Energetics and Ionic–Electronic and Geometric Variabilities of Hydroxylammonium-Based Salts

In the present work, we study the formation energy of energetic ionic salts (EISs), as well as the volumetric and electric variabilities of ions involved in EISs, by employing 25 hydroxylammonium cations (HA⁺) in 22 salts as examples. We identify the viability of a dispersion-corrected density functional theory method, PBE-D3(BJ), to describe the lattice parameters, with a mean relative deviation of 1.26% and a maximal relative deviation of 2.5%. Thereby, the ionization energy, formation energy, and lattice energy are predicted. The variability of volume (V_HA+) is confirmed in different HA⁺-based salts, as well as between two HA⁺ of the same lattice with Z′ > 1. The largest difference of V_HA+ among the 25 HA⁺ reaches ∼9%. This variability of V_HA+ is originated from the difference in crystal field effects, or intermolecular interactions. That is, the stronger intermolecular hydrogen bond tends to exhibit a stronger drag and pull effect on HA⁺ and leads to a larger V_HA+. In comparison, it exhibits a more significant variability of electricity, as the maximal total Mulliken charge of HA⁺ (Q_HA+) is >1.67 times the minimal one. Moreover, the observed Q_HA+ varying from 0.164 to 0.438 e is much less than the apparent charge of 1 e, suggesting the highly insufficient charge of HA⁺ in EISs. This means that the ionic bonding does not necessarily govern the intermolecular interactions in common EISs. Besides, an electrostatic potential (ESP) minimum-oxygen balance (OB) correlation is found in the anions countering HA⁺, as the more negative OB tends to cause a more negative ESP minimum. All of these findings are expected to richen the insight into the nature of EISs as a special group of EMs.