High-Pressure Studies of Hydrogen-Bonded Energetic Material 3,6-Dihydrazino-s-tetrazine Using DFT

Hydrolytically Degradable PEG-Based Inverse Electron Demand Diels-Alder Click Hydrogels

Hydrogels cross-linked by inverse electron demand Diels–Alder (iEDDA) click chemistry are increasingly used in biomedical applications. With a few exceptions in naturally derived and chemically modified macromers, iEDDA click hydrogels exhibit long-term hydrolytic stability, and no synthetic iEDDA click hydrogels can undergo accelerated and tunable hydrolytic degradation. We have previously reported a novel method for synthesizing norbornene (NB)-functionalized multiarm poly(ethylene glycol) (PEG), where carbic anhydride (CA) was used to replace 5-norbornene-2-carboxylic acid. The new PEGNB_CA-based thiol-norbornene hydrogels exhibited unexpected fast yet highly tunable hydrolytic degradation. In this contribution, we leveraged the new PEGNB_CA macromer for forming iEDDA click hydrogels with [methyl]tetrazine ([m]Tz)-modified macromers, leading to the first group of synthetic iEDDA click hydrogels with highly tunable hydrolytic degradation kinetics. We further exploited Tz and mTz dual conjugation to achieve tunable hydrolytic degradation with an in vitro degradation time ranging from 2 weeks to 3 months. Finally, we demonstrated the excellent in vitro cytocompatibility and in vivo biocompatibility of the new injectable PEGNB_CA-based iEDDA click cross-linked hydrogels.

Recent advances in studying the nonnegligible role of noncovalent interactions in various types of energetic molecular crystals

This highlight summarizes the research progress on the considerable effects of noncovalent interactions on diverse types of energetic materials and enlighten us to explore new factors that affect the key performance of explosives.

Quantum chemistry for molecules at extreme pressure on graphical processing units: Implementation of extreme-pressure polarizable continuum model

Pressure plays essential roles in chemistry by altering structures and controlling chemical reactions. The extreme-pressure polarizable continuum model (XP-PCM) is an emerging method with an efficient quantum mechanical description of small- and medium-sized molecules at high pressure (on the order of GPa). However, its application to large molecular systems was previously hampered by a CPU computation bottleneck: the Pauli repulsion potential unique to XP-PCM requires the evaluation of a large number of electric field integrals, resulting in significant computational overhead compared to the gas-phase or standard-pressure polarizable continuum model calculations. Here, we exploit advances in graphical processing units (GPUs) to accelerate the XP-PCM-integral evaluations. This enables high-pressure quantum chemistry simulation of proteins that used to be computationally intractable. We benchmarked the performance using 18 small proteins in aqueous solutions. Using a single GPU, our method evaluates the XP-PCM free energy of a protein with over 500 atoms and 4000 basis functions within half an hour. The time taken by the XP-PCM-integral evaluation is typically 1% of the time taken for a gas-phase density functional theory (DFT) on the same system. The overall XP-PCM calculations require less computational effort than that for their gas-phase counterpart due to the improved convergence of self-consistent field iterations. Therefore, the description of the high-pressure effects with our GPU-accelerated XP-PCM is feasible for any molecule tractable for gas-phase DFT calculation. We have also validated the accuracy of our method on small molecules whose properties under high pressure are known from experiments or previous theoretical studies.

High pressure structural behaviour of 5,5'-bitetrazole-1,1'-diolate based energetic materials: a comparative study from first principles calculations

Pressure on the scale of gigapascals can cause incredible variations in the physicochemical and detonation characteristics of energetic materials. As a continuation of our earlier work (B. Moses Abraham, et al., Phys. Chem. Chem. Phys., 2018, 20, 29693-29707), here we report the high pressure structural and vibrational properties of 5,5'-bitetrazole-1,1'-diolate based energetic ionic salts via dispersion-corrected density functional theory calculations. Remarkably, these energetic materials exhibit anisotropic behavior along three crystallographic directions with progressing pressure; especially, the maximum and minimum reduction in volume is observed for HA-BTO and TKX-50, respectively. The large bulk modulus of TKX-50 (28.64) indicates its hard nature when compared to other BTO-based energetic salts. The effect of pressure on hydrogen bonded D-H⋯A energetic materials induces spectral shift (lengthening/shortening) in the donor group (D-H) of the stretching vibrations and is widely recognized as the signature of hydrogen bonding. We observed unusual contraction of the D-H bond under compression due to the short range repulsive forces encountered by the H atom while the molecule attempts to stabilize. The Hirshfeld surface analysis highlights the pressure induced stabilization of HA-BTO due to increased N⋯H/H⋯N and O⋯H/H⋯O close contact of hydrogen bond acceptors and donors. These studies provide theoretical guidance as a function of pressure, on how the micro-structures and intermolecular interactions can tune macroscopic properties to enhance the energetic performance.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Possible pre-phase transition of the α-HMX crystal observed by the variation of hydrogen-bonding network under high pressures

The variation of non-covalent interactions in the HMX crystal under high pressures was investigated through disassembling the hydrogen-bonding network.

Molecular dynamics simulations of a cyclotetramethylene tetra-nitramine/hydrazine 5,5'-bitetrazole-1,1'-diolate cocrystal

An energetic ionic salt (EIS)-based cocrystal formation, cyclotetramethylene tetra-nitramine (HMX)/hydrazine 5,5′-bitetrazole-1,1′-diolate (HA·BTO), is predicted based on molecular dynamics simulations. HA·BTO is a newly-synthesized environmentally friendly energetic ionic salt with good detonation performance and low sensitivity. Calculated powder X-ray diffraction patterns and intermolecular interactions deduce the formation of the new cocrystal structure. Radial distribution function analysis suggests that hydrogen bonds and van der Waals (vdW) forces exist between the H⋯O pairs of HMX and HA·BTO, while the hydrogen bonds between the H of HA·BTO and the O of HMX play a prominent role. The cohesive energy density and mechanical properties are also analyzed. The cohesive energy density of the HMX/HA·BTO cocrystal is larger than that of the composite of HMX and HA·BTO, indicating an improvement in crystal stability by cocrystalization. Compared to both HMX and HA·BTO, HMX/HA·BTO has smaller Young modulus, bulk modulus and shear modulus values, but larger K/G values and a positive Cauchy pressure, suggesting decreased stiffness and improved ductibility. Moreover, the calculated formation energy is −405.79 kJ mol⁻¹ at 298 K, which implies that the proposed cocrystal structure is likely to be synthesized at ambient temperature. In summary, we have predicted an EIS-based cocrystal formation in which the safety and mechanical properties of HMX have been improved via cocrystalization with HA·BTO, and this provides deep insight into the formation mechanism of the EIS-based cocrystal.

An energetic ionic salt-based cocrystal formation, HMX/HA·BTO, is predicted based on molecular dynamics simulations.

The nature of the chemical bond in oxyanionic crystals based on QTAIM topological analysis of electron densities

The QTAIM topological analysis of the calculated electron densities in oxyanionic crystals revealed the covalency criteria for metal–oxygen and hydrogen bonds.