
mechanisms of the dehydrogenation by quinones depend [32,42]
on the structure of the substrates and, in particular, on the stability
or reactivity of the intermediates.
which characteristics [35–37] to the stretching vibrations of C
O
Herein, in our case of interaction between T2T and DDQ, we are
not yet in a position to prove unambiguously one or the other sug-
gested mechanisms which could be applied to this T2T–DDQ redox
reaction. However, time dependency of the established dehydro-
genation of T2T with DDQ, to yield T2T-2H and DQQH2 via first
formation of a CT interaction followed by a redox reaction, has
found an adequate explanation.
of the free DDQ; i.e., the acceptor has no longer been in the quinone
form. Instead, a broad peak has been observed at 3200 cm−1 which
characteristics [35–37] to the OHs (of DDQH2) that might be asso-
ciated with a hydrogen-bonding with ca the exocyclic S atom of
the donor (T2T-2H) in its thione form and/or it might be over-
lapped with the stretching vibration with first –NH– peaks (of the
thione form of T2T-2H) usually appeared at around 3300 cm−1
.
In turn, we believe that the following reaction pathway (Eqs.
(15)–(19)) might be hypothesized for the interaction between the
T2T, as a donor, with the DDQ, as an acceptor, meanwhile the
multiple and sequential CT complexations involved through this
T2T–DDQ interaction has been suggested too:
However, it is worthwhile to mention that none of the characteris-
tic [35–37] peaks for any of DDQ·−, DDQ2−, DDQH− (anion radicals
or semiquinones) have been observed in the IR spectra of the solid
CT complex which provides an evidence for the complete reduction
of the DDQ to the corresponding hydroquinone (DDQH2). In turn,
the IR spectral measurements have provided a further evidence for
our suggested reaction pathway for the interaction of T2T with
DDQ. Accordingly, the structure of the formed (T2T-2H·DDQH2)
CT complex might be hypothesized as shown in the following
T2T
T2T
→
(15)
(16)
Thioamide dimer complex
Individual free molecules
T2T(thione) ꢀ T2T(thiole) (tautomerism)

U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
1415
3.4. Association constant (KCT) and molar absorption coefficient
(εCT) of the CT complex resulting from the interaction of T2T with
DDQ; (T2T-2H·DDQH2)
Under the condition [T2T] ≫ [DDQ], the electronic absorption
spectra of the T2T/DDQ mixture solutions in MeOH were mea-
sured. The spectra were measured after elapsing 70 min from the
time of mixing the two interacting species, whereas this period
of time is necessary to reach the maximum CT band intensity. The
absorbance values obtained from the CT band, at ꢁmax = 350 nm and
at different temperatures, 10, 15, 20, and 25 ◦C, were used for deter-
mination of the formation constants (KCT) and molar absorption
coefficients (εCT) of the CT complex (T2T-2H·DDQH2), making use
the well known Benesi–Hildebrand [23] and Scott [24] equations
(Eqs. (20) and (21)):
[A]0
A
1
1
1
εCT
=
+
(20)
(21)
KCTεCT [D]0
Fig. 5. Electronic absorption spectra of the CHCl3 solution of the synthesized
(T2T–DDQ) solid CT complex at ◦C.
[D]0[A]0
1
[D]0 + [A]o
[C]
=
+
−
εCT
A
KCTεCT
εCT
(Structure IV).
where A is the absorbance of the CT mixture [D]o and [A]o are the
initial concentrations of the donor and acceptor respectively, and
[C] represents the concentration of the CT complex.
OH
NH
S
NC
NC
Cl
Cl
.
The obtained low values of the formation constant (KCT), listed
in Table 1, indicates that the CT complex (T2T-2H·DDQH2) has a
low stability; i.e., weak CT complex, and this CT complex might be
classified as a one of the contact-type charge-transfer complexes
[26] in which the CT interaction results during random encounters
whenever the donor (T2T-2H) and the acceptor (DDQH2) molecules
are sufficiently close to one another. The computed KCT values of
(T2T-2H·DDQH2) CT complex (Table 1) at different temperatures
were utilized to determine the enthalpy change (ꢂH) of this com-
plex by using a Van’t Hoff plot [25]. Gibbs free energy change (ꢂG)
was calculated from the equation: ꢂG = −RT ln KCT. The obtained
results of the thermodynamic parameters are listed in Table 1.
The data listed in Table 1 reveal that the formed CT complexes
are better stabilized as the temperature is increasing. This indi-
cates the endothermic nature of the (T2T-2H/DDQH2) CT complex.
Moreover, the high value of ꢂS (cf. Table 1) indicates the random
encounters of the two reacting species (T2T-2H and DDQH2) char-
acterizing to the contact-type CT complex interactions [26]. This
could be supported by the determined Mulliken’s ratio (b/a)2, using
the proposed relation [47,48] b2/a2 = −ꢂH/hꢀ. In this relation, hꢀ is
the energy of the absorption band of the complex, where a and b
are the coefficients of the dative bond and the nonbonding wave
S
OH
Structure (IV): (T2T-2H·DDQH2) CT complex.
The electronic absorption spectra of the CHCl3 solution of the
synthesized solid CT complex (cf. Fig. 5) have shown a spectral band
at around 350 nm. This band has been previously seen in the absorp-
tion spectra of the T2T–DDQ mixture solution in CHCl3 which have
been measured after elapsing the time necessary to reach the max-
imum band intensity. This provides an additional evidence for the
suggested (T2T–DDQ) reaction pathway.
On the other hand, the 1H NMR and mass spectral measurements
of the formed CT complex have provided further evidences for con-
firming the structure of the formed CT complex (T2T-2H·DDQH2).
In turn, the aforementioned suggested reaction pathway of the
interaction between T2T and DDQ has found adequate evidence,
too.
The 1H NMR spectrum (300 MHz) of the CT product in DMSO
displays distinct signals characterizing [45] to the suggested (T2T-
2H·DDQH2) CT complex namely: ı (ppm); 3.46 (H, s, NH of thione
form), 5.62 (H, broad, 2OH of DDQH2), 6.97 (H, d, C5 proton of thia-
zole) and 7.43 (H, d, C4 proton of thiazole). However, broadening of
the proton signals of the OHs might be referred to a hydrogen bond
[45,46] arising with ca exocyclic S atom of the donor (T2T-2H).
Convincing evidence for the structure of the (T2T-2H·DDQH2)
CT complex has been provided by the mass spectra. The mass spec-
tra of the synthesized CT complex have displayed the molecular
ion peaks m/z (%) = 345 (0.3), 228 (100) and 117 (0.5) which repre-
sent the molecular weights of the species T2T-2H·DDQH2 (the CT
complex), DDQH2 (the acceptor; hydroquinone) and T2T-2H (the
donor), respectively. Moreover, the expected corresponding frag-
ments of the DDQH2 and T2T-2H species have also been indicated
from the other remaining mass spectral ion peaks.
It is worthy to mention that the TLC measurement of the syn-
thesized complex under investigation, T2T-2H·DDQH2, has shown
a single peak which indicates the existence of a single compound.
Thus, one can suggest that the synthesized CT complex, T2T-
2H·DDQH2, has a low stability that is readily decomposed under
the influence of the applied high energy of the Mass spectroscopy
machine [45] (ca ≥ 70 eV) to its two constituent species; DDQH2
and T2T-2H.
functions («D
+
−
and
«
, respectively) of that CT complex.
D−A
A
The obtained ratios (b2/a2) herein are close to those of many CT
complexes of weak strength [26].
The ionization potential (IP) of the T2T-2H donor has been esti-
mated from the energy of the CT transition using the empirical
relation of Alosi and Pignataro [49]. The calculated IP value of the
donor, T2T-2H (IP = 10.1 eV, cf. Table 1) indicates that the compound
T2T-2H is a CT donor of weak strength and the formed complex is of
the contact-type CT complexes. This provides a further convincing
evidence for the weak T2T-2H/DDQH2 CT interaction.
It is worthy to mention that, on measuring the electronic
absorption spectra of the T2T/DDQ mixture solutions in CHCl3, the
non-polar solvent, cf. Fig. 6, the obtained values of the formation
constants, thermodynamic parameters and ionization potential are
close to those obtained in the case of using MeOH as a solvent,
cf. Table 1. This consistency of the T2T/DDQ interaction in case of
using either CHCl3 or MeOH as a solvent means that the T2T/DDQ
redox reaction does not depend largely on the solvent polarity and,
moreover, the probable interaction of DDQ and MeOH by them-
selves [50] is excluded too. Thus, methanolysis of DDQ by MeOH

1416
U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
Table 1
Spectral characteristics, formation constants (KCT), molar extinction coefficients (εCT), thermodynamic parameters, and (b2/a2) values for the CT T2T-2H·DDQH2 molecular
complex in a polar (MeOH) and a non-polar (CHCl3) solvents at different temperatures and, as well as, the ionization potential of the donor.
-1
-1
-1
-1
-1
)
Solvent
ꢁmax (nm)
IP (eV)
KCT (dm3mol
)
εCT at 20 ◦C (dm3 mol-1 cm
)
−ꢂH0 (kJ mol
)
−ꢂS0 (J mol
)
−ꢂG0 (kJ mol
(b2/a2)
10 ◦
C
15 ◦
C
20 ◦
C
25 ◦
C
CHCl3
MeOH
350
350
10.1
10.1
25.41
–
–
45.61
24.78
56.13
36.27
5105.2
49541.5
37.55
86.11
159.67
319.44
9.98
8.89
0.11
0.24
10.87
the corresponding T2T-2H, whereas DDQ is reduced to the corre-
sponding hydroquinone (DDQH2). However, this redox reaction is
not the terminal outcome of the interacting system T2T–DDQ. A
second CT interaction has been occurred between the two species
resulting in situ; the oxidized form (T2T-2H) as a new donor and
the reduced form (DDQH2) as an unconventional new acceptor. The
product (T2T-2H·DDQH2) is a CT complex of the contact-type CT
complexes characterizing by the low formation constants and low
stability that readily breaks down in polar solvents to its interacting
species.
Acknowledgments
The authors thank Dr. M.A. Hassan and Dr. R.F. Fandy (Chemistry
Department, Faculty of Science at Qena, South Valley University) for
help in assignment of 1H NMR and mass spectra.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.04.078.
References
[1] A.R. Fakhari, S.S. Hosseiny Davarani, H. Ahmar, S. Makarem, J. Appl. Elec-
trochem. 38 (2008) 1743–1747.
Fig. 6. Electronic absorption spectra of (T2T–DDQ) mixture solutions in CHCl3
[2] D. Nematollahi, E. Tammari, J. Org. Chem. 70 (2005) 7769–7772.
[3] C. Laurence, M.J. El Ghmari, J.Y. Le Questel, M. Berthelot, R. Mokhlisse, J. Chem.
Soc.: Perkin Trans. 2 (1998) 1545–1551.
[4] J. Leclere, J. Orgiazzi, B. Rousser, J.L. Schlienger, J.L. Wemeau, La Thyroïde, Expan-
sion Scientifique Fran␨aise, Paris, 1992.
at 25 ◦C. ([DDQ] = 2.0 × 10−4 mol dm−3 (constant), [T2T] = (a) 2.0 × 10−3 mol dm−3
,
,
(b) 1.8 × 10−3 mol dm−3
,
(c) 1.6 × 10−3 mol dm−3
,
(d) 1.4 × 10−3 mol dm−3
(e) 1.2 × 10−3 mol dm−3
,
(f) 1.0 × 10−3 mol dm−3
,
(g) 0.8 × 10−3 mol dm−3
, (h)
0.6 × 10−3 mol dm−3
,
(i) 0.4 × 10−3 mol dm−3
,
and (j) 0.2 × 10−3 mol dm−3). The
[5] J.C. Thomes, F. Comby, J.F. Lagorce, J. Buxeraud, C. Raby, Jpn. J. Pharmacol. 58
(1992) 201–207.
spectra have been measured after elapsing 60 min of mixing the reactants.
[6] H. Engler, A. Taurog, T. Nakashima, Biochem. Pharmacol. 31 (1982) 3801–3806.
[7] A. Taurog, Endocrinology 98 (1976) 1031–1046.
(interaction of DDQ and MeOH by themselves) is well known [50],
however, this interaction is characterized by two remarkable obser-
vations: it occurs in a period of 48 h [50], and, in addition, one of the
DDQ/MeOH reaction intermediates has a visible absorption spec-
tral band at 465 nm [50]. None of these observations has been seen,
herein in our case, for the interaction of T2T/DDQ in presence of
MeOH as a solvent.
[8] G.T. Zitouni, S. Sezgin, Acta Pharm. Turc. 40 (1998) 151–154.
[9] (a) D.A. Mc Ginty, W.G. Bywater, J. Pharmacol. 84 (1945) 342;
(b) D.A. Mc Ginty, W.G. Bywater, J. Pharmacol. 85 (1946) 129–139.
[10] O. Gawron, L. Keil, Arch. Biochem. Biophys. 89 (1960) 293–295.
[11] G.R. Handrick, E.R. Atkinson, F.E. Granchelli, R.J. Bruni, J. Med. Chem. 8 (1965)
762–766.
[12] H.S. Mahal, T. Mukherjee, Radiat. Phys. Chem. 54 (1999) 29–37.
[13] C.L. Fang, General Concepts of Additives in the Electroplating Solution, Finishing
Science Publication, Taipei, Taiwan, 1996.
Thus, except the lapse of time required to reach the maximum
intensity of the CT band, the solvent polarity has no effect on the
T2T-2H/DDQH2 CT complex formation. This means that the formed
CT complex does not have a dative structure (i.e., the CT complex
does not exist in the dative D+·A− inner sphere). The CT complex
(T2T-2H·DDQH2) exists, then, in the neutral species (D and A) hav-
ing the structure of the nonbonding outer sphere (i.e., D·A outer
sphere) which gives a supporting evidence for the weak strength
of both the new donor (T2T-2H) and the new acceptor (DDQH2),
resulting in situ through the T2T/DDQ interaction.
[14] L.H. Little, R.H. Ottewill, Can. J. Chem. 40 (1962) 2110–2121.
[15] R. Solmaz, G. Kardas, M. Culha, B. Yazici, M. Erbil, Electrochim. Acta 53 (2008)
5941–5952.
[16] N.L.D. Filho, D.R. do Carmo, F. Gessner, A.H. Rosa, Anal. Sci. 21 (2005)
1309–1316.
[17] U.M. Rabie, M.H.M. Abou-El-Wafa, H. Nassar, Spectrochim. Acta Part A 78 (2011)
512–517.
[18] U.M. Rabie, M.H.M. Abou-El-Wafa, R.A. Mohamed, J. Mol. Struct. 871 (2007)
6–13.
[19] U.M. Rabie, Collect. Czech. Chem. Commun. 71 (2006) 1359–1370.
[20] U.M. Rabie, H.M.A. Salman, M.H.M. Abou-El-Wafa, Bull. Chem. Soc. Jpn. 77
(2004) 681–686.
[21] H.M.A. Salman, M.R. Mahmoud, M.H.M. Abou-El-Wafa, U.M. Rabie, R.H. Crab-
tree, Inorg. Chem. Commun. 7 (2004) 1209–1212.
[22] H.M.A. Salman, U.M. Rabie, E.M. Abd-Alla, Can. J. Anal. Sci. Spectrosc. 49 (2004)
1–7.
4. Conclusions
[23] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2704.
[24] R.L. Scott, Recl. Trav. Chim. Pays-Bas 75 (1956) 787–789.
[25] G.N. Lewis, M. Randall, in: K.S. Pitzed, L. Brewer (Eds.), Thermodynamics, 2nd
ed., McGraw-Hill, NY, 1961.
[26] R. Foster, Organic Charge-Transfer Complexes, Academic Press, London, 1969.
[27] S.N. Bhat, C.N.R. Rao, J. Am. Chem. Soc. 88 (1966) 3216–3217.
The T2T–DDQ system has performed multiple interactions: Two
charge transfer interactions and a redox reaction. A first CT inter-
action has been occurred between T2T and DDQ. This interaction
has led to a redox reaction resulting in dehydrogenation of T2T to

U.M. Rabie et al. / Spectrochimica Acta Part A 79 (2011) 1411–1417
1417
[28] M.R. Mahmoud, M.M.A. Hamed, H.M.A. Salman, Spectrochim. Acta 44 (1988)
1185–1188.
[41] A.A. Hassan, N.K. Mohamed, E.H. El-Tamany, B.A. Ali, A.E. Mourad, Monatsh.
Chem. 126 (1995) 653–662.
[29] M.R. Mahmoud, A.M. El-Nady, A.A. Mohamed, H.M.A. Salman, Gazz. Chim. Ital.
119 (1989) 157–161.
[42] S. Patai, The Chemistry of the Quinonoid Compounds Part 1, John Wiley and
Sons, NY, 1974.
[30] R.D. Srivastava, G. Prasad, Spectrochim. Acta 22 (1966) 1869–1875.
[31] E.M. Kosower, Prog. Phys. Org. Chem. 3 (1965) 81–164.
[32] P. Muller, D. Joly, Helv. Chim. Acta 66 (1983) 1110–1118.
[33] U.M. Rabie, B.P. Patel, R.H. Crabtree, M.R. Mahmoud, Inorg. Chem. 36 (1997)
2236–2238.
[34] U.M. Rabie, M.H. Abou-El-Wafa, R.A. Mohamed, Spectrochim. Acta Part A 68
(2007) 605–611.
[35] Y. Iida, Bull. Chem. Soc. Jpn. 43 (1970) 345–349.
[36] Y. Matsunaga, J. Chem. Phys. 41 (1964) 1609–1613.
[37] J.S. Miller, P.J. Krusic, D.A. Dixon, W.M. Reiff, J.H. Zhang, E.C. Anderson, A.J.
Epstein, J. Am. Chem. Soc. 108 (1986) 4459–4466.
[43] E.A. Braude, L. Jackman, R.P. Linstead, J. Chem. Soc. (1954) 3548–3563,
3564–3568.
[44] C. Preti, G. Tosi, Can. J. Chem. 54 (1976) 1558–1562.
[45] J.B. Lambert, H.F. Shurvell, D.A. Lightner, R.G. Cooks, Introduction to Organic
Spectroscopy, Macmillan Publishing Company, NY, 1987.
[46] A. Lycka, Z. Vrba, M. Vrba, Dyes Pigments 47 (2000) 45–51.
[47] (a) R.S. Mulliken, J. Am. Chem. Soc. 74 (1952) 811–813;
(b) R.S. Mulliken, J. Phys. Chem. 56 (1952) 801–805;
(c) R.S. Mulliken, Recl. Trav. Chim. Pays-Bas 57 (1956) 845–852.
[48] R.S. Mulliken, W.B. Pearson, Molecular Complexes – A Lecture and Reprint
Volume, Wiley Interscience, NY, 1969.
[38] E. Gebert, A.H. Reis Jr., J.S. Miller, H. Rommelmann, A.J. Epstein, J. Am. Chem.
Soc. 104 (1982) 4403–4410.
[39] R.L. Brandon, J.H. Osiecki, A. Ottenberg, J. Org. Chem. 31 (1966) 1214–1217.
[40] R.D. Srivastava, G. Prasad, Acta Chim. (Bp.) 75 (1973) 281–286.
[49] G.G. Aloisi, S. Pignataro, J. Chem. Soc.: Faraday Trans. 69 (1) (1973) 534–539.
[50] K. Tanemura, Y. Nishida, T. Suzuki, K. Satsumabayashi, T. Horaguchi, J. Chem.
Res. (S) (1999) 40–41.