Understanding Trigger Linkage Dynamics in Energetic Materials Using Mixed Picramide Nitrate Ester Explosives.

The ability to predict the handling sensitivity of new organic energetic materials has been a longstanding goal. We report the synthesis and characterization of six new nitropicramide energetic materials with mixed functional groups that mimic known explosives such as nitroglycerin, erythritol tetranitrate (ETN), and pentaerythritol tetranitrate (PETN). The molecules have been studied theoretically using quantum molecular dynamics (QMD) simulations and density functional theory (DFT) calculations to identify the weakest bond in the reactants - the trigger-linkages - which control handling sensitivity, and to quantify their specific enthalpies of explosion.

Machine Learning Models for High Explosive Crystal Density and Performance.

The rate of discovery of new explosives with superior energy density and performance has largely stalled. Rapid property prediction through machine learning has the potential to accelerate the discovery of new molecules by screening of large numbers of molecules before they are ever synthesized. To support this goal, we assembled a 21,000-molecule database of experimentally synthesized molecules containing energetic functional groups.

Transport Properties of Liquid Pentaerythritol Tetranitrate (PETN) from Molecular Dynamics Simulations.

We have used molecular dynamics simulations to determine the transport properties of liquid pentaerythritol tetranitrate (PETN), an important energetic material. The density, ρ, self-diffusion coefficient, , thermal conductivity, κ, and shear viscosity, μ, have been computed over pressures and temperatures relevant to the subshock regime (up to 1000 K and a few GPa), where PETN is known to melt prior to initiation. We find that the thermal conductivity κ(, ) can be represented by a simple analytical function that fits the data points with very good accuracy, even beyond the subshock regime, up to 2000 K and 20 GPa.

An Integrated Experimental and Modeling Approach for Assessing High-Temperature Decomposition Kinetics of Explosives.

We present a new integrated experimental and modeling effort that assesses the intrinsic sensitivity of energetic materials based on their reaction rates. The High Explosive Initiation Time (HEIT) experiment has been developed to provide a rapid assessment of the high-temperature reaction kinetics for the chemical decomposition of explosive materials. This effort is supported theoretically by quantum molecular dynamics (QMD) simulations that depict how different explosives can have vastly different adiabatic induction times at the same temperature.

An updated technique to obtain explosive kinetics data on microsecond timescales.

There are few techniques available for chemists to obtain time-to-explosion data with known temperature inputs at the early stages of the design and synthesis of new explosives. In the 1960s, a technique was developed to rapidly heat milligram-quantities of confined explosives to ∼1000 K on microsecond timescales. Wenograd [Trans.

Efficient Parameterization of Density Functional Tight-Binding for 5-Elements: A Th-O Case Study.

Density functional tight binding (DFTB) models for -element species are challenging to parametrize owing to the large number of adjustable parameters. The explicit optimization of the terms entering the semiempirical DFTB Hamiltonian related to orbitals is crucial to generating a reliable parametrization for -block elements, because they play import roles in bonding interactions. However, since the number of parameters grows quadratically with the number of orbitals, the computational cost for parameter optimization is much more expensive for the -elements than for the main group elements.

Properties of Erythritol Tetranitrate from Molecular Dynamics Simulation.

The nonpolarizable force field for alkyl nitrates developed by Borodin et al. [, , , 734-742] has been employed to calculate selected properties of crystalline and liquid erythritol tetranitrate (ETN). The set of partial charges proposed by Borodin for pentaerythritol tetranitrate (PETN) was used except for a small correction to the H atom charges to ensure charge neutrality owing to the absence of the neopentyl carbon in ETN.

Halogenated PETN derivatives: interplay between physical and chemical factors in explosive sensitivity.

Determining the factors that influence and can help predict energetic material sensitivity has long been a challenge in the explosives community. Decades of literature reports identify a multitude of factors both chemical and physical that influence explosive sensitivity; however no unifying theory has been observed. Recent work by our team has demonstrated that the kinetics of "trigger linkages" (, the weakest bonds in the energetic material) showed strong correlations with experimental drop hammer impact sensitivity.

Understanding Explosive Sensitivity with Effective Trigger Linkage Kinetics.

We present a simple linear model for ranking the drop weight impact sensitivity of organic explosives that is based explicitly on chemical kinetics. The model is parameterized to specific heats of explosion, , and Arrhenius kinetics for the onset of chemical reactions that are obtained from gas-phase Born-Oppenheimer molecular dynamics simulations for a chemically diverse set of 24 molecules. Reactive molecular dynamics simulations sample all possible decomposition pathways of the molecules with the appropriate probabilities to provide an effective reaction barrier.

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.

Identifying the Molecular Properties that Drive Explosive Sensitivity in a Series of Nitrate Esters.

Energetic materials undergo hundreds of chemical reactions during exothermic runaway, generally beginning with the breaking of the weakest chemical bond, the "trigger linkage." Herein we report the syntheses of a series of pentaerythritol tetranitrate (PETN) derivatives in which the energetic nitrate ester groups are systematically substituted by hydroxyl groups. Because all the PETN derivatives have the same nitrate ester-based trigger linkages, quantum molecular dynamics (QMD) simulations show very similar Arrhenius kinetics for the first reactions.

Systematic literature review assessing the overall costs and societal impacts of moderate-to-severe atopic dermatitis in Europe.

Atopic dermatitis (AD) is a chronic inflammatory disease, driven by type 2 inflammation. The condition manifests as moderate-to-severe disease in approximately 20% of adults with AD across Europe and is associated with a substantial burden on patients, society and healthcare systems. However, systematic assessments capturing the totality of disease burden associated with moderate-to-severe AD are limited; therefore, the overall impacts of the disease may be underestimated.

Atom Equivalent Energies for the Rapid Estimation of the Heat of Formation of Explosive Molecules from Density Functional Tight Binding Theory.

Atom equivalent energies have been derived from which the gas-phase heat of formation of explosive molecules can be estimated from fast, semiempirical density functional tight binding total energy calculations. The root-mean-square deviation and maximum deviation of the heats of formation from the experimental values for the set of 45 energetic molecules compiled by Byrd and Rice [ , 2006, 110, 1005-1013] are 10.4 and 25.

SCC-DFTB Parameters for Fe-C Interactions.

We present an optimized density-functional tight-binding (DFTB) parameterization for iron-based complexes based on the popular set of parameters. The transferability of the original and optimized parameterizations is assessed using a set of 50 iron complexes, which include carbonyl, cyanide, polypyridine, and cyclometalated ligands. DFTB-optimized structures predicted using the parameters show a good agreement with both experimental crystal geometries and density functional theory (DFT)-optimized structures for Fe-N bond lengths.

Insight into the Chemistry of PETN Under Shock Compression Through Ultrafast Broadband Mid-Infrared Absorption Spectroscopy.

Thin films of pentaerythritol tetranitrate (PETN) were shock compressed using the laser driven shock apparatus at Los Alamos National Laboratory (LANL). Two spectroscopic probes were available to this apparatus: visible white light transient absorption spectroscopy (VIS) from 400 to 700 nm and mid-infrared transient absorption spectroscopy (MIR) from 1150 to 3800 cm. Important PETN vibrational modes are the symmetric and antisymmetric NO stretches at 1280 and 1650 cm, respectively, as well as CH stretches at ∼2900 cm.

Tight-Binding Modeling of Uranium in an Aqueous Environment.

The first density functional tight-binding (DFTB) parameters for uranium, oxygen, and hydrogen chemistry are reported, which enable quantum molecular dynamics simulations that will be instrumental in understanding actinide speciation, reaction mechanisms, and kinetics. These parameters were fitted to atomization energies and forces obtained from density functional theory with a training set of small molecules that includes various oxidation states. The energetic results with these DFTB parameters for various reactions of hydration, hydrolysis, dimerization, and isomerization demonstrate that the DFTB method can qualitatively capture the correct chemistry with a small systematic deviation from the density functional theory reference values.

Development of Density Functional Tight-Binding Parameters Using Relative Energy Fitting and Particle Swarm Optimization.

We provide a strategy to optimize density functional tight-binding (DFTB) parameterization for the calculation of the structures and properties of organic molecules consisting of hydrogen, carbon, nitrogen, and oxygen. We utilize an objective function based on similarity measurements and the Particle Swarm Optimization (PSO) method to find an optimal set of parameters. This objective function considers not only the common DFTB descriptors of binding energies and atomic forces but also incorporates relative energies of isomers into the fitting procedure for more chemistry-driven results.

Ranking the Drop-Weight Impact Sensitivity of Common Explosives Using Arrhenius Chemical Rates Computed from Quantum Molecular Dynamics Simulations.

Drop-weight impact tests are used routinely to characterize the handling safety of explosives. Numerous studies have sought to connect various physical and chemical properties of the energetic molecules and materials to their measured impact sensitivities. Wenograd in the early 1960s demonstrated that there is a strong dependency of the drop-heights on the critical temperatures required for explosives to undergo prompt reactions.

Parallel replica dynamics simulations of reactions in shock compressed liquid benzene.

The study of the long-term evolution of slow chemical reactions is challenging because quantum-based reactive molecular dynamics simulation times are typically limited to hundreds of picoseconds. Here, the extended Lagrangian Born-Oppenheimer molecular dynamics formalism is used in conjunction with parallel replica dynamics to obtain an accurate tool to describe the long-term chemical dynamics of shock-compressed benzene. Langevin dynamics has been employed at different temperatures to calculate the first reaction times in liquid benzene at pressures and temperatures consistent with its unreacted Hugoniot.

Probing ultrafast shock-induced chemistry in liquids using broad-band mid-infrared absorption spectroscopy.

We probe shock-induced chemistry in two organic liquids by measuring broadband, midinfrared absorption in the 800-1400 cm frequency range. To test this new method and understand the signatures of chemical reactions in time resolved vibrational spectra, we compared liquid benzene shocked to unreactive conditions (shocked to a pressure of 18 GPa for a duration of 300 ps) to nitromethane under reactive conditions (25 GPa). We see clear signatures of shock-induced chemistry that are distinguishable from the pressure- and temperature-induced changes in vibrational mode shapes.

Transferable density functional tight binding for carbon, hydrogen, nitrogen, and oxygen: Application to shock compression.

A new parameterization for density functional tight binding (DFTB) theory, 31, has been developed for molecules containing carbon, hydrogen, nitrogen, and oxygen. Optimal values for the Hubbard s, on-site energies, and the radial dependences of the bond integrals and repulsive potentials were determined by numerical optimization using simulated annealing to a modest database of -calculated atomization energies and interatomic forces. The transferability of the optimized DFTB parameterization has been assessed using the CHNO subset of the QM-9 database [R.

Linear Scaling Pseudo Fermi-Operator Expansion for Fractional Occupation.

Recursive Fermi-operator expansion methods for the calculation of the idempotent density matrix are valid only at zero electronic temperature with integer occupation numbers. We show how such methods can be modified to include fractional occupation numbers of an approximate or pseudo Fermi-Dirac distribution and how the corresponding entropy term of the free energy is calculated. The proposed methodology is demonstrated and evaluated for different electronic structure methods, including density functional tight-binding theory, Kohn-Sham density functional theory using numerical orbitals, and quantum chemistry Hartree-Fock theory using Gaussian basis functions.

Examining the chemical and structural properties that influence the sensitivity of energetic nitrate esters.

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity and chemical intramolecular effects to mesoscale structure and defects, and has been a topic of extensive study for over 50 years. Due to these complex competing chemical and physical elements, a unifying relationship between molecular framework, crystal structure, and sensitivity has yet to be developed. In order to move towards this goal, ideally experimental studies should be performed on systems with small, systematic structural modifications, with modeling utilized to interpret experimental results.

Numerical Optimization of Density Functional Tight Binding Models: Application to Molecules Containing Carbon, Hydrogen, Nitrogen, and Oxygen.

New parametrizations for semiempirical density functional tight binding (DFTB) theory have been developed by the numerical optimization of adjustable parameters to minimize errors in the atomization energy and interatomic forces with respect to ab initio calculated data. Initial guesses for the radial dependences of the Slater-Koster bond integrals and overlap integrals were obtained from minimum basis density functional theory calculations. The radial dependences of the pair potentials and the bond and overlap integrals were represented by simple analytic functions.