Electron donor-acceptor complexes of 2,3-dichloro-5-nitro-1,4- naphthoquinone with some methyl substituted anilines: Formation of 1:2 (A:D) complexes

Article abstract of DOI:10.1016/j.saa.2003.09.017

The donor-acceptor complexes between 2,3-dichloro-5-nitro-1,4- naphthoquinone (DClNNQ) and some methyl substituted anilines were investigated by spectrophotometric method. This investigation was carried out in three different chlorinated solvents viz., chloroform, dichloromethane and 1,2-dichloroethane. Experimental evidences shows the formation of 1:2 (A:D) complexes but not 1:1 complexes. The calculated values of the oscillator strength and transition moment confirms this suggestion. The stoichiometry of the complexes was unaffected by the variation of temperature over a small interval and also change in solvent. The thermodynamic and spectroscopic parameters were evaluated in all the three solvents for the formation of CT-complexes. The influence of used solvents on the thermodynamic and spectroscopic parameters was investigated. The strength of the donors was also investigated.

Full text of DOI:10.1016/j.saa.2003.09.017

Spectrochimica Acta Part A 60 (2004) 1793–1803
Electron donor–acceptor complexes of 2,3-dichloro-5-nitro-
1,4-naphthoquinone with some methyl substituted anilines:
formation of 1:2 (A:D) complexes
∗
Gururaj M. Neelgund , M.L. Budni
P.G. Department of Studies in Chemistry, Karnatak University, Dharwad-580003, India
Received 11 July 2003; received in revised form 19 August 2003; accepted 3 September 2003
Abstract
The donor–acceptor complexes between 2,3-dichloro-5-nitro-1,4-naphthoquinone (DClNNQ) and some methyl substituted anilines were
investigated by spectrophotometric method. This investigation was carried out in three different chlorinated solvents viz., chloroform,
dichloromethane and 1,2-dichloroethane. Experimental evidences shows the formation of 1:2 (A:D) complexes but not 1:1 complexes.
The calculated values of the oscillator strength and transition moment confirms this suggestion. The stoichiometry of the complexes was
unaffected by the variation of temperature over a small interval and also change in solvent. The thermodynamic and spectroscopic parame-
ters were evaluated in all the three solvents for the formation of CT-complexes. The influence of used solvents on the thermodynamic and
spectroscopic parameters was investigated. The strength of the donors was also investigated.
©
2003 Elsevier B.V. All rights reserved.
Keywords: 2,3-Dichloro-5-nitro-1,4-naphthoquinone (DClNNQ); CT-complex; Spectrophotometric method; Thermodynamic parameters; 1:2 (A:D)
complexes
1
. Introduction
that Benesi–Hildebrand plots are linear even where a sec-
ond equilibrium corresponding to 1:2 (A:D) complex is
involved, this linearity has been taken in a number of
instances as evidence for 1:1 complex formation.
For a reaction involving 1:1 stoichiometric complex for-
mation between acceptor A and donor D, viz.,
Atoms combine in a definite proportion to form a
molecule. Similarly, two or more molecules can combine to
give a new complex molecule. Benesi and Hildebrand [1]
discovered this 55 years ago while studying the effect of
various solvents on the absorption spectrum of molecular
iodine. Recently, charge-transfer (CT) complexes are re-
ceiving great deal of attention for their potential nonlinear
optical activity [2–5]. In the determination of equilibrium
constants of weak complexes of the donor–acceptor type by
the use of optical data, the method of Benesi–Hildebrand
A + D ꢀ AD
The equilibrium constant, K CA D, may be expressed as:
CAD
AD
C
K
=
(1)
0
0
D
C C
A
[
1] or some modification thereof [6,7] has, for the most
0
0
where C and C are the initial molar concentrations of the
A
D
part, been employed, assuming 1:1 stoichiometry for the
complex. There have been several anomalies in the re-
sults thus obtained, some of which have been attributed
to the presence of termolecular complexes [6,7]. Quantita-
tive treatment of complexes other than 1:1 stoichiometry,
has rarely been attempted. Although it has been noticed
acceptor and donor respectively and CAD is the concentra-
0
D
0
A
tion of the complex. Assuming that C ꢀ C in all mea-
surements, Rose and Drago [8], have derived an equation of
the form:
0
0
D
1
d
C C
A
0
A
0
D
AD
λ
=
− (C + C ) +
ε
(2)
AD
AD
λ
K
ε
d
C
∗
where ‘d’ is the measured absorbance of the complex at
its λmax in a cell of 10 mm path length, ελ , the molar
Corresponding author.
AD
E-mail address: gneelgund@yahoo.com (G.M. Neelgund).
1
386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2003.09.017

1
794
G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
0
0
1
,2-dichloroethane (Ranbaxy) were used without further
absorptivity of the complex at ␭max. A plot of C C /d
A
D
0
0
purifications.
versus (C +C ) would then be expected to give a linear plot
A
D
AD −1
AD AD −1
. The
The acceptor used in the present investigation, 2,3-
dichloro-5-nitro-1,4-naphthoquinone (DClNNQ) was pre-
pared by a modified method due to Wilbur and Day [10]
and the detailed procedure is described further.
with slope of (ε
)
and an intercept of (K
ε )
␭
␭
C
linearity of such a plot can be considered to be a test for 1:1
stoichiometry of the CT-complexes. Optical measurements
in many of the above cases are made under the condition
0
D
0
A
C ꢀ C , and it has been found that the Eq. (2) is often
2
2
.1. Preparation of
,3-dichloro-5-nitro-1,4-naphthoquinone
wavelength dependent and the experimental plots are not
always linear.
0
D
0
A
Under the condition C ꢀ C , there is a possibility of
Fifty grams of 2,3-dichloro-1,4-naphthoquinone (Fluka)
formation of 1:2 (A:D) stoichiometric complexes. A few
was mixed with 65 ml of concentrated H2SO4. One hun-
dred and thirty millilitre of fuming HNO3 was then added
drop-wise with constant stirring. A vigorous exothermic re-
action sets in after the addition of about 31 ml of nitric acid
so that cooling was necessary to keep the temperature of the
reaction below 100 C. Above the temperature of 120 C,
there is a possibility of decomposition. The remaining nitric
acid was added slowly by maintaining the temperature be-
attempts have been made to treat them on the basis of
:2 (A:D) stoichiometry. Jayadevappa and Nagendrappa [9]
have suggested an equation for systems of 1:2 (A:D) stoi-
chiometry under simplifying assumption that AD2 complex
is the only one of significance at the wavelength of mea-
surement. For an equilibrium of the kind.
1
◦
◦
A + 2D ꢀ AD2
◦
tween 80 and 90 C. The mixture was heated at this temper-
CAD2
AD2
C
K
=
(3)
(4)
ature for about 7 h and then poured on to crushed ice when
a yellow product was separated. The solid was filtered off
and washed thoroughly with water and stirred well for about
2
0
0
D
C C
A
The equation derived by them [9] has the form:
8
h with 1N sodium carbonate solution. The solid, then, was
0
02
D
0
D
0
A
0
D
separated, washed with water and dried. The product was
repeatedly recrystallized from chloroform using decoloriz-
ing carbon to get the yellow needle-like crystals. The purity
of the compound was confirmed by TLC, which showed a
sharp single spot without tail. The m.p. of the compound
C C
A
1
(C + 4C )C
=
+
AD2 AD2
C
AD2
d
K
ε
ε
λ
λ
2
0
A
0
0
D
0
A
0
D
A plot of C C /d versus (C +4C )C , would then be
D
AD2 −1
AD2
expected to give a straight line with a slope (ε
)
and an
◦
λ
was found to be 176–177 C, which agrees very well with
the reported value [10].
AD2 AD2
AD2 −1
intercept, (KC
ε
and the values of KC and ελ
)
λ
may be determined.
In the present investigation, a less studied accep-
tor, 2,3-dichloro-5-nitro-1,4-naphthoquinone (DClNNQ)
and its CT-interaction with some methyl substituted
anilines, viz., 2,5-dimethylaniline, 2,6-dimethylaniline, 3,4-
dimethylaniline, 3,5-dimethylaniline and 2,4,6-trimethylan-
iline (mesidine), has been undertaken to establish the sto-
ichiometry of the complexes and to study their properties.
These studies have been made in three different solvents such
as chloroform, dichloromethane and 1,2-dichloroethane.
The spectroscopic and thermodynamic parameters of these
complexes have also been determined in all the three sol-
vents used.
The values of C, H and N obtained from ThermoQuest
elemental analyzer are given below, which agree very well
with the expected values (shown in brackets).
C : 44.18 (44.15); H : 1.23 (1.10); N : 5.21 (5.15) (%)
Further, the formation of syntesised compound was con-
firmed by the FT-IR (Nicolet Impact 410) and NMR
(Brucker, 300 MHz) techniques.
2
. Experimental
The electronic absorption measurements were made on
the Cary-50 double beam Bio UV-Vis Spectrophotometer
using a matched pair of quartz cuvettes of 10 mm path length
with airtight teflon stoppers. Stock solutions of DClNNQ
and donors were prepared by directly weighing the required
quantities of the respective compound in a ‘A’ grade vol-
umetric flask. Freshly prepared solutions of DClNNQ and
donors were mixed just before recording the spectra. An in-
stantaneous colour developed on mixing the donor and ac-
ceptor solutions indicating the formation of CT-complexes
Out of five different donors used in the present investi-
gation, 2,5-, 2,6-, 3,4- and 3,5-dimethylanilines were Fluka
samples and 2,4,6-trimethylaniline was an Aldrich sample.
All the donors except 3,4-dimethylaniline were dried over
caustic potash (Merck) and distilled twice just before use,
whereas 3,4-dimethylaniline was distilled under reduced
pressure in the dry conditions. The spectral grade solvents,
viz., chloroform (s.d. fine), dichloromethane (Merck) and

G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
1795
and were stable for several hours. The concentration of
DClNNQ was held constant while that of the concentration
of aromatic anilines were varied for each set of measure-
ments. The precision in reading and recording the spectrum
was ± 0.2 nm. The photometric value could be read with
an accuracy of ± 0.0004. The peltier accessory was used to
maintain the temperature at the desired value and was main-
is enhanced by the introduction of electron withdraw-
ing substituents like -chloro, -nitro and -cyano, in the
1,4-naphthoquinone ␲-system.
The broad CT-bands were observed in the visible region
for all the cases studied where neither the acceptor nor the
donor absorbed, pure solvent was used in the reference beam
for all the systems and solvents. The concentration of donor
was in large excess over that of the acceptor for every mea-
surement, in which always maintained the condition that
◦
tained constant to within ± 0.1 C. The absorption spectra
of the complex solutions were recorded at a low scan speed.
Sufficient time was allowed for the solution to attain ther-
mal equilibrium before each spectrum was recorded. As far
as possible, all the solutions were protected from direct sun-
light.
0
0
C ꢀ C .
D
A
In the present study, the pale yellow coloured solu-
tion of DClNNQ and colourless solutions of di- and
trimethylanilines in all the three solvents, viz., chloroform,
dichloromethane and 1,2-dichloroethane, form character-
istic colours instantaneously after mixing. The change of
colour is attributed to the formation of CT-complexes be-
tween the DClNNQ and methyl substituted anilines. The
typical absorption spectra of CT-complexes formed be-
tween DClNNQ and 3,4-dimethylaniline in chloroform at
3
. Results and discussion
The DClNNQ is a ␲-acceptor and forms CT-complexes
with aromatic hydrocarbons [11] and amines [12]. The
average electron affinity of DClNNQ is 1.18 eV [12], as
compared to 0.72 eV for 1,4-naphthoquinone [12]. It has
been established [12] that the electron affinity of DClNNQ
◦
20 C is shown in Fig. 1, wherein the concentration of
DClNNQ was held constant and the concentration of the
3,4-dimethylaniline was varied.
Fig. 1. Absorption spectra of 2,3-dichloro-5-nitro-1,4-naphthoquinone-3,4-dimethylaniline complex at various concentrations of the donor in chloroform
◦
at 20 C.

1
796
G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
Fig. 2. Rose–Drago plot for 2,3-dichloro-5-nitro-1,4-naphthoqui-
none-3,4-dimethylaniline complex in chloroform for (1:1) stoichiometry
at 50 C.
Fig. 3. Rose–Drago plot for 2,3-dichloro-5-nitro-1,4-naphthoquinone-
3,4-dimethylaniline complex in chloroform for 1:2 (A:D) stoichiometry.
◦
In all the systems studied, the absorption spectra are of
similar nature except for the position of absorption maxima
AD2
Although the formation constant, KC and molar extinc-
tion coefficient, ε Aλ D2 , of the CT-complexes can be calcu-
lated by the use of Rose–Drago linear plots for 1:2 (A:D)
stoichiometry, also, we have estimated these parameters for
all the systems by using the Rose–Drago intersection plots
[8] because of greater reliability in spite of tedious na-
ture. A typical Rose–Drago intersection plot for DClNNQ
(λmax) of the complexes. In the present investigation, the
λmax was found to appear on the longer wavelength side of
the visible region. The wavelength (λmax), frequency (νmax)
and wave numbers (νmax) corresponding to the absorption
maxima of these complexes in all the three solvents studied
have been presented in the Table 1. A closer look at the
Table 1 indicating the stronger interaction between DClNNQ
and aromatic anilines [13,14]. The experimental data shows
the change in dielectric constant has little effect on the λmax
values of the complexes.
◦
with 3,4-dimethylaniline in chloroform at 30 C is shown in
Fig. 4. The values of these parameters thus obtained along
with the associated errors are given in Table 2. The λmax for
each system remains almost constant irrespective of change
in temperature in a given solvent, this shows the stoichiom-
etry of the complexes may be 1:2 (A:D) and is not altered
by the change in temperature.
It has been reported that the 1,4-naphthoquinone forms 1:2
ꢁ
A:D) stoichiometric complexes with N-methyl- and N,N -
(
dimethylaniline [9], whereas 2,3-dichloro-1,4-naphthoquin-
one forms 1:1 stoichiometric complexes for the same set
of donors [15]. In view of this, it was of interest to study
the CT-complex formation between DClNNQ with aro-
matic anilines. When the experimental data was subjected
to Rose–Drago treatment for 1:1 stoichiometry (Eq. 2), the
plots were found to be scattered. Such a typical plot for
one of the systems studied is shown in Fig. 2. This sug-
gests the assumption of 1:1 stoichiometry would have to
be reviewed. Hence the experimental data was, therefore,
analyzed in accordance with Eq. (4), under the assumption
that the equilibrium:
A + 2D ꢀ AD2
is virtually the only one of significance. Typical Rose–Drago
plot for 1:2 (A:D) stoichiometry obtained from the Eq. (4)
for one of the systems studied is shown in Fig. 3. All the com-
plexes of DClNNQ shows good linear plots when a graph
2
0
A
0
0
D
0
A
0
D
of (C C )/d versus C (C + 4C ) was plotted at all the
D
Fig. 4. Rose–Drago intersection plot for 2,3-dichloro-5-nitro-1,4-naphtho-
quinone-3,4-dimethylaniline complex in chloroform for 1:2 (A:D) stoi-
four temperatures and in all the solvents used indicating the
◦
stoichiometry of the complexes is 1:2 (A:D).
chiometry at 30 C.

G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
1797
Table 1
The absorption maxima of the charge-transfer complexes of 2,3-dichloro-5-nitro-1,4-naphthoquinone with different aromatic anilines in different solvents
◦
at 30 C
νmax × 10− (s
14
−1
)
−1
νmax (cm )
Donor
Solvent
λmax (nm)
2,4,6-Trimethylaniline
3,4-Dimethylaniline
3,5-Dimethylaniline
2,5-Dimethylaniline
2,6-Dimethylaniline
Chloroform
Dichloromethane
498.9
496.7
497.4
6.0132
6.0398
6.0314
20056
20132
20358
1,2-dicloroethane
Chloroform
Dichloromethane
562.4
560.6
565.0
5.3343
5.3514
5.3097
17815
17908
17857
1,2-Dicloroethane
Chloroform
Dichloromethane
530.2
526.8
530.6
5.6582
5.6948
5.6538
18942
19007
18939
1,2-Dicloroethane
Chloroform
Dichloromethane
550.0
547.0
548.8
5.4545
5.4844
5.4664
18195
18261
18248
1,2-Dicloroethane
Chloroform
Dichloromethane
556.2
554.2
558.4
5.3937
5.4132
5.3725
18005
18001
17943
1,2-Dicloroethane
The observation of the Table 2 reveals the molar extinc-
tion coefficient, ελ , of all the complexes remains almost
group and also the two –CH3 groups present in this donor
are in para position to each other, 2,5-dimethylaniline
occupies the next lower position in the series in respect
of donor strength. The 2,6-dimethylaniline appears to be
weaker than 2,5-dimethylaniline. The two –CH3 groups in
AD2
constant at different temperatures in a given solvent and the
AD2
large values of formation constants, KC were itself indica-
tive of the 1:2 (A:D) stoichiometry of these complexes. The
AD2
2,6-dimethylaniline are ortho position to –NH group, thus
2
high values of formation constants (KC ) as are evident in
one would expect that this is to be stronger donor than the
the Table 2, are a pointer to the strength of the binding be-
tween DClNNQ with methyl substituted aromatic anilines
and the greater stability of the resultant CT-complexes. The
2
–
,5-dimethylaniline but strong steric hindrance between the
CH3 and –NH2 groups reverse the expectation. Thus the
AD2
electron releasing property of 2,6-dimethylaniline is some
what further reduced and it appears to be weaker than
relative magnitude of KC at the lowest temperature of
measurement in chloroform was used to determine the rel-
ative strength of donors. On the basis of this, the order of
donor strength was found to be:
2,5-dimethylaniline.
In all the five donors studied, the relative strength of the
donors was found to be as per expectation. Of the all the
donors, 2,4,6-trimethylaniline appears to be the strongest
due to the combined effect of the three –CH3 groups. The
methyl groups are well known for their electron repelling
effects and in this donor they are symmetric to each other,
they contribute maximally to the ␲-electron density of the
ring. 3,4-Dimethylaniline appears to be next in the series
due to the combined effect of two –CH3 groups, one of
which is in para position to the –NH2 group. In case of
3
.1. Thermodynamic parameters
The formation of CT-complexes is temperature depen-
dent, the temperature dependence of the formation constants,
K
AD2
C
, may be used to calculate the thermodynamic param-
eters. The van’t Hoff plots were used to calculate the ther-
modynamic parameters. Hence, good linear plots were ob-
AD2
−1
tained when log KC versus T was plotted, the values
of ꢀH, ꢀG and ꢀS were calculated using these plots. A
typical plot is shown in Fig. 5.
3,5-dimethylaniline both the –CH3 groups are in meta po-
sition to the –NH2 group, thus its electron repelling effect
is some what reduced and making it next in the order
of donor strength. In case of 2,5-dimethylaniline, one of
the methyl groups is in the ortho position to the –NH2
The molar extinction coefficients, ε Aλ D2 , were found to be
independent of temperature, it may be taken as the further
proof that the stoichiometry of these complexes is unaffected

1
798
G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
Table 2
AD2
AD2
Formation constants (KC ) and molar extinction coefficients (ελ ) of 2,3-dihloro-5-nitro-1,4-naphthoquinone with various aromatic anilines in different
solvents at different temperatures
◦
AD2
C
−1
AD2
λ
−1
−1
Donor
Solvent
Temperature ( C)
K
(l mol
)
ε
(l mol cm )
2,4,6-Trimethylaniline
3,4-Dimethylaniline
3,5-Dimethylaniline
2,5-Dimethylaniline
2.6-Dimethylaniline
Chloroform
20
273.76 ± 2.40
269.66 ± 2.49
261.28 ± 2.38
257.50 ± 2.30
283.57 ± 2.09
272.90 ± 2.05
269.38 ± 2.03
262.23 ± 2.00
306.91 ± 4.62
291.66 ± 3.64
288.87 ± 3.43
279.96 ± 3.29
237.80 ± 3.07
233.78 ± 2.75
232.94 ± 2.76
230.76 ± 2.53
312.47 ± 4.16
308.67 ± 3.75
306.76 ± 3.23
305.54 ± 3.69
216.63 ± 2.42
215.42 ± 2.23
214.39 ± 2.20
213.73 ± 2.20
128.47 ± 1.17
127.36 ± 1.13
125.81 ± 1.00
123.01 ± 0.90
132.05 ± 1.19
129.57 ± 1.11
125.07 ± 0.98
123.96 ± 0.97
142.35 ± 1.36
140.06 ± 1.19
138.19 ± 1.07
134.50 ± 1.11
84.57 ± 0.68
83.71 ± 0.57
82.82 ± 0.53
81.49 ± 0.49
84.88 ± 0.61
83.93 ± 0.57
82.44 ± 0.53
81.72 ± 0.51
87.41 ± 0.64
86.86 ± 0.55
85.20 ± 0.53
84.96 ± 0.48
255.25 ± 3.4
251.46 ± 3.3
249.00 ± 3.3
246.44 ± 3.3
251.89 ± 3.7
247.27 ± 3.6
243.75 ± 3.3
241.42 ± 3.6
301.50 ± 2.4
212.37 ± 2.2
201.05 ± 2.2
195.84 ± 2.2
344.69 ± 2.8
294.22 ± 2.7
248.94 ± 2.4
227.43 ± 2.4
275.12 ± 2.6
252.00 ± 2.6
226.70 ± 2.7
214.27 ± 2.4
372.35 ± 4.4
304.78 ± 3.4
257.81 ± 3.1
225.11 ± 2.9
362.82 ± 3.7
306.77 ± 3.4
259.91 ± 3.4
230.56 ± 3.3
319.31 ± 3.5
294.20 ± 3.5
262.89 ± 3.4
246.89 ± 3.3
349.98 ± 4.4
284.85 ± 3.4
233.47 ± 3.1
211.35 ± 2.9
351.58 ± 4.2
304.25 ± 4.3
263.14 ± 4.1
228.56 ± 4.0
317.94 ± 4.2
292.73 ± 4.2
268.00 ± 4.1
248.64 ± 4.0
301.25 ± 4.1
258.14 ± 4.1
221.60 ± 3.8
187.36 ± 3.7
30
40
50
Dichloromethane
20
25
30
35
1,2-Dichloroethane
20
30
40
50
Chloroform
20
30
40
50
Dichloromethane
20
25
30
35
1,2-Dichloroethane
20
30
40
50
Chloroform
20
30
40
50
Dichloromethane
20
25
30
35
1,2-Dichloroethane
20
30
40
50
Chloroform
20
3
4
5
0
0
0
Dichloromethane
20
2
3
3
5
0
5
1,2-Dichloroethane
20
3
4
5
0
0
0
Chloroform
20
44.01 ± 0.24
43.88 ± 0.22
43.13 ± 0.21
42.18 ± 0.20
59.36 ± 0.34
54.00 ± 0.28
52.57 ± 0.28
51.02 ± 0.26
56.40 ± 0.29
55.76 ± 0.28
54.79 ± 0.26
50.92 ± 0.30
327.81 ± 4.6
290.18 ± 4.5
256.56 ± 4.3
225.09 ± 4.1
273.26 ± 4.1
258.77 ± 4.2
252.81 ± 4.1
238.89 ± 4.0
315.07 ± 4.7
276.73 ± 4.5
232.51 ± 4.2
206.25 ± 3.5
3
4
5
0
0
0
Dichloromethane
20
2
3
3
5
0
5
1,2-Dichloroethane
20
3
4
5
0
0
0

G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
1799
where ε mλ ax is the molar extinction coefficient of the
CT-complex at the wavelength of maximum absorption and
ν
is the half band-width in wave number units. The val-
.5
0
ues of f and µEN thus obtained are given in the Table 4. The
closer inspection of this table reveals the near constancy
of both the parameters in all the four temperatures in a
given solvent. This is an additional support for 1:2 (A:D)
stoichiometry of all the complexes studied.
3.3. Ionization potential of the donors
One of the very important applications of the CT-
complexes is to calculate the ionization potentials of the
donors. Briegleb and Czekalla [19] have described a method
of determining the ionization potentials of the electron
donors from νCT, of the complexes. It may be difficult
to determine ionization potentials by other methods such
as Rydberg series, photoionization and electron impact,
because of the low vapour pressure of some of these sub-
stances. Based on the theoretical relationship, Parrel and
Newton [20] suggested an empirical relation between ion-
ization potential and hνCT. The ionization potentials of
the donors which form complexes with DClNNQ, may be
calculated from the Eq. (7),
Fig. 5. van’t Hoff plot for 2,3-dichloro-5-nitro-1,4-naphthoquinone-3,4-
dimethylaniline complex in 1,2-dichloroethane.
by the variation of the temperature which is in agreement
with the earlier reports [16,17]. The thermodynamic param-
eters thus evaluated are given in the Table 3.
3
.2. Spectral properties of CT-complexes
D
Further evidence in support of our findings in respect of
hνCT = 0.21 I − 4.28 eV
(7)
the stoichiometry of the CT-complexes investigated were the
spectroscopic parameters like oscillator strength, f and the
transition moment, µEN. These properties were evaluated
by using the approximate equations due to Tsubumora and
Lang [18].
D
where I is the ionization potential of the donor molecule.
The hνCT values of the complexes were determined
in three different halocarbon solvents, viz., chloroform,
dichloromethane and 1,2-dichloroethane. The values of
ionization potentials thus determined are given in Table 5.
The relation between h␯CT of the lowest intermolecular
CT-band and the ionization potentials of the donors with a
certain acceptor has been given by McConnel et al. [21], in
the form:
f ≈ 4.32 × 10− × ελ
9
max
ν
(5)
0.5
and
ꢀ
ꢁ1/2
max
ε
× ꢀν0.5
λ
µEN(Debyes) ≈ 0.0958
(6)
νmax
D
A
hνCT = I − E − W
(8)
Table 3
◦
Thermodynamic parameters of the complexes of 2,3-dichloro-5-nitro-1,4-naphthoquinone with various aromatic anilines in different solvents at 20 C
−ꢀH (kJ mol−1)
−ꢀS (J mol−1
K
−1
)
−ꢀG (kJ mol−1
)
Donor
Solvent
2,4,6-Trimethylaniline
3,4-Dimethylaniline
3,5-Dimethylaniline
2,5-Dimethylaniline
2,6-Dimethylaniline
CHCl3
CH2Cl2
C2H4Cl2
1.6920 ± 0.004
3.7207 ± 0.008
2.2542 ± 0.004
0.7412 ± 0.004
1.1055 ± 0.008
0.3568 ± 0.004
1.1126 ± 0.008
3.3794 ± 0.016
1.4372 ± 0.012
0.9533 ± 0.004
1.9806 ± 0.012
0.8512 ± 0.004
40.88 ± 1.43
34.25 ± 1.12
39.91 ± 1.76
42.95 ± 2.15
43.99 ± 2.36
43.49 ± 2.17
36.57 ± 1.89
29.06 ± 1.61
36.31 ± 1.16
33.63 ± 1.80
30.16 ± 1.63
34.30 ± 1.93
13.67 ± 0.133
13.75 ± 0.117
13.95 ± 0.121
13.33 ± 0.104
13.99 ± 0.121
13.10 ± 0.112
11.82 ± 0.079
11.89 ± 0.071
12.08 ± 0.066
10.80 ± 0.100
10.82 ± 0.108
11.07 ± 0.104
CHCl3
CH2Cl2
C2H4Cl2
CHCl3
CH2Cl2
C2H4Cl2
CHCl3
CH2Cl2
C2H4Cl2
CHCl3
CH2Cl2
C2H4Cl2
1.1252 ± 0.004
7.2537 ± 0.025
1.6543 ± 0.008
27.62 ± 1.55
09.19 ± 0.82
27.87 ± 1.52
09.22 ± 0.158
09.95 ± 0.175
09.82 ± 0.171

1
800
G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
Table 4
Spectroscopic parameters of the complexes of 2,3-dichloro-5-nitro-1,4-naphthoquinone with some aromatic anilines in different solvents at different
temperatures
◦
−1
−1
ν0.5 (cm )
Donor
Solvent
CHCl3
Temperature ( C)
νmax (cm
)
f
µEN (Debyes)
2,4,6-Trimethylaniline
3,4-Dimethylaniline
3,5-Dimethylaniline
2,5-Dimethylaniline
2,6-Dimethylaniline
20
19924.76
20044.42
20084.84
20165.68
20112.64
20082.37
20131.30
20206.10
20086.79
21104.55
20106.24
20167.84
56811.02
54922.43
54338.65
56101.88
56199.38
55799.04
56370.98
55450.82
54933.73
61056.75
56588.11
57131.66
0.08356
0.07110
0.06245
0.05839
0.05583
0.07772
0.07522
0.07016
0.05220
0.06632
0.05865
0.05870
2.9848
2.7451
2.5700
2.4801
2.4284
2.8670
2.8174
2.7161
2.3496
2.5838
2.4893
2.4865
3
4
5
0
0
0
CH2Cl2
20
2
3
3
5
0
5
C2H4Cl2
20
3
4
5
0
0
0
CHCl3
20
18804.20
18860.98
19013.73
19092.08
18917.97
18983.29
18998.48
19135.39
18853.21
18846.87
18948.04
19113.19
51009.08
50498.81
48935.08
48529.62
50282.23
49375.44
50085.05
49059.28
50235.99
50656.11
49771.74
48457.15
0.07995
0.06692
0.05494
0.04833
0.06936
0.06275
0.05688
0.05232
0.07595
0.06233
0.05020
0.04424
3.0053
2.7455
2.4777
2.3191
2.7908
2.6500
2.5220
2.4199
2.9254
2.6506
2.3724
2.2175
3
4
5
0
0
0
CH2Cl2
C2H4Cl2
20
2
3
3
5
0
5
20
3
4
5
0
0
0
CHCl3
20
18804.20
18860.98
19013.73
19092.08
18917.97
18983.29
18998.48
19135.39
18853.21
18846.87
18948.04
19113.19
51009.08
50498.81
48935.08
48529.62
50282.23
49375.44
50085.05
49059.28
50235.99
50656.11
49771.74
48457.15
0.07995
0.06692
0.05494
0.04833
0.06936
0.06275
0.05688
0.05232
0.07595
0.06233
0.05020
0.04424
3.0053
2.7455
2.4777
2.3191
2.7908
2.6500
2.5220
2.4199
2.9254
2.6506
2.3724
2.2175
3
4
5
0
0
0
CH2Cl2
C2H4Cl2
20
2
3
3
5
0
5
20
3
4
5
0
0
0
CHCl3
20
18187.49
18181.18
18208.46
18199.70
18201.05
18272.27
18280.25
18287.16
18169.52
18222.39
18268.26
18327.96
48321.16
48668.03
48968.86
49535.73
49620.06
47961.03
48481.93
48589.93
49236.18
48953.42
49109.69
48667.43
0.07339
0.06396
0.05566
0.04891
0.06815
0.06065
0.05613
0.05219
0.06407
0.05459
0.04701
0.03939
2.9278
2.7339
2.5484
2.3894
2.8204
2.6553
2.5540
2.4623
2.7370
2.5227
2.3382
2.1367
3
4
5
0
0
0
CH2Cl2
C2H4Cl2
20
2
3
3
5
0
5
20
3
4
5
0
0
0
CHCl3
20
17889.18
17979.55
18050.61
18106.98
17914.43
17966.24
18043.59
18090.06
17831.11
17907.70
17928.29
18103.27
50718.91
49155.68
48094.27
48303.99
49933.40
49828.96
48584.89
48497.23
51259.53
49924.97
50424.35
48411.18
0.07182
0.06162
0.05330
0.04697
0.05894
0.05570
0.05306
0.05005
0.06977
0.05968
0.05065
0.04109
2.9205
2.6982
2.5047
2.3475
2.6438
2.5664
2.4994
2.4243
2.8831
2.6608
2.4498
2.1959
3
4
5
0
0
0
CH2Cl2
C2H4Cl2
20
2
3
3
5
0
5
20
3
4
5
0
0
0

G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
1801
Table 5
D
Ionization potentials (I ) of donors, dissociation energies (W) of the charge-transfer excited states and hνCT of the 2,3-dichloro-5-nitro-1,4-naphthoquinone
with different aromatic anilines in different solvents
Donor
Solvent
ID (eV)
hνCT (eV)
W (eV)
2,4,6-Trimethylaniline
3,4-Dimethylaniline
3,5-Dimethylaniline
2,5-Dimethylaniline
2,6-Dimethylaniline
Chloroform
Dichloromethane
8.26
8.27
8.30
2.4917
2.5012
2.5287
4.5864
4.5885
4.5946
1,2-Dichloroethane
Chloroform
Dichloromethane
7.92
7.93
7.92
2.2133
2.2248
2.2185
4.5253
4.5279
4.5265
1,2-Dichloroethane
Chloroform
Dichloromethane
8.09
8.10
8.09
2.3534
2.3619
2.3529
4.5561
4.5579
4.5560
1,2-Dichloroethane
Chloroform
Dichloromethane
7.98
7.99
7.98
2.2605
2.2687
2.2671
4.5357
4.5375
4.5371
1,2-Dichloroethane
Chloroform
Dichloromethane
7.95
7.95
7.94
2.2369
2.2364
2.2292
4.5305
4.5304
4.5288
1,2-Dichloroethane
where W is the dissociation energy of the charge-transfer
excited state. Though this relation gives a linear plot when
hνCT versus ID is plotted which, some times, can be used
to determine the ionization potentials of weak donors, the
ear and typical plot of such type in chloroform is shown in
Fig. 7.
3.4. Effect of solvent on the formation of CT-complexes
D
linearity is questionable over a wide range of I values
[
22].
The experimental results of CT-interaction between
It was reported that the plot of hνCT versus ID is linear
DClNNQ with various di- and trimethyl anilines in differ-
for complexes of iodine [21], 1,3,5-trinitrobenzene [19] and
chloranil [19] for a number of donors. It may be consid-
ered as independent of donors [22]. A more detailed con-
sideration of the energetics of CT-process has resulted in
an alternative relationship between hνCT and I for com-
plexes with a given acceptor by Masten and co-workers
AD2
ent solvents shows the values of formation constant, KC
,
molar extinction coefficient, ε Aλ D2 , spectroscopic and ther-
modynamic properties were markedly affected by the varia-
tion in solvent polarity in which measurements were carried
out.
D
[23].
2
2
β
D
hνCT = I − C +
(9)
(
ID − C)
where both ‘C’ and ‘β’ are approximately constant. Mulliken
and Person [24] have identified ‘C’ with the sum of vertical
electron affinity of the acceptor and the attraction energy
between the component molecules in the CT-process and ‘β’
with the resonance integral between electron donating and
accepting orbitals. While the equation is one of a parabola
D
rather than a straight line, a plot of hνCT versus I is expected
D
to give a straight line when (I − C) is greater than β.
Fig. 6 is such a plot for the acceptor studied with different
donors, which shows a linear relationship. The lower slope is
interpreted [25] as being characteristic of higher resonance
stabilization.
The ionization potentials of methylated anilines obtained
from the longer wavelength CT-bands were found to be
nearly constant and ranged between 7.99 and 8.37 eV. The
dissociation energy of the charge-transfer excited state, W,
of the complexes was evaluated by using the Eq. (8) and are
D
Fig. 6. The plot of hνCT vs.
2
3
I
of a series of donors for
,3-dichloro-5-nitro-1,4-naphthoquinone systems in chloroform. (ꢂ)
,4-Dimethylaniline; (ᮀ) 2,6-dimethylaniline; (+) 2,5-dimethylaniline;
D
included in the Table 5. The plot of W versus I is also lin-
(᭺) 3,5-dimethylaniline and (᭛) 2,4,6-trimethylaniline.

1
802
G.M. Neelgund, M.L. Budni / Spectrochimica Acta Part A 60 (2004) 1793–1803
medium. The present investigations reveal that the charac-
teristic properties of the complexes are solvent dependent.
4. Conclusions
The UV-Vis spectrophotometric method for the study
of CT-complexes of DClNNQ reveals that it forms 1:2
A:D) complexes with various aromatic anilines in all
(
the three halocarbon solvents studied, viz., chloroform,
dichloromethane and 1,2-dichloroethane. In all the sys-
tems, the stoichiometry is unaltered by the variation of
temperature and also change in solvent. The formation
AD2
AD2
constants, KC
and molar extinction coefficients, ελ
of all the complexes were evaluated by the Rose–Drago
method. The thermodynamic and spectroscopic parameters
have been calculated which confirms the stoichiometry of
the CT-complexes is 1:2 (A:D). The ionization potential
values for all the donors studied were evaluated. From
the study of these CT-complexes in different solvents,
it may be concluded that the magnitudes of formation
of CT-complexes are highly influenced by the solvent.
Fig. 7. The plot of W vs. ID of a series of donors for 2,3-dic-
hloro-5-nitro-1,4-naphthoquinone systems in chloroform. (᭺) 3,4-Dime-
thylaniline; (ᮀ) 2,6-dimethylaniline; (+) 2,5-dimethylaniline; (ꢂ)
3
,5-dimethylaniline and (᭛) 2,4,6-trimethylaniline.
AD2
It is observed in the present investigations that the λmax
Based on the values of KC at the lowest temperature of
values of the complexes are not linearly dependent on the
dielectric constant of the medium. It is generally expected
that the equilibrium constant and ꢀH for a molecular com-
plex formation will decrease or remain nearly the same as
the medium is changed from a non-polar to a polar solvent.
Such a situation prevails in a few CT-systems for which
both gas phase and solution data are available [26–28]. How-
ever, the data reported in the present studies shows that
DClNNQ interacts more strongly with aromatic anilines in
dichloromethane among the other two solvents used, namely,
chloroform and 1,2-dichloroethane. All solvents used are
polar and due to the solubility problem of the acceptor in
non-polar solvents, such solvents cannot be employed for
the study.
measurement in chloroform, the order of donor strength
was found to be: 2, 4, 6 − trimethylaniline (mesidine) >
3
2
, 4 − dimethylaniline
>
3, 5 − dimethylaniline
>
, 5 − dimethylaniline > 2, 6 − dimethylaniline.
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[
[
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H with the solvent polarity. The only complex for which
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AD
both ꢀH and KC have been reported to increase with
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spectroscopic and thermodynamic properties are influenced
to a greater extent by the change in dielectric constant of the

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[
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[
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