Classical and Quantum Mechanical Studies of Crystalline Ammonium Dinitramide

Plane-wave ab initio calculations based on density functional theory and the pseudopotential method have been used to investigate the structural properties of crystalline ammonium dinitramide (ADN). The optimization of the crystal structure has been done with full relaxation of atomic positions and lattice parameters under P 2 1 / c symmetry. The calculations were performed using periodic boundary conditions in all three directions. The predicted crystal structure is in good agreement with X-ray data and indicates no internal symmetry for the dinitramide ion, in contradistinction to the gas phase theoretical results. The two halves of the dinitramine ion are twisted with respect to each other while the NO 2 groups are rotated out of the N−N−N plane. We have developed an intermolecular potential that describes the structure of the ADN crystal in the approximation of rigid ions. This potential is composed of pairwise Lennard−Jones, hydrogen-bonding terms, and Coulombic interactions. Crystal-packing calculations without symmetry constraints performed with the potential reproduce accurately the main crystallographic features and yield very good agreement with the estimated lattice energy. This potential was further tested in isothermal−isobaric molecular dynamics simulations at atmospheric pressure and in the temperature range 4.2−350 K. The main temperature effect is an increase in rotational disorder of the ammonium ions, without any significant change of the translational order of the ions. The thermal expansion coefficients calculated for the model indicate anisotropic behavior.