Molecular structure, thermal behavior and adiabatic time-to-explosion of 3,3-dinitroazetidinium picrate

Journal of Molecular Structure 981 (2010) 103–110

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Molecular structure, thermal behavior and adiabatic time-to-explosion of 3,3-dinitroazetidinium picrate Haixia Ma a,*, Biao Yan a,b, Junfeng Li a, Yinghui Ren a, Yongshi Chen a, Fengqi Zhao c, Jirong Song a,d,**, Rongzu Hu c a

School of Chemical Engineering/Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi’an, Shaanxi 710069, PR China College of Chemistry and Chemical Engineering, Yulin University, Yulin, Shaanxi 719000, PR China Xi’an Modern Chemistry Research Institute, Xi’an 710065, PR China d Conservation Technology Department, The Palace Museum, 4 Jingshan Qianjie, Beijing 100009, PR China b c

a r t i c l e

i n f o

Article history: Received 8 May 2010 Received in revised form 22 July 2010 Accepted 26 July 2010 Available online 1 August 2010 Keywords: 3,3-Dinitroazetidine (DNAZ) Picric acid (PA) Molecular structure Thermal behavior Adiabatic time-to-explosion Electron spin resonance (ESR)

a b s t r a c t 3,3-Dinitroazetidinium picrate (DNAZPA) was synthesized by adding 3,3-dinitroazetidine (DNAZ) to picric acid (PA) in methanol, the single crystals suitable for X-ray measurement were obtained by recrystallization at room temperature. The compound crystallises orthorhombic with space group P212121 and crystal parameters of a = 0.7655(1) nm, b = 0.8962(2) nm, c = 2.0507(4) nm, V = 1.4069(5) nm3, Dc = 1.776 g cm3, Z = 4, F(0 0 0) = 768 and l = 0.166 mm1. The thermal behavior of DNAZPA was studied under a non-isothermal condition by DSC and TG–DTG methods. The kinetic parameters of the first exothermic thermal decomposition process were obtained from analysis of the DSC and TG curves by Kissinger method, Ozawa method and the integral method. The specific heat capacity of DNAZPA was determined with a continuous Cp mode of micro-calorimeter and the standard mole specific heat capacity was 436.56 J mol1 K1 at 298.15 K. Using the relationship of Cp with T and the thermal decomposition parameters, the time of the thermal decomposition from initialization to thermal explosion (adiabatic time-to-explosion) was evaluated to be 40.7 s. The free radical signals of DNAZPA and 1,3,3-trinitroazetidine (TNAZ) were detected by electron spin resonance (ESR) technique to estimate its sensitivity. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Picric acid (2,4,6-trinitrophenol, PA) is a strong-acid nitrophenol compound used as an explosive in munitions, bombs and rocket warheads, and is a primary hydrolysis product of tetryl in seawater [1,2]. It can form salts of organo-alkali compound [3] as well as metal salts [4–7] owing to its acid nature. Its salts can be used as a component in explosives and detonating powders [8]. Moreover, it is well-known as TNP which is an organic nonlinear optics crystal by its shorter cutoff wavelength, optical quality, sufficiently large nonlinear coefficient, transparency in UV region and high damage threshold [9]. Highly nitrated small-ring heterocycles are good candidates of energetic materials because of the increased performance from the additional energy release upon opening of the strained ring system during decomposition [10]. Azetidine-based explosives, such as 1,3,3-trinitroazetidine (TNAZ) [11,12] demonstrate excellent performance partly because of the high strain asso-

* Corresponding author. ** Corresponding author at: School of Chemical Engineering, Northwest University, Xi’an, Shaanxi 710069, PR China. E-mail addresses: [email protected] (H. Ma), [email protected] (J. Song). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.07.036

ciated with the four-membered ring. 3,3-Dinitroazetidine (DNAZ, pKb = 6.5) is an important derivate of TNAZ and it can prepare a variety of solid energetic DNAZ salts [12–16] with high oxygenbalance [12]. In this paper, DNAZPA was synthesized and its thermal behavior was studied by DSC and TG–DTG techniques and its kinetic model function and kinetic parameters of the first exothermic thermal decomposition reaction were obtained. This is quite useful in the evaluation of the thermal stability under non-isothermal condition and in the analysis of compatibility of energetic materials. The specific heat capacity of DNAZPA was determined with continuous a Cp mode of micro-calorimeter (Micro-DSCIII). The time of the thermal decomposition from initialization to thermal explosion (adiabatic time-to-explosion) was also estimated for evaluating the safety performance of DNAZPA. Free radical (FR) is a molecule, atom or group without paired electrons and it plays an important role in the sensitivity and energization of energetic materials. The initial and propagation of detonation are usually related with the formation and torrent diffusion of free radicals. Therefore, the electron spin resonance (ESR) technique was carried out to find the free radical signals for estimating the sensitivity of DNAZPA compared with TNAZ.

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2. Experimental 2.1. Material DNAZ was synthesized and purified by a reported method [12]. Other chemicals were of analytical grade. DNAZPA used in this work was prepared according to the following method: an appropriate amount of PA was dissolved into methanol, then stirred and the same equimolar of DNAZ was added at room temperature. The brown precipitates were collected by filtration after the solution was concentrated in vacuo. Single crystals suitable for X-ray measurement were obtained by recrystallization in methanol for 10 d. A crystal with cross-dimensions of 0.40  0.50  0.45 mm3 was chosen for X-ray determination. 2.2. Apparatus and determination of crystal structure X-ray intensities were recorded at room temperature on a Bruker SMART APEX CCD X-ray diffractometer (Bruker, Germany) using Mo Ka radiation (k = 0.071073 nm) graphite monochromation. In the range of 1.99° < h < 27.61°, 9 < h < 8, 11 < k < 11, 25 < l < 20, 3146 independent reflections were obtained. The final conventional R1 is 0.0405 and xR (unit weight) is 0.1134 for 2604 observable independent reflections with reflection intensity I > 2r(I). The C, N, O atoms were obtained by direct method and the hydrogen atoms were obtained by the geometric calculation after their existences were approved by difference Fourier synthesis, the structure was refined by full-matrix least-squares on F2. 2.3. Thermal decomposition condition The DSC and TG–DTG experiments for DNAZPA were performed using a model Q600SDT (TA, USA) under a nitrogen atmosphere, at a flow rate of 100 ml min1. The sample mass was about 0.7 mg. The

heating rates used were 2.5, 5, 10 and 15 °C min1 from ambient temperature to 500 °C. The temperature and heat were calibrated using pure indium and tin particles. The DSC and TG–DTG curves obtained under the same conditions overlap with each other, indicating that the reproducibility of tests was satisfactory. 2.4. The determination of the specific heat capacity The specific heat capacity of DNAZPA was determined by a continuous Cp mode within 283–353 K at a heating rate of 0.15 K min1 on Micro-DSCIII (Seteram, France) with the sample mass of 149.89 mg. The Micro-calorimeter was calibrated with a-Al2O3 (calcined) and the math expression of Cp (J g1K1) was 0.1839 + 1.9966  103T within 283–353 K, the standard heat 1 1 capacity C H K which is p;m ða-Al2 O3 Þ at 298.15 K was 79.44 J mol in an excellent agreement with the reported value in the literature [17] (79.02 J mol1). 2.5. The detection of free radicals The free radical experiment was performed on an ESR equipment EMX-10/12 (Bruker, Germany) with illumination frequency and energy of 9.780 GHz and 19.971 mW, respectively within a scanning range of 2980–4000 G in 30 min. 3. Result and discussion 3.1. Crystal structure The bond lengths and bond angles are summarized in Table 1. The molecular structure and atom labeling are shown in Fig. 1, one-dimension monolayer structure, two-dimension monolayer structure and the packing of the molecular in the crystal lattice are illustrated in Figs. 2–4.

Table 1 Selected bond lengths (nm), bond angles (°) and dihedral (°) of DNAZPA. Bond lengths C1–C2 C1–N1 C2–C3 C2–N2 C2–N3 C3–N1 N2–O1 N2–O2 N3–O3 Bond angles C2–C1–N1 C1–C2–C3 C1–C2–N2 C1–C2–N3 C3–C2–N2 C3–C2–N3 N2–C2–N3 N1–C3–C2 C1–N1–C3 O1–N2–O2 C2–N2–O1 C2–N2–O2 Bond dihedrals O1–N2–O2–C2 O3–N3–O4–C2 N2–C2–N3–N1 C4–C5–N4–O7 C4–C9–N6–O10 C5–C4–C9–N6 C5–C6–C7–N5

0.1512(3) 0.1502(3) 0.1522(3) 0.1505(3) 0.1497(3) 0.1503(3) 0.1206(3) 0.1199(3) 0.1201(4) 89.7(2) 89.3(2) 114.1(2) 116.5(2) 114.6(2) 116.1(2) 106.1(2) 89.4(2) 90.4(2) 124.6(3) 117.3(2) 118.1(2) 177.4 179.4 179.9 174.8(2) 148.2(2) 177.5(2) 176.7(2)

N3–O4 C4–C5 C4–C9 C4–O5 C5–C6 C5–N4 C6–C7 C7–C8 C7–N5 O3–N3–O4 C2–N3–O3 C2–N3–O4 C5–C4–C9 C5–C4–O5 C9–C4–O5 C4–C5–C6 C4–C5–N4 C6–C5–N4 C5–C6–C7 C6–C7–C8 C7–C8–C9 C6–C5–C4–O5 C6–C5–N4–O6 C6–C7–N5–O9 C7–C6–C5–N4 C7–C8–C9–N6 C8–C9–C4–O5 C8–C7–N5–O8

0.1198(4) 0.1445(3) 0.1453(3) 0.1260(3) 0.1379(3) 0.1456(3) 0.1379(3) 0.1380(3) 0.1445(3) 126.8(3) 117.7(3) 115.5(3) 111.2(2) 125.5(2) 123.2(2) 124.2(2) 120.4(2) 115.4(2) 119.2(2) 121.3(2) 118.8(2) 173.3(2) 171.1(2) 163.6(19) 179.4(2) 177.6(2) 175.8(2) 161.9(2)

C8–C9 C9–N6 N4–O6 N4–O7 N5–O8 N5–O9 N6–O10 N6–O11

C4–C9–C8 C4–C9–N6 C8–C9–N6 O6–N4–O7 O6–N4–C5 O7–N4–C5 O8–N5–O9 O8–N5–C7 O9–N5–C7 010–N6–O11 O10–N6–C9 O11–N6–C9 C8–C9–N6–O11 C9–C4–C5–N4 C9–C8–C7–N5 O6–N4–O7–C5 O8–N5–O9–C7 O10–N6–O11–C9

0.1368(3) 0.1455(3) 0.1218(3) 0.1224(2) 0.1224(2) 0.1228(3) 0.1213(3) 0.1227(3)

124.9(2) 119.2(2) 115.9(2) 121.7(2) 120.1(2) 118.3(2) 123.8(2) 118.4(2) 117.9(2) 123.2(2) 117.5(2) 119.3(2) 145.0(2) 177.0(2) 174.3(2) 179.3 179.6 177.8

H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110

Fig. 1. Molecular structure of DNAZPA.

105

The crystal structure is orthorhombic with space group P212121. Crystal data: a = 0.7655(1) nm, b = 0.8962(2) nm, c = 2.0507(4) nm, V = 1.4069(5) nm3, Dc = 1.776 g cm3, Z = 4, F(0 0 0) = 768 and l = 0.166 mm1. The analytical results indicate that the molecule is made up of a  +  cation C3N3O4Hþ 6 (DNAZ ) and an anion C7N3O7H 3 (PA ). The atoms of C1, C2, C3 and N1 in the four-member ring of DNAZ+ are nearly in a plane. The planes of two nitro-groups of azetidine ring are almost vertical. All atoms in PA are nearly coplanar except for the oxygen atoms of the three corresponding nitro-groups. The bond length of C2–N2 is 0.1505(3) nm, is longer than that of the conventional C–N bond (0.147–0.150 nm) [18], suggesting that the nitro group connected with C2 will be firstly lost when DNAZPA is heated to some temperature. This is quite consistent with the former study that –NO2 on N–NO2 or C–NO2 will be firstly lost on TNAZ [19]. Due to the repulsion effect between O5 atom and nitro group in PA, the bond lengths of C4–C5 and C4–C9 are longer than those of others in the benzene ring, and the bond lengths of C5–N4 and C9–N6 are longer than that of C7–N5. The intermolecular hydrogen bonds C1–H1A  O7, N1– H1C  O5, N1–H1C  O11, N1–H1D  O5 and N1–H1D  O6 form a one-dimensional chain (Fig. 2).

Fig. 2. One-dimensional structure of DNAZPA.

Fig. 3. Two-dimensional structure of DNAZPA.

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H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110

It can be seen from Fig. 3 that a one-dimensional chain and four surrounding one-dimensional chains crosswise connect by the hydrogen bond C3–H3B  O8 to form an interlaced three-dimensional structure (Fig. 4).

where a is the conversion degree, T the absolute temperature, Ea the apparent activation energy, b the heating rate, R the gas constant, Tp the peak temperature on DSC curve, A the pre-exponential factor, G(a) the integral mechanism function.

3.2. Thermal behavior and analysis of kinetic data for the exothermic main decomposition reaction of DNAZPA

From the original data in Table 2, Ea obtained by Kissinger’s method [20] is determined to be 92.23 kJ mol1. The pre-exponential constant (A) is 108.95 s1. The linear correlation coefficient (rk) is 0.9965. The value of Ea obtained by Ozawa’s method [21] is 94.54 kJ mol1 with the linear correlation coefficient (ro) of 0.9970. By substituting the original data, bi, T0i, Ti, ai and (da/dT)i, i = 1, 2, . . . , n, tabulated in Table 3 from TG curves into Eqs. (2) and (3), the values of Ea for any given value of a in Table 3 are obtained. The average value of Ea in the a range of 0.175 to 0.875 in Fig. 7 is in good agreement with the calculated values obtained by Kissinger’s method and Ozawa’s method. The activation energy (E) values calculated using Eqs. (2) and (3) were used to check the validity of activation energy by other methods. Eqs. (4)–(7) are cited to obtain the values of Ea, A and the most probable kinetic model function [f(a)] from a single non-isothermal TG curve [23]. Mac Callum-Tanner equation

Typical DSC and TG–DTG curves for DNAZPA are shown in Figs. 5 and 6. The DSC curve indicates that the thermal decomposition of DNAZPA is composed of two exothermic processes with peak temperatures of 166.92 °C and 292.51 °C, respectively. The TG–DTG curves also show two stages of mass loss processes. The first stage begins at about 157.62 °C and completes at 184.10 °C with a mass loss of 26.98% and the second stage begins at 184.10 °C and completes at 433.84 °C with a mass loss of 50.55%. In order to obtain the kinetic parameters [apparent activation energy (Ea) and pre-exponential factor (A)] of the first decomposition reaction for DNAZPA, three model-free isoconversional methods (Eqs. (1)–(3)) were employed. These methods are as follows: Differential method Kissinger equation [20]

ln

b T 2p

¼ ln

AR Ea  Ea RT p

ð1Þ

Integral method Flynn–Wall–Ozawa (F–W–O) equation [21]

 AEa Ea  2:315  0:4567 lg b ¼ lg RGðaÞ RT

lg½GðaÞ ¼ lg

ð4Þ

Satava–Sestak equation



ð2Þ

lg½GðaÞ ¼ lg

  AEa Ea  2:315  0:4567 bR RT

ð5Þ

Agrawal equation

integral isoconversional non-linear [NL–INT] equation [22]

   n X n X bj IðEa ; T a;i Þ    nðn  1Þ ¼ min    i j–i bi IðEa ; T a;j Þ

  AEa 0:449 þ 0:217Ea 1   0:4828E0:4357 a T bR 0:001

 ð3Þ

ln

GðaÞ T2



 39 8 2 < AR 1  2 RT = E Ea 4  5  a ¼ ln :bEa 1  5 RT ; RT Ea

ð6Þ

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H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110

4

0.6

2 0 -2

0.4 DTG

0.3

0 0.2 TG

0.1

-4 -6

1

0.5

Mass (mg)

Exo

0.7

0.0 0

100

200

300

400

500

0

100

300

400

500

-1

T (°C)

T (°C)

Fig. 6. TG–DTG curves for DNAZPA at a heating rate of 10 °C min1.

Fig. 5. DSC curve for DNAZPA at a heating rate of 10 °C min1.

The General Integral equation

3   Gð a Þ AR Ea 5 ¼ ln  ln 4  bEa RT T 2 1  2RT Ea

200

Deriv.Weight (%/min)

2

6

Endo

Heat Flow (W/g)

Fig. 4. The packing diagram of DNAZPA.

2

ð7Þ

where G(a) is the integral model function, da/dt is the rate of conversion, T the temperature (K) at time t, a the conversion degree, R the gas constant. Forty-one types of kinetic model functions in reference [24] and the original data tabulated in Table 3 were put into Eqs. (4)–(7) for

Table 2 Calculated values of the kinetic parameters for the exothermic decomposition reaction for DNAZPA determined from the DSC curves at various heating rates and a flowing rate of N2 gas of 100 ml min1. b (°C min1)

Tesx (°C)

E0e (kJ mol1)

2.5 141.86 108.96 5.0 149.29 10.0 160.92 15.0 164.14 Mean: E0 = (108.96 + 92.23 + 94.54)/3 = 98.58 kJ mol1

r0e

Tp (°C)

Ek (kJ mol1)

log(Ak) (s1)

rk

Eo (kJ mol1)

r0

0.9940

145.47 153.97 166.92 172.52

92.23

8.95

0.9965

94.54

0.9970

b, heating rate ; Te, onset temperature in the DSC curve; Tp, maximum peak temperature; E, apparent activation energy; A, pre-exponential constant; r, linear correlation coefficient; subscript k, data obtained by Kissinger’s method.

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H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110

Table 3 Data of DNAZPA determined by TG at different heating rates and apparent activation energies (Ea) of thermal decomposition obtained using isoconversional methods. Data point

a

T2.5 (K)

T5 (K)

T10 (K)

T15 (K)

Eo (kJ mol1)

EN (kJ mol1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

1.000 0.975 0.950 0.925 0.900 0.875 0.850 0.825 0.800 0.775 0.750 0.725 0.700 0.675 0.650 0.625 0.600 0.575 0.550 0.525 0.500 0.475 0.450 0.425 0.400 0.375 0.350 0.325 0.300 0.275 0.250 0.225 0.200 0.175 0.150 0.125 0.100 0.075 0.050 0.025

439.59 436.78 434.06 431.52 429.03 426.96 425.38 424.47 423.82 423.33 422.93 422.55 422.21 421.89 421.59 421.31 421.04 420.79 420.58 420.36 420.14 419.91 419.68 419.44 419.20 418.94 418.64 418.34 418.01 417.62 417.16 416.63 415.83 414.63 411.38 402.73 380.64 354.81 333.04 313.26

442.75 440.72 438.64 436.82 435.08 433.60 432.52 431.87 431.36 430.93 430.54 430.19 429.86 429.55 429.25 428.96 428.69 428.43 428.18 427.95 427.71 427.48 427.26 427.02 426.76 426.48 426.18 425.86 425.51 425.12 424.72 424.42 424.08 423.42 422.59 421.28 418.98 413.74 396.80 351.22

457.23 453.18 450.51 447.47 444.92 442.72 441.21 440.32 439.66 439.12 438.64 438.20 437.78 437.37 436.97 436.59 436.21 435.85 435.50 435.17 434.85 434.55 434.25 433.94 433.62 433.29 432.95 432.56 432.12 431.63 431.06 430.39 429.49 428.19 425.93 420.45 407.28 379.71 349.90 317.53

467.34 464.34 461.18 458.25 455.65 453.26 451.31 449.78 448.87 448.31 447.86 447.46 447.11 446.77 446.46 446.16 445.87 445.59 445.31 445.05 444.80 444.57 444.35 444.12 443.89 443.65 443.4 443.12 442.81 442.44 442.04 441.55 440.93 440.07 438.86 437.49 435.08 425.43 386.46 330.29

95.21 97.43 99.28 101.47 102.37 103.84 105.43 107.94 108.94 109.11 109.19 109.16 109.10 109.08 109.02 109.10 109.10 109.02 109.21 109.29 109.31 109.21 109.07 108.91 108.72 108.48 108.10 107.83 107.54 107.24 106.80 106.60 105.84 104.11 95.76 71.34 37.64 20.44 13.82 6.44

92.86 95.25 97.22 98.88 100.54 102.11 103.80 106.45 107.51 107.70 107.79 107.77 107.70 107.69 107.64 107.61 107.61 107.65 107.85 107.94 107.96 107.86 107.72 107.56 107.36 107.11 106.72 106.44 106.14 105.83 105.37 105.18 104.39 102.59 93.88 68.32 33.1 15.4 9.18 3.74

T with the subscript 2.5, 5, 10, 15 were the temperature obtained at the heating rates of 2.5, 5, 10, 15 °C min1 respectively.

Table 4. Their values of E are very close to each other. The values of Ea and A obtained from a single non-isothermal DSC curve are in good agreement with the calculated values obtained by Kissinger’s method and Ozawa’s method. Therefore, the reaction mechanism of the first exothermic decomposition process of the compound is classified as Avrami–Erofeev equation f(a) = 3 (1  a)[ln(1  a)]2/3. Substituting f(a) with 3(1  a)[ln (1  a)]2/3, Ea with 107.92 kJ mol1 and A with 1010.90 s1 in Eq. (8),

120

Ea / kJmol

-1

100 F-W-O NL-INT

80 60

da=dT ¼ 40 20 0

0.0

0.2

0.4

α

0.6

0.8

1.0

Fig. 7. Ea–a curves for the decomposition of DNAZPA by F–W–O and NL–INT.

calculation, respectively. The kinetic parameters and the probable kinetic model function were selected by the logical choice method and satisfying the ordinary range of the thermal decomposition kinetic parameters for energetic materials (Ea = 80–250 kJ mol1, log A = 7–30 s1) together with their appropriate values of linear correlation coefficient (r), standard mean square deviation (S) and believable factor (d, where d = (1  r) S), were presented in

A f ðaÞ expðEa =RTÞ b

ð8Þ

the kinetic equation of the exothermic decomposition reaction may 10:90 be described as da=dT ¼ 310b ð1  aÞ½ lnð1  aÞ2=3 expð1:30 4 10 =TÞ. The values (Teo and Tpo) of the onset temperature (Te) and peak temperature (Tp) corresponding to b ? 0 obtained by Eq. (9) taken from reference [23] are 131.66 °C and 134.82 °C respectively.

T eo

rp

¼ T eo

or po

þ abi þ bb2i ;

i¼14

ð9Þ

where a and b are coefficients. The corresponding critical temperatures of thermal explosion (Tb) obtained from Eq. (10) taken from reference [25] are 145.00 °C and 150.61 °C, respectively.

Tb ¼

EO 

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E2O  4EO RT eo or po 2R

ð10Þ

109

H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110 Table 4 Calculated values of kinetic parameters of decomposition reaction for DNAZPA. b (°C min1)

E (kJ mol1)

Equation

log(A) (s1)

r

S

d

2.5

Mac Callum-Tanner Satava-Sestak Agrawal The General Integral

106.22 108.47 107.07 107.07

10.61 10.94 10.76 10.76

0.9833 0.9833 0.9811 0.9811

8.86  10 8.86  103 4.71  102 4.71  102

1.48  104 1.48  104 8.88  104 8.88  104

5

Mac Callum-Tanner Satava-Sestak Agrawal The General Integral

121.40 122.76 121.98 121.98

12.55 12.77 12.68 12.68

0.9905 0.9905 0.9893 0.9893

5.08  103 5.08  103 2.70  102 2.70  102

4.84  105 4.84  105 2.88  104 2.88  104

10

Mac Callum-Tanner Satava-Sestak Agrawal The General Integral

90.58 93.70 91.30 91.30

8.80 9.26 8.92 8.92

0.9917 0.9917 0.9903 0.9903

4.44  103 4.44  103 2.36  102 2.36  102

3.69  105 3.69  105 2.88  104 2.88  104

15

Mac Callum-Tanner Satava-Sestak Agrawal The General Integral Mean

110.02 112.06 110.42 110.42 107.92

11.05 11.35 11.15 11.15 10.90

0.9655 0.9655 0.9610 0.9610

1.82  102 1.82  102 9.66  102 9.66  102 –

6.27  104 6.27  104 3.77  103 3.77  103

1.45

0.0

1.35

A¼

-0.1

1.30

-0.2

1.25 1.20

-0.3

1.15 1.10 1.05 1.00

280

290

300

310

320

330

340

350

Heat Flow (mW)

1.40

Cp (J/g.K)

and A = Ak obtained by Eqs. (11)–(13) are 76.19 J mol1 K1, 92.23 kJ mol1 and 123.31 kJ mol1, respectively.

0.1

1.50

3

kB T DS– =R e h

ð11Þ

A expðEa =RTÞ ¼



kT DS exp h R

–

  DH – exp  RT

DG– ¼ DH–  T DS–

ð12Þ ð13Þ

where kB is the Botzman constant and h the Plank constant.

-0.4

3.3. Determination of the specific heat capacity

-0.5

Fig. 8 shows the determination result of DNAZPA using a continuous specific heat capacity mode of Micro-DSC apparatus. The specific heat capacity of DNAZPA presents a good quadratic relationship with temperature (T) in the determining temperature range. Specific heat capacity equation is shown as

360

T (K) Fig. 8. Determination results of the continuous specific heat capacity of DNAZPA.

C p ðJ g1 K1 Þ ¼  4:4887 þ 3:2466  102 T þ 0:4476  104 T 2  ð283 K < T < 353 KÞ 1200

The standard molar specific heat capacity for DNAZPA is 436.56 J mol1 K1 at 298.15 K.

1000

Intensity

800

3.4. Estimation of the adiabatic time-to-explosion

600

TNAZ DNAZPA

400 200 0 -200 -400 -600

ð14Þ

3000

3200

3400

3600

3800

4000

The adiabatic time-to-explosion (t, s) of energetic materials is the time of thermal decomposition transiting to explosion under the adiabatic conditions, and is an important parameter for assessing the thermal stability and the safety of energetic materials. The estimation equation of the adiabatic time-to-explosion for energetic materials is showed as Eq. (16) taken from [25,26], and t value obtained by the definite integral equation is 40.7 s, longer than that of TNAZ [27] and NTODNAZ (3-nitro-1,2,4-triazol-5-one 3,3dinitroazetidinium) [16], shorter than that of DNAZ3,5-DNSA (3,3-dinitroazetidinium 3,5-dinitrosalicylate) [14].

Frequency (G) Fig. 9. Spectra of free radical intensity of DNAZPA and TNAZ under light at 30 min.

where R is the gas constant (8.314 J mol1 K1), Eo is the value of E obtained by Ozawa’s method. The entropy of activation (DS–), enthalpy of activation (DH–) and free energy of activation (DG–) corresponding to T = Tpo, Ea = Ek

dT ¼ QA expðE=RTÞf ðaÞ dt Z T 1 C p expðE=RTÞ dT t¼ QA T 0 f ðaÞ

Cp

ð15Þ ð16Þ

where Cp as Eq. (14) expressed, and the temperature range extrapolating from 283 to 353 K; f(a), differential mechanism function f(a) = 3(1  a)[ln(1  a)]2/3; E, activation energy, 107.92 kJ mol1;

110

H. Ma et al. / Journal of Molecular Structure 981 (2010) 103–110

A, pre-exponential constant, 1010.90 s1; Q, decomposition heat, 1190.5 J g1; n, decomposition reaction order,1/3; R, the gas constant, 8.314 J mol1 K1; a, the conversion degree, and

a¼

Z

T

T0

Cp dT Q

ð17Þ

where the integral upper limit T = Tbp = 423.76 K and the lower limit T0 = Teo = 404.81 K. In the calculation process of adiabatic time-toexplosion, a little change of the activation energy can make a great difference to the result, and a small increase of the activation energy can induce adiabatic time-to-explosion to rise greatly. 3.5. The free radicals of DNAZPA Free radicals cannot be detected in normal, spin-paired molecules and the sample must possess unpaired electron spins. ESR technique is used to study radicals formed during chemical reactions or by radiation, radicals that act as probes of biological structure, many d-metal complexes and molecules in triplet states. When DNAZPA and TNAZ were put into the magnetic field under light respectively, both of them have no signal. The free radical intensity of TNAZ slightly strengthens along the time while that of DNAZPA has no obvious growth. The spectra of free radical intensity of DNAZPA and TNAZ under light at 30 min were shown in Fig. 9. From the figure, one can see that the free radical intensity of TNAZ was a bit stronger than that of DNAZPA. Therefore, both of the compounds are stable under light and TNAZ is a little more sensitive than DNAZPA in the experimental condition. 4. Conclusions (1) The compound, DNAZPA was synthesized and its structure was determined by single-crystal X-ray diffraction. The structure solution indicated that DNAZPA is an ionic compound which is made up of a cation DNAZ+ and an anion PA. (2) The thermal behavior of the title compound under the nonisothermal condition by DSC, TG–DTG methods was studied. The apparent activation energy and pre-exponential factor of the first exothermic thermal decomposition reaction are 107.92 kJ mol1 and 1010.90 s1, respectively. (3) The specific heat capacity was determined by Micro-DSC method. Specific heat capacity equation is C p ðJ g1 K1 Þ ¼ 4:4887 þ 3:2466  102 T þ 0:4476  104 T 2 (283 K < T < 353 K) and the standard molar specific heat capacity is 436.56 J mol1 K1 at 298.15 K. The adiabatic time-to-explosion was evaluated to be 40.7 s. (4) DNAZPA and TNAZ are stable under light and the former is less sensitive than the latter.

Supplementary material Crystallographic data for the structural analysis have been deposited in the Cambridge Data Center (CCDC), CCDC No. 774057 for C9 H8 N6 O11. Acknowledgments This work is supported by the National Natural Science Foundation of China (20603026), and the Provincial Natural Foundation of Shaanxi (2009JQ2002). We would like to thank Prof. Yunxia Sun (Center of Modern Analysis, Nanjing University, Nanjing, 210093 PR China) for critically discussion on the ESR experiment. References [1] S.L. Yost, J.C. Pennington, J.M. Brannon, C.A. Hayes, Mar. Pollut. Bull. 54 (2007) 1262. [2] H. Zhang, F. Chen, F. Zhao, C.M. Meng, J. Mol. Struct. (THEOCHEM) 857 (2008) 33. [3] J. Janczak, G.J. Perpétuo, J. Mol. Struct. 975 (2010) 166. [4] Y.W. Wang, W.S. Liu, N. Tang, M.Y. Tan, K.B. Yu, J. Mol. Struct. 660 (2003) 41. [5] J. Seo, M.R. Song, K.F. Sultana, H.J. Kim, J. Kim, S.S. Lee, J. Mol. Struct. 827 (2007) 201. [6] M.I. Saleh, E. Kusrini, R. Adnan, B. Saad, B.M. Yamin, H.K. Fun, J. Mol. Struct. 837 (2007) 169. [7] J.R. Zheng, N. Yang, W.S. Liu, K.B. Yu, J. Mol. Struct. 873 (2008) 89. [8] M.J. Liou, M.C. Lu, J. Hazard Mater. 151 (2008) 540. [9] P. Srinivasan, M. Gunasekaran, T. Kanagasekaran, R. Gopalakrishnan, J. Cryst. Growth 289 (2006) 639. [10] M.B. Talawar, Alok Singh, N.H. Naik, B.G. Polke, G.M. Gore, S.N. Asthana, B.R. Gandhe, J. Hazard Mater. 134 (1–3) (2006) 8. [11] T.G. Archibald, R. Gilardi, K. Baum, C. George, J. Org. Chem. 55 (1990) 2920. [12] M.A. Hiskey, M.D. Coburn, M.A. Mitchell, B.C. Benicewicz, J. Heterocycl. Chem. 29 (1992) 1855. [13] M.A. Hiskey, M.M. Stincipher, J.E. Brown, J. Energy Mater. 11 (1993) 157. [14] H.X. Ma, B. Yan, Z.N. Li, J.R. Song, R.Z. Hu, J. Therm. Anal. Calorim. 95 (2009) 437. [15] R. Gao, H.X. Ma, B. Yan, J.R. Song, Y.H. Wang, Chem. J. Chinese U. 30 (2009) 577 (in Chinese). [16] H.X. Ma, B. Yan, Z.N. Li, Y.L. Guan, J.R. Song, K.Z. Xu, R.Z. Hu, J. Hazard Mater. 169 (2009) 1068. [17] D.A. Ditmars, S. Ishihara, S.S. Chang, J. Res. Natl. Bur. Stand. 87 (1982) 159. [18] M.D. Harmony, V.W. Laurie, R.L. Kuczkowski, R.H. Schwendeman, D.A. Ramsay, F.J. Lovas, W.J. Lafferty, A.G. Maki, J. Phys. Chem. Ref. Data 8 (1979) 619. [19] Q.H. Zhao, S.W. Zhang, Q.S. Li, Chem. Phys. Lett. 412 (2005) 317. [20] H.E. Kissinger, Anal. Chem. 29 (11) (1957) 1702. [21] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1) (1965) 1881. [22] S. Vyazovkin, D. Dollimore, J. Chem. Inf. Comput. Sci. 36 (1996) 42. [23] R.Z. Hu, Z.Q. Yang, Y.J. Liang, Thermochim. Acta 123 (1988) 135. [24] R.Z. Hu, Q.Z. Shi, Thermal Analysis Kinetics, Science Press, Beijing, 2001 (in Chinese). [25] T.L. Zhang, R.Z. Hu, Y. Xie, F.P. Li, Thermochim. Acta 244 (1994) 171. [26] L.C. Smith, Thermochim. Acta 13 (1975) 1. [27] H.A. Zhao, R.Z. Hu, X.J. Wang, F.Q. Zhao, H.X. Gao, H. Zhang, X.L. Zhang, Y. Feng, H.X. Ma, Acta Chim. Sinica 67 (22) (2009) 2536 (in Chinese).