The DFT Studies on Structure and Property of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

The compound octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine is a well known energetic material called cyclotetramethylene-tetranitramine (commonly known asHMX in the explosives field). It is an eight-numbered cyclic nitramine with best comprehensive properties and with applications ranging from explosives and rocked propellants to automobile air bags. It is unusual among high explosives as it has four readily formed and well-recognized polymorphs.β-HMX is the thermodynamically stable from under ambient condition. Despite the many experimental investigations, very few theoretical studies have previously been reported for HMX. The high accurate calculation for HMX has been carried out based on ab initio quantum mechanics. Geometry optimization and normal-mode analysis of HMX inβform are performed using nonlocal density functional theory(DFT) method. The density functional used in this study is B3LYP and the basis set employedis 6-31G*. Normal-mode analyses are used to characterize the stable point and to determine harmonic vibrational frequencies forβ-HMX conformer. The molecular geometry, electronic structures, IR spectra and thermodynamic properties are obtained. The fully optimized geometric parameters and the net electric charges on atoms all have Cisymmetric property. C-N bond lengths are among 0.144~0.148 nm. This indicates that all cyclic C-N bonds are single bond. The N-NO2bond Mulliken population is the smallest among the bonding atoms. The fact indicates that there are less electronics between them and predicts that this bond is an initial bond for pyrolysis and explosive. The IR spectra are in good agreement with the corresponding experiment data. Half of the 78 harmonic vibrational frequencies forβ-HMX are IR active and belong to Au representation, the rest are Raman active and belong to Ag representation. The calculated standard thermodynamic functions, heat capacities at standard conditionC0p,m, standard entropySm0and standard enthalpyHm0between 298~1200 K increase with the increasing temperature. These functions are helpful to further study on the reaction and properties forHMX.