Bridged Effects of Various Heterocyclic Linkages in Bis-1,2,4-triazoles①

WU Jin-Ting XU Jin LI Hong-Bo② ZHANG Jin-Guo

a (School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China)

b (State Key Laboratory of Explosion Science and Technology,Beijing Institute of Technology, Beijing 100081, China)

ABSTRACT There are numerous studies on nitrogen-rich heterocycles explosive design and synthesis due to their good detonation activity. A series of bistriazoles with different heterocyclic linkages were designed and calculated by density functional theory (DFT) b3lyp/6-311+G* method. The structure, detonation properties and stability of the energetic compounds have been investigated. According to the results from heats of formation(HOFs), the HOF values of bistriazole with heterocycle linkage (M1～M4) are higher than those of the corresponding diamino-heterocycle bridged ones (M5～M8). By analyzing the bond dissociation energy (BDE),-NH- is not conducive to increase the stability of the derivatives. In terms of detonation performances and stability of bistriazole derivatives, the combination of furazan or tetrazole linkages with bis-triazoles may be considered as potential candidates for energetic materials.

Keywords: density functional theory, effect of heterocyclic linkage, energetic material, detonation performance; DOI: 10.14102/j.cnki.0254-5861.2011-3181

1 INTRODUCTION

Recently, a recent trend in the development of energetic materials field is replacing the traditional C–C frameworks energetic materials of nitrogen-rich heterocycles[1-3]. Indeed,heterocycles show more efficient than traditional energetic materials, and they tend to possess important energetic properties, such as good oxygen balance (OB), positive heat of formation (HOF), high density and rational sensitivity towards external forces like impact or friction[4,5].Furthermore, owing to the high nitrogen content, the decomposition product of this kind of compounds is mainly eco-friendly nitrogen gas, which makes them promising candidates as green energetic materials[6,7]. Among those heterocycles, furazan[8-10], triazole[11,12], tetrazole[13,14],tetrazine[15]and so on are normally used in modern energetic materials.

Bis-triazole is one of the most promising nitrogen-rich heterocyclic backbones in energetic fields because its properties can be controlled by the selection of various substituents on two positions of each ring[16,17]. Better detonation properties can be achieved by introducing energy-rich functional groups such as nitro (-NO2)[18,19]and nitramine (-NHNO2)[20,21]in heterocyclic bis-triazoles based energetic materials. In addition, the nitrogen-rich heterocyclic linkages can also improve the properties of this kind of compound. Thus, the combination of bistriazole backbone with other energetic heterocyclic fragments as linkage is a promising strategy to acquire materials with excellent detonation performance[22,23].

Inspired by this, we attempted to design novel energetic materials by combining varying nitro-rich heterocyclic linkages with bis-triazoles rings (Fig. 1) and their optimized geometries in the potential energy surface were obtained using density functional theory. Then, theHOFs, density,detonation properties as well as the bond dissociation energies (BDE) were calculated. The relationship between the characteristics and the structure was also discussed so as to supply some useful data for the molecular design and synthesis of new energetic materials.

2 COMPUTATIONAL METHODS

All calculations were performed by using the Gaussian 09 quantum program[24]. The DFT B3LYP/6-311+G* calculations[25]were carried out all through the geometric optimization and frequency analysis, in which all optimized structures are energy minima on the potential energy surface(no imaginary frequencies). The gas phaseHOFswere obtained via designed isodesmic reactions[26], and the condensed phaseHOFswere calculated through Hess’ law[27,28].The calculations involve HOMO-LUMO orbitals, electrostatic potential (ESP), thermodynamic parameters and BDE were implemented at the same level of theory based on the optimized structures. The empirical Kamlet-Jacob equations were famous to estimate the values of detonation velocity and detonation pressure for CHON energetic materials[29].

Fig. 1. Molecular structures of bridged bistriazole derivatives

3 RESULTS AND DISCUSSION

A series of new nitrogen-rich energetic molecules was designed by combining bis(1-nitramino-3-nitro-triazoles)derivatives with varying linkage groups (tetrazine M1,tetrazole-5-one M2, furazan M3, tetrazole M4,3,6-diamino-tetrazine M5, 1,4-diamino-tetrazole-5-one M6,4,5-diamino-furazan M7, 1,5-diamino-tetrazole M8). The optimized geometries of these new bridged bis(1-nitramino-3-nitro-triazoles) are shown in Fig. 2. The C–N and N–N bond lengths of trizole ring skeleton range from 1.23 to 1.38 ? and 1.34 to 1.39 ?, indicating these triazole rings are aromatic. The bond angles (C(1)–N(4)–C(2)) of triazole rings are between 101.7 and 107.1o, which implies a certain tension in the triazole ring. Most nitro groups on the C atom are coplanar with the triazole rings, while the nitramine groups on the N atom twist out of the triazole rings. A systematic structure-property relationship has been established in the present study. The predicted energetic properties of bridged bis(1-nitramino-3-nitro-triazoles) derivatives are compared with the benchmark explosives like TNT (2,4,6-trinitrotoluene) and RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine).

Fig. 2. Optimized geometries at the B3LYP/6-311+G* level

3. 1 Electronic structure

The electrostatic potential (ESP), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the title compounds were calculated in order to obtain their substantial electronic structures and properties.

For studies of the intermolecular interaction and sensitivities, the electrostatic potentials (ESP) of the designed compounds were obtained based on the optimized structures and visualized in Fig. 3. Positive regions (blue part) are observed energetic compounds, which mean weak bond strength that can be related to the increased impact sensitivities. As expected, the major negative regions occur in the nitramino and nitro moieties due to the lone pair electrons of oxygen atoms. The introduction of bridged ring that contents N–O bond into the bis(1-nitramino-3-nitro-triazoles)makes the ring a little more negative. However, such a difference is not obvious, which shows the difference between various bridged bis(1-nitramino-3-nitro-triazoles)towards impact stimulus is subtle. The major concentration of positive potential appears in those regions occupied by triazoles. As shown in Fig. 3, the positive values over the triazole moieties in compounds M5 and M8 are higher than the other ones, thus reflecting these two compounds may be sensitive to impact.

Fig. 3. Electrostatic potentials of studied compounds, ranging from –0.111 a.u. (red) to 0.111 a.u. (blue)

The HOMO-LUMO orbitals of the title molecules are illustrated in Fig. 4, while the red and blue colors denote the positive and negative phases, respectively. Both the HOMO and LUMO orbitals of M1 are contributed by the tetrazine bridged group. Conversely, the HOMOs of molecules M5～M8 occupy the whole molecule except the nitro groups, while the LUMO of M5 is mainly contributed by the linkage group,that of M6 occupies one 1-nitramino-3-nitro-triazole and that of M7 comprises two 1-nitramino-3-nitro-triazoles. The HOMO of M8 is made up of bridged group and triazole which in the right part of the molecule and LUMO comprises bridged group and another triazole. The values of energy gap between HOMOs and LUMOs of the title compounds range from 3.66 to 4.59 eV, which is relative to a comparably stable range.

Fig. 4. HOMO-LUMO orbitals of the studied compounds

3. 2 Detonation performance

In order to quantitatively evaluate the detonation performance of studied structures, the predicted density (d),solid phase enthalpy of formation (?fH298K(s)), oxygen balance (OB) and detonation detonations (Q,D,P) were calculated systematically, with the results listed in Table 1.

Table 1. Properties of M1～M6 Compared with TNT and RDX

Table 1 shows that the densities of the studied compounds are ranging from 1.83 to 1.87 g/cm3, higher than that of RDX.This may be attributed to the incorporation of nitro groups into the designed molecule species. Due to the high nitrogen content in the molecule, the series of the title compounds exhibits strongly positive solid phase enthalpy of formation(743.6～978.6 kJ/mol) that is a typical feature for most high-energy-density compounds.

These new compounds possess high heats of detonation,most of which are above 1400 cal/g, and compound M5 has the highest value of 1567.5 cal/g, which indicates that the diamino-tetrazole bridged group can effectively increase the heats of detonation of the system. All the studied compounds possess calculated detonation velocities and detonation pressures which are superior to those of TNT. For M3 and M4, the detonation parameters are comparable to the RDX explosive. Taking into account the density and impact sensitivity, compounds M3 and M5 illustrate their high value of application.

3. 3 Thermal stability

The weakest bond in a molecule is widely regarded as the“trigger bond”, and its breaking is the common primary step of energetic materials toward external heat, impact, and electric spark stimulus[30,31]. Generally, the smaller the energy needed for breaking a bond, the weaker the bond. The BDE of the trigger bond is obtained to evaluate the thermal stability of energetic compounds, which reflects their practicability. For the compounds studied, two possible bond dissociations were considered: (1) the C–NO2bond in the side chain; (2) the N–NHNO2bond in the side chain. Table 2 presents the BDEs of the relatively weaker bonds of the compounds studied. Based on our calculations, all of the bond dissociation energies are far larger than 120 kJ/mol,which indicated the molecules designed meet the stability requirement of a practical energetic material. In general, the stability of C–NO2bond is not affected by changing different linkages, whereas, the BDE value of N–NHNO2bond varied with the bridged groups. When the heterocyclic linkages with amino group, the nitramino groups are less stable, suggesting the whole molecules have lower stability. Thus, introduction of -NH- tends to decrease the stability of the derivatives that may be due to the longer bridged group.

Table 2. Calculated Bond Dissociation Energies (BDE, kJ/mol)

4 CONCLUSION

In this work, eight novel bis(1-nitramino-3-nitro-triazoles)compounds were designed by introducing various heterocyclic linkages into the molecule. Then, their heats of formation, energetic properties, electronic structure, molecule stability and impact sensitivity were investigated using the DFT B3LYP/6-311+G*. Based on our calculations, the adding of functional groups and nitrogen-rich heterocyclic linkages show positive influence onHOFof the title molecules. All of the studied compounds possess positiveHOFswhich confirm their energetic nature. In addition, the heterocyclic linkages have more favorable effect on the detonation performance than the corresponding diaminoheterocyclic ones. By analyzing BDE of the weakest bond of the designed compounds, the BDE values are much higher than 120 kJ/mol, with relatively ideal thermal stability.However, the addition of amino groups is not conducive to improve the stability of derivatives. In terms of detonation performance and stability, compounds M3 and M4 yield promising results and illustrate their high value of application.