Influence of the aromatic substitutes in the thermal and kinetic behavior of mesoionic compounds of the 1,3-thiazole-5-tiolate system

Article abstract of DOI:10.1016/j.jallcom.2009.12.002

In this work, three mesoionic compounds of the 1,3-thiazole-5-tiolat system were studied, derived from amino acids of the glycerin through 1,3-dipolar cyclo-addition/reversion reaction. The mesoionic compounds were characterized as: MI-1 (mesoionic 2-(4-chlorophenyl)-3-methyl-4-phenyl-1,3-thiazole-5-tiolat); MI-2 (mesoionic 2-(4-chlorophenyl)-3-methyl-4-(4-isopropylphenyl)-1,3-thiazole-5-tiolat) and MI-3 (Mesoionic 2-(4-clorophenyl)-3-methyl-4-(methoxyphenyl)-1,3-thiazole-5-tiolate). These compounds were characterized by infrared spectroscopy (IR), nuclear magnetic resonance (NMR), thermogravimetry (TG) and differential scanning calorimeter (DSC). Also, the kinetic study of the thermal decomposition by non-isothermal thermogravimetry has been realized, presenting, the kinetic and thermal behavior of these compounds. The results of the spectroscopic analysis confirmed the structure of the synthesized mesoionic compounds. The DSC curves of the mesoionic compounds MI-1, MI-2, and MI-3 indicated the fusion of two of them followed by a subsequent decomposition. The TG/DTG curves showed that the decomposition of the mesoionic compounds MI-1, MI-2 and MI-3 occurred in several steps.

Full text of DOI:10.1016/j.jallcom.2009.12.002

Journal of Alloys and Compounds 495 (2010) 598–602
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Influence of the aromatic substitutes in the thermal and kinetic behavior of
mesoionic compounds of the 1,3-thiazole-5-tiolate system
Soraya Alves de Moraisa , Crislene Rodrigues da Silva Moraisb,∗ , Petrônio Filgueiras de Athayde Filhoc ,
Bruno Freitas Lirac, Marcos Antonio feitosa de Souzac
a Universidade Estadual da Paraíba, Departamento de Química, Av. Baraúnas, s/n, Campina Grande, PB, Brazil
b Universidade Federal de Campina Grande, Unidade Acadêmica de Engenharia de Materiais, Av. Aprígio Veloso, 882, Campina Grande, PB, Brazil
c Universidade Federal da Paraíba, Departamento de Química - CCEN, 58.081-970, João Pessoa, PB, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, three mesoionic compounds of the 1,3-thiazole-5-tiolat system were studied, derived from
amino acids of the glycerin through 1,3-dipolar cyclo-addition/reversion reaction. The mesoionic com-
pounds were characterized as: MI-1 (mesoionic 2-(4-chlorophenyl)-3-methyl-4-phenyl-1,3-thiazole-
5-tiolat); MI-2 (mesoionic 2-(4-chlorophenyl)-3-methyl-4-(4-isopropylphenyl)-1,3-thiazole-5-tiolat)
and MI-3 (Mesoionic 2-(4-clorophenyl)-3-methyl-4-(methoxyphenyl)-1,3-thiazole-5-tiolate). These
compounds were characterized by infrared spectroscopy (IR), nuclear magnetic resonance (NMR), ther-
mogravimetry (TG) and differential scanning calorimeter (DSC). Also, the kinetic study of the thermal
decomposition by non-isothermal thermogravimetry has been realized, presenting, the kinetic and ther-
mal behavior of these compounds. The results of the spectroscopic analysis confirmed the structure of
the synthesized mesoionic compounds. The DSC curves of the mesoionic compounds MI-1, MI-2, and
MI-3 indicated the fusion of two of them followed by a subsequent decomposition. The TG/DTG curves
showed that the decomposition of the mesoionic compounds MI-1, MI-2 and MI-3 occurred in several
steps.
Received 8 September 2008
Received in revised form
26 November 2009
Accepted 1 December 2009
Available online 6 December 2009
Keywords:
Mesoionic compounds
Thermal analysis
Kinetics
© 2010 Published by Elsevier B.V.
1. Introduction
ied as promising structures for nonlinear optical devices with
application in the storage and transmission of technological
Mesoionic compounds are defined as planar five-member hete-
rocyclic betaines with at least one side-chain whose ␣-atom is also
in the ring plane with dipole moments of the order of 5D [1–5].
A generic representation of the mesoionic compounds of the 1,3-
thiazolium-5-thiolate system is shown by the Structure 1 where
R1, R2 and R3 can be alkyl or aryl groups.
information [2,4,9–13]. The main applicability of the mesoionic
compounds lies in the large variety of biological activities pre-
sented by this kind of betainic compound. One believes that
this diversity is related to structural and electronic factors such
as: the betainic characteristics which lead to strong interactions
with many biomolecules, structural similarity with many active
pharmacologic drugs or electrostatic interaction with two comple-
mentary positions, such as a protein helice. In view of the fact that
no previous results exist on the kinetic and thermal behavior of
these compounds, this study aims to study the thermal behavior of
three mesoionic compounds of the 1,3-thiazolium-5-thiolate sys-
tem by infrared spectroscopy (IR) and thermal analysis (TG and
DSC), as well as, to determine the kinetic parameters (activation
energy, pre-exponential factor and reaction order) using integral
mathematical treatments and non-isothermal thermogravimetric
analysis.
Studies show that the mesoionic compounds exhibit a high
potential for useful biological activities, such as, antitumor,
anti-inflammatory, antimalaric and analgesic [6–8]. Besides the
therapeutic applications, nowadays, mesoionics are being stud-
2. Experimental
∗
Corresponding author. Tel.: +55 031 83 3310 1183; fax: +55 031 83 3310 1178.
E-mail addresses: sorayaamorais@yahoo.com.br (S.A. de Morais),
crislene@pq.cnpq.br, crislene@dema.ufcg.edu.br (C.R. da Silva Morais),
athayde-filho@pq.cnpq.br (P.F.d.A. Filho), brunofrlira@hotmail.com (B.F. Lira),
ramcos31@yahoo.com.br (M.A.f.d. Souza).
2.1. Synthesis of mesoionic compounds
All reagents and solvents were obtained from commercial sources and used as
supplied. Mass spectra were obtained on a Finnigan GCQ Mat type quadrupole-ion
0925-8388/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2009.12.002

S.A. de Morais et al. / Journal of Alloys and Compounds 495 (2010) 598–602
599
Scheme 1.
trap spectrometer. IR spectra (KBr) were obtained on a Bruker IFS66 spectrom-
eter. 1H and 13C NMR spectra (DMSO-d6 with internal TMS) were recorded on
a Varian Unity Plus 300 MHz spectrometer. Elemental analysis was carried out
on a PerkinElmer elemental microanalyser. Melting points were determined on
131.4, 129.5, 125.5, 122.5, 113.5, 55.3, 40.3; Anal. Calcd for C17H14ClNOS2, C, 58.69;
H, 4.06; N, 4.03; Found: C, 58.39; H, 3.82; N, 4.20; yield 56%, m.p. 215–217 ◦C
[recryst. EtOH:H2O (1:1, v/v)].
a
Kofler hot-plate apparatus combined with
a Carl–Zeiss microscope and are
2.2. Thermal measurements
uncorrected.
Mesoionic compounds (MI) were synthesized according to the methodol-
ogy described by Athayde-Filho et al. [2] starting from amino acids, obtained
through Strecker synthesis (i), that were treated with aroyl chlorides (ii), cyclized
for the action of acetic anhydride (iii) and finally submitted to 1,3-dipolar
cyclo-addition/reversion with carbon disulphide (iv) (Scheme 1). The struc-
tures of compounds were elucidated and compared with the data described in
the literature [2,4] through elementary analysis studies, infrared spectroscopy
and RMN of 1H and 13C. Four mesoionics of the 1,3-thiazolium-5-thiolate sys-
tem were obtained: 2,4-diphenyl-3-methyl-1,3-thiazolium-5-thiolate (MI-1);
2-(4-chlore-phenyl)-3-methyl-4-phenyl-1,3-thiazolium-5-thiolate (MI-2) and
2-(4-chlore-phenyl)-3-methyl-4-(4-methoxy-phenyl)-1,3-thiazolium-5-thiolate
(MI-3).
Thermogravimetric curves were obtained using a Shimadzu model TGA-50 ther-
mobalance with an alumnae crucible, and with heating rates of 10 ◦C min−1 in a
temperature range from ambient 1 to 900 ◦C and under a nitrogen atmosphere with
a flow rate of 50 mL min−1. The sample mass was 2.0 0.5 mg. The TG curves were
analyzed with the aid of the TASYS software from Shimadzu.
The DSC curves were achieved in
a SHIMADZU, DSC-50 calorimeter. The
analyses were realized in an aluminum crucible with a nitrogen atmosphere,
flux 50 mL min−1, temperature range from ambient 1 to 500 ◦C and heat rates
10 ◦C min−1
.
2.3. Spectroscopy
The infrared spectra were achieved in a BOMEM-MICHELSON SERIES instru-
ment, employing KBr pellets and registering amplitude waves ranging from 400 to
2.1.1. Preparation of mesoionics compounds—general procedure
4000 cm−1
.
The N-aroyl-N-methyl-C-arylglycines (1) were dissolved in Ac2O (20 mL) and
heated, with stirring, at 55 ◦C for 15 min. After cooling to ambient temperature, CS2
(20 mL) was added and the reaction mixture allowed to stand for 48 h. MeOH/H2O
(1:1) was then added until the mixture became cloudy. After standing for 24 h,
the desired products precipitated as orange–red crystals, which were recrystallized
from MeOH.
2.4. Other measurements
The kinetic parameters, activation energy, reaction order and frequency factor
were obtained from the thermogravimetric data by the integral methods proposed
by Coats and Redfern (CR) [14] and Madhusudanan (MD) [15], and the approxi-
mation methods proposed by Horowitz–Metzger (HM) [16] and Van Krevelen et
al. (VK) [17]. The kinetic parameters in the non-isothermal heating method were
determined according to the Coats and Redfern [14] equation, using the thermal
decomposition model suggested by the data obtained in the isothermal heating
experiments [18].
2.1.1.1. 2,4-Diphenyl-3-methyl-1,3-thiazolium-5-thiolate
(MI-1). N-benzoyl-N-
methyl-C-phenylglycine 5.00 g (19.40 mmol); IR, KBr (ꢀ cm−1): 3025 (ꢀCAr–H),
2948 (ꢀC–H), 1482 (ꢀN–CH3—asymmetric), 1424 (ꢀN–CH3–symmetric), 1291
(ꢀC–S− thiol group); 1H NMR (300 MHz, CDCl3): ı = 3.64 (s, 3H, H-10), 7.33 (m,
3H—aromatics) and 7.52 (m, 7H—aromatics). 13C NMR (75 MHz, CDCl3): ı = 159.8,
140.7, 154.3, 139.1, 131.1, 132.8 (C-7 and C-7ꢀ), 129.7, 129.6 (C-11), 129.4, 128.2,
123.8, 40.3; Anal. Calcd for C16H13NS2: C, 53,13; H, 5.82; N, 6.21. Found: C, 53.20;
H, 5.70; N, 6.29; yield 51%, m.p. 183–184 ◦C [recryst. EtOH:H2O (1:1, v/v)].
2-(4-chloro-phenyl)-3-methyl-4-phenyl-1,3-thiazolium-5-thiolate (MI-2)—N-(4-
chloro-benzoyl)-N-methyl-C-phenylglycine 0.5 g (1.4 mmoles); IR, KBr (ꢀ cm−1):
3049 (ꢀCAr(H); 2981 and 2853 (ꢀC(H); 1580, 1468 (ꢀC C and C N); 1434 (ꢀC(N,
N–CH3); 1282 (ꢀC–S−, thiol group); 1087 (ꢀCAr(Cl); 1H NMR (300 MHz, CDCl3): ı
3.62 (s, 3H; H-10); 7.44 (dd; 1H; H-14); 7.46 (t; 2H; H-13 and 13ꢀ); 7.52 (s, H-7 and
7ꢀ); 7.52 (s, H-8 and 8ꢀ) and 7.57 (dd, 2H; H-12 and 12ꢀ); 13C NMR (75 MHz, CDCl3):
ı = 161.2 (C-5); 152.3 (C-2); 141.2 (C-4); 138.0 (C-9); 131.1 (C-12 and C-12ꢀ); 130.7
(C-7 and C-7ꢀ); 129.9 (C-8 and C-8ꢀ); 129.8 (C-11); 129.3 (C-14); 128.8 (C-13 and C-
13ꢀ); 125.2 (C-6), 40.5 (C-10). Anal. Calcd for C16H12ClNS2, C, 60.46; H, 3.81; N, 4.41;
Found: C, 60.82; H, 4.73; N, 4.22; yield 61.3% m.p. 179–181 ◦C [recryst. EtOH:H2O
(1:1, v/v)].
3. Results and discussion
3.1. Spectroscopic results
All the products gave satisfactory elemental analyses. In all
of the infrared spectra, characteristic bands can be observed of
the functional groups, such as, the absorption band of exocyclic
tiolat (ꢁC–S−) between 1280 and 1291 cm−1 which certifies the
existence of a lateral chain and consequent generation of the
mesoionic compound. Table 1 shows the main attributes of the
absorption bands, observed in the infrared spectra in the range
2-(4-chloro-phenyl)-3-methyl-4-(4-methoxy-phenyl)-1,3-thiazolium-5-thiolate
(MI-3)—N-(4-chloro-benzoyl)-N-methyl-C-(4-methoxy-phenyl)-glycine
4000–400 cm−1
.
0.5 g
(1.5 mmoles); IR, KBr (( cm−1): 3043, 3007 (ꢀCAr(H,); 2989 and 2831 (ꢀC(H); 1603,
1597, 1482 (ꢀC N); 1433 ( N(CH3); 1280 (ꢀC–S−, thiol group); 1251 (ꢀCAr–O–C);
1098 (ꢀCAr(Cl); 1H NMR (300 MHz, CDCl3): ı 3.66 (s, 3H; H-10); 3.82 (s; 3H, H-15);
7.07 (d, 2H; H-13, 13ꢀ); 7.52 (d, 2H; H-8, 8ꢀ); 7.68 (d, 2H; H-12, 12ꢀ) and 7.76 (d,
2H; H-7, 7ꢀ); 13C NMR (75 MHz, CDCl3): ı = 161.2, 159.8, 151.1, 141.2, 136.2, 132.6,
Studies of 1H NMR of mesoionic compounds are not very infor-
mative when there is no hydrogen in the heterocyclic ring, however
these are important in the detection of hydrogens in the substituent
groups. In contrast, the 13C NMR spectroscopy is an important tool

600
S.A. de Morais et al. / Journal of Alloys and Compounds 495 (2010) 598–602
Table 1
Infrared absorption bands of mesoionic compounds.
Compounds
Attributions ꢀ (cm−1
)
CAr–H
C–H (as)
C–H (s)
N–CH3
C–S−
C–Cl
C–O–C
C–F
MI-1
MI-2
MI-3
3025
3049
3025
2948
2981
2948
14251482
1434
14241482
1291
1282
1291
–
1087
–
–
–
–
–
–
–
2853
–
to characterize the structure of mesoionic compounds due to the
large difference in electronic densities between the two regions of
the betainic ring (positive and negative). This difference in elec-
tronic density manifests itself in shielding or deshielding effects
of the carbons of the heterocyclic ring and of the carbons of aro-
matic or aliphatic groups linked to the mesoionic ring. The three
peaks at ca. 160, 150, and 140 ppm are attributed to C5, C2, and
C4 respectively (see Scheme 1). The reason that C4 is at high field
compared with C5; although both are in the negative region of the
mesoionic ring can be related to the significant partial double bond
character of the carbon-thiolate bond (C5–S5). The methylic car-
bon linked to N-3 is characterized by a peak at 40 ppm and the
methylic protons have chemical shifts around 3.60 ppm, there is a
down-field shift as a result of the positive charge on N3. The car-
bon atoms with chemical shift at low field, 159.6 ppm, are affected
by deshielding effects caused by the existence of the methoxy
group.
of the heterocyclic ring in 3.66 ppm and three were assigned to
the methoxy group in 3.82 ppm. In the region between 8.34 and
7.07 ppm we observed corresponding signals for the eight aromatic
hydrogens.
The results above are in agreement with the results observed by
Athayde-Filho et al. [4,5].
3.2. Thermal behavior
The mesoionic DSC curves for MI-1 and MI-2 (Fig. 1) present
endothermic peaks at 185.5 and 170.7 ◦C (respectively) immedi-
ately followed by exothermic peaks at 192.1 ◦C (MI-1) and 175.7 ◦C
(MI-2), characteristics of a fusion process followed by the com-
pounds decomposition. After that, two endothermic bands can be
observed with a maximum at 266.8 and 296.1 ◦C, for MI-1 and an
endothermic band with a maximum at 265.9 ◦C (MI-2), a character-
istic of thermal decomposition. The mesoionic DSC curve presents
five endothermic bands with maximum at 43.6, 146.6, 191.6, 207.4
and 335.2 ◦C. The first band refers to the loss of humidity of the
compound which can be confirmed through the infrared spectra.
The other bands refer to its own decomposition.
Figs. 2–4 present TG/DTG curves of the mesoionic compounds
(heat rate 1.0 ◦C min−1). It can be observed that the MI-1 mesoionic
compoundpresent only one decomposition step. TheMI-2 and MI-3
present 5 and 4 decomposition steps, respectively.
Table 2 presents the results obtained in TG/DTG curves of the
studied compounds. The experimental mass loss (%), the tempera-
ture ranges (◦C)andthe attributions referredto each decomposition
step of the mesoionic compounds is defined in the table.
The 13C NMR (APT, 50 MHz, DMSO-d6) study of the compound
MI-1 showed that the carbons of the mesoionic ring have chemi-
cal shifts of 154.3; 140.7 and 159.8 ppm assigned, respectively, to
C2, C4 and C5 (see Scheme 1). The carbon C10 has chemical shifts
of 40.7 ppm, while the aromatic carbons have chemical shifts of
139.1 (C-9), 131.1 (C-12 and C-12ꢀ), 132.8 (C-7 and C-7ꢀ), 129.7 (C-
8 and C-8ꢀ), 129.6 (C-11), 129.4(C-14), 128.2(C-13 and C-13ꢀ), and
123.8 ppm (C-6).
The 1H NMR (200 MHz, DMSO-d6) spectrum of MI-1 shows two
hydrogen signals. Of these, one intense singlet was observed in the
aliphatic region with an integral of 3H for the hydrogens H10 of the
methyl group in 3.64 ppm. There was also a signal between 8.27
and 7.23 assigned to the eight aromatic hydrogens.
For compound MI-2 we observed a similar chemical behavior.
The 13C NMR (APT, 50 MHz, DMSO-d6) spectrum showed the car-
bons of the mesoionic ring in chemical shifts of 152.3; 141.2 and
161.2 for C2, C4 and C5, respectively (see Scheme 1). Carbon C10
shows chemical shifts that are compatible with the N-methyl group
in 40.5. The aromatic carbons have chemical shifts of 138.0 (C-9),
131.1 (C-12 and C-12ꢀ), 130.7 (C-7 and C-7ꢀ), 129.9 (C-8 and C-8ꢀ),
129.8 (C-11), 129.3 (C-14), 128.8 (C-13 and C-13ꢀ), and 125.2 ppm
(C-6).
For MI-1 only one decomposition step can be observed with
mass loss of 99.1% in a temperature range between 191 and 347 ◦C.
The MI-2 compound presents 5 decomposition steps with mass
loss of 6.6%; 71.9%; 8.1%; 6.5% and 3.1% for the temperature ranges
152–210; 210–354; 354–435; 435–577 and 634–787 ◦C, respec-
tively. It can be observed that the higher mass loss occurred in the
The 1H NMR (200 MHz, DMSO-d6) spectra of MI-2 were
observed as one intense singlet in the aliphatic region with an
integral of three hydrogen assigned to the N-methyl group of the
heterocyclic ring in 3.69 ppm. In the region between 8.34 and
7.07 ppm we observed corresponding signals for the nine aromatic
hydrogens.
For compound MI-3 we observed a similar chemical behavior.
The 13C NMR (APT, 50 MHz, DMSO-d6) spectrum showed the car-
bons of the mesoionic ring in chemical shifts of 151.1; 141.2 and
161.2 for C2, C4 and C5, respectively (see Scheme 1). Carbons C10
and C15 show chemical shifts that are compatible with the N-
methyl group in 40.8 and the methoxy group in 55.3. The aromatic
carbons have chemical shifts of 136.2 (C-9), 132.6 (C-12 and C-12ꢀ),
131.4 (C-7 and C-7ꢀ), 129.5 (C-8 and C-8ꢀ), 125.5 (C-6), 122.5 (C-11),
and 113.5 ppm (C-13 and C-13ꢀ).
The 1H NMR (200 MHz, DMSO-d6) spectrum of MI-3 showed
two intense singlets in the aliphatic region with an integral of six
hydrogens, three hydrogens were assigned to the N-methyl group
Fig. 1. DSC curves of the mesoionic compounds MI-1, MI-2 and MI-3 obtained using
a heating rate of 10 ◦C min−1
.

S.A. de Morais et al. / Journal of Alloys and Compounds 495 (2010) 598–602
601
Table 2
Thermal decomposition data for mesoionic compounds at 10 ◦C min−1
.
Compounds
MI-1
Stage
Temperature range (◦C)
Mass loss (%)
Species lost
1
191–347
>347
99.1
0.9
C8H8N + C8H5S2
–
Residue
MI-2
MI-3
1
2
3
4
152–210
210–354
354–435
435–577
634–787
>787
6.6
71.9
8.1
6.5
3.1
3.8
CH3
C8H7ClN + CS2
C2H2
C2H2
C
5
Residue
–
1
2
3
4
138–227
227–379
380–425
495–871
>871
3.7
CH3
70.1
13.6
6.8
C7H5ClN + C3H5 + CS2
C4H4
C2H2
–
Residue
5.8
Table 3
Kinetic parameters of compounds MI-1, MI-2 and MI-3 calculated starting from the non-isothermal thermogravimetric data (heating rates of 10 ◦C min−1).
Compounds
Stage
Parameters
Methods
CR
MD
0.32
HM
0.34
VK
0.23
MI-1
1
n
Ea
A
r
0.12
91.41
95.01
115.21
98.00
1.74 × 106
4.51 × 106
1.000
4.20 × 106
2.46 × 1012
0.978
1.000
0.999
MI-2
MI-3
2
2
n
Ea
A
r
0.52
98.60
0.51
98.53
0.68
0.56
120.31
120.31
5.07 × 106
1.000
5.37 × 106
1.000
6.47 × 108
4.79 × 1012
0.981
0.999
n
Ea
A
r
0.50
113.86
4.30 × 107
1.000
0.56
116.63
8.54 × 107
1.000
0.74
140.70
0.69
132.44
1.26 × 1010
8.32 × 1014
1.000
0.999
ꢂ = heat rate (◦C/min); n = reaction order; r = linear correlation coefficients; Ea = activation energy (kJ mol−1); A = frequency (s−1); CR = Coats–Redfern; MD = Madhusudanam
et al.; HM = Horowitz–Metzger and VK = Van Krevelen.
second decomposition step. For MI-3 four decomposition steps can
be observed with mass loss of 3.7%; 70.1%; 13.6% and 6.8% for the
temperature ranges138–227; 227–379; 380–425 and 495–871 ◦C,
respectively. In this case as well a higher mass loss occurred in the
second decomposition step. Based on the mesoionic decomposition
temperatures, the following increasing order of thermal stability
can be proposed: MI-3 < MI-2 < MI-1.
3.3. Kinetic studies of non-isothermal decomposition
Kinetic studies of non-isothermal decomposition were realized
using the best experimental conditions according to the relation
between the thermogravimetric profile and the heating rate, using
decomposed fraction (˛) from 0.15 to 0.85. The kinetic parameters
how reaction order (n), apparent activation energies (Ea) and fre-
quency factor (A) for the stage considered more significant in each
Fig. 2. TG/DTG curve of the mesoionic MI-1 obtained using a heating rate of
Fig. 3. TG/DTG curve of the mesoionic MI-2 obtained using a heating rate of
10 ◦C min−1
.
10 ◦C min−1
.

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S.A. de Morais et al. / Journal of Alloys and Compounds 495 (2010) 598–602
Table 4
Kinetic parameters determined using non-isothermal thermogravimetric method of Coats-Redfern’s equation for ꢂ = 10 ◦C min−1
.
Compounds
MI-1
Stage
Parameters
Models
R1
D1
184.04
D2
1
Ea
A
r
s
Ea
87.54
203.65
6.82 × 105
1.20 × 1015
6.30 × 1016
0.9997
0.0098
97.79
0.9998
0.0191
103.59
0.9999
0.0153
200.68
MI-2
MI-3
2
2
A
r
s
2.10 × 106
0.9999
0.0036
5.46 × 106
0.9998
0.0089
1.85 × 1015
0.9999
0.0135
Ea
102.12
108.16
209.32
A
r
3.65 × 106
0.9999
9.76 × 106
0.9998
5.40 × 1015
0.9999
s
0.0051
0.0102
0.0151
ꢂ = heat rate (◦C/min); r = linear correlation coefficients; Ea = activation energy (kJ mol−1); A = frequency (s−1) and s = standard deviation.
Thermal decomposition of the mesoionic compounds, in nitrogen
atmosphere, occurs in various stages. According to the thermo-
gravimetric curves and kinetic studies, the order of thermal stability
was as follows: MI-3 < MI-2 < MI-1. The kinetic parameters obtained
by approximation and integral methods were satisfactory and pre-
sented good correlation. Generally, the activation energy values
obtained using the integral methods are lower than the values
obtained by the approximation methods. The kinetic models that
better described the thermal decomposition mechanisms, obtained
by the non-isothermal method were R1 (MI-1) and R2 (MI-2 and
MI-3) (based on geometric models).
Acknowledgments
The authors acknowledge UEPB, UFPB and UFCG for scholarship
and financial support of this work.
References
Fig. 4. TG/DTG curve of the mesoionic MI-3 obtained using a heating rate of
10 ◦C min−1
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The DSC curve of the mesoionic compounds MI-1 and MI-2
indicates that they suffer a fusion process followed by a decom-
position process. This mechanism was not observed in the case
of MI-3 mesoionic that presented only decomposition events.