H NMR, IR and UV/VIS spectroscopic studies of some Schiff bases derived from 2-aminobenzothiazole and 2-amino-3-hydroxypyridine

Article abstract of DOI:10.1002/jccs.200800131

By condensing 2-aminobenzothiazole with 2-hydroxy-1-naphthaldehyde, 2-hydroxybenzaldehyde, 4-methoxybenzaldehyde, 4-hydroxybenzal-dehyde, benzaldehyde and 4-dimethylaminobenzaldehyde, and five Schiff bases Ia-Ie are prepared. Also, two Schiff bases IIa an

Full text of DOI:10.1002/jccs.200800131

Journal of the Chinese Chemical Society, 2008, 55, 875-884
875
1H NMR, IR and UV/VIS Spectroscopic Studies of Some Schiff Bases
Derived From 2-Aminobenzothiazole and 2-Amino-3-hydroxypyridine
Raafat M. Issa, Abdalla M. Khedr* and Helen Rizk
Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
By condensing 2-aminobenzothiazole with 2-hydroxy-1-naphthaldehyde, 2-hydroxybenzaldehyde,
4-methoxybenzaldehyde, 4-hydroxybenzal-dehyde, benzaldehyde and 4-dimethylaminobenzaldehyde,
and five Schiff bases Ia-Ie are prepared. Also, two Schiff bases IIa and IIb are prepared by condensation of
2-amino-3-hydroxypyridine with 2-hydroxy-1-naphthaldehyde and 2-hydroxybenzaldehyde. The 1H
NMR, IR and UV/Vis spectra of these seven Schiff bases are investigated. The signals of the 1H NMR
spectra as well as the important bands in the IR spectra are considered and discussed in relation to molecu-
lar structure. The UV/Vis absorption bands in ethanol are assigned to the corresponding electronic transi-
tions and the electronic absorption spectra of Schiff bases Ib and IIb are studied in organic solvents of dif-
ferent polarities. The UV/Vis absorption spectra of 2-amino-3-hydroxypyridine Schiff bases IIa and IIb
are investigated in buffer solutions of different pH values containing 5% (v/v) methanol, and the results
are utilized for the determination of pKa and DG* of the ionization of the phenolic OH-groups. The fluo-
rescence spectra of IIa and IIb are studied in organic solvents of different polarities.
The obtained spectral results are confirmed by some molecular calculations using the atom super posi-
tion and electron delocalization molecular orbital theory for the Schiff base IIb.
Keywords: 2-Aminobenzothiazole; 2-Amino-3-hydroxypyridine; Benzaldehyde derivatives;
Schiff bases; Spectral studies; Molecular orbital calculations.
1. INTRODUCTION
considered. The UV/Vis absorption spectra of Schiff bases
Ib and IIb as well as the fluorescence spectra of IIa and IIb
are investigated in organic solvents of different polarities
and discussed with respect to different solvent parameters.
The ionization of phenolic OH-groups of 2-amino-3-hy-
droxypyridine Schiff bases IIa and IIb is studied in aqueous
buffer solutions of varying pH values. The results of spec-
tral studies are supported by some molecular orbital calcu-
lations using the ASED-MO theory for the Schiff base IIb.
Although there are wide applications of Schiff bases
and their metal chelates in biological systems,1,2 cataly-
sis,3,4 dying processes,5,6 and analytical applications,7 the
spectral studies of the Schiff bases containing a hetero-
cyclic ring are comparatively minor.7-9
1
In the present article, the H NMR, IR and UV/Vis
spectra of seven Schiff bases derived from 2-aminobenzo-
thiazole and 2-amino-3-hydroxypyridine (Scheme I) are
2. EXPERIMENTAL
General formulae of the Schiff bases under
investigation
Scheme I
All compounds used in the present study were of pure
grade available from BDH, Aldrich or Sigma. The solvents
used for the spectral study were spectroscopic grade from
Aldrich.
The Schiff bases were obtained by refluxing equi-
molar quantities of the aromatic amine and the aldehyde
(0.01 mole of each in 25 mL ethanol) on a water bath for
2-5 hrs.10 The Schiff base compounds precipitated on cool-
ing, were then filtered and purified by recrystallisation
* Corresponding author. E-mail: abkhedr2001@yahoo.com; Tanta, P. O. 31527.

876 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Issa et al.
from the appropriate solvent (yield 70-80%).
RF 510 spectrofluorometer and referenced to fluorescence
in 0.1 N NaOH (ff = 0.93).11 Molecular orbital calculations
were carried out on Schiff base IIb using the ASED-MO
method.
1
The H NMR spectra were obtained by the aid of a
Bruker DMX 750 (500 MHz) spectrometer using d6 DMSO
as a solvent and TMS as internal standard. The IR spectra
were recorded on a Perkin Elmer 1430 ratio recording in-
frared spectrometer using the KBr wafer technique. The
UV/Vis spectra were measured at room temperature within
the 200-600 nm range on a Perkin Elmer Lambda 4B using
1.0 cm matched silica cells. The fluorescence quantum
yield for solutions having an absorbance of less than 0.1 at
the excitation wavelength were determined using a Shimadzu
3. RESULTS AND DISCUSSION
1
3.1. H NMR spectra
The signals observed in the 1H NMR spectra of the
Schiff bases under study (Figs. 1 and 2) are collected in Ta-
ble 1. The spectra exhibit a multiplet at 6.50-8.18 ppm for
Fig. 1. 1H NMR spectrum of 2-aminobenzothiazole-2-hydroxy-1-naphthaldehyde (Ia).

1H NMR, IR, UV/VIS of Benzothiazole and Pyridine Schiff
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 877
Fig. 2. 1H NMR spectrum of 2-amino-3-hydroxypyridine-2-hydroxy-1-naphthaldehyde (IIa).
the hydrogens of the aromatic rings. The azomethine hy-
drogen (-CH=N-) leads to a singlet of integration intensity
equivalent to one hydrogen at 8.32-9.71 ppm. The spectra
of Ic and If display signals at 3.41 ppm or 3.08 and 3.12
ppm due to the hydrogens of the OCH3 or –N(CH3)2 groups.
The spectra of Id, IIa and IIb show a singlet with an integra-
tion equivalent to one hydrogen at 10.62, 8.70 and 9.33
ppm corresponding to the hydrogen of the free OH-group,
while Ia, Ib, IIa and IIb exhibit a signal due to the hydrogen
bonded hydroxyl group at 9.28-10.03 ppm.
It is worth mentioning that the signal of the azome-
thine hydrogen appears as two singlets in the spectra of Ic
and Ie (Dd = 0.03 ppm) with a total integrated intensity cor-
responding to one hydrogen. This behavior can be ascribed
to the existence of these compounds as a mixture of anti
and syn forms.
The higher field singlet is assigned to the azomethine
hydrogen in the syn form while that at lower field is due to
the anti form. Also, the high-resolution spectra of Ia and IIa
(Fig. 1) show the signal of the N=CH hydrogen as a doublet

878 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Issa et al.
Table 1. Data from 1H NMR spectra of 2-aminobenzothiazole and 2-amino-3-hydroxypyridine
Schiff bases
Compound number
Assignment
Ia
Ib
Ic
Id
Ie
IIa
---
IIb
---
---
---
3.41
---
---
CH3
C-H (Ar)
N=CH
OH free
OH bonded
7.73-8.78 6.97-7.70 6.82-7.72 6.93-8.05 6.89-7.72 6.30-8.15 7.50-7.75
8.81
---
10.03
9.0
---
9.28
8.92, 8.95
9.05
10.62
---
8.96, 8.99
8.50
9.70
9.85
8.03
9.33
9.72
---
---
---
---
tending within the 3300-2700 cm-1 region. The stretching
vibrations of the aromatic C-H groups give medium to
weak bands within the 3072-3043 [n(C-H)asym] and 3057-
3026 cm-1 [n(C-H)sym] ranges. The azomethine hydrogen
gives three bands at 3004-2915 cm-1 [n(N=C-H)], 1279-
1200 cm-1 [d(N=C-H)] and 1046-1010 cm-1 [g(N=C-H)].
The n(C=N) bands are observed at 1645-1557 cm-1; the po-
sition of these bands varies with the molecular structure,
though no regularity can be pointed out. The spectra of
compounds Ia-If exhibit a medium to sharp band at 1606-
1560 cm-1 corresponding to n(C=N) of the thiazole ring
The anti and syn forms in Schiff bases Ic
and Ie
Scheme II
Scheme III Keto-enol toutomerism in Schiff base Ia
at 8.81 ppm (J = 7.8 Hz) and 8.50 ppm (J = 10.20 Hz) of a
total integration intensity equivalent to one hydrogen. This
splitting can be explained on the bases of a coupling with a
neighboring –NH hydrogen, which resonates as a doublet
at 7.27 ppm (J = 7.8 Hz), and the existence of keto-enol
toutomerism (Scheme III). The presence of the singlet at
10.03 ppm, due to the H-bonded OH-group, indicates a
high contribution of the enolic structure.
3.2. IR Spectra
The IR spectra of the Schiff bases under study (Figs. 3
and 4) are recorded in the solid state using the KBr disc
technique. Selected bands of diagnostic importance are
collected in Table 2.
The spectra of compounds Id, IIa and IIb having free
OH groups display somewhat broadened bands within the
3434-3425 cm-1 region. The presence of OH-groups in-
volved in intramolecular hydrogen bonding for Ia, Ib, IIa
and IIb give the n(OH) as a shallow, very broad band ex-
Fig. 3. IR spectra of Schiff bases Ia, Ib, Ic and Id.

1H NMR, IR, UV/VIS of Benzothiazole and Pyridine Schiff
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 879
Table 2. Selected bands of diagnostic importance from the IR spectra of 2-aminobenzothiazole and 2-amino-
3-hydroxypyridine Schiff bases
Compound number
Assignment
Ia
Ib
Ic
Id
Ia
If
IIa
IIb
---
---
---
---
3425
---
---
---
---
---
3434
3434
n(OH) free
not defined not defined
not defined not defined n(OH) bonded
3059
3047
2969
1620
1597
1560
1280
1237
1170
1033
921
3064
6050
2915
1642
1622
1606
1268
1235
1170
1010
889
3069
3057
3004
---
1637
1595
---
1235
---
1023
---
3043
3026
2917
---
1645
1600
1340
1279
1148
1013
929
3065
3053
2956
---
1636
1599
---
1256
---
1017
---
3069
3052
2916
---
1635
1612
---
1258
---
1015
---
3072
3055
2929
1624
1579
1534
1300
1200
1148
1046
935
3070
3055
2982
1598
1557
1539
1289
1234
1191
1029
913
n(CH) asym
n(CH) sym
n(N=CH)
n(C=O)
n(C=N) (Schiff)
n(C=N) (ring)
d(OH)
d(N=CH)
n(C-OH)
g(CH=N)
g(OH)
while IIa and IIb have the C=N vibrations of the pyridine
ring at 1534 and 1539 cm-1, respectively. Compounds con-
taining OH groups display bands at 1340-1268 cm-1 [d(OH)],
1191-1148 cm-1 [n(C-OH)] and 935-889 cm-1 [g(OH)]. The
spectra of all compounds show medium to sharp bands
within the 870-630 cm-1 range due to the out-of-plane de-
formations of the aromatic C-H groups.12
3.3. The electronic absorption spectra
3.3.1. Spectra in ethanol
The electronic absorption spectra of the Schiff bases
included in the present study (Fig. 5 and Table 3) exhibit
mainly four bands. The two bands on the higher energy
side, within the 210-231 and 252-270 nm ranges, are due to
the excitation of the p electrons (p¾p* transitions) of the
aromatic rings. The third band within the 275-353 nm
range is assigned to the p¾p* transition within the C=N
group, while the longer wavelength band at 325-471 nm is
due to an intramolecular charge transfer (CT transition) in-
volving the whole molecule. The CT transition is influ-
enced by the aldehyde rest. This is confirmed by calculat-
ing the ECT values from lmax using the relation:
CT
ECT =1241.6/ lmax (eV)
(1)
and comparing the values obtained with the data calculated
using the Briegleb equation:13
ECT = (IP -EA )+C
(2)
in which IP is the ionization potential of the donor, EA is the
electron affinity of the C=N acceptor (-1.3 eV) and C is the
coulomic force between the electron transferred and the
positive hole left behind (-5.2 or -5.6 eV).13
Fig. 4. IR spectra of Schiff bases Ie, If, IIa and IIb.

880 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Issa et al.
Table 3. UV/Vis spectral data of 2-aminobenzothiazole and 2-amino-3-hydroxypyridine Schiff bases in ethanol
Compound number
Cal.
Values
for IIb
Assignment
Ia
Ib
Ic
Id
Ie
If
IIa
IIb
221
2.4
252
0.7
306
0.3
217
3.3
2.65
1.2
278
1.2
221
2.8
265
1.3
290
0.6
218
2.6
268
0.6
310
1.0
360
2.5
221
2.9
266
1.6
275
0.7
221
3.3
255
1.2
297
0.6
210, 230 214, 231
2.1, 2.8 1.9, 1.3
230, 255 255, 270
2.8, 1.4 0.9, 0.6
324, 353 306, 334
0.8, 0.6 0.6, 0.8
215,
235
252,
270
307,
329
359
lmax Band A (p-p* Ar)
emax
lmax Ban d B (p-p* Ar)
emax
lmax Band C (p-p* of
emax
lmax
emax
C=N group)
Band D (CT)
349, 427 325, 375 356
344 341, 427 446, 471 362, 433
0.5, 0.9 1.1, 1.4
3.6, 2.9 3.8, 3.3
---, 3.2 3.6, 3.1
---, 2.7
---, 0.15 ---, 0.25 0.21
---, 0.21 ---, 0.23
---, 7.41 ---, 7.7
---, 6.88 ---, 7.2
---, 7.14 ---, 7.4
---, 7.14 ---, 7.5
0.7
3.5
3.7,
3.2
0.4
3.6
3.8,
3.3
2.5, 0.5 2.0, 2.2
3.6, 2.9 2.8, 2.6
---, 3.2
---, 2.7
0.8, 0.2
3.4, 2.9
3.4
3.7,
3.2
ECT (ev) from equation (1)
3.2, 3.0
2.7, 2.5
0.23,
3.7, 3.2 ECT from Breigleb equation (2)13
3.2, 2.7
0.80
0.08 ---, 0.45
---, 0.09
8.0
7.5
7.6
7.7
0.34, 0.03
(f)
0.19
7.9
7.4
7.5
7.6
7.8
7.3
7.5
---, 7.41 7.3, 7.2
---, 6.88 6.8, 6.6
---, 7.14 7.1, 7.0
---, 7.14 7.1, 6.9
7.8, 7.4
7.3, 6.9
7.5, 7.1
7.6, 7.1
I
P1 (ev)15
IP2 (ev)16
IP3 (ev)17
7.56
Mean IP (ev) values
lmax in (nm), emax (L mol-1 cm-1 ´ 10-4) and f is the oscillator strength (L mol-1 cm-2)
The data calculated for ECT from Eq. 2 using the two
values of C are in satisfactory agreement with the ECT ob-
tained from the experimental data using Eq. 1 (Table 3).
It is worth mentioning that the spectra of the Schiff
bases Ia, Ib, IIa and IIb display the bands due to the p¾p*
of the C=N linkage and the CT bands as two adjacent
peaks. This can be ascribed to the existence of these com-
pounds in two tautomeric forms (Scheme III). The bands at
shorter wavelength are assigned to the absorption of the
hydroxy aryl form while those at longer wavelength sides
are due to the quinine-like structure.14
The spectra of If display a broad shallow band with
lmax 427 nm which originates from the contribution of a
p-quinone-like structure,14 originating from the possible
mesomeric shift (Scheme IV).
The electronic absorption spectra were used to calcu-
Fig. 5. Electronic absorption spectra of the Schiff bases under study.

1H NMR, IR, UV/VIS of Benzothiazole and Pyridine Schiff
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 881
3.3.2. Spectra in organic solvents of different polari-
ties
Scheme IV Hydroxy aryl and quinonid structures of If
The electronic absorption spectra of Ib and IIb were
measured in six organic solvents of different polarities
namely ethanol, methanol, DMF, chloroform, tetrachloro-
methane and cyclohexane (Table 4). These spectra indi-
cated that the bands due to localized electronic transitions
are slightly influenced by the nature of the solvent used. On
the other hand, the CT band displayed in general a red shift
with increasing the solvent polarity which reflects an in-
creased solvent stabilization of the excited state in more
polar solvents.
late the ionization potentials of the compounds under study
using the relation:
The application of the dielectric relations of Gati and
Szalay19 or those of Suppan20 to the data of the CT band did
not give linear relationships denoting that the dielectric
Ip = a + b ECT
(3)
in which a and b are constants having the values (4.93 &
0.857),15 (5.156 & 0.778)16 or (5.11 & 0.701).17 The calcu-
lated values together with the mean values are depicted in
Table 3.
constant of the medium did not play an important role in the
max
CT
CT band solvent shift. Also, the plot of E
in different
solvents as a function of the solvent parameters Z,21 ET,22
(acidity), b (basicity) and p* (dipolarity),23 showed nonlin-
a
The oscillator strength (f) of the CT band for some
compounds was also determined (Table 3) using the rela-
tion:18
ear relationships. This behaviour denotes that each solvent
max
CT
parameter will have its effective contribution to the E
band which depends also on the nature of the solute under
f = 4.6 ´ 10-9 emax Dn1/2
(4)
study. Accordingly, it can be concluded that the change of
max
CT
E
with solvent is the result of the influence of all these
parameters. These effects can be additive, counteracting or
even may cancel out each other.
where emax is the molar extinction coefficient at lmax and
Dn1/2 is the band width at half absorbance values.
Table 4. UV/Vis spectral data of Schiff bases Ib and IIb in organic solvents of varying
polarities
Band (A)
Band (B)
Band (C)
Band (D)
Com.
Ib
Solvent
l
e
l
e
l
e
l
e
Methanol
DMF
CHCl3
221
---
30.3
---
269
268
273
13.8
38.8
10.0
332
329
333
349
332
349
329*
344*
339
7.6
374
373
377
9.2
46.2
18.5
38.0
14.1
15.2
14.9
16.0
20.9
15.5
12.2
240
11.2
CCl4
Cyclohexane
Methanol
DMF
232
248
216
232*
213
229*
---
8.3
9.0
273
271
268
269
11.1
11.0
9.5
378
377
20.8
20.0
22.1
15.5
17.6
15.9
---
IIb
358
453
368
436
362
366
362
13.7
1.8
15.1
3.1
16
10.4
---
---
CHCl3
CCl4**
240
---
9.9
---
---
---
270
269
269
10.2
---
---
341
341
12.8
---
---
---
Cyclohexane** 215
230
322*
341*
---
---
* Shoulder, ** Saturated solution, emax ´ 10-3 (l mol-1 cm-1), lmax (nm).

882 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Issa et al.
3.3.3. Absorption spectra of IIa and IIb in buffer so-
lutions of varying pH
ergy changes (DG*) of each ionization step. The DG* val-
ues were found to be 31.3 and 48.2 kJ mol-1 in the case of
IIa and 41.2 and 55.7 kJ mol-1 in the case of IIb for the first
and second ionization steps, respectively.
2-Amino-3-hydroxypyridine Schiff bases IIa and IIb
are considered as dibasic acids. The dissociation of these
acids can be represented by the following equilibrium:
3.4. Fluorescence spectra
The fluorescence spectra of 2-amino-3-hydroxypyri-
dine Schiff bases IIa and IIb were investigated in organic
solvents of different polarities (Table 5). The results ob-
tained reveal that both the fluorescence quantum yield (ff)
and emission lmax depend on the molecular structure of the
Schiff base and the solvent used as medium. Also, Schiff
bases IIa and IIb give good fluorescence spectra and satis-
factory values for the fluorescence quantum yield (ff). This
behavior is good evidence for the formation of strong hy-
drogen bonding in these compounds (Scheme III) which in-
creased the stability of the molecule in the excited state.26
The solvent effect on emission spectra of IIb and IIe
is actually the result of the influence of various solvent pa-
The acid-base equilibrium of the Schiff
bases under study
Scheme V
where K1 and K2 are the dissociation constants for each
step.
The absorption spectra in the UV/Vis region (200-
600 nm) of Schiff bases IIa and IIb are studied in universal
buffer solutions of varying pH values containing 5% (v/v)
methanol. The spectra obtained reveal that the pH of the
medium has an obvious effect on both the position and ex-
tinction of the absorption peaks which can be explained in
the following points:
rameters. This fact appeared clearly on plotting the values
emm
of E
as a function of macro- or microscopic solvent pa-
a. The spectra of IIa showed two peaks with lmax at
300 and 395 in the strong acidic medium (pH < 4) corre-
sponding to the H2L form. The absorbance of these peaks
decreased with increasing the pH of solution with the ap-
pearance of a new absorption peak with lmax = 480 nm cor-
responding to formation of HL- and L2-. The extinction of
this peak increases with increasing pH of the medium.
b. For Schiff base IIb; the spectra show only one peak
with lmax = 310 nm in solution having pH < 6 correspond-
ing to the H2L form. The extinction of this peak decreased
with increasing pH value of the solution while a new ab-
sorption peak appeared at pH ³ 6 with lmax = 380 nm corre-
sponding to the HL- form. The absorbance of this peak in-
creased with increasing pH value of the solution until pH =
8 and remains constant to pH = 9, then it began to increase
after pH = 9 until pH = 12 which can be attributed to the
formation of L2-.
max
rameters, which gave nonlinear relationships, as in the case
of the ground state spectra.
3.5. Molecular orbital calculations
The atom superposition and electron delocalization
molecular orbital (ASED-MO) method27,28 is used to inves-
The pH-absorbance curves of Schiff bases IIa and IIb
for the longer wavelength bands are typical S-shaped (Fig.
6). The presence of two steps in each pH-absorbance curve
supports the existence of two types of equilibria (Scheme
V). These curves were utilized for the determination of the
two acid dissociation constants of these Schiff bases using
the half-height methods24 and the limiting absorbance
methods.25 The values of pK1 equal 5.4 and 7.1, whereas
pK2 is equal to 8.3 and 9.6 for IIa and IIb, respectively.
Also, the pK values were used in calculating the free en-
Fig. 6. The pH-absorbance curves of Schiff bases IIb
(l = 380 nm) and IIe (l = 480 nm).

1H NMR, IR, UV/VIS of Benzothiazole and Pyridine Schiff
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 883
Table 5. Data obtained from the emission spectra of Schiff bases
IIa and IIb in organic solvents of varying polarities
Table 6. Atomic parameters used in the calculations: principle
quantum numbers, n; ionization potentials, IP (eV);
orbital exponents, z (au)
Schiff base IIa
Schiff base IIb
Solvent
s
p
lemm (nm)
ff
lemm (nm)
ff
Atom
n
IP
z
n
IP
z
Ethanol
Methanol
DMF
Acetone
CH2Cl2
Toluene
492
496
505
495
501
510
0.039
0.036
0.023
0.018
0.061
0.015
500
502
506
498
504
527
0.035
0.031
0.19
0.013
0.058
0.009
C
N
O
H
2
2
2
1
16.590 1.8580
18.330 1.9240
28.480 2.2459
13.600 1.2000
2
2
2
11.260 1.8180
12.530 1.9170
13.620 2.2266
lemm is the emission wavelength and ff is the fluorescence
quantum yield.
spectively, with the double bond character for N7-C8 bond.
Also, the proposed keto-enol tautomerism (Scheme III) is
supported by the partial double bond character of C14-O15
with high contribution of the enolic form as mentioned be-
fore. The calculations reveal the formation of a hydrogen
bond (N7-H18) as assumed before with bond order = 0.0403
and bond length 1.89 Å. The calculated bond orders and
bond lengths over the whole-investigated molecule IIb are
shown in Table 7.
tigate the geometric and electronic structures as well as the
types of electronic transitions in the Schiff base IIb (Fig. 7).
This technique has already been applied for predicting mo-
lecular structures, reaction mechanism, and electronic and
vibrational properties.29-34 The input data consists of ion-
ization potentials35 (VSIP) and valence-state Slater orbital
exponents36 for the constituent atoms. Atomic parameters
used in the calculations are reported in Table 6; in all calcu-
lations the C-H bond lengths are kept constant at 1.10 Å.
As shown in Fig. 4, the trans form is found to be the
stable structure of Ib. The calculations indicated that the
phenyl ring is out of plane with respect to the pyridine moi-
ety. Strong distribution and rotation begin from C8 and con-
tinue for C9, C10, C11, C12, C13, C14 and O15. The Schiff base
formation is confirmed by the values of bond orders (bond
lengths) of C1-N7, N7-C8, and C8-C9 bonds which are equal
to 0.8545 (1.40), 1.0723 (1.33) and 0.8430 (1.44 Å), re-
The ASED-MO calculations for the different types of
electronic transitions in Schiff base IIb denote the follow-
ing points:
a. The lowest energy electronic transition between the
HOMO (-13.39562 eV) and the LUMO levels (-9.93915
eV) equals 3.45647 eV corresponding to CT electronic
transition. The calculated wavelength of this transition is
equal to 358.5 nm in satisfactory agreement with the exper-
Table 7. The calculated bond orders and bond lengths (Å) of
Schiff base IIb obtained from ASED-MO theory
Bond
Bond order
Bond length
C1-C2
C2-C3
C2-O16
C3-C4
C4-C5
C5-N6
C1-N6
C1-N7
N7-C8
C8-C9
C9-C10
C10-C11
C11-C12
C12-C13
C13-C14
C9-C14
C14-O15
0.9136
1.0012
0.7653
0.9718
0.9753
0.9771
0.9560
0.8545
1.0723
0.8430
0.9274
0.9970
0.9656
0.9960
0.9862
0.9818
0.7424
1.4100
1.3600
1.3000
1.3800
1.3800
1.3500
1.3704
1.4000
1.3300
1.4400
1.4200
1.3800
1.3800
1.3700
1.3600
1.3663
1.3200
Fig. 7. The investigated molecular structure of the
Schiff base IIb using ASED-MO theory.

884 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Issa et al.
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b. The p®p* transition within the C=N group gives
two bands at 306.9 and 329.1 nm with high coefficients on
N7, C8, C14 and O15 atoms. The experimental wavelengths
are equal to 306 and 334 nm, respectively, which is good
evidence for the proposed keto-enol tautomerism (Scheme
III).
9. Issa, R.-M.; Khedr, A.-M.; Rizk, H.-F. Spectrochimica Acta
A, 2005, 62, 621.
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c. The third electronic transition is a p-p*(Ar) transi-
tion with greater coefficients on C1, C3, C4, and N6 atoms
within 252 and 269.7 nm where the experimental values are
255 and 270 nm, respectively.
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d. The fourth type is p-p*(Ar) transition at 214.8 and
225 nm which could be described by an electron excitation
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characterized by coefficients of C10, C12, C13, C14 and C14
atoms to the LUMO level with a complete p* character. The
experimental wavelengths are 214 and 231 nm, respec-
tively.
17. Wheast, C. D. Hand Book of Chemistry and Physics, 5th ed.;
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ACKNOWLEDGMENT
The authors are grateful to Prof. Dr. Mohamed Khaled
Awad for his assistance in the molecular orbital calculation
carried out in this work.
26. Forster, T. Z. Elektochem. (Ber. Bunsengesell. Physik.
Chem.) 1950, 54, 43.
Received December 25, 2007.
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