Charge-transfer complexes of pyrimidine Schiff bases with aromatic nitro compounds

Article abstract of DOI:10.1016/j.saa.2011.03.022

Charge-transfer (CT) complexes of pyrimidine Schiff bases, derived from condensation of 2-aminopyrimidine and substituted benzaldehydes, with some aromatic polynitro compounds were prepared and investigated using IR, UV, visible and ¹H NMR spec

Full text of DOI:10.1016/j.saa.2011.03.022

Spectrochimica Acta Part A 79 (2011) 513–521
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Charge–transfer complexes of pyrimidine Schiff bases with aromatic nitro
compounds
Yousry M. Issa∗, A.L. El Ansary, O.E. Sherif, H.B. Hassib
Chemistry Department, Faculty of Science, Cairo University,Gamaa Street, Giza, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2010
Received in revised form 21 February 2011
Accepted 11 March 2011
Charge–transfer (CT) complexes of pyrimidine Schiff bases, derived from condensation of 2-
aminopyrimidine and substituted benzaldehydes, with some aromatic polynitro compounds were
prepared and investigated using IR, UV, visible and 1H NMR spectroscopy. For all solid complexes, the
main interaction between the donor and acceptor molecules takes place through the ␲–␲* interaction.
Strong and some weak acidic acceptors, in addition interact through proton transfer from the accep-
tor molecule to the basic centre of the electron donor. Also, an n–␲* transition was detected in some
complexes.
Keywords:
Charge–transfer complexes
Pyrimidine Schiff bases
Spectroscopy
© 2011 Elsevier B.V. All rights reserved.
Donor–acceptor interaction
Molecular complexes
1. Introduction
The present study deals with the preparation of charge–transfer
complexes of some pyrimidine Schiff bases with di- and
Schiff bases attracted the attention of many researchers due
to their wide applications and importance, as corrosion inhibitors
[1], catalyst carriers [2,3], thermo-stable materials [4–6], metal ion
complexing agents [7] and in biological systems [8,9].
tri-nitrobenzene derivatives. They were investigated using IR,
UV–visible and 1H NMR spectroscopy to identify the type of bond-
ing between the donor and acceptor molecules in the CT complexes.
Recently, Schiff-base model compounds with different central
groups and various side-group substitutions have been synthesized
and characterized by elemental analysis, DSC technique, 1H NMR,
FTIR and UV–vis spectroscopy measurements [10].
2. Experimental
All chemicals used in this investigation were pure laboratory
grade BDH (England) and Fluka chemicals. The aminopyrimidine
Schiff bases used were prepared by condensation of equimolecular
amounts of 2-aminopyrimidine and the corresponding aldehy-
des. The products were recrystallized from ethanol till constant
m.p. [25]. The prepared Schiff bases have the following formulae
(Scheme 1).
Where x = H (a), o-OH (b), p-OH (c), p-OCH3 (d), o-NO2 (e), p-
NO2 (f), p-Cl (g), m-Cl (h), p-Br (i), p-N(CH3)2 (j) and o-OH (Naph.)
(k).
Several investigations have been made concerning the
charge–transfer complexes of aromatic nitro compounds with
various aromatic donor compounds [11–15]. Such studies were
devoted to evaluate the ionization potential of donor, or electron
affinities of acceptors as well as the importance of ␲–␲* bonding
in complex formation. Few studies are concerned with CT com-
plexes of Schiff bases [16,17]. Hindawey et al. [18] investigated the
solid complexes of p-nitrophenol as well as some dinitro and trini-
trobenzenes with p-substituted benzylidenaniline. The molecular
complexes of some hydroxy Schiff bases with polynitrobenzenes
were studied [19].
The acceptors used are picric acid (1), 3,5-dinitrosalicylic acid
(2), 3,5-dinitrobenzoic acid (3), 2,4-, 2,5- and 2,6-dinitrophenols
Molecular complexes of the donor–acceptor type formed
between aromatic amines and ␲-acceptors were the subject of
extensive studies [20–22]. Recently, N-heterocyclic compounds,
were used as efficient donors in the preparation of CT complexes
with different p-benzoquinone derivatives [23,24].
∗
Corresponding author. Tel.: +20 2 35868094; fax: +20 2 35728843.
E-mail address: yousrymi@yahoo.com (Y.M. Issa).
Scheme 1. Structural formula of pyrimidine Schiff bases.
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.03.022

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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
disappearance of the bands corresponding to these acidic centres
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-
gen atom ( +N–H) [27]. However, for the 1:2 complexes with picric
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.

Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
515
Scheme 3. Structure of 1:1 CT complex of 3,5-dinitrobenzoic acid with the investi-
gated Schiff bases.
complexes of some representative Schiff bases with 2,4- and 2,6-
dinitrophenols were prepared, Table 4.
Scheme 2. Structure of 1:1 CT complex of picric acid with a–i and k Schiff bases.
The infrared spectra reveal that the complexes formed between
the investigated donors and the weak acidic acceptors may involve,
in some cases, proton transfer besides the ␲–␲* electronic interac-
tion.
The ꢀasym. NO2 bands of the acceptors display some interest-
ing changes. The three bands in the spectrum of picric acid (1555,
1540 and 1530 cm−1) are reduced to two bands in the complex.
This is due to the rupture of the intramolecular hydrogen bond
between the OH-group and the neighboring NO2 group. One of
these two asym. NO2 bands, is shifted to lower wave number while,
the second exhibits a counter shift. This is explained by invoking
the participation of the two NO2 groups in positions 2 and 6 in
n–␲* electronic interaction with the two nitrogen atoms of the
pyrimidine ring. This interaction is allowed by the orientation of
the picric acid ion to a position facilitating the formation of the elec-
trostatic bond between the positive and negative centres resulting
from proton transfer. In the case of dinitrosalicylic acid (1540 and
1530 cm−1) the orientation of the molecule permits the participa-
tion of only one nitro group in n–␲* electronic transition. The asym.
NO2 bands of 3,5-dinitrobenzoic acid (1555 and 1540 cm−1) display
shifts to lower wave numbers. This is attributed to an increased
electron density on the acceptor molecules as a result of the ␲–␲*
interaction.
The molecular complexes of almost all investigated Schiff bases
with acceptors 4, 5 and 7 exhibit new intense broad bands within
the 3040–2200 cm−1 corresponding to N+–H stretching mode of
a proton linked to quaternary positively charged nitrogen [17].
Meanwhile, the bands corresponding to the ꢀOH of the free accep-
tors appearing at 3270, 3270 and 3610 cm−1, respectively are no
more observed. This may be clarified by assuming the transfer of a
proton from the acidic OH group of the acceptor to a basic centre on
the donor molecule. Mention is to be made that complexes of both
(e and f) with acceptors (4) and (7) and (e) with acceptor (6) do not
show the NH+ band. The incapability of these donors to exhibit pro-
ton transfer is due to the presence of the electron withdrawing nitro
group which causes a decrease in the basicity of the nitrogen atoms.
Further evidence of the above conclusion is the observed shift of the
␥CH band of the CH N linkage to lower wave number values in the
case of complexes showing proton transfer, while in the other type
of complexes, showing no proton transfer, a shift to higher values
is observed. The latter is attributed to decreased electron density
on the CH N centre as a result of complex formation.
The ꢀsym. NO2 bands are mostly shifted to lower wave numbers
in accordance with the increased ␲–electron density on the ring of
the acceptor molecule of the CT complexes.
Based on the above findings the charge–transfer complexes may
be represented by the following formulae (Schemes 2–5).
On the other hand, the IR spectra of the complexes of 2,5-
dinitrophenol with the exception of (j), denotes the absence of
proton transfer. This is based on the appearance of the bands corre-
sponding to the ꢀOH in the spectra of the complexes. These bands
show slight shifts to higher wave number indicating the destruc-
tion of the intramolecular hydrogen bonding between the OH and
the neighboring nitro group in the free acceptor on complexation.
3.1.2. Complexes with weak acidic acceptors
The main IR bands of some representative 1:1 complexes of the
investigated Schiff bases with 2,4-, 2,5-, 2,6-dinitrophenols and p-
nitrophenol (4–7) are collected in Tables 2 and 3. The 1:2 (D:A)
Table 2
Characteristic IR bands of charge–transfer complexes of pyrimidine Schiff bases with 2,4-dinitrophenol (1:1).
No
Color
m.p. (◦C)
NH+
NO2asym.
NO2sym.
8CHacceptor
Bands of free acceptor
1540,1540
1539,1530
1538
1540
1538
1538
1540
1540b
1540b
1535
1350
1530
1345
1340
1342
1350
1350
1340
1345
1342
1345
1335
928,825
924,820
925,793
919,810
925,825
925,795
928,790
926,827
925,–
a
b
c
d
e
f
g
h
i
Canary yellow
Yellow
Canary yellow
Canary yellow
Brown
Light yellow
Canary yellow
Canary yellow
Canary yellow
Orange
132
124
104
133
102
79
131
126
130
123
83
2800–2200
2750–2305
2800–2300
2810–2200
–
–
2700–2200
2790–2200
2800–2200
2820–2200
–
925,970
925,825
924,822sh
j
k
1540
1540
Brown

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Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
Scheme 4. Structure of 1:2 CT complex of picric acid with a–i and k Schiff bases.
Scheme 5. Structure of 1:2 CT complex of picric acid with Schiff base j.
Table 3
Characteristic IR bands of charge–transfer complexes of pyrimidine Schiff bases with p-nitrophenol (1:1).
No
Color
m.p. (◦C)
NH+
NO2asym.
NO2sym.
8CHacceptor
Bands of free acceptor
1520
1520
1525
1520
1520
1520
1535
1535
1540
1530
1520
1525
1350
1350
1340
1340
1338
1337
1335
1335
1350
1340
1335
1340
855,760
855,760
849,755
840,756
852,758
850,745
850,735
855,748
850,760
850,755
753
a
b
c
d
e
f
g
h
i
Brownish white
Buff
Brownish yellow
Buff
Brown
Buff
Buff
Buff
Yellow
Dark brown
Dark Brown
85
84
175
85
163
75
2920–2380
2860–2240
2860–2300
2870–2400
–
–
90
2860–2200
1860–2260
2860–1460
3000–2500
2860–2260
162
Oily
52
j
k
115
842,755

Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
517
Table 4
Characteristic IR-bands of 1:2 charge–transfer complexes of pyrimidine Schiff base with 2,4-and 2,6-dinitrophenol.
No.
Color
m.p. (◦C)
NH+
(OH
NO2asym.
NO2sym.
8CHacceptor
Complexes of 2,4-dinitrophenol
Bands of free acceptor
3270
3290
3295
3340
3280
–
1540,1520
1540
1555,1530
1550,1540
1540
1350
928,825
922.818
926,810
926,811
925,818sh
818
a
c
d
g
j
Canary yellow
Yellow
Yellow
Canary yellow
Orange yellow
112
101
130
136
104
2740–2300
2700–2200
2700–2200
2700–2200
2800–2200
1348.1338
1348,1333
1344
1342
1345,1335
1555,1538
Complexes of 2,6-dinitrophenol
Bands of free acceptor
3270
3240b
3220
3240b
3250b
–
1543,1537
1354
924.812
908,815
915,820
925sh,815
, 815
a
c
d
g
j
Orange
Orange
Orange
Orange
116
106
123
118
87
2800–2360
2820–2340
2840–2200
2800–2200
2800–2400
1540,1530,1525
1540,1530,1525
1540,1530
1538,1530,1520
1555,1545
1350,1340
1350,1335
1340s
1335s
1350,1320
Brownish orange
928,820
The NO2-bands of the acceptor part generally shift to lower val-
ues except in some complexes a counter shift is observed. The
later observation denotes the possible existence of n–␲* interac-
tion between one NO2 group with the hetero nitrogen facing it, if
the orientation of the acceptor molecule allows.
Accordingly, thedifferent typesofinteractionbetween the Schiff
bases and the weak acidic acceptors can be represented by the
formula given in Schemes 6 and 7.
The bonding in the complexes may be represented as shown in
Scheme 8.
3.1.3. Complexes with non-acidic acceptors
This class comprises the CT complexes with acceptors deprived
from any acidic centres (8–13). The main IR bands of some rep-
resentative charge–transfer 1:1 and 1:2 complexes with picryl
chloride are given in Table 5.
The 1:2 (D:A) spectra reveal the formation of two types of molec-
ular complexes, the same as in the 1:1 complexes. In the complexes
formed through ␲–␲* and proton transfer, the OH band of the sec-
ond 2,4-dinitrophenol molecule is observed at higher wave number
value in comparison to that of free acceptor. This shift may be
attributed to the cleavage of the intramolecular hydrogen bond
between the OH group and the o-NO2 group. In the IR spectra of the
p-N(CH3)2 Schiff bases, the second OH band vanishes also denoting
the participation of the OH group of the second acceptor molecule
in acid-base interaction with the basic N(CH3)2 group. The observed
shift of the ꢀC N band to lower values is an evidence of the proton
transfer interaction.
The asym NO2 bands shift to lower values in the complexes
of 2,6-dinitrophenol showing an increase of electron density on
the acceptor moiety. For complexes of 2,4-dinitrophenol the ꢀNO2
bands shift to either higher or lower wave number. This confirms
the existence of n–␲* interaction.
The only possible way of CT complex formation is electron trans-
fer either from the ␲–electron system of the aromatic ring or the
n–electrons of the azomethine linkage.
The NO2 bands of the acceptors display different behavior on
complex formation, depending on the type of the donor and accep-
tor used. In most cases the asym. NO2 bands become broader and
show some splitting in the spectra of CT complexes. This behavior
indicates a higher differentiation of the energy states of the NO2
groups in the CT complexes than in the free acceptors.
The single asym. NO2-band of free 1,3,5-dinitrobenzene
(1552 cm−1) and of 1,5-difluoro-2,4-dinitrobenzene (1548 cm−1
)
splits either to two or three bands on complex formation, one of
the splitted bands appear at higher wave number and the others
are displayed to lower field. In case of picryl chloride complexes
the two adjacent asym. NO2 bands (1553, 1540 cm−1) split to
three peaks, two bands appear at higher and the third at lower
Scheme 6. Structure of 1:1 CT complex of 2,4-dinitrophenol with a–d and h–k Schiff
Scheme 7. Structure of 1:1 CT complex of 2,4-dinitrophenol with e and f Schiff
bases.
bases.

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Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
Scheme 8. Structure of 1:2 CT complex of 2,4-dinitrophenol with a–d and h–k Schiff bases.
wave number values, while those for 1-chloro- and 1-fluro-2,4-
dinitrobenzene (1550, 1533 cm−1, 1548 cm−1), become broader
and shift to higher values. The shift of the NO2 to higher values
is attributed to the occurrence of secondary n–␲* bonding forces
in the complex molecule besides ␲–electron overlap, Such interac-
tion is facilitated if the acceptor ring is slightly twisted so that the
azomethine N-atom is positioned in front of one of the nitro groups
of the acceptor (CH N → NO2). In other cases, the orientation of
the molecules make possible for one of the N-hetero ring atoms to
face another NO2 group on the acceptor molecule, thus a second
n–␲* interaction takes place. The latter case clarifies the fact that
two NO2 bands are observed at higher values. The observed shift to
lower wave numbers indicates a stronger polarization which origi-
nates from increased electron density as a result of HOMO → LUMO
interaction and absence of n–␲* interaction. The formula is demon-
strated in (Scheme 9).
The IR spectra of 1:2 (D:A) complexes with picryl chloride dis-
play the normal behaviors encountered in case of 1:1 CT complexes.
The C N bands shift to higher wave number on complex forma-
tion, the magnitude of such shift is smaller than that observed
Scheme 9. Structure of 1:1 CT complex of trinitrobenzene with non-acidic accep-
tors.
Table 5
Characteristic IR bands of 1:1 and 1:2 charge–transfer complexes of pyrimidine Schiff bases with picryl chloride.
No.
Color
m.p. (◦C)
NO2asym.
NO2sym.
1348
8CHacceptor
Bands of free acceptor
1553, 1540
928, 825, 778
1:1 Complexes
a
b
c
d
e
f
g
h
i
Brownish yellow
Brownish orange
Yellow
Brownish orange
Brown
Brown
Brown
Orange
Brown
140
135
157
115
Low melting
75
128
193ch
138
1560, 1545, 1533
1560, 1554, 1540
1565, 1549, 1540
1560, 1550, 1540
1560, 1550, 1540
1560, 1550, 1540
1560, 1554, 1540
1550, 1540
1347
918, 815sh, 752
922, 807, 757
922, 805, 758
922, 808, 754
922, 810, 755
922, 812, 750
919s, –, 750
912, 810, 742
922, 809, 760
923, 810, 757
928, 918, 757
1353, 1345
1350, 1348
1350s
1350
1350
1355, 1350
1343s
1346
1349
1342, 1338
1560, 1552, 1540
1557, 1542, 1537
1556, 1550, 1540
j
k
Brown
Brown
Low melting
77
1:2 Complexes
a
d
g
j
Dark brown
Dark brown
Dark brown
Dark brown
153
163
157
63
1565, 1555, 1540
1555, 1545
1536, 1550, 1540
1560, 1552, 1540
1354, 1340
1350
1350
922, 805sh, –
922, –, 775
923, –, 760
922, –, 756
1345

Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
519
Fig. 1. Electronic absorption spectra of 1:1 charge–transfer complexes of pyrimidine
Schiff bases with picric acid.
for 1:1 complexes. This may be explained on the increased local-
ization of the ␲–electrons on the donor molecules as a result of
the participation of both rings in complex formation as mentioned
before.
Fig. 2. Electronic absorption spectra of some charge–transfer complexes (1:1) of
pyrimidine Schiff bases with 2,6-dinitrophenol.
3.2. Electronic absorption spectra
The electronic absorption spectra of 1:1 CT complexes of some
of the investigated Schiff bases with picric acid (strong acidic accep-
tor), 2,5-, 2,6- and p-nitrophenol (weak acidic acceptors) are given
in Figs. 1 and 2. The spectra display new sets of bands located on the
longer wavelength side which are not observed in either the free
donor or acceptor. These new bands may be broad or may appear
as a shoulder. The shoulder or band may be assigned as ␲–␲* elec-
tronic transition. The broad shape of some bands may be attributed
to the contribution of the weak n–␲* electronic interaction in the
complex formation.
Table 6
Electronic absorption spectral data of some 1:1 charge–transfer complexes.
No.
X
(
max
(nm)
ECT (eV)
Obs.
Calc.
For the verification of this assumption, the ECT values were cal-
culated from the absorption spectra using the equation:
Complexes with picric acid
1
2
3
4
5
6
7
H
380
3.26
2.88
3.02
3.26
9.95
3.09
2.95
2.63
2.68
2.74
2.70
2.94
2.63
3.25
o-OH
p-OH
p-OCH3
p-NO2
p-Cl
430
410
380
420
400
420
1239.9
ꢁCT (nm)
ECT
=
eV
The results are given in Table 6 and compared with the theoret-
ical values calculated using the relation given by Briegleb [28,29].
p-N(CH3)2
Complexes with 2,5-dinitrophenol
a
c
d
f
2.88
2.82
–
2.88
–
ECT = Ip − EA + C
430sh
440
–
where Ip is the ionization potential of the donor, EA is the elec-
tron affinity of the acceptor and C is the coulomb force between
the electron transferred and the positive hole left behind (4.7 eV)
[28,29]. The observed and calculated ECT values, are nearly concor-
dant favoring their assignments as charge–transfer bands.
g
430sh
Complexes with 2,6-dinitrophenol
a
c
f
g
j
450
450
446
450
440
2.76
2.76
2.78
2.76
2.82
3.3. Nuclear magnetic resonance studies
The 1H NMR spectra of some of the prepared Schiff bases
and some selected nitrobenzene acceptors are studied to throw
more insight on the molecular structure and types of interaction
occurring in such complexes. Based on the IR studies, the CT com-
plexes are classified to two main types, complexes formed through
Complexes with p-nitrophenol
c
d
g
j
350sh
390
350sh
350sh
3.54
3.18
3.54
3.54

520
Y.M. Issa et al. / Spectrochimica Acta Part A 79 (2011) 513–521
electron transfer only and those formed through proton transfer in
addition to electron transfer.
The 1H NMR spectra of the CT complexes formed through elec-
tron transfer only compared to those of the free components reveal
a shift of the signals due to the protons of the acceptor towards
higher fields, while those of the donor are shifted to lower fields.
This behavior is interpreted on the basis of increased shielding of
the acceptor protons and decreased on those of the donor which
results from the intermolecular ␲–␲* CT interaction. It is clear
from Table 7 that, the chemical shifts of the hetero ring protons
are greater than those for the benzal ring protons. This confirms
the previous assumption that the hetero ring is the moiety con-
tributing to the CT interaction. An exception is the p-N(CH3)2 Schiff
base where the protons of the N-dimethyl group exhibit a pro-
nounced shift, confirming that the benzal ring is the origin of the
␲–␲* electronic interaction for this derivative (j). The above behav-
ior comprises the CT complexes formed with non-acidic and some
weak acidic acceptors.
The 1H NMR spectra of complexes involving proton transfer dis-
play some changes in comparison to those of their constituents.
In the spectra, the signal due to the OH-group of acceptor is no
more observed, meanwhile, a new signal with an integration value
equivalent to one proton is observed at about 4.1 ppm. This signal
is assigned to the new centre ( +N–H) formed through the trans-
fer of the OH proton of the acceptor to the azomethine group of
the donor. Such conclusion is in good agreement with that reached
previously through IR studies.
It is noteworthy that, in the 1:1 complex of (j) with picric acid,
the signals of the protons of the methyl group in N(CH3)2 become
broad and shift to lower field, this is taken as a further proof that the
proton of the acceptor is attached to the dimethylamine nitrogen
and not to the azomethine group. For the1:2 complex, the two rings,
the benzal and the hetero, are contributing to the charge–transfer
interaction and both the azomethine and the dimethylamine nitro-
gens are involved in proton transfer which is supported by the
disappearance of the acceptor OH-proton signal and the existence
of two signals at 3.8 and 4.1 ppm due to the +NH(CH3)2 and C +NH
protons, respectively.
4. Conclusion
The IR spectra reveal that all strong acidic and some of the weak
acidic acceptors form charge–transfer complexes with the donors
through ␲–␲* electronic transfer from the donor (hetero-ring for
a–i and benzal ring for j) to the acceptor molecule in addition to
proton transfer from the acceptor to the donor.
For non acidic and some weak acidic acceptors interactions are
only through ␲–␲* electron transfer. In some complexes secondary
bonding forces, namely n–␲* interaction, are operative besides
the aforementioned interactions whenever the orientation of the
molecules permits.
The ultraviolet absorption spectra of some representative
charge–transfer complexes reveal the presence of a new band at
the longer wavelength region which was not observed in the spec-
tra of either the free donor or free acceptor such band is assigned to
the (HOMO–LUMO) ␲–␲* interaction (CT band). In some cases, the
observed band is very broad and of composite nature. This is taken
as an indication for n–␲* interaction for complexes showing it.
The nuclear magnetic resonance studies were in good agree-
ment with those obtained from IR studies.
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