Control of Explosive Chemical Reactions by Optical Excitations: Defect-Induced Decomposition of Trinitrotoluene at Metal Oxide Surfaces.

Interfaces formed by high energy density materials and metal oxides present intriguing new opportunities for a large set of novel applications that depend on the control of the energy release and initiation of explosive chemical reactions. We studied the role of structural defects at a MgO surface in the modification of electronic and optical properties of the energetic material TNT (2-methyl-1,3,5-trinitrobenzene, also known as trinitrotoluene, CHNO) deposited at the surface. Using density functional theory (DFT)-based solid-state periodic calculations with hybrid density functionals, we show how the control of chemical explosive reactions can be achieved by tuning the electronic structure of energetic compound at an interface with oxides.

Recruiting Perovskites to Degrade Toxic Trinitrotoluene.

Everybody knows TNT, the most widely used explosive material and a universal measure of the destructiveness of explosions. A long history of use and extensive manufacture of toxic TNT leads to the accumulation of these materials in soil and groundwater, which is a significant concern for environmental safety and sustainability. Reliable and cost-efficient technologies for removing or detoxifying TNT from the environment are lacking.

Understanding Dimethyl Methylphosphonate Adsorption and Decomposition on Mesoporous CeO.

The increased risk of chemical warfare agent usage around the world has intensified the search for high-surface-area materials that can strongly adsorb and actively decompose chemical warfare agents. Dimethyl methylphosphonate (DMMP) is a widely used simulant molecule in laboratory studies for the investigation of the adsorption and decomposition behavior of sarin (GB) gas. In this paper, we explore how DMMP interacts with the as-synthesized mesoporous CeO.

Degradation of Fatal Toxic Nerve Agents on Dry TiO.

Despite a recent dramatically increased risk of using chemical warfare agents in chemical attacks and assassinations, fundamental interactions of toxic chemicals with other materials are poorly understood, and micromechanisms of their chemical degradation are yet to be established. This represents an outstanding challenge in both fundamental science and practical applications in combat against chemical weapons. One of the most versatile and multifunctional oxides, TiO, has been suggested as a promising material to quickly adsorb and effectively destroy toxins.

Achieving tunable chemical reactivity through photo-initiation of energetic materials at metal oxide surfaces.

Known applications of high energy density materials are impressively vast. Despite this, we argue that energetic materials are still underutilized for common energy purposes due to our inability to control explosive chemical reactions releasing energy from these materials. The situation appears paradoxical as energetic materials (EM) possess massive amounts of energy and, hence, should be most appropriate for applications in many energy-intensive processes.

Dimethyl methylphosphonate adsorption and decomposition on MoO as studied by ambient pressure x-ray photoelectron spectroscopy and DFT calculations.

Organophosphonates range in their toxicity and are used as pesticides, herbicides, and chemical warfare agents (CWAs). Few laboratories are equipped to handle the most toxic molecules, thus simulants such as dimethyl methylphosphonate (DMMP), are used as a first step in studying adsorption and reactivity on materials. Benchmarked by combined experimental and theoretical studies of simulants, calculations offer an opportunity to understand how molecular interactions with a surface changes upon using a CWA.

Adsorption and decomposition of dimethyl methylphosphonate on size-selected (MoO) clusters.

The adsorption and decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected molybdenum oxide trimer clusters, i.e. (MoO), was studied both experimentally and theoretically.

Electron Spectroscopy and Computational Studies of Dimethyl Methylphosphonate.

Dimethyl methylphosphonate (DMMP) is one of the most widely used molecules to simulate chemical warfare agents in adsorption experiments. However, the details of the electronic structure of the isolated molecule have not yet been reported. We have directly probed the occupied valence and core levels using gas phase photoelectron spectroscopy and the unoccupied states using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.

Photochemistry of the α-Al₂O₃-PETN Interface.

Optical absorption measurements are combined with electronic structure calculations to explore photochemistry of an α-Al₂O₃-PETN interface formed by a nitroester (pentaerythritol tetranitrate, PETN, C₅H₈N₄O12) and a wide band gap aluminum oxide (α-Al₂O₃) substrate. The first principles modeling is used to deconstruct and interpret the α-Al₂O₃-PETN absorption spectrum that has distinct peaks attributed to surface F⁰-centers and surface-PETN transitions. We predict the low energy α-Al₂O₃ F⁰-center-PETN transition, producing the excited triplet state, and α-Al₂O₃ F⁰-center-PETN charge transfer, generating the PETN anion radical.

Molecular Theory of Detonation Initiation: Insight from First Principles Modeling of the Decomposition Mechanisms of Organic Nitro Energetic Materials.

This review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation. As a science and engineering community we are still far away from having a comprehensive molecular detonation initiation theory in a widely agreed upon form. However, recent advances in experimental and theoretical methods allow for a constructive and rigorous approach to design and test the theory or at least some of its fundamental building blocks.

Effect of polar surfaces on decomposition of molecular materials.

We report polar instability in molecular materials. Polarization-induced explosive decomposition in molecular crystals is explored with an illustrative example of two crystalline polymorphs of HMX, an important energetic material. We establish that the presence of a polar surface in δ-HMX has fundamental implications for material stability and overall chemical behavior.

Topography of photochemical initiation in molecular materials.

We propose a fluctuation model of the photochemical initiation of an explosive chain reaction in energetic materials. In accordance with the developed model, density fluctuations of photo-excited molecules serve as reaction nucleation sites due to the stochastic character of interactions between photons and energetic molecules. A further development of the reaction is determined by a competition of two processes.

Decomposition mechanisms and kinetics of novel energetic molecules BNFF-1 and ANFF-1: quantum-chemical modeling.

Decomposition mechanisms, activation barriers, Arrhenius parameters, and reaction kinetics of the novel explosive compounds, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (BNFF-1), and 3-(4-amino-1,2,5-oxadiazol-3-yl)-4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (ANFF-1) were explored by means of density functional theory with a range of functionals combined with variational transition state theory. BNFF-1 and ANFF-1 were recently suggested to be good candidates for insensitive high energy density materials. Our modeling reveals that the decomposition initiation in both BNFF-1 and ANFF-1 molecules is triggered by ring cleavage reactions while the further process is defined by a competition between two major pathways, the fast C-NO₂ homolysis and slow nitro-nitrite isomerization releasing NO.

Surface-Accelerated Decomposition of δ-HMX.

Despite extensive efforts to study the explosive decomposition of HMX, a cyclic nitramine widely used as a solid fuel, explosive, and propellant, an understanding of the physicochemical processes, governing the sensitivity of condensed HMX to detonation initiation is not yet achieved. Experimental and theoretical explorations of the initiation of chemistry are equally challenging because of many complex parallel processes, including the β-δ phase transition and the decomposition from both phases. Among four known polymorphs, HMX is produced in the most stable β-phase, which transforms into the most reactive δ-phase under heat or pressure.

Formation and migration of oxygen vacancies in La(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) perovskites: insight from ab initio calculations and comparison with Ba(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ).

The formation and migration of oxygen vacancies in the series of (La,Sr)(Co,Fe)O(3-δ) perovskites, which can be used as mixed conducting SOFC cathode materials and oxygen permeation membranes, are explored in detail by means of first principles density functional calculations. Structure distortions, charge redistributions and transition state energies during the oxygen ion migration are obtained and analyzed. Both the overall chemical composition and vacancy formation energy are found to have only a small impact on the migration barrier; it is rather the local cation configuration which affects the barrier.

Rapid materials degradation induced by surfaces and voids: ab initio modeling of β-octatetramethylene [corrected] tetranitramine.

A computational strategy based on coupling density functional theory, variational transition state theory, and a microscale materials morphology description unravels details of the defect-induced effect on the surface decomposition of molecular crystals. The technique allows us to resolve the earliest stages of decomposing solids, even for very complex materials and for ultrafast chemical reactions. A comparative analysis of chemical decomposition reactions in HMX with progressively increasing system complexity (an isolated HMX molecule; a perfect single HMX crystal; a defect-containing, porous, and granular HMX crystal) demonstrates that the initiation of the material's degradation can be effectively manipulated by changing the crystal morphology.

Modeling thermal decomposition mechanisms in gaseous and crystalline molecular materials: application to β-HMX.

Exploration of initiation of chemistry in materials is especially challenging when several coexisting chemical mechanisms are possible and many reactions' products are produced. It is even more difficult for complex materials, such as molecular, supramolecular, and hierarchical materials and systems. A strategy to draw a complete picture of the earliest stages of rapid decomposition reactions in molecular materials is presented in this study.

Ab initio kinetics of gas phase decomposition reactions.

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process.

Ab initio equation of state of the organic molecular crystal: beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine.

We apply a simple strategy for calculating from first principles a thermodynamically complete equation of state for molecular crystals using readily available quantum chemistry techniques. The strategy involves a combination of separate methods for the temperature-independent mechanical compression and the thermal vibrational contributions to the free energy. A first principles equation of state for beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) has been calculated for temperatures between 0 and 400 K and for specific volumes from 0.

Vibration-induced inelastic effects in the electron transport through multisite molecular bridges.

We theoretically analyzed inelastic effects in the electron transport through molecular junctions originating from electron-vibron interactions. The molecular bridge was simulated by a periodical chain of identical hydrogenlike atoms with the nearest neighbors interaction thus providing a set of energy states for the electron tunneling. To avoid difficulties inevitably arising when advanced computational techniques are employed to study inelastic electron transport through multilevel bridges, we propose and develop a semiphenomenological approach.

Effect of molecular and lattice structure on hydrogen transfer in molecular crystals of diamino-dinitroethylene and triamino-trinitrobenzene.

We have studied the intra- and intermolecular hydrogen transfer in a crystalline 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by means of an embedded cluster method and density functional theory (DFT). We found that, even though both of these materials have similar amino- and nitro- functional groups and layered crystalline structures, there are important differences in the mechanisms of hydrogen transfer. In particular, our calculations suggest that the proton migration from an amino-group to a nitro-group of the same molecule is a feasible process in TATB but not in DADNE.

Effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules.

The authors have calculated the electronic structure of individual 1,1-diamino-2,2-dinitroethylene molecules (FOX-7) in the gas phase by means of density functional theory with the hybrid B3LYP functional and 6-31+G(d,p) basis set and considered their dissociation pathways. Positively and negatively charged states as well as the lowest excited states of the molecule were simulated. They found that charging and excitation can not only reduce the activation barriers for decomposition reactions but also change the dominating chemistry from endo- to exothermic type.

Ab initio studies of crystalline nitromethane under high pressure.

We have studied the mechanical compressibility and band structure of solid nitromethane both in equilibrium and compressed states using Hartree-Fock and density functional theory (DFT) with atom-centered all-electron linear combination of atomic orbitals basis sets. Hartree-Fock calculations with a 6-21G basis set, uncorrected for basis set superposition error, gave the best agreement with experimental compression studies. These results may be due to the cancellation of basis set superposition error with dispersion force errors.

Ab initio equation of state of an organic molecular crystal: 1,1-diamino-2,2-dinitroethylene.

A complete equation of state for the molecular crystal 1,1-diamino-2,2-dinitroethylene has been calculated from first principles for temperatures between 0 and 400 K, and for specific volumes from 61 to 83 cm3/mol, corresponding to relative volumes from 0.78 to 1.06.

First principles calculation of the mechanical compression of two organic molecular crystals.

The mechanical compression curves for the organic molecular crystals 1,1-diamino-2,2-dinitroethylene and beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX) are calculated using the Hartree-Fock approximation to the solutions of the many-body Schrödinger equation for a periodic system as implemented in the computer program CRYSTAL. No correction was made for basis set superposition error. The equilibrium lattice parameters are reproduced to within 1% of reported experimental values.