The DFT Predictions of the Structure and Property of Cyclopentamethylenepentanitramine

The compounds hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octohydro-1,3, 5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are two well known energetic materials. The Cyclopentamethylenepentanitramine is called CRX. The full geometry optimization and normal-mode analysis of CRX are performed using nonlocal density functional theory (DFT) method. The density functional used in this study is B3LYP and the basis set empolyed is 6-31G*. Normal mode analyses are used to characterize the stable point and to determine the harmonic vibrational frequencies. The standard thermodynamic properties (C0p,m, S0mandH0m) within 200~1200 K are calculated using the statistical thermodynamic method. The conformation of CRX is aboutC2symmetry. The dipole moment is 6.137 Dabye. C-N bond lengths and N-NO2bond lengths are among 0.144~1.48 nm, 0.139~0.142 nm, respectively. The difference between the energy of the highest occupied molecular orbital (HOMO) and that of the lowest unoccupied molecular orbital (LUMO) is 5.2054 eV, which predicts that CRX is more stable. All the thermodynamic quantities increase as the temperature goes up. This is because when the temperature is lower the main contributions to the thermodynamic functions are from the translations and rotations of molecules. But at the higher temperatures, the vibrations contribute more to their thermodynamic functions, which results in the increase of the thermodynamic functions. In addition, the added extent for both C0p,m and S0m decreases with the increasing temperature, but increases forH0m. Defined as the volume inside a contour of 0.001 electrons/bohr3density, the molar volume of CRX is computed, using the Monte-Carlo method based on the spatial structure obtained from full optimization at B3LYP/6-31G*level. The obtained molar volume is 200.499 cm3/mol. Then the calculated density is 1.846 g/cm3. It is greater than that of both RDX and HMX. The detonation velocity and pressure of the titled compound are predicted to be 9169 m/s and 37.88 GPa respectively by Kamlet formula. They are higher than those of RDX and HMX.