Study of picrate salts with amines

Article abstract of DOI:10.1016/j.molstruc.2012.12.020

The reaction of picric acid (2,4,6-trinitrophenol) with amines [urea, cyclohexane-1,2-diamine, 1H-1,2,4-triazole-3,5-diamine, 6-phenyl-1,3.5-triazine- 2,4-diamine] yielded the corresponding picrate salts 1-4. Theoretical studies reveal that the hydrogen-b

Full text of DOI:10.1016/j.molstruc.2012.12.020

Journal of Molecular Structure 1036 (2013) 427–438
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Study of picrate salts with amines
b
b
Nidhi Goel a, Udai P. Singh a, , Gurdip Singh , Pratibha Srivastava
⇑
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Department of Chemistry, DDU Gorakhpur University, Gorakhpur 273009, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The reaction of picric acid (2,4,6-trinitrophenol) with amines [urea, cyclohexane-1,2-diamine, 1H-1,2,4-
triazole-3,5-diamine, 6-phenyl-1,3.5-triazine-2,4-diamine] yielded the corresponding picrate salts 1–4.
Theoretical studies reveal that the hydrogen-bond interaction energy decreases on increasing the steric
hindrance in amines. The solid state structure of compounds 1–4 was measured by X-ray techniques and
compared to the gas phase optimized geometries (DFT/B3LYP). Thermal stability of these salts has been
studied.
"
Synthesis of four new salts of picric
acid with amines.
Comparison of solid state and
gaseous phase structure for picrate
salts.
The mechanism of thermal
decomposition was studied.
Study of the explosive behavior.
"
"
"
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of picric acid (2,4,6-trinitrophenol) with amines [urea, cyclohexane-1,2-diamine, 1H-1,2,4-
triazole-3,5-diamine, 6-phenyl-1,3.5-triazine-2,4-diamine] yielded the corresponding picrate salts 1–4.
Theoretical studies reveal that the hydrogen-bond interaction energy decreases on increasing the steric
hindrance in amines. The solid state structure of compounds 1–4 was measured by X-ray techniques and
compared to the gas phase optimized geometries (DFT/B3LYP). Thermal stability of these salts has been
studied by means of thermogravimetric–differential scanning calorimetry (TG–DSC) while kinetic param-
eters have been evaluated using models fitting and isoconversional methods. Thermolytic pathways have
also been suggested.
Received 29 September 2012
Received in revised form 8 December 2012
Accepted 11 December 2012
Available online 20 December 2012
Keywords:
Picric acid
Amines
Hydrogen bond interaction energy
Thermolysis
Ó 2013 Elsevier B.V. All rights reserved.
Explosion delay
1. Introduction
groups, which decompose and explode at elevated temperature
[1–5]. The crystal structures of picrate salts with amines have been
Picric acid (2,4,6-trinitrophenol) is used to produce very sensi-
tive compounds since the nitro groups are the powerful oxidizing
studied in the past [6–10] but the mechanism of thermal analysis,
explosion delay measurement are not yet reported in literature.
When the picric acid is combined with amines, the resultant salts
undergoes self propagative decomposition reaction due to the
presence of both oxidizing (picric acid) and reducing (amines)
⇑
Corresponding author. Tel.: +91 1332 285329 (O); fax: +91 1332 273560.
E-mail address: udaipfcy@iitr.ernet.in (U.P. Singh).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.12.020

428
N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
groups in the same molecule. A proton transfer mechanism has
been postulated to play an important role in the thermal decompo-
sition of almost all the ammonium salts as this was suggested as a
primary step in the thermal decomposition [11–16]. Due to these
attractive features, amines have been combined with picric acid
to see the effect on explosive properties in comparison with picric
acid. In the present article, the preparation, characterization and
thermal study of picrate salts with amines are described. Thermo-
analytical techniques provide the information about the overall
kinetics of thermally induced reaction, which reveals the decom-
position and mechanism of picrate salts via solid-state reactions.
2.2.4. Synthesis of [6-phenyl-2,4 diamino-1,3,5-triazinium picrate] (4)
Salt 4 was prepared by the same procedure as outlined above
for 2 with 6-phenyl-1,3,5-triazine-2,4-diamine (0.18 g, 1.0 mmol)
in 67.0% (0.27 g) yield. Anal. Calcd. (%) for C15H12N8O7 (416.33):
C, 43.28; H, 2.90; N, 26.91. Found: C, 43.19; H, 2.83; N, 26.49. IR
(KBr, cmꢀ1): 3378, 3105, 2874, 1856, 1673, 1535, 1429, 1336,
1154, 1082, 976, 855, 778, 704, 534. 1H NMR (DMSO-d6, ppm) d:
8.79 (s, 2H, picrate), 12.58 (s, br, 1H, NH), 7.49-8.31 (m, 5H, Ph),
6.81 (s, br, 4H, –NH2). 13C NMR (DMSO-d6, ppm) d: 170.62,
167.88, 159.60, 140.93, 137.52, 131.50, 128.60, 128.12, 126.57,
125.16.
2.3. Instrumentation
2. Experimental
Crystallized salts were carefully dried under vacuum for several
hours prior to elemental analysis on Elementar Vario EL III ana-
lyzer. IR spectra were recorded on a Thermo Nikolet Nexus FT-IR
spectrometer on KBr pellets. 1H and 13C NMR spectra were
recorded on Bruker-D-Avance 500 MHz spectrometer with Fourier
transform technique using tetramethylsilane as internal standard.
2.1. Materials
All manipulations were performed in air using commercial
grade solvents. Picric acid, cyclohexane-1,2-diamine, 1H-1,2,4-tria-
zole-3,5-diamine and 6-phenyl-1,3,5-triazine-2,4-diamine were
purchased from Aldrich Chemical Company, USA while urea was
commercially available from SD Fine-Chem Limited, Mumbai.
2.3.1. X-ray crystallography
The X-ray data collection were performed on a Bruker Kappa
Apex four circle-CCD diffractometer using graphite monochromat-
2.2. Syntheses of organic salts
ed Mo K
a radiation (k = 0.71070 Å) at 100 K. In the reduction of
2.2.1. Synthesis of [uronium picrate] (1)
data Lorentz and polarization corrections, empirical absorption
corrections were applied [17,18]. Crystal structures were solved
by direct methods. Structure solution, refinement and data output
were carried out with the SHELXTL program [18,19]. Non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were
placed in geometrically calculated positions by using a riding mod-
el. Images and hydrogen bonding interactions were created in the
crystal lattice with DIAMOND and MERCURY software [20,21].
Picric acid (0.22 g, 1.0 mmol) and urea (0.06 g, 1.0 mmol) were
mixed in a water-methanol mixture (v/v%, 1:4, 10 ml). The result-
ing solution was stirred for 6 h and filtered through celite. The fil-
trate was evaporated to dryness under vacuum and the yellow
solid obtained was redissolved in methanol. The yellow crystals
of salt 1 were obtained by slow evaporation (0.21 g, 71.5% yield).
Anal. Calcd. (%) for C7H7N5O8 (289.18): C, 29.07; H, 2.43; N,
24.22; Found: C, 28.93; H, 2.34; N, 24.17. IR (KBr, cmꢀ1): 3412,
3103, 2882, 1856, 1709, 1635, 1534, 1432, 1345, 1148, 1082,
926, 826, 774, 729, 530. 1H NMR (DMSO-d6, ppm) d: 8.75 (s, 2H,
picrate), 6.49 (s, br, 4H, –NH2). 13C NMR (DMSO-d6, ppm) d:
159.61, 140.95, 126.56, 125.17, 116.61.
2.3.2. Computational study
Geometry optimization of the different compounds in this work
was carried out using Density Functional Methods with
6-31G(d,p) basis set using Gaussian 03 suite of program [22,23].
The input for the simulation was the Z matrix generalized by
Gaussview that was also used for visualizing the molecules with
optimized geometries. Lastly, we performed a frequency calcula-
tion using the same method.
a
2.2.2. Synthesis of [1/2cyclohexane-1,2-diaminium picrate] (2)
Picric acid (0.22 g, 1.0 mmol) and cyclohexane 1,2-diamine
(0.11 g, 0.5 mmol) were mixed in a water-methanol mixture (v/
v%, 1:4, 10 ml). The resulting solution was refluxed for 2 h and fil-
tered through celite. The filtrate was evaporated to dryness under
vacuum and the yellow solid obtained was redissolved in metha-
nol. The yellow diamond shaped crystals of salt 2 were obtained
by slow evaporation (0.39 g, 69.7% yields). Anal. Calcd. (%) for
2.3.3. Thermal analysis
2.3.3.1. TG–DSC. Thermogravimetry and differential scanning calo-
rimetry (TG–DSC) was carried out at a rate of 10 °C/min (sample
mass = 5.5 mg) under a nitrogen atmosphere at a flow rate of
200 ml/min on a Perkin–Elmer’s (Pyris Diamond) thermogravime-
try analyzer.
C18H20N8O14 (572.42): C, 37.77; H, 3.52; N, 19.57; Found: C,
37.53; H, 3.41; N, 19.47. IR (KBr, cmꢀ1): 3429, 2926, 1633, 1547,
1489, 1428, 1342, 1265, 1084, 910, 780, 702, 628. 1H NMR
(DMSO-d6, ppm) d: 8.76 (s, 2H, picrate), 2.89–3.23 (m, 2H, CH),
1.65–2.21 (m, 4H, CH2), 1.31–1.81 (m, 4H, CH2), 7.9 (s, br, 6H,
NH3). 13C NMR (DMSO-d6, ppm) d: 159.59, 140.94, 126.53,
125.15, 57.15, 51.22, 34.99, 30.38, 24.98, 21.33.
2.3.3.2. Isothermal TG. The isothermal TG studies (wt. 0.03 g, 100–
200 mesh) of salts 1–4 were performed at appropriate tempera-
tures (230–270 °C) in static air using indigenously fabricated TG
apparatus [24] fitted with a temperature cum controller.
2.3.3.3. Explosion delay measurements. The explosion delay (DE)
data were recorded using the tube furnace technique [25] (mass
0.02 g, 100–200 mesh) in the temperature range 380–420 °C
( 1 °C). Each run was repeated five times and the mean DE values
were calculated. The DE data were found to fit in the following
equation [26–28].
2.2.3. Synthesis of [3,5-diamino-1,2,4-triazolium picrate] (3)
Salt 3 was prepared by the same procedure as outlined above
for 2 with 1H-1,2,4-triazole-3,5-diamine (0.09 g, 1.0 mmol) in
77.8% (0.25 g) yield. Anal. Calcd. (%) for C8H8N8O7 (328.22): C,
29.37; H, 2.15; N, 34.25. Found: C, 29.23; H, 2.05; N, 34.11. IR
(KBr, cmꢀ1): 3460, 3418, 3354, 3168, 1839, 1689, 1627, 1548,
1430, 1329, 1272, 1163, 1076, 999, 912, 788, 711, 656. 1H NMR
(DMSO-d6, ppm) d: 8.77 (s, 2H, picrate), 12.55 (s, br, 2H, NH),
6.43 (s, br, 4H, –NH2). 13C NMR (DMSO-d6, ppm) d: 159.58,
156.02, 152.15, 140.92, 126.54, 125.21.
ꢁa=RT
DE ¼ AeE
where Eꢁa is the activation energy for thermal explosion, A is the
pre-exponential factor and T is the absolute temperature. Eꢁa was

N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
429
determined from the slope of a plot of ln (DE) vs. 1/T (Fig. 13). The
percent oxygen balance (OB) was calculated by a general equation
suggested by Martin and Yallop [29].
on the extent of reactions. The value of
a
is experimentally derived
from the global mass loss in TG experiments. The reaction model
may take various forms; the temperature dependence of k(T) can
be satisfactorily described by the Arrhenius equation, whose substi-
tution into Eq. (1) yields
2.3.4. Kinetics analysis of isothermal TG data
The kinetic analysis of a solid state decomposition is usually
based on the following single step kinetic equation [30].
da
=dt ¼ A expðꢀE=RTÞ ꢂ fð
a
Þ
ð2Þ
where A is pre exponential factor, E is activation energy and R is the
gas constant.
da=dt ¼ kðTÞfð
aÞ
ð1Þ
where t is the time, T is the temperature,
a
is the extent of conver-
sion (0 <
a
< 1), k(T) is the rate constant and f(
a
) is the reaction
2.3.4.1. Model fitting method. Rearrangement and integration of Eq.
model [31], which describes the dependence of the reaction rate
(1) for isothermal conditions gives
Scheme 1. General method for preparation of salts 1–4.
Table 1
Crystal data and structure refinement parameters of salts 1–4.
Salts
(1)
(2)
(3)
(4)
Formula
C7H7N5O8
289.18
Monoclinic
P21/c
3.886(11)
25.824(8)
11.167(3)
90
93.71(11)
90
1118.4(6)
4
C18H20N8O14
572.42
Monoclinic
C2/c
22.105(8)
8.165(3)
14.524(5)
90
117.74(16)
90
C8H8N8O7
328.22
Monoclinic
C2/c
22.834(14)
4.808(3)
22.557(14)
90
94.17(3)
90
2470(3)
8
C15H12N8O7
416.33
Triclinic
P ꢀ 1
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
7.604(16)
7.940(19)
14.596(4)
88.56(9)
81.72(9)
77.43(9)
851.3(3)
2
a
(°)
b (°)
c
(°)
V (Å3)
2319.8(14)
4
Z
Temperature
296(2)
1.717
0.158
296(2)
1.639
0.143
296(2)
1.765
0.156
296(2)
1.624
0.133
DCalc (g/cm3)
l
(Mo K ) (cmꢀ1
)
a
F(000)
592
1184
1344
428
Crystal size
0.32 + 0.26 + 0.21
1.58-28.29
2754
1934
2754/0/182
1.222
0.31 + 0.27 + 0.21
2.88-28.43
2699
1650
2699/0/182
1.563
0.24 + 0.21 + 0.17
1.79-29.56
3353
2137
3353/0/224
0.917
0.29 + 0.19 + 0.13
1.41-27.21
3678
1609
3678/0/275
1.084
Theta for data collection (°)
No. of measured reflections
No. of observed reflections
Data/restraints/parameters
Goodness-of-fit
a
Final R indices[I > 2(I) R1
0.046
0.146
0.078
0.223
0.044
0.125
0.090
0.168
b
wR2
a
R1 =
wR2 = {
R
||Fo| ꢀ |Fc||/
R|Fo|.
R
[wðF2o ꢀ F2c Þ ]/
R
wðF2o Þ }1/2
.
b
2
2

430
N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
gj ð
a
Þ ¼ kjðTÞt
ð3Þ
ꢀ ln ta ¼ ln½Aa=gð
a
Þꢃ ꢀ Ea=RTi
ð5Þ
;i
R
a
where g(
a) = 0 [f(
a
)]ꢀ1
d
a
is the integrated form of the reaction
where E is evaluated from the slope of the plot of ꢀln t against
a
a,i
Tꢀ1. Thus, E at various ai for salts 1–4 have been evaluated.
model. The subscript j has been introduced to emphasize that
substituting a particular reaction model in Eq. (3) results in evalu-
ating the corresponding rate constant, which is determined from
a
i
3. Results and discussion
the slope of a plot of gj (a) vs. t. For each reaction model selected,
the rate constants are evaluated at several temperatures Ti and
Arrhenius parameters are determined using the Arrhenius Eq. (4)
in its logarithmic form
The reaction of picric acid with the following amines [urea,
cyclohexane-1,2-diamine, 1H-1,2,4-triazole-3,5-diamine, 6-phe-
nyl-1,3,5-triazine-2,4-diamine] in methanol–water mixtures
resulted in the formation of ionic salts (1–4) according to Scheme 1.
The formation of salts 1–4 was confirmed by elemental analysis, IR,
NMR and crystallographic studies. The asymmetric and the
symmetric stretching vibrations of –NO2 group show bands at
1536 cmꢀ1 and 1334 cmꢀ1, respectively [32,33]. The shift of the
ln kjðTiÞ ¼ ln Aj ꢀ Ej=RTi
ð4Þ
Arrhenius parameters were evaluated for isothermal experi-
mental data by the model fitting method.
m
as(NO2) vibration to lower frequency (1581–1530 cmꢀ1) in the
2.3.4.2. Isoconversional method. This method allows the activation
energy to be calculated without making any assumptions about
the reaction model. Additionally, the method evaluates the effec-
tive activation energy as a function of the extent of conversion
which allows one to explore multistep kinetics. The basic assump-
tion of the isoconversional method [31] is that the reaction model
as defined in Eq. (1) is not dependent on temperature or heating
rate. Under isothermal conditions, on combining Eqs. (3) and (4)
we obtain:
spectrum of the salts (1–4) compared with the free picric acid
(1607 cmꢀ1) suggested the large electron density on the picric acid.
The NH stretching vibration is normally observed at 3500–
3400 cmꢀ1 which is shifted to lower wave number due to the
attraction of the –NH protons by the picrate anion leading to an in-
crease in –NH bond length in salts (1–4) [34]. The shifting towards
lower frequency in both cases, is due to the hydrogen bonded non-
covalent interactions between donor (–NH protons) and acceptor
(picrate anion) [35]. The hydrogen bonding characteristic of salts
Table 2
Non-covalent interactions for 1–4 [distances (Å) angles (°)].
S.n
1
D–Hꢂ ꢂ ꢂA
(1)
d(D–H)
d(H–A)
d(D–A)
h(DHA)i
O11–H11ꢂ ꢂ ꢂO1
O11–H11ꢂ ꢂ ꢂO7
N6–H6Aꢂ ꢂ ꢂO1
N6–H6Aꢂ ꢂ ꢂO2
N6–H6Bꢂ ꢂ ꢂO6
N6–H6Bꢂ ꢂ ꢂO7
N7–H7Aꢂ ꢂ ꢂO2
N7–H7Aꢂ ꢂ ꢂO3
N7–H7Bꢂ ꢂ ꢂO4
N7–H7Bꢂ ꢂ ꢂO6
C3–H3ꢂ ꢂ ꢂO3
C5–H5ꢂ ꢂ ꢂO5
0.821
0.821
0.861
0.861
0.860
0.860
0.860
0.860
0.860
0.860
0.930
0.930
1.740(2)
2.340(9)
2.135(6)
2.651(4)
2.701(2)
2.373(10)
2.246(8)
2.968(2)
2.625(5)
2.224(3)
2.587(7)
2.999(15)
2.547
2.762
2.817
3.453
3.384
3.218
3.076
3.753
3.106
3.024
3.478
3.884
167.0
112.6
135.7
155.4
137.2
167.5
161.9
152.7
116.5
154.5
160.6
159.4
2
3
(2)
N4–H4Aꢂ ꢂ ꢂO1
N4–H4Aꢂ ꢂ ꢂO7
N4–H4Bꢂ ꢂ ꢂO7
N4–H4Cꢂ ꢂ ꢂO1
N4–H4Cꢂ ꢂ ꢂO2
C2–H2Bꢂ ꢂ ꢂO6
0.889
0.889
0.890
0.890
0.890
0.969
2.092(18)
2.385(6)
2.293(16)
1.928(8)
2.428(10)
2.431(22)
2.755
2.928
3.126
2.755
2.940
3.347
141.6
119.5
155.9
153.9
116.9
157.3
(3)
N1–H1ꢂ ꢂ ꢂO1
N2–H2ꢂ ꢂ ꢂO4
N4–H4Aꢂ ꢂ ꢂO5
N4–H4Bꢂ ꢂ ꢂO5
N5–H5Aꢂ ꢂ ꢂO3
N5–H5Bꢂ ꢂ ꢂO1
N4–H4Bꢂ ꢂ ꢂN3
C7–H7ꢂ ꢂ ꢂO6
0.877
0.894
0.867
0.867
0.861
0.860
0.867
0.929
1.932(27)
2.357(29)
2.689(35)
2.688(49)
2.203(26)
2.234(28)
2.115(31)
2.699(6)
2.674
3.229
3.037
3.034
3.034
2.914
2.967
3.042
141.4
165.0
105.4
105.5
161.0
136.0
167.5
102.7
4
(4)
N6–H6ꢂ ꢂ ꢂO1
N6–H6ꢂ ꢂ ꢂO7
N7–H7Aꢂ ꢂ ꢂO2
N7–H7Aꢂ ꢂ ꢂO3
N7–H7Bꢂ ꢂ ꢂO1
N7–H7Bꢂ ꢂ ꢂO2
N8–H8Bꢂ ꢂ ꢂO7
N8–H8Aꢂ ꢂ ꢂN5
C12–H12ꢂ ꢂ ꢂO4
C13–H13ꢂ ꢂ ꢂO4
C14–H14ꢂ ꢂ ꢂO5
C15–H15ꢂ ꢂ ꢂO3
0.810
0.810
0.859
0.850
0.860
0.860
0.860
0.861
0.930
0.930
0.930
0.930
1.920(37)
2.567(63)
2.389(15)
2.377(9)
2.685
3.159
3.144
3.172
2.710
2.902
2.925
3.062
3.223
3.297
3.376
3.906
157.3
131.0
146.9
154.0
144.4
135.2
157.0
157.8
125.5
120.3
150.2
161.8
1.964(8)
2.227(12)
2.113(11)
2.248(13)
2.593(8)
2.728(11)
2.537(11)
3.012(16)

N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
431
1–4 in the solution was studied by 1H NMR spectroscopy. The
prominent downfield chemical shift in the NMR spectra observed
in each case with respect to the free ligand [(DMSO-d6, 500 MHz)
Picric acid, 8.72 (s, 2H); urea, 5.61 (s, br, 4 H, –NH2); cyclohex-
ane-1,2-diamine, 7.2 (s, br, 6H, –NH3), 2.80–3.10 (m, 2H, CH),
1.55–2.10 (m, 4H, CH2), 1.01–1.55 (m, 4H, CH2); 1H-1,2,4-tria-
zole-3,5-diamine, 5.2 (s, br, 4H, –NH2), 12.52 (s, br, 1H, NH); 6-phe-
nyl-1,3,5-triazine-2,4-diamine, 6.76 (s, br, 4H, –NH2), 7.44–8.26
(m, 5H, Ph] suggested the existence of hydrogen bonding in the
solution. The crystallographic and the selected hydrogen bonding
data are given in Tables 1 and 2, respectively.
(donor) of picrates are self dimerized with R22(10) motif involving
0
C–Hꢂ ꢂ ꢂO [C3–H3ꢂ ꢂ ꢂO3, 2.587(7) Å; C5–H5ꢂ ꢂ ꢂO5, 2.999(15) ÅA] inter-
actions and weak Oꢂ ꢂ ꢂO [O5ꢂ ꢂ ꢂO6, 2.889(8) Å; O3ꢂ ꢂ ꢂO4,
3.198(16) Å] intermolecular interactions, which are responsible
for one dimensional spiral molecular arrangement as shown in
Fig. S1a. The protonated oxygen (O11) of urea shows the strong
hydrogen bonding with the phenolic oxygen atom (O1) of picrate
via O–Hꢂ ꢂ ꢂO [O11–H11ꢂ ꢂ ꢂO1, 1.740(2) Å] interaction. Both organic
moieties make three dimensional zig-zag arrangement along the
‘c’ axis through N–Hꢂ ꢂ ꢂO and O–Hꢂ ꢂ ꢂO [O11–H11ꢂ ꢂ ꢂO7,
2.340(9) Å] non-covalent interactions (Fig. 2, S1b, Table 2).
3.1. Structure description
3.1.2. Crystal structure of [1/2cyclohexane-1,2-diaminium picrate] (2)
According to Fig. 3, the asymmetric unit of salt 2 contains one
molecule of picrate and half molecule of ½ cyclohexane-1,2-dia-
minium which crystallizes in monoclinic system with C2/c space
group. In 1,2-diaminocyclohexane, both amino nitrogens (N4) are
protonated by the hydroxyl group of both picric acid molecules.
Picrate anions are self assembled to form a hydrogen bonded
homomeric synthons through Oꢂ ꢂ ꢂO [O2ꢂ ꢂ ꢂO6, 3.109(10) Å;
O2ꢂ ꢂ ꢂO7, 3.109(10) Å; O5ꢂ ꢂ ꢂO7, 3.160(12) Å; O4ꢂ ꢂ ꢂO6, 3.139(5) Å]
3.1.1. Crystal structure of [uronium picrate] (1)
Salt 1 crystallizes in monoclinic with the space group P21/c
(Z = 4). The unit cell contains one uronium cation and one picrate
anion. The molecular structure is depicted in Fig. 1, where it shows
that the acidic hydrogen of the hydroxyl group on picric acid prot-
onates the oxygen atom (O11) of urea and making the cation. The
molecular packing analysis shows that the N–O (acceptor) and C–H
Fig. 1. Molecular structure of salt 1.
Fig. 2. Three dimensional zig-zag molecular arrangements along the ‘c’ axis in 1.

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N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
Fig. 3. Molecular structure of salt 2.
Fig. 4. Perspective view of the supramolecular motif in 2.
and C8–H8ꢂ ꢂ ꢂ
p
[3.250(3) Å] intermolecular interactions. The four
O7, 2.293(16) Å, N4–H4Cꢂ ꢂ ꢂO1, 1.928(8) Å; N4–H4Cꢂ ꢂ ꢂO2,
2.428(30) Å]. The extended hydrogen bonding network is also
formed which involves R22(8) graph-set association through C–
Hꢂ ꢂ ꢂO [C2–H2Bꢂ ꢂ ꢂO6, 2.431(22) Å] intermolecular interactions
(Fig. S3). The various non-covalent interactions present in the
salt, help in the formation of pseudocavity through the self
assembled interactions of acid and base where ½ cyclohexane-
1,2-diaminium act as a pseudoguest (Fig. 4).
picrates non-covalently interact via Oꢂ ꢂ ꢂO, generate a cavity of
size 1.54 Å (Fig. S2a), resulting in the three dimensional spiral
packing as shown in Fig. S2b. On the other side, ½ cyclohex-
ane-1,2-diaminium acts as a base and does not assemble with
the same molecule but shows non-covalent interactions with
four molecules of picrate anions through N–Hꢂ ꢂ ꢂO [N4–
H4Aꢂ ꢂ ꢂO1, 2.092(18) Å; N4–H4Aꢂ ꢂ ꢂO7, 2.385(6) Å; N4–H4Bꢂ ꢂ ꢂ

N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
433
3.1.3. Crystal structure of [3,5-diamino-1,2,4-triazolium picrate] (3)
zole molecules are stacked one above other via N–Hꢂ ꢂ ꢂ
H4Aꢂ ꢂ ꢂ 3.347(47) Å; N4–H4Bꢂ ꢂ ꢂ
p
[N4–
Salt 3 crystallizes in a monoclinic space group C2/c with one
molecule of anionic deprotonated picric acid and one molecule of
cationic protonated 1H-1,2,4-triazole-3,5-diamine in an asymmet-
ric unit (Fig. 5). In this structure, two molecules of picrate anion are
p
,
p
,
3.302(34) Å; N5–H5Bꢂ ꢂ ꢂ
p,
3.304(6) Å] interactions, forming two dimensional layer along ‘c’
axis (Fig. S4a and b). One molecule of cationic 1H-1,2,4-triazole-
3,5-diamine attracts three molecules of picrate anions and one
N–H group of it shows the strong hydrogen bonding with the phe-
nolic oxygen (O1) of picrate anion [N1–H1ꢂ ꢂ ꢂO1, 1.932(27) Å].
Other non-covalent interactions N–Hꢂ ꢂ ꢂO, are in the range of
2.202(26) ꢀ 2.689(39) Å (Fig. S4c). The resulting heteromeric syn-
thon due to these hydrogen bonds connects the acid and base into
an infinite three dimensional packing as alternate channels of pic-
rate anions and 1H-1,2,4-triazole-3,5-diamine cations along the ‘c’
axis (Fig. 6).
self dimerized with R22(10) motif involving C–Hꢂ ꢂ ꢂO [C7–H7ꢂ ꢂ ꢂO6,
0
2.699(6) ÅA] interaction and also a picrate anion is interconnected
to the adjacent picrate via weak non-covalent interactions Oꢂ ꢂ ꢂO
[O2ꢂ ꢂ ꢂO4, 3.117(43) Å; O3ꢂ ꢂ ꢂO4, 2.948(17) Å], create a cavity of size
1.3 Å, resulting the ribbon shaped one dimensional packing along
‘ac’ plane. Meanwhile, two molecules of protonated 1H-1,2,4-tria-
zole-3,5-diamine are self dimerized with R22(8) motif involving
0
N–Hꢂ ꢂ ꢂN [N4–H4ꢂ ꢂ ꢂN3, 2.115(31) ÅA] interaction and also two tria-
Fig. 5. Molecular structure of salt 3.
N
HN
NH2
N
H
H2N
Fig. 6. Three dimensional packing as alternate channels of picrates and 3,5-diamino-1,2,4-triazolium along the ‘c’ axis in 3.

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N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
3.1.4. Crystal structure of [6-phenyl-2,4-diamino-1,3,5-triazinium
picrate] (4)
nitrogen atom (N6) of 6-phenyl-2,4-diamino-1,3,5 triazine
(Fig. 7). Crystal packing reveals that the four molecules of picrate
anions are intermolecularly hydrogen bonded and make cavity of
The crystal structure of salt 4 showed that the asymmetric unit
contains one molecule of picrate anion and one molecule of 6-phe-
nyl-2,4-diamino-1,3,5 triazine cation, crystallizes in triclinic with
space group P ꢀ 1. The ionic species is formed by the transfer of
the proton from the hydroxyl group of picric acid to the one
size 1.9 Å. Also two molecules of picrates are self dimerized with
0
R22(10) motif through C–Hꢂ ꢂ ꢂO [C5–H5Aꢂ ꢂ ꢂO5, 2.722(15) ÅA] interac-
tions, forms other cavity of size 1.1 Å (Fig. S5a). The non-covalent
interactions among different picrate anions cause the formation
Fig. 7. Molecular structure of salt 4.
N
N
H2N
N
H
NH2
Fig. 8. Three dimensional packing as alternate channels of picrates and 6-phenyl-2,4-diamino-1,3,5 triazinium in 4.

N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
435
Table 4
TG–DSC phenomenological data of salts 1–4 in an N2 atmosphere.
S.n
Salts
TG
DSC
Ti (°C)
Tf (°C)
a
Peak temp. (°C)
Nature
1
2
3
4
(1)
(2)
(3)
(4)
244
190
247
268
381
307
349
377
96.6
95.6
98.4
95.1
341 °C
292 °C
331 °C
319 °C
Exo
Exo
Exo
Exo
salt 1
salt 2
7
salt 3
Fig. 9. Optimized geometry of salts: (a) 1, (b) 2, (c) 3, and (d) 4.
salt 4
Table 3
Hydrogen bond interaction energy (HBE) in Kcal/mol.
S.n
Salts
HBE (Kcal/mol)
1
2
3
4
(1)
(2)
(3)
(4)
19.83
17.32
15.33
10.26
Fig. 10. Simultaneous TG–DSC of salts 1–4 in nitrogen atmosphere.

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N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
Fig. 11. Isothermal TG in static air atmosphere for salts 1–4: (a) 1, (b) 2, (c) 3, and (d) 4.
of three dimensional zig-zag molecular arrangement along ‘bc’
plane as shown in Fig. S5b. The base 6-phenyl-2,4-diamino-1,3,5-
triazinium also interconnected with other same molecule through
C–Hꢂ ꢂ ꢂN [C11–H11ꢂ ꢂ ꢂN8, 2.572(15) Å] and N–Hꢂ ꢂ ꢂN [N8–H8ꢂ ꢂ ꢂN5,
2.248(13) Å] intermolecular interactions, resulted in the formation
of spiral molecular structure along the ‘a’ axis (Fig. S5c). These mol-
ecules are partially stacked one above the other via weak
pꢂ ꢂ ꢂp
[3.504(15) Å] interactions as shown in Fig. S5d. The protonated
nitrogen (N6) of 6-phenyl-2,4-diamino-1,3,5-triazinium shows
the strong hydrogen bonding with the phenolic oxygen (O1) of pic-
rate, i.e., N6–H6ꢂ ꢂ ꢂO1 [1.920(57) Å]. Both organic moieties are held
together by various non-covalent interactions viz., C–Hꢂ ꢂ ꢂO and N–
Hꢂ ꢂ ꢂO (Fig. S6, Table 2). These non-covalent interactions present
between cation and anion resulted in the formation of entirely dif-
ferent three dimensional packing as alternate channels of picric
acid and 6-phenyl-2,4-diamino-1,3,5 triazine (Fig. 8).
Fig. 12. Dependence of activation energy (Ea) on the extent of conversion (
salts 1–4.
a) for
3.2. DFT energy calculation
The optimized structural parameters of all individual amine,
acids and their salts were calculated at B3LYP/6-31G(d,p) basis
sets and are summarized in Table S1. Each optimized geometries
showed positive vibrational frequencies suggesting that optimized
structure is the global minimum on the potential energy surface.
Single point energy calculations were performed and zero point
corrected total energies for various species were recorded. In addi-
tion to the characterization of these salts, the gas phase geome-
tries, harmonic vibrational frequencies and binding energies of a
series of amine were computed. The B3LYP predicted structure of
salts is shown in Fig. 9 and geometrical parameters are given in
Table S1.
The hydrogen bond interaction energies were determined
according to the following equation.
D
E ¼ ESalt ꢀ ðEamine þ EacidÞ
Fig. 13. Graph of ln DE vs. 1/T.

N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
437
Scheme 2. Proton transfer mechanism leading to explosion in salts 1–4 PT = Proton Transfer, ORR = Oxidation-Reduction Reaction.
where ESalt, Eamine and Eacid are the zero point corrected total ener-
gies of salt, amine and acid calculated at DFT(B3LYP)/6-31G(d,p) le-
vel of theory.
to oxidation-reduction reactions and can be associated with explo-
sive decomposition processes.
The kinetics of thermal decomposition of salts 1–4 were evalu-
ated using fourteen mechanisms based kinetic models as given in
Table S3. The set of reaction models [31] were used to analyze
the isothermal TG data in the range of 230-270 °C for 1–4
(Fig. 11), to calculate the Ea values for thermal decomposition.
The activation energy values are reported in Table S4. In the model
fitting method, the kinetics is analyzed by choosing a ‘‘best fit’’
model based on the value of the correlation coefficient r close to
1. Among various values of r calculated for different models, the
highest values of r for salt 1, 4 is model 11 (three dimensional dif-
fusion), for salt 2, model 7 (Avrami–Erofeev) and for salt 3, model
14 (Ginstling–Brounshtein). The corresponding value of Ea are re-
ported in Table S4 for salt 1–4 are 199.4(1), 152.1(2), 109.8(3)
and 108.2(4) kJ molꢀ1 respectively. The isoconversional method is
known to permit estimation of activation energy independent of
the model used. This method is being used to establish a relation
The trend observed for the hydrogen bond interaction energy is
given in Table 3. In case of salts 1–4, the steric hindrance plays an
important role to describe the order of hydrogen bond interaction
energy. It is important to point out that the salt 4 shows the lowest
hydrogen bond interaction energy of all compounds as the substi-
tuted phenyl group on the triazine ring produce the greater steric
hindrance. Along with this, the negligible difference in the energy
of the optimized structure and the crystal structure of salts suggest
that the orientation and interaction remain almost the same for the
solid state as for the gas phase structure (Table S2). The optimized
bond lengths and angles for the salts 1–4 are given in Table S1, and
further supports that the molecules are arranged in same fashion
in both solid and gaseous phases.
3.3. Thermal analysis
between activation energy and extent of conversion (
ple. According to Fig. 12, for a particular salt each activation energy
has a separate value at different . Although these salts are stable
a) of the sam-
The thermal stability of salts 1–4 is demonstrated by a thermo-
gravimetry (TG) and differential scanning calorimetry (DSC). The
thermoanalytical data of these salts are listed in Table 4 and the
TGA–DSC curves are shown in Fig. 10. All salts are stable up to
200 °C and show single step decompositions. The salt 1 shows a
weight loss of 96.6% between 244 and 381 °C, corresponding to
the release of both picrate anion and uronium cation. This stage
corresponds to single exotherm in the DSC curve at 341 °C. In salt
2 the decomposition starts at 190 °C and continue up to 307 °C
with 95.6% mass loss due to the release of picrate anion and cyclo-
hexane-1,2-diaminium. The exothermic peak is observed at 292 °C
in the DSC curve. Salts 3 and 4 also follow a similar pathway for
their decomposition (Table 4). Beyond 405 °C, explosion occurs
with the formation of gaseous products. The exotherms are due
a
at room temperature but experience has shown that when sub-
jected sudden to high temperature, they explode. Freeman and
Gordon [28] have suggested the heat balance equation in order
to evaluate the pre-explosion reactions as given in supporting
Information.
Further to evaluate the sensitivity of these salts, their explosion
delay measurement was carried out in temperature ranges 380–
420 °C for salts 1–4 (Table S5). For salts 1–4, Eꢁa was determined
from the slope of a plot of ln (DE) vs. 1/T as shown in Fig. 13. The
energy of activation for explosion is 92.8, 69.4, 47.3 and
43.4 kJ molꢀ1 for salts 1–4, respectively. It has also been observed
experimentally that salt 4 has the least activation energy which

438
N. Goel et al. / Journal of Molecular Structure 1036 (2013) 427–438
could be due to high aromaticity of the amine, i.e., it is less stable.
The activation energy (calculated by isothermal TG) and explosion
delay measurement has different values which may be due to the
different temperature ranges. The oxygen balance values reported
in Table S5, suggest that these salts belong to the low explosives
class. The thermal decomposition process of energetic materials of-
ten involves a concert of bond breaking and bond forming steps
under condensed or gas phase reaction. The weakening of a partic-
ular bond seems to enhance the tendency to have a predominant,
identified path to decomposition. The overall process of decompo-
sition takes place at higher temperature by transfer of proton (N–H
bond cleavage) from amines to picrates and form the correspond-
ing Oꢂ ꢂ ꢂH bond in the condensed phase prior to explosion to form
finally gaseous products leaving a black carbon residues [36]
(Scheme 2).
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In summary, we synthesized four new salts of picric acid with
amines. The X-ray studies shows that the hydrogen bonding gives
entirely different three dimensional packing views, i.e., salt 1 give
zig-zag arrangement, salt 2 pseudocavity, while salts 3 and 4 in-
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gaseous phase, structure is same and hydrogen bond interaction
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picrate anion. At higher temperature, interaction between cation
and anion leads to an explosion. From the explosion delay mea-
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The authors gratefully acknowledge the CSIR (India) for a Senior
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
12.020.