Theoretical Insight Into The Explosive Sensitivity Of Several Explosive In External Electric Field

It has been anticipated for a long time that the introduction of external electric field into energetic material can increase the energy content of the conduction gases in the vicinity of detonation front, and consequently accelerate the detonation velocity and lead to an increase of detonation pressure. Therefore, there has been the work devoted to the investigation on how to add the external electric field into energetic material system. The actual experimental work in which the external electric field is imposed to energetic material system is hazardous. Except for the dangers from the experimental operations and equipments(e.g., strong electric field installations), high energy molecules may be essentially more unstable in the presence of the external electric fields. For example, the “trigger linkage”, the breaking of which is a key factor in the initiation of detonation, may be easier to be opened due to the reduced C–NO2, N–NO2 or O–NO2 bond dissociation energy in external electric field in comparison with that in no external electric field. Thus, the sensitivity may be increased in the external electric field. In fact, trigger linkage rupture is often a dominant factor in determining sensitivity, and the explosive sensitivity of nitro explosive is closely related to the detonation initiation, i.e., the first step in the decomposition process of explosive.In general, the weaker the trigger linkage or the lower the activation energy, the more easily the detonation initiation occurs, leading to the higher explosive sensitivity. Furthermore,recently many studies have indicated that the design of cocrystal explosives is one of the most promising approaches to decrease sensitivity by intermolecular interactions and maintain detonation performance of existing explosives. Therefore, recently much attention has been paid to the investigation on the synthesis, characterization and application of cocrystal explosives. It is extremely urgent to further reduce sensitivity for cocrystal. Some prototypical explosives, such as the modeling explosives CH3NO2, NH2NO2, CH3ONO2 and their complexes, and some cocrystal explosives, such as HMX/FOX-7, HMX/NQ and CL-20/FOX-7, has attracted lots of attention in theoretical and experimental investigations.This investigation can help us understand the initiation mechanism of energetic materials in external electric field. It must be useful to design rationally experimental equipment and add efficiently electric field to energetic material as well as avoid the catastrophic explosion in external electric field.In this work, a comparison of the effect of external electric field on the C/N–NO2 bond with the C/N–H or N–O bond in CH3NO2 and NH2NO2 was firstly carried out using the DFT-B3 LYP and MP2 methods with the 6-311++G(2d,p) and aug-cc-p VTZ basis sets. The result shows that the fields have a minor effect on the C–N or C–H bond but a major effect on the N–O bond in CH3NO2, while in NH2NO2 the fields greatly affect the N–N bond but the N–O or N–H bond is slightly affected. Thus, in CH3NO2 oxygen transfer or unimolecular isomerization to methyl nitrite might be prior to the breaking of the C–N bond in the initial stage of decomposition, and in this case, the N–O bond can be the possible trigger bond in electric fields. In NH2NO2, however, the N–N bond rupture may be preferential in electric field and consequently the N–N bond might be always the real trigger bond. The analyses of AIM(atoms in molecules), NBO delocalization, shifts of electron densities, APT charges and frequencies support the above viewpoints. Sixty-nine good linear correlations were found along the different field orientations at the different levels of theory, including those between the field strengths(E) and the changes of the N-O/N-N bond length(ΔRN-O/N-N), ρ(N-O/N-N)values(Δρ(N-O/N-N)), APT charges in C, O, N or nitro group(ΔQC/O/N/NO2), or stretching frequencies of the N-O/N-O bond(Δ?N-O/N-O).The effects of external electric field on the detonation initiation reaction dynamics of TNT(C-NO2 �' C-ONO, intermolecular hydrogen transference and intramolecular hydrogen transference) were investigated by the MP2 and CCSD methods, and a comparison of the effect of external electric field on those in CH3NO2 was also carried out. The results show that a new bimolecular intermolecular hydrogen transference mechanism, from a side-by-side geometry TNT dimer formed by two intermolecular H-bonding interactions to a side-by-side geometry aci-TNT dimer, is the most favored bimolecular reaction pathway. Furthermore,this pathway occurs in the most competition with the C–NO2 bond rupture at the initial stage of TNT decomposition in no external electric field. For three reaction paths, some linear correlations between the changes of bond lengths(or electron densities) and field strengths in different field orientations are obtained. In particular, when the field strength is larger than+0.0060 a.u. in the +z-direction(along the C-NO2 bond axis), or the field is in the-y-direction of dimer(the orientation perpendicular to the O???N???N plane), the barriers of the isomerization or intermolecular hydrogen transference are lower than the C–NO2 bond dissociation energies. Therefore, when the field strength is larger than +0.0060 a.u. in the+z-direction, the isomerization reaction occurs preferentially over the C–NO2 bond scission,and the field in the-y-direction causes preference of the intermolecular hydrogen transference reaction over the C-NO2 bond dissociation. In the-y-direction, the explosive sensitivity of TNT can be evaluated by the strength of the C–H bond and the C–H bond may be trigger linkage in external electric field. Furthermore, in these two cases, the explosive sensitivities are increased in comparison with those in no external electric field. Therefore, the field strengths should be controlled under +0.0060 a.u. in the +z direction of TNT or the field should not be in the-y-direction of dimer in experimental work. However, in the other oriented fields, the barriers of three reaction paths are all higher than the C–NO2 bond dissociation energies. Thus, the fields do not induce the change of the reaction pathways relative to those in no external electric field and the C–NO2 bond scission is kinetically favored in initiating detonation. In these cases, the explosive sensitivities of TNT can be evaluated by the strength of the C–NO2 bond and they are almost equal to those in no external electric field. All those results are similar those in CH3NO2. The change of the directions and strengths of external electric fields can change the reaction pathways and sensitivities.Therefore, the introduction of external electric field into energetic material will be an available way to adjust explosive sensitivity. This theoretical investigation can help us understand the initiation mechanism of more complex energetic materials in external electric field. It must be useful to design rationally experimental equipment, and add efficiently and safely external electric field into energetic material to increase the energy, detonation velocity and detonation pressure.The effects of external electric field on the detonation initiation reaction dynamics of RDX???H2O(intramolecular hydrogen transference and intermolecular hydrogen transference)were investigated by the MP2 and CCSD methods, and a comparison of the effect of external electric field on the detonation initiation reaction dynamics of NH2NO2???H2O was also carried out. In the x-, y-, and z-oriented fields, the barriers of the intramolecular hydrogen transference and intermolecular hydrogen transference reaction paths are all lower than the C–NO2 bond dissociation energies. The barriers are increased and the sensitivities are decreased in the-x-,-y-, and-z-oriented fields. When the field strength is larger than-0.0130 a.u. in the z-direction, the barriers of the intramolecular hydrogen transference reaction are lower than those of intermolecular hydrogen transference reaction. In this case, the intramolecular hydrogen transference reaction occurs preferentially, and the sensitivities are further decreased. The change of the directions and strengths of external electric fields canchange the reaction pathways and decrease the explosive sensitivity. Thus, the actual experimental works imposing the external electric fields to energetic materials in the x-orientation are as safe as those in no external electric field. When the field strength is lower than-0.0130 a.u. in the y- or z-directions, similar to that in no field, intermolecular hydrogen transference reaction occurs preferentially.The effects of external electric field on the structures and properties of HMX and its complex with HF were investigated by the B3 LYP and MP2(full) methods. The results show that the external electric fields have a minor effect on the N–NO2 trigger linkages and intermolecular interactions. As the increases of the external electric fields, bond dissociation energies of trigger linkages in complexes turn small.Molecular dynamics method was employed to study the binding energies and mechanical properties of the selected crystal planes of the HMX:FOX-7, HMX/NQ or CL-20/FOX-7cocrystal in different molecular molar ratios. The results indicate that the cocrystals, in which molecules of β-HMX super cells were substituted by FOX-7, prefer cocrystalizing in a 1:1molar ratio, which has good mechanical properties, and the cocrystallization is dominated by the(0 2 0) and(1 0 0) facets. The cocrystals, in which molecules of FOX-7 super cells were substituted by β-HMX, prefer cocrystalizing in a 1:2 molar ratio, with the cocrystallization by the(0 2 0) and(1 0 0) facets of FOX-7. Similarly, the cocrystals, in which molecules ofβ-HMX super cells were substituted by NQ, prefer cocrystalizing in a 1:1 molar ratio, which has good mechanical properties, and the cocrystallization is dominated by the(0 2 0) and(1 00) facets. The cocrystals, in which molecules of NQ super cells were substituted by β-HMX,prefer cocrystalizing in a 1:2 molar ratio, with the cocrystallization by the(0 1 0) and(1 0 0)facets of NQ, and it has good mechanical properties. CL-20/FOX-7 cocrystal also prefers cocrystalizing in a 1:1 molar ratio. The N–NO2 bond turns strong upon the formation of complex and the sensitivity of HMX or CL-20 might decrease in cocrystal. The origin of the sensitivity change of cocrystal originates from not only the formation of the intermolecular interaction but also the increment of the N–NO2 bond dissociation energy. The cocrystals exhibit good detonation performance and meet the requirement of high density energetic materials. The external electric fields in the positive orientation increase the binding energies of the selected crystal planes, leading to a possible decreased explosive sensitivity, while the external electric fields in the negative orientation decrease the binding energies, leading to an increased explosive sensitivity. In fact, this field effect is minor for the binding energies of theselected crystal planes in HMX:FOX-7, HMX/NQ or CL-20/FOX-7 cocrystal. The field effects on the binding energies in the cocrystals with the large molecular molar ratios are larger that those in the small molecular molar ratios. Under the external electric fields in the positive orientation, the values of G and E are increased, leading to a better ductility and a possible decreased explosive sensitivity, while in those of the negative orientation, the values of G and E are decreased with the possible increased explosive sensitivity. The external electric fields in the positive orientation increase the values of the impact h50, leading to a possible decreased explosive sensitivity of HMX and CL-20, while the external electric fields in the negative orientation decrease the values of h50, with an increased explosive sensitivity of HMX and CL-20.