Structural and thermal behavior of imidazolium N,N′-dinitrourea

Article abstract of DOI:10.1016/j.molstruc.2012.02.001

The structural and thermal behavior of energetic salts based on N,N′-dinitrourea (DNU) should be fully investigated for their further applications. In the present paper, the crystal structure and vibrational spectrum of imidazolium N,N′-dinitrourea ([IMI][DNU]) were investigated in detail. A slight twist or torsion was found in the cation and anion because of their interactions. These interactions also formed a stabilized hydrogen bond network. The vibrations of the functional groups were mainly exhibited in an infrared (IR) spectrum, and the skeletal stretching vibrations mainly appeared in a Raman spectrum. The non-isothermal kinetics of the title compound and its mixture with polyethylene glycol 10000 (PEG10000) were investigated with the methods of Kissinger and Ozawa. In situ IR analysis, mass spectral fragmentation, and bond dissociation enthalpy calculations showed that the initial decomposition step of [IMI][DNU] was the breakage of the NN bond in the anion. The in situ IR of gaseous phase products and mass spectral fragmentation indicated that the main final products of [IMI][DNU] decomposition were N ₂O and imidazole.

Full text of DOI:10.1016/j.molstruc.2012.02.001

Journal of Molecular Structure 1015 (2012) 67–73
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structural and thermal behavior of imidazolium N,N0-dinitrourea
b,
b
Long Liu a,b, Zengxi Li a, , Chunshan Li , Suojiang Zhang
⇑
⇑
a Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
b Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
The structural and thermal behavior of energetic salts based on N,N0-dinitrourea (DNU) should be fully
investigated for their further applications. In the present paper, the crystal structure and vibrational
spectrum of imidazolium N,N0-dinitrourea ([IMI][DNU]) were investigated in detail. A slight twist or
torsion was found in the cation and anion because of their interactions. These interactions also formed
a stabilized hydrogen bond network. The vibrations of the functional groups were mainly exhibited in
an infrared (IR) spectrum, and the skeletal stretching vibrations mainly appeared in a Raman spectrum.
The non-isothermal kinetics of the title compound and its mixture with polyethylene glycol 10000
(PEG10000) were investigated with the methods of Kissinger and Ozawa. In situ IR analysis, mass spectral
fragmentation, and bond dissociation enthalpy calculations showed that the initial decomposition step of
[IMI][DNU] was the breakage of the NAN bond in the anion. The in situ IR of gaseous phase products and
mass spectral fragmentation indicated that the main final products of [IMI][DNU] decomposition were
N2O and imidazole.
Article history:
Received 11 November 2011
Received in revised form 1 February 2012
Accepted 1 February 2012
Available online 10 February 2012
Keywords:
Imidazolium N,N0-dinitrourea
Vibrational spectroscopy
Energetic salts
Crystal structure
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
density as well as high heat of formation, have been synthesized.
The crystal structure and physical properties of triazolium, includ-
Energetic salts are of considerable interest due to their advanta-
ges over neutral molecules. These salts often possess lower vapor
pressures and higher thermal stabilities than similar nonionic ana-
logues [1–6]. The synthesis and characterization of new energetic
ionic salts, including nitrogen rich salts [2,7–11], high density
energetic salts [12], coordination complexes [13–15], and high
oxygen balance salts [5,16,17], have been successfully carried out
in the past decade. To study the application of energetic salts as
propellants, explosives, or pyrotechnics, the thermal behavior of
some energetic salts have also been systematically studied [18].
For example, non-isothermal kinetic studies of the coordination
complexes [Co(DAT)6](ClO4)2 and [Cd(DAT)6] (ClO4)2 have shown
that they can be used as a primary explosive [13,15]. The thermal
decomposition behaviors of 1,2,3-triazole nitrate and hydrazine,
3-nitro-1,2,4-triazole-5-one were investigated by thermal analyses
and quantum chemical calculations [19,20].
ing the heat of formation and detonation parameters, have also
been calculated in literature [12]. However, the investigation of
imidiazolium salts, another typical nitrogen heterocycle energetic
salt, has not been reported.
The present work aimed to synthesize imidazolium DNU
([IMI][DNU]) and its precursor according to existing methods
[21,22], as well as to investigate further its structural and thermal
behaviors. The structural properties were extensively studied by
single X-ray crystal diffraction, infrared (IR), and Raman spectrum
analysis. The non-isothermal kinetics of the title compound and its
mixture with polyethylene glycol (PEG) 10000 (PEG10000) were
investigated with the methods of Kissinger [25] and Ozawa [26].
The compatibility of [IMI][DNU] with PEG10000 was analyzed
according to the peak temperature of differential thermal analysis
(DTA). The possible decomposition mechanisms and initial decom-
position step were predicted by combining in situ IR analyses and
mass spectral fragmentation with bond dissociation enthalpy
(BDE) calculations. The obtained results are beneficial in the design
of thermally stable energetic salts.
Energetic salts based on the N,N0-dinitrourea (DNU) anion have
been synthesized because of the high density (1.98 g/cm3) of DNU
[12,21–24]. The structural and thermal behaviors of its potassium
and ammonium salts have been studied, and several tautomeric
forms with high thermal stabilities are found [21,23]. Energetic
salts based on nitrogen heterocycles, which often possess high
2. Experimental section
Caution: DNU and its imidazolium salt, which are powerful and
sensitive explosives, were handled with extreme care using the
best safety practices. The preparation involved small amounts
and strict reaction temperature control.
⇑
Corresponding authors.
E-mail addresses: zxli@home.ipe.ac.cn (Z. Li), csli@home.ipe.ac.cn (C. Li).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2012.02.001

68
L. Liu et al. / Journal of Molecular Structure 1015 (2012) 67–73
2.1. General methods for characterization
level. The frequency values were scaled down by a factor 0.9668
[36].
1H NMR spectra were recorded on Bruker AV-600 at 600 MHz.
The spectra were measured in [D6] dimethyl sulfoxide (DMSO).
The chemical shifts were given in ppm relative to tetramethylsil-
ane. IR spectra were recorded on a Thermo Nicolet 6700 spectrom-
eter as KBr pellets at room temperature. Microanalyses were
performed using a Vario EL element analyzer. ESI/MS analyses were
carried out using an FT-ICR mass spectrometer.
2.5. X-ray analyses
X-ray single-crystal data were collected on a Bruker CCD X-ray
diffractometer equipped with an area detector using graphite
monochromated Mo K
a radiation (k = 0.7103 Å). The structures
were solved and refined using the SHELXTL software package
[37]. Absorption corrections were made using SADABS [38]. The
structures were refined by full-matrix least squares on F2. All
non-hydrogen atoms were anisotropically refined. The hydrogen
atoms of NAH were found from difference Fourier maps and iso-
tropically refined without restraint. The hydrogen atoms were
placed in a calculated position with CAH = 0.95 Å. CCDC-829453
contained the supplementary crystallographic data for the present
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic DataCentre via http://www.ccdc.cam.a-
c.uk/data_request/cif.
2.1.1. Synthesis of DNU
DNU was synthesized according to literature [22]. Urea (3 g,
0.05 mol) was added to a mixture of 98% sulfuric acid (6 mL) and
100% nitric acid (8 mL) at 0–5 °C. The mixture was then stirred for
about 30 min at 0 °C, and DNU as a white solid was precipitated
in 60% yield. 1H NMR (600 MHz, [D6]DMSO): d = 11.2 (s, 2H);
elemental analysis calcd. (%) for CN4O5H2: C 8.00%, H 1.34%, N
37.34%; found: C 8.18%, H 1.53%, N 37.04%; MS m/z: DNUꢀ, 148.9.
UV: 223 nm. IR (KBr):
m .
= 1762, 1705, 1610, 1310, and 1080 cmꢀ1
2.1.2. Synthesis of [IMI][DNU]
3. Results and discussion
Imidazole (0.68 g, 10 mmol) was added to a solution of DNU
(1.8 g, 12 mmol) dissolved in acetonitrile (20 mL), and stirred at
room temperature for 12 h. A white product was obtained after
filtration, and was washed with CH3CN as well as diethyl ether as
in literature [12], yield 76%. 1H NMR 600 MHz, [D6]DMSO:
d = 13.85 (s, 1H), 9.06 (s, 1H), 7.63 (s, 2H); elemental analysis calcd.
(%) for C4H6N6O5: C 22.03%, H 2.77%, N 38.53%; found: C 22.24%, H
3.1. Crystal structure
Suitable [IMI][DNU] crystals for X-ray diffraction were obtained
by slow recrystallization from ethyl acetate at room temperature.
The structures and crystallographic data are shown in Table 1
and Fig. 1a, respectively.
2.88%, N 37.69%; IR (KBr):
m = 3161, 2878, 1710, 1592, 1293, 1172,
.
The structure of [IMI][DNU] at 153 K crystallizes in the ortho-
rhombic space group (P212121) with one molecule in the unit cell.
The calculated density is 1.64 g/cm3 (Table 1). There are also chiral
configurations as DNU and triazolium dinitrourea. The bond length
of N(3)AN(4) is also lowered to 1.316(2) Å because of the deproto-
nation of the DNU molecule [12]. However, the torsion angles of
O(1)AN(3)AN(4)AC(4) = 1.9(3)° is smaller than that of O(7)A
N(8)AN(9)AC(10) = 8.9(2)° for triazolium dinitrourea, and O(1)A
N(1)AN(2)AC(1) = ꢀ11.2(2)° for DNU [12]. These values mean that
the [IMI][DNU] anion is nearly coplanar because of the delocaliza-
tion of the anionic charge and the hyper-conjugative effect that
exists in nitramines [12,39]. A slightly twisted hydrogen atom
H(1N) that connects to the nitrogen atoms out of the imidazole ring
plane by 3° may be the consequence of competition among
1069, 1003, and 769 cmꢀ1
2.2. In situ pyrolytic FT-IR spectroscopy
The experiments were carried out in a grease-free sodium boro-
silicate glass cell. The cells were cylindrical, with a length of
13.5 cm and internal diameter of 2.3 cm. ZnSn windows were fixed
with Crystal sealant. IR spectra were recorded using a Nicolet 6700
IR spectrometer. The light source was a glow bar from which the
interferometer analyzed the spectral region from 650 cmꢀ1 to
4000 cmꢀ1. The cell was heated using a thermolyne heating tape,
which was controlled by an analogue temperature controller. The
condensed and gas phase products from the thermal decomposition
of DNU were collected every 5 °C at a heating rate of 5 °C/min in a
vacuum.
Table 1
Crystal data and structure for title compound.
2.3. Compatibility of [IMI][DNU] with PEG10000
Formula
Mol. wt.
C4H6N6O5
218.15
[IMI][DNU] was composed of PEG10000 at a mass ratio of 1:1.
The decomposition processes of [IMI][DNU] and [IMI][DNU] + -
PEG10000 were determined by a SHIMADZU DTG-60H system at
the heating rates of 2, 5, 10, 15, and 20 °C/min. The kinetic param-
eters were calculated according to the Kissinger and Ozawa
methods.
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
153
Orthorhombic
P212121
5.4173(12)
8.162(2)
19.978(5)
90
a
(°)
b (°)
90
c
(°)
90
883.3(4)
2.4. Computational procedures
V (Å3)
Z
4
1.64
0.149
448
qcalcd (mg/m3)
Density functional theory (DFT) calculations are used to study
the thermal decomposition of energetic compounds, such as
HMX, RDX, TNT, and HNTO [19,27–31]. The Becke three-parameter
hybrid density functional approximation with the Lee–Yang–Parr
correlation functional approximation (B3LYP) [32] is adequate
for calculating the thermal decomposition mechanism of similar-
sized systems [33,34]. In the present study, all calculations were
performed with the Gaussian 09 program package [35]. The optimi-
zation and frequency determinations of the isolated DNU structure
were carried out using the 6-311G (d, p)/B3LYP basis set at the DFT
l
(mmꢀ1
)
F (000)
Crystal size (mm3)
h Range (°)
0.24 ꢁ 0.29 ꢁ 0.36
3.22–27.49
ꢀ7 6 h 6 5
ꢀ10 6 k 6 10
ꢀ24 6 l 6 25
1208/0.025/6905
R1 = 0.0290, wR2 = 0.0702
0.19 and ꢀ0.14
Limiting indices
Data/restraints/param.
Final R indices (I > 2
r
(I))a
Largest diff. peak and hole (eÅꢀ3
)

L. Liu et al. / Journal of Molecular Structure 1015 (2012) 67–73
69
Fig. 1. Crystal structure of [IMI][DNU]. (a) ORTEP representation of the molecular structure. (b) Close contacts around each anion. (c) Close contacts around each cation.
(d) The three-dimensional hydrogen-bonding network.
Table 2
intramolecular hydrogen bonds. The detailed hydrogen bonds,
torsion angles and bond length are listed in Supporting information.
The packing structure of the title compound built by hydrogen
bonds is also depicted in Fig. 1. Fig. 1b shows that each DNU anion
is connected to four cations via the terminal peripheral oxygen
atoms through hydrogen bonds: N1AH(1 N)ꢂ ꢂ ꢂO(1) = 3.051(2) Å,
122(2)°; N1AH(1N)ꢂ ꢂ ꢂO(2) = 2.793(2) Å, 177(2)°; N(2)AH(2N)ꢂ ꢂ ꢂ
O(1) = 2.818(2) Å, 151(2)°; N(2)AH(2N)ꢂ ꢂ ꢂO(3) = 2.902(2) Å, 134(2)°;
N(5)AH(5N)ꢂ ꢂ ꢂO(2) = 2.804(2) Å, 170(2)°; C(2)AH(2) ꢂ ꢂ ꢂO(4) =
3.252(3) Å, 170°; C(3)AH(3)ꢂ ꢂ ꢂO(4) = 3.006(2) Å, 121°. These hydro-
gen bonds also connect a cation to four anions, as shown in Fig. 1c.
Another peripheral oxygen atom of an anion is connected to another
anion by the hydrogen bond of N(5)AH(5N)ꢂ ꢂ ꢂO(2). The above inter-
actions enable the cations and anions to pack into complicated
three-dimensional hydrogen bond networks (Fig. 1d).
The wavenumbers (cmꢀ1) of IR and Raman spectra and the calculated results for title
compound.
Correction
Assignment
IR spectra
Raman spectra
528
526
594
725
784
940
q
NO2
N2H
CN
x
x
769
ds NO2
ds NO2
782
972
976
996
ms N4C4N5
1006
x
N1H
1003
1069
1035
1153
1174
1223
1242
1315
1364
1445
1523
1575
1598
1700
2730
3133
3480
3529
mas C4N5N6
1060
1164
1187
1204
m
m
C1N2, N2C3
N1C3N2
1172
1293
mas N3N4C4
mas NO2
ms N6N5H
d C4N5H
mas C1C2N2
NO2
1331
1455
3.2. Vibrational spectroscopy
m
C1C2
mas NO2
1592
1710
2878
3161
The observed IR and Raman bands with their calculated wave-
numbers and assignments are given in Table 2. The derivation of
some wavenumbers may be attributed to the intermolecular
hydrogen bonds, which were found in the crystal structure of the
solid phase. The hydrogen bonds also formed a dominant broad
band ranging from 2200 cmꢀ1 to 3600 cmꢀ1, and weakened the
vibrations of NH at 3480 and 3529 cmꢀ1 [40,41]. The absorption
bands at 1592 and 1710 cmꢀ1 correspond to the stretching vibra-
tions of N@O and C@O in the anion, respectively. The asymmetric
stretching vibrations of the C4AN5AN6 and N6AN5AH bonds in
the anion were observed at 1069 and 1293 cmꢀ1. The CAN wag-
ging mode in the anion was assigned to the medium strength band
m
m
m
m
m
CO
1715
N1H
C3H
N5H
N2H
at 769 cmꢀ1. For the cationic vibrations, only those of CAH and
NAH bonds were observed in the IR spectrum. For example, the
in-plane rocking vibrations of C1AH and N2AH were assigned at
1172 cmꢀ1. These stretching vibrations were confirmed within
2880–3235 cmꢀ1
.

70
L. Liu et al. / Journal of Molecular Structure 1015 (2012) 67–73
The skeletal stretching vibrations were mainly exhibited in the
materials on the decomposition temperatures of energetic materi-
als [48,49]. PEG is widely used in explosives and propellants for
reducing sensitivity [50] and improving mechanical properties
[51]. Therefore, the compatibility of [IMI][DNU] with PEG10000
was studied by TG/DTA at the heating rates of 2, 5, 10, 15, and
20 K/min. The results are listed in Table 4. The kinetic parameters
were calculated according to Eqs. (1)–(4). The peak temperature
changes of [IMI][DNU] in the mixture decreased with increased
heating rate, and its values ranged from ꢀ16 K to ꢀ5 K. The average
apparent activation energy calculated from both Kissinger and
Ozawa methods was 88.7 kJ/mol, which was lower than that of
the pure products. These results meant that [IMI][DNU] was
incompatible with PEG10000 [47,52,53], which decreased the ther-
mal stability of [IMI][DNU]. However, the pre-exponential factor A
and the rate constant k were still in the same order of magnitude as
that of pure [IMI][DNU].
Raman spectrum. The strong band at 1187 cmꢀ1 was assigned to
the symmetric stretching vibration of N1AC3AN2 bonds in the
cation. Another strong band at 1006 cmꢀ1 was associated with
the symmetric stretching vibration of N4AC4AN5 bonds in the
anion. The rocking vibrations of ANO2 were assigned at 528, 782,
and 972 cmꢀ1
.
The asymmetric stretching vibrations of
C4AN5AN6, N3AN4AC4, and C1AC2AN2 were associated at
1060, 1204, and 1455 cmꢀ1, respectively.
3.3. Non-isothermal kinetic analysis
The methods of Kissinger [25] (Eq. (1)) and Ozawa [26] (Eq. (2))
were applied to study the kinetic parameters of the energetic
materials [42–44]. The related equations were:
d½lnðb=T2mÞꢃ
dð1=TmÞ
E
R
¼ ꢀ
;
ð1Þ
ð2Þ
ð3Þ
3.5. In situ IR analysis
0:4567E
RTm
In situ Fourier transform infrared (FT-IR) was used to analyze
the thermal decomposition processes of energetic materials and
other organic compounds [19,44,54,55]. The thermal decomposi-
tion process of [IMI][DNU] was analyzed from 20 °C to 200 °C.
The curves of the characteristic band intensity for the condensed
and gas phase products with different pyrolysis temperatures are
shown in Figs. 2 and 3, respectively.
log b þ
¼ C;
ꢀ
ꢁ
Eb
RT2m
E
RTm
A ¼
exp
;
ꢀ
ꢁ
E
RT
k ¼ A exp
ꢀ
;
ð4Þ
Fig. 2 shows that the band intensity of [IMI][DNU] at 1069 and
1293 cmꢀ1 initially weakened when the temperature was increased
to 100 °C. At the same time, new bands within 2218–2371 cmꢀ1
were assigned to the vibrations of the AN@C@O group [56], which
may have been formed by the breakage of one of the NAN bonds.
The weakened intensities of these bands with increased tempera-
ture may be attributed to its further reaction. These reactions also
enabled the blue shift of the in-plane rocking vibrations of C1AH
where Tm is the peak temperature (K); R is the gas constant, 8.314
J/(mol K); b is the linear heating rate (K/min); A is the pre-exponen-
tial factor; and k is the rate constant.
Thermogravimetric (TG)/DTA analysis was carried out at the
heating rates of 2, 5, 10, 15, and 20 °C/min. The calculated results
and TG/DTA plot of [IMI][DNU] are shown in Table 3 and the ther-
mal analysis plots are listed in Supporting information. The calcu-
lated apparent activation energies were 161.6 kJ/mol by the
Kissinger method and 159.7 kJ/mol by the Ozawa method. The
average of these two values is 160.7 kJ/mol. The values of A and
k calculated with this average number were all within the
normal range of the kinetic parameters of the thermal decomposi-
tion reaction of solid materials [15].
and N2AH at 1172 cmꢀ1
. The stretching vibrations of C@O
(1710 cmꢀ1) and N@O (1592 cmꢀ1) weakened at high tempera-
tures, which meant that these groups reacted or rearranged after
the initial decomposition step. Nearly all the bands disappeared
or weakened at 200 °C, indicating that the decomposition became
complete at this temperature, in agreement with the TG results.
Fig. 3 shows nearly all the decomposition gas phase products of
[IMI][DNU]. The gas products simultaneously appeared at around
105 °C, and reached the highest band intensity at about 150 °C.
The dominant product from the [IMI][DNU] decomposition was
N2O, which can be assigned at about 2230, 2220, 1300, 1250, and
3.4. Compatibility of [IMI][DNU] with PEG10000
Compatibility is an important safety property of energetic
materials related to their production, storage and application
[45–47]. Differential scanning calorimetry (DSC), TG, and DTA
curves are effective methods for evaluating the effect of contact
Table 4
Peak temperature of the exothermic stage at different heating rates and the kinetic
parameters of [IMI][DNU] in mixture.
Table 3
ba
Tm1
Tm
D
Td
Ae
kf
b
c
Peak temperature of the exothermic stage at different heating rates and the kinetic
parameters of pure [IMI][DNU].
2
5
370
375
380
384
387
354
362
370
375
382
ꢀ16
ꢀ13
ꢀ10
ꢀ9
1.75 ꢁ 10ꢀ4
4.19 ꢁ 10ꢀ4
8.02 ꢁ 10ꢀ4
1.17 ꢁ 10ꢀ3
1.50 ꢁ 10ꢀ3
1.70 ꢁ 10ꢀ4
4.07 ꢁ 10ꢀ4
7.79 ꢁ 10ꢀ4
1.14 ꢁ 10ꢀ3
1.46 ꢁ 10ꢀ3
b
ba
Tm
Ac
kd
10
15
20
2
5
370
375
380
387
2.99 ꢁ 10ꢀ4
7.27 ꢁ 10ꢀ4
1.42 ꢁ 10ꢀ3
2.74 ꢁ 10ꢀ3
161.6
2.84 ꢁ 10ꢀ4
6.90 ꢁ 10ꢀ4
1.35 ꢁ 10ꢀ3
2.60 ꢁ 10ꢀ3
ꢀ5
g
10
20
Ek
Eo
Ea
87.9
h
i
89.4
88.7
e
Ek
Eo
Ea
f
159.7
160.7
a
b
c
d
e
f
g
Heating rate (°C/min).
Peak temperature of pure [IMI][DNU] (K).
Peak temperature of [IMI][DNU] in mixture.
change of peak temperature of [IMI][DNU] between pure and mixture.
Pre-exponential factor (sꢀ1).
a
b
c
d
e
f
Heating rate (°C/min).
Peak temperature (K).
Pre-exponential factor (sꢀ1).
Rate constant at Tm
.
Rate constant at Tm.
g
h
i
Activation energy in Kissinger method (kJ/mol).
Activation energy in Ozawa method (kJ/mol).
Average activation energy in both method (kJ/mol).
Activation energy in Kissinger method (kJ/mol).
Activation energy in Ozawa method (kJ/mol).
Average activation energy in both method (kJ/mol).
g

L. Liu et al. / Journal of Molecular Structure 1015 (2012) 67–73
71
Fig. 4. Mass spectra of [IMI][DNU].
Fig. 2. IR spectra of the condensed phase products by thermal decomposition of
[IMI][DNU] at various temperatures.
Table 5
Computed BDE (kcal/mol, 298 K, with the 6-311G+(d, p)
basis set) for initial decomposition of [IMI][DNU].
Bonds
BDE
N1(2)AH
C3AH
C1(2)AH
N3AN4
N5AN6
N5AH
112.7
126.3
125.9
57.6
47.7
92.8
C4AN5
C4AN4
88.3
87.1
that DNU and [IMI][DNU] had similar decomposition pathways
[22].
3.7. Quantum chemical computation
BDE is frequently and successfully used to measure the bond
strength and relative stability of a compound, as well as those of
the corresponding radicals [61–65]. In the present study, the geom-
etries of molecules and related radical species were optimized at
the B3LYP/6-311+G (d, p) level. The optimized structures were
demonstrated to be ground states without imaginary frequencies
(NImag = 0) with the help of subsequent frequency calculations at
the same level.
Fig. 3. IR spectra of the gas phase products by thermal decomposition of
[IMI][DNU] at various temperatures.
670 cmꢀ1 [57]. This gas may have been formed from the rearrange-
ment of anions after the breakage of one of the NAN bonds. The
multi-peaks range from 2300 cmꢀ1 to 2400 cmꢀ1 can be attributed
to the vibrations of NCO. Strong bands at 2189 and 2204 cmꢀ1 were
the absorption spectra of HCNO [58,59]. These products may have
also been formed from the breakage of the NAN bond. The broad
band ranging from 2968 cmꢀ1 to 3416 cmꢀ1 was assigned to the
vibrations of CH and NH in the cation [56].
Therefore, BDEs were calculated using the thermo-chemical
scheme shown in the following equation:
BDEðR1—R2Þ ¼ ½Df HðR1Þ þ Df HðR2Þꢃ ꢀ Df HðR1—R2Þ:
where R1–R2 stands for the neutral molecules. DfH (R1), DfH (R2),
and DfH (R1–R2) are the standard heats of formation for the mole-
cule and radicals.
3.6. Electron impact ionization (EI) analysis
The experimental results showed that with increased tempera-
ture, the bonds in the anion were most probably cleaved, and
the cation lost a hydrogen atom by hydrogen migration. The corre-
sponding BDEs were calculated, and the results are listed in Table 5.
The minimum BDE was only 47.7 kcal/mol, indicating that the
N5AN6 bond may have been the initial step in the thermolysis or
explosion in the gas phase [63], in agreement with the RSFTIR anal-
ysis results. The BDE of N1(2)AH was 112.7 kcal/mol lower than
that of the CAH bond, which meant that the m/z of 68 found from
the EI spectrum may have been formed by one of the NAH bond that
cleaved.
The mass spectral fragmentation could represent the thermal
decomposition pathway [22,58,60]. Hence, mass spectroscopy
(MS) was conducted to confirm the final decomposition products
of [IMI][DNU] (Fig. 4). The most intensive peak of m/z 68 was as-
signed to the products of iminazole (C3N2H4), which indicated that
the cation (C3N2H5) decomposed into iminazole by hydrogen
migration with temperature increase. The m/z of 69 was assigned
to the cation (C3N2H5) fragment. The m/z of 67 (C3N2H3) may have
been formed by the further delocalization of one hydrogen atom
from iminazole. The m/z of 44, 41, 40, 39, and 30 may have been
formed from the decomposition of the anion. The corresponding
products were assigned to N2O, CN2H, CN2, C2NH, and NO. Most
of them were also found in the DNU mass spectrum, which meant
Therefore, the possible decomposition pathway of [IMI][DNU]
was proposed according to Scheme 1. The thermal decomposition
of [IMI][DNU] was initiated by hydrogen migration or N5AN6 bond

72
L. Liu et al. / Journal of Molecular Structure 1015 (2012) 67–73
Scheme 1. Proposed decomposition pathway of [IMI][DNU].
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homolysis forming stable iminazole (C3N2H4) and unstable inter-
mediate derivatives (CN3H2Oꢀ and NO2ꢀ ). The intermediate
3
CN3H2Oꢀ further decomposed into the products HNCO, N2O, and
3
ꢀ
ꢀ
ꢀOH. The radicals OH and NO2 then recombined to HNO3, which
further decomposed into NO, NO2, and H2O.
4. Conclusions
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The crystal structure and thermal behavior of [IMI][DNU] were
fully investigated by experimental and theoretical methods. A
slight twist or torsion was found in the cation and anion because
of the interaction between them. This interaction also formed a
stabilized hydrogen bond network. Non-isothermal studies
showed that the average apparent activation energy from the
methods of Kissinger and Ozawa was 160.7 kJ/mol. The corre-
sponding value of [IMI][DNU] and the PEG10000 mixture was
88.7 kJ/mol. The peak temperature of [IMI][DNU] in the mixture
decreased from ꢀ16 K to ꢀ5 K with increased heating rate. In situ
IR analysis and BDE calculation results indicated that the initial
decomposition step of [IMI][DNU] was the breakage of the NAN
bond in the anion. The in situ IR of the gas phase products and
mass spectral fragmentation also showed that the main final prod-
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Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Grant No. 21076221), Innovation Fund of
Chinese Academy of Sciences (Grant No. CXJJ-11-M52), Supporting
Projects (Grant No. GH-125-080).
Appendix A. Supplementary material
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Supporting online materials provides further details about the
crystal parameters; the assignment of IR and Raman spectrum,
the thermal analysis plots. These materials can be found in the
online version. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.molstruc.2012.
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