Theoretical Study On The Mechanism Of HMX Synthesized By Bachmann Method

Now,HMX is the best single explosive with great comprehensive performance in application field,which is manufactured by Bachmann method,but the problems of high cost and low yield of this method have not been thoroughly resolved.The essential reason is that Bachmann method is complex with many by-products,the reaction mechanism is unclear and theoretical basis for process parameters is insufficient.Therefore,it is of great significance to further clarify the reaction mechanism of synthesizing HMX by Bachmann method for process and technical improvement.In this paper,the main reaction mechanism of HMX by Bachmann method was studied by combination of theoretical and computational methods in depth.The main functions of CH₃COOH、Ac₂O and NH₄NO₃ in HMX synthetizing were revealed.Finally,the decomposition mechanism of the key intermediate DPT and the effects of NO₂ and NH₄NO₃on its decomposition were studied.The specific contents is are as follows:（1）By M06-2X/def-TZVP and M06-2X-D3（BJ）/def-TZVP,the stable configurations and Raman spectra of CH3COONO2 and[CH3COOHNO2]⁺were calculated.The Raman spectra of HNO₃ and Ac₂O reaction solutions measured by experiments was compared with the calculated Raman spectra of CH3COONO2 and[CH3COOHNO2]⁺,confirming the presence of CH3COONO2 in HNO₃ and Ac₂O system.So,the activated nitrating agent for the synthesis of HMX by Bachmann method is CH3COONO2.The reaction channel generating CH3COONO2 has been constructed,and the reaction energy barrier is 83.33 k J/mol.It was confirmed that the electrophilic reaction site and the nucleophilic reaction site of CH3COONO2are N atom and the single bond O atom respectively by reaction site research.（2）The single crystal structure measurement of HAMN show that the configuration of[HA-H]⁺formed by HA and H⁺.The main reaction channels of[HA-H]⁺in acetic anhydride system were constructed.The results show that the main channel for reaction of[HA-H]⁺and CH₃COONO₂ to generate 3-methylene-7-nitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane cation（MNT）is single bond atom O in CH₃COONO₂ attacks the proton of[HA-H]⁺firstly,then atom N in[HA-H]⁺attacks atom N in CH₃COONO₂;the main channel for reaction of[HA-H]⁺and Ac₂O to generate 3-methylene-7-nitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane cation（MNT）is single bond atom O in CH₃COONO₂ attacks the proton of[HA-H]⁺firstly,then atom N in[HA-H]⁺attacks atom C in Ac₂O.The formation energy barrier of MNT is 21.44 k J/mol lower than that of MAT.Therefore,the formation of MNT may be accompanied by the formation of MAT,and temperature increasement is conducive to the formation of MAT,However,the reducing of Ac₂O adding appropriately is beneficial to the formation of MNT.（3）The channels for the formation of DPT（3,7-dinitro-1,3,5,7-tetraazabicyclic[3,3,1]nonane）and its main reactions in the acetic anhydride system were constructed.The results showed that the reaction energy barrier for the formation of DPT from 3-acetoxymethylene 7-nitro-1,3,5,7-tetraazabicyclo[3,3,1]nonane（AMNT）was the lowest,which is 113.81 k J/mol,and the main product of AMNT in the acetic anhydride system is DPT.The main product of DPT in acetic anhydride system is 1-acetoxymethylene-3,5,7-trinitro-1,3,5,7-tetraaza-cyclooctane（PHX）;At high acidity,[DPT-H]⁺formed from DPT and H⁺,and the main product of[DPT-H]⁺in the acetic anhydride system is a six-membered cyclic compound.So,the addition of CH₃COOH reduces the energy barrier of DPT formation.At high acidity,it is not conducive to formation of the 8-membered ring intermediate PHX.The addition of CH₃COOH、Ac₂O and NH₄NO₃ can all reduce acidity of the system.（4）The main reaction channels of PHX in acetic anhydride system were constructed.The results showed that in the absence of NH₄NO₃,the main product of PHX in the acetic anhydride system is 1,9-diacetoxy-2,4,6,8-tetranitro-2,4,6,8-tetraazanonane（Ac An）,and it is difficult for the formation of HMX.In the presence of NH₄NO₃,the main product of PHX in the acetic anhydride system is 1-hydroxymethylene 3,5,7-trinitro 1,3,5,7-tetraazacyclooctane（HMNT）,and the main product of HMNT in the acetic anhydride system is HMX.The"ring protecting"mechanism of NH₄NO₃ in the synthesis of HMX by Bachmann method was revealed:transforming PHX into HMNT and reducing the reaction energy barrier of HMX formation.The main rotation for synthesizing HMX by Bachmann method is HA→[HA-H]⁺→MNT→NMNT→AMNT→DPT→PHX→HMNT→HMX.（5）The decomposition mechanism of DPT,an important intermediate in the synthesis of HMX by Bachmann method,was studied by Reax FF MD,and the simulated decomposition products of DPT were verified by TG-FTIR-MS.The results showed that the decomposition of DPT produced a variety of intermediate products,and the reaction mechanism was complex.There were three decomposition routes,among which route b was the main route,that is,the breaking of the N-N bond as starting reaction route.The activation energy of decomposition was 174.6 k J/mol.The decomposition products were H₂O,N₂,NH₃ and carbon-net.The gaseous products were consistent with the results obtained by TG-FTIR-MS.The effects of NO₂ and NH₄NO₃ on the decomposition of DPT were studied by Reax FF MD method.It was found that NO₂ has the effect of promoting the decomposition of DPT,while NH₄NO₃ can absorb NO₂ and eliminate this effect.