Structural and thermal characterization of cerium, thorium and uranyl complexes of sulfasalazine

Article abstract of DOI:10.1016/j.saa.2005.04.006

Cerium(IV), Thorium(IV) and Uranyl(II) complexes with the ammonium salt of sulfasalazine drug (H₂SSZ, HL^-) have been studied. The structures of the complexes were elucidated using elemental analysis, IR and mass spectroscopy and ther

Full text of DOI:10.1016/j.saa.2005.04.006

Spectrochimica Acta Part A 62 (2005) 1095–1101
Structural and thermal characterization of cerium, thorium
and uranyl complexes of sulfasalazine
Gehad G. Mohameda,∗, Ahmed A. Solimanb, Mohamed A. El-Mawgooda
a
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
b
Department of Chemistry, Faculty of Science, UAE University, P.O. Box 17551 Al-Ain, UAE
Received 22 February 2005; received in revised form 1 April 2005; accepted 6 April 2005
Abstract
Cerium(IV), Thorium(IV) and Uranyl(II) complexes with the ammonium salt of sulfasalazine drug (H2SSZ, HL−) have been studied. The
structures of the complexes were elucidated using elemental analysis, IR and mass spectroscopy and thermal analysis. The complexes were
isolated in 1:1 and 1:2 (M:L) ratios. The solid monocomplexes (1:1) (M:H2SSZ) were isolated in the general formulae [UO2(L)(H2O)2]·2H2O
and [M(L)(X)z(H2O)n]·yH2O (M = Ce(IV) and Th(IV) (X = NO3, z = 2, n = 2, y = 0–3)). The biscomplexes (1:2) (M:H2SSZ) solid chelates
found to have the general formulae [UO2(HL)2]·2H2O and [M(L)2(H2O)2] (M = Ce(IV) and Th(IV)). The thermal decomposition of the
complexes should be discussed in relation to structure, and the thermodynamic parameters of the decomposition stages were evaluated
applying Coats–Redfern and Horwitz–Mitzger methods.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Sulfasalazine complexes; IR; Thermogravimetric analysis; Cerium; Thorium; Uranyl complexes
1. Introduction
UO2(II) which may help in deeper understanding of the role
of metal ions in biological systems. The stability of the metal
chelates have been calculated pH-metrically and the solid
complexes characterized using elemental and thermal analy-
ses, IR and molar conductance measurements. The thermal
decomposition of the complexes have been also used to infer
the structure.
Sulfa drugs have attracted special attention from their ther-
apeutic importance as they were used against a wide spectrum
of bacterial ailments [1–9]. Also, some sulfa drugs have been
used in the treatment of cancer, malaria, leprosy and tubercu-
losis [4]. The importance of metal ions in biological systems
is well known. One of the very interesting features of metal
coordinated systems is the concerted spatial arrangement of
the ligands around the metal ions [10]. Although the com-
plexes of the sulfa drugs have been investigated in the solid-
state, relatively was known about their solution chemistry in
particular their mixed-ligand complexes [8,11]. In Previous
work, the synthesis and characterization of Fe(III), Co(II),
Ni(II), Cu(II), Zn(II) and Cd(II) complexes with sulfasalazine
have been described [12]. In this article, we continue describ-
ing the coordination chemistry of sulfasalazine complexes
with some f-block elements namely Ce(IV), Th(IV) and
2. Experimental
2.1. Materials and measurements
All chemicals used throughout the work have been of
the highest purity available. They included sulfasalazine
(Sigma), potassium hydroxide (BDH), sodium chloride
(Merck), hydrochloric acid (Merck), EDTA-disodium salt
(Sigma), ammonium chloride and hydroxide (Merck).
Cerium ammonium nitrate, thorium nitrate tetrahydrate and
uranyl acetate dihydrated were from BDH. The organic sol-
vents used included absolute ethyl alcohol (BDH) and diethyl
ether (Aldrich). Elemental analyses (C, H, N) have been
∗
Corresponding author.
E-mail address: ggenidy@hotmail.com (G.G. Mohamed).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.04.006

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G.G. Mohamed et al. / Spectrochimica Acta Part A 62 (2005) 1095–1101
performed in the Microanalytical Center at Cairo Univer-
sity, the analyses have been repeated twice. The IR spectra
have been recorded using 1430 Perkin-Elmer FT-IR spec-
trometer and in wave number region 4000–400 cm−1. The
spectra have been recorded as KBr disks. The thermal anal-
ysis of the complexes has been carried out using a Shimadzu
thermogravimetric analyzer TGA-50H in a dynamic nitro-
gen atmosphere (flow rate 20 ml min−1) with a heating rate
of 10 ◦C min−1. The wt% loss has been measured from the
ambient temperature up to 1000 ◦C. Highly sintered ␣-Al2O3
was used as a reference. The molar conductance measure-
ments of the complexes have been carried out in DMF using
a Genway 4200 conductivity meter. The pH-metric titrations
have been carried out in an alcohol–water mixture (50%, v/v)
at 25 ◦C and ionic strength 0.1 M (achieved by the addition of
appropriate amounts of 1 M NaCl) using Genway 3020 pH/T
meter type. The pH meter was calibrated before each titration
using standard buffers of pH 4, 7 and 9. The stability con-
stants of its metal chelates with Ce(IV), Th(IV) and UO2(II)
have been determined using the technique of Sarin and Mun-
shi [13] and Irving and Rossitti [14]. Metal contents of the
complexes have been determined by titration against standard
EDTA after complete decomposition of the complexes with
aqua regia in 50 ml digestion flasks.
Fig. 2. HL− ((R = −N2PhSO2NHPy); Py = pyridine).
3. Results and discussion
3.1. Elemental analyses
The results of the elemental analyses (C, H, N and metal
content, mass (m/e)) with the proposed molecular formulae
are presented in Table 1. The metal content values are the
average values of the metal contents determined by titration
and those deduced from the metallic residues. The results
obtained are in good agreement with those calculated for the
suggested formulae. The molar conductance of the solid com-
plexes (Λm ꢀ2 cm−1 mol−1) has been measured. The H2SSZ
ligand under study form a 1:1 complexes with the di-, tri-
and tetravalent metal ions where it loses two hydrogen atoms
from phenolic and carboxylic OH groups (L2− form). The
solid monocomplexes (1:1) (M:HL−) were isolated in the
general formulae [M(L)(H2O)2]·yH2O (M = UO2(II); y = 2)
and [M(L)(X)z(H2O)n]·yH2O (M = Ce(IV) (X = NO3, z = 2,
n = 2, y = 0) and Th(IV) (X = NO3, z = 2, n = 2, y = 3). The
biscomplexes (1:2) (M:H2SSZ) solid chelates found to have
the general formulae [UO2(HL)2]·2H2O and [M(L)2(H2O)2]
(M = Ce(IV) and Th(IV)).
It is also clear from Table 1 that sulfasalazine has a high
affinity for chelation with metal ions under study. The overall
stability constants of the complexes (1:1); logK1logK2 have
been found in the order of UO2(II) < Th(IV) < Ce(IV). This is
with accordance to the increasing charge density of the metal
ions and hence the increasing of their coordination affinities.
The free energy changes values (ꢁG) for the formation of
the complexes (Table 1) are of high negative values which
reflect the spontaneous nature of the complex formation
reaction.
2.2. Synthesis of the complexes
Metal complexes have been synthesized by the addition
of a hot ethanolic solution (60 ◦C) of the appropriate metal
nitrate or acetate (25 ml, 0.1 mmol) to a hot ammoniacal
ethanolic solution (25 ml) of H2SSZ (25 ml, 0.1 (0.2) mmol).
The resulting mixture was stirred under reflux for 1 h, and
left to cool whereby the complexes were precipitated. The
solid complexes were filtered, washed firstly by ethanol and
then by diethyl ether and dried in vacuum desiccator over
anhydrous calcium chloride.
It is worthy to mention here also that the free carboxylic
form of sulfasalazine (Fig. 1) is insoluble in water or ethanol
so that ammonical ethanolic solution are used (pH 6.0) in
which the carboxylic acid group is ionized [12].
In addition, the sulfonamide NH (pKa 11.1) does not par-
ticipate in bonding due to structure complication. Hence,
sulfasalazine can be symbolized as HL− as shown in Fig. 2
in which R is the rest of sulfsalazine molecule.
Fig. 1. Structure of sulfasalazine.

G.G. Mohamed et al. / Spectrochimica Acta Part A 62 (2005) 1095–1101
1097
Table 1
Analytical data of salfasalazine complexes
Compound
Molar mass
Colour
Found (calculated) (%)
Λm (ꢀ2 cm−1 mol−1
)
log β
−ꢁG
(kJ mol−1
)
Calculated (m/e) (% yield)
C
H
N
M
[Ce(L)(H2O)2(NO3)2]
C18H16N6O13SCe
696.3
(697.2)
Dark brown 31.54
(70)
2.56
11.77
19.88
6.9
23.23
10.2
4.68
7.18
13.46
17.44
97.43
(31.02) (2.29) (12.06) (20.09)
Dark brown 44.62 3.58 11.65 14.46
(44.59) (2.89) (11.56) (14.44)
[Ce(L)2(H2O)2]
968.69
(966.6)
33.99 189.89
C
36H30N8O12S2Ce
(72)
[Th(L)(H2O)2(NO3)2]·3H2O 842.43
Yellow
(75)
25.14
2.95
9.93
27.93
(27.55)
13.71
76.59
C18
H22N6O16STh
(842.34)
(25.65) (2.61) (9.98)
[Th(L)2(H2O)2]
C36H30N8O12S2Th
1060.82
(1062.1)
Yellow
(82)
40.90 2.50 10.86
(40.72) (2.62) (10.56) (21.87)
21.55
26.44 147.71
[UO2(L)(H2O)2]·2H2O
738.42
(740.1)
Yellow
(76)
30.20
2.77
7.90
32.45
(31.96)
10.91
60.95
C18H20N4O11SU
(29.25) (2.71) (7.59)
[UO2(HL)2]·2H2O
1098.81
(1098.3)
Yellow
(82)
39.61 2.52 10.13
(39.32) (2.73) (10.19) (21.66)
21.09
19.02 106.26
C
36H30N8O14S2U
3.2. Molar conductance data
sulfasalazine, which would be overlapped by those of the
water molecules. The participation of the phenolic group
is further confirmed by clearifing the effect of chelation on
the ν(C O) stretching vibration. The shift of the ν(C O) of
phenolic group, from 1281 cm−1 in the free H2SSZ [12,16]
ligand to 1271–1256 cm−1 in the complexes indicated the
participation of the phenolic group in complex formation
[17,18]. Also the participation of the OH group is apparent
from the shift in position of the δ(OH) in-plane bending from
1394 cm−1 in the free H2SSZ ligand to 1388–1374 cm−1 in
the complexes [18].
The presence of water molecules in the above mentioned
complexes is assisted by the appearance of a broad band
within the range 3450–3300 cm−1, which is attributed to
ν(OH2) of the water molecules associated with the complex
formation. Also a bending vibration of the water molecules;
δ(OH2), is found in the range 965–914 cm−1. The other bend-
ing vibration of the water molecules; δ(OH2), is usually
around 1600 cm−1 which always interferes with the skele-
ton vibration of the benzene ring (C C vibration).
The molar conductance values (Table 1) for Ce(IV),
Th(IV) and UO2(II) chelates have been found to be 6.90,
10.20 and 7.18 ꢀ2 cm−1 mol−1, respectively. The relatively
low values indicate the non-electrolytic nature of these com-
plexes [15]. This can be accounted for by the deprotonated
carboxylic OH group (used as ammonium salt) in all com-
plexes and deprotonation of the phenolic OH group in Ce(IV)
and Th(IV) complexes. The neutrality of the UO2(II) com-
plexes has been interpreted by the deprotonated carboxylic
group and the coordination of the phenolic OH group to the
metal without proton displacement.
3.3. Infrared spectra and mode of bonding
The IR spectra of the free ligand and its metal complexes
have been carried out in the range of 4000–400 cm−1 and
the important bands are listed in Table 2. The IR spectrum of
H2SSZ showed a medium broad band at 3439 cm−1 which
attributed to OH of the phenolic and carboxylic OH groups.
The carboxylic OH group is not considered in the spectra
of complexes since ammonical solution of sulfasalazine was
used in which the carboxylic was used as the ammonium
salt. The existence of water of hydration and/or water
of coordination in the spectra of the complexes render it
difficult to get conclusion from the phenolic OH group of the
The participation of the carboxylic group in chelation can
be indicated from the changes of the bands of the asymmetric
and symmetric stretching vibrations of the carboxylate group
upon complex formation. The spectrum of H2SSZ ligand
shows sharp bands at 1618 and 1427 cm−1 assigned for asym-
metric and symmetric stretching vibrations of the carboxy-
late moiety, respectively. These two bands are either slightly
Table 2
Selected IR data (4000–400 cm−1) for sulfasalazine and its complexes
Compound
ν(OH)H O
ν(C O)
δ(OH)
νasym.(COO)
νsym.(COO)
δ(H2O)
ν(M O)
2
[Ce(L)(H2O)2(NO3)2]
[Ce(L)2(H2O)2]
3450 b
3520 b
3300 b
3400 vb
3330 b
3450 vb
1268 s
1270 s
1256 s
1265 s
1271 s
1261 s
1384 s
1384 s
1388 s
1374 s
1382 s
1384 s
1618 s
1618 s
1595 s
1595 s
1608 s
1614 s
1425 s
1429s
1436 s
1436 s
1421 s
1427 s
965 m
956 m
914 s
918 s
930 m
933 m
447 s
470 s
447 s
520 s
439 s
518 s
[Th(L)(H2O)2(NO3)2]·3H2O
[Th(L)2(H2O)2]
[UO2(L)(H2O)2]·2H2O
[UO2(HL)2]·2H2O
s, Strong; m, medium; b, broad; vb, very broad.

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G.G. Mohamed et al. / Spectrochimica Acta Part A 62 (2005) 1095–1101
shifted to lower frequencies or remained decreased markedly
in intensity. This indicates that the carboxylate group partic-
ipated in complex formation with the metal ions [19].
estimated mass loss 69.82% (calculated mass loss 70.14%)
which is reasonably accounted for the liberation of two nitrate
moieties (as NO2) along with the ligand molecule leaving
CeO2 as the metallic residue, with a total estimated mass
loss of 75.08% (total calculated mass loss 75.31%).
The thermogram of the biscomplex of Ce(IV) with the
general formula [Ce(L)2(H2O)2] give decomposition pattern
of three stages. The first estimated mass loss 3.64% occurred
within the temperature range 25–250 ◦C is corresponded to
the loss of two coordinated water molecules (calculated mass
loss 3.71%). The second and third steps occurred within the
temperature range 250–900 ◦C with an estimated mass loss
79.31% (calculated mass loss 78.54%) which was accounted
for by the liberation of two ligand molecules leaving CeO2
residue with total estimated mass loss 82.95% (total calcu-
lated mass loss = 82.18%), as given in Table 3.
Participation of the phenolic and carboxylic OH groups is
also confirmed by the appearance of new bands in the com-
plexes in the 472–439 cm−1 regions which may be assigned
to the ν(M O) stretching vibration [16,20]. Coordination of
the nitrate anion to the Ce(IV) and Th(IV) ions were also sup-
ported by the IR spectra of the nitrate complexes. The nitrato
complexes of Ce(IV) and Th(IV) ions displayed three strong
bands due to the nitrate group νasym.(NO2) at 1500 cm−1
,
νsym.(NO2) at 1270 cm−1 and ν(NO) at 987 cm−1 assigned
to monodentate nitrate group [21].
3.4. Thermal analysis data
Th(IV) complex with the general formula
[Th(L)(H2O)2(NO3)2]·3H2O was thermally decomposed in
three successive decomposition steps. The first estimated
mass loss of 6.58% (calculated mass loss 6.41%) found as
very wide, slow and decline step within the temperature
range 30–150 ◦C could be attributed to the liberation of
three hydrated water molecules. The second step found
within the temperature range 250–310 ◦C with an estimated
weight loss 15.50% (calculated mass loss 15.21%) was
attributed to the removal of two coordinated water molecules
and two nitrate moieties (as NO2). The third step occurred
within the temperature range 310–650 ◦C with an estimated
mass loss 47.30% (calculated mass loss 47.03%) which
was accounted for the liberation of ligand molecule leaving
ThO2 as metallic residue with a total estimated mass loss
69.38% (total calculated mass loss 68.65%).
The TG and DTG curve of the mono- and biscomplexes
of sulfasalazine with Ce(IV) are shown in Fig. 3 and the
data are listed in Table 3. The thermodynamic parameters
for the different decomposition steps are deduced from both
Coats–Redfern [22] and Horowitz–Metzger [23] plots and
listed in Table 4, where the average values of these parame-
ters are those considered in the discussion. The correlations
between the different decomposition steps of the complexes
with the corresponding weight losses are discussed in terms
of the proposed formulae of the complexes as follows:
Ce(IV) complex with a general formula [Ce(H2L)
(H2O)2(NO3)2] wasthermallydecomposed in twosuccessive
decomposition steps. The first decomposition step was found
as a split DTG peak in the temperature range 180–350 ◦C
with an estimated mass loss of 5.26% (calculated mass loss
5.17%) could be assigned to the successive loss of two coor-
dinated water molecules. The second step occurred as broad
DTG peak within the temperature range 350–900 ◦C with an
The thermogram of thorium biscomplex with the gen-
eral formula [Th(L)2(H2O)2] gave a decomposition pattern
Fig. 3. TG and DTG curves of Ce(IV) complexes.

G.G. Mohamed et al. / Spectrochimica Acta Part A 62 (2005) 1095–1101
1099
Table 3
Thermoanalytical data of sulfasalazine complexes
Complex
TG range (◦C)
Mass loss
Total mass loss
Assignment
Metallic
residue
Estimated (calculated %)
Estimated (calculated %)
[Ce(L)(H2O)2(NO3)2]
[Ce(L)2 (H2O)2]
219–350
519–820
5.26 (5.17)
69.82 (70.14)
Loss of 2H2O (coord.)
Loss of two nitrate groups and
ligand molecule
Loss of 2H2O (coord.)
Loss of (two ligand
molecules-2O)
CeO2
CeO2
ThO2
75.08 (75.31)
82.35 (82.18)
30–250
250–900
3.64 (3.71)
79.31 (78.54)
[Th(L)(H2O)2 (NO3)2]·3H2O
30–150
250–310
6.58 (6.41)
15.50 (15.21)
Loss of 3H2O (hyd.)
Two nitrates (as
NO2) + 2H2O(coord.)
Loss of ligand molecule
510–650
47.30 (47.03)
69.38 (68.65)
75.78 (75.11)
[Th (L)2 (H2O)2]
[UO2L (H2O)2]·2H2O
[UO2(L)2]·2H2O
35–165
165–1000
3.18 (3.39)
72.60 (73.71)
Loss of 2H2O molecules
Loss of (two ligand
molecules-2O)
ThO2
UO2
UO2
50–150
150–650
4.96 (4.88)
54.87 (54.22)
Loss of 2H2O (hyd.)
Loss of 2H2O
(coord) + (ligand-2O)
Loss of 2H2O (hyd.)
Loss of two ligands
59.83 (59.10)
75.58 (75.61)
30–220
220–607
3.30 (3.28)
72.28 (72.33)
of two stages. The first estimated mass loss 3.18% occurred
within the temperature range 35–165 ◦C corresponding to the
loss of two coordinated water molecules (calculated mass
loss 3.39%). The second step (165–1000 ◦C) was slow one
and corresponded to three successive and close DTG peaks
with an estimated mass loss sum to 72.60% (calculated
mass loss 71.71%) was attributed to the loss of two ligand
molecules leaving ThO2 residue. The total estimated mass
loss is 75.78% (total calculated mass loss 75.11%).
(calculated mass loss 4.88%) within the temperature range
50–150 ◦C could be attributed to the liberation of two
hydrated water molecules. The second step, composed of
two successive steps with two DTG peaks, occurred within
the temperature range 150–650 ◦C with an estimated mass
loss 58.87% (calculated mass loss 58.54%) was reasonably
accounted for by the liberation of two coordinated water
molecules along with the ligand molecule leaving UO2 as
the metallic residue with a total estimated mass loss 63.83%
(total calculated mass loss 63.41%).
The uranyl complex with the formula [UO2(L)
(H2O)2]·2H2O was thermally decomposed in two decom-
position steps. The first estimated mass loss of 4.96%
The uranium biscomplex, [UO2(HL)2]·2H2O, was ther-
mally decomposed in three successive decomposition steps.
Table 4
Thermodynamic data of the thermal decomposition of sulfasalazine complexes
Complex
Decomposition HM (CR)
range (◦C)
E* (kJ mol−1
)
A (Arrhenius factor) (s−1
)
ꢁS* (kJ/(mol K)) ꢁH* (kJ mol−1
−11.12 (−62.34) 132.8 (109.7)
)
ꢁG* (kJ mol−1
)
[CeL(H2O)2(NO3)2]
[Ce(L)2(H2O)2]
219–350
350–820
137.5 (114.4)
66.54 (53.48)
3.1 × 1012 (6.6 × 109)
139.1 (144.6)
214.7 (128.8)
2.1 × 103 (9.6 × 107)
−189.5 (−100.5)
59.74 (46.68)
25–250
143.5 (134.1)
89.57 (74.48)
1.8 × 1013 (4.1 × 1011
3.1 × 104 (6.4 × 106)
1.0 × 106 (1.1 × 105)
1.9 × 1024 (5.0 × 1022
1.1 × 106 (2.6 × 105)
)
)
−27.71 (−32.14) 138.9 (129.5)
136.9 (144.8)
229.4 (175.4)
250–900
−167.9 (−123.6)
82.29 (67.19)
[Th(L)(H2O)2(NO3)2]·3H2O 30–150
41.48 (33.27)
270.6 (259.9)
91.81 (81.19)
−131.2 (−149.3)
38.65 (30.44)
265.8 (255.1)
85.87 (75.24)
83.29 (81.26)
140.9 (147.9)
183.3 (181.3)
250–310
510–650
214.7 (184.2)
−136.4 (−148.5)
[Th(L)2(H2O)2]
35–165
25.32 (20.20)
44.23 (35.03)
160.1 (89.19)
88.70 (69.61)
5.2 × 107 (2.9 × 106)
2.4 × 108 (7.7 × 106)
4.2 × 106 (1.8 × 105)
2.5 × 104 (1.4 × 106)
−185.0 (−210.1)
22.42 (17.30)
39.62 (30.41)
86.87 (90.62)
142.3 (150.5)
221.3 (210.6)
304.7 (315.7)
165–350
350–580
580–1000
−186.1 (−216.3)
−78.37 (−149.1) 153.7 (82.02)
−198.5 (225.1)
79.36 (60.17)
[UO2L(H2O)2]2H2O
[UO2(L)2]2H2O
50–150
150–450
450–650
30.31 (36.12)
208.3 (199.4)
394.2 (380.9)
2.4 × 1016 (4.8 × 1014
)
)
−161.6 (−128.2)
27.60 (26.24)
80.27 (89.74)
162.0 (174.6)
199.1 (213.4)
2.9 × 1025 (4.6 × 1023
62.47 (−29.65) 202.8 (193.9)
2.7 × 108 (4.6 × 1023
)
234.2 (199.9)
387.5 (374.2)
30–220
220–670
55.81 (49.72)
112.8 (102.1)
2.7 × 108 (6.0 × 106)
−48.57 (−116.2)
53.01 (46.92)
81.50 (86.04)
141.38 (181.0)
1.7 × 108 (3.9 × 106)
−88.80 (−125.6) 109.84 (96.49)

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G.G. Mohamed et al. / Spectrochimica Acta Part A 62 (2005) 1095–1101
Fig. 4. Structure of metal complexes (water of hydration is omitted).
The first estimated mass loss of 3.30% within the temperature
range 100–220 ◦C which might be attributed to liberation of
two hydrated water molecules (calculated mass loss 3.28%).
The ligand molecules were eliminated in the second and third
steps found within temperature range 220–670 ◦C with an
estimated mass loss sum to 72.28% (calculated mass loss
72.33%) which is responsibly accounted for liberation of
ligand molecule leaving UO2 residue within total estimated
mass loss 75.58% (total calculated mass loss 75.61%).
that the Ce(IV), Th(IV), UO2(II) complexes are hexacoordi-
nated and sulfasalazine ligand acted as bidentate ligand via
the deprotonated phenolic and carboxylic OH groups. The
extra coordination positions were provi−ded by the coordi-
nated water molecules and Cl− or NO3 anions. The pro-
posed structures are given in Fig. 4.
References
3.5. Kinetic studies
[1] D.B. Clyson, J.A.S. Pringle, G.M. Ranses, Biochem. Pharmacol. 16
(1967) 614.
The kinetic parameters such as activation energy (ꢁE*),
enthalpy (ꢁH*), entropy (ꢁS*) and free energy change of
the decomposition (ꢁG*) have been evaluated graphically
by employing the Coats–Redfern and Horwitz–Mitzger rela-
tions [22,23]. The calculated values of E*, A, ꢁS*, ꢁH* and
ꢁG* for the decomposition steps are given in Table 4. The
most significant result is the considerable thermal stability of
the complexes reflected from the high values of the activa-
tion energy of the decomposition. The second essential result
from Table 4 is that the entropy change ꢁS* for the forma-
tion of the activated complexes from the starting reactants is
in most cases of negative values. The negative sign of the ꢁS*
suggests that the degree of structural “complexity” (arrange-
ment, “organization”) of the activated complexes was lower
than that of the starting reactants and the decomposition reac-
tions are fast reactions [24].
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