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Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
(4, 5 and 6), p-nitrophenol (7), picryl chloride (8), trinitroben-
zene (9), m-dinitrobenzene (10), 1-chloro-2,4,-dinitrobenzene
(11), 1-fluoro-2,4-dinitrobenzene (12) and 1,5-difluoro-2,4-
dinitrobenzene (13).
the highest occupied -level on the donor molecule (HOMO) to
the lowest unoccupied level on the acceptor (LUMO). The hetero
ring is the centre of charge–transfer as noticed from the observed
shift to much lower values with respect to the benzal ring. A pecu-
liar behavior is observed for the p-N-dimethyl derivative (j) as the
benzal ring is the centre of electron donation.
2.1. Preparation of charge–transfer complexes
Depending on the nature of the acceptor used, the
charge–transfer complexes are classified into three groups.
The CT, 1:1 and 1:2, complexes were prepared as described
previously [26] by mixing a hot saturated ethanolic solution of
the donor with an equivalent amount of the acceptor. The solid
complexes were either separated immediately, e.g. picric acid, or
on standing. The ethanol soluble complexes were recrystallized
from ethanol while the insoluble complexes were just boiled with
ethanol to ensure freeing from contaminations of unreacted prod-
ucts. The resulting products were subjected to elemental analysis
in the Micro Analytical Centre, Faculty of Science, Cairo University.
The obtained results were in high agreement with those theoreti-
cally calculated.
3.1.1. Complexes with strong acidic acceptors
These include the CT complexes with acceptors (1), (2) and (3)
in molar ratio 1:1. The 1:2 (D:A) complexes of some donors with
picric acid were also studied. The main IR bands of some representa-
tive 1:1 and 1:2 complexes with picric acid are collected in Table 1.
In this class of compounds, the acceptors exhibit acidic character
while the donors reveal basic character. Thus, an acid-base inter-
action involving a proton transfer from the acceptor to the donor
is to be expected. The basic centre or the proton acceptor would be
the azomethine nitrogen in case of Schiff bases a–i and k, and the
dimethylamino nitrogen in case of j. The proton transfer originates
from the OH group of the acceptors (1) and (2) and the COOH group
of (3). Such mechanism of proton transfer is deduced from the
observed at 3110, 3190 and 3570 cm−1 for the free acceptors in the
spectra of the 1:1 CT complexes. Also the spectra display new broad
intense bands within the wave number range 3300–2100 cm−1
which correspond to a proton linked to a positively charged nitro-
acid, the –+NH bands are observed within the wave number range
3020–2360 cm−1. This band is formed through the transfer of a pro-
ton from one of the two acceptor molecules to the basic centre of the
donor. This is supported by the appearance of the ꢀOH band of the
second picric acid molecule at 3110 cm−1 [27]. It is noteworthy to
mention that, for donor j with p-N(CH3)2 group on the benzal ring,
the ꢀOH band of the second picric acid molecule also disappears
as a result of the existence of two acid–base interactions, the first
with the N(CH3)2 and the second with the azomethine nitrogen.
The C N band undergoes the same behavior in the case of 1:1
and 1:2 complexes. The ꢀC N is shifted to higher values for most
CT complexes except for donor j a counter shift is observed. This
is attributed to the attachment of the proton in such case to the
p-N(CH3)2 group and not to the azomethine nitrogen.
2.2. Apparatus
The IR spectra were obtained by using a PYE UNICAM SP 1000
infrared spectrometer as KBr discs. The electronic absorption spec-
tra were recorded on a PYE UNICAM SP 1750 spectrophotometer
using the Nujol mull technique to obtain the electronic absorption
spectra of some charge–transfer complexes in the solid state as they
dissociate in polar solvents. The 1H NMR spectra were recorded
using a VARIAN EM-390 (90 MHz) spectrometer using tetramethyl-
silane (TMS) as internal reference and Merk-d6 dimethylsulphoxide
(DMSO) as a solvent.
3. Results and discussion
3.1. Infrared spectral studies
The IR spectra of the complexes are studied and the main bands
compared with their analogues in free donors [25] and acceptors.
From these studies the type of interaction observed in these com-
plexes is elucidated. For all studied complexes, it is noticeable
that the ␥CH bands of the donor part are shifted to higher values
while those of the acceptor are displaced in the opposite direc-
tion. This is a result of intermolecular –* electron transfer from
Table 1
Characteristic IR bands of charge–transfer complexes of pyrimidine Schiff bases with picric acid.
No
X
Color
m.p. (◦C)
NH+
(OH
NO2asym.
NO2sym.
1350
8CHacceptor
784
Bands of free acceptor
1555,1540,1530
1:1 (D:A) complexes
a
b
c
d
e
f
g
h
i
H
o-OH
p-OH
p-OCH3
o-NO2
p-NO2
p-C1
m-C1
p-Br
Canary yellow
Yellow
Yellow
Lemon yellow
Lemon yellow
Brownish yellow
Lemon yellow
Yellow
229
215
>240
198
226
224
230
235
>240
202
226
3940–2100
3980–2600
3000–2360
3000–2600
2940–2630
2940–2640
2930–2600
2900–2400
3030–2480
2940–2600
3000–2500
1571,1550
1570,1552
1570,1552
1575,1555,1540
1570,1550
1570,150,1583
1572,1552
1552
1340,1330
1338,1325
1340,1330
1335,1335
1340,1327
1340,1338sh
2347,1330
1325
745
748
748
747
747
745
745
745
747
747
742
Yellow
Orange yellow
Lemon yellow
1572,1552
1571,1550
1570,1540
1340,1328
1340,1328
1342,1332
j
k
p-N(CH3)2
o-OH naph
1:2 (D:A) complexes
a
b
d
g
i
H
o-OH
p-OCH3
p-C1
p-N(CH3)2
Lemon yellow
Canary yellow
Canary yellow
Yellow
220
205
227
219
163
3000–2360
3000–2400
3020–2480
3000–2500
3000–2360
3100
3095b
3100
3100
–
1578,1548,1522
1570,1560
1570,1550
1572,1550
1570,1549
1342
772
747s
745
745
745
1338,1328,
1345,1550
1348,1325
1340,1330
Canary yellow
sh = shoulder, s = strong, b = broad, w = weak.
Compd: (OH, (-OH, (C–OH.
b: 3140b 1280 1174w.