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

Area of Science:

• Computational Chemistry
• Materials Science
• Crystallography

Background:

• The explosive properties of energetic materials are intrinsically linked to the charge distribution of nitro (NO2) groups within their crystalline structures.
• Accurate modeling of these materials requires understanding how molecular conformation and intermolecular interactions influence charge distribution.

Purpose of the Study:

• To investigate the approximations applicable to modeling crystalline polymorphs and their physical properties using advanced charge partitioning methods.
• To analyze the influence of the nitro group's torsion angle and its environment on charge distribution and molecular properties.

Main Methods:

• Utilized basis-space iterated stockholder atom partitioning for high-quality charge distribution analysis.
• Examined exemplar energetic materials: RDX, TNT, TNB, and HNB.
• Analyzed the effect of NO2 torsion angle variations on charge, dipole, and conformational energy.

Main Results:

• Nitro group charge distribution is significantly influenced by neighboring atoms, molecular context, and torsion angle variations observed in crystal structures.
• Established a correlation between NO2 torsion angle, crystal structure occurrence, conformational energy, and atomic charge/dipole magnitudes.
• Demonstrated that analytically rotating atomic multipole moments is viable for crystal structures but insufficient for modeling lattice energies.

Conclusions:

• The conformation-dependent nature of NO2 charge distribution necessitates careful consideration in structure-property relationship studies for energetic materials.
• The findings provide a basis for developing transferable NO2 charge distributions for accurate nonempirical intermolecular potentials in simulations.