Kinetics of Thermal Decomposition of 3,7-Dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane

Article abstract of DOI:10.1134/S107036321902004X

The kinetics of thermal decomposition of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane has been studied in solid phase and in solution. The mechanism, kinetic parameters of decomposition, and activation parameters of the rate-limiting step have been determined.

Full text of DOI:10.1134/S107036321902004X

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 2, pp. 194–198. © Pleiades Publishing, Ltd., 2019.
Russian Text © L.A. Kruglyakova, R.S. Stepanov, Yu.V. Kekin, K.V. Pekhotin, 2019, published in Zhurnal Obshchei Khimii, 2019, Vol. 89, No. 2,
pp. 191–196.
Kinetics of Thermal Decomposition
of 3,7-Dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane
L. A. Kruglyakovaa*, R. S. Stepanova, Yu. V. Kekinb, and K. V. Pekhotina
a Reshetnev Siberian State University of Science and Technology,
pr. Gazety Krasnoyarskii Rabochii 31, Krasnoyarsk, 660037 Russia
*e-mail: lakruglyakova@sibsau.ru
b Siberian Law Institute of the Ministry of Internal Affairs of Russia, Krasnoyarsk, Russia
Received July 19, 2018; revised July 19, 2018; accepted July 26, 2018
Abstract—The kinetics of thermal decomposition of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane has been
studied in solid phase and in solution. The mechanism, kinetic parameters of decomposition, and activation
parameters of the rate-limiting step have been determined.
Keywords: dinitrotetraazabicyclo[3.3.1]nonane, cyclic nitramines, octogen, thermal decomposition, activation
parameters
DOI: 10.1134/S107036321902004X
3,7-Dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane is
an intermediate product of nitrolysis of urotropine into
1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (octogen),
and the yield of octogen is mainly determined by the
yield of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]-
nonane [1]. It should be noted that while the kinetics
and mechanism of thermal decomposition of secondary
nitramines have been investigated for more than
70 years [2], thermal decomposition of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane has been scarcely
studied [3], and the available data are not sufficient for
reliable conclusions on the structural and kinetic
features or the decomposition mechanism as well as
safety issues. In view of the above, we studied the
kinetics of thermal decomposition of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane in different states:
in solid phase and in solutions in dibutyl phthalate and
dinitrobenzene.
to the vessel volume (m/V), although shortening the
induction period, had practically no effect on the rate
of decomposition during the induction period.
Therefore, the decomposition of 3,7-dinitro-1,3,5,7-
tetraazabicyclo[3.3.1]nonane occurred exclusively in
the condensed phase, and smooth increase in the rate
of decomposition at the acceleration step was due to
autocatalytic interaction of the decomposition products,
in particular, nitrogen dioxide, with the starting
substrate. The initial rate and the induction period were
not affected by the variation of the size of the crystals
within the 0.01–0.2 mm range (Fig. 2). That was also
indicative of the fact that the topochemical effects
practically did not affect the acceleration of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane decomposition. In
the case of octogen (β-form) 3 the final volume of
gaseous decomposition products was larger because
here the intracrystalline solvent also evolved which
was captured by 3,7-dinitro-1,3,5,7-tetraazabicyclo
[3.3.1]nonane during crystallization (Fig. 2) [4].
Thermal decomposition of 3,7-dinitro-1,3,5,7-tetra-
azabicyclo[3.3.1]nonane in solid state proceeded with
acceleration and was expressed by the S-shaped kinetic
curves (Fig. 1). Variation of the ratio of the reaction
vessel surface to its volume (S/V) did not affect the rate
and the kinetic equation of decomposition, evidencing
absence of heterogeneous reaction on the walls of the
vessel as well as of chain processes. At the same time,
20-fold increase in the ratio of the compound loading
The kinetic curves of thermal decomposition of 3,7-
dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane in 2 wt %
solutions in dibutyl phthalate and dinitrobenzene
differing in the dielectric constant (ε 6.4 and 20.6,
respectively) revealed slightly pronounced S-shape,
typical of secondary nitramines. The thermal decom-
position in dibutyl phthalate and dinitrobenzene
194

KINETICS OF THERMAL DECOMPOSITION
195
τ, min
τ, min
Fig. 2. Effect of the crystal size on thermal decomposition
of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane at 160°С,
m/V = 4.2×10–4 g/cm3: (1) ground (freshly prepared);
(2) ground (aged); and (3) non-ground.
Fig. 1. Kinetics of thermal decomposition of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane: (1) 140, (2) 150,
(3) 160, (4) 170, and (5) 180°C.
solutions was faster than in the solid state (Table 1)
and, in general, faster than for other cyclic nitramines
given for comparison in Table 1.
the data for compound 1 (Table 1) showed an opposite
trend: the decomposition was sharply decelerated in
comparison with 3,7-dinitro-1,3,5,7-tetraazabicyclo-
[3.3.1]nonane. For solid compound 1, the rate was
decreased by ~62 times. At the same time, the eight-
membered cyclic nitramine octogen which can exist as
polymorphic crystalline modifications differing in the
ring conformation (compounds 1–4), was the most
At first glance, one could assume that accumulation
of nitramine groups in the ring should have favored the
acceleration of the substrate decomposition, as has
been observed for aromatic nitro compounds. However,
Table 1. Kinetic parameters of thermal decomposition of cyclic nitramines
Solid state
Solution
k180º×105,
k180º×105,
Compound
Solvent
log A
log A
s–1
s–1
1
2
3
Octogen
158.6 11.20
158.9 11.30
0.082
0.095
[5]
[6]
187.9 15.00
0.22
Dinitrobenzene
[5]
Octogen (δ-form)
Octogen (β-form)
150.6 10. 80
280.3 25.00
0.280
0.048
[7]
[8]
4
Octogen (α-form)
171.5 12.60
159.0 10.80
0.067
0.029
[9]
[6]
5
6
N,N-Dinitropiperazine
155.6 12.00
0.12
Nitrobenzene
[11]
[5]
Hexogen
165.3 11.70
167.4 11.30
217.6 19.10
0.043
0.010
0.103
[5]
[6]
[5]
166.1 14.30
167.5 14.62
1.42
2.04
Dinitrobenzene
Dibutyl phthalate [10]
7
8
Pentogen
155.9 13.60
4.23
Dinitrobenzene
Dinitrobenzene
[11]
[12]
Bicyclooctane
210.2 18.00
172.9 15.64
0.058
5.08
[12]
170.0 13.77
165.0 14.96
14.79
8.65
Dibutyl phthalate [12]
9
3,7-Dinitro-1,3,5,7-tetra-
azabicyclo[3.3.1]nonane
151.6 13.71
150.6 13.62
17.06
18.10
Dibutyl phthalate
Dinitrobenzene
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196
KRUGLYAKOVA et al.
Table 2. Composition of non-condensing gaseous products of thermal decomposition of 3,7-dinitro-1,3,5,7-tetraazabicyclo-
[3.3.1]nonane in solid state
Products of decomposition, mol %
Temperature, °C
Degree of conversion, %
N2
N2O
CO
CO2
1.93
2.01
2.69
4.11
5.10
2.18
2.23
7.94
160
160
160
160
160
150
150
170
10
20
0.98
0.69
0.64
0.66
0.64
0.68
0.46
0.95
96.19
96.47
95.57
93.94
91.87
96.32
96.55
88.22
0.90
0.83
1.09
1.29
2.39
0.83
0.76
2.87
50
75
100
10
20
100
stable. The decomposition rate constant was varied by
approximately an order of magnitude depending on the
conformation of the ring. As follows from the data in
Table 1, the most thermally stable modification was
the tightly packed β-form of octogen, taking the
conformation of twisted ring in which one pair of
adjacent nitro groups was located on the same side of
the average plane of the ring, and the other pair was
located on the other side. The structure of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane, analog of octo-
gen, in the crystal was similar to the β-modification of
octogen, and the N-nitramine groups in the molecule
were almost planar [13, 14].
angles in the ring are of 113°–118°, approximately the
same as in the α-form of hexogen (an intermediate in
the synthesis of octogen) but lower than in the
molecule of octogen (121°–125°). Many intramole-
cular contacts are shortened in the molecule of 3,7-di-
nitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane. The distance
between the oxygen atoms and the adjacent methylene
carbon atoms is 2.62–2.69 Å [13], which is less than
the sum of the van der Waals radii of carbon and
oxygen atoms (3.16 Å). Besides, there are some short
intermolecular С···О contacts in the structure of 3,7-
dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane crystals
(3.2–3.3 Å). In general, it could be concluded that the
structure of bicyclo[3.3.1]nonane was much more rigid
than that of cyclooctane, and the conversion of the ring
was substantially hampered. Let us notice that,
according to the conformational simulations, the
structures of the chair-boat and boat-boat type are
substantially less favorable, and low barriers of the
formation make such conformations practically
unstable. The above reasoning could explain the low
thermal stability of 3,7-dinitro-1,3,5,7-tetraazabicyclo-
[3.3.1]nonane, its rate constant of thermal decom-
position being ~18 times higher than that of the
β-conformer of octogen.
Theoretically, three conformations are possible for
the molecule of 3,7-dinitro-1,3,5,7-tetraazabicyclo-
[3.3.1]nonane: chair-chair (A), chair-boat (B), and
boat-boat (C), differing in the type of angular
deformations and nonvalent interactions (Scheme 1).
In the crystal, the chair-chair conformation (A) is
realized, in which the nitramine groups have pyramidal
structure (deviation of the amine nitrogen from the
plane of its surrounding γ = 26°–24°) and the CNC
Scheme 1.
O2N
N
GC analysis of non-condensing gaseous products of
thermal decomposition of 3,7-dinitro-1,3,5,7-tetra-
azabicyclo[3.3.1]nonane (Table 2) showed that nitrous
oxide was the major one, and nitrogen amount was
100–200 times lower, whereas thermal decomposition
of hexogen and octogen gives comparable amounts of
nitrous oxide and nitrogen [15].
O2N
N
N
N
N
N NO2
N
N
NO2
N
N
N
NO2
N
O2N
C
А
B
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 2 2019

KINETICS OF THERMAL DECOMPOSITION
197
Scheme 2.
N
N
N
N
N
O2N N
N
NO2
O2N N
N NO2
O2N N
N
+ NO2
(1)
(2)
N
N
N
N
CH2=N NO2
+
2CH2=N
CH2=N NO2
CH2=N CH2
+
O2N N
(3)
(4)
CH2O
N2O
+
+
CH2=N
CH2O
NO
+
CH2=N CH2 NO2
+
N
N
(5)
(6)
+
NO
O2N N
N
O2N N
N NO
N
N
N
O2N N
N
NO
+
CH2=N CH2=N
NO + CH2=N⎯CH2⎯N⎯NO2
N
CH2=N NO
(7)
(8)
+ CH O
N2
+
CH2=N
2
+
CH O
N2O
CH2=N⎯NO2
+
CH2=N
+
2
CH2=N⎯CH2⎯N⎯NO2
CH2=N⎯CH2⎯N⎯NO2
CH2=N NO2
(9)
CH2=N
(10)
(11)
CH2=N +
NO2
CO +
CH2O NO2
+
NO + CO2 +
H2O
According to the LC–MS analysis data for 3,7-di-
nitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane and condensed
products of its thermal decomposition at 5 and 30%
conversion, the molecular ion peak in the mass
spectrum of the starting 3,7-dinitro-1,3,5,7-tetra-
azabicyclo[3.3.1]-nonane was weak (1.81%), the major
peaks being assigned to the fragment ions with m/z 128
(100%) and 142 (64%). For condensed products of the
thermal decomposition, the intensity of those peaks
was different depending on conversion, and new peaks
of fragment and rearrangement ions appeared (m/z 202,
172, 144, 130, 114, 112, 88, 74, 72, 58, 46, 44, 42, 30,
28), suggesting the destruction of the ring during
decomposition.
hexogen and octogen), with the rate-determining step
being the N–NO2 bond rupture in one of the rings of
the molecule (Scheme 2). For such mechanism, the
transition state should be more ordered than the initial
one, and the activation energy should be lower than the
energy of the weakest N–NO2 bond (1.375 Å) [15].
The activation entropy of thermal decomposition of
3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane in a
solution at 180°С was as low as 5.75 J mol–1 K–1
(dibutyl phthalate) and 4.02 J mol–1 K–1 (dinitro-
benzene). One could assume that the transition state
was close to the initial state due to more rigid structure
and the presence of short С···О contacts in the 3,7-di-
nitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane molecule
with planar configuration of the nitramine group.
In view of the above-mentioned and the symbate
variation of the activation parameters of 3,7-dinitro-
1,3,5,7-tetraazabicyclo[3.3.1]nonane decomposition in
solution, the homolytic mechanism of thermal decom-
position could be suggested (also by analogy with
Mechanism (1)–(11) in Scheme 2 adequately ex-
plained the formation of gaseous and condensed pro-
ducts of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 2 2019

198
KRUGLYAKOVA et al.
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thermal decomposition as well as low activation
parameters of the Arrhenius equation in a solution as
compared to those of hexogen and octogen (Table 1).
3. Zeman, S. and Dimun, M., Propel. Expl. Pyrotechn.,
In summary, the kinetic features, activation param-
eters, and the mechanism of thermal decomposition of
3,7-dinitrotetraazabicyclo[3.3.1]nonane in the solid
state and in solutions were determined. It was shown
that the acceleration of decomposition was due to
autocatalysis by the products of the reaction. The
effect of the structure of the 3,7-dinitrotetraazabicyclo
[3.3.1]nonane molecule on the rate and the mechanism
of thermal decomposition were revealed.
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mp 207°С (205–206°С [1]).
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The kinetics of thermal decomposition was studied
by manometric method using the Burdon type glass
manometer with residual air pressure in the reaction
volume of 10–2–10–1 mmHg [16]. Gaseous products of
decomposition were analyzed using an LKhM-80 chro-
matograph equipped with Polysorb 1 column. Com-
position of the condensed products of decomposition
was analyzed using a Shimadzu LCMS-2020 chromato–
mass spectrometer (solvent: acetonitrile, energy of
ionization 70 eV). Rate constants of thermal decom-
position were calculated from the starting rates of
decomposition. The error in determination of the rate
constant did not exceed 6–8 %, activation energy was
determined with accuracy of ±3–4 kJ/mol.
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CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
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