Quantum Mechanics Study Of The Intermediates In Thermal Decomposition Of Nitromethane

Energetic materials have been in extensive use in various fields. Experimentaland theoretical investigations concerning the properties and reactions mechanism ofenergetic material have been carried out extensively for the great value of energeticmaterial in the field of military and industrial.The thermal decompositionmechanism of energetic material at high pressure and temperature is of considerableimportance in processing, transportation, use and storage of explosives.The most widely studied and archetypical example of energetic materials isnitromethane（CH3NO₂）. Nitromethane is the simplest organic-nitro compound,whichis usually used to study the carbon nitro （C-NO₂） kind of explosives or othernitrogroup derivatives model.Nitromethane could be used as an explosive and apropellant.Despite numerous studies have been performed on the decompositionbehavior of nitromethane,the thermal decomposition mechanism of nitromethane isstill largely unknown.In this paper, we studied the thermal decomposition processes of nitromethanefrom two aspects,and all calculations have been carried out using the Gaussian03program package:Firstly,we analyzed the thermal decomposition processes of nitromethane,andthe geometries of intermediates have been identified. The geometries of all the longlife（more than0.005ps） intermediates which formed in the thermal decompositionof nitromethane were optimized at the UB3LYP level using3-21G*、6-31G*and6-311G（d） basis set.The vibration frequencies and total energies were calculatedbased on the optimized geometries at the same level,and all the intermediates havebeen identified by vibrational analysis. Final computation gets63stableintermediates,and some of them played an important role during the course of thedecomposition by acting as a hydrogen transfer,such as H2O、NO₂、CO₂、CNO andNCO.Secondly,we studied the primary thermal decomposition reaction ofnitromethane.The geometries of reactants and products were optimized at the UB3LYP level using3-21G^*、6-31G^*and6-311G（d） basis set,and the transitionstates on the reaction path were searched by STQN method.All the transition stateshave been identified by vibrational analysis for the only imaginary frequency at thesame level. Finally,we calculated the energy of reasonable transition states to get theenergy barrier of the reaction path.The results show that,nitromethane can isomerizeto cis-CH₃ONO and trans-CH₃ONO via different reaction path on the stage ofprimary thermal decomposition.But the energy barrier of isomerization is slightlyhigher than the the C-N bond dissociation energy of nitromethane. So is more likelyfor nitromethane to break the C-N bond in the thermal decomposition.