A Computational Study of Density of Some High Energy Molecules

Abstract

The detonation pressure depends quadratically on the loading density of the explosives. A precise estimate of the density is thus crucial to decide if a novel energetic material is worth pursuing. In this work we investigate theoretically the crystal densities of the energetic compounds RDX, TNT, NTO, DNAM, CL‐20, DADNE, and HMX. We calculate the crystal densities by using Materials Studio 7.0 Polymorph Predictor, employing force fields and exploring molecular packing arrangements with minima in total energy. Geometry optimized molecular structures computed by density functional theory (DFT) are used as input to the density predictions. In an additional DFT study we apply two functionals, B3LYP and M06 with the 6‐31G(d) and the 6‐31G(d,p) basis sets, and the program package GAUSSIAN09. In this part of the work crystal densities are calculated by using the molecular isosurface volume (defined by the volume within a surface with an electron density of 0.001 electrons per Bohr3) alone or combined with the variance of the electrostatic potential (ESP). The Polymorph Predictor seems to overestimate the densities, but the values are very dependent on the force field strength determined by charges assigned to atoms. In the GAUSSIAN09 DFT study the densities derived by using the M06 functional are in similar agreement with experimental data as what we experienced for the B3LYP results, although both functionals appear to give slightly lower densities than reported experimentally for the majority of the molecules. On average, the densities derived by the ESP method correlate equally well with measured values as the results obtained by the isosurface method.

Charge Distributions of Nitro Groups Within Organic Explosive Crystals: Effects on Sensitivity and Modeling

“…This suggests that the anarot methodology could be used to determine the range of low-energy crystal structures that could be adopted by a nitroaromatic energetic molecule within the Ψ mol approach to crystal structure prediction 88 and thus could improve the prediction of crystal packing from the molecular diagram of new energetic molecules. 74 This method is suitable as an initial approximation of the potential energy landscape, but the relative energy differences in Table 1 are large in comparison with the lattice energy differences between polymorphs, which are usually much less than 5 kJ mol –1 for small molecules. 89 The conclusion that analytical rotation does not reproduce relative electrostatic energies well was also reached in the study on the transferability of electrostatic models between polypeptides.…”

“…It seems that cell geometries can be obtained relatively accurately and very quickly without recalculating the charge density as the nitro groups rotate (Table ), by analytically rotating the multipole moments. This suggests that the anarot methodology could be used to determine the range of low-energy crystal structures that could be adopted by a nitroaromatic energetic molecule within the Ψ mol approach to crystal structure prediction and thus could improve the prediction of crystal packing from the molecular diagram of new energetic molecules . This method is suitable as an initial approximation of the potential energy landscape, but the relative energy differences in Table are large in comparison with the lattice energy differences between polymorphs, which are usually much less than 5 kJ mol –1 for small molecules .…”

“…This surface has recently been used to define the molecular volume for use in estimating the density of energetic materials. 74 The electrostatic potential on this isodensity surface is approximately 3 Å from the nearest nucleus in the molecule and so gives the electrostatic potential sampled by the nonhydrogenic nuclei that would be in van der Waals contact with the molecule in the crystal. Lattice energies were calculated using a combination of the ISA distributed multipoles, to obtain the electrostatic energy, and the exp-6 FIT potential, for all other contributions using DMACRYS.…”

“…This isodensity surface was chosen as the majority of electrostatic potential studies on energetic materials and their sensitivities use this surface, following the suggestion by Bader, , as it can include more than 95% of a molecule’s true electronic density. This surface has recently been used to define the molecular volume for use in estimating the density of energetic materials . The electrostatic potential on this isodensity surface is approximately 3 Å from the nearest nucleus in the molecule and so gives the electrostatic potential sampled by the nonhydrogenic nuclei that would be in van der Waals contact with the molecule in the crystal.…”

Molecule Design and Properties of Cyclic Energetic Materials Based on Nitroguanidine and 1,1-Diamino-2,2-Dinitroethene

“…The optimized structures and vibrational analyses of the designed molecules were carried out by Gaussian 03 software 23 combined with B3LYP method at 6-31G (d,p) level of density functional theory (DFT) which have demonstrated as a reliable way to calculate the accurate energy. [24][25][26][27] The optimizations were performed without any symmetry restrictions using the default convergence criteria in the Gaussian program. All of the optimized structures were characterized to be true local energy minima on the potential energy surfaces without imaginary frequencies.…”