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

Abstract

The charge distribution of NO 2 groups within the crystalline polymorphs of energetic materials strongly affects their explosive properties. We use the recently introduced basis-space iterated stockholder atom partitioning of high-quality charge distributions to examine the approximations that can be made in modeling polymorphs and their physical properties, using 1,3,5-trinitroperhydro-1,3,5-triazine, trinitrotoluene, 1-3-5-trinitrobenzene, and hexanitrobenzene as exemplars. The NO 2 charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO 2 torsion angle within the possible variations found in observed crystal structures. Thus, the proposed correlations between the molecular electrostatic properties, such as trigger-bond potential or maxima in the electrostatic potential, and impact sensitivity will be affected by the changes in conformation that occur on crystallization. We establish the relationship between the NO 2 torsion angle and the likelihood of occurrence in observed crystal structures, the conformational energy, and the charge and dipole magnitude on each atom, and how this varies with the neighboring groups. We examine the effect of analytically rotating the atomic multipole moments to model changes in torsion angle and establish that this is a viable approach for crystal structures but is not accurate enough to model the relative lattice energies. This establishes the basis of transferability of the NO 2 charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials.

Unimolecular Decomposition Reactions of Picric Acid and Its Methylated Derivatives─A DFT Study

“…The crystal structures of mPA and dmPA are, to our knowledge, unknown. Properties related to the crystal structure of solid explosives have been shown to affect C–NO 2 trigger bond BDEs as well as molecular charge distributions, 94 two quantities that are routinely employed for correlation studies on impact sensitivity. The orientation of a molecule within the crystal with respect to its neighboring molecules affects the feasibility of intermolecular hydrogen transfer, while twisting of a single molecule’s functional groups affects the feasibility of intramolecular hydrogen transfer.…”

A non-empirical intermolecular force-field for trinitrobenzene and its application in crystal structure prediction

“…Ideally, a further CSP would be done in which the NO 2 groups of TNB could change conformation in response to the packing forces. This could be done by analytically rotating the multipoles during the structure relaxation, 42 but a final revaluation of at least the multipoles for each crystalline conformation would be needed for the final lattice energies. 42…”

“…This could be done by analytically rotating the multipoles during the structure relaxation, 42 but a final revaluation of at least the multipoles for each crystalline conformation would be needed for the final lattice energies. 42…”

“…An anticipated advantage of the ISA partitioning of the charge density is that the changes in the atomic potential parameters with conformation may be limited to the physical redistribution of charge with conformation 42 (i.e., the changes that go beyond the geometric changes that can be done using analytical tensor rotation). This could greatly improve the efficiency of modeling the changes in conformation that occur on crystallization.…”

“…40,41 Indeed, larger changes in the NO 2 torsion angles would affect the charge distribution of the nitro group to an extent that would require reparameterization of the model potential for accurate lattice energies. 42 We can develop non-empirical derived intermolecular forcefields (DIFFs) for TNB with an atomic multipolar expansion of long-range components VLR and an exponential model for the shortrange terms VSR. The functional form 1 for the interaction between molecules M and N containing atoms i and k of types ι and κ (types are C H , C N , N, O, or H) at a separation R ik and orientation Ω ik can be written as…”