Destructive Effect of Zinc on TEX-A DFT Treatment

Various metal components like Al, B, Zr etc., as energetic particles are employed in thermobaric explosives. In composite systems compatibility of ingredients with each other is an important point to be considered. In the present study, effect of zinc on TEX, which is a caged explosive of nitramine type is investigated within the constraints of density functional theory at the levels of B3LYP/6-31+G(d), ωB97X-D/6-31G(d) and ωB97XD/6-31+G(d). Various quantum chemical properties have been calculated for the TEX+Zn composite and compared with TEX. The zinc atom interacts with TEX molecule via destructive reduction of the explosive. The B3LYP/6-31+G(d) level of calculation predicts cleavage of one of the etheric bond of the cage as well as N-NO2 bond. Whereas, ωB97X-D/6-31G(d) and ωB97X-D/6-31+G(d) level of treatments show cleavage of only one of the N-NO2 bonds. In all the cases the zinc atom acquires some positive charge development.


Introduction
4,10-Dinitro-2,6,8,12-tetraoxa-4,10-diazawurtzitane, known as TEX (see Figure 1) stands for one of the nitramine type energetic materials which has attracted attention in recent years as insensitive and thermally stable energetic materials [1]. Moreover, its cage structure with etheric (acetal) bonds as well as two nitramine linkages makes it a candidate for various scientific investigations. The first synthesis of TEX was managed by Boyer and coworkers in two-step synthesis [2]. They started with form amide and glyoxal to obtain TEX. The synthesis involved a piperazine derivative as an intermediary 2 product [2]. TEX has two nitramine bonds and additionally contains two embedded fivemembered cyclic dietheric structures which can also be considered as acetal, thereby resembling to 1,3-dioxalane. An improved and scalable synthesis of TEX has been reported [3].
The presence of nitro groups as a part of nitramine moiety in TEX structure recalls reduction reactions of organic chemistry. Zinc in different media is often used to reduce nitro groups on aromatic structures to produce variety of compounds [9][10]13]. However, the reduction of aliphatic nitro compounds to amines is of little importance because the amino group is easily introduced in a number of different ways [13] contrary to the case in aromatic series.

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In the present study, interaction of TEX with Zn has been investigated. Note that, such kind of investigation is absent to the best of knowledge of the author.

Method of Calculations
Geometry optimizations of all the presently considered systems (TEX and TEX+Zn) leading to energy minima were initially achieved by using MM2 method followed by semi-empirical PM3 self-consistent fields molecular orbital (SCF MO) method [23,24] at the restricted level [25,26]. Subsequent optimizations were achieved at Hartree-Fock level using various basis sets hierarchically. Then, geometry optimizations were managed within the framework of density functional theory (DFT) using B3LYP [27,28] and ωB97X-D [29,30] functionals at the levels of 6-31G(d) and 6-31+G(d,p). The exchange term of B3LYP consists of hybrid Hartree-Fock and local spin density (LSD) exchange functions with Becke's gradient correlation to LSD exchange [31,32]. Note that the correlation term of B3LYP consists of the Vosko, Wilk, Nusair (VWN3) local correlation functional [32] and Lee, Yang, Parr (LYP) correlation correction functional [33]. Presently, the vibrational analyses have been also done at the same level of calculations which had been performed for the optimizations. The total electronic energies (E) are corrected for the zero point vibrational energy (ZPE) to yield E c values.
The normal mode analysis for each structure yielded no imaginary frequencies for the 3N-6 vibrational degrees of freedom, where N is the number of atoms in the system. This indicates that the structure of each molecule corresponds to at least a local minimum on the potential energy surface. All these calculations were done by using the Spartan 06 package program [34].

Results and Discussion
Zinc atom has 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 electronic configuration in its ground state. It is a closed shell system and donating its 4s electrons it may reduce certain organic functional groups. The ionization potential of zinc is 9.36 eV and 17.89 eV, respectively for the first and second oxidation states [35].     Figure 5 shows the ESP and natural charges on the atoms of TEX. Note that the ESP charges are obtained by the program based on a numerical method which generates charges that reproduce the electrostatic potential field from the entire wavefunction [34].  Figure 6 shows the electrostatic potential map of TEX (WB97X-D/6-31+G(d)). In the figure the blue and red regions stand for positive and negative potential regions, respectively. Thus, the base ring of TEX possesses highly positive potential.    After the reduction by zinc, nitrogen of the broken nitramine bond remaining in the cage structure has more negative (increased in absolute value) ESP and natural charges as compared to the nitramine bond remained intact (on the other side of the cage). The zinc atom in all the cases possesses some positive charge. The departing NO 2 moiety in all the cases has some negative over all charge. Some energies of the composite are tabulated in Table 1 and give some idea how the energies vary according to the functional and the basis set employed. Some molecular orbital energy levels of TEX and TEX+Zn composite are displayed by Figure 9. Comparison of the respective parts of the figure for TEX and the composite at the level of B3LYP/6-31+G(d) indicates that the composite has higher HOMO but lower LUMO energy levels (see also Table 2). On the other hand, the calculations based on ωB97X-D functional estimate lower HOMO but higher LUMO energy values as compared to B3LYP functional. Table 2 also includes the interfrontier molecular orbital gap (∆ε) of the composite which has the order of B3LYP/6-31+G(d) < ωB97X-D/6-31+G(d) < ωB97X-D/6-31G(d).

Conclusion
The present study within the constraints of the density functional theory at the applied level of calculations has investigated the action of zinc on TEX structure. Although some of the results are depended on the functional and basis set employed, the common point is that zinc is not compatible with TEX molecule and one of the nitramine bonds in the presence of Zn undergoes cleavage expelling a NO 2 group, irrespective of the DFT functionals and basis sets employed. The decomposition process is a reduction, the zinc atom supplies some electron population to the cage and the NO 2 group, gaining itself some positive charge. In the light of the results, it seems that zinc is not a suitable material to be used for the purpose of enhancing heat characteristics of TEX molecule.