Molecular Design Of High Energy Density Materials (HEDM)-Cyclic Nitramines

The present dissertation is devoted to systematic research on the structures andproperties of multi-series of the cyclic nitramine energetic compotmds by using moderntheoretical and computational chemistry methods, mainly including quantum mechanics(QM) and molecular dynamics (MD). From the gas molecules, the crystals, and to thecomposite materials (such as polymer-bonded explosives), the whole study process ofsearching for high energy density compounds (HEDCs) and high energy density materials(HEDMs) has been completed. The whole work can be divided into three parts:The first part concentrates on "gas" molecular design for HEDC. Based on the QMcalculations, the quantitative criteria of detonation performance as a HEDC (densityρ≈1.9 g/cm³, detonation velocity D≈9.0 km/s, and detonation pressure P≈40.0 GPa) andthe stability requirement (bond dissociation energy of the initial step in thermolysis BDE＞84 kJ/mol) are employed to recommend several decades of potential HEDC objectivesfrom the title compounds.Based on the QM computations, a novel method for conveniently and exactlypredicting the crystalline densities of energetic compounds is proposed and testified. 45energetic nitramines with experimental crystalline densities are selected for research, andtheir molecular densities (ρ_cal, defined as inside a contour of 0.001 e/Bohr³ density) wereobtained from the calculations of different QM methods and basis sets. In comparison withthe experimental densities (ρ_exp), it is found that the results (ρ_cal calculated by the hybriddensity functional theory (DFT) B3LYP method with the 6-31G^** basis set agree wellwith the experiments. Based on the predicted densities (ρ_cal and heats of formation, thedetonation velocity (D) and detonation pressure (P) of the energetic compounds areestimated by the Kamlet-Jacobs formula, which establishes a novel way to quantitativelyevaluate the HEDC.The fully optimized structures, infrared (IR) spectra, and thermodynamic properties(C°_p,m, S°_m and H°_m) in the temperature range 200～800 K of two types of monocyclicnitramines polynitroazacyclohexane and polynitroazacyclopentane are obtained at theDFT-B3LYP/6-31G^** level. Theoretical crystal density (ρ), detonation velocity (D), anddetonation pressure (P) of each compound are predicted. Bond dissociation energies (BDE)of four possible trigger bonds in their thermolyses are computed by the B3LYP/6-31G^**method under the unrestricted Hartree-Fock model, and their pyrolysis mechanisms areascertained to be the homolysis of N-NO₂ bond. Relative stabilities of these monocyclic nitramines are also judged from the BDE. It is found that 1,2,4-trinitrotriazacyclohexane,1,3,5-trinitrotriazacyclohexane i.e. RDX, and 1,2,4,5-tetranitrotetraazacyclohexane agreewith the forementioned quantitative criteria of a HEDC and stability requirement.Similar studies are performed on the bicyclo-HMX derivatives and TNAD isomers.According to the quantitative criteria of a HEDC and stability requirement, it is found thatbicyclo-HMX and its derivatives (â…¡-3～Ⅱ-5,Ⅳ～Ⅶ, andâ…©) are worth recommendation aspotential candidates of HEDCs. Although the detonation properties of TNAD and itsisomers are not large enough, the isomers with N-NO₂ groups at the meta- andpara-positions have better stabilities than RDX. Thus, they can be recommended asenergetic insensitive explosives.The studies on the structures and properties of some tricyclic nitramines provide basicinformation and discipline for the molecular design of novel HEDCs. The results show thatadding a cyclobutane between two cyclic nitramines of bicyclo-HMX results in detonationproperties decreasing much in comparison with bicyclo-HMX, while increasing thenumber of combined cyclic nitramines in bicyclo-HMX and TNAD will improve theirdetonation performances markedly. When changing the tricyclic nitramines into thecorresponding glycoluril derivatives, the oxygen balance and detonation performanceimprove remarkably. The chair and boat conformations of hexanitrohexaazatricyclododecane (HHTD) and hexanitrohexaazatricyclododecanedione (HHTDD) have excellentexplosive performance, but their stabilities are less satisfied.The fully optimized structures of a series of spiro nitramines are reported, and theirthermodynamic stabilities are evaluated according to the electronic structures. The IRspectra and thermodynamic properties (C°_p,m, S°_m, and H°_m) in the temperature range200～800 K are computed. Their crystalline densities, heats of formation, detonationvelocities, and detonation pressures are predicted. Pyrolysis mechanisms of the spironitramines are investigated and ascertained by comparing the BDE of four possible triggerbonds (C-C, C-N, N-N and N-NO₂) and the activation energies (Ea) in the thermolysis.The molecular stability or sensitivity of spiro nitramines is correlated well with their staticand dynamic electronic structures. According to the quantitative criteria of HEDCs andstability demand, TNSHe (tetranitrotetraazaspirohexane), TNSH (tetranitrotetraazaspiroheptane), and TNSO (tetranitrotetraazaspirooctane) are recommended as HEDCcandidates with superior exploitation and application values.Similar calculations have also been performed on a series of furazan andfuroxan-fused cyclic nitramines to study their structures and properties. The results indicate that all of the furoxan-fused cyclic nitramines have largerρ, Q, D, and P than thecorresponding furazan-fused cyclic nitramines. When the molecule only contains onep-dinitropiperazine and two furazan rings, it has met the quantitative demand of a HEDC.With changing the furazan into the furoxan or the number of fused p-dinitropiperazineincreasing, density and detonation properties all increase and exceed the HEDC'squantitative criteria much. However, the N-O bond in the furazan or furoxan ring is weakerin comparison with other energetic compounds, which leads the Stabilities of furazan andfuroxan-fused cyclic nitramines to decrease and hence restricts their exploitations andapplications.The second part focuses on theoretical studies of bicyclo-HMX and TNAD. Periodicquantum mechanics (QM) calculations and molecular dynamics (MD) simulations are firstcarried out to study their crystalband structures and properties. The influences of pressureand temperature on the structures and properties have also been investigated.The density functional theory (DFT) methods in the CASTEP program package areused to compute the crystal band structures. The results demonstrate that LDA/CA-PZ ismore proper and satisfactory than GGA/PBE and GGA/PW91 for studying the bicyclicnitramine molecular crystals. Crystal structure, molecular structure, charge distribution,frontier band structure, band gap, and density of states (DOS) of bicyclo-HMX and TNADare first reported. The correlations between various parameters and their conductivity,sensitivity are discussed in detail. An analysis of the valence and conduction bands showsthat the N-NO₂ bond is the trigger bond during their thermolyses, which are consistentwith the conclusions drawn from the experiments and theoretical studies on the thermolysismechanism of molecules in the gas phase. Bicyclo-HMX with larger band gap is morestable than TNAD, which is consistent with the conclusions drawn from the activationenergies (Ea) and BDE, and also accord with "the principle of the easiest electrontransition" (PET). Considering the superior detonation performances of bicyclo-HMX, wesuggest that much more attention should be particularly paid to the bicyclo-HMX, andexploitation and application researches about it should be intensified. Moreover, theinfluence of pressure on the crystal structures and properties of bicyclo-HMX and TNADhas also reported. Their band structures and electronic structures change scarcely as thepressure less than 10 GPa, while in the pressure range of 10～400 GPa these parameterschange largely. When the pressure is larger than 400 GPa, the band gaps of both them areclose to zero and the DOS almost becomes a continuum curve, showing a metalliccharacter. Applying the DISCOVER module of Materials Studio (MS) 3.0.1 software package,periodic NPT-MD simulations in the temperature range of 5～400 K are performed onbicyclo-HMX and TNAD to study their general crystal structures and properties.Applicability Of the COMPASS force field is demonstrated. The effects of temperature onthe crystal structures, thermal expansion, and mechanical properties of bicyclo-HMX andTNAD are observed. Their lattice parameters and cell volume all increase linearly with theincreasing temperature, while the linear and volume expansion coefficients decreaseslightly as the temperature increases. Elastic constants and mechanical moduli all decreasewith the increasing temperature, indicating that the system's rigidity weakens andductibility decreases. Bicyclo-HMX has the larger Poisson ratio than TNAD, showing thatthe former has lager plasticity than the latter. The lattice energy of bicyclo-HMX is nearlytwice as much as that of TNAD, implying the former is more stable than the latter. Thecrystalline studies indicate that bicyclo-HMX has more exploitation and application values.The third part centers on the MD simulations of the structures and properties ofbicyclo-HMX and TNAD-based polymer bonded explosives (PBXs). The effects of thetype and content of polymer binders and its orientation along different crystalline surfaceson the PBXs' mechanical properties, binding energies, and detonation performances areinvestigated. These provide information and establish the basis for the practicalformulation design of energetic composite materials, namely HEDM.The "cutting surface" method as implemented in the MS program package is chosento cleave the bicyclo-HMX and TNAD supercells along three different crystalline surfaces(001), (010), and (100). Four types of typical fluorine polymer binders PVDF, PCTFE,F₂₃₁₁, and F₂₃₁₄ are then put on above three crystalline surfaces of bicyclo-HMX and TNAD,respectively, and a dozen of bicyclo-HMX and TNAD-based PBX models are obtained,respectively. MD simulations are carried out on them with the COMPASS force field toget their general structures and properties. Mechanical properties, binding energies,detonation performances, and the discipline of them changing with the type and content ofpolymer binders and different crystalline surfaces are first reported for the bicyclo-HMXand TNAD-based PBXs. The results show that the mechanical properties of the basalexplosives can be effectively improved by adding small amounts of fluorine polymers. Theeffects of improvement on different crystalline surfaces of bicyclo-HMX are in the order of(010)＞(001)≈(100), and those of TNAD are in the sequence of (001)＞(010)＞(100).Considered various aspects comprehensively, the mechanical properties of the PBXsbicyclo-HMX(010)/F₂₃₁₄ and TNAD(001)/PVDF are better. The interactions between each fluorine polymer and different crystalline surfaces of bicyclo-HMX and TNAD alldecrease in the order of (010)＞(100)＞(001), and those of the polymers with the samecontent decrease in the sequence of PVDF＞F₂₃₁₁＞F₂₃₁₄＞PCTFE. Detonation propertiesdecrease slightly compared with the pure crystal by the additions of small amounts offluorine polymers, but they are still superior to the commonly used explosive TNT andhence can be used as good energetic materials with high exploitation and applicationvalues.In a word, a set of convenient methods are developed in this thesis based on thequantum chemistry to compute the density (ρ), detonation velocity (D), detonation pressure(P), and bond dissociation energy (BDE) of energetic compounds. According toquantitative criteria of energy and stability demand for a HEDC, a series of potentialHEDC targets are recommended among various series of cyclic nitramine derivativesbased on the systematic theoretical studies on their structures and properties, and theinsensitive energetic compound bicyclo-HMX is especially recommended to exploit. Theperiodic DFT and classical MD methods are first employed to calculate and simulate thecrystal structures and properties of typical bicyclic nitramines: bicyclo-HMX and TNAD.The disciplines of the changing of structures and properties with the pressure andtemperature are investigated. MD method is also used to simulate the structures andproperties of bicyclo-HMX and TNAD-based PBX with various polymer binders ondifferent crystal surfaces, which provide a demonstration and plentiful information forchoosing optimal polymer binders and formulation design of HEDM. These studies areprecursory and originally innovative in front of multi-subject crossing research fields ofHEDC/HEDM, and successfully complete the scientific research tasks of basicapplications assigned by the national "973" projects and National Natural ScienceFoundations.