3-Picrylamino-1, 2, 4-Triazole (PATO) has been widely used for blasting and destroying in the wars, owing to its relatively high density. Synthesis and characterization of PATO have been reported in literatures [1-3]. The explosion phenomenon occurs because of the PATO’s molecular structure decomposition and its oxidation reaction with the oxygen of the atmosphere [4-6]. PATO is a solid nitroaromatic compound that is synthesized by implementing nitration process on the toluene. This molecule is a toxic yellow solid with high formation heat, due to the existence of many N-N and C-N bonds in its chemical structure [7-10]. One of the most useful properties of PATO is that it can be melted and mixed with other high energy density materials safely [11-13]. Due to the fact that PATO mixtures with oxygen-wealthy compounds can release more energy in comparison to pure PATO, in the 20th century, often a mixture of PATO with ammonium nitrate was used in the construction of military explosives [14-15]. Generally, the heat released from PATO is considered as a reference for evaluating other explosives because the heat of PATO burning is generated by the reaction between carbons and the atmospheric oxygen. PATO can be considered as a green and environmental friendly material because, after its combustion excessive N2 gas will be produced in the atmosphere instead of harmful and pollutant gases which are common products of other explosives’ combustion [16-19]. Adsorption energy of explosive molecules like 1, 3, 5-Trinitro perhydro-1, 3, 5-triazine (RDX), Octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX), and so on, with graphene and boron nitride (BN) sheet have been studied by density functional theory methods. These theoretical studies demonstrated that the BN sheet binding with molecules is firmer than graphene ones [20-22]. Also, the incitement of energetic materials has been investigated by time-dependent density functional theory (TD-DFT) [23-27].
The aim of this study was to evaluate the effect of the nanostructure of B12N12 on energetic properties of PATO by density functional theory (DFT) method.
Geometrical optimizations and thermodynamic parameters calculations were done by Spartan 10 software . The calculations of systems contain C, O, and N atoms described by the standard 6-31G(d) basis set . Geometry optimization was performed utilizing with the Becke, three-parameter, Lee-Yang-Parr method (B3LYP) .
RESULTS AND DISCUSSIONS
Fig. 1(a, b, c) presents the structures of PATO, B12N12, and the products of the reaction between these molecules (I-isomer and II-isomer). Absolute energy values of these molecules are gathered in Table 1. As it can be seen, I-isomer is more stable than II-isomer.
The dipole moment values of PATO and B12N12 … PATO isomers are listed in Table 1. It can be found, the dipole moment values are increased in B12N12 … PATO isomers in compared to PATO. Dipole moment has a direct relationship with solubility in water and a substance with higher dipole moment is more soluble in polar solvents. Hence, it can be deduced that the solubility of I-isomer and II-isomer is better than pure PATO. In more details, it is obvious that the dipole moment of II-isomer is larger than I-isomer. So, II-isomer has a stronger solubility in comparison to the other one.
Density of molecules
Density values of the studied molecules are gathered in Table 1. As it is clear, there is not a considerable discrepancy between the density of the pure explosive and its derived products with B12N12. On the other hand, there are no significant changes in similar bindings of explosive properties such as the bonds between oxygen and nitrogen in nitrite groups and carbon-bonded bonds of benzene ring and nitrite groups of the compounds studied (Table 2 and Fig. 2 (a, b)). Hence, it could be expected that I and II isomers exhibit similar explosive properties to the pure PATO.
Electronic structure and thermal stability
The energy gap between the frontier orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are significant properties in numerous chemical processes. The energy values of the HOMO (EHOMO) and LUMO (ELUMO) reveals the electron donating and electron accepting characters of a molecule, respectively. It has been explored in numerous investigations that the HOMO-LUMO gap may be a significant stability index of the molecules. Exploring of the frontier orbitals is useful to illustration whether the reaction is possible or not and the relative thermal stability of an individual molecule in the gas phase. High HOMO–LUMO gap directly related to the high stability of compounds so these compounds are less reactive in chemical reactions. The energy gap, chemical hardness, chemical potential, electrophilicity and maximum transmitted charge to the system were calculated by the subsequent equations:
HOMO-LUMO gap =ELUMO - EHOMO
η = (ELUMO - EHOMO)/2
µ ≈ (EHOMO + ELUMO)/2
ω = µ2/2η
∆Nmax=- µ/ η
Table 1, lists these calculated parameters for the PATO and its derivatives with B12N12.
It can be found, the HOMO and LUMO energy levels increase, and the HOMO–LUMO gap decreases in PATO derivatives with B12N12 in compared to the single PATO. Therefore, it can be concluded that the stability of I and II- isomers are less than PATO molecule. Moreover, energy gap has a direct relationship with the electronic conductivity. Indeed, the materials with lower HOMO–LUMO gap show better conductance than the substances with higher HOMO–LUMO gap values and in this research, the electronic conductivity of I-Isomer and II-isomer is lower than pure PATO. Thus pure PATO is less conductive than its derivatives with B12N12.
Chemical hardness is the next investigated variable which can estimate the softness of a molecule. In other words, a hard molecule has a large HOMO-LUMO gap and a soft molecule has a small HOMO-LUMO gap. As it is obvious from the table, the chemical hardness of PATO has decreased remarkably from 5.235 (eV) to 2.975 and 2.595 in I and II isomers, respectively. So, after binding with B12N12 the structure of PATO has become more chemically smoother.
Electrophilicity index and maximum amount of electronic charge index were also investigated. The electrophilicity index is a measure of the electrophilic power of a compound when two molecules react with each other. One of them acts as a nucleophile, while the other one behaves as an electrophone system. A compound with higher electrophilicity index demonstrates more electrophilicity.
The most accepted electron charge can be calculated from ∆Nmax parameter. The maximum amount of electronic charge index (ΔNmax) is the most electron charge which a system can accept. A positive ∆Nmax indicates that charge flows to the system or in other words, the system acts as an electron acceptor. But a negative ∆Nmax value indicates that the system likes to donate its electrons and acts as a lewis base. As it can be observed from the table, the electrophilicity and ∆Nmax of PATO has enhanced significantly after its attachment to the studied nanostructure. In other words, its affinity for accepting electron has defused dramatically.
Owing to the fact that, the purpose of this study is to find out the effects of B12N12 on the chemical and energetic properties of PATO, It seems logical to investigate the feasibility of the reaction between the explosive substance and B12N12. In this regards, thermodynamic parameters of the reaction between PATO and B12N12 (∆Hf, ∆Gf, Kth and Cv) were calculated and checked out on the basis of the following reaction:
PATO + B12N12 → PATO… B12N12 +1/2 H2 (reaction 1)
These parameters are evaluated by the following equations (X=H and G):
ΔXf (T K)=Σ(ε0+X corr)Products–Σ(ε0+X corr)Reactants (1)
Sum of the electronic energy (ε0) and thermal enthalpies or free energy (Xcorr) = ε0 + X
ΔXf = ΔX formation
ΔXf (T K) =Σ (X) Products–Σ (X) Reactants (2)
According to the equations 1-2 can be written:
ΔHf =[X PATO… B12N12 +1/2X H2]–[X PATO + X B12N12] (3)
The thermodynamic equilibrium constant of the desired procedure was also calculated from the succeeding equation:
K = exp (- ΔG f / RT) (4)
Thermodynamic parameters of the reaction are listed in Table 2. As it can be witnessed, ∆Hf values are negative and positive for I-isomer and II-isomer, respectively. Therefore, it can be deduced that B12N12 reaction with the carbon atom of PATO is exothermic while the B12N12 reaction with the nitrogen atom of PATO is endothermic. The effect of temperature variation on ∆Hf values was also inspected in the temperature range of 300-400 K. As it is obvious from the table, by increasing the temperature ∆Hf values of the II-isomer have also risen. But by contrast, the ∆Hf values of the I-isomer have reduced proportionally by incrementing of temperature. This phenomenon reveals that the formation process of the I-isomer has become more exothermic by temperature increasing whereas the formation procedure of the II-isomer has become more endothermic by temperature incrementing. In fact, B12N12 reacts with PATO through the carbon atom (Fig. 2). There are good linear relationships between ∆H and temperature:
I-isomer: ∆H = 0.2165 T - 237.82; R² = 0.9995
II-isomer: ∆H = 0.2193 T + 507.87; R² = 0.9996
Gibbs free energy changes (ΔGf) and thermodynamic equilibrium constant (Kth) of the desired reactions are calculated and listed in Table 2. It can be found, ΔGf and Kth values for the formation reaction of the I-isomer are considerably negative. Therefore, it can be predicted this process is spontaneous. It should be noted that the temperature has a substantial influence on the formation reaction of I-isomer because the acquired results show that by enhancing the temperature the values of ΔGf and Kth have become more negative and positive respectively. Hence, temperature rising can catalyze this reaction. However, it seems the formation reaction of the II-isomer is non-spontaneous due to the achieved positive ΔGf and small Kth values (Fig. 2 (a, b)). It can be observed good linear relationships between ∆G and temperature:
I-isomer: ∆G = -0.2092 T - 183.54; R² = 0.9997
II-isomer: ∆G = -0.2105 T + 557.55; R² = 0.9991
The calculated Specific heat capacity (Cv) Values for the derivative products of PATO and B12N12 at different temperatures are presented in Table 2 and (Fig. 3). The obtained heat capacity values indicate that the heat capacity of PATO is lower than its derivatives with B12N12 in both of the I-isomer and II-isomer at all of evaluated temperatures. The specific heat capacity is the amount of heat per unit mass required to raise the temperature by one degree Celsius, so it causes low energy increases the material temperature. Less specific heat capacity values of PATO shows that energetic properties of PATO are greater than its derivations. Heat capacity is increased with increasing molecular weight which is illustrated in Table 3 and Fig. 3. Therefore, the sensitivity of PATO to the shock and heat has reduced sharply after junction to B12N12.
Researching on explosive materials is a significant challenge in front of the scholars because it can endanger the safety and health of the scientists. In addition, it needs some expensive and sophisticated instruments which are not affordable in every laboratory. But fortunately, theoretical methods can provide valuable information about this type of substance which is in a good agreement with experimental methods. Moreover, computational techniques are economical and extremely safe especially in this field of science. In this regard, the reaction of PATO with B12N12 was investigated in this research in the temperature range of 300-400 K at two configurations. The obtained ΔHf and ΔGf values were negative for I-isomer. But on the other hand theses parameters are positive for II-isomer. Therefore, B12N12 reaction with the carbon atom of PATO is experimentally feasible because it is exothermic and spontaneous. But on the other hand, the B12N12 reaction with the nitrogen atom of PATO is an endothermic and non-spontaneous. The density of pure PATO and its derivatives with B12N12 were very close to each other. Whilst, the specific heat capacity of the I-isomer and II-isomer were considerably greater than single PATO and this phenomenon is evidence which proved that the sensitivity to heat and shock of PATO derivatives with the evaluated nanostructure is lower than single PATO. Therefore, I-isomer and II-isomer have higher safety than PATO. So, PATO derived products with B12N12 has the ability to be used as an potential explosive in the construction of war weapons.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this review article.