Spectral regression and correlation coefficients of some benzaldimines and salicylaldimines in different solvents

Article abstract of DOI:10.1016/j.saa.2005.04.020

Sixteen Schiff bases obtained from the condensation of benzaldehyde or salicylaldehyde with various amines (aniline, 4-carboxyaniline, phenylhydrazine, 2,4-dinitrophenylhydrazine, ethylenediamine, hydrazine, o-phenylenediamine and 2,6-pyridinediamine) are

Full text of DOI:10.1016/j.saa.2005.04.020

Spectrochimica Acta Part A 63 (2006) 255–265
Spectral regression and correlation coefficients of some benzaldimines
and salicylaldimines in different solvents
Hassan H. Hammuda,∗, Amer Ghannouma, Mamdouh S. Masoudb
a
Chemistry Department, Faculty of Science, Beirut Arab University, P.O. Box 11-5020 Beirut, Lebanon
b
Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt
Received 27 December 2004; accepted 6 April 2005
Abstract
Sixteen Schiff bases obtained from the condensation of benzaldehyde or salicylaldehyde with various amines (aniline, 4-carboxyaniline,
phenylhydrazine, 2,4-dinitrophenylhydrazine, ethylenediamine, hydrazine, o-phenylenediamine and 2,6-pyridinediamine) are studied with
UV–vis spectroscopy to observe the effect of solvents, substituents and other structural factors on the spectra. The bands involving different
electronic transitions are interpreted. Computerized analysis and multiple regression techniques were applied to calculate the regression and
correlation coefficients based on the equation that relates peak position λmax to the solvent parameters that depend on the H-bonding ability,
refractive index and dielectric constant of solvents.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Schiff bases; Salicylaldimine; Benzaldimine; Electronic spectra; Solvent effects; Regression coefficients
taken: N,Nꢀ-bis(salicylidene)-1,2-diaminoethane and N,Nꢀ-
bis(salicylidene)-1,6-hexanediamine [9], N-salicylidene-1-
hexadecylamine [10], N,Nꢀ-bis(salicylidene)-1,2-cyclohexa-
nediamine [11] and N-salicylidene-␣-methylbenzylamine
[12]. The tautomeric equilibrium of salicylidene Schiff
bases is the subject of considerable interest from both
theoretical and practical point of view [13–15]. The
position of proton transfer equilibrium in solution is
affected by interactions with the solvent molecules. The
tendency of interconversion to ketoamine has been observed
in polar solvents for N-(R-salicylidene)-benzylamine
[16], N-(R-salicylidene)-methylamines [17,18] and N,Nꢀ-
bis(salicylidene)-2,6-pyridinediamine [19].
Although UV–vis spectral behavior of Schiff bases have
been extensively investigated in recent years, only spare
information is available about the effect of different solvent
parameters on the position of observed absorption peak [17].
In a continuation of our previous studies of computerized
correlation data for the solvent effects on the electronic spec-
tra of organic compounds (barbiturate and azo compounds)
[20–25], the present work was undertaken. This paper deals
with solvent and structural effects on the UV–vis absorption
spectra of 16 Schiff bases derived from benzaldehydes or
1. Introduction
The research on Schiff base compounds is growing
because of their biological importance [1–3]. Recently,
Co(III) complexes with Schiff bases were reported as a new
anticancer agents with radio/thermosensitizing activities [4].
It was also found that Co–salen complexes bound to DNA
in an intercalative model supported by fluorescence spectral
studies (salen is bis(salicylidene)ethylenediamine) [5].
The study of solvent effects on the UV absorption spectra
of Schiff bases has received great deal of attention. The
benzaldimines studied are: substituted N-(R-benzylidene)
benzidine and N-(R-furfurylidene)benzidine [6], N-benzy-
lidene and N-naphthylbenzylidene-2-aminopyrimidine
[7] bis(X-benzylidene-o-phenylenediamine [8]. Salicy-
laldimines show important photochromism where light
absorption causes interconversion between enol-imine and
keto-amine tautomers through intramolecular hydrogen
transfer. Several spectroscopic studies on the photochrom-
isms of salicylaldimines of different structures were under-
∗
Corresponding author. Tel.: +961 3 381862; fax: +961 1 818402.
E-mail address: h.hammud@bau.edu.lb (H.H. Hammud).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.04.020

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H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
Table 1
Numbers and names of Schiff bases
IA
IIA
N-Benzylideneaniline
N-(4-Carboxyphenyl)benzylideneimine
Benzaldehydephenylhydrazone
2,4-Dinitrobenzaldehydephenylhydrazone
N,Nꢀ-bis(Benzylidene)ethylenediamine
N,Nꢀ-bis(Benzylidene)hydrazine
N,Nꢀ-bis(Benzylidene)-o-phenylenediamine
N,Nꢀ-bis(Benzylidene)-2,6-pyridinediamine
IB
IIB
N-Salicylideneaniline
N-(4-Carboxyphenyl)salicylideneimine
Salicylaldehydephenylhydrazone
2,4-Dinitrosalicylaldehydephenylhydrazone
N,Nꢀ-bis(Salicylidene)ethylenediamine
N,Nꢀ-bis(Salicylidene)hydrazine
N,Nꢀ-bis(Salicylidene)-o-phenylenediamine
N,Nꢀ-bis(Salicylidene)-2,6-pyridinediamine
IIIA
IVA
VA
VIA
VIIA
VIIIA
IIIB
IVB
VB
VIB
VIIB
VIIIB
salicylaldehydes and various amines (Table 1; Figs. 1–4).
Correlation data was computed in order to evaluate the
solvent–solute interaction effects on the electronic absorption
spectra of Schiff bases. Several absorption peaks in a single
compound was studied independently by regression analy-
sis and high positive correlation coefficients were obtained.
The effect of molecular structures of the compounds on their
spectral behavior is also discussed.
Fig. 1. Structure of Schiff bases IA, IB, IIA and IIB.
Fig. 2. Structure of Schiff bases IIIA, IIIB, IVA and IVB.

H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
257
Fig. 3. Structure of Schiff bases VA, VB, VIA and VIB.
Fig. 4. Structure of Schiff bases VIIA, VIIB, VIIIA and VIIIB.
2. Experimental
hyde or salicylaldehyde with the amine, (aniline) giving IA
[26] or IB [27]; (4-carboxyaniline), IIA or IIB; (phenylhy-
drazine), IIIAorIIIB[28];(2,4-dinitrophenylhydrazine), IVA
or IVB (Table 1; Figs. 1 and 2). The bis-benzylidenimines
and -salicylidenimines Schiff bases were also synthesized
in ethanol by the condensation of 2–1 molar amounts
of the benzaldehyde or salicylaldehyde with the amine,
All chemicals used in the present work were Merck
products. The organic solvents used were spectroscopic
grade products (>99.8%). The mono-benzylidenimines and -
salicylidenimines Schiff bases were synthesized in ethanol
by the condensation of equimolar amounts of benzalde-

258
H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
Table 3
(ethylenediamine) giving VA or VB [29,30]; (hydrazine)
VIA or VIB; (o-phenylenediamine) VIIA or VIIB [28,31];
(2,6-pyridinediamine) VIIIA or VIIIB [19,28,32] (Table 1;
Figs. 3 and 4). The crude products were purified by recrystal-
lization from ethanol. The structures (Figs. 1–4) and purity
of the compounds were checked by mp, elemental analysis,
IR and NMR spectra [33].
λmax (nm) values for the Schiff bases IA–IVB compounds in different
solvents
Solvents
IA
IB
IIA IIB IIIA IIIB IVA IVB
Carbontetrachloride 315 343 271 347 345 350 367 373
263 323
325
307
277
304
297
The electronic spectra of the solution (concentration
1 × 10−5 to 1 × 10−6 mol/L) were investigated in various
organic solvents of different polarities: carbon tetrachlo-
ride (CCl4), diethylether, dioxane, chloroform (CHCl3),
acetone, N,Nꢀ-dimethylformamide (DMF), dimethylsulfox-
ide (DMSO), acetonitrile, ethanol (EtOH) and methanol
(MeOH). The electronic spectra were recorded on a Ciba
Corning 2800 Spectrophotometer using 1 cm quartz cell at
25 ◦C. The electronic absorption spectra were recoded imme-
diately after preparing the solutions in order to obtain the
spectra of mainly the enolimine tautomer and avoiding inter-
ference from the second ketoamine tautomer in the case of
salicylaldimines.
Diethylether
Dioxane
328 340
316
344 344 350 380 389
321
300
268
275
320 339 267 344 345 348 376 381
263 320
322 303 302 296
306 246 245
277
Chloroform
Acetone
315 340 272 345 341 345 379 382
264 319
321 303 302 297 297
307 245 246
278
268
327 339
341 345 346 380 387
Dimethylformamide 308 340 272 345 349 353 390 400
268 318
322 305 303 298
306 257
278
3. Results and discussion
Dimethylsulfoxide
Acetonitrile
Ethanol
312 340 274 344 353 356 395 408
3.1. Method of calculations and results
268 320
324 306 304 299
306 260 260
280
The absorption maximum (λmax) of Schiff bases
molecules is shifted due to solvent effects (Tables 2–4). The
absorption spectra of Shiff bases in a single solvent are shown
in Figs. 5–8. While Fig. 9 shows the influence of different
solvents on the UV–vis absorption spectra of compound IVB
and VB.
271
319 336 268 342 341
380 383
296 294
260 316
320
305
275
312 338 274 344 346 348 378 388
An empirical expression (1) have been developed for the
evaluation of solvent effects [20,23–25]:
263 316
301
321 303 302 295 295
305 242 245
277
268
Y = a0 + a1X1 + a2X2 + a3X3 + . . . + anXn
(1)
Methanol
310 336 275 341 344 346 376 387
261 315
300
320 303 300 293 294
304 240 243
275
The observed peak location λmax (Y) is considered as the
dependentvariablewhiletheindependentvariableshavebeen
selected to be the solvent interaction mechanisms E, K, M,
N (Tables 2–4). The constant a1, a2 and a3 are the different
regression coefficients and the constant a0 is the regression
intercept.
268
The parameter E is an empirical solvent polarity parameter
sensitive to both solvent–solute hydrogen bonding and dipo-
lar interactions, E = 2.859 × 10−3 ν, where ν is the wavenum-
ber of the absorption maximum in the given solvent.
K depends on the solvent dielectric constant D and is a
measure of the polarity of the solvent:
Table 2
Solvent dielectric constant, refractive index and parameters used in spectral
correlation equations
Solvent
D
n
E
K
M
N
ꢀꢁ
ꢂꢃ
Carbon tetrachloride
Diethylether
Dioxane
Chloroform
Acetone
Dimethyl formamide
Dimethyl sufoxide
Acetonitrile
Ethanol
2
4.2
2.2
1.426
1.353
1.422
1.443
1.359
1.427
1.478
1.344
1.361
1.329
32.5
34.6
36
39.1
42.2
43.8
45
46
51.9
55.5
0.2
0.22
0.18
0.2
0.21
0.18
0.2
0.22
0.18
0.18
0.17
0.01
0.3
D − 1
K =
0.34
0.22
0.36
0.46
0.48
0.49
0.48
0.47
0.48
2D + 1
0.03
0.29
0.65
0.67
0.66
0.71
0.67
0.71
4.7
M depends on the solvent refractive index n and is a measure
of solute permanent dipole–solvent induced dipole interac-
tions:
20.7
36.7
48.9
37.5
24.3
32.6
ꢀꢁ
ꢂꢃ
n2 − 1
M =
2n2 + 1
Methanol

H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
259
Table 4
λmax (nm) values for the Schiff bases VA–VIIIB compounds in different
solvents
Solvents
VA VB VIA VIB VIIA VIIB VIIIA VIIIB
Carbon
tetrachloride
256 323 302 363 298
295
340
273
320
373
278
Diethylether
245 331
355 291
335
241
Dioxane
245 320 302 358 299
257 293
335
271
320
247
372
278
Chloroform
Acetone
249 318 304 360 298
337
270
319
247
374
279
258
295
328
355
336
373
Dimethyl-
formamide
261 316 303 356 295
296
335
273
322
325
379
279
Dimethyl-
sulfoxide
262 316 305 358 298
334
272
380
278
264
296
Acetonitrile
244 315 298 355 301
255
331
268
319
244
374
280
Ethanol
245 318 300 356 291
332
269
321
248
376
277
256
249 316 298 355 290
255 293
294
Methanol
330
270
320
248
373
276
Fig. 6. Absorption spectra of Schiff bases IIIA, IIIB and IVA, IVB in ethanol.
N is a measure of permanent dipole–permanent dipole inter-
actions:
ꢀꢁ
ꢂꢃ ꢀꢁ
ꢂꢃ
D − 1
D + 2
n2 − 1
n2 + 2
N =
−
A multiple regression analysis has been performed. In each
case, fits are obtained as a function of one, two or three param-
eters. The results are listed in Tables 5 and 6.
In a complementary study, the coefficients K1, K2, ν vapor
(cm−1) were calculated using multiple regression technique
based on Equation (2) [22,25]:
ꢁ
ꢂ
ꢁ
ꢂ
2D − 2
2D + 2
2n2 − 2
ν (solution) = ν (vapor)+K1
+K2
2n2 + 1
(2)
ν (vapor) is the wavenumber of the maximum in absence of
solvents. D is the dielectric constant and n is the refractive
index of the solvents. As an indication of the goodness of
the fit, the multiple correlation coefficient and the square of
correlation were calculated for each r2 (ν, n2) and r2 (ν, D).
The results are summarized in Table 7.
3.2. Discussion
The present work study the roles played by different
solvents on the spectral properties of benzylaldimines and
Fig. 5. Absorption spectra of Schiff bases IA, IB and IIA, IIB in ethanol.

260
H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
Fig. 7. Absorption spectra of Schiff bases VA, VB and VIA, VIB in ace-
tonitrile.
Fig. 8. Absorption spectra of Schiff bases VIIA, VIIB and VIIIA, VIIIB in
dioxane.
salicylaldimines and compare their spectra according to dif-
ference in structures.
[6,36]. This band is more important in salicylaldimines com-
pounds due to the presence of strong intramolecular hydrogen
bond between the hydroxyl group and the azomethine nitro-
gen [37] that causes planarity of the molecules and facilitates
the charge transfer to take place within the whole molecule.
These charge transfer bands are more sensitive to solvent
changes than bands resulting from local transition [6].
The effect of solvents on the shift of λmax for the enolimine
tautomer of salicylaldimines has been studied with minimum
interference from the other ketoamine tautomer by running
the absorption spectra of freshly prepared solutions in a min-
imum time. This decreases the proton transfer of hydroxyl
group to the imine nitrogen of enolimine tautomer in H-
bonding solvent and thus reduces subsequent formation of
ketoamine tautomer [9,17,38]. Hence, no intense new peaks
appear in the spectrum of salicylaldimines in H-bonding sol-
vent compared to non-polar solvents (Tables 3 and 4; Fig. 9).
The previous studies absorption spectrum of N-
benzylideneaniline IA in 95% ethanol contained three peaks
(not well resolved) and interpreted to be at 260 and
300 nm (␲–␲*) and 360 nm (n–␲*), relatively very small in
absorbency [26]. Fig. 5 shows the absorption spectra of IA
in absolute ethanol. The peaks are at 263 nm (Band B) due
to ␲–␲* localized on the phenyl rings, 312 nm (Band C) due
3.2.1. Electronic absorption spectral peaks
The absorption spectra of the investigated Schiff bases
in different solvents are composed of several bands in the
200–500 nm region (Figs. 5–9). It is evident that the molecu-
lar structure of the compound and the polarity of the medium
both are of great importance to affect the spectral behavior
of Schiff bases. The longest λmax is λ1 followed by λ2, λ3
and λ4 (Tables 3 and 4). Those bands indicate that differ-
ent electronic transitions are involved. The first one or two
bands in the wavelength range 230–280 nm (Bands A and
B) could be assigned to the excitation of the ␲-electrons of
the aromatic system. These bands are sensitive to substitu-
tion at the aromatic rings and their positions are little influ-
enced by changing the solvent polarity confirming the local
␲–␲* nature of the electronic transition [34]. The third band
observed within the wavelength range 270–310 nm (Band
C) can be due to transition between the ␲-orbital localized
on the central bond of azomethine group (>CH N ) [6,35].
The fourth band located within the wavelength 320–400 nm
(Band D) can be the effect of an intramolecular charge trans-
fer (CT) transistion within the whole Schiff base molecule

H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
261
peaks by about 2–5 nm with respect to those of IB (Table 3).
The electronic-withdrawing nature of –COOH group seems
to stabilize the excited state of the involved electronic
transitions.
The absorption spectra of benzaldehydephenylhydra-
zone IIIA and salicylaldehydephenylhydrazone IIIB shows
three peaks corresponding to Bands B–D at ␭ ∼245, 303
and 347 nm, respectively, in ethanol (Fig. 6). Band D
(λ1) of compound IIIB shows a bathochromic shift of
2–6 nm with respect to IIIA in different solvents (Fig. 6;
Table 3). Also, the peak at λ1 (Band D) of 2,4-dinitrosalicyl-
aldehydephenylhydrazone IVB is red shifted of 3–13 nm with
respect to 2,4-dinitrobenzaldehydephenylhydrazone IVA in
various solvents (Fig. 6; Table 3). The H-bond between
hydroxyl O–H and imine (>CH N ) groups forces planarity
of the whole molecule in the case of IIIB and IVB. This action
stabilizes ␲* orbital and decreases its energy, thus facilitating
intramolecular charge transfer and causing red shift [17].
It is also observed that the charge transfer of IVA and IVB
at λ1 is red shifted in all solvents by an average ꢀλ1 = 35
and 39 nm relative to the CT band of the other compounds
IIIA and IIIB, respectively (Fig. 6; Table 3). The red shift
is attributed to the presence of electron-withdrawing –NO2
substituents in IVA and IVB supporting the CT nature of
the band at λ1 [36]. The high electron-withdrawing power
of the –NO2 group makes it behave as a good CT acceptor
center with the n-orbital of azomethine group being the main
participant of the electron donor group.
Fig. 9. Absorption spectra of Schiff bases IVB and VB in different solvents.
*
to ␲–␲ localized on the azomethine (>CH N ) group and
the expected third peak at 360 (n–␲*) is submerged under
the more intense C band [26] (Table 3; Fig. 5). The peak at
λ2 (Band B) of IA shifts to longer wavelengths 268 nm in
highly polar solvent DMF (D = 36.7) and DMSO (D = 48.9)
compared to 263 nm in non-polar solvent CCl4 (D = 2.0) and
dioxane (D = 2.2) (Tables 2 and 3). This confirms that the
New peak at the longer wavelength region
appears in the electronic absorption spectra of N,Nꢀ-
bis(salicylidene)ethylenediamine (salen) VB and N,Nꢀ-bis-
(salicylidene)hyrazine VIB compared to N,Nꢀ-bis(benzy-
lidene)ethylenediamine VA and N,Nꢀ-bis(benzylidene)-
hydrazine VIA, respectively (Figs. 3 and 7; Table 4). The
peaks observed in different solvents are: Band A (λ2 at
225 nm) and Band B (λ1 at 244–262 nm) for compound VA
compared to Band A (λ3 at 230 nm), B (λ2 = 255–264 nm)
and C (λ1 = 315–328 nm) for compound VB. Band A
(λ2 at 225 nm) and Band C (λ1 = 298–305 nm) for VIA
compared to Band A (λ4 at 230 nm), B (λ3 at 240 nm),
C (λ2 = 293–296 nm) and D (λ1 = 355–363 nm) for VIB
(Table 4). The peaks of salen VB at ∼257 and 318 nm
observed in different solvents are due to the enolimine
tautomer, the other peaks at ∼290 nm and 370 nm due to the
ketoamine tautomer are negligible (Figs. 7 and 9; Table 4).
The ketoamine tautomer become important when salen VB
was irradiated at λex 276 nm in chloroform where spectral
changes resulted in intensification of the peaks at 291 and
369 nm and reduction of 259 and 320 nm [9].
*
absorption peak at λ2 is due to ␲–␲ electronic transition.
The shift to longer wavelength on going to more polar sol-
vents is due to more stabilization of the excited state ␲* than
that of the ground state ␲. The spectrum of the related com-
pound IIA in absolute ethanol (Fig. 5) indicates the presence
of one peak at λ1 = 271 nm (Band B) (Table 3) and two unre-
solved peaks at λ = 290 and 325 nm. The solvent effect has
been studied for only λ1 (Table 3). There has been a small
red shift of λ1 to longer wavelength in DMSO, ethanol and
*
methanol also confirming ␲–␲ transition. The presence of
–COOH auxochrome in the structure of IIA causes red shift
of Band B at ∼270 nm by 7–10 nm with respect to those for
IA at ∼263 nm. Fig. 5 indicates that four peaks appear in the
spectrum of salicylaldimines IB and IIB with intensification
of longest wavelength peaks compared to the corresponding
peaks of benzaldimines IA and IIA. Both Band A (λ4) and
Band B (λ3) of compounds IB and IIB are little affected by
change in solvent polarity and involve ␲–␲* transition of aro-
matic rings [6], while Band C (λ2) and Band D (λ1) slightly
shift to shorter wavelength with increasing H-bonding abil-
ity of solvent (solvent parameter E) (Tables 2 and 3). The
presence of –COOH in IIB caused a red shift of all four
Fig. 4 shows the structures of N,Nꢀ-bis(benzylidene)-
o-phenylenediamine
VIIA,
N,Nꢀ-bis(salicylidene)-o-
phenylenediamine (salophen) VIIB, N,Nꢀ-bis(benzylidene)-
2,6-pyridinediamine VIIIA and N,Nꢀ-bis(salicylidene)-2,6-
pyridinediamine VIIIB. The structure of VIIB has been
1
confirmed by mass spectra and HNMR [31]. The crystal
structure of VIIIB was studied by X-ray diffraction revealing

262
H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
Table 5
Regression analysis values of multiple correlation coefficients MCC for solvent parameters at different λmax of Schiff bases
Parameters IA IB IIA IIB IIIA IIIB IVA IVB VA VB VIA VIB
0.521 0.758 0.708 0.609 0.204 0.024 0.264 0.366 0.093 0.674 0.605 0.546 0.617 0.818 0.159
VIIA
VIIB
VIIIA VIIIB
E
0.2
0.179 0.672
0.516 0.049 0.651
0.889 0.309 0.162
0.109
0.306
0.349
0.613 0.57
0.766
K
0.297 0.666 0.462 0.601 0.413 0.245 0.643 0.66
0.02
0.565 0.322 0.682 0.631 0.706 0.383
0.049 0.159 0.365 0.145
0.019
0.514
0.169 0.667
0.454 0.519 0.287
0.622 0.068 0.334
0.064
M
0.295 0.758 0.422 0.775 0.388 0.545 0.159 0.091 0.749 0.04
0.908 0.817 0.681 0.722 0.397
0.791 0.853 0.175
0.49
0.578
0.629 0.926
0.858 0.569 0.927
0.904 0.787 0.755
0.809
0.873
N
0.289 0.698 0.478 0.64
0.092 0.684 0.457 0.468 0.325
0.419 0.226 0.572 0.604 0.012 0.577 0.428 0.709 0.612 0.739 0.349
0.051
0.504
0.062
0.086
0.445 0.154
0.698 0.018 0.249
0.013
EK
EM
EN
KM
KN
MN
EKM
EKN
EMN
KMN
0.564 0.763 0.752 0.636 0.465 0.459 0.78
0.721 0.214 0.647 0.7
0.582
0.682 0.652 0.821 0.492
0.933 0.676 0.845
0.317
0.805
0.625 0.699
0.518 0.866 0.824
0.921 0.552 0.86
0.299
0.858 0.865 0.738 0.808 0.613 0.761 0.444 0.501 0.853 0.845 0.923 0.827 0.737 0.884 0.848
0.736
0.903
0.803 0.944
0.858 0.789 0.939
0.859 0.766
0.911
0.972
0.95
0.921
0.857 0.902
0.588 0.767 0.766 0.654 0.5
0.543 0.702 0.517 0.918 0.854
0.499 0.839
0.462 0.697 0.658 0.18
0.674 0.64
0.493
0.904
0.715 0.636 0.824 0.463
0.933 0.641 0.908
0.294
0.858
0.197
0.558 0.844 0.514 0.828 0.732 0.79
0.808 0.783 0.851 0.638 0.918 0.893 0.784 0.842 0.772
0.937 0.969 0.855 0.393
0.555
0.635
0.833 0.979
0.865 0.864 0.936
0.927 0.839 0.924
0.917
0.299 0.72
0.572 0.688
0.485 0.683 0.419 0.27
0.458 0.592 0.416
0.855 0.541 0.598
0.548
0.791 0.747 0.225 0.579 0.845 0.724 0.638 0.761 0.442
0.678 0.545 0.705 0.161
0.231
0.515
0.582 0.844 0.513 0.829 0.772 0.831 0.767 0.754 0.865 0.674 0.913 0.889 0.758 0.844 0.797
0.828 0.969 0.86 0.896 0.942 0.943 0.971 0.854 0.454
0.596
0.617
0.932 0.848 0.945
0.907
0.884 0.869 0.849 0.828 0.732 0.802 0.858 0.798 0.862 0.847 0.923 0.922 0.785 0.887 0.85
0.776
0.827
0.836 0.986
0.869 0.877 0.939
0.954 0.859 0.993
0.925
0.974
0.977
0.857 0.998
0.626 0.773 0.775 0.687 0.503 0.464 0.835 0.763 0.247 0.68
0.882 0.735 0.689 0.829 0.497
0.938 0.767 0.936
0.333
0.906
0.682 0.702
0.527 0.939 0.865
0.961 0.609 0.979
0.919
0.703
0.883 0.868 0.845 0.829 0.777 0.834 0.816 0.768 0.869 0.846 0.926 0.919 0.758 0.886 0.855
0.832 0.974 0.86 0.928 0.942 0.972 0.984 0.859 0.996
0.751
0.86
0.951 0.861 0.957
0.548
0.606 0.844 0.514 0.829 0.827 0.92
0.834 0.999 0.93 0.981 0.981
0.845 0.799 0.881 0.709 0.966 0.895 0.865 0.844 0.819
0.956 0.971 0.866 0.616
0.812
0.728
0.946 0.883 0.998
0.924

H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
263
Table 6
Regression analysis coefficients for Schiff bases at different λmax using three-parameters equation E, K, M or (K, M, N)
a0
a1
a2
a3
IA
IB
415.36 (355.04)
224.46 (229.25)
−1.16 (82.6)
22.33 (−253.07)
−300.62 (−47.27)
157.97 (−2.58)
0.077 (20.94)
11.91 (138.66)
334.11 (329.58)
296.13 (303.75)
−0.104 (−5.67)
0.079 (−43.52)
−2.68 (63.66)
55.16 (−0.788)
−11.63 (126.52)
118.36 (14.76)
IIA
IIB
185.82 (281.51)
1.42 (13.01)
−39.78 (−71.21)
206.29 (0.686)
333.47 (332.63)
306.37 (312.56)
304.2 (297.25)
254.08 (255.09)
0.005 (0.346)
0.0376 (−41.35)
−0.091 (12.13)
0.055 (20.13)
−5.2 (62.14)
−3.66 (92.59)
1.49 (32.84)
4.19 (83.83)
64.56 (−2.39)
77.92 (16.31)
24.96 (−5.98)
96.35 (−5.49)
IIIA
IIIB
315.29 (326.34)
297.68 (293.42)
111.66 (166.9)
0.004 (−69.16)
−0.111 (−54.83)
0.607 (−230.6)
18.99 (152.47)
13.54 (101.98)
1.8 (632.2)
115.78 (36.78)
26.32 (25.4)
547.69 (102.58)
297.89 (316.12)
283.29 (289.55)
0.156 (−110.1)
0.058 (−25.84)
14.16 (256.78)
−0.814 (88.05)
195.8 (55.36)
83.71 (11.64)
IVA
IVB
VA
335.03 (298.16)
325.76 (299.01)
171.04 (189.86)
−0.544 (193.19)
−0.363 (183.35)
0.22 (−66.82)
85.47 (159.16)
91.46 (191.6)
12.89 (353.33)
176.6 (−54.95)
213.26 (−44.92)
328.68 (37.89)
VB
391.46 (348.33)
201.65 (222.8)
−0.84 (46.61)
10.79 (−165.74)
−204.51 (31.13)
0.278 (−60.84)
0.094 (232.73)
223.2 (29.41)
VIA
VIB
266.59 (269.52)
0.094 (60.13)
−0.17 (96.8)
155.1 (−24.02)
103.25 (4.46)
54.54 (2.76)
337.09 (344.25)
283.59 (277.08)
0.145 (−20.06)
−0.094 (0.161)
−16.04 (95.54)
10.46 (80.41)
VIIA
VIIB
271.06 (290.49)
0.07 (−166.87)
−24.79 (229.42)
167.67 (60.05)
331.92 (322.48)
250.29 (252.3)
−59.99 (189.55)
−0.197 (−0.648)
−3.14 (74.05)
62.06 (−4.85)
94.95 (−7.87)
0.046 (19.6)
−0.75 (72.81)
3.4 (−200.31)
−135 (483.32)
1061.18 (94.16)
VIIIA
VIIIB
281.59 (302.491)
204.94 (224.6)
0.276 (−26.75)
1.66 (107.11)
−22.25 (211.67)
133.84 (16.51)
134.31 (45.85)
0.56 (−99.53)
206.75 (334.53)
297.99 (275.08)
1.7 (−501.87)
−47.32 (694.16)
580.41 (220.32)
−33.55 (19.97)
−0.38 (−51.69)
10.1 (74.6)
coplanarity of whole molecule because of intramolecular H-
bond between hydroxyl and imino groups [19]. There is large
shift to longer wavelength in λmax1(av) (av: average in all sol-
vents) in the spectra of salicylaldehyde derived Schiff bases
relative to the corresponding benzaldehyde derived Schiff
bases (λ1(av) = 334 nm for VIIB compared to λ1(av) = 296 nm
for VIIA, ꢀλ = 38 nm) and (λ1(av) = 377 nm for VIIIB
compared to λ1(av) = 321 nm for VIIIA ꢀλ = 56 nm) (Fig. 8;
Table 4). The expected peak (n–␲*) at λ1 = 370 nm of VIIB
is submerged under the peak (␲–␲*) at λ1(av) = 335 nm
(Fig. 8) [31]. Replacement of the central phenyl ring of VIIA
and VIIB with a pyridine ring causes a red shift of λ1(av) by
ꢀλ = 25 nm for VIIIA with respect to VIIA and a red shift of
λ1(av) by ꢀλ = 43 nm for VIIIB with respect to VIIB (Fig. 8;
Table 4). There is a reduced tendency to tautomeric intercon-
version in the case of Schiff bases of aminopyridine VIIIB.
The tautomeric constant Kt = [ketoamine]/[enolimine] = 0.02
in methanol. The electron delocalization causes a decrease
in imino nitrogen basicity, followed by a weakening of the
intramolecular H bond OH···N C and reduced tendency to
tauomeric interconversion [19].
3.2.2. Correlation studies
TheelectronicabsorptionspectraoftheinvestigatedSchiff
bases show shift in λmax in different solvents (Tables 3 and 4).
Such behavior is depicted in Fig. 9, where λmax1 shifts to
longer wavelengths in the order dioxane, ethanol and DMSO
for compound IVB and ethanol, dioxane and CCl4 for com-
pound VB.
The value of λmax1 of the studied benzaldimine and
salicylaldimine (except IA, IIA and VB) is slightly red
shiftedwithchangingthesolventphysicalparametersaccord-
ing to the following order: MeOH < EtOH < DMF < DMSO
(Tables 3 and 4). This suggests that this band is the effect of
an intramolecular charge transfer transition within the whole
molecule, where azomethine group is the primary center con-
tributing to CT. In ethanol and methanol, the electron charge
transferisdifficultthroughthesolutemoleculebecauseoffor-

264
H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
Table 7
K1, K2, ν (vapor) and correlation analysis data for Schiff bases at different ν cm−1
ν (vap) cm−1
K1
K2
MCC
r2 (ν, D)
r2 (ν, n)
IA
IB
27980.69
42851.88
1456.64
−771.11
43156.3
505.9
6817.21
−11101.2
0.634
0.923
0.254
0.025
0.055
0.636
417091.7
32739.94
−1022
0.473
0.92
0.065
0.482
0.209
0.592
−4336.53
IIA
IIB
35501.86
−975.33
282.48
168.12
162.98
−282.64
4831.78
0.516
0.211
0.144
29861.93
32138.41
33412.67
38715.93
−2547.37
−3055.71
−2119.91
−6155.78
0.803
0.773
0.912
0.961
0.368
0.193
0.405
0.003
0.475
0.534
0.695
0.825
IIIA
IIIB
31274.9
34520.11
54473.83
−632.28
−453.72
−2492.23
−4786.68
−3039.06
−30735.3
−5232.63
−3539.82
−24330.4
0.539
0.913
0.918
0.193
0.284
0.062
0.218
0.357
0.642
31014.57
34465.34
51985.28
−435.3
0.647
0.85
0.946
0.284
0.718
0.107
0.053
0.079
0.554
37.44
−2654.54
IVA
IVB
VA
31373.77
31179.09
49542.45
−1785.51
−2137.5
−1192.01
−9419.21
−9775.83
−22182.5
0.875
0.84
0.754
0.414
0.443
0
0.133
0.078
0.487
VB
28882.89
44553.11
1278.76
−935.75
3885.96
−12800.5
0.649
0.962
0.347
0.001
0.01
0.763
VIA
VIB
36267.51
4.74
−7860.22
0.983
0.966
0.119
28780.04
35771.93
438.18
−219.89
−2838.86
0.831
0.894
0.474
0.023
0.432
0.662
−4141.36
VIIA
VIIB
35419.36
1311.47
−7030.74
0.705
0.366
0.263
30704.31
39130.02
596.66
85.72
−3231.99
0.82
0.87
0.499
0.749
0.377
0.145
−5781.98
VIIIA
VIIIB
33049.08
45852.33
−461.68
−3881.66
−10985.2
0.796
0.526
0.152
0.023
0.515
0.048
−1450.19
27815.48
36090.92
333.87
385.12
−3945.12
0.6
0.534
0.192
0.252
0.298
0.119
−1257.58
mation of intermolecular hydrogen bond between the hydro-
gen bond donor solvent and the solute molecule [37]. This
results in blocking the n-electrons of the azoethine nitrogen
atom and a small blue shift is observed compared to DMF
and DMSO.
interaction between solute and solvent molecules (hydro-
gen bonding) are major factors to explain the spectral
behavior of compounds. The shift in λmax1 due to differ-
ences in solvents range from ꢀλ1 = 6 to 28 nm for ben-
zaldimines and from ꢀλ1 = 6 to 35 nm for salicyladlimines,
with 2,4-dinitrobenzaldehydephenylhydrazone IVA and 2,4-
dinitrosalicylaldehydephenylhydrazone IVB showing the
maximum ranges, respectively.
The influence of empirical solvent polarity on the position
of λmax is evident by the high correlation coefficients MCC
(greater than 0.5) computed with E parameter for absorp-
tion peaks of salicylaldimines: λ1 of VB and VIB; λ2 of
IIIB and VIIIB; λ1 and λ2 of IB and VIIB; λ1, λ2 and λ3
of IIB. Also, high correlations with E parameter are found
for benzaldimines absorption peaks: λ1 of IA, IIA, VIA and
VIIA and λ2 of VIIIA (Table 5). The high correlation values
obtained using the parameter K indicates high influence of
the dielectric constant on the position of absorption peak at
λ1 of IVA, IVB, VB, VIB, VIIA and VIIB; λ2 of IIIA and
VIIIB; λ1 and λ2 of IB; and λ1 and λ3 of IIB (Table 5).
*
The peaks due to ␲–␲ transitions show bathochromic
shift in going to more polar solvent due to the stabilization
of the excited state more than that of the ground state. Red
shift in λmax values is observed in solvents in the following
increasing order: dioxane < chloroform < DMF < DMSO for
peaks at λ2 of IA, IVA, IIIB, VIB, VIIB and VB; λ3 of IB;
λ2 and λ4 of IIB; and λ2 and λ3 of IIIA.
The spectral data are good evidence for the presence of
solute–solvent interactions between the active solvent and
Schiff bases. The absorption spectra are influenced by the
physical properties of the solvent (dipole moment, dielec-
tric constant and refractive index), the change in the polar-
ity and dipole moment of the solute through excitation
and the difference in solvation energy from one solvent
to another. The solvent effects and the effect of specific

H.H. Hammud et al. / Spectrochimica Acta Part A 63 (2006) 255–265
265
The solute permanent dipole–solvent induced dipole inter-
actions (parameter M that depends on refractive index) plays
a major role in the spectra of Schiff bases: λ2 of IA; λ1 and
λ2 of IB; λ1, λ2, λ3 and λ4 of IIB; λ2 and λ3 of IIIA; λ1, λ2
and λ3 of IIIB; λ1 of VA; λ2 of VB; λ1 of VIA; λ1 of VIIA;
λ1 and λ2 of VIIB and λ2 of VIIIB. The permanent dipole-
permanent dipole interactions (parameter N) has important
effect on λ1 of IVA, IVB, VIB, VIIA and VIIB; λ2 of VIIIB;
λ1 and λ2 of IB; λ1 and λ3 of IIB. Table 5 also indicates
improvement of the fits when using three-parameters equa-
tion over two-parameters and one-parameter equations, since
higher correlation coefficients are obtained.
Table 6 lists the coefficients for the regression analysis
when using three-parameter equations E, K, M or K, M, N.
The value of a0 is the intercept. The solvent effects due to E, K
and M or K, M and N parameters produce bathochromic shift
for positive values of coefficients a1, a2 and a3, respectively,
andhypsochromic shift fornegativevaluesofthe coefficients.
The values of K1, K2, ν (vapor), r2 (ν, D), r2 (ν, n) and
MCC for the Schiff bases are computed and listed in Table 7.
The data indicates that both the dielectric constant and the
refractive index of solvents affect the electronic absorption
spectra of compounds but with varying degrees. The nega-
tive values for K1 and K2 indicate the occurrence of strong
solute–solvent interaction and causes decrease in energy of
electronic transition from LUMO to HOMO in comparison
with the vapor phase. The correlation data between (ν, D) is
good for VIA at λ1 and IIIB and VIIB at λ2. The correlation
data between (ν, n2) is good for IA, VB and VIB at λ2; IIIA
at λ3; IIB at λ3 and λ4.
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