Dinitramide ion: Robust molecular charge topology accompanies an enhanced dipole moment in its ammonium salt

The electron density, rho(r), of crystalline ammonium dinitramide (ADN) was determined from low-temperature X-ray diffraction data and electronic structure calculations. Single molecule wave functions were also computed for comparison. Bader's atoms in molecules (AIM) method was used to partition rho(r). The same number and kinds of critical points in rho(r) for the dinitramide ion are found in ADN as were found in previous studies of other salts with different amounts of nitro group twist. An atomic interaction line (AIL) is always observed between the two inner oxygens. Topological characterization of the negative Laplacian of the charge distribution (-del(2)rho(r)) was also performed to locate (3,-3) critical points in the valence-shell charge concentration (VSCC) region. Such points may be associated with the presence and location of lone pairs of electrons as predicted by the Lewis and VSEPR models. As with rho(r), the same number of (3,-3) critical points are found in the Laplacian, -del(2)rho(r), with experimental, single-molecule B3LYP, and crystal B3LYP models, however, comparison of experimental and theoretical results show some differences in the location of these points. These differences are shown to arise mostly from limited flexibility in the multipole model used to fit rho(r) to the experimental data. Nonetheless, both B3LYP modeling and experiment agree that there is a single (3,-3) critical point in the VSCC associated with a lone pair of electrons on the central nitrogen. The hybridization of the central nitrogen of the dinitramide ion is, therefore, sp(2)-like, as observed in the two biguanidinium salts, [((NH2)(2)C)(2)N] [N3O4] and [((NH2)(2)C)(2)NH][N3O4](2). Despite this robust topology, the dipole moment obtained from both experiment and crystal modeling is larger than that computed for a single dinitramide ion. Significant differences in the direction of the dipole from theory and experiment are found, as are differences in the atomic charges. These are also attributed to the limited flexibility of the multipole model. Electron densities obtained from crystal wave functions demonstrate that strong hydrogen bonding polarizes the dinitramide ion, increasing the negative charge on the most strongly hydrogen bonded oxygen atom. Decomposition of the theoretical molecular dipole moment into atomic charge and atomic dipole contributions reveals that the atomic dipoles are nearly equal in both the crystal and single molecular ion. Changes in the atomic charge contribution to the molecular dipole moment principally account for the induced dipole.