JIA Jing-Xin PANG Yu YANG Jing② LI Min-Xin MENG Xing-Jun GAO Xio-Zhen LIU Li-Hu LIU Meng-N

a (Department of Chemistry, Tangshan Normal College, Tangshan 063000, China)

b (Yangquan Municipal Key Laboratory of Quantum Manipulation,Shanxi Institute of Technology, Yangquan 045000, China)

ABSTRACT The geometric and electronic structures of the derivatives of 4-nitro-5-(5-nitroimino-1,2,4-triazol-3-yl)-2H-1,2,3-triazolate (named A～J) are explored employing density functional theory (DFT) calculations at the B3LYP/6-311G** level of theory. Based on the optimized molecular structures, the heats of formation (HOF) are obtained, and the electronic properties, density and molecular sensitivity by characteristic heights (H50) are discussed. Besides, the detonation performances (detonation velocity, detonation pressure) are estimated via Kamlet-Jacobs (K-J) formula. Compounds B (H50 = 29.4 cm, ρ = 1.91 g/cm3, Q = 1563.04 cal/g, P = 36.05 GPa, D= 8.95 km/s) and H (H50 = 31.9 cm, ρ = 1.80 g/cm3, Q = 1610.09 cal/g, P = 37.31 GPa, D = 9.12 km/s) have positive HOFs and remarkable insensitivity and good detonation performance, strongly suggesting them as the acceptable new-type explosive. The initiating power surpasses conventional primary explosives, such as HMX. The outstanding detonation power of compounds B and H contributes to its future prospects as a promising green primary explosive.

Keywords: 4-nitro-5-(5-nitroimino-1,2,4-triazol-3-yl)-2H-1,2,3-triazolate, high energy density materials,density functional theory, explosive; DOI: 10.14102/j.cnki.0254-5861.2011-3143

1 INTRODUCTION

High energy density materials (HEDCs) are extensively used in the fields of rocket propulsion systems, gas generators,and explosives[1-10]. It’s known that HEDCs should exhibit excellent detonation performance and favorable stability or low sensitivity. It is a pity that the requirements of low sensitivity and high energy are inconsistent with each other,which makes the development of new HEDCs a challenge[11].Recently, conventional explosives including octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and cyclo-1,3,5-trimethylene-2,4,6-trinitr-amine (RDX) are widely used as a secondary explosive in the military. However, these typical explosives face similar environmental pollution problems in terms of composition and decomposition products[12]. Therefore, developing green decomposition products as a substitute for classic explosives is an exceedingly good choice. According to most studies,nitrogen-enriched compounds are the choice for synthesizing modern HEDMs as they fulfill most of the requirements, such as high positive heat of formation and high density, while generating environmentally friendly nitrogen gas upon detonation. The molecular framework of nitrogen-enriched compound consists mainly of nitrogen-based heterocyclic rings, such as triazine, tetrazine, triazole, tetrazolium, and other heterocyclic rings[13-15].

Many triazole derivatives have been studied extensively since triazole was first described[16-18]. Meanwhile, extensive efforts have been made to develop new HEDCs based on triazole in heterocyclic energetic materials due to their high and positive heat of formation. Nowadays, Yang research group conduct the study on the synthesis of different neutral compounds consisting of 1,2,3-2H-triazole and 1,2,4-triazole rings carrying energetic moieties like amino, nitroimino, nitro as well as azo, but unfortunately their detonation performances need to be improved[19]. Thus, we choose the promising 4-nitro-5-(5-nitroimino-1,2,4-triazol-3-yl)-2H-1,2,3-triazolate in these neutral compounds as the starting material to develop new high energy density materials by the introduction of popular high-energy group, such as -NO2, -NH2, -NHNO2,-NHNH2, -N(NO2NH2) and -N3groups. Based on density functional theory (DFT), we designed ten molecules (Fig. 1)and evaluated their detonation performance, electronic properties and sensitivity. We sincerely hope that the exploration and calculation of triazole bicyclic derivatives will provide useful information for the development of HEDCs in the future.

Fig. 1. Derivatives of 4-nitro-5-(5-nitroimino-1,2,4-triazol-3-yl)-2H-1,2,3-triazolate designed in this paper

2 CALCULATION METHODS

where (SA) is the molecular surface area for this structure,σ2totis interpreted as an indicator of the variability of the electrostatic potential on the molecular surface, andνis described as showing the degree of balance between the positive and negative potentials on the molecular surface.

The significant parameters of an explosive, including the detonation velocity and pressure, were determined by the

Table1.Calculated Methods for the Values of N,?,and Q of Explosive CaHbOcNd.Is the Molecular Weight ing/moland?the Solid Phase HOF inkcal/mol

Table1.Calculated Methods for the Values of N,?,and Q of Explosive CaHbOcNd.Is the Molecular Weight ing/moland?the Solid Phase HOF inkcal/mol

Parameters Explosives components conditions c≥2a+b/2 2a+b/2>c≥b/2 b/2>cimages/BZ_7_423_1376_464_1417.png (b+2c+2d)/4M (b+2c+2d)/4M (b+d)/2Mimages/BZ_7_423_1430_468_1472.png4M/(b+2c+2d) (56d+88c-8b)/(b+2c+2d) (2b+28d+32c)/(b+d)Q*10-3 (28.9b+94.05a+0.239images/BZ_7_909_1544_988_1592.png )/M[28.9b+94.05(c/2-b/4)+0.239images/BZ_7_1269_1543_1350_1592.png]/M (57.8c+0.239images/BZ_7_1907_1515_1997_1567.png)/M

Impact sensitivities (H50) were also estimated as it is an important parameter for predicting the safety of an energetic material during use or storage. For all compounds, impact sensitivity was calculated by a simple method recommended by Pospíšil et al.[32].

Another indicator of explosives, oxygen balance (OB100), is a parameter used to describe the degree of which an explosive can be oxidized. For a compound with molecular formula CaHbOcNd, the oxygen balance can be represented as equation(9)[33].

3 RESULTS AND DISCUSSION

3. 1 Heats of formation

The heats of formation (HOF), frequently referred to be indicative of the “energy content” of high energetic compounds, is substantial for calculating the detonation performances of energetic compounds. In this work, for convenient discussion, all derivatives are set as derivatives A～L displayed in Fig. 1. Table 2 presents the calculated total energies (E0, a. u), zero-point energies (ZPE, a. u), values of thermal correction (HT, a. u) and HOF (kJ/mol) of the derivatives. All compounds possess positive HOFSin the range of 137.86 to 996.11 kJ/mol. At the same time, the values of all HOFs are significantly higher than that of RDX (95.14 kJ/mol) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB =102.67 kJ/mol)[34]. With the presence of a large number of N-N or C-N bonds in these heterocyclic compounds,especially the two nitrogen-enriched rings linked by C-C bond have the highest heats of formation spanning 996.11 kJ/mol (compound G), exceeding that of hexanitrohexaazaisowurtzitane (CL-20 = 397.8 kJ/mol)[34].This indicates derivative G is more energetic but more unstable thermodynamically. However, kinetic stability is more important than thermal stability for high energy density molecules. Compared with nitroso compound (B), amino compound (A) has higher heats of formation. What’s more,for these neutral compounds, when the -NHNO2group is converted to -NHNH2, the heat of formation increases 164.06 kJ/mol from compound D to C. When the methoxy group is substituted by azido, the heat of formation for E increases to 996.11 kJ/mol, but unlike with other energetic groups, the introduction of -NNO2ONO2lowers the heat formation. In general, comparing all derivatives, the presence of azido group gives rise to higher heats of formation than other groups. Thus we can conclude that -N3is the superior group for increasing heat of formation, followed by the -NHNH2and-NNO2NO2. The general trends of heats of formations of HOFs in all substituted derivatives are arranged as -N3>-NHNH2> -NNO2NO2> -NH2> -NNH2NO2> -NHNO2>-NO2> -OCH3> -ONO2> -NNO2ONO2.

Table 2. Total Energies (E0), Zero Point Energies (ZPE), Thermal Correction Values (HT), and Heat of Formation(HOF) of All the Derivatives at the B3LYP/6-311G** Level of Theory Compared to RDX and TATB

3. 2 Electronic properties

Table 3. Energies of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO),and Energy Gaps ( ΔEL UMO-HOMO) for All Derivatives at the B3LYP/6-311G** Level of Theory.The Impact Sensitivity (H50) Is also Shown and Compared with the RDX, HMX, and CL-20

3. 3 Impact sensitivity

In addition to the heat of formation and electronic properties, predicting the impact sensitivity of new organic molecules which are candidates to be energetic materials has been the topic of several studies in the past few years. The impact sensitivity can be evaluated by using characteristic height (H50). Pospíšil et al. have correlated the explosive characteristics of an energetic material to the electrostatic potential of the molecule and put forward an empirical formula relatingH50and electrostatic potential of the molecule, as given by equation (8). Table 4 shows a comparison of the impact sensitivity (H50) for derivatives with experimental and calculatedH50parameters for commonly used explosives. It can be seen from the table that all the explosive molecules exhibit relatively acceptable impact sensitivities withH50values ranging from 24.5 to 42.3 cm,which means all derivatives surpass CL-20 (12.0 cm) and RDX (26.0 cm) except for new molecule J[33,37]. Among them,compound A displays very high impact sensitivity (TheH50value is 42.3 cm). In contrast, A is much higher than HMX(32 cm)[37]. More importantly, derivatives A, C, E and I display lower sensitivity than the parent (H50= 32 cm). This may suggest that properly replacing the hydrogen atom by high-energy group in explosives with super high energy may be a useful way to make them less sensitive without reducing too much energy.

3. 4 Detonation performance

Apart from sensitivity issues, the detonation performance of energetic materials is also of critical importance.Detonation velocity (D) and pressure (P) are elementary parameters for estimating the detonation performance of HEDMs. In order to assess the detonation velocities (D) and detonation pressures (P) of unsynthesized energetic complexes, we used the method based on K-J method equations (5) and (6), which has been widely used to predict the pressures and detonation velocities of many energetic complexes. The detonation velocity (D) and detonation pressure (P) are bound up with density (ρ) and heat of detonation (Q). Thus, Table 4 collected the predicted crystal densities (ρ), detonation velocities (D), detonation pressure (P)and heat of detonation (Q) of the title derivatives. For comparison, theρ,DandPof the well-known explosives RDX and HMX are also listed in this table.

The densities of prepared compounds fall in the range of 1.56～1.91 g/cm3. We noticed that the actual densities of compound A may be higher, because -NH2and -NO2groups of adjacent molecules can be electrostatically attracted each other and pull the molecular units closer together in the condensed phase. Besides, the densities of molecules B, D, F,and J are higher than RDX (1.82 g/cm3)[38], meeting the standards for high-density-energy materials (HEDMs). As summarized in Table 4, compound B possesses the highest density of 1.91 g/cm3, which is over HMX (1.90 g/cm3)[38].Similar to density, these new compounds exhibit high detonation heat (Q), and compounds B and H have the high values of 1563.04 and 1610.09 cal/g, indicating that the NH2and NO2groups can effectively increase the detonation heat of the system. By comparing physicochemical properties of the parent and derivatives (Table 4), we found that the introduction of one nitro to replace the hydrogen atom can greatly improve the density, which proved the validity of our original hypothesis. In our work, molecule B is inferior to H in view of its detonation heat (Q= 1563.04 cal/g), detonation velocity (D= 8.97 km/s) and detonation pressure (P= 36.05 GPa). It is ascribed to the less nitrogen content in compound B than in H. Thus, quite promisingly, with the increased nitrogen content, the molecules have much betterD,PandQvalues. At the same time, the detonation pressure and detonation velocity of molecule H are higher than that of the parent compound. The calculatedDandPsuggest that it has the potential to be a high-performance energetic material. Moreover, the cyclic organic molecule with polynitro functionality may be ideal energetic materials that can solve the long-standing inherent contradiction between detonation performance and stability.

Table4. Predicted Density,ExplosiveHeats,Detonation Pressure and DetonationVelocityofAllDerivatives t ogether with RDX and HMX(IsMolecularDensity,, andRectificatory DensityValueis).Oxygen Balance (OB) and Nitrogen Content Are also Calculated at the B3LYP/6-311G** Level of Theory

Table4. Predicted Density,ExplosiveHeats,Detonation Pressure and DetonationVelocityofAllDerivatives t ogether with RDX and HMX(IsMolecularDensity,, andRectificatory DensityValueis).Oxygen Balance (OB) and Nitrogen Content Are also Calculated at the B3LYP/6-311G** Level of Theory

eData from Ref. [19].fData from Ref. [38].

Compound images/BZ_10_649_1485_699_1527.png(g/cm3) images/BZ_10_852_1482_893_1527.png(g/cm3) Q(cal/g) P(GPa) D(km/s) OB100 N%Parente 1.88 36.20 9.07 A 1.78 1.79 891.70 24.11 7.40 -1.56 54.69 B 1.84 1.91 1563.04 36.05 8.95 0.70 48.95 C 1.69 1.72 878.41 22.03 7.18 -1.85 56.83 D 1.83 1.83 1036.39 28.30 7.95 0.33 51.16 E 1.55 1.56 1126.10 19.60 6.97 -2.06 40.33 F 1.85 1.85 1151.73 30.68 8.25 1.32 46.36 G 1.76 1.74 906.91 23.90 7.39 -0.71 59.57 H 1.83 1.80 1610.09 37.31 9.12 0.00 53.16 I 1.82 1.78 1320.83 32.52 8.54 1.73 48.55 J 1.90 1.90 1218.66 34.39 8.66 2.21 46.41 RDXf 1.82 1345.57 39.00 9.10 -21.61 37.84 HMXf 1.91 1343.81 34.00 8.75 -21.61 37.84

Oxygen balance (OB) is another important index to evaluate the deficiency or excess of oxygen in a molecule required to convert all carbon into carbon dioxide and all hydrogen into water. As can be seen from Table 4, arising from the multiple nitro and nitramino functionalities,derivatives B, D, F, I and J have positive oxygen balances,whereas the OB value of H is zero. Other compounds exhibit negative OB values ranging from -2.06% to -0.71%. It is important to note that the OB value of compound H is zero,which indicates that it can fully use its chemical energy.Notably, the detonation performance (Q= 1610.09 cal/g,D=9.12 km/s) of derivative H is much superior to the typically high explosives including RDX (Q= 1345.57 cal/g,D= 9.12 km/s)[38]. This is due to conjugate action and intramolecular hydrogen bonds in the system, and the high nitrogen content is also good for improving detonation performance. For energetic propellant fuels, high nitrogen content is advantageous for smokeless combustion. The nitrogen content of all derivatives ranged from 40.33% (compound E) to 59.57% (compound G), which is higher than those of RDX and HMX (N% = 37.84%). What’s more, the excellent heat of formation of compound G is attributed to their high nitrogen content (N% = 59.57%), as well as the large number of high-energy bonds (C-N, N-N and N-O bonds), which is due to the unique structure. At the same time, the derivatives of title compounds are suggested to have potential for further study.

4 CONCLUSION

In this work, a series of novel nitrogen-enriched energetic derivatives with dicyclic structures have been designed. And the density, heat of formation, electronic properties, properties of detonation and impact sensitivity were calculated using the DFT method at the B3LYP/6-311G** level of the theory. In the design of new energetic materials, these compounds of nitrogen-rich heterocycles exhibit some favorable features:

(1) Our results showed that all derivatives possess positive HOFs. This changing trend of HOFs in substituted can be arranged in the sequence as -N3> -NHNH2> -NNO2NO2>-NH2> -NNH2NO2> -NHNO2> -NO2> -OCH3> -ONO2>-NNO2ONO2. Quite noteworthy in this respect is molecule G with the most positive enthalpy of formation (996.11 kJ/mol)than that of other common energetic compounds and a large enthalpy of formation are very beneficial to the detonation heat.

(2) The triazole-based materials have relatively high stability due to the appropriate value of energy separation(ΔELUMO-HOMO=0.1167～0.2026 a.u.) between the HOMO and LUMO. And this allows the molecular orbitals to overlap to have a delicate electronic communication conjugation,which is a sign of the intramolecular charge transfer from the electron donating group through theπ-conjugation system to the electron accepting group.

(3) The impact sensitivity (H50) of the new compounds fall in the range of 24.5 to 42.3 cm which is less sensitive than CL-20 (12.0 cm). The results indicate that these derivatives have preferable thermal stability. Aside from derivatives D, E,F and J, theH50of the new organic molecules is higher than those for RDX and HMX, which implies that the sensitivities of the new compounds are lower than those of RDX and HMX. The stability of energetic materials is very momentous because they need long-term preservation and adaptation to various environments.

(4) The predicted results for these newly designed compounds exhibit good detonation performance (especially compounds B and H). Besides, compounds B and H (1.91 and 1.80 g/cm3) also show higher density than TATB (1.79 g/cm3)[39]and are comparable with HMX (1.91 g/cm3) and RDX (1.82 g/cm3)[38]. It is a remarkable fact that the nitrogen contents of all compounds have high nitrogen contents between 40.33% (molecule E) and 59.57% (molecule G).Compounds bearing high nitrogen content in compounds often cause energetic materials to exhibit good performance and show great potential as additives in gas generators, as insensitive ammunition, and as smoke-free pyrotechnics.

Based on the above results, we can conclude that organic compounds B (H50= 29.4 cm,ρ= 1.91 g/cm3,Q= 1563.04 cal/g,P= 36.05 GPa,D= 8.95 km/s) and H (H50= 31.9 cm,ρ= 1.80 g/cm3,Q= 1610.09 cal/g,P= 37.31 GPa,D= 9.12 km/s) can be considered a potential candidate for an HEDM.Our observations indicate that the combination of triazole derivatives and oxygen balance to zero is a very effective way to obtain potential energetic compounds with outstanding detonation performance. These compounds present good explosive potentials and are worthy of synthesis and further investigation. Our results should also provide some useful information for the molecular design of novel HEDCs.