Molecular polarizabilities of some energetic compounds.

The dependence of sensitivity of an explosive on its molecular structure may be mainly attributed to the molecular deformability, which can be expressed by some characteristic parameters, resonance energy for aromatic an explosive, strain energy for a strained-ring or strained-cage explosive, large π-π separation energy for a large π-π linked-explosive, bond rotational energy barriers of C-NO2, N-NO2, O-NO2 for C-NO2, N-NO2, O-NO2 bond-based explosives, and so on. Molecular polarizability of an explosive is also an important molecular deformability index, which can be effectively used to compare impact sensitivities of explosive's isomers, isoelectronic species, and similar structures. Interestingly, comparing the molecular polarizabilities under external electric fields with different energy levels of isomeric N20(Ih) and N20(D3d) clusters and the Mo2N20 and Re2N20 complex compounds, it is found that there are different energy thresholds of significant molecular expansion.

Novel insensitive energetic-cocrystal-based BTO with good comprehensive properties.

Combining a layer construction strategy with cocrystallization techniques, we designed and prepared a structurally unusual 1H,1'H-5,5'-bistetrazole-1,1'-diolate (BTO) based energetic cocrystal, which we also confirmed by single-crystal X-ray diffraction and powder-crystal X-ray diffraction. The obtained cocrystal crystallizes in a triclinic system, P-1 space group, with a density of 1.72 g cm-3. The properties including the thermal stability, sensitivity and detonation performance of the cocrystal were analyzed in detail. In addition, the thermal decomposition behavior of the cocrystal was studied by differential calorimetry and thermogravimetry tandem infrared spectroscopy. The results indicated that the cocrystal exhibits strong resistance to thermal decomposition up to 535.6 K. The cocrystal also demonstrates a sensitivity of >50 J. Moreover, its formation enthalpy was estimated to be 2312.0 kJ mol-1, whereas its detonation velocity and detonation pressure were predicted to be 8.213 km s-1 and 29.1 GPa, respectively, by applying K-J equations. Therefore, as expected, the obtained cocrystal shows a good comprehensive performance, which proves that a high degree of layer-by-layer stacking is essential for the structural density, thermal stability and sensitivity.

Insight into electrostatic initiation of nitramine explosives.

The electrostatic safety of explosives is of great importance. However, the mechanism for the transfer of energy from an electrostatic spark to the reactive center of an explosive material is not well understood. Thus, in this work, we attempted to clarify the mechanism associated with the static-electricity-initiated detonation of explosives using a model of the interaction that incorporated relevant parameters. Nitramine explosives were considered as examples to study the relationship between electrostatic spark energy and 32 relevant parameters. The four parameters that were most closely correlated with the electrostatic spark energy were the standard deviation of the negative electrostatic potential, the minimum surface electrostatic potential, the minimum ionization energy, and the detonation pressure. A model for the dependence of the electrostatic spark energy on these four parameters was derived using the theoretical method known as genetic function approximation. The electrostatic spark energy values predicted using this model were in good agreement with the corresponding experimental values. The results of this work should lead to a deeper understanding of the electrostatic initiation mechanism of nitramines, and help to inspire the design of new explosives.

A novel 3D energetic MOF of high energy content: synthesis and superior explosive performance of a Pb(ii) compound with 5,5'-bistetrazole-1,1'-diolate.

The development of high-performance insensitive energetic materials is important because of the increasing demand for these materials in military and civilian applications. A novel 3D energetic metal-organic framework (MOF) of exceptionally high energy content, [Pb(BTO)(H2O)]n, was synthesized and structurally characterized by single crystal X-ray diffraction, featuring a three-dimensional parallelogram porous framework, where BTO represents 5,5'-bistetrazole-1,1'-diolate. The thermal stability and energetic properties were determined, exhibiting good thermostability (Td = 309.0 °C), excellent detonation pressure (P) of 53.06 GPa, a detonation velocity (D) of 9.204 km s(-1), and acceptable sensitivity to confirmed impact (IS = 7.5 J). Notably, the complex possesses unprecedented superior density than the reported energetic MOFs. The results highlight this new MOF as a potential energetic material.

Quantitative correlation between facets defects of RDX crystals and their laser sensitivity.

In this work, the {210} facets of cyclotrimethylenetrinitramine (RDX) single crystals with different quality were studied by scanning electron microscopy and atomic force microscopy. Their laser sensitivity was then assessed using a direct laser ignition test irradiated with ultraviolet laser (wavelength: 355nm, pulse width: 6.4ns). Quantitative relationships between laser sensitivity and surface defects of RDX (210) and (2¯1¯0) facets were investigated. It is determined that the laser sensitivity exhibits significant correlation with the surface roughness, size of which is comparable with scales of laser wavelength. 3D FDTD simulations disclose that this relationship can be well explained with light intensity modulation effects induced by micro-defects on the initial plane wave.

Ultraviolet Laser-induced ignition of RDX single crystal.

The RDX single crystals are ignited by ultraviolet laser (355 nm, 6.4 ns) pulses. The laser-induced damage morphology consisted of two distinct regions: a core region of layered fracture and a peripheral region of stripped material surrounding the core. As laser fluence increases, the area of the whole crack region increases all the way, while both the area and depth of the core region increase firstly, and then stay stable over the laser fluence of 12 J/cm(2). The experimental details indicate the dynamics during laser ignition process. Plasma fireball of high temperature and pressure occurs firstly, followed by the micro-explosions on the (210) surface, and finally shock waves propagate through the materials to further strip materials outside and yield in-depth cracks in larger surrounding region. The plasma fireball evolves from isotropic to anisotropic under higher laser fluence resulting in the damage expansion only in lateral direction while maintaining the fixed depth. The primary insights into the interaction dynamics between laser and energetic materials can help developing the superior laser ignition technique.

Computational assessment of several hydrogen-free high energy compounds.

Tetrazino-tetrazine-tetraoxide (TTTO) is an attractive high energy compound, but unfortunately, it is not yet experimentally synthesized so far. Isomerization of TTTO leads to its five isomers, bond-separation energies were empolyed to compare the global stability of six compounds, it is found that isomer 1 has the highest bond-separation energy (1204.6kJ/mol), compared with TTTO (1151.2kJ/mol); thermodynamic properties of six compounds were theoretically calculated, including standard formation enthalpies (solid and gaseous), standard fusion enthalpies, standard vaporation enthalpies, standard sublimation enthalpies, lattice energies and normal melting points, normal boiling points; their detonation performances were also computed, including detonation heat (Q, cal/g), detonation velocity (D, km/s), detonation pressure (P, GPa) and impact sensitivity (h50, cm), compared with TTTO (Q=1311.01J/g, D=9.228km/s, P=40.556GPa, h50=12.7cm), isomer 5 exhibites better detonation performances (Q=1523.74J/g, D=9.389km/s, P=41.329GPa, h50= 28.4cm).

Nitrogen-Rich Energetic Metal-Organic Framework: Synthesis, Structure, Properties, and Thermal Behaviors of Pb(II) Complex Based on N,N-Bis(1H-tetrazole-5-yl)-Amine.

The focus of energetic materials is on searching for a high-energy, high-density, insensitive material. Previous investigations have shown that 3D energetic metal-organic frameworks (E-MOFs) have great potential and advantages in this field. A nitrogen-rich E-MOF, Pb(bta)·2H2O [N% = 31.98%, H2bta = N,N-Bis(1H-tetrazole-5-yl)-amine], was prepared through a one-step hydrothermal reaction in this study. Its crystal structure was determined through single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, and elemental analysis. The complex has high heat denotation (16.142 kJ·cm-3), high density (3.250 g·cm-3), and good thermostability (Tdec = 614.9 K, 5 K·min-1). The detonation pressure and velocity obtained through theoretical calculations were 43.47 GPa and 8.963 km·s-1, respectively. The sensitivity test showed that the complex is an impact-insensitive material (IS > 40 J). The thermal decomposition process and kinetic parameters of the complex were also investigated through thermogravimetry and differential scanning calorimetry. Non-isothermal kinetic parameters were calculated through the methods of Kissinger and Ozawa-Doyle. Results highlighted the nitrogen-rich MOF as a potential energetic material.

Insight into shock-induced chemical reaction from the perspective of ring strain and rotation of chemical bonds.

Density functional theory BLYP/DNP and hyperhomodesmotic equations were employed to calculate ring strain energy, the bond dissociation energy of X-NO(2) (X=C, N) and the charges on the nitro groups of several four-membered and six-membered heterocycle compounds. BLYP/DNP and LST/QST + CG method were also applied to calculate bond rotational energy of X-NO(2) (X=C, N) of above mentioned compounds. It indicated that ring strain energy of four-membered heterocycle nitro compounds is apparently higher than that of six-membered heterocycle nitro compounds. Predictably, ring-opening reactions may preferentially occur for those compounds containing higher ring strain energy under shock. In addition, C-NO(2) bonds in these compounds may rotate easier than N-NO(2) bonds in response to the external shock. As for N-NO(2) bonds in these compounds, they also respond to the external shock by the rotation of N-NO(2) bonds, once to the saddle point of the rotational energy barrier, the whole molecule will become relaxed, N-NO(2) bond becomes weaker and eventually leads to the breakage. When one -C=O, -C=NH or -NH(2) group is introduced to the six-membered heterocycle, the charges on the nitro groups of the new compound decrease drastically, and ring strains increase remarkably. It can be predicted that the new compounds will be more sensitive to shock, and the viewpoint is confirmed by the experimental results of shock sensitivity (small scale gap test) of several explosives.

An important factor in relation to shock-induced chemistry: resonance energy.

With density function theory BLYP/DNP method, together with homodesmotic reactions and isodesmic reactions, we calculated the resonance energies of some explosives, including eight nitro compounds which contains benzene rings, three nitro compounds which contains azaheterocycles (2,4-dinitroimidazole (2,4-DNI), 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) and 2,4,6-trinitro-1,3,5-triazine) and one nitrogen-rich energetic compound of 3,3'-azobis(6-amino-s-tetrazine) (DAAT). The results indicate that their resonance energies are in relation to their shock sensitivity which measuring their threshold pressures of initiation, that is, the lower the resonance energy is, the higher the shock sensitivity of the explosive behaves. And this measuring method according to resonance energy is based on the global property of the molecule instead of the local one, such as one nitro group in the molecule. It is meaningful to calculate resonance energies of these kind of compounds quickly and accurately because resonance structures exist widely in these organic compounds and resonance energies may play a significant role in determining their shock sensitivity, and it is helpful in the rational design or synthesis of high energy and insensitive materials.

On the shock sensitivity of explosive compounds with small-scale gap test.

In this work an improved set of small-scale gap tests was applied to measure the shock sensitivity of 13 explosive compounds, and a Mn-Cu manometer was also employed to measure the output pressures of shock waves passed through aluminum gaps with different thicknesses to draw a standard curve. The critical initiation thicknesses of aluminum gaps of different explosive compounds (244 shots in total) were treated according to statistical method, and the Mulliken charges of nitro groups, bond dissociation energies of X-NO(2) (X = C, N), resonance energies, and ring strain energies of these explosive compounds were calculated with the means of DFT/BLYP/DNP calculations and homodesmotic reactions designs. Genetic function approximation was used to construct a relationship between the critical initiation thicknesses of aluminum gaps of different explosive compounds and their forementioned molecule structural parameters.

The studies on the aromaticity of fullerenes and their holmium endohedral compounds.

Density functional theory BLYP/DNP was employed to optimize a series of fullerenes and their holmium endohedral compounds, including C(20), Ho@C(20), Ho(3+)@C(20), C(60), Ho@C(60), Ho(3+)@C(60),C(70), Ho@C(70), Ho(3+)@C(70) C(78), Ho@C(78), Ho(3+)@C(78), C(82),Ho@C(82) and Ho(3+)@C(82). DFT semi core pseudospot approximation was taken into consideration in the calculations of the element holmium because of its particular electronic structure. Fullerenes and their holmium endohedral compounds' aromaticity were studied in terms of structural criteria, energetic criteria, and reactivity criteria. The results indicate that the aromaticity of fullerenes was reduced when a holmium atom was introduced into the carbon cage, and the endohedral fullerenes' reactive activity enhance; but the aromaticity of the carbon cage increased when a Ho(3+) cation was encapsulated into a fullerene. Calculations of aromaticity and stability indicate that two paths can lead to the similar aim of preparing holmium endohedral fullerenes; that is, they can form from either a holmium atom or a holmium cation (Ho(3+)) reacting with fullerenes, respectively, and the latter is more favorable.

Two important factors influencing shock sensitivity of nitro compounds: Bond dissociation energy of X-NO2 (X = C, N, O) and Mulliken charges of nitro group.

DFT/BLYP/DNP is employed to calculate bond dissociation energy of X-NO(2) (X = C, N, O) and Mulliken charges of nitro group of 14 kinds of nitro compounds, and partial least squares approximation is applied to linearly fit their shock initiation pressure (p(90%,TMD)). It is found that the fitted values are in good agreement with the experimental shock initiation pressures. The fitted model is used to predict the shock initiation pressures of two kinds of explosives, TNB and TNETB. The predictive values are in accordance with experimental ones. It reflects that bond dissociation energy of X-NO(2) (X = C, N, O) and Mulliken charge of nitro groups may be the important factors influencing the shock sensitivity of nitro compounds. On the basis of the fitted model, bond dissociation energy of X-NO(2) (X = C) and Mulliken charges of nitro groups of another 14 kinds of heterocyclic nitro compounds are in consideration to predict shock sensitivity. This work is meaningful in further understanding the shock mechanism and helpful to the design and synthesis of novel energetic materials.

Theoretical investigation of an energetic fullerene derivative.

A self-consistent estimation method for the thermochemical properties of N-methyl-3-(2',4',6'-trinitrobenzene)-fulleropyrrolidine (MTNBFP) is presented. This method is based on enthalpy of formation (Delta(f)H(m)(minus sign in circle)) and enthalpy of combustion obtained from BLYP/DNP calculations of the total energies and frequencies for MTNBFP. The enthalpy of formation was calculated by an optimized set of isodesmic reactions given the available experimental Delta(f)H(m)(minus sign in circle) of relative compounds. MTNBFP has a high enthalpy of formation, 2782.2 kJ/mol. Detonation velocity and detonation pressure were also presented in terms of Kamlet and Jacobs equations. Drop hammer impact sensitivity tests and blasting point per 5 s tests indicate MTNBFP may be a potential candidate primary explosive. To understand the test results well, we proposed a series of chemical reaction mechanisms and interpreted the relationship between impact sensitivity and electronic structures from the viewpoint of nitro group charge, electrostatic potential, and vibrational modes.

A new method to evaluate the stability of the covalent compound: by the charges on the common atom or group.

A new method of comparing and analyzing the electrostatic potential (ESP) charges of the common atom or group to evaluate and compare the stabilities of covalent compounds was introduced. That is, covalent compounds will become more stable when the electron acceptors accept adequate electrons and possess adequate negative charges, and the electron donors donate adequate electrons and possess adequate positive charges accordingly. All calculations were performed by density functional theory (DFT) and the general gradient approximation (GGA) method with the Beck-LYP hybrid functional and the DNP basis set in Acceryls' code Dmol3. Calculation results verified the method considering the molecular structure is well applied in the covalent molecule systems of hydrides, oxides, alkyl radicals, and nitro compounds. Furthermore, the method has good operability, for the charges can be easily obtained by simple calculation.