Theoretical study on the trans–cis isomerization and initial decomposition of energetic azofurazan and azoxyfurazan
Introduction
Energetic azofurazan and azoxyfurazan compounds are very important class of furazan-based energetic materials, which have received much attention due to their favorable properties such as high energy density, low sensitivity, good heat stability, and high percentage of nitrogen content. Energetic azofurazan and azoxyfurazan compounds have found great application as an insensitive explosive [1], as well as an energetic additive to modify the properties of rocket propellant and explosive formulations [2], [3]. Unlike conventional high energy materials (e.g. nitroamine, nitroester, etc.), energetic azofurazan and azoxyfurazan compounds originate their energy from the positive heat of formation and generation of large volume of N2. A large quantity of C–N and N–N bonds and low percentage of carbon and hydrogen also endow these compounds with higher density and need of lesser amount of oxygen for combustion.
Azofurazan and azoxyfurazan compounds are made up of two aromatic furazan rings linked by an azo- or azoxy-group, which are generally produced by the oxidation of 3,4-diaminofurazan (DAF) with oxidizing agents (e.g. hydrogen peroxide, sulfuric acid, etc.). Since the 3,3′-diamino-4,4′-azoxyfurazan (DAAF) and 3,3′-diamino-4,4′-azofurazan (DAAzF) were firstly synthesized by Russian scientists in 1981 [4], some works have been focused on the properties of these energetic azofurazan and azoxyfurazan compounds, including thermal stability [1], [5], thermal decomposition [6], [7], heat of formation [8], detonation [1], sensitivity [8], and so on. Some explosive and propellant formulations containing DAAzF, 3,3′-dinitro-4,4′-azofurazan (DNAF), or 3,3′-dinitro-4,4′-azoxyfurazan (DNOAF) have also been designed and it has found that the addition of these azofurazan or azoxyfurazan compounds can evidently decrease the sensitivity of the explosive formulation [9] or improve the specific impulse of solid propellant [10], [11].
Energetic azofurazan and azoxyfurazan compounds with different substituent groups exhibit different sensitivity. For example, Lei et al. [12] have found that, measured under the same conditions, the friction sensitivity (probability of initiating an explosion) of DAAzF, DAAF, and DNOAF are 4%, 36%, and 100%, respectively. In addition, Li et al. [13] also compared the impact sensitivity (H50, the height in centimeters in which a given weight falling upon the compound gives a 50% probability of initiating an explosion) of DAAzF and DAAF. They found that the H50 value of DAAzF with 5 kg drop hammer is greater than 140 cm while the value of DAAF is 112 cm. In another paper [14], the H50 value of DNOAF was found to be only 7 cm. Clearly, the difference in the molecular structure has effect on the sensitivity for the azofurazan and azoxyfurazan compounds.
It is well known that azobenzene has two isomers: the thermally stable trans-form and metastable cis-form. These two forms can be converted reversibly into each other by heat or light irradiation [15]. Similar to azobenzene, energetic azofurazan and azoxyfurazan compounds also have the characteristic of the reversible trans–cis isomerization. In our previous work [16], we have found that a correlation exists between the sensitivity and the relative energy of trans- and cis-isomers of the energetic azo-compounds. The higher the relative energy, the lower the sensitivity. The relative energy can be regarded as one of the parameters that evaluate the sensitivity of energetic azo-compounds. However, it is still not clear that how the thermal isomerization mechanism and activation energy affect the sensitivity of these energetic azo-compounds. In addition, the initial thermal decomposition reaction is usually considered to be essential for the sensitivity of energetic compound. In the case of energetic azofurazan and azoxyfurazan compounds, the thermal trans–cis isomerization and initial decomposition belong to a pair of competing reactions. Which process dominates in the initial stage of stimulation will be determined by the activation energies of these reactions. However, little attention has been paid on the competitive relationship of initial decomposition and trans–cis isomerization.
In this work, four azofurazan derivatives were studied by using ab initio calculation based on a hybrid density functional theory. The initial decomposition and isomerization process were analyzed to explore the possible reason that energetic azofurazan and azoxyfurazan compounds have different sensitivity. Moreover, a self-desensitization effect was proposed to understand the general characteristic that most energetic azofurazan and azoxyfurazan compounds belong to insensitive energetic material.
Section snippets
Computational details
Density functional theory (DFT) has been utilized to study the energetic materials [17], [18], [19], [20]. The calculations on azobenzene, whose structure is very similar to that of azofurazan, have shown that the hybrid B3LYP/6-31G* method can accurately reproduce the experimental geometries [21], [22]. Accordingly, the geometrical parameters of DAAzF, DAAF, DNAF, and DNOAF were optimized at the B3LYP/6-31G(d) level of theory in this work. The transition state (TS) of the thermal trans–cis
Molecular structure and relative energy of trans- and cis-isomer
The B3LYP/6-31G* optimized geometrical parameters of trans-DAAzF are given in Fig. 1, where the experimental values (in italics) obtained by the X-ray crystallography are also provided [25]. It can be seen that the theoretical calculation result and the experimental observation indicate that the trans-DAAzF has C2 symmetry. The molecular backbone of trans-DAAzF is approximately a planar structure, whereas the two hydrogen atoms of the –NH2 group are out of the molecular plane because of the
Conclusion
In the present study, the thermal trans–cis isomerization and initial decomposition of DAAzF, DAAF, DNAF, and DNOAF were studied using B3LYP hybrid DFT method. Our calculation results show that, for the azofurazan and azoxyfurazan compounds, the relative energies between trans- and cis-isomer are in the following order: DAAzF > DAAF > DNAF > DNOAF. This order is reverse to the ordering of the sensitivity of these energetic compounds. In addition, it is found that the thermal trans–cis isomerization
Acknowledgment
This research was supported by the Key Project of Chinese Ministry of Education (Grant No. 209080).
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