Theoretical Insight Into Cocrystal Explosives Based On HMX And CL-20

Power and safety are both the most concern in the field of energetic materials,but there is an essential power-safety contradiction between them: the highly energetic materials are often not safe,and at present the rareness of pure low-sensitive and highly energetic explosive has been found.Due to the stringent requirements for the low sensitivity and high power of energetic materials,the cocrystallization of explosive has attracted great interest since it can alleviate to a certain extent the power-safety contradiction.Furthermore,recently a lot of cocrystal explosives have been synthesized and characterized.Moreover,an evaluation of the power and safety of energetic cocrystals has been carried out.The intermolecular interactions in energetic cocrystals have also been discussed.The cocrystallization of explosive provides a new method to alleviate the power-safety contradiction.A lot of experimental results have shown that the cocrystal based on HMX or CL-20 is of great significance for increasing the energy?in comparison with the low energy explosive?and reducing the sensitivity of explosives?in comparison with the high energy explosive HMX or CL-20?.Few theoretical investigations were reported.According to the rule of supramolecule or cocrystal formation,some cocrystal models,such as HMX/DMI,HMX/FOX-7,HMX/NQ,CL-20/DMF,CL-20/FOX-7 and CL-20/?-HMX,were designed.Density functional theory and molecular dynamics method are employed to investigate their structure,performance,molecular ratio and polymorph.Follows are the main works in this paper:In order to find out the theoretical method to calculate the structure and property of explosive,the changes of the strength of N–NO₂ bonds,ring strains and electrostatic potentials were studied upon the formation of intermolecular H-bond between HF and the nitro group in nitrogen heterocyclic rings CnH2nN-NO₂?n=2–5?,RDX and HMX by using the DFT-B3 LYP and MP2?full?methods with the 6-311++G?2df,2p?and aug-cc-pVTZ basis sets.The analysis of electron density shifts was carried out.The results indicate that,in most cases,the H-bonding energy correlates well with the increment of ring strain energy.Upon the complex formation,the strength of the N–NO₂ trigger-bond is enhanced,suggesting the reduced sensitivity,while the ring strain energy is increased,showing the possible increased sensitivity.However,as a global feature of molecule,molecular surface electrostatic potentials,such as local maximum of above N-NO₂ bond and ring,2?+ and electrostatic balance parameter ?,remain essentially unchanged upon the complex formation.The little change of the calculated impact sensitivities h₅₀ is also suggested.The molecular surface electrostatic potentials should be taken into account in the analysis of explosive sensitivity in the H-bonded complexes.Molecular dynamics method was employed to study the binding energies of the selected crystal planes of the HMX/DMI cocrystal in different molecular molar ratios.The mechanical properties were estimated in different molar ratios.Solvent effects were evaluated and the cooperativity effects were discussed in the HMX???HF???DMI ternary by using the M06-2x/6-311+G?2df,2p?and MP2?full?/6-311+G?2df,2p?methods.The results indicate that the substituted patterns?0 2 0?and?1 0 0?may own the highest binding energies and the stabilities of cocrystals in the 1:1 and 2:1 ratios are very close.The HMX/DMI cocrystal prefers cocrystalizing in the 1:1 and 2:1 molar ratios,which have good mechanical properties.The sensitivity change of the HMX/DMI cocrystal originates from not only the formation of intermolecular interaction but also the increment of the N–NO₂ BDE.The cooperativity effect appears possibly in the linear complex while the anti-cooperativity effect might be found in the cyclic system.DMI binding to HMX is not energetically and structurally favored in the presence of HF.This is perhaps the reason that the solvent with large dielectric constant weakens the stability of the HMX/DMI cocrystals.Therefore,the solvents with low dielectric constants should be chosen in preparation of the HMX/DMI cocrystal.Molecular dynamics method was employed to study the binding energies and mechanical properties of the selected crystal planes of HMX/FOX-7 and HMX/NQ cocrystal explosives with different molecular molar ratios.For the HMX/FOX-7?1:1?and HMX/NQ?1:1?complexes,the N–NO₂ bond dissociation energies?BDEs?and impact h₅₀ were investigated by using the B3 LYP and MP2?full?methods with the 6-311++G?d,p?and 6-311++G?2df,2p?basis sets.The results indicate that HMX/FOX-7?1:1?or HMX/NQ cocrystal prefers cocrystalizing in a 1:1 molar ratio,which has good mechanical properties.The cocrystallization might be dominated by the?0 2 0?and?1 0 0?facets.The N–NO₂ bond might turn strong and the sensitivity of HMX might decrease in cocrystal.Molecular dynamics method was employed to study the binding energies on the selected crystal planes in the ?-,?-,?-conformation 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane??-,?-,?-CL-20?cocrystal explosives with 1,1-diamino-2,2-dinitroethylene?FOX-7?,1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane with ?-conformation??-HMX?and N,N-dimethylformamide?DMF?in different molar ratios.The oxygen balance,density,detonation velocity and surface electrostatic potential were analyzed.The results indicate that,for all the cocrystals,the binding energies Eb* and stabilities are in the same order of 1:1 > 2:1 > 3:1 > 5:1 > 8:1?CL-20:FOX-7/?-HMX/DMF?.The values of Eb* and stabilities of the energetic-nonenergetic CL-20/DMF cocrystals in all the substituted patterns with all the ratios are far larger than those of the energetic-energetic CL-20/FOX-7 and CL-20/?-HMX cocrystals,for which the values of Eb* and stabilities of CL-20/?-HMX are the smallest.For CL-20/FOX-7 and CL-20/?-HMX,the largest Eb* might appear in the cocrystals with the 1:1,1:2 or 1:3 molar ratio.Furthermore,the values of Eb* and stabilities of the cocrystals with the excess ratio of CL-20 are weaker than those in the cocrystals with the excess ratio of FOX-7 or CL-20/?-HMX.In the CL-20/FOX-7 cocrystal,CL-20 prefers adopting the ?-form,and ?-CL-20 is the preference in CL-20/?-HMX,and ?-CL-20 and ?-CL-20 can be found in CL-20/DMF.The CL-20/FOX-7 and CL-20/?-HMX cocrystal explosives with low molar ratios can meet the requirements of the low sensitive high energetic materials.Surface electrostatic potential reveals the nature of the sensitivity change upon the formation of cocrystal.These studies can provide some novel insight for the design of the HMX and CL-20 cocrystal explosives.It must be useful for alleviating the power-safety contradiction in explosives to some extend,synthesis and safe using of the novel cocrystal explosives.