A comparative study of the structure, energetic performance and stability of nitro-NNO-azoxy substituted explosives

Article abstract of DOI:10.1039/c4ta04716h

2,4-Dinitro-NNO-azoxytoluene and 2,6-dinitro-4-nitro-NNO-azoxytoluene were synthesized as energetic compounds. Their structures and properties were studied by X-ray diffractometry, nuclear magnetic resonance and infrared spectroscopy. The differences betw

Full text of DOI:10.1039/c4ta04716h

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Journal of Materials Chemistry A
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DOI: 10.1039/C4TA04716H
A Comparative Study of the Structure, Energetic Performance and
Stability of Nitro-NNO-azoxy Substituted Explosives.
Yuan Wang, Shenghua Li , Yuchuan Li , Rubo Zhang*, Dong Wang and Siping Pang*
Nitro-NNO-azoxy group: The unique structure that contains one nitro group, one
N-oxide group and one azo-linkage, could improve the density, heat of formation,
detonation velocity and detonation pressure of an explosive. Compared with nitro
group, the nitro-NNO-azoxy group has a stronger energetic and electron-attracting
property.

Journal of Materials Chemistry A
Page 2 of 9
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ARTICLE TYPE
A Comparative Study of the Structure, Energetic Performance and
Stability of Nitro-NNO-azoxy Substituted Explosives.
a
a
a
b
a
ac
Yuan Wang , Shenghua Li , Yuchuan Li , Rubo Zhang* , Dong Wang and Siping Pang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
2,4-Di-nitro-NNO-azoxytoluene and 2,6-dinitro-4-nitro-NNO-azoxytoluene were synthesized as energetic
compounds. Their structures and properties were studied by X-ray diffractometry, nuclear magnetic
resonance and infrared spectroscopy. The differences between the nitro-NNO-azoxy and the nitro groups
are discussed. The detonation properties as predicted using EXPLO5 indicate that the detonation velocity
and pressure of 2,4-di-nitro-NNO-azoxytoluene increased by 21.7% and 74.3%, respectively, compared
with 2,4-dinitrotoluene. Nucleus independent chemical shift analysis was used to investigate skeleton
aromaticity and the effect of the nitro-NNO-azoxy and nitro groups on ring aromaticity. Electrostatic
potential, bond dissociation energy, Mulliken charges and Wiberg bond order were estimated by density
functional theory to establish the molecular electron distribution and their stabilities. The nitro-NNO-
azoxy group has a stronger electron-withdrawing property than that of the nitro group.
1
0
1
5
1
4
azoxy-benzenes as intermediates . Bis-3,3’-(nitro-NNO-azoxy)-
1
5
Introduction
difurazanyl ether (1) , and bis-3,3’-(nitro-NNO-azoxy)-4,4’-
16
azofurazan (2) were then synthesized (see Scheme 1).
Energetic compounds are one of the most important organic
components because of their unique energy storage and stability
5
5
6
6
7
0
5
0
5
0
However, the nitro-NNO-azoxy group has not been studied
thoroughly as an energetic functional group, and the influence of
the group on the structural and energetic properties has not yet
been clarified. Differences between the nitro-NNO-azoxy and
nitro groups must still be explored. To investigate the above
problems systematically, we designed two nitro-NNO-azoxy
derivatives by substituting nitro groups of energetic compounds
with nitro-NNO-azoxy groups, and studied their properties (see
Scheme 2). Two traditional explosives, 2,4,6-trinitrotoluene
TNT) and 2,4-dinitrotoluene (DNT), were selected since they are
used widely and are obtained easily. Consequently, 2,4-di-nitro-
NNO-azoxytoluene (3) and 2,6-dinitro-4-nitro-NNO-
azoxytoluene (4) were synthesized and their structures were
confirmed by mass spectrometry, H nuclear magnetic resonance
1-3
properties . It is a challenge to design and synthesize novel
energetic compounds with improved energetic performance and
stability since propellants must transport increasing payloads and
more powerful explosives are required. The performance of
energetic compounds is linked closely to structure. In general,
energetic compounds are composed of backbones and functional
2
2
3
3
4
4
0
5
0
5
0
5
4-6
groups . Only a few energetic compounds contain oxygen atoms
in their backbones, and the oxygen balance is usually improved
by introducing oxygen-rich groups. A nitro group is often
introduced to improve the oxygen balance of an explosive. Other
(
nitro-containing groups, such as the nitrato (ONO ), nitramino
2
(NHNO
C(NO
backbones to produce novel energetic derivatives with excellent
2
), geminal dinitro (CH(NO
2
)
2
)
and trinitromethyl
1
(
2
)
3
) groups are also introduced frequently into parent
13
(
NMR), C NMR, infrared spectroscopy (IR) and elemental
analysis. Changes in structural and performance features that
resulted from the introduction of the new group onto the parent
compounds are discussed in detail. X-ray diffractometry (XRD)
and density functional theory (DFT) were used to reveal
differences between the nitro-NNO-azoxy and nitro groups such
7-10
detonation performance . Recently, the introduction of N→O
on nitrogen-heterocycles has been found to be useful to increase
the energetic performance of energetic materials
these new oxygen-rich groups have drawbacks in that they may
be difficult to synthesise or they may be sensitive to thermal
conditions, impact and friction. Therefore, further research is
required to develop new oxygen-rich groups.
1
1, 12
. However,
as configuration and electrostatic potential (ESP)
O
N
N
O2N
O
O
NO2
O2N
O
We have become interested in the nitro-NNO-azoxy group,
which has not been studied fully. Its unique structure that contains
one nitro group, one N-oxide group and one azo-linkage, could
improve the oxygen balance and heat of formation (HOF) of
energetic compounds. The nitro-NNO-azoxy group was first
N
N
N
N
N
N
N
N
N
N
O
O
NO2
N
N
N
N
N
N
O
O
O
1
2
Scheme
derivatives.
1
Reported nitro-NNO-azoxy substituted furazan
1
3
reported by Churakov et al. in 1996 . After the first synthesis,
Churakov modified the synthesis route by using tert-butyl-NNO-
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Page 3 of 9
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C4TA04716H
Fig. 1
Oak Ridge
Thermal Ellips
oid Plot ORTEP
representation
of the molecular structure of 3 (a) and 4 (b). Displacement
ellipsoids are shown at the 50% probability level. Selected bond
length/Å: (a) C1-N1 1.47, N1-O1 1.24, N1-N2 1.28, N2-N3 1.47,
N3-O2 1.21, N3-O3 1.21; (b) C5-N1 1.46, N1-O1 1.24, N1-N2
6
5
1
.28, N2-N3 1.47, N3-O2 1.21, N3-O3 1.21, C1-N4 1.47, C3-N5
Scheme 1 Comparison of nitro-NNO-azoxy and nitro groups
1.48.
substituted with toluene.
Results and discussion
X-Ray crystallography
5
0
5
0
5
Single crystals of 3 and 4 suitable for crystal structure analysis
were obtained by recrystallization from hexane and hexane/ethyl
acetate 1:1, respectively, at room temperature. The molecular
structures, as determined by XRD are shown in Figs 1 and 2.
Some selected bond lengths are listed in Fig. 1. The N-NO
2
bond
1
1
2
2
in the nitro-NNO-azoxy group has a standard N-N bond length of
1
7
1
.47 Å , which is slightly shorter than the same bond in 1 (1.50
Å). In general, a shorter bond could result in increased stability.
Dihedral angles of O1-N1-N2-N3 in the nitro-NNO-azoxy groups
of 3 and 4 are less than 4°. The nearly planar conformation is
1
8, 19
quite common in azo compounds
. A similar planar structure
is also observed in the N-NO segment and the two planes are
2
almost perpendicular (the dihedral angle of O3-O2-O1-N1 is 86°).
The dihedral angles between the O←N=N plane and the benzene
ring, like O1-N1-C1-C6, range from 13° to 43°. The
configuration indicates that the NNO-azoxy group and the ring do
not share the same π system. The nitro-NNO-azoxy group tends
to refrain from making a larger conjugated system with the ring.
The packing diagram of two crystals shows that 3 packs in
parallel layers, whereas 4 has a “V-shaped” packing. The XRD
data and the parameters of compounds 3 and 4 are summarized in
Table 1.
4
0
Fig. 2 Unit cell packing of 3 (a) and 4 (b).
2
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Journal of Materials Chemistry A
Page 4 of 9
Table 1 X-Ray data and parameters of compounds 3 and 4.
View Article Online
DOI: 10.1039/C4TA04716H
3
4
CCDC
1023014
1023015
Empirical formula
Formula mass
Crystal system
Space group
Z
C
7
H
6
N
6
O
6
7 5 5 7
C H N O
270.03
Monoclinic
271.02
Monoclinic
P2
4
1
/n
P 2
4
1
/n
a (Å)
5.3336(13)
7.9688(19)
25.494(6)
90
5.2932(15)
29.236(8)
6.907(2)
90
b (Å)
c (Å)
α (°)
β (°)
94.454(3)
90
98.746(4)
90
γ (°)
3
Volume (Å )
1080.3(5)
1.661
1056.4(5)
1.705
Fig. 3 DSC spectra of 3 and 4.
-3
D
calc (g·cm )
Temperature (K)
F(000)
153 (2)
552
153 (2)
552
Sensitivity
The impact sensitivity (IS) and friction sensitivity (FS) of 3 and 4
were determined according to general methods. The ISs of 3 and
4 are 5 J and 15 J, respectively. Both compounds are classified as
h, k, l
7, 10, 32
0.147
7, 37, 9
0.155
-
1
μ (cm )
[I > 2σ(I)]
2
5
R
1
0.0393
9291
0.0461
8887
20
“sensitive” explosives . The results indicate that 4 has a similar
Reflections collected
IS to its parent compound TNT (IS = 15 J), whereas 3 is more
sensitive towards impact than DNT (measured IS > 40 J). The IS
is not affected significantly when one nitro group is substituted
by a nitro-NNO-azoxy group. However, it decreases significantly
when the two nitro groups are replaced by nitro-NNO-azoxy
groups. The FSs for 3 and 4 are 120 N and 160 N, respectively.
These compounds are less stable than DNT (FS > 360 N) and
Completeness to theta full (%)
0.994
0.998
wR
2
(all data)
2
0.1061
0.999
0.1290
1.003
S on F
3
0
Differential scanning calorimetry
T To determine the thermal behavior of the compounds,
differential scanning calorimetry (DSC) measurements were
2
1
TNT (FS = 353 N) . Detailed sensitivity information is given in
-
1
5
carried out at 5°C·min using dry oxygen-free nitrogen at 20 mL 35 the Electronic Supplementary Information (ESI†).
-1
min . As shown in Fig. 3, an endothermic peak is observed at
Density, heat of formation and detonation properties
5
1
8.5°C, which indicates the melt of 3. The exothermic peak at
39.5°C illustrates that 3 decomposed. It is likely that 4 melts at
Density and HOF are important parameters used to predict the
detonation performance of an explosive . Their precise values
play a significant role in obtaining accurate results. The crystal
2
2
1
14.0°C and decomposes at 137.0°C. Their decomposition
temperature decreases compared with the parent compounds DNT
250°C) and TNT (295°C). This is presumably because the long
1
0
-
3
4
4
5
5
0
5
0
5
densities of 3 and 4 measured at 153 K are 1.661 g·cm and
(
-
3
1
.705 g·cm , which is higher than the measured values of DNT
nitrogen chain is more vulnerable to heat, and conjugation of the
N-NO bond in the chain is less stable than the C-NO bond
-
3
-3
(
1.559 g·cm , 173 K) and TNT (1.704 g·cm , 123 K) at low
2
2
2
3, 24
temperature
. The nitro-NNO-azoxy group is more powerful
between the benzene ring and the nitro group. However,
compared with previously reported benzene derivatives (85–
than the nitro group in improving the density. An increase of 0.1
1
5
-3
1
3, 16
g·cm was obtained when both nitro groups were replaced with
nitro-NNO-azoxy groups. To obtain more reliable results for
detonation performance, the low temperature densities are
110°C) and 2 (132°C)
, the decomposition temperature of
1
39.5°C of 3 makes it the most stable compound in this series.
This provides a clue for the synthesis of new energetic nitro-
NNO-azoxy compounds with better thermal stabilities.
2
5
converted to those at room temperature by using Sun’s method .
Densities used for the calculation of the detonation performance
of 3 and 4 are 1.63 g·cm and 1.67 g·cm , respectively.
HOFs were estimated by isodesmic reaction and molecule
optimization was accomplished by using the Gaussian 03
program package . The calculation was carried out using the
density functional theory (DFT) B3LYP method
311++G(d,p) basis set
2
0
-3
-3
2
6
2
7, 28
and the 6-
2
9, 30
. Vibrational analysis confirmed that
the structures obtained correspond to the minima on their
potential energy hypersurfaces. To guarantee the accuracy, the
solid-phase HOF was calculated by subtracting the heat of
sublimation (ΔH ) from the gas-phase HOF by Politzer’s
s
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 3

Page 5 of 9
Journal of Materials Chemistry A
3
1, 32
-1
method
. The calculated HOFs of the title compounds are
kJ·mol , respectively, and are higher than those of DNT and TNT.
-1
summarized in Table 2. More HOF calculation details are shown
in the ESI†.
The HOF of toluene (50.1 kJ·mol ) decreased when toluene was
nitrated, and increased when the nitro-NNO-azoxy groups were
View Article Online
DOI: 10.1039/C4TA04716H
1
5
introduced. According to our calculations, when one nitro group
∆
HꢀSolidꢁ ꢂ ∆HꢀGasꢁ ꢃ ∆HꢀSublimationꢁ (1)
is displaced by one nitro-NNO-azoxy group in TNT, an
-1
increment of 135.9 kJ·mol in HOF is obtained. The phenomenon
is more evident when the nitro groups in DNT are displaced
completely by nitro-NNO-azoxy groups. Compare with DNT, 3
ꢄ
ꢄ
5
∆HꢀSublimationꢁ ꢂ aꢀSAꢁ ꢅ bꢆσ ν ꢅ c (2)
Tꢇꢈ
∆
H(Solid), ∆H(Gas) and ∆H(Sublimation) refer to the solid-phase
-1
HOF, gas-phase HOF and heat of sublimation, respectively; a, b, 20 obtains an extra 480 kJ·mol in HOF. The nitro-NNO-azoxy
and c are fitting parameters, respectively; SA is the molecular
group enhances the HOF in the skeleton more positively than the
2
33,
surface area; σ
is the total variance of the calculated ESP on
nitro group. The azo and N-N bonds are the main contributors
Tot
3
4
1
0
the molecular surface and ν is a balance parameter.
.
-1
The calculated HOFs of 3 and 4 are 415.7 kJ·mol and 55.4
Table 2 Properties of title compounds.
-3
-1
-1
compound
ρ/g·cm
OB/%
-65.2
-56.1
-114.3
-74
N/%
31.1
25.8
15.4
18.5
HOF/kJ·mol
415.7
D/km·s
7.78
P/GPa
24.4
23.9
14.0
19.5
Tm/°C
58.5
114
Td/°C
139.5
137
IS/J
5
FS/N
120
3
4
1.63
1.67
1.52
1.65
55.4
7.64
15
40
160
a
c
DNT
-64.3
6.39
70
250
360
b
c
TNT
-80.5
6.88
80.4
295
15
353
a
b
35
c
36
2
5
Record of 2,4-dinitrotoluene in the GESTIS Substance Database from the IFA, accessed on 9. October 2007. Reference
.
Reference .
The detonation velocity and pressure of the title compounds
were calculated using EXPLO5 v 6.01 with a rectified density
and HOF . As expected, with a better density, oxygen balance,
was therefore proposed to reflect contributions arising from the zz
3
9, 42
component of the shielding tensor
imply a stronger molecule aromaticity. NICS(1)zz is believed to
nitrogen content and HOF, 3 and 4 achieved better detonation 60 offer an improved interpretation of aromaticity with fewer errors.
. Larger negative values
3
7
3
3
4
4
0
5
0
5
performance than their corresponding nitro-compounds. The new
nitro-NNO-azoxy energetic group improves the energetic
performance, especially for the disubstituted 3, with its
To compare the aromatic differences that result from the
substitution of the nitro and nitro-NNO-azoxy groups, the
NICS(1)zz values of 3, 4, DNT, TNT and toluene were calculated
using the GIAO/B3LYP/6-311++G** method. The results are
-3
performance compared with DNT being as follows: 0.11 g·cm
increase in density; 49.1% increase in oxygen balance; additional 65 summarized in Table 3.
-1
-1
480 kJ·mol HOF; and most importantly, 1.39 km·s and 10.4
GPa increase in detonation velocity and pressure, respectively.
The nitro-NNO-azoxy group therefore enhanced every energetic
level parameter. However, as shown in Table 2, adverse effects
also result. In 3, for example, the decomposition temperature
drops by 110°C after substitution. There is a simultaneous
decrease in IS and FS of 35 J and 240 N, respectively. Thus, the
nitro-NNO-azoxy group could improve the energetic performance
of a compound but weakened its heat and mechanical stability. To
explain the origin of the instability of the nitro-NNO-azoxy group,
a series of computations were carried out that addressed structural
Table 3 NICS(1)zz values of title compounds.
DNT
3
4
TNT
Toluene
NICS(1)zz
ppm
/
-24.2309 -23.0447 -24.5468 -23.0761 -27.8042
The NICS(1)zz values of 3 and 4 are higher than those of
DNT and TNT even though the increases are not very evident.
Both 3 and 4 have a slightly weaker aromaticity than DNT and
and electronic differences between the nitro-NNO-azoxy and 70 TNT, respectively. Compared with toluene, the four other
nitro groups.
compounds have high NICS values, which means that the
aromaticity of the benzene ring decreases after the H atoms are
substituted by electron-withdrawing groups. The nitro-NNO-
azoxy group has a stronger ability than the nitro group to weaken
Nucleus independent chemical shift
The nucleus independent chemical shift (NICS) is a useful tool to
5
0
describe compound aromaticity or the magnetic properties of a 75 the aromaticity of the toluene skeleton.
structure. The original definition of the NICS is the negative
To investigate the shielding distribution of the zz component
value of the absolute magnetic shielding computed at ring
1 Å above the plane, the NICS of 125000 points around the
molecule was computed using the same method. Color-filled
3
8, 39
centers
illustration of the π-electron structure character since it is defined
as an NICS of 1 Å above the ring plane. However, NICS(1) is 80 4). The colors range from -45 to +40 ppm with dark blue denoting
. The NICS(1) concept is more practical for
4
3
shielding maps were sketched using Multiwfn v 3.3.4
(see Fig.
5
5
4
0, 41
still based on the total isotropic shielding value
. NICS(1)zz
extremely anti-shielding regions and red denoting shielding
4
|
Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Journal of Materials Chemistry A
Page 6 of 9
regions. The white regions have shielding values larger than 40
Electrostatic potentials
ppm. Since shielding and NICS share the same value of opposite
sign, a darker red color in the figure means that the structure has
better aromaticity. The scale on the right refers to the shielding of
a zz component. The curves in the maps are contour lines.
5
3
7
0
5
5
Determination of an ESP is an efficient way to relate compound
View Article Online
44
stability to outer stimulations . Murray et
D
a
O
l.I:sh10
o
.
w
10
e
3
d
9/
t
C
ha
4
t
TAth
0
e
4
r
7
e
16is
H
a link between ESP, IS and the C-NO
energy (BDE) of an energetic compound . The ESP at any point r
is given in the following equation.
2
/N-NO bond dissociation
2
5
45
ꢎꢀꢋ ꢁꢐꢋꢏ
ꢏ
Z
ꢉ
Vꢀrꢁ ꢂ ∑ꢌ |
ꢃ ꢍ
(3)
ꢏ
Rꢉꢊꢋ|
|ꢋ ꢊꢋ|
Where Z is the charge on nucleus A, located at R , and ρ(r) is
A A
the molecule’s electronic density. ESP was computed based on
the optimized structures of the title compounds. Fig. 5 shows the
3
ESPs for the 0.001 electron/bohr isosurfaces of electron density
evaluated at the B3LYP/6-311++G(d,p) level. The colors range
from -0.015 to +0.04 hartrees with dark blue denoting extremely
electron-deficient regions (V(r) ≥ 0.04), red denoting electron-
rich regions (V(r) ≤ -0.015) and yellow denoting neutral regions.
As shown in Fig. 5, the four calculated compounds have a
1
90 strongly positive center and negative periphery. This is a common
phenomenon for traditional explosives such as HMX and CL-20 .
8
Because only two electron-withdrawing groups exist on the ring,
the ESP of 3 and DNT is divided into three parts. The blue part on
the left refers to an electron-deficient region, whereas the red part
95 on the right represents an electron-rich region. The cyan part in
the middle is a transition region. In contrast with compound 3 and
DNT, 4 and TNT show electron-deficient skeletons surrounded
by electron-rich groups. Since electron-withdrawing groups are
attached to the toluene ring, the blue color in the middle of 4 is
1
Fig. 4 Shielding maps of toluene (a), 3 (b), 4 (c), DNT (d) and 200 darker than that of 3. The same result is also observed in TNT and
TNT (e).
DNT. We are more concerned with differences caused by
replacing the nitro group with the nitro-NNO-azoxy group. Fig.
Fig. 4a shows that the ring is filled with red, which means
5
b and 5d shows that the prolonged nitrogen chain leads to an
that toluene has a strong and uniform aromaticity. The dark red
color is well distributed on the six carbon atoms, and the contour
line on these atoms is a regular hexagon. Nevertheless, results of
other compounds in the study are different from those of toluene.
The contour line on the carbon atoms has a “nut-like” shape. The
red areas have decreased in size compared with toluene,
especially for the positions connected with an electron-
withdrawing group. These groups not only affect the magnetic
property of the center, but also that of the periphery. The red on
the ring of 3 is less uniformly distributed than that of DNT. The
strong electron-withdrawing property and the chain configuration
of the nitro-NNO-azoxy group probably result in a less regular
magnetic field spread. The results of 4 and TNT did not differ
much, mainly because 4 has only one nitro-NNO-azoxy group
and the effect is limited.
enhancement of the electron-rich part at the bottom of the
05 molecule. Furthermore, the blue color over the middle of 4 is
darker than that of TNT. These phenomena indicate that the
electron-withdrawing capability of the nitro-NNO-azoxy group is
stronger than that of the nitro group, which concurs with the
results of NICS(1)zz. The charge distribution on the surface of 4 is
10 more scattered than that on TNT. Generally, such as structure is
not conducive to the stabilization of one molecule. The ESP
figures of 3 and DNT depict the phenomenon clearly. Compared
with DNT, the red part of 3 is distributed irregularly and the blue
color on the left is darkened. In terms of ESP, the nitro-NNO-
15 azoxy group could not perform better than the nitro group in
stabilizing the molecules. This is probably another way to
illustrate why the nitro-NNO-azoxy group derivatives have a
relatively lower stability.
1
1
2
0
5
0
2
2
2
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Journal of Materials Chemistry A
Mulliken charges and Wiberg bond order
To illustrate how electrons are distributed on different atoms, the
View Article Online
Mulliken charges and Wiberg bond order of the two compounds
DOI: 10.1039/C4TA04716H
were calculated. Mulliken charges may reflect the electron
distribution on different atoms whereas the Wiberg bond order
indicates bond stability . Both analyses methods can be used to
investigate the interactions between atoms in the nitro-NNO-
azoxy group. Selected values are shown in Table 5.
6
5
4
7
Table 5 Selected Mulliken atomic charges and Wiberg bond
4
5
orders of title compounds.
3
4
DNT
0.206
0.182
-0.205
-0.122
0.269
-0.174
TNT
0.182
0.190
0.182
-0.141
0.264
-0.141
Mulliken
atomic charges
C1
C2
0.223
0.202
-0.211
-0.119
0.298
-0.150
-0.395
-0.318
-0.323
0.161
-0.132
0.566
0.918
1.428
1.472
0.869
1.563
0.181
0.189
0.185
-0.136
0.304
-0.143
-0.416
-0.313
-0.314
0.155
-0.113
0.569
0.927
1.424
1.469
0.868
1.560
0.913
0.913
C3
C4
C5
C6
Fig. 5 ESPs of 3 (a), 4 (b), DNT (c) and TNT (d). Red: negative,
orange: slightly negative, yellow: neutral, green: slightly positive,
light blue: positive, dark blue: very positive.
O1
O2
O3
Bond dissociation energy
N1
N2
5
0
5
0
To test the firmness of the N-NO and N→O bonds in the nitro-
2
N3
NNO-azoxy group and the C-N bond between the ring and the
group, the corresponding bond dissociation energies were
Wiberg bond
orders
C-NNO
N1-O1
N1-N2
N2-N3
N3-O2
4
6
calculated according to the following equation :
A-B (g) → A• (g) + B• (g)
1
1
2
The BDE values of the C-N bonds between the ring and the nitro-
2
NNO-azoxy group are large whereas the N-NO BDE is relatively
-1
small (Table 4). BDE values larger than 280 kJ·mol indicate that
the connection between the ring and the group is strong and firm.
The C-NNO bonds in 3 or 4 share similar BDE values with the C-
C1-NO
2
0.927
0.927
0.914
0.914
0.921
C3- NO
C5- NO
2
2
2
NO bonds in the same position of DNT or TNT, which indicates
that the two groups have a close stability in terms of linking with
The N3 in the nitro group is positively charged since it is
connected with two electron-withdrawing O atoms. Nevertheless,
the nitro group, which is treated as an individual part, is a
negatively charged moiety. Interestingly, the charge on N2 is
-1
the skeleton. BDE values larger than 500 kJ·mol for N→O BDE
imply that the bond is stable. However, the BDEs of N-NO range
2
-1
from 130-140 kJ·mol , and the N-NO bond is deemed to be the
2
vulnerable bond for the title compounds. To stabilize the nitro-
1
1
1
1
50 negative, with two O atoms and the N2 that stretches the N3 in a
different direction. The N-NO bond therefore becomes the
2
NNO-azoxy containing compounds, N-NO bond deactivation
2
should be considered.
weakest bond in the molecule. This is also seen from the Wiberg
bond order of N2-N3, which is the lowest value in the molecule.
The Wiberg bond orders for N1-N2 and N1-O1 are 1.47 and 1.43,
55 respectively, and lie between the single and double bonds. The π-
electron is delocalized on the three atoms, which makes the bonds
firm and stable. These results agree with the BDE conclusions.
The Mulliken charges for the O1-N1-N2-N3 structure are
characteristic of alternately distributed positive and negative
60 values. The O1 stabilized the nitro-NNO-azoxy group and is
crucial. The total Mulliken charge of atoms in the nitro-NNO-
azoxy group (-0.43) is larger than that of the nitro group (-0.36).
After being affected by the strong electron-attracting property of
the nitro-NNO-azoxy group, C1 and C5 on compound 3 possess
65 higher charges than those of DNT. Similarly, the charge of C5 on
compound 4 (0.304) is higher than that of TNT (0.264). This is
because the nitro-NNO-azoxy group has a stronger electron-
Table 4 Selected bond dissociation energies of title compounds.
a
b
c
c
3
3
4
DNT
TNT
-
1
C-N/kJ·mol
283.9
134.6
551.7
299.3
140.1
557.3
289.9
138.2
555.9
299.1
289.6
-
1
N-NO
2
/kJ·mol
-
1
N→O/kJ·mol
a
b
Values refer to the group connected with C1 in compound 3. Group
c
2
5
connected with C5 in compound 3. BDEs of C5-NO
2
in DNT and TNT.
6
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Journal Name, [year], [vol], 00–00
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Journal of Materials Chemistry A
Page 8 of 9
attracting property than that of the nitro group. The bond order of 45 22.93 (1C, Me), 114.99 (1C, Ph), 124.18 (1C, Ph), 131.45 (1C,
-
1
the C-NNO bond on 3 or 4 and the C-NO
in the same place did not show much difference. The rebalance of
electrons between the ring and the group made the C-NNO bond
2
bond on DNT or TNT
Ph), 139.81 (1C, Ph), 145.63 (1C, Ph). IR KBr ν/cm 1617, 1484,
1275, 860, 828, 710, 576.
View Article Online
DOI: 10.1039/C4TA04716H
2,6-Dinitro-4-nitro-NNO-azoxytoluene could be synthesized
in a similar way and only half the moles of zinc powder,
ammonium chloride, N,N-dibromo-tert-butylamine and NO BF
were needed. Yield 38.6%, yellow solid. (Found: C, 30.52; H,
1.96; N, 26.07; O, 41.45; calc. for C H N O : C, 31.01; H, 1.86;
5
as stable as the C-NO bond.
2
5
0
2
4
Experimental
The synthesis method of 3 and 4 is shown in Scheme 3.
7
5
7
1
8
N, 25.83; O, 41.30) m/z 181, 165, 89. H NMR: (400 MHz,
Zn NH
4
Cl
H
2
O
MnO
2
1
3
AR
NO
2
AR
NHOH
AR
NO
acetone-d
6
Me
4
Si) δ 2.72 (3H, s, Me), 9.04 (2H, s, Ph) C NMR:
Me Si) δ 15.77 (1C, Me), 110.71 (1C, Ph),
22.70 (2C, Ph), 134.72 (1C, Ph), 152.70 (2C, Ph). IR KBr ν/cm
090, 1603, 1536, 1487, 1352, 1272, 909, 852, 723. All spectra of
CH Cl
2
2
5
5
(400 MHz, acetone-d
1
3
6
4
-1
Br
O
N
O
N
3 and 4 are shown in the ESI†.
N
t-Bu
2 4
NO BF
Br
AR
N
t-Bu
AR
N
NO2
CH
2
Cl
2
3
CH CN
Conclusions
3
or 4
6
6
7
7
8
0
5
0
5
0
2,4-Di-nitro-NNO-azoxytoluene and 2,6-dinitro-4-nitro-NNO-
azoxytoluene were designed and synthesized. Differences
between the nitro-NNO-azoxy and nitro groups were compared
systematically. The structures of the two new compounds were
determined by mass spectrometry, IR, NMR, elemental analysis
and XRD. Their intramolecular interactions were studied by
investigating their crystal structures and their electron distribution
properties were obtained by calculating their NICS, ESP, BDE,
Mulliken atomic charges and Wiberg bond order. Substitution of
the nitro-NNO-azoxy group could enhance the energetic levels of
the toluene system such as density, HOF, detonation velocity and
pressure. The nitro-NNO-azoxy derivatives also have an adverse
effect in terms of lower decomposition temperature, higher
impact and friction sensitivities than the traditional nitro-
containing counterparts. The nitro-NNO-azoxy group has a
stronger electron-withdraw capability than the nitro group. Its
higher electronegativity makes the nitro-NNO-azoxy group more
able to influence the aromaticity and electron distribution of the
benzene ring than the nitro group. The BDE and Wiberg bond
AR
NO
2
=
DNT or TNT
Scheme 3 Synthesis methods used in the study.
,4-Dihydroxaminotoluene: DNT (3 g, 16.5 mmol),
dichloromethane (18 mL), Ethanol (75 mL) and water (18 mL)
were added to a 250 mL three-necked flask followed by zinc
powder (27 g, 412.8 mmol). A dropwise solution of 3 g (56.1
mmol) ammonium chloride and 30 mL water was added to the
vigorously stirred mixture under nitrogen. The reaction was
2
1
1
2
2
3
3
4
0
5
0
5
0
5
0
maintained at 15°C in an ice bath for 30 min. MgSO was used to
4
remove residual water after the reaction. The precipitate was
filtered, washed with dichloromethane and a solution of 2,4-
dihydroxaminotoluene was obtained. The solution was
concentrated to 50 mL using a rotary evaporator.
2,4-Dinitrosotoluene: Activated MnO2 (150 g) and 250 mL
dichloromethane were added to a 500 mL three-necked flask. A
dropwise solution of 2,4-dihydroxaminotoluene was added to the
stirred mixture under nitrogen and at ambient temperature. Thirty
minutes after the solution had been added, the MnO was filtered
off and the solution was concentrated to 50 mL.
,4-Di-tert-butyl-NNO-azoxytoluene: Under nitrogen, N,N-
dibromo-tert-butylamine (8 g, 35.1 mmol) was added to the
vigorously stirred 2,4-dinitrosotoluene solution at room
temperature. After one hour, the solution was evaporated in a
rotary evaporator and the residue passed through a silicagel
column to yield 2,4-di-tert-butyl-NNO-azoxytoluene (3.14 g,
5.1%, three steps).
,4-Di-nitro-NNO-azoxytoluene: 2,4-Di-tert-butyl-NNO-toluene
4.4 g, 15 mmol) was dissolved in 50 mL dry acetonitrile, to
which 5 g (37.5mmol) NO BF was added in a nitrogen
atmosphere. The solution was stirred for 5 h at 0°C, poured into
00 mL water, and extracted with dichloromethane three times.
The solvent was removed and 2,4-di-nitro-NNO-azoxytoluene
2.13 g, 52.3%), a yellow solid, was obtained after passing it
through a silicagel column. (Found: C, 31.26; H, 2.47; N, 30.65;
O, 35.62; calc. for C
5.53). m/z 270, 180, 91. H NMR: (400 MHz, acetone-d Me Si)
δ 2.58 (3H, s, Me), 7.91-7.93 (1H, m, Ph), 8.48-8.50 (1H, m, Ph),
.88 (1H, m, Ph), C NMR: (400 MHz, acetone-d Me Si) δ
order results indicate that the N-NO bond is the most vulnerable.
2
The comprehensive performance of the nitro-NNO-azoxy group
proves it is a competitive energetic group for designing new
compounds with high energetic levels. We anticipate that the
results from this paper could contribute to further studies of
compounds of this kind.
2
2
8
5
Acknowledgements
The authors gratefully acknowledge the support of the National
Natural Science Foundation of China (No.11176004) and the
opening project of the State Key Laboratory of Science and
Technology (Beijing Institute of Technology, No. ZDKT12-03).
6
2
(
2
4
9
0
Notes and references
2
a
School of Materials Science & Engineering, Beijing Institute of
Technology, Beijing 100081 (China), E-mail: invincibly@yeah.net.
(
b
School of Chemistry, Beijing Institute of Technology, Beijing 100081
(
China), E-mail: zhangrubo@bit.edu.cn.
c
7
H
6
N
6
O
6
: C, 31.12; H, 2.24; N, 31.11; O, 95 State Key Laboratory of Explosion Science and Technology, Beijing
1
Institute of Technology, Beijing 100081 (China), E-mail:
pangsp@bit.edu.cn
3
6 4
1
3
8
6 4
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Journal Name, [year], [vol], 00–00 | 7

Page 9 of 9
Journal of Materials Chemistry A
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This journal is © The Royal Society of Chemistry [year]