UV-vis, IR and 1H NMR spectroscopic studies of some 6-chloro,2-pyridyl hydrazones

Article abstract of DOI:10.1016/j.saa.2005.10.032

The electronic absorption spectra of some 6-chloro,2-pyridyl hydrazones are studied in seven organic solvents of different polarity. The absorption bands are assigned to the corresponding electronic transitions and the effect of solvent parameters on the charge transfer energy (E_CT) is investigated. The spectra in buffer solutions of varied pH are also studied and utilized for the determination of the acid dissociation constants of the compounds under study. The fluorescence spectra were recorded for one of the studied compounds in six solvents, the solvent effect on the photoquantum yield and spectral pattern are also studied. Bands of diagnostic importance in the IR spectra and signals in the ¹H NMR spectra are assigned. The results of the present investigation are supported by some MO calculations using the atom super position and electron delocalization molecular orbital theory (ASED-MO) and Gaussian 94 program. The geometry is optimized using the PM3 method.

Full text of DOI:10.1016/j.saa.2005.10.032

Spectrochimica Acta Part A 65 (2006) 206–214
UV–vis, IR and 1H NMR spectroscopic studies of some
6-chloro,2-pyridyl hydrazones
Raafat M. Issaa, Ahmed A. Hassaneinb,
Ibrahim M. El-Mehassebc,∗, Reham I. Abed. El-Wadouda
a Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
b Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt
c Chemistry Department, Faculty of Education at Kafr El-Sheikh, Tanta University, Kafr El-Sheikh Branch, Kafr El-Sheikh, Egypt
Received 14 July 2005; received in revised form 10 October 2005; accepted 10 October 2005
Abstract
The electronic absorption spectra of some 6-chloro,2-pyridyl hydrazones are studied in seven organic solvents of different polarity. The absorption
bands are assigned to the corresponding electronic transitions and the effect of solvent parameters on the charge transfer energy (ECT) is investigated.
The spectra in buffer solutions of varied pH are also studied and utilized for the determination of the acid dissociation constants of the compounds
under study. The fluorescence spectra were recorded for one of the studied compounds in six solvents, the solvent effect on the photoquantum
yield and spectral pattern are also studied. Bands of diagnostic importance in the IR spectra and signals in the 1H NMR spectra are assigned. The
results of the present investigation are supported by some MO calculations using the atom super position and electron delocalization molecular
orbital theory (ASED-MO) and Gaussian 94 program. The geometry is optimized using the PM3 method.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrazone Schiff bases; Solvent effects; MO calculations
1. Introduction
optimized applying the PM3 method and the IR spectral data are
calculated from the optimal geometry. Also the UV–vis spectra
are calculated using the ASED-MO method.
Hydrazones and Schiff bases (azomethines) were the subject
of many interesting studies due to their important biological
andtechnicalapplications[1–4]. Inadditions, someazomethines
have been used as dyestuffs [5].
2. Experimental
The pyridylhydrazones under study were found to function as
chelating agents for transition metal ions [6] whereby the formed
complexes act as good catalysts for some chemical reactions.
The study of the molecular structure of these metal complexes
by spectral methods requires a good understanding of the spectra
of the ligands themselves which is the aim of the present study.
Accordingly, the present investigation includes an electronic
The hydrazones under study were prepared by the conden-
sation of 2,6-dichloro pyridine with hydrazine hydrate to obtain
6-chloro,2-hydrazopyridine. This latter was further condensed
with the required aldehyde or ketone in ethanolic solutions
using the conventional method for the preparation of hydra-
zones [7]. The formed hydrazones were then filtered off, washed
several times with ethanol, ether, pet. ether then dried under
vacuum. The purity of the compounds was tested by constancy
of melting points and data of elemental analysis. The hydra-
zones included in the present study have the general structural
formula (I):
1
absorption, fluorescence, IR and H NMR spectra of pyridyl-
hydrazones. The data of the spectral investigation are supported
by some MO calculations using the Gaussian 94 program and as
well the atom super position and electron delocalization molec-
ular orbital theory. The geometry of compound (1) was fully
∗
Corresponding author.
E-mail address: elmehasseb@yahoo.com (I.M. El-Mehasseb).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.10.032

R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
207
In which X and Y are
The spectral measurements were carried out at room tem-
perature (25 1.0 ◦C) as described before [8,9]. The UV–vis
spectra were recorded on a Shimadzu 240 spectrophotometer in
the 200–600 nm range while the IR spectra were obtained by the
aid of a Perkin-Elmer 1430 IR spectrophotometer as KBr discs
where IP is the ionisation potential of the donor part, EA the
electron affinity of the acceptor part namely the C N group
(−1.3 eV) and C is the coulomic force of attraction between
the electron transferred and the positive hole left behind it
(−5.6 eV).
1
within the range 4000–200 cm−1 region. The H NMR spec-
The ECT values thus obtained are in satisfactory agreement
with those calculated from the λmax values, Table 1, using the
equation
tra were recorded on a Bruker AM 300 MHz spectrometer using
TMS as internal standard. The fluorescence spectra were utilized
to calculate the photo-quantum yield referred to quinine sulphate
in 0.1N sulphuric acid (Φ = 0.55), the solutions used should have
absorption less than 0.1 at the exciting wavelength [10].
MO calculations were performed on a PC using Gaussian 94
program [11]. The geometry was fully optimized using the PM3
method [12]. The IR spectra were calculated for the optimal
geometry. Also the data of UV–vis spectra are analysed using
the atom super position and electron delocalization molecular
orbital theory (ASED-MO).
1241.6
ECT(eV) =
λmax(nm)
The oscillator strength (f) for the CT band of the pyridyl
hydrazones under study were determined applying the relation
[15]
f = (4.6 × 10−9)εmaxꢀν1/2
For which εmax is the molar extinction coefficient
(1 mol−1 cm−1) and ꢀν1/2 is the bandwidth at half value
of the absorbance. For compounds (1) and (2), the CT band
overlaps with the neighboring one on the shorter wavelength
side, hence its envelope was completed by considering the
3. Results and discussion
3.1. Electronic absorption spectra in organic solvents
The electronic absorption spectra of the pyridyl hydra-
zones were recorded in seven organic solvents of different
polarity, namely ethanol, methanol, DMF, dichloromethane,
trichloromethane, acetonitrile and ethyl acetate (Fig. 1).
The UV–vis spectra of compounds (3), (4) and (5) display
four bands, (Table 1). Bands A and B within the ranges 224–258
and 272–316 nm are assigned to the ␲–␲* transitions within
the aromatic rings. Band C (300–330) is assigned to the ␲–␲*
transitions within the C N bond and C O group if present, while
band D (332–368) is due to the intramolecular charge transfer
involving the whole molecule. Bands C and D in the spectra of
(3), (4) and (5) appear doubled or as a band and a shoulder due
to the probable existence of a tautomeric shift of the type.
The bands at shorter wavelength are assigned to the absorp-
tion of the enolic form (II) while the absorption at longer wave-
length would be due to the ketonic form (III) in analogy to a
previous observation on Schiff bases [13].
In the spectra of compounds (1) and (2), bands A and B or
C and D strongly overlap giving a broad band or a band and
shoulder. The CT character of the longer wavelength band is
gathered from its high extinction values and broadening of its
envelope. The intramolecular charge transfer would originate
from the hydroxyaryl moiety as a source to the C N bond as
a sink. This can be confirmed by calculating the energy charge
transfer (ECT) using the Briegleb relation [14]
ECT = (IP − EA) + C
Fig. 1. The electronic absorption spectra of (3) in different organic solvents.

208
R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
Table 1
Data from UV–vis spectra in ethanol
Cal. val., eV (nm)
Compound number
Item and assignment
(1)
(2)
(3)
(4)
(5)
4.82 (257)
4.29 (289)
3.74 (327)
3.59 (344)
250
2.90
258
2.01
230
9.80
234
8.73
224
9.60
λmax (nm) (Band A) (␲–␲* Ar)
εmax × 10−3
λmax (nm) (Band B) (␲–␲* Ar)
εmax × 10−3
λmax (nm) (Band C) (␲–␲* C N)
εmax × 10−3
λmax (nm) (Band D) CT
εmax × 10−3
284
5.06
272
2.80
290
2.20
288
1.26
316
2.00
328
9.60
326
3.80
310
3.20
300
1.13
330
2.53
336
10.00
334
4.00
336
4.00
332
1.60
368
3.33
3.70
4.21
0.689
3.72
3.70
4.56
0.924
3.74
3.37
ECT (ev) (from λmax CT)
ECT (calculated from Briegleb equation)
f
0.569
0.809
0.483
7.58
8.05
7.72
7.78
7.58
8.05
7.72
7.78
7.56
8.03
7.70
7.76
7.62
8.07
7.73
7.10
7.28
7.78
7.47
7.5
IP [15]
[16]
[17]
Mean value (ev)
band envelope to follow a Gaussian curve. The f values for
compounds (1) and (2) would be the subject to some over
estimation. The values of f thus obtained are collected in
(Table 1).
The electronic absorption spectra of the pyridyl hydrazones
under study were made use of to calculate the ionization poten-
tials of the molecules using the general equation:
Based on these results, the solvent shift of the CT band would
be the resultant of the various factors governed by the different
solvent parameters. These may be additive, counter acting or
even may cancel out each other. This assumption was tested by
applying a modified form of the relation given by Krgyowiski
and Fawcett [24] as follows:
ECST = EC0 T + x0D + x1Z + x2α + x3β + x4π
In which ECST is the energy of the CT band at λmax in a given
solvent, EC0 T is the energy of the CT band at λmax in the gas
phase and x0, x1, . . . are the slopes of ES, solvent parameter
plots.
IP = a + bEmax
where IP is the ionization potential of the molecule, a and b are
constants having the values 4.39 and 0.857 [16], 5.15 and 0.778
[17] or 5.11 and 0.701 [18], and Emax is the energy of the lowest
electronic transition.
The IP values calculated using the different values for a and
b are in good agreement with each other. The mean values for
IP are also determined and collected in Table 1.
This relation gave very low correlation coefficient (r) but
on decreasing the number of solvent parameters in the equa-
tion, the value of (r) increased but were still unsatisfactory
(r = 0.799–0.935).
On using the linear regression calculation for the various ECT
,
solvent parameters plots, it was found that the correlation coeffi-
cient (r) increased with regression of the solvents falling far from
linearity. Satisfactory values of (r) were obtained for three to
four solvents only with the various solvent parameters (Table 2).
Thus, it seems that there are poor correlations between ECT and
the solvent parameters which indicates that the solvent effect
mechanism is complex.
The electronic absorption spectra recorded in seven solvents
of different polarity showed that bands A, B and C, those due
to localized ␲–␲* electronic transitions, displayed only small
changes in their λmax and εmax values. On the other hand, the
CT band (D) displayed obvious shifts in the various solvents
(Fig. 1).
The application of the Gati and Szalay [19] or Suppan [20]
dielectric relations to the data of the CT band did not give linear
relations indicating that the dielectric constant of the solvent is
not the main factor affecting the CT band shift. This reveals that
the specific solute solvent interactions involving solvation of
both the ground and excited states as well as hydrogen bonding
can be the dominant factors.
3.2. Electronic absorption spectra in buffer solutions
The UV–vis spectra of the hydrazones were studied in buffer
solutions of different pH (2–12) containing 20% (v/v) ethanol to
affect the solubility of the organic compounds. The bands due
to localized ␲–␲* transitions displayed slight changes, while
the CT band showed interesting changes with pH increase. The
spectra of compounds (1) and (2), not containing the phenolic
OH group, exhibited changes in the extinction and position of the
CT band only within the acid range of pH. The band absorbance
The plot of ECT as a function of the various molecular micro-
scopic solvent parameters essentially Z [21], E [22], α (acidity),
β (basicity) and π* (dipolarity) [23] also showed nonlinear rela-
tions. This indicates that these solvent parameters are not the
main cause of the band solvent shifts.

R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
209
Table 2
Values of the correlation coefficient in different solvents
Solvent pararmeter
Values of (r) and solvents included in its calculation
All solvents
Five solvents
Four solvents
Three solvents
Results for compound (1)
D
Z
0.049
0.441
0.490
0.329
0.065
0.952(1,2,5,6,7)
0.851(1,2,3,6,7)
0.490(1,2,4,5,6)
0.793(1,2,4,5,6)
0.961(1,2,4,6,7)
0.943(1,2,5,6)
0.920(2,3,6,7)
0.693(1,2,5,6)
0.899(1,2,5,6)
0.960(1,2,4,7)
0.999(1,5,6)
0.970(2,3,7)
0.831 (1,5,6)
0.996(1,2,5)
0.991(1,2,7)
α
β
π*
Results for compound (2)
D
Z
0.205
0.220
0.352
0.050
0.636
0.960(1,2,5,6,7)
0.437(1,2,4,5,7)
0.352(1,2,4,5,6)
0.557(1,2,4,5,6)
0.838(2,3,5,6,7)
0.986(1,2,6,7)
0.981(1,2,5,7)
0.466(1,2,5,6)
0.721(1,2,5,6)
0.997(2,3,6,7)
0.991(1,6,7)
0.981(1,2,7)
0.987(1,2,6)
0.915(1,2,6)
0.996(2,3,6,)
α
β
π*
Results for compound (3)
D
Z
0.006
0.050
0.137
0.121
0.475
0.489(1,2,4,6,7)
0.779(1,2,3,6,7)
0.137(1,2,4,5,6)
0.120(1,2,4,5,6)
0.918(1,2,4,6,7)
0.991(1,2,6,7)
0.933(1,2,3,6)
0.962(1,2,4,5)
0.373(1,2,5,6)
0.895(1,2,4,6)
0.991(1,6,7)
0.966(1,3,6)
0.983(1,2,5)
0.915(1,2,6)
0.980(2,4,6)
α
β
π*
Results for compound (4)
D
Z
0.734
0.172
0.741
0.762
0.597
0.956(1,2,3,5,6)
0.634(1,2,4,5,7)
0.740(1,2,4,5,6)
0.942(2,3,5,6,7)
0.630(1,3,4,6,7)
0.993(1,2,5,6)
0.986(1,2,4,5)
0.961(1,2,4,5)
0.962(3,5,6,7)
0.941(1,3,4,6)
0.992(2,5,6)
0.986(1,2,4)
0.992(1,2,4)
0.989(5,6,7)
1.000(3,4,6)
α
β
π*
Results for compound (5)
D
Z
0.856
0.244
0.513
0.503
0.761
0.966(1,3,5,6,7)
0.713(3,4,5,6,7)
0.513(1,2,4,5,6)
0.743(1,3,4,6,7)
0.991(2,4,5,6,7)
0.971(1,3,5,7)
0.935(3,4,5,7)
0.642(2,4,5,6)
0.968(3,4,6,7)
0.996(4,5,6,7)
0.998(1,5,7)
0.960(3,5,7)
0.871(2,5,6)
0.993(3,6,7)
1.000(4,5,7)
α
β
π*
Solvents used: 1, ethanol; 2, methanol; 3, DMF; 4, ethyl acetate; 5, dichloromethane; 6, acetonitrile; 7, chloroform.
increased and the band was slightly shifted to red with increasing
the pH of the medium. The pH-absorbance curves showed a
single step on the acid side.
longer wavelength. The change of absorbance with pH at some
wavelengths showed also two steps; one on the acid side and
the other on the alkaline range of pH (Fig. 3). On considering
the behaviour of all five hydrazones studied, it can be concluded
that the acid base equilibrium on the acid side would involve the
dissociation of a proton from the protonated pyridine ring occur-
ring with all five compounds. On the alkaline side, the ionization
For compound (3), the absorbance of the CT band decreased
with rise of pH while a new band is developed at longer wave-
length; clear isosbestic points are observed denoting the exis-
tence of an equilibrium of the acid base type (Fig. 2). The pH
absorbance curves display two clear steps indicating the estab-
lishment of two acid base equilibria over the whole pH range. For
compounds (4) and (5), the absorbance of the CT band increased
with rise of pH and a new absorption shoulder developed at
Fig. 3. The change of absorbance with pH of (3) at λmax = 335 nm.
Fig. 2. Effect of pH on the electronic absorption spectra of (3).

210
R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
Table 3
Data of UV–vis spectra in buffer solutions and pKa values
(1)
(2)
(3)
(4)
(5)
λ(nm)
250
340
250
335
295
335
290
330
325
365
Data for the first ionisation step (pKa1) protonated pyridine ion ring
3.20
3.60
3.90
–
3.40
3.70
3.10
–
4.50
4.20
4.60
–
4.50
4.90
4.40
–
6.10
6.20
6.50
6.30
6.20
6.50
6.10
5.90
6.10
6.30
6.40
–
6.30
5.10
4.90
5.20
–
4.80
5.10
4.60
–
HHM
LAM
MLAM
MIPM
6.40
6.20
–
3.40
3.55
4.43
4.60
6.30
6.18
6.27
6.30
6.07
4.83
Mean value
3.48
4.52
6.23
6.28
4.95
9.60
Mean of total
Data for the second ionisation step (pKa2) ionisation of the phenolic group
10.40
10.80
10.40
10.50
10.50
10.20
10.70
10.50
–
10.10
10.80
9.90
–
9.90
9.50
9.30
–
9.50
9.80
9.60
–
HHM
LAM
MLAM
MIPM
10.90
10.80
10.80
10.53
10.75
10.26
9.56
9.63
Mean value
Mean of total
10.64
10.37
of the phenolic OH group takes place in case of compounds (3),
(4) and (5). These equilibria can be represented as follows:
ThepHabsorbancechangeswereutilizedtocalculatetheacid
dissociation constants of the protonated species and the phenolic
OH groups. This was achieved by applying the half height, limit-
ing absorbance, modified limiting absorbance and modified isos-
besticpointmethods [25,26]. Thevaluesobtainedaredepictedin
(Table 3). The data reflect that the presence of the CH3 group on
the azomethine carbon has but little influence on the pK values,
compounds (3) and (4), while for compound (5) the naphthyl
moiety lowers the pK values obviously. This behaviour recalls
that observed for the ECT and IP values of this compound.
3.3. IR spectra
The IR spectra of the hydrazones (1–5) were recorded in the
solid state as KBr discs (Fig. 4). The bands of diagnostic impor-
tance are listed in (Table 4), together with the values calculated
for the optimized geometry.
The spectra of all compounds exhibit an intense sharp band at
3337–3189 cm−1 due to the stretching mode of the N–H group.
For compounds (3), (4) and (5), the stretching vibration of the
phenolic OH group leads to a very broad band extending over the
spectral range 3300–2700 cm−1 falling below the νNH and νCH
bands. The low values for the νOH bands denote the involvement
of the OH groups in hydrogen bonding which is stronger in case
of (4) and (5) than for (3). The weak bands at 3190–3050 cm−1
are due to νCH of the aromatic rings. Compounds (2) and (4) give
the νCH within the range 2990–2860 cm−1 as two weak bands.
3
The C N band is observed at 1660–1620 cm−1, the lower
values for compounds (3), (4) and (5) is due to the contribution
Fig. 4. The IR spectra of hydrazone derivatives of 6-chloro,2-hydrazopyridine.

R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
211
Table 4
Some selected bands of diagnostic importance from the IR spectra of the pyridyl hydrazones 1–5
Compound number
Assignment
Calculated values
(1)
(2)
(3)
(4)
(5)
3264
3189
3032
–
3337
3181
3085
2995
2924
–
1660
1326
–
1280
1171
–
981
876
788
725
698
3264
3180
3044
–
3273
3154
3050
–
νNH
νCH (Ar) asym.
νCH (Ar) sym.
3323
3111–3127
3082–3011
3329
3052
2980 (sh)
2923
1678 (sh)
1590
1373
1301
1254
1179
1143
979
νCH (CH3) asym.
3
–
–
–
–
νCH (CH3) sym.
3
1660
1615
–
1678
1592
–
νC (toutomeric)
O
1640
–
νC
δCH
1627
N
3
–
1306
1213
1190
1133
978
871
783
–
1320
1246
1159
1159 (sh)
983
864
779
–
δOH
νC–N
1216
1180
–
982
857
771
732
692
1286–1221
1168–1185
νN–N
νC–OH
Py ring breath
γCH
975–999
839–859
763–797
711–763
700
840
773
–
649
γCH
νC–S
685
650
γCH
Table 5
Assignment of the 1H NMR signals
of the C N group to hydrogen bonding with the phenolic OH
group on the aromatic ring in o-position to the C N group. The
medium to strong bands at 1320–1306 and 1159–1133 cm−1
are assigned to the δOH and νC–OH modes of the phenolic group.
The stretching mode of the C–N is observed at 1254–1213 cm−1
while that of the N–N lies at 1180–1133 cm−1. The CH3 groups
(1)
(2)
(3)
(4)
(5)
–
Assignment
CH3
–
2.56
–
2.63
7.06–8.19 7.92–8.46 7.10–8.30 7.33–8.34 7.39–7.80 Aromatic ring
8.80
11.16
–
–
9.70
10.30
12.20
–
8.27
10.85
13.10
N
CH
of (2) and (4) give the δCH at 1326 and 1373 cm−1, respectively.
10.39
–
10.54
13.12
NH
OH
3
The sharp medium intensity band at 983–978 cm−1 is assigned
to the pyridine ring breath mode of vibration [27]. It is worthy
mentioning that the spectra of compounds (4) and (5) exhibit
a strong band at 1678 cm−1, which appears as a shoulder in
the spectrum of (3) at 1660 cm−1. This band can be assigned
to the stretching mode for the C O group resulting from the
tautomeric shift between the OH and C N groups involved in
the intramolecular hydrogen bonding. This is in accordance with
the observation gained from the electronic absorption spectra
mentioned above.
is assigned to the hydrogen of the NH group, with integration
equivalent to one H. The singlet at 8.27–9.7 ppm with integra-
tion of one H is due to the azomethine hydrogen (CH N), while
the group of multi signals at 7.06–8.46 ppm corresponds to the
hydrogens of the aromatic rings. The spectra of (2) and (4)
exhibit a sharp singlet with integration equivalent to 3 hydro-
gens at 2.56 and 2.63 ppm, respectively, due to the CH3 groups.
The sharp medium bands within the 960–650 cm−1 range
can be assigned to the various out of plane deformations of the
aromatic C–H groups [28,29]. The assignment of the main bands
is supported by the data of MO calculations as given in Table 4
for compound (1), using the PM3 method [12] and Gaussian 94
program [11].
3.5. Fluorescence spectra
The flourescence spectra of compound (1), as a representative
example, have been measured in six solvents of different polarity
and hydrogen bonding donor ability. Fig. 5 shows the fluores-
cence spectra and the data are given in Table 6. As can be seen
from (Fig. 5), the flourescence spectra in the hydrogen bonding
donor solvents ethanol, CHCl3 and CH2Cl2 display one band
whileintheother(aprotic)solventstwobandsareobservedwhen
excited at 330 nm. The high Stokes shift of the band appearing
in aprotic solvents can be attributed to an excited state with
3.4. 1H NMR spectra
The hydrogen magnetic resonance spectra of the hydrazones
involved in the present investigation were recorded using TMS
as internal standard (Table 5), the signals at 12.2–13.12 ppm
equivalent to one hydrogen in the spectra of (3), (4) and (5) is due
to the OH group. The strong down field shift of this signal sup-
ports the contribution of the OH– group to the intramolecular H–
bond with the C N linkage. The signals of (4) and (5) are down
field shifted relative to that of (3) denoting stronger intramolec-
ular hydrogen bonding for (4) and (5). This is in conformity
with the results of the IR spectra. The signal at 10.3–11.16 ppm
Table 6
Fluorescence maxima and quantum yield of the azomethine (1) measured in
some solvents at room temperature, λexc = 330 nm
Solvent
EtOH
CHCl3
CH3CN
Et. acetate
CH2Cl2
DMF
λf (nm)
φf
382
0.0073
373
0.031
375.502
0.025
378.484
0.0119
376
0.0094
392.490
0.0289

212
R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
Table 7
Atomic parameters used in the calculations: principle quantum numbers, n; ion-
ization potentials, IP (ev); orbital exponents, ζ (AU)
Atom
S
P
n
IP
ζ
n
IP
ζ
C
N
S
Cl
H
2
2
3
3
1
16.590
18.330
21.200
24.540
13.600
1.8850
1.92400
2.7200
2.5562
1.2000
2
2
3
3
11.260
12.530
11.360
13.010
1.8180
1.9170
2.3273
2.0388
Table 8
˚
The calculated bond lengths (A), and bond angle obtained from ASED-MO
method of compound (1)
Fig. 5. The flourescence spectra of (1).
Bond
Bond length
Bond
Bond angle
1, 2
3, 4
5, 6
11, 12
14, 15
17, 18
18, 22
1, 6
1.353
2, 1, 6
3, 2, 7
4, 3, 8
5, 4, 9
6, 5, 10
5, 6, 11
6, 11, 13
11, 13, 14
13, 14, 16
14, 16, 17
16, 17, 21
17, 18, 19
19, 18, 22
18, 19, 23
1, 2, 3
2, 3, 4
3, 4, 5
4, 5, 6
1, 6, 5
6, 11, 12
12, 11, 12
13, 14, 15
15, 14, 16
16, 17, 18
18, 17, 21
17, 18, 22
18, 19, 20
20, 19, 23
1, 2, 7
118.7201
120.1734
121.3049
120.0583
121.2019
119.8398
117.5206
123.3534
127.1197
125.7062
125.1481
112.2645
124.9902
125.5178
122.6727
118.3731
119.8857
118.7969
121.5509
112.1444
113.8694
114.9936
117.8831
112.3345
122.5076
122.7451
112.4994
121.9828
117.1538
120.322
intramolecular proton transfer. This is confirmed by the dis-
appearance of such a band in hydrogen bond donor solvents
such as ethanol. Also, a cursory glance at Table 4 indicates that
the solvent strongly affects the moderate flourescence quantum
yield calculated for this compound. The flourescence is strongly
quenched in hydrogen bond donor solvents relative to the other
ones. This is due to the enhanced radiationless internal conver-
sion assisted by hydrogen bonding. The relatively low φf-values
obtained here may be due to the high flexibility of the compound
under investigation which allows free rotations responsible for
flourescence quenching and dissipation of excitation energy.
1.3915
1.4082
1.0008
1.0992
1.4329
1.0903
1.3607
1.0945
1.0959
1.3859
1.4588
1.0909
1.7202
1.3971
1.3893
1.4399
1.2956
1.3372
1.3666
1.0893
1.6919
1.0951
3, 8
5, 10
11, 13
14, 16
17, 21
19, 20
2, 3
4, 5
3.6. Quantum chemical calculations
6, 11
13, 14
16, 17
18, 19
19, 23
2, 7
The atom superposition and electron delocalization molecu-
lar orbital (ASED-MO) theory [30,31] is used. This is a semiem-
pirical theoretical approach based on partitioning the electronic
charge density into free atom parts and a component due to
electron delocalization bond formation. Using the electrostatic
theorem, the forces on the nuclei are integrated as the atoms
are brought into a molecular configuration to yield a repul-
sive energy due to electron delocalization. The sum is the exact
molecular binding energy.
In the ASED-MO method the atom superposition energy was
calculated from actual atomic densities and the electron delocal-
ization is approximated as the change in the total one-electron
valence orbital energy obtained by using a modified extended
4, 9
2, 3, 8
3, 4, 9
4, 5, 10
1, 6, 11
120.056
120.0007
118.436
¨
Huckel Hamiltonian. The ASED-MO theory has been applied
in numerous diverse studies of molecular structures, reaction
mechanism as well as electronic and vibrational properties (for
recent papers, see references [32–36]). The input data consist of
valence-stateSlater-orbital-exponents[37]andionizationpoten-
tials [38] (VSIP) for the constituent atoms. These parameters are
sometimes altered, particularly in treating ionic heteronuclear
molecules, to ensure reasonably accurate calculations of ionic-
ities and bond lengths in diatomic fragment molecules. In this
way, electronic charge self-consistency is taken into account.
The diatomic parameters are used in studying larger
molecules. The atomic parameters used for C, N, S, Cl and H in
all calculations in this paper are in (Table 7). Because of ionic-
ity, the ionization potentials for N are decreased by 2.0 eV and
increased for S by 1.0 eV from the atomic values [38], so that
the charge transfer in diatomic molecule is close to that deduced
from Pauling’s ionicity relationship. The Slater exponents of C
2s and 2p are increased by 0.2 AU and by 0.5 for S 3s and 3p from
the atomic values of Clementi and Raimondi [37]. Unshifted
atomic parameters for Cl and H atoms given in (Table 8), are
used. In all reported results for equilibrium structures, the bond
˚
lengths are variationally optimized to the nearest 0.01 A, and the
bond angles to the nearest degree. In all calculations the C H
˚
bond lengths are kept constant at 1.10 A.

R.M. Issa et al. / Spectrochimica Acta Part A 65 (2006) 206–214
213
to −0.5071e. The hydrazo form is also confirmed from the cal-
culations of small negative charge on N9, −0.2946e and electron
deficiency on C10, 0.2574e. A slight electron deficiency is also
found on S atom, 0.1018e while a slight charge density on C18
and N7 atoms, −0.1868 and −0.0172e, respectively. The high-
est electron deficiency is calculated for C2 and C4 atoms with
charges equal to 0.3791 and 0.4903e, respectively.
Semiempirical calculations are used to study the electronic
structure and types of transitions of the investigated compound.
The calculated orbital energies are shown in (Fig. 6). The cal-
culations show that the lowest energy transition between the
HOMO–LUMO levels is equal to 1.460 eV. This transition cor-
responds to ␲ → ␲* transition mainly described by one electron
excitation from the HOMO level at −11.261 eV, significantly
characterized by coefficient of lone-pair on N7 atom, to the
LUMO level at −9.801 eV, with complete ␲* character with
greater coefficients on C13 and C14 atoms. Similar n → ␲*
transitions are calculated with excitation energies equal to 1.727
and 2.232 eV corresponding to a transition from the lone pair
orbital on S atom to the LUMO level with complete ␲* character.
The calculations indicate excitations with energies of 3.740 eV
(327 nm) and 4.290 eV (289.0 nm), which are in a good agree-
ment with the experimental ones (Table 1); these correspond to
␲ → ␲* transitions; this is mainly described by electronic exci-
tations from the levels at −13.087 and −14.090 eV, respectively.
These are significantly characterized by greater amplitude coef-
ficients on C5 and C6 atoms to the LUMO level with complete
␲* character.
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