DENSITY FUNCTIONAL THEORY STUDIES ON THE STRUCTURE, THERMODYNAMIC FUNCTIONS AND HEAT OF FORMATION OF N_8 CUBANE

摘要

With the increasing interest in high energy density materials(HEDM), there are more and more theoretical studies on mono-nitrogen clusters such as Ng which is predicted to be a potential HEDM. In this article, we have optimized the geometries of Ng and its charged and protonated species using the density functional theory (DFT) at the B3LYP level. Geometry optimizations have also been performed on Ng using ab initio method at the MP2 level for comparison. The standard 6-31G~* and 6-311G~* gaussian basis sets were employed. The geometries obtained by the DFT methods are very close to the high-level ab initio results. The relative energies of Ng to 4N2 derived from the DFT calculations are also quite close to those from the ab initio methods. The vertical and adiabatic ionization potential, electron and proton affinities have been calculated. Vibrational analyses have been performed and the standard thermodynamic functions of Ng were computed in the range of 200～600 K. The heat of formaion has been derived from the atomization (a), decomposition (b) and isodesmic (c) reactions, and the obtained results are 1919.90, 1863.03 and 1900.10 kJ/mol, respectively. To check the reliability of the calculated results, the heats of formation of CgHg and the species involved in the reactions have been calculated too from the atomization reactions (d) and the isodesmic reaction (e) (for C_8H_8 only). It is found that the heats of formation calculated from the atomization energies are always much larger than the experimental results, while those from the isodesmic reaction are comparatively much better. For C_8H_8, the Δ_fH_(298°) obtained from the isodesmic reaction is 37.3 kJ/mol larger than the experimental value, while that from the atomization reaction is 154.8 kJ/mol too larger. If the deviation 37.3 kJ/mol is used to correct the Δ_fH_(298°) of N_8 calculated from the isodesmic reaction (c), the obtained result 1865.70 kJ/mol is almost identical to that from the decomposition reaction (b) (1863.03 kJ/mol). This seems to show that the Δ_fH_(298°) derived from the decomposition reaction at the B3LYP/6-311G~* level is rather reliable. The high energy content and specific impulse show that N_8 is a potential explosive with high performance.