Abstract
The charge distribution of NO2 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 NO2 charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO2 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 NO2 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 NO2 charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials.
Highlights
Energetic materials, which decompose explosively under various external stimuli, are intrinsically difficult to study experimentally, and the design of new materials with desirable property combinations such as low sensitivity but high detonation performance can benefit from accurate molecular modeling.[1−3] The explosive properties are sensitive to the arrangement of molecules in the crystal and to the polymorph formed.[1,4−6] crystal structure prediction (CSP) methods may be used to determine whether an energetic molecule can crystallize in a dense structure with good energetic properties, helping to focus synthetic efforts
We focus on the electrostatic potential surface maximum (Vmax) and minimum (Vmin), as many studies have focused on the correlation of impact sensitivity with these molecular properties.[1,3,7,21,51−53,56−59] The electrostatic maxima and minima in molecules have been used in a method of estimating the likelihood of cocrystal formation,[60] i.e., as a simple and approximate surrogate for likely relative crystal energies
We use iterated stockholder atom (ISA)[29] partitioned distributed multipoles to determine whether changes in torsion angle due to crystallization affect some proposed correlations with impact sensitivity. We investigate whether this change is important to the molecular modeling of nitroenergetics, for crystal structure prediction (CSP) studies and the development of nonempirical anisotropic atom−atom intermolecular potentials
Summary
Energetic materials, which decompose explosively under various external stimuli, are intrinsically difficult to study experimentally, and the design of new materials with desirable property combinations such as low sensitivity but high detonation performance can benefit from accurate molecular modeling.[1−3] The explosive properties are sensitive to the arrangement of molecules in the crystal and to the polymorph formed.[1,4−6] crystal structure prediction (CSP) methods may be used to determine whether an energetic molecule can crystallize in a dense structure with good energetic properties, helping to focus synthetic efforts. Correlations have been found between the heat of fusion of the crystal and the N−N bond dissociation energy[19] or crystalline void space,[20] but the prediction of impact sensitivity and other processes is clearly a complex combination of molecular and crystalline properties.[21−23] In particular, the crystalline conformation can differ from the isolated molecule conformation and vary between polymorphs and with temperature and pressure. We examine how changes in NO2 torsion angles that can occur on crystallization can affect the molecular charge distribution and associated electrostatic properties. This is a preliminary and necessary step toward the goal of developing methods of predicting impact sensitivity and other explosive properties, using CSPgenerated crystal structures
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