Intermolecular Weak Hydrogen Bonding (Het-H-N/O): an Effective Strategy for the Synthesis of Monosubstituted 1,2,4,5-Tetrazine-Based Energetic Materials with Excellent Sensitivity

Obtaining superior high-density fused-ring energetic materials via the introduction of carbonyl, o-NH2-NO2 and nitroamino groups

Two carbonyl and o-NH2-NO2-containing energetic materials and their analogues were effectively designed, synthesized and fully characterized with multinuclear NMR, IR and elemental analyses. Their structures were also further confirmed via X-ray diffraction. Among them, compound 7 exhibits good potential for application as a secondary explosive with extremely high density (2.04 g cm-3), good sensitivity (IS > 40 J, FS > 360 N), and excellent calculated detonation performance (Dv = 8943 m s-1, P = 35.0 GPa). Furthermore, a detailed comparative study based on X-ray diffraction, Hirshfeld surfaces and 2D fingerprint plots among compounds 4, 7 and 9 has demonstrated that the density and detonation performance could be effectively improved via introducing a carbonyl group into fused-ring compounds. More importantly, the sensitivity of the resulting energetic materials did not deteriorate. Obviously, this strategy via introducing carbonyl, o-NH2-NO2 and nitroamino groups into fused-ring energetic compounds will help in the design of next-generation high-energy and insensitive fused-ring energetic materials.

Can Catenated Nitrogen Compounds with Amine-like Structures Become Candidates for High-Energy-Density Compounds?

The worthwhile idea of whether amine-like catenated nitrogen compounds are stable enough to be used as high-energy materials was proposed and answered. Abstracting the NH3 structure into NR3 (R is the substituent) yields a new class of amine-like catenated nitrogen compounds. Most of the azole ring structures have a high nitrogen content and stability. Inspired by this idea, a series of new amine-like catenated nitrogen compounds (A1 to H5) were designed, and their basic energetic properties were calculated. The results showed that (1) amine-like molecular structures are often characterized by low density; however, the density of these compounds increases as the number of nitrogens in the azole ring increases; (2) these catenated nitrogen compounds generally have extremely high enthalpies of formation (882.91-2652.03 kJ/mol), and the detonation velocity of some compounds exceeds 9254.00 m/s; (3) the detonation performance of amine-like catenated nitrogen compounds designed based on imidazole and pyrazole rings is poor due to their low nitrogen content; and (4) the bond dissociation enthalpy of trigger bonds of most compounds is higher than 84 kJ/mol, indicating that these compounds have a certain thermodynamic stability. In summary, amine-like catenated nitrogen compounds have the potential to become energetic compounds with excellent detonation properties and should be considered to be synthesized by experimental chemists.

A multi-fused heat-resistant energetic compound constructed by hydrogen bonds

The design of heat-resistant energetic compounds generally employs symmetry, planarity, and multi-hydrogen bonds to obtain compounds with high density, good thermal stability, and low sensitivity. In this paper, a heat-resistant hydrazine-bridged compound, 6,6'-(hydrazine-1,2-diyl)bis(5-nitropyrimidine-2,4-diamine) (PHP), was designed and synthesized with the strategy of multi-fused conjugated structure constructed by hydrogen bonds. The compound featured high symmetry, high planarity, and strong conjugation with good thermal stability (364 °C). This strategy provides a basis for the design of heat-resistant energetic compounds.

Chemical Descriptors for a Large-Scale Study on Drop-Weight Impact Sensitivity of High Explosives

The drop-weight impact test is an experiment that has been used for nearly 80 years to evaluate handling sensitivity of high explosives. Although the results of this test are known to have large statistical uncertainties, it is one of the most common tests due to its accessibility and modest material requirements. In this paper, we compile a large data set of drop-weight impact sensitivity test results (mainly performed at Los Alamos National Laboratory), along with a compendium of molecular and chemical descriptors for the explosives under test. These data consist of over 500 unique explosives, over 1000 repeat tests, and over 100 descriptors, for a total of about 1500 observations. We use random forest methods to estimate a model of explosive handling sensitivity as a function of chemical and molecular properties of the explosives under test. Our model predicts well across a wide range of explosive types, spanning a broad range of explosive performance and sensitivity. We find that properties related to explosive performance, such as heat of explosion, oxygen balance, and functional group, are highly predictive of explosive handling sensitivity. Yet, models that omit many of these properties still perform well. Our results suggest that there is not one or even several factors that explain explosive handling sensitivity, but that there are many complex, interrelated effects at play.

An Alliance of Polynitrogen Heterocycles: Novel Energetic Tetrazinedioxide-Hydroxytetrazole-Based Materials

Energetic materials constitute one of the most important subtypes of functional materials used for various applications. A promising approach for the construction of novel thermally stable high-energy materials is based on an assembly of polynitrogen biheterocyclic scaffolds. Herein, we report on the design and synthesis of a new series of high-nitrogen energetic salts comprising the C-C linked 6-aminotetrazinedioxide and hydroxytetrazole frameworks. Synthesized materials were thoroughly characterized by IR and multinuclear NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction and differential scanning calorimetry. As a result of a vast amount of the formed intra- and intermolecular hydrogen bonds, prepared ammonium and amino-1,2,4-triazolium salts are thermally stable and have good densities of 1.75–1.78 g·cm⁻³. All synthesized compounds show high detonation performance, reaching that of benchmark RDX. At the same time, as compared to RDX, investigated salts are less friction sensitive due to the formed net of hydrogen bonds. Overall, reported functional materials represent a novel perspective subclass of secondary explosives and unveil further opportunities for an assembly of biheterocyclic next-generation energetic materials.

Crystal Structure and Noncovalent Interactions of Heterocyclic Energetic Molecules

Nitrogen-rich heterocyclic compounds are important heterocyclic substances with extensive future applications for energetic materials due to their outstanding density and excellent physicochemical properties. However, the weak intermolecular interactions of these compounds are not clear, which severely limits their widespread application. Three nitrogen-rich heterocyclic compounds were chosen to detect their molecular geometry, stacking mode and intermolecular interactions by crystal structure, Hirshfeld surface, RDG and ESP. The results show that all atoms in each molecule are coplanar and that the stacking mode of the three crystals is a planar layer style. A large amount of inter- and intramolecular interaction exists in the three crystals. All principal types of intermolecular contacts in the three crystals are N···H interactions and they account for 40.9%, 38.9% and 32.9%, respectively. Hydrogen bonding, vdW interactions and steric effects in Crystal c are stronger than in Crystals a and b. The negative ESPs all concentrate on the nitrogen atoms in the three molecules. This work is expected to benefit the crystal engineering of heterocyclic energetic materials.

A property-oriented adaptive design framework for rapid discovery of energetic molecules based on small-scale labeled datasets

It remains an important challenge to apply machine learning in material discovery with limited-scale datasets available, in particular for the energetic materials. Motivated by the challenge, we developed a Property-oriented Adaptive Design Framework (PADF) to quickly design new energetic compounds with desired properties. The PADF consists of a search space, machine learning model, optimization algorithm and an evaluator based on quantum mechanical calculations. The effectiveness and generality of the PADF were assessed by two case studies on the heat of formation and heat of explosion as the target properties. 88 compounds were selected as the initial training dataset from the search space containing 84 083 compounds generated. SVR.lin/Trade-off coupled with E-state + SOB and KRR/KG coupled with CDS + E-state + SOB were determined to be the best combination pairs for the heat of formation and the heat of explosion, respectively. Most of the ten compounds selected from the first ten iterations exhibit better properties than the optimal sample in the initial dataset. Besides, the heat of explosion as the target property outperforms the heat of formation in designing energetic compounds with high detonation performance. In particular, a new compound selected at the 3rd iteration exhibits high potential as an explosive. Our strategy could be extended to other domains limited by small-scale datasets labeled.

In this work, we construct a self-adaptive design framework to efficiently screen energetic compounds with the desired heat of formation and heat of explosion from the vast chemical space unexplored.

Can the sensitivity of energetic materials be tuned by using hydrogen bonds? Another look at the role of hydrogen bonding in the design of high energetic compounds

Strongly positive electrostatic potential in the central areas of molecules of energetic materials is one of the most important factors that determines the sensitivity of these molecules towards detonation. Quantum chemical and density functional theory calculations were used to reveal the influence of hydrogen bonding on the values of electrostatic potential above the central areas of molecules of three conventional explosives: 1,3,5-trinitrobenzene, 2,4,6-trinitrophenol, and 2,4,6-trinitrotoluene. Both the case when energetic molecules act as hydrogen atom donors and when they act as hydrogen atom acceptors were considered. Results of the calculations performed using the M06/cc-PVDZ level of theory showed that there are significant differences in the influence of hydrogen bonding on the electrostatic potential of energetic molecules acting as hydrogen atom donors and hydrogen atom acceptors. In the case when energetic molecules act as hydrogen acceptors, an increase of 10% in the strength of positive electrostatic potential was identified. In the case when energetic molecules act as hydrogen atom donors, a significant decrease (20-25%) in the strength of the positive potential on the molecular surface was calculated. These differences give an opportunity for fine-tuning the impact sensitivities of energetic compounds and provide new guidelines for the design of explosives with desirable characteristics.

Mono-N-oxidation of heterocycle-fused pyrimidines

Mild oxidation reactions of nitrogen-rich heterocyclic rings lead to the formation of energetic compounds with the mono-N-oxide moiety which show good thermal stabilities and detonation performances.