Primary Investigations On The Molecular And Formulation Design Of Organic Cage High Energy Density Materials (HEDM)

The dissertation is devoted to systematic researches on the structures and properties of the two types of important organic cage compounds (a series of the high energy derivatives of adamantane and hexaazaadamantane) and the well-known high energy density compound (HEDC) hexanitrohexaazaisowurtzitane (CL-20), using the theoretical and computational chemistry methods, mainly including quantum mechanics (QM), molecular mechanics (MM), molecular dynamics (MD) and so on. We have completed the whole process of searching for high energy density materials (HEDM) from gas molecules to crystal structures, and then to complex materials (such as polymer bonded explosives, PBX). The dissertation is divided into three parts:The first part concentrates on the"molecular design"of HEDC. Based on the QM calculations, both the quantitative standard (densityρ>1.9g·cm^-3, detonation velocity D>9.0km·s^-1, and detonation pressure p>40.0GPa.) as an HEDC and the stability requirement (the dissociation energy of trigger bond, BDE>120kJ·mol^-1) are first time suggested to screen out seven potential HEDC from the above referred organic cage compounds.First, based on the fully optimized structures at the DFT-B3LYP/6-31G* level, the infrared (IR) spectra and thermodynamic properties (C_p,m°, S_m°, and H_m°) in the range of 298⁸00K are computed for polynitroadamantanes (PNA); their gas heats of formation (HOF) are obtained by designing isodesmic reactions; according to the volume inside a contour of 0.001e·Bohr^-3, their theoretical crystal densities (ρ) are evaluated; and their D and p are estimated according to the Kamlet-Jacobs equations. The UHF-PM3 method is employed to evaluate the activation energy (Ea) for the homolysis of each possible trigger bond, and to predict the pyrolysis initiation mechanism and relative stability of PNA; the dissociation energy (E_C–N) of each trigger bond C–NO₂ is calculated at the B3LYP/6-31G* level; and it is found that all the bond order of the trigger bond (BC–N), EC–N, Ea, and the net charge on–NO₂ (QNO₂) can be equally used to predict the stability or sensitivity of PNA. According to the suggested quantitative standard and stability requirement as an HEDC, 1,2,3,4,5,6,7,8-octanitroadamantane, 1,2,3,4,5,6,7,8,9-nonanitr -oadamantane, and 1,2,3,4,5,6,7,8,9,10-decanitroadamantane are worth recommending as potential HEDC, and this denies the foreigners'conclusion from group additivity method that the PNA with 11 nitro groups was the target compound. Besides, similar studies are carried out on the–ONO₂ derivatives of adamantane; and in the light of the standard, 1,2,4,6,8,9,10-adamantylheptanitrate also is considered as an HEDC candidate.Second, at the B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G* level, the reaction mechanism on the nitration of adamantane with NO₂ is investigated. All the transition states of each possible reaction path are found, and the most possible reaction path is confirmed by comparing each reaction barrier (Ea). The obtained conclusion agrees well with that from the experiment, presenting the availability of theoretical computation in guiding HEDC synthesis.Third, in the same way, on the basis of the studies on the structures and properties of the polynitrohexaazaadamantanes (PNHAA), their HOF obtained by isodesmic reaction are reported; theirρ, D and p are evaluated; by comparing the dissociation energies (EC–N and EN–N) of two possible trigger bonds (C–N and N–NO₂), their thermolysis mechanisms is revealed, and the N–NO₂ bond is predicted as the trigger bond; their stability or sensitivity has been correlated with various theoretical structure parameters. Similarly, according to theρ, D and p criteria as an HEDC and the value of EN–N, three PNHAA with 4, 5 and 6–NO₂ are recommended as the potential HEDC.Finally, as an extension of the studies on PNHAA, further studies on the structures and properties of the other three kinds of HAA derivatives (–CN,–NC, and–ONO₂) are made to provide more basis data for the molecular design of new type of HEDM. The calculation results indicate that although the–CN and–NC derivatives of HAA show high heats of formation and high heats of detonation, they cannot meet the criteria as an HEDC, due to their negative oxygen balances and lowerρ, D and p. (Accordingly, none of these two types of adamantane derivatives can be predicted as HEDC). 2,4,6,8-, 2,4,6,8,10-, and 2,4,6,8,9,10-hexaazaadamantylnitrates satisfy theρ, D, and p standard as an HEDC, their dissociation energies (EO–N) of trigger bond O–NO₂, however, are too small (for example, EO–N of 2,4,6,8-hexaazaadamantyltetranitrate is only 29.81 kJ.mol-1), thus they all cannot be recommended as practical HEDC, too.The second part focuses on molecular packing predictions and theoretical studies on the structures and performance of crystals. The predictions of spacial packing are made for the seven HEDC obtained from the above molecular design; and the band structures and performances of the reasonably predicted crystals are calculated from periodic calculations. Periodic Ab inito calculations are also carried out on the four polymorphs of the currently most important HEDC–CL-20, and the influences of pressure on the most stableε-CL-20 crystal is investigated. First, the Polymorph module in MS program package is employed to search for the most possible spatial packings forε-CL-20 and the above recommended 7 HEDC among seven most possible space groups (P21/c, P-1, P212121, P21, C2/c, Pbca and Pna21), and their reasonable crystal structures are predicted. By comparing the optimized molecular structures from the Dreiding and Compass force fields with that using the DFT-GGA-PBE method, and it is found that the Dreiding force field is suitable for nitro and nitrate compounds, but the Compass for the nitramine and nitrate series. Considering the results from both the two force fields,ε-CL-20, 2,4,6,8-tetranitrohexaadamantane, 2,4,6,8,10-pentanitrohexaazaadamantane, 2,4,6,8,9,10-hexanitrohexaadamantane, and 1,2,4,6,8,9,10-adamantylheptanitrate are predicted to pack in P21/c, P-1, P-1, P21/c, and P21/c space groups, respectively, and 1,2,3,4,5,6,7,8-octanitroadamantane, 1,2,3,4,5,6,7,8,9-nonanitroadamantane, and 1,2,3,4,5,6,7,8,9,10-decanitroadamantane are predicted to crystallize in C2/c, P21/c, and C2/c, respectively.Second, periodic ab initio calculations are performed to obtain the band structures of the predicted crystals ofε-CL-20 and the seven potential HEDC using the DFT-GGA-PBE method in Dmol3 program package.The method is proved reliable by the similarity of the band structures and density of states (DOS) between the predicted and experimentalε-CL-20 crystal. Seen from the DOS, especially the constitutions of the valence and conductor bands near Fermi energy of each crystal, the N–NO₂, C–NO₂, and O–NO₂ bonds are suggested as the trigger bonds of the nitramine compounds (ε-CL-20 and PNHAA), PNA, and adamantantylnitrates, respectively. This is consistent with both that drawn from the theoretical studies on their gas molecules and experiment. It is strongly suggested that more attention should be paid on the 2,4,6,8,9,10-hexanitrohexaazaadamantane. It has similar DOS and comparable band gap (ΔEg) with those ofε-CL-20, presenting its similar sensitivity withε-CL-20. This agrees well with that drawn from the bond dissociation energy (EN–N) of N–NO₂; meanwhile, due to its larger predicted density (2.315g.cm-3) than that ofε-CL-20 (2.173g.cm-3), it is believed that `if it can be successfully synthesized, its energetic properties may be prior to those ofε-CL-20, which is currently the most heated point.Finally, periodic ab initio calculations are carried out to obtain the band structures of the four CL-20 existing polymorphs (α·H2O,β,γandε-CL-20), and according to the calculated band gap (?Eg), the sensitivity order of the four polymorphs is predicted asε<β<γ<α·H2O, which is consistent with that by the experiment and the principle of easiest transition (PET). This provides the first example of using the theoretical criterion to predict the sensitivity of molecular crystals. The most stableε-CL-20 crystal is chosen to investigate the influences of pressure on its geometry, electronic structures, and performances. The results show thatε-CL-20 always exhibits its anisotropic properties from lower to higher pressure; when the pressure is at lower than 10 GPa, its cell parameters, band structure, DOS, andΔE_g are little affected by the pressure; while the pressure increases from 10 to 400GPa, these parameters change a lot; and when it is at 400 GPa, theΔE_g trends to be zero, the DOS become smooth curves, and great distortions happened to the dihedral angles of N-NO₂, indicating thatε-CL-20 has already owned some metal properties.The third part is mostly the MD simulations on the structures and properties of theε-CL-20-based PBX, also named HEDM. The effects of different types of binders, the proportion of binder (W%), temperature (T), crystal defects and so on, on the four basic properties (mechanic properties, energetic properties, compatibility, and safety) are investigated. These provide information and establish the basis for the practical formulation design.First, based on the"cutting"model, in which four types of fluorine binders, polyvinylidene difluoride (PVDF), polychlorotrifluoroethylene (PCTFE), F2311, and F2314, are put on the three different crystal surfaces (001), (010), and (100) ofε-CL-20 crystal, respectively, and MD simulations are performed on the obtained 12 PBX models. From the static mechanic analysis, their mechanic properties (elastic constants, tensile modulus, bulk modulus, shear modulus, and Poisson's ratio) are obtained. The results show that the additions of small fluorine binders can improve the mechanical properties ofε-CL-20; due to the higher molecular density on the (001) surface ofε-CL-20, this surface can more strongly interact with the polymer binders, and therefore the binders on the (001) surface can improve more the mechanic properties; among the four binders, the PBX consisting ofε-CL-20 and F2314 has the best comprehensive mechanical properties; and the four binders have similar influences on the energy properties (heat of detonation, detonation velocity, and detonation pressure).Second, the better model,ε-CL-20(001)/F2314, is selected as an example to investigate the influences of T, W%, and crystal defects (vacancy and doping) on the performances of the PBX by MD simulations. It is found that T has effects on the mechanical properties, but little on the binding energy (Ebind). With W% increasing, the total Ebind between F2314 andε-CL-20 molecules increases, but the average binding energy per polymer chain (Eaver) decreases; the PBX with 4.69% F2314 is found with the better mechanical properties and detonation properties, this proportion (W%)of F2314 matches well with that (<5%) in practical formulation. In terms of the two models of crystals with defects, only the mechanic properties (mainly refers to tensile modulus, bulk modulus, and shear modulus) of the crystal with vacancy and the corresponding PBX change greatly compared with the perfect crystal, but little change happens to the structures and properties of the models with doping.Third, three types of energetic binders, polyurethane (Estane), glycidyl azide polymer (GAP), and hydroxy terminated polybutadiene (HTPB) are chosen to constituteε-CL-20-based PBX for MD simulations. It is the first time to correlate the binding energies (Ebind) with their physic compatibility between the binders and body explosives, and this well differentiates and interprets the experimental results. By the pair correlation function g(r) analysis, the manners of interactions between the binders andε-CL-20 are revealed. As to theε-CL-20/polyglycol (PEG) PBX, without experiment data is similarly simulated, and its related performances are predicted. Comprehensively comparison of the mechanical and explosive properties among the various simulated PBX will provide information and reference evidence for choosing optimum binder.In all, in this thesis, based on quantum chemistry, a set of simple and convenient method is developed to evaluate the density (ρ), detonation velocity (D), and detonation pressure(p) of high energy materials; a quantitative standard of energy (ρ, D, and p) and stability(BDE of trigger bond) is suggested to discriminate a practical HEDC; and seven potential HEDC targets are recommended among various derivatives of adamantane and hexaazaadamantane. By combining the MM and QM methods, the space packings and band structures of the predicted crystals for the seven HEDC are predicted, and hexanitrohexaazaadamantane is strongly recommended to synthesize for its performance superior to CL-20. Besides, the MD method is employed to simulate the structures and many performances of various CL-20-based PBX (namely, HEDM) at different temperatures, with different proportions of binder, and with"perfect"or"defective"CL-20 crystal. These provide information and reference in choosing optimum binder and guiding formulation design of HEDM. This work is pioneer and innovative in the front and crossing research field of HEDM, and successfully complete the various tasks assigned by the national defence 973 and National Nature Science Finance Projects.