Crystal structure of 3,4-dinitropyrazole in water

Article abstract of DOI:10.1080/15421406.2019.1681753

3,4-dinitropyrazole (DNP) was synthesized by N-nitration, thermal rearrangement and C-nitration with pyrazole as the raw material. A pure DNP single crystal was obtained by solvent evaporation using water as a solvent, and its structure was characterized by X-ray single crystal diffraction. The results showed that the single crystal structure of DNP contained water, and the molar ratio of DNP/water was 4:1. The DNP molecules existed stably due to the presence of intermolecular and intramolecular hydrogen bonding, as well as π-π stacking between DNP molecules. This study was valuable to the production and application of DNP.

Full text of DOI:10.1080/15421406.2019.1681753

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Crystal structure of 3,4-dinitropyrazole in water
Haoqi Guo, Siyuan Yu, Yongxiang Li, Jianlong Wang, Duanlin Cao & Zexin Qin
To cite this article: Haoqi Guo, Siyuan Yu, Yongxiang Li, Jianlong Wang, Duanlin Cao & Zexin
Qin (2019) Crystal structure of 3,4-dinitropyrazole in water, Molecular Crystals and Liquid Crystals,
690:1, 43-49, DOI: 10.1080/15421406.2019.1681753
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
019, VOL. 690, NO. 1, 43–49
2
https://doi.org/10.1080/15421406.2019.1681753
Crystal structure of 3,4-dinitropyrazole in water
a
a
a
a
a
Haoqi Guo , Siyuan Yu , Yongxiang Li , Jianlong Wang , Duanlin Cao , and
Zexin Qin
b
a
Department of Chemical Engineering and Technology, North University of China, Taiyuan, Shanxi,
b
China; Department of Humanities and Social Sciences, North University of China, Taiyuan,
Shanxi, China
ABSTRACT
KEYWORDS
DNP; melt-casting
explosives carrier; crystal
water; hydrogen bond
3
,4-dinitropyrazole (DNP) was synthesized by N-nitration, thermal
rearrangement and C-nitration with pyrazole as the raw material. A
pure DNP single crystal was obtained by solvent evaporation using
water as a solvent, and its structure was characterized by X-ray sin-
gle crystal diffraction. The results showed that the single crystal
structure of DNP contained water, and the molar ratio of DNP/water
was 4:1. The DNP molecules existed stably due to the presence of
intermolecular and intramolecular hydrogen bonding, as well as p-p
stacking between DNP molecules. This study was valuable to the
production and application of DNP.
1. Introduction
Improving energy and safety is an eternal theme for the development of explosive tech-
nology. The explosive should meet the requirements of high energy and insensitivity
when charging. At present, 90% of the melt-casting explosives use TNT as a liquid car-
rier. However, TNT has the disadvantages of oil impregnate, contraction and expansion
during melt-casting [1], and the pollutants will pose a serious threat to the environ-
ment. Therefore, searching a new generation of high-energy insensitive melting cast
explosive carrier with superior performance as well as environmentally friendly technol-
ogy to replace TNT has become a hot spot.
Nitrogen heterocyclic energetic compounds contain high electronegative nitrogen and
oxygen atoms, as well as, high nitrogen contents. The presence of p-electron system
makes nitrogen heterocyclic compounds have the advantages of insensitivity, high
enthalpy of formation, high density, good thermal stability and environmental friendli-
ness [2]. DNP is an outstanding representative of nitrogen heterocyclic energetic com-
pounds, which has a broad prospect in melt-casting explosives carrier [3]. David et al.
[4] reported a series of properties of DNP such as melting point and detonation speed,
and it was shown that the performance was superior than that of TNT (Table 1).
Refining the crude DNP with water to remove residual acid can reduce cost and
environmental pollution. However, the collected DNP solid particles will melt when
CONTACT Yongxiang Li
liyongxiang@nuc.edu.cn
School of Chemical Engineering and Technology, North
University of China, Taiyuan, Shanxi 030051, China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

4
4
H. GUO ET AL.
Table 1. The properties of TNT and DNP.
Melting
point
( C)
Detonation
speed
(m/s)
Explosive
pressure
(Gpa)
Explosive
heat
(kJ/kg)
Oxygen
balance
(%)
Energy
density
(kJ/cm )
Density
(g/cm )
3
ꢀ
3
Materials
TNT
DNP
1.65
1.87
80.9
85
6900
8108
21
29.4
3335
–
ꢁ74
6.997
8.211
ꢁ25.48
Figure 1. The synthetic route of DNP.
dried, which indicates that DNP contains a large amount of water. Therefore, it is
necessary to explore the crystal form of DNP, the hydrogen bond network of DNP mol-
ecules and water molecules, as well as the type of water in DNP (crystal water or
free water).
In this study, DNP was synthesized via N-nitration, thermal rearrangement and C-
nitration. DNP crystal was obtained via water evaporation, and its crystal structure was
analyzed by X-ray single crystal diffraction and further compared with those grown in
different solvents.
2. Experimental Section
2.1. Materials
Pyrazole was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd.; Acetic
acid and nitric acid (98%) were purchased from Tianjin Chemical Reagent Research
Institute; Sulfuric acid was purchased from Beijing Lianhua Yongxing Technology
Development Co., Ltd.; Acetic anhydride, sodium chloride and ether were purchased
from Sinopharm Chemical Reagent Co., Ltd.; Phenylacetonitrile and petroleum ether
were purchased from Tianjin Beichen Fangzheng Reagent Factory; Deionized water was
prepared from laboratory. All reagents used were of analytic grade.
2.2. Synthesis of DNP
The process for synthesizing 3,4-dinitropyrazole (DNP) can be divided into three steps
including N-nitration, thermal rearrangement and C-nitration. The synthetic route is
presented in Figure 1.
Pyrazole was selected as the starting material [5], to which, nitric acid (98%) and
acetic anhydride were added. The N-nitration reaction was carried out at room tem-
perature for 30 min. With N-nitropyrazole in hand, thermal rearrangement was pro-
ceeded at a reaction temperature of 438.15 K and a reaction time of 30 min using

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
45
mesitylene as a solvent to obtain 3-nitropyrazole, after which, added with nitric acid
and sulfuric acid. C-nitration reaction was carried out at 375.15 K for 30 min, affording
3
,4-dinitropyrazole. The transparent crystal of DNP was obtained by slow evaporation
from deionized water at room temperature.
2.3. Instrumentation
X-ray single crystal diffraction of DNP was recorded on a Bruker APEX-II CCD
Diffractometer. The crystal was kept at 220.0 K during data collection. Using Olex2 [6],
the structure was solved with the ShelXT [7] structure solution program using Intrinsic
Phasing and refined with the ShelXL [8] refinement package using Least Squares
minimization.
Melting point of DNP crystallized in different solvents including ether, benzene and
water was measured by BUCHI M-565 Melting Point Meter (Switzerland). The tempera-
ꢀ
ꢀ
ture was ranging from 50 to 100 C, with a heating rate of 2 C/min.
The impact sensitivity of DNP was determined by its explosion probability, using a
calibrated Type 12 Vertical Drop Hammer, with a drop weight of 2.5 kg and a drop
ꢀ
height of 80 cm. Experiment was carried out at an ambient temperature of 25 C and a
relative humidity of 35%. A total of 25 same samples was selected, and each sample was
35 mg. The explosion probability was calculated by Eq (1).
X
P ¼ 2 ꢂ 100%
(1)
5
Where P is the explosion probability, X is the number of explosion in 25
experiments.
3. Results and Discussion
3.1. Crystal Structure of DNP
The crystal and instrumental parameters used in the unit cell determination together
with the data collection and structure refinement parameters are shown in Table 2.
DNP crystal was a yellow, transparent and uniform bulk substance, and its molecular
structure is shown in Figure 2.
The structure of crystal was symmetrical, and the ratio of DNP/water was 4:1. The
oxygen atom of the water molecule located on a twofold axis, and the occupancy factor
of the water molecule was 0.5. It can be founded that the pyrazole parent ring was pla-
nar, and there were 5 atoms in the least-squares plane (C1, C2, C3, N3, N4; C4, C5, C6,
N8, N7). The dihedral angle between plane 1 (C1, C2, C3, N3, N4) and two nitro
groups were 15.402˚ (O2, N1, O1) and 32.217˚ (O3, N2, O4), respectively. The dihedral
angle between plane 2 (C4, C5, C6, N7, N8) and two nitro groups were 17.522˚ (O7,
N5, O8) and 25.288˚ (O5, N6, O6), respectively. In addition, the dihedral angle of the
plane in which two DNP molecules located was 17.378˚. Furthermore, water molecules
and DNP molecules were connected by hydrogen bonds, and the ratio was 1:4.
The network of hydrogen bonds and the specific information regarding the hydrogen
bond of DNP crystal are shown in Figure 3 and Table 3, respectively.

4
6
H. GUO ET AL.
Table 2. Crystal data and structure refinement parameters of DNP.
Item
Value
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
12 10 16 17
C H N O
650.36
220.0
monoclinic
P2/n
11.5494(18)
7.1894(11)
15.166(2)
90
99.843(3)
90
b/Å
c/Å
ꢀ
a/
ꢀ
b/
ꢀ
c/
3
Volume/Å
1240.7(3)
Z
q
2
3
calcg/cm
1.741
0.163
-
1
l/mm
F (000)
Crystal size/mm
Radiation
660.0
3
0.28 ꢂ 0.15 ꢂ 0.12
MoKa (k ¼ 0.71073)
ꢀ
2h range for data collection/
4.112 to 53
Index ranges
Reflections collected
ꢁ14 ꢄ h ꢄ 14 -8 ꢄ k ꢄ 8, ꢁ19 ꢄ l ꢄ 18
6832
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F
2507 [Rint¼ 0.0231, Rsigma¼ 0.0304]
2507/2/216
1.126
2
Final R indexes [I> ¼2r (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å
R
R
1
¼ 0.0389, wR
2
2
¼ 0.1157
¼ 0.1211
1
¼ 0.0451, wR
-3
0.34/-0.26
Figure 2. The molecular structure of DNP.
From Table 3, it can be concluded that there are intermolecular hydrogen bonds (N-
HꢃꢃꢃN and C-HꢃꢃꢃO) and intramolecular hydrogen bonds between DNP molecules, as
well as, intermolecular hydrogen bonds (O-HꢃꢃꢃN) between DNP molecules and water
molecules. Due to the action of hydrogen bond, the stability and security of DNP can
be improved.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
47
Figure 3. The network of hydrogen bonds.
Table 3. The length and angle of hydrogen bond.
D-HꢃꢃꢃA
d(D-H)/Å
d(HꢃꢃꢃA)/Å
d(DꢃꢃꢃA)/Å
d(DꢃꢃꢃA)/Å
N(3)-H(3)ꢃꢃꢃO(9)﹟1
N(8)-H(8)ꢃꢃꢃN(4)﹟2
O(9)-H(9)ꢃꢃꢃN(7)﹟3
C(3)-H(3A)ꢃꢃꢃO(2)﹟3
C(3)-H(3A)ꢃꢃꢃO(6)﹟3
C(6)-H(6)ꢃꢃꢃO(4)﹟5
0.886
0.887
0.860
0.940
0.940
0.940
1.923
2.082
2.055
2.505
2.565
2.565
2.787
2.964
2.913
3.353
3.293
3.326
174.32
172.51
175.97
150.08
134.55
155.99
#1: 1-x, 2-y, 1-z; #2: 1/2 þ x, 1-y, -1/2 þ z; #3: x, 1 þ y, z; #5: x, 1 þ y, z.
3.2. Comparison of Crystal Structure of DNP Crystallized in Different Solvents
As shown in Table 4, there is little difference in the crystal system, bond length and
bond angle of DNP grown in different solvents, on the contrary, the space group, dens-
ity and dihedral angle between nitro plane and pyrazole mother-ring plane of DNP vary
with the solvent. It can be founded that the space group of DNP crystals containing no
solvent molecules was P2 /c, and that of containing water and benzene were P2/n and
1
P2 /n respectively. The crystal density of the solvent-free DNP was greater than 1.8 g/
1
3
3
cm , and that of the solvent-containing DNP was less than 1.8 g/cm . The presence of
solvent molecules reduced the crystal density. In addition, for those DNP crystals con-
taining no solvent molecules, the dihedral angle between the plane of the nitro group
crystal and pyrazole ring was greater than 40˚, and those solvent-containing DNP crys-
tals showed an opposite result, which is due to the action of the hydrogen bond
between the solvent molecule and the DNP molecule.

4
8
H. GUO ET AL.
Table 4. Crystal structure of DNP crystallized in different solvents.
Solvent
Ether
Ether
Ether
Benzene
ꢃ0.5C
Water
ꢃH
Formula
C
3
H
2
N
4
O
4
C
3
H
2
N
4
O
4
C
3
H
2
N
4
O
4
2C
355.23
Monoclinic
P2 /n
0.40 ꢂ 0.30 ꢂ 0.15
0.28 ꢂ 0.15 ꢂ 0.12 Color/Habit
Colourless block Colourless block
3
H
2
N
4
O
4
6
H
6
4C
650.36
3
H
2
N
4
O
4
2
O
FW [g/mol]
Crystal system
Space Group
Size [mm]
158.09
Monoclinic
P2 /c
158
Monoclinic
P2 /c
158.09
Monoclinic
P2 /c
Monoclinic
P2/n
1
1
1
1
0.47 ꢂ 0.45 ꢂ 0.41
0
.10 ꢂ 0.22 ꢂ 0.28
0.30 ꢂ 0.20 ꢂ 0.20
Light yellow block
Colourless block Light yellow block
a [Å]
b [Å]
c [Å]
9.9801 (8)
11.9959 (9)
9.7192 (7)
90
94.232 (1)
90
1160.41 (15)
0.71
8
9.7013 (13)
12.0797 (10)
9.7587 (7)
90
93.962 (11)
90
1140.88 (19)
0.71073
8
9.8326 (4)
12.0559 (4)
9.7190 (4)
90
93.959 (4)
90
1149.35 (8)
0.71073
8
7.4579 (15)
9.787 (2)
19.534 (4)
90
94.87 (3)
90
1420.7 (5)
0.71073
4
11.5494 (18)
7.1894 (11)
15.166 (2)
90
99.843 (3)
90
1240.7 (3)
0.71073
2
ꢀ
a [ ]
ꢀ
b [ ]
ꢀ
c [ ]
3
V [Å ]
kMoKa [Å]
Z
Dataset h; k; l
ꢁ11–11;
–
ꢁ12–12;
ꢁ12–14;
ꢁ11–11
1.827
0.0324
0.0930
1.04
ꢁ9–9;
ꢁ12–1;
ꢁ25–25
1.661
0.060
0.105
ꢁ14–14;
1
4–10;
ꢁ8–8;
ꢁ10–11
ꢁ19–18
1.741
0.0389
0.1211
1.126
ꢁ3
q [g/cm
(obs)
(all data)
]
1.80969
0.1012
0.1377
–
1.84
0.0408
0.1001
–
R
1
wR
S
2
0.92
T [K]
F (000)
l [mm
Ref.
298
640
–
[9]
105.3
–
0.17
[10]
123
640
0.17
[11]
123
724
0.15
[12]
220
660
0.163
This study
ꢁ
1
]
Table 5. Melting point of DNP crystallized in different solvents.
Solvent
Water
87.3
Benzene
88.2
Ether
87.7
Melting Point
Table 6. The explosion probability of DNP crystallized in different solvents.
1
2
3
4
5
6
7
8
9
10
11
12
13
Water
Benzene
Ether
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
᭺
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
᭺
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
᭺
14
15
16
17
18
19
20
21
22
23
24
25
Water
Benzene
Ether
ꢂ
ꢂ
ꢂ
ꢂ
᭺
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
᭺
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
3.3. Comparison of Melting Point of DNP Crystallized in Different Solvents
Table 5 compares the melting point of DNP crystallized in water, benzene and ether.
The melting points of DNP crystal crystallized in different reagents were different,
which indicated that the properties of crystal were influenced by solvent.
3
.4. Comparison of Impact Sensitivity of DNP Crystallized in Different Solvents
From Table 6, the number of explosion for the sample crystallized in water, benzene
and ether is 0, 3 and 2, respectively. Therefore, the explosion probability of DNP

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
49
crystallized in three different solvents was different, namely, benzene (12%) > ether
8%) > water (0%). The dihedral angle between the nitro and pyrazole plane was differ-
(
ent, and the impact sensitivity of DNP was directly influenced by the dihedral angle.
Therefore, water can be determined as an excellent solvent for recrystallization of DNP.
4. Conclusions
In this study, DNP was synthesized by N-nitration, thermal rearrangement and
C-nitration using pyrazole as the raw material, and its single crystal was obtained by
solvent evaporation using water as solvent. X-ray single crystal diffraction confirmed
that the single crystal structure of DNP contained water, which can be removed by
melt-drying. Through the analysis of hydrogen bond of DNP, the stability and security
of DNP can be improved owing to the presence of intermolecular and intramolecular
hydrogen bonds. Furthermore, it was founded that the crystal system, bond length and
bond angle of DNP crystal grown in different solvents were same, and the space group,
dihedral angle, melting point and impact sensitivity were influenced by solvent. Water
is a promising solvent for recrystallization of DNP due to its environmental friendliness
and low-cost, and the properties of DNP recrystallized in water were all in line with the
standards. This study is of great significance to the production and application of DNP.
Disclosure statement
No potential conflict of interest was reported by the authors.
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