Çaglar CELIK BAYAR, Mert BAYTAR, Kudret Batuhan OKSEM
Abstract: Introduction: An explosive of RDX, known as 1,3,5-Trinitroperhydro-1,3,5-triazine, was discovered by Henning in 1899. However, at that time Henning was not aware of that RDX was an explosive and he used it for medical purposes. Herz used RDX as an explosive in 1920. It is a white crystalline solid at melting point of 204oC. Pure RDX is so sensitive towards impact and friction that it is generally desensitized by coating the crystals with wax, oils or grease. It can also be combined with mineral jelly or similar materials to have plastic RDX explosives. RDX has a greater explosive power than TNT and picric acid (Akhavan, 2004: 41-42). The stable RDX crystals at room temperature has orthorhombic α-RDX structure. α-RDX is a stable phase of RDX at low pressures (Miller and Garroway, 2001: 2). The desired properties in modern explosives are having higher crystal density, higher standard heat of formation, higher detonation velocity and pressure and higher thermal stability while having lower sensitivity and favorable oxygen balance (Wei, Zhang, He, Shreeve, 2015: 8607-8612). Tetrazole based energetic materials are popular in this manner that these materials have a great potential to produce environmentally friendly gas nitrogen. The second advantage of tetrazole ring is having an aromatic structure which provides additional kinetic and thermal stabilities to the energetic materials it is attached. However, the tetrazole ring alone or substituted tetrazole rings are sensitive energetic materials. 5-nitrotetrazole based materials such as 5-nitro-2-H-tetrazole, 1-methyl-5-nitrotetrazole and 2-methyl-5-nitrotetrazole can be given as examples. Their sensitivities were found to be lower than 2 J. However, 5-nitrotetrazole has higher crystal density, detonation velocity and pressure than RDX while the methyl derivatives had lower values (Klapötke, Sabate´, Stierstorfer, 2009: 136-147). On the other hand, functionalization of energetic materials with tetrazole ring can provide them extra ballistic strength and insensitivity. Furazan-functionalized tetrazole salts were synthesized and the higher decomposition temperatures, impact and friction sensitivities and ballistic performances were obtained with respect to RDX (Wei, Zhang, He, Shreeve, 2015: 8607-8612). Tetrazole modified 1,3,5-triamino-2,4,6-trinitrobenzene derivatives were designed and investigated theoretically. There found to be higher crystal density, detonation velocity and specific impulse properties in addition to lower sensitivity with respect to RDX in some of these derivatives (Ma, Liao, Cheng, Fan, Huang, Wang, 2016: 639-655). Purpose: The purpose of this research is to investigate the effect of attached tetrazole ring on the properties of α-RDX explosive, computationally. Scope: First, the study was performed on α-RDX structure and then tetrazole ring attached α-RDX structure in order to compare the differences between the two in terms of insensitivity, ballistic and chemical properties. Limitations: The results of the study can be evaluated in the limitations of the theoretical method, functional and basis set used. Method: The DFT method with UB3LYP functional and 6-31G(d,p) basis set was used throughout the study (DFT UB3LYP/6-31G(d,p)) (Kohn and Sham, 1965: A1133-A1138). Gaussian 09 software was used for all the computations (Frisch et al., 2013). Findings: Contrary to expectations, tetrazole ring attachment increased the sensitivity of RDX. The weakest N−NO2 bond dissociation energy of 131,7 kJ/mol in RDX decreased to 111,3 kJ/mol in RDX-tetrazole. The results showed that the electrostatic potential around RDX N−NO2 bonds shifted to more positive region in RDX-tetrazole structure which indicated that the electron density around RDX N−NO2 bonds decreased in a significant amount in RDX-tetrazole. Consequently, this deficiency caused to more sensitive bonds. The competing ballistic performances (detonation velocity (D) and detonation pressure (P)) with respect to RDX were obtained in a new compound (D(RDX) = 9,02 km/s, D(RDX-tetrazole) = 8,89 km/s, P(RDX) = 35,82 GPa, P(RDX-tetrazole) = 35,02 GPa). The chemical hardness (η) of RDX is a little bit more than RDX-tetrazole (η(RDX) = 0,27 eV, η(RDX-tetrazole) = 0,22 eV). Additionally, the electrophilicity index (ω) of RDX is more than RDX-tetrazole showing that RDX has a more electrophilic character (ω(RDX) = 0,072 eV, ω(RDX-tetrazole) = 0,063 eV). However, the frontier molecular orbital energy ratios (EHOMO/ELUMO: fH/L) showed that RDX-tetrazole was less resistant to oxidation compared to RDX (fH/L(RDX-tetrazole) = -6,98; fH/L(RDX) = -6,46). Conclusion: Ballistically powerful, thermally more sensitive and less oxidizable energetic material was obtained via modification of RDX with tetrazole ring.

Anahtar Kelimeler: Keywords: RDX, Tetrazole, Ballistic Performance, Bond Dissociation Energy, Frontier Molecular Orbital