Temperature and substituent effects on the dissociation constants of 5-azorhodanine derivatives. Semi-empirical quantum mechanical calculation

Article abstract of DOI:10.1016/S1386-1425(01)00564-9

Detailed analysis of the electronic structure and properties of some rhodanine derivatives (RDs) is presented. The aim of the present investigation is to pinpoint the electronic structural similarities and differences, among the series of the studied RDs

Full text of DOI:10.1016/S1386-1425(01)00564-9

Spectrochimica Acta Part A 58 (2002) 447–455
www.elsevier.com/locate/saa
Temperature and substituent effects on the dissociation
constants of 5-azorhodanine derivatives. Semi-empirical
quantum mechanical calculation
Tarek M. El-Gogary *, A.A. El-Bindary, Azza S. Hilali 1
Chemistry Department, Faculty of Science (Damietta), Mansoura Uni6ersity, Domyat Al-Gideda, 34517 Damietta, Egypt
Received 12 March 2001; received in revised form 20 May 2001; accepted 28 May 2001
Abstract
Detailed analysis of the electronic structure and properties of some rhodanine derivatives (RDs) is presented. The
aim of the present investigation is to pinpoint the electronic structural similarities and differences, among the series
of the studied RDs that govern and determine their acidic, basic and co-ordinative properties. The geometries of the
studied rhodanine were fully optimized at the level of AM1 semi-empirical method. Relative stabilities of the
enol/keto isomers have been calculated. Proton affinities and proton detachment energies were computed for the series
of rhodanine studied, at the level of AM1 method and compared with the potentiometrically-determined proton–lig-
and dissociation constants. Zero-point energy and electron correlations have been taken into consideration. pKH have
been found to increase with increasing electron-donating nature of the substituents. The resulting linear Hammett
plots of pKH versus the Hammett constant | values indicate the co-planarity of the investigated molecules. The
evaluated thermodynamic parameters (DG, DH and DS) indicate that the dissociation processes are non-spontaneous,
endothermic and entropically unfavourable. © 2002 Published by Elsevier Science B.V.
Keywords: Semi-empirical calculation; Electronic structure; Geometry optimization; 5-azorhodanine; Dissociation constants; Ther-
modynamic parameters
large sizes of some molecular systems. For the
above-mentioned problem, the semi-empirical
1. Introduction
methods are essential. These methods allow the
The value of ab initio calculations for molecu-
study of valence electronic structure of molecules
lar geometries are well established, particularly by
with appreciable saving in computational effort.
the systematic work of Pople [1]. However, ab
The different semi-empirical approaches have
initio methods have been faced by the problems of
been reviewed by Berthier [2] and critically dis-
cussed by De Brouckere [3].
* Corresponding author. Fax: +20-57-403-868.
E-mail address: asmasomy@mum.mans.eun.eg (T.M. El-
Gogary).
1 Abstracted from her Ph.D. thesis.
Azo compounds based on rhodanine was syn-
thesized for the first time at the L’vov Medical
Institute as potential medicinal preparations [4]
1386-1425/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S1386-1425(01)00564-9

448
T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
and play an important role in analytical chemistry
as chromogenic agents [5]. Furthermore, from a
biological point of view, the study of the binding
properties of organic chelating ligands containing
both ꢀNꢁNꢀ and ꢀOH groups is significant due to
their use as models for certain metal–enzyme
interactions and also for the possible transport of
metal ions in biological media [6–8]. Previously
different methods, e.g. IR matrix-isolation spectra
[8], spectrophotometry [9], potentiometry [10–12],
were used to evaluate the dissociation constants
and the corresponding thermodynamic functions.
The aim of the present investigation is to pin-
point the electronic structural similarities and dif-
ferences, among the series of the studied
rhodanine derivatives (RDs) that govern and de-
termine their acidic, basic and co-ordinative prop-
erties. On the other hand, potentiometric studies
of 5-phenylazo-3-phenyl-2-thioxo-4-thiazolidinone
and its substituted derivatives in 0.1 M KCl and
50% (v/v) ethanol–water mixture were performed.
The influence of substituents on the proton
affinity (PA) of the ligands was examined on the
basis of inductive and mesomeric effects. The
corresponding thermodynamic parameters are
derived and discussed.
sodium nitrite to a concentrated hydrochloric acid
solution of 0.01 mol of aniline, p-chloroaniline,
p-nitroaniline, p-methylaniline and p-methoxyani-
line with stirring and kept for about 20 min in the
ice bath. The formed diazonium chloride solu-
tions were added gradually with stirring to a 0.01
mol cold solution of 3-phenyl-2-thioxo-4-thiazo-
lidinone in 50 ml pyridine. After dilution, the
5-azorhodanine compounds formed were filtered
off and washed with water. The crude material
were recrystallized from ethanol and then dried in
a vacuum desiccator over anhydrous calcium
chloride. The purity was checked by elemental
1
analyses, IR and H NMR spectra.
3.2. Reagents and materials
The ligands solution (0.01 M) were prepared by
dissolving the accurate weight of the solid in
ethanol (Analar). Solutions of 0.005 M HCl and 1
M KCl were also prepared in doubly distilled
water. A carbonate-free sodium hydroxide solu-
tion in 50% (v/v) ethanol–water mixture was used
as titrant and standardized against Analar oxalic
acid.
3.3. Potentiometric measurements
2. Calculations
The apparatus, general conditions and methods
of calculation were the same as in the previous
work [7–9]. The following mixtures were prepared
and titrated potentiometrically at 298 K against
standard 0.02 M NaOH in 50% (v/v) ethanol–wa-
ter mixture:
A Pentium II computer machine with 300 MHz
processor and 128 MB RAM was utilized to run
all the calculations. Calculations were performed
using the HyperChem package version 5.1 of Hy-
percube, Inc., 1998. Semi-empirical AM1 method
[13] was used to run the calculations with root
1. 5 ml 0.005 M HCl+5 ml 1 M KCl+25 ml
ethanol;
2. 5 ml 0.005 M HCl+5 ml 1 M KCl+20 ml
ethanol+5 ml 0.01 M ligand.
−1
,
mean square gradient of 0.01 kcal (A mol)
using Polak-Ribiere algorithm.
For each mixture, the volume was made up to
50 ml with doubly distilled water before the titra-
tion. These titrations were repeated for tempera-
3. Experimental
3.1. Synthesis of
tures of 308 and 318 K and a constant
5-phenylazo-3-phenyl-2-thioxo-4-thiazolidinone
and its deri6ati6es
temperature was maintained to 90.05 K by using
an ultrathermostat (Neslab 2 RTE 220). The pH
meter readings in 50% (v/v) ethanol–water mix-
ture were corrected according to the Van Uitert
and Hass relation [14].
The title compounds were prepared [4] by grad-
ual addition of an aqueous solution of 0.01 mol of

T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
449
tively. The donating–accepting nature of the sub-
stituent strongly affects the distribution of charge.
If we consider, as example, the charge on the
carbonyl oxygen in the keto form (Table 3), the
amount of negative charge accumulated could be
ranked as p-CH3\p-OCH3\p-H\p-Cl\
p-NO2.
Total energy, heat of formation, ionization po-
tential, HOMO–LUMO energy gap and dipole
moment of keto and enol forms of RD are de-
picted in Tables 5 and 6. Inspection of these
Tables reveals that
Fig. 1. Enol–keto tautomerism of RDs.
4. Results and discussion
1. RD shows ionization energy around 8.0 eV
indicating comparable donating power. This
could be attributed to the accumulation of the
charge densities of the HOMO’s on the rhoda-
nine moiety rather than the other party of the
molecule. However, the nature of the sub-
stituent still has a considerable effect on the
donating power of the RD which could rank
the IP energies as p-NO2\p-Cl\p-H\
p-CH3\p-OCH3.
4.1. Geometrical features of the studied rhodanine
deri6ati6es
The study of the geometry and preferred con-
formation of RDs are important because of the
possible usefulness of such information as an aid
to the understanding of their mode of action as
acids or bases and co-ordinative species.
The geometries of all the studied RD were fully
optimized using AM1 semi-empirical method.
Two isomeric forms, namely enol and keto
forms, of the RD are possible (Fig. 1). The opti-
mized geometrical parameters of keto and enol
forms of the studied RD are represented in Tables
2. RD also shows almost constant energy gaps of
about 8.08 eV indicating comparable activities.
3. The binding energy increases, as expected, with
the size of the molecule.
4. p-NO2 has the greatest value of dipole moment
(9.312) indicating high polarizability, while
other derivatives have comparable dipole
moments.
5. The keto form is always more stable (less
energetic) than the enol form for all derivatives
(Table 7).
1
and 2, respectively. The nature of the
p-substituted has but a slight effect on the
geometry of the compounds under investigation.
The strong withdrawing nitro group has the max-
imum effect.
Mulliken charge distributions are shown in
(Tables 3 and 4) for keto and enol forms, respec-
Fig. 2. Molecular structure of RD showing atomic numeration.

450
T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
Table 1
,
Selected bond lengths (A) and bond angles (in degrees) for the studied RDs (keto form) calculated using AM1 semi-empirical
method
p-NO2
p-Cl
1.5454
p-OCH3
p-CH3
1.5463
1.3961
1.4272
1.4154
1.2309
1.5392
1.7991
1.1285
1.4677
1.2276
1.4333
p-H
1.5462
Description
1.5436
1.5464
1.396
1.4273
1.4153
1.231
C5ꢀS6
C5ꢀN4
N4ꢀC7
N4ꢀC3
C3ꢀO18
C3ꢀC2
C2ꢀS1
C2ꢀH19
1.3974
1.4279
1.4127
1.2309
1.5403
1.796
1.3967
1.4276
1.4139
1.2309
1.5398
1.7965
1.2847
1.4679
1.2274
1.4339
1.7351
124.911
124.912
110.177
121.98
116.041
121.347
117.306
121.347
101.385
111.504
111.453
120
1.3965
1.4272
1.4147
1.2309
1.5391
1.7972
1.1289
1.4684
1.227
1.4338
1.7768
124.474
127.836
107.677
121.091
117.667
123.058
117.31
119.379
99.239
109.613
109.93
117.772
117.772
109.777
121.233
97.766
113.543
110.212
113.474
1.5385
1.7975
1.1298
1.4686
1.2282
1.4288
1.7352
124.425
127.894
107.68
121.034
117.695
122.973
117.638
119.387
99.104
111.026
109.983
117.71
116.645
109.622
121.271
97.826
111.766
109.586
114.602
1.1291
1.4674
1.2262
1.4372
1.7358
124.417
127.88
107.68
121.034
117.695
122.973
117.672
119.259
99.099
111.026
109.983
117.71
116.265
109.685
121.271
97.826
112.11
109.687
114.551
C2ꢀN20
N20ꢀN21
N21ꢀC22
1.7345
S1ꢀS5
124.407
127.775
107.642
121.124
117.696
122.998
117.61
S6ꢀC5ꢀS1
S6ꢀC5ꢀN4
S1ꢀC5ꢀN4
C5ꢀN4ꢀC7
C5ꢀN4ꢀC3
N4ꢀC3ꢀO18
N4ꢀC3ꢀC2
C2ꢀC3ꢀO18
C3ꢀC2ꢀS1
S1ꢀC2ꢀN20
C3ꢀC2ꢀN20
N20ꢀN21ꢀC22
N21ꢀC22ꢀC23
C3ꢀC2ꢀH19
C7ꢀN4ꢀC3
C2ꢀS1ꢀC5
H19ꢀC2ꢀN20
C2ꢀN20ꢀN21
H19ꢀC2ꢀS1
119.389
99.109
110.462
109.761
117.714
116.265
109.847
121.169
97.794
112.178
109.95
114.682
120
111.405
121.98
95.092
109.469
109.857
111.453
See Fig. 2 for atomic numeration.
4.2. Gas-phase acidities
A large negative DH° value corresponds, to a
strong acid. The PDE may be written as
In solution, RDs used may act as a base and
undergo a proton abstraction on the carbonyl
oxygen forming a positive species, or may act as
an acid when it loses the enol form hydroxyl
proton forming a negative ion. To compare the
experimental acidity with that computed, PA and
proton detachment energy (PDE) were calculated.
The PA of an anion A− or the PDE of an acid
&
PDE= −DEe0l−DZPE− DCp dt,
where DE is the electronic energy difference
E(A−)−E(HA).
DZPE is the zero-point energy difference
ZPE(A−)−ZPE(AH+) and DCp is the difference
of constant-pressure molar heat capacities
Cp(H)+Cp(A−)−Cp(AH).
may be defined as DH° for the gas phase
298
reaction:
If A− and AH possess the same number of
rotational degrees of freedom and if the vibra-
tional and electronic contribution of 5/2RT
A−+H+“AH

T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
451
(ꢀ6.2 kJ mol−1 at 298 K), this small contribu-
tion could be omitted.
DZPE contributions which are typically 25–40
kJ mol−1 should be considered in our
calculations.
PDE for the series of the studied RDs are re-
ported in Table 8. PA and PDE values are corre-
sponding to isogyric processes (in which the
number of unpaired election spins does not
change). For such processes, the correlation effect
is expected to be small. Since more negative values
of PDE mean high acid dissociation and hence
stronger acid, according to Table 8, the calculated
gas acidity of the different RD showed that RD
are weak acids and their acidity could be ranked
as p-OCH3\p-CH3\p-NO2\p-H\p-Cl.
On the other hand, more negative PA values
mean stronger proton acceptor on the carbonyl
oxygen. The calculated gas basicity of the differ-
ent RD could be ranked as p-NO2\p-Cl\
p-H\p-CH3\p-OCH3.
Gas phase acidity is frequently defined as DG°
298
instead of DH° . The difference between the two
298
acidity scales is approximately constant, viz.,
DG° is less than DH° by TDS°, the largest
298
298
contribution, to which is TS°(H+)=32.4 kJ
mol−1. This contribution, associated with the
translational entropy of H+, is augmented by
small contribution from the differences in rota-
tional and/or vibrational entropies of A− and
AH. In the present work, DH°298 values are used as
a measure of acidity and basicity. The PA and
Table 2
,
Selected bond lengths (A) and bond angles (in degrees) for the studied RDs (enol form) calculated using AM1 semi-empirical
method
p-NO2
1.5535
p-Cl
1.55495
1.41068
1.4262
1.39628
1.36246
1.40001
1.71148
1.40044
1.23543
1.43353
1.72371
1.41481
p-OCH3
p-CH3
p-H
1.55705
1.40739
1.423984
1.40557
1.3625
1.39657
1.71755
1.40385
1.235
1.43374
1.72191
0.973848
Description
1.55745
1.40676
1.42378
1.40612
1.36284
1.3961
1.71759
1.40422
1.23528
1.43148
1.72223
0.973725
123.199
127.84
108.961
124.287
113.393
123.554
113.748
122.697
109.349
128.553
122.098
119.722
118.087
122.319
94.5483
1.55728
C5ꢀS6
C5ꢀN4
N4ꢀC7
N4ꢀC3
C3ꢀO18
C3ꢀC2
C2ꢀS1
C2ꢀN20
1.41047
1.42448
1.40221
1.36039
1.40027
1.71751
1.39871
1.23644
1.4349
1.40717
1.42379
1.4058
1.36264
1.39637
1.71757
1.4041
1.23508
1.43285
1.72188
0.973791
N20ꢀN21
N21ꢀC22
S1ꢀC5
1.72237
0.974597
123.337
O18ꢀH19
S6ꢀC5ꢀS1
S6ꢀC5ꢀN4
S1ꢀC5ꢀN4
C5ꢀN4ꢀC7
C5ꢀN4ꢀC3
N4ꢀC3ꢀO18
N4ꢀC3ꢀC2
C2ꢀC3ꢀO18
C3ꢀC2ꢀS1
S1ꢀC2ꢀN20
C3ꢀC2ꢀN20
N20ꢀN21ꢀC22
C2ꢀN20ꢀN21
C7ꢀN4ꢀC3
C2ꢀS1ꢀC5
122.967
127.81
123.23
123.237
127.8
127.757
108.906
124.181
113.463
123.733
113.733
122.533
109.329
128.714
121.956
119.324
118.449
122.355
94.5692
127.803
108.968
124.253
113.392
123.57
113.737
122.693
109.357
128.513
122.13
109.223
123.198
112.966
117.542
114.097
123.36
108.963
124.255
113.396
123.577
113.736
122.687
109.358
128.529
122.113
119.725
118.029
122.347
94.5462
109.42
129.265
121.314
119.707
118.597
123.189
94.2923
119.754
118.051
122.355
94.5459
See Fig. 2 for atomic numeration.

Table 3
Calculated Mulliken charges of selected atoms for the studied RDs (keto form) using AM1 semi-empirical method
C22
N21
N20
O18
C7
S6
C5
N4
C3
C2
S1
Compound
−0.0558
−0.04611
−0.04692
−0.04394
−0.04351
−0.04696
−0.07531
−0.0781
−0.08103
−0.07586
−0.05959
−0.25716
−0.25932
−0.25909
−0.25708
−0.25386
0.03645
0.037169
0.037127
0.035564
0.03313
0.0095
−0.16777
−0.16508
−0.1654
−0.16847
−0.17441
−0.25232
−0.25124
−0.25334
−0.25176
−0.25097
0.308642
0.315082
0.311479
0.309473
0.305873
−0.23779
−0.23359
−0.23507
−0.23693
−0.24184
0.271138
0.263242
0.26632
0.270225
0.274419
p-H
−0.06441
−0.09496
−0.05638
−0.00807
0.00835
0.006457
0.014705
0.029732
p-CH3
p-OCH3
p-Cl
p-NO2
/
See Fig. 2 for atomic numeration.
Table 4
Calculated Mulliken charges of selected atoms for the studied RDs (enol form) using AM1 semi-empirical method
5
)
C22
N21
N20
0.006144
−0.0003
−0.00092
−0.02993
0.02364
O18
C7
0.000381
−0.000047
0.000391
S6
C5
N4
C3
C2
S1
Compound
4
−0.0391
−0.04789
−0.07604
−0.0403
−0.01312
−0.09604
−0.09536
−0.08949
0.082734
−0.19059
−0.18904
−0.19085
−0.21161
−0.1888
−0.07148
−0.0817
−0.07495
−0.05523
−0.04404
−0.22027
−0.24212
−0.2188
−0.22695
−0.2254
−0.19904
−0.195
−0.19873
−0.15967
−0.19902
0.193572
0.197321
0.189836
0.216068
0.214189
−0.3886
0.480895
0.496336
0.477834
0.487216
0.482395
p-H
−0.38167
−0.38448
−0.45565
−0.40412
p-CH3
p-OCH3
p-Cl
455
0.033844
−0.006943
−0.10974
p-NO2
See Fig. 2 for atomic numeration.

T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
453
Table 5
Some calculated parameters for the studied RDs (keto form) calculated using AM1 semi-empirical method
Dipole moment
(Debye)
E (eV)
IP (eV)
Heat formation
Total energy
(kcal mol−1
)
Compound
(kcal mol−1
)
3.575
4.074
4.629
2.657
3.601
7.306
7.32
7.346
6.241
6.686
−8.783
−8.751
−8.723
−8.067
−9.019
200.189
193.718
164.761
204.976
212.801
−78 286.381
−81 879.69
−89 257.788
−86 578.937
−97 437.877
p-H
p-CH3
p-OCH3
p-Cl
p-NO2
Table 6
Some calculated parameters for the studied RDs (enol form) calculated using AM1 semi-empirical method
Dipole moment
(Debye)
E (eV)
IP (eV)
Heat formation
Total energy
(kcal mol−1
)
Compound
(kcal mol−1
)
5.102
5.838
6.494
4.618
9.312
6.45
−8.18
218.432
316.129
182.488
209.963
230.949
−78 268.138
−81 757.279
−89 240.016
−86 573.95
−97 419.729
p-H
6.426
6.379
6.329
6.277
−8.142
−8.065
−8.324
−8.66
p-CH3
p-OCH3
p-Cl
p-NO2
Table 7
PA and PDE (kcal mol−1) for the studied RDs calculated using AM1 semi-empirical method
Description
p-NO2
p-Cl
p-CH3
p-H
p-OCH3
PA
−249.782
−231.375
−222.565
−219.637
−122.103
PDE
128.262
163.388
43.931
147.757
4.397
Table 8
Calculated energetic properties for the studied RDs (enol and keto) forms using AM1 semi-empirical method
Keto form
Enol form
Compound
Ef (kcal mol−1
)
Et (kcal mol−1
)
Ef (kcal mol−1
)
Et (kcal mol−1
)
−78 144.363
−81 721.076
−89 094.697
−86 440.72
−97 295.01
−78 286.381
−81 879.69
−89 257.788
−86 578.937
−97 437.877
−78 124.049
−81 595.148
−89 074.288
−86 436.071
−97 272.409
−78 268.138
−81 757.279
−89 240.061
−86 573.95
−97 419.729
p-H
p-CH3
p-OCH3
p-Cl
p-NO2
These results could be simply accounted for
considering the electron withdrawing properties of
p-NO2 and p-Cl and electron donating behaviour
of p-H, p-CH3 and p-OCH3. On the other hand,
these results agree well with the potentiometri-
cally-measured pKH (Table 9).
4.3. Proton–ligand stability constants
The average number of protons associated with
the ligands at different pH values, nA, was calcu-
lated from the titration curves of the acid in the
absence and presence of ligand. Thus, the forma-

454
T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
tion curves (nA versus pH) for the proton–ligand
systems were constructed and found to extend
between 0 and 1 in the nA scale. This means that
ligands have one ionizable proton (the enolized
hydrogen ion of the carbonyl oxygen in the rho-
danine moiety). It can be seen that for the same
volume of NaOH added the ligand titration
curves have a lower pH value than the acid titra-
tion curve. The displacement of a ligand titration
curve along the volume axis with respect to the
acid titration curve is an indication of proton
dissociation. The proton–ligand stability con-
stants were calculated using the method of Irving
and Rossotti [15].
The phenolic ꢀOH group is known to be
weakly acidic, indicating a stronger bonding be-
tween the proton and the oxygen donor. This
means that the proton–ligand stability constant
of 5-azorhodanine derivatives should be high due
to the dissociation of ꢀOH group [16].
stituents. The p-OCH3 and p-CH3 derivatives
have a lower acidic character (higher pKH values)
than the p-Cl and p-NO2 derivatives.
This is quite reasonable because the presence of
p-OCH3 and p-CH3 groups i.e., an electron do-
nating effect will enhance the electron density,
whereby stronger OꢀH bond is formed. The pres-
ence of p-Cl and p-NO2 groups (i.e., an electron
withdrawing effect) will lead to the opposite
effect.
The results are also in accordance with Ham-
mett’s para-substituent constant values |x [17].
Straight lines are obtained on plotting pKH values
at different temperatures versus |x. The para sub-
stituents in the phenyl moiety have a direct influ-
ence on the pKH values of the investigated
compounds, revealing the co-planarity of the
molecule and thus affording a maximum reso-
nance via delocalization of its p-system.
4.5. Effect of temperature
4.4. Substituent effect on pKH
The dissociation constants of RD have been
evaluated at 298, 308, and 318 K and are given in
Table 9.
An inspection of the results in Table 9 reveals
that the pKH values of the RD are influenced by
the inductive and mesomeric effects of the sub-
The slope of the plot pKH versus 1/T was
Table 9
Thermodynamic functions for the dissociation of RD and its derivative in 50% (v/v) ethanol–water mixture and 0.1 M KCl at
different temperatures
Compound Temperature (K) Dissociation
Free energy change DG1 Enthalpy change DH
Entropy change DS
constant pKH
(kJ mol−1
)
(kJ mol−1
)
(J mol−1 K−1
)
1
p-NO2
p-Cl
298
308
318
7.10
7.00
6.85
40.46
41.23
41.78
117.35
116.04
114.12
5.49
298
308
318
7.81
7.66
7.56
44.50
45.10
45.97
129.43
127.17
125.91
5.93
6.21
6.69
6.88
p-H
298
308
318
8.16
8.02
7.90
46.50
47.23
48.04
135.20
133.18
131.54
p-CH3
p-OCH3
298
308
318
8.35
8.20
8.07
47.58
48.29
49.07
137.21
135.06
133.27
298
308
318
8.50
8.35
8.21
48.43
49.18
49.92
139.43
137.34
135.34

T.M. El-Gogary et al. / Spectrochimica Acta Part A 58 (2002) 447–455
455
utilized to evaluate the enthalpy change (DH) for
References
the dissociation process. From the energy change
(DG) and (DH) values, one can deduce the en-
tropy change (DS) using the relationships (1) and
(2):
[1] J.A. Pople, in: H.F. Schaefer III (Ed.), Modern Theoreti-
cal Chemistry, vol. 4, McGraw-Hill, New York, 1986.
[2] G. Berthier, Adv. Quantum Chem. 8 (1983) 1974.
[3] G. De Brouckere, Adv. Chem. Phys. 37 (1978) 203.
[4] S.B. Savvin, R.F. Gureva, Talanta 34 (1) (1987) 87.
[5] S. Shibata, in: H. Flaschka, A.J. Barnard (Eds.), Chelates
in Analytical Chemistry, vol. 4, Marcel Dekker, New
York, 1972.
[6] D.E. Fenton, in: A.J. Sykes (Ed.), Advances in Inorganic
and Bioorganic Mechanisms, vol. 2, Academic Press,
London, 1983.
[7] G.N. Ledesma, M. Gonzales Sierra, G.M. Escander,
Polyhedron 17 (1998) 1517.
DG=2.303RTpK,
DH−DG
(1)
(2)
DS=
.
T
All thermodynamic parameters of the dissociation
process of RD are recorded in Table 9. From
these results, the following conclusions can be
made.
[8] A.A. El-Bindary, P. Klaeboe, C.J. Nielsen, Acta Chem.
Scand. 45 (1991) 877.
1. The pKH values decrease with increasing tem-
perature, i.e., the acidity of the ligands in-
creases, independent of the nature of the
substituent [10].
[9] A.A. El-Bindary, Monatsh. Chem. 125 (1994) 811.
[10] A.A. El-Bindary, M.M. Ghoneim, A.Z. El-Sonbati, S.A.
Barakat, Monatsh. Chem. 129 (1998) 1259.
[11] A.A. El-Bindary, A.Z. El-Sonbati, H.M. Kera, Can. J.
Chem. 77 (1999) 1305.
[12] A.A. El-Bindary, A.Z. El-Sonbati, N.A. El-Deeb, Appl.
Polym. Sci. 77 (2000).
[13] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P.J. Stew-
art, J. Am. Chem. Soc. 107 (1985) 3902.
[14] L.G. Van Uitert, C.G. Hass, J. Am. Chem. Soc. 75 (1953)
451.
[15] H. Irving, H.S. Rossotti, J. Chem. Soc. (1954) 2904.
[16] V.D. Athawale, S.S. Nerkar, Monatsh. Chem. 131 (2000)
267.
2. A positive value of DH indicates that its disso-
ciation is accompanied by absorption of heat
and the process is endothermic.
3. A large positive value of DG indicates that the
dissociation process is not spontaneous.
4. The dissociation processes for RD have nega-
tive values of DS due to increased order as a
result of the solvation processes. Furthermore,
the order increases as the electron donating
nature of the substituent increases.
[17] J. Hine, Structural Effects of Equilibria in Organic Chem-
istry, Wiley, New York, 1975, p. 667.