Edward F. C. Byrd

Evaluation of electrostatic descriptors for predicting crystalline density

This study evaluates the importance of electrostatic corrections to earlier quantum‐mechanically based methods to predict crystal densities of neutral and ionic molecular energetic materials. Our previous methods (B. M. Rice et al., J. Phys. Chem. A 2007, 111, 10874) use the molecular volumes of the isolated molecule or formula unit to estimate the crystal density; this volume is defined to be that inside the quantum‐mechanically determined 0.001 a.u. isosurface of electron density surrounding the isolated molecule. The electrostatic corrections to these volumetric estimates are based on features of the electrostatic potential mapped onto this isosurface of electron density, and have been parameterized using information from 180 neutral and 23 ionic CHNO molecular systems. The quality of the electrostatically corrected methods was assessed through application to 38 neutral and 48 ionic compounds not used in the parameterization. The root mean square (rms) percent deviation and average absolute error of predictions for the 38 neutral species relative to experiment are 2.7% and 0.035 g/cm3, respectively, decreases of 0.9% and 0.015 g/cm3 from the earlier predictions (3.6% and 0.050 g/cm3, respectively). The rms percent deviation and average absolute error of predictions for the 48 ionic compounds relative to experiment are 3.7% and 0.045 g/cm3, respectively, decreases of 2.6% and 0.043 g/cm3 from the earlier predictions that used the formula unit volumes only. The results clearly show a significant improvement to the earlier method upon inclusion of electrostatic corrections.

Development of quantitative structure–property relationships for predictive modeling and design of energetic materials

A quantitative structure-property relationship (QSPR) based on the AM1 semiempirical quantum mechanical method was derived using the program, CODESSA, to describe published drop height impact sensitivities for 227 nitroorganic compounds. An eight-descriptor correlation equation having R(2)=0.8141 was obtained through a robust least median squares regression. The resulting model is the most comprehensive and systematic quantum mechanically derived QSPR for energetic materials of those that have been published. The predictive capability of the model is also presented and discussed.

Structural and electrical analysis of epitaxial 2D/3D vertical heterojunctions of monolayer MoS2 on GaN

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2u2009μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which correspon...

Simple and Efficient Synthesis of Explosive Cocrystals containing 3,5‐Dimethylpyrazol‐1‐yl‐substituted‐1,2,4,5‐tetrazines

The reaction of 3,4-dinitropyrazole, 5-nitrotetrazole, or 4-nitro-1,2,3-triazole with 1,2,4,5-tetrazines substituted with 3,5-dimethylpyrazolyl (dmp) groups results in energetic cocrystals after 1 minute of reflux and cooling to room temperature in yields of 89-92u2009%. Hydrogen-bonding between the dmp group to the N-H of the energetic heterocycles are the predominant interaction that stabilizes the new cocrystals. Each cocrystal packs in a different lattice structure and the cocrystals with sheet-like and herring-bone crystal packing orientations are less sensitive than the cocrystal with the interlocked structure. Electrostatic potential mapping helps rationalize why dmp-substituted tetrazines readily form cocrystals, whereas more electron-deficient pyrazolyl tetrazines do not. The calculated energetic performance of the new cocrystals approaches that of 2,4,6-trinitrotoluene (TNT) and importantly, these materials will aid in the rational design of new cocrystalline energetic materials.

Development of quantitative structure property relationships for predicting the melting point of energetic materials

The accurate prediction of the melting temperature of organic compounds is a significant problem that has eluded researchers for many years. The most common approach used to develop predictive models entails the derivation of quantitative structure-property relationships (QSPRs), which are multivariate linear relationships between calculated quantities that are descriptors of molecular or electronic features and a property of interest. In this report the derivation of QSPRs to predict melting temperatures of energetic materials based on descriptors calculated using the AM1 semiempirical quantum mechanical method are described. In total, the melting points and experimental crystal structures of 148 energetic materials were analyzed. Principal components analysis was performed in order to assess the relative importance and roles of the descriptors in our QSPR models. Also described are the results of k means cluster analysis, performed in order to identify natural groupings within our study set of structures. The QSPR models resulting from these analyses gave training set R(2) values of 0.6085 (RMSE = ± 15.7 °C) and 0.7468 (RMSE = ± 13.2 °C). The test sets for these clusters had R(2) values of 0.9428 (RMSE = ± 7.0 °C) and 0.8974 (RMSE = ± 8.8 °C), respectively. These models are among the best melting point QSPRs yet published for energetic materials.

Effect of a core-softened O–O interatomic interaction on the shock compression of fused silica

Isotropic soft-core potentials have attracted considerable attention due to their ability to reproduce thermodynamic, dynamic, and structural anomalies observed in tetrahedral network-forming compounds such as water and silica. The aim of the present work is to assess the relevance of effective core-softening pertinent to the oxygen-oxygen interaction in silica to the thermodynamics and phase change mechanisms that occur in shock compressed fused silica. We utilize the MD simulation method with a recently published numerical interatomic potential derived from an ab initio MD simulation of liquid silica via force-matching. The resulting potential indicates an effective shoulder-like core-softening of the oxygen-oxygen repulsion. To better understand the role of the core-softening we analyze two derivative force-matching potentials in which the soft-core is replaced with a repulsive core either in the three-body potential term or in all the potential terms. Our analysis is further augmented by a comparison ...

Computational chemistry models leading to mediation of gun tube erosion

Understanding the interaction of propellant gas products with transition-metal surfaces, and in particular with iron surfaces, is of primary importance to understanding the erosion process in gun tubes. By modeling these interactions, and the ensuing reactions, we shall gain a better understanding of the physics and chemistry underlying gun tube erosion. Using spin-polarized density functional theory (DFT) with the generalized gradient approximation (GGA), calculations were performed on the interactions of a wide variety of small product gases with the (100) and (111) Miller surfaces of iron. For the (100) surface, we have located the adsorption sites and determined atomic configurations of HCO, HOC, H/sub 2/CO, OH, CHOH, H/sub 2/, H, N, and NO as well as the coadsorption of CO+H, NO+C, and CO+N. We have also inspected the dissociative chemisorption of H/sub 2/ on the iron (100) surface, and dissociation pathway of CO and HCO (the latter resulting in surface bound O+HC) using the nudged elastic band (NEB) method. For the iron (111) surface, we have predicted the adsorption sites of CO, NH/sub 3/, NO, N, C, and O, the coadsorption sites of C+O and N+CO, as well as mapping the reaction paths and energetic barriers for the migration and dissociation of CO on the iron (111) surface. The interaction of CO with a nitrited iron surface is also in the process of being studied for both the (100) and (111) iron surfaces. Ab initio direct molecular dynamics (DMD) has been used to model the collision of CO at various temperatures with a small iron cluster to compute, by a second means, the energy required for dissociation. DMD is also being used to model the dynamics of the CO/sub (gas/adsorbed)/+Fe/sub 14//spl rarr/O/sub (g)/+C/sub (ads)/-Fe/sub 14/ reaction. Monte Carlo MD simulations with embedded atom models (EAM) are being carried out on the interactions of H/sub 2/ with large iron surfaces to measure hydrogen diffusion rates into the iron, with particular interest in the relative rates of diffusion through a perfect surface versus through surface defects (e.g., grain boundaries). Through many different computational tools and approaches, and the inspection of a wide array of different systems, we are gaining a better understanding of the chemistry involved in gun tube erosion. Using the knowledge gained of the energetics of binding and surface dissociation, coupled with macroscopic (thermodynamic and kinetic) erosivity models being used or developed at ARL, we should be able to predict the erosivity of different propellant blends as a function of the propellant composition.