Theoretical Studies on the Structures, Thermodynamic Properties, Detonation Properties, and Pyrolysis Mechanisms of Spiro Nitramines

Density function theory (DFT) has been employed to study the geometric and electronic structures of a series of spiro nitramines at the B3LYP/6-31G** level. The calculated results agree reasonably with available experimental data. Thermodynamic properties derived from the infrared spectra on the basis of statistical thermodynamic principles are linearly correlated with the number of nitramine groups as well as the temperature. Detonation performances were evaluated by the Kamlet-Jacobs equations based on the calculated densities and heats of formation. It is found that some compounds with the predicted densities of ca. 1.9 g/cm3, detonation velocities over 9 km/s, and detonation pressures of about 39 GPa (some even over 40 GPa) may be novel potential candidates of high energy density materials (HEDMs). Thermal stability and the pyrolysis mechanism of the title compounds were investigated by calculating the bond dissociation energies (BDE) at the B3LYP/6-31G** level and the activation energies (E a) with the selected PM3 semiempirical molecular orbital (MO) based on the unrestricted Hartree−Fock model. The relationships between BDE, E a, and the electronic structures of the spiro nitramines were discussed in detail. Thermal stabilities and decomposition mechanisms of the title compounds derived from the B3LYP/6-31G** BDE and the UHF−PM3 E a are basically consistent. Considering the thermal stability, TNSHe (tetranitrotetraazaspirohexane), TNSH (tetranitrotetraazaspiroheptane), and TNSO (tetranitrotetraazaspirooctane) are recommended as the preferred candidates of HEDMs. These results may provide basic information for the molecular design of HEDMs.