Experimental and theoretical studies on the structure and vibrational properties of nitropyrazoles

Precise control of the degree and regioselectivity of functionalization in nitro- and amino-functionalized di(trispyrazolylborato)iron(II) spin crossover complexes

Di(trispyrazolylborato)iron(II) ([Tp₂Fe]) complexes represent one of the most robust classes of spin-crossover complexes. Their stability renders them particularly suitable for integration in nanoscale devices, e.g. as sensors or information storage units. While prior studies of the functionalization of those derivatives have been focused on the electronic and steric effects of alkyl and -CF₃ groups in position 3, a pyrazole exchange reaction between nitropyrazole and either trispyrazolylborate or its iron complex allows the regioselective installation of nitro substituents in positions 3, 4 and 5 of the [Tp₂Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine-functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of di- and tetra-3-NO₂ substituted complexes, all derivatives display spin crossover properties in the solid state, with transition temperatures ranging from 180 to 380 K and showing different degrees of abruptness but no hysteresis. The Slichter-Drickamer model was used to extract the empirical thermodynamic transition parameters, allowing a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substitution on the magnetic properties of the complexes. The steric effects dominate for substitution in position 3 but the electronic effects are significant for the other positions.

Recent Advances in Synthesis and Properties of Nitrated-Pyrazoles Based Energetic Compounds

Nitrated-pyrazole-based energetic compounds have attracted wide publicity in the field of energetic materials (EMs) due to their high heat of formation, high density, tailored thermal stability, and detonation performance. Many nitrated-pyrazole-based energetic compounds have been developed to meet the increasing demands of high power, low sensitivity, and eco-friendly environment, and they have good applications in explosives, propellants, and pyrotechnics. Continuous and growing efforts have been committed to promote the rapid development of nitrated-pyrazole-based EMs in the last decade, especially through large amounts of Chinese research. Some of the ultimate aims of nitrated-pyrazole-based materials are to develop potential candidates of castable explosives, explore novel insensitive high energy materials, search for low cost synthesis strategies, high efficiency, and green environmental protection, and further widen the applications of EMs. This review article aims to present the recent processes in the synthesis and physical and explosive performances of the nitrated-pyrazole-based Ems, including monopyrazoles with nitro, bispyrazoles with nitro, nitropyrazolo[4,3-c]pyrazoles, and their derivatives, and to comb the development trend of these compounds. This review intends to prompt fresh concepts for designing prominent high-performance nitropyrazole-based EMs.

Properties and Promise of Catenated Nitrogen Systems As High-Energy-Density Materials

The properties of catenated nitrogen molecules, molecules containing internal chains of bonded nitrogen atoms, is of fundamental scientific interest in chemical structure and bonding, as nitrogen is uniquely situated in the periodic table to form kinetically stable compounds often with chemically stable N-N bonds but which are thermodynamically unstable in that the formation of stable multiply bonded N₂ is usually thermodynamically preferable. This unique placement in the periodic table makes catenated nitrogen compounds of interest for development of high-energy-density materials, including explosives for defense and construction purposes, as well as propellants for missile propulsion and for space exploration. This review, designed for a chemical audience, describes foundational subjects, methods, and metrics relevant to the energetic materials community and provides an overview of important classes of catenated nitrogen compounds ranging from theoretical investigation of hypothetical molecules to the practical application of real-world energetic materials. The review is intended to provide detailed chemical insight into the synthesis and decomposition of such materials as well as foundational knowledge of energetic science new to most chemists.

Multimolecular Complexes of CL-20 with Nitropyrazole Derivatives: Geometric, Electronic Structure, and Stability

The multimolecular complexes formed between 2,4,6,8,10,12-hexanitro-2,4,6,6,8,10,12-hexaazaisowurtzitane (CL-20) and nitropyrazole compounds were investigated using B3LYP-D3/6-311G(d,p) and B97-3c methods. CL-20 in these complexes was surrounded by methyl, nitro, and amino derivatives of 4-nitropyrazole. The influence of substituents on the molecular electrostatic potential distribution of nitropyrazoles was investigated to figure out the potential electrostatic interaction sites. For the complex, the O···H hydrogen bond was popular in the intermolecular interactions, and dispersion interaction played an essential role, especially in Cx/CL-20 multimolecular complexes. Trigger bond analysis showed that their strength increased upon the formation of intermolecular weak interactions. Nitro group charge calculations stated that the negative charge on almost all nitro groups showed a significant increase. Therefore, the sensitivity of CL-20 seemed to be lower than the original. In addition, the transfer of electron density between CL-20 and nitropyrzoles in complexes was investigated, revealing the influence of weak interactions on the electron density of CL-20.

Real-time fingerprinting of structural isomers using laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) has surfaced as an attractive alternative to mass spectrometry and wet chemistry methods for chemical identification, driven by its real-time, label-free nature. Rapid analysis needs, especially in high-energy materials and pharmaceutical compounds, have further fueled an increasing number of refinements in LIBS. Yet, isomers are seldom identifiable by LIBS as they generate nearly identical spectra. Here we employ a suite of chemometric approaches to exploit the subtle, but reproducible, differences in LIBS spectra acquired from structural isomers, a set of pyrazoles, to develop a sensitive and reliable segmentation method. We also investigate the possible mechanistic principles (causation) behind such spectral variations and confirm their statistically significant nature that empowers the excellent classification performance.

Theoretical investigation on the structure and performance of N, N'-azobis-polynitrodiazoles

Six novel high energy density compounds of N, N'-azobis-polynitrodiazoles were designed. Their optimized geometric and electronic structures, band gaps, and heats of formation were explored at B3LYP/aug-cc-pVDZ level of density functional theory (DFT). Detonation properties were predicted by Kamlet-Jacobs equations. Results show that the designed compounds have high densities (1.80 to 1.84 g · cm⁻³) and excellent detonation performance (D 8.51 to 9.02 km · s⁻¹, P 32.16 to 36.58 GPa). In addition, the bond dissociation energies of C-NO₂ bonds were found to range from 223.59 to 240.46 kJ · mol⁻¹. All of them appear to be potential explosives compared with the well known ones, 1,3,5-trinitro-1,3,5-triazine (RDX, 8.75 km · s⁻¹, 34.70 GPa) and octahydro- 1,3,5,7-tetranitro-1,3,5,7-tetraazocane (HMX, 8.96 km · s⁻¹, 35.96 GPa), especially R3 (8.98 km · s⁻¹, 36.19 GPa) and R6 (9.02 km · s⁻¹, 36.58 GPa). Finally, the position and number of nitro groups in the N, N'-azobis-polynitrodiazoles determine the heat of formation, stability, sensitivity, density, and detonation performance of these compounds.