Interaction of cis -1,4-Dinitroglycoluril and its Isomers with Magnesium-A DFT Treatment

cis -1,4-Dinitroglycoluril, known as DINGU, is an insensitive explosive. To investigate the compatibility of it and its nitramine isomers with magnesium, some model composites of them have been subjected to density functional treatment at the level of B3LYP/6-31++G(d,p). Within the restrictions of the theory and the level of calculations performed, it has been found that most of the composites considered are unstable and undergo N-NO 2 bond rupture. Moreover, it has been in one case found that the elongation of nitramine bond is dictated by the way of approach of the Mg atom to the organic component. Some physical and molecular orbital properties of the systems are obtained and discussed. 1 in the spectrum of cis -DINGU+Mg are the carbonyl C=O stretchings. It occurs at 1617 cm - 1 in the case of cis -DINGU+2Mg. Carbonyl stretchings of isomer-A+Mg happens at 1896 cm - 1 and 1810 cm - 1 . The sharp peaks of cis -DINGU+Mg', occurring at 1834 and 1684 cm - 1 are the carbonyl C=O stretchings. In the spectrum of isomer-B+Mg the C=O stretching occurs at 1864 cm - 1 . In all the cases peaks above 3500 cm - 1 belong to N-H stretchings.


Introduction
The date of synthesis of nitrourea explosives goes back to 1974. N-nitroureas are very attractive because mono-nitroureas and di-nitroureas were proved to have a good explosive performance. Some N-nitroureas such as TNGU and 1,4-dinitroglycouril (DINGU) are to be mentioned. An explosive that has been of interest to the HEMs (high energy materials) community recently is cis-1,4-Dinitroglycoluril (cis-DINGU) [1][2][3]. It is an important explosive. cis-DINGU was first prepared as early as 1888 by Franchimont and Klobbie [4,5]. vulnerability ammunition (LOVA) applications [9]. cis-DINGU is an insensitive material which has been an alternative to RDX (hexahydro-1,3,5-trinitro-s-triazine) and TNT (trinitro-toluene) [10]. cis-DINGU based polymer bonded explosives (PBXs) are prepared which possess high explosion energy, good physico-chemical stability and low vulnerability. Because of all these properties cis-DINGU is comparable to TATB (triamine-trinitro-benzene)-based PBXs. Note that the preparation of cis-DINGU is very simple and requires inexpensive starting materials as compared to TATB. Therefore, cis-DINGU based PBXs are highly preferable over TATB based PBXs [11]. Also toxicological hazards of cis-DINGU were investigated [12] and according to the classical guidelines, it would be considered only slightly toxic. The literature, contains many studies on cis-DINGU including its synthesis [6][7][8], structure determination (X-ray diffraction for cis-DINGU) [13], evaluation of the solid-state formation enthalpy [14], spectro-thermal decomposition [15], mass-spectral fragmentation pathways [16]. Some of the studies have concentrated on modelization by molecular mechanics [17] and modelization by using AM1and PM3 methods [18]. A computational study performed using DFT and ab initio methods which revealed some structural, quantum chemical and thermal properties of DINGU configurational isomers (cis-and trans-DINGU) in the gas phase [19]. Some DFT calculations even on trans-dinitroglycoluril isomers have been published [20]. However, to the best of the knowledge of the author, there is no study on interaction of metals with cis-DINGU. In the present study, interaction of magnesium with cis-DINGU isomers has been investigated within the constraints of density functional theory (DFT).

Method of Calculation
The geometry optimizations of all the structures presently considered, leading to energy minima were achieved first by using MM2 method [21,22] for the equilibrium conformer. Subsequent optimizations were achieved at Hartree-Fock level using various basis sets hierarchically. Then, the geometry optimizations were managed within the framework of density functional theory [23,24], finally at the levels of RB3LYP /6-31++G(d,p). Note that 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 [24,25]. Note that the correlation term of B3LYP consists of the Vosko, Wilk, Nusair (VWN3) local correlation functional [26] and Lee, Yang, Parr (LYP) correlation correction functional [27]. The vibrational analyses have also been done. The total electronic energies are corrected for the zero point vibrational energy (ZPE). The stationary points to energy minima were proved in all the cases by calculation of the second derivatives of energy with respect to the atom coordinates. 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 system corresponds to at least a local minimum on the potential energy surface. All these calculations were done by using the Spartan 06 package program [28].

Results and Discussion
DINGU structure has two configurational isomers, cis-and trans-DINGU. Of those cis-DINGU is in use as an explosive material.
Magnesium atom has 1s 2 2s 2 2p 6 3s 2 electronic configuration in its ground state. It is a closed shell system and donating its 3s electrons it may reduce certain organic functional groups. Magnesium atom also forms some organomagnesium compounds, such as Grignard compounds, most of them exhibit spontaneous ignition in air and makes them difficult to handle [29]. The ionization potential of magnesium is 7.61 eV and 14.96 eV, respectively for the first and second oxidation states [30].    unstable. Note that the difference between composites labeled as isomer-A+Mg and isomer-A+Mg' is the location of the Mg atom. In isomer-A+Mg case the Mg atom approaches the organic component from the embedded urea moiety which does not have any nitramine group in contrast to isomer-A+Mg' case where Mg atom approaches from the site of nitramine moieties. Isomer-B+Mg composite is unstable and undergoes N-NO 2 bond rupture irrespective of from where the Mg atom approaches. Both of isomers-A and -B cannot bear two Mg atoms just at the initial construction stage. So, their optimizations could not be carried out.  Note that the ESP charges are obtained by the program based on a numerical method that generates charges that reproduce the electrostatic potential field from the entire wavefunction [28]. In all the cases Mg atom acquires some partial positive charge. All the expelled NO 2 moieties have partial overall negative charges.  Table 1 tabulates some properties of the composites considered. The relatively high dipole moment value for isomer-B+Mg case is noticeable.   11 factor contribution but also stabilization of the charges and ionic attractions present between the decomposed organic components and partially positively charged Mg atom. Table 3 shows the HOMO, LUMO energies and the interfrontier molecular orbital energy gaps (∆ε) of the composites. Variation of the energies from one system to another in Table 3 arises from structural peculiarities of the organic component present in the optimized composites which is most of the cases a decomposed structure. Isomer-A+Mg has the smallest interfrontier molecular orbital energy gap (∆ε). Note that it is not decomposed composite however it is less stable system as compared to isomer-A+Mg' (see Table 2). Figure 7 shows some of the molecular orbital energy levels of the composites.

Conclusion
The present modeling study, within the constraints of DFT and the level of calculations performed reveals that cis-DINGU and its constitutional nitramine isomers are incompatible with magnesium. The metal transfers some electron population to the organic component causing the cleavage of N-NO 2 bond. The expelled NO 2 moiety and the remnant of the organic component possess some partial negative while the magnesium acquires the opposite charge.