| Polynitrocubanes are one of the most important derivatives of cubane and belong to the new type of high energetic density materials (HEDM) with high strain and cage structure. They have considerable potential importance in practical applications such as explosives, pharmaceuticals, liquid crystals, and specialty polymers, etc. In order to elucidate the structures and properties of polynitrocubanes, ab initio, density functional theory (DFT) and several semiempirical MO methods are employed to study their molecular geometries, electronic structures, infrared vibrational spectra, heats of formation and the other thermodynamic properties, kinetic acidities pKa, pyrolysis mechanism, and detonation properties. In addition, for comparison, the heats of formation for polycyanocubanes, polyisocyanocubanes, and cubylnitrates are calculated. The structures and properties for hexanitrohexaazaisowurtzitane (CL-20), a very important energetic material with cage structure, are also studied. Brief summary is listed as follows:1. The molecular geometries, electronic structures, IR frequencies, and thermodynamic properties (H m, C P, m and S m) in the temperature range 298 ~ 1000 K are calculated for 10 polynitrocubanes and ( ), p, and s-conformations of CL-20, using the DFT method at the B3LYP/6-31G* level. The accurate IR frequency scaling factor 0.9501 suitable for polynitrocubanes is obtained at the B3LYP/6-31G* level, and the calculated IR frequencies of polynitrocubanes are scaled.2. For the first time the accurate heats of formation for four kinds of derivatives of cubane C8H8-m(R)m (R = -NO2, -ONO2, -CM, and -NC; 1 m 8) and a total of 84 compounds (21 4) are systematically calculated, using both DFT-B3LYP/6-31G* method via designed isodesmic reactions in which the cubane cage skeleton has been kept and semiempirical MO methods. The relationships between the heats of formation and molecular structures are discussed. It is found that with increasing in the number of nitro groups, the trend of changes for the heats of formation of polynitrocubanes is.exceptional. Therefore, the previous data of heats of formation of polynitrocubanes estimated from groupadditivity method are corrected.3. Calculations indicate that, for polynitrocubanes, there are parallel relationships between their experimental kinetic acidities pKa, the average s characters (sa) in the exocyclic carbon orbital used for the C-H bonds within each molecule, and the average C-H stretching frequencies (va) within each molecule. These calculated results are helpful to the estimations of the kinetic acidities pKa of polynitrocubanes and the other type of compounds based on the values of sa or va , which has reference value for organic syntheses.4. The pyrolysis mechanism of polynitrocubanes is elucidated using various theoretical methods, i.e. MP2, DFT and selected MINDO/3 semiempirical MO, based on the unrestricted model. The calculated results show that the pyrolysis initiation reaction of polynitrocubanes, i.e. rate-controlling step, is to form a diradical by the single C-C bond breaking in the cube. The second C-C bond breaking is easily followed. The calculated activation energy for the pyrolysis initiation reaction of octanitrocubane, obtained from B3LYP/6-31G* method, is 155.30 kJ/mol, indicating that octanitrocubane has better thermal stability and less sensitivity. A series of calculated results are in good agreement with the experimental results.5. Based on the calculations from quantum chemistry, the sublimation enthalpy, density and heat of formation for octanitrocubane crystal are 220.63 kJ/mol, 2.189 g/cm3 and 505.84 kJ/mol, respectively. According to Kamlet-Jacobs equations, the estimated detonation velocity and detonation pressure of octanitrocubane are 10.26 km/s and 520.86 kbar, respectively, indicating that octanitrocubane has highly exploitable values as HEDM.6. Based on the B3LYP/6-31G* calculations, it is found that, in CL-20, t...
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