A theoretical library of N1s core binding energies of polynitrogen molecules and ions in the gas phase

Abstract

Polynitrogen molecules and ions are important building blocks of high energy density compounds (HEDCs). High energy bonds formed at the N sites can be effectively probed by X-ray photoelectron spectroscopy (XPS) at the N K-edge. In this work, with the density functional theory and the ΔKohn–Sham scheme, we simulated the N1s ionic potentials (IPs) of 72 common polynitrogen molecules [tetrazoles, pentazole (N₅H), diazines, triazines, tetrazines, furazans, oxazoles and oxadiazoles], ions [pentazolate anion (cyclo-N₅⁻), pentazenium cation (N₅⁺), etc.], and molecular (NH₃⋯N₅H, H₂O⋯N₅H) and ionic (NH₄⁺⋯N₅⁻, H₃O⁺⋯N₅⁻) pairs, as well as mononitrogen relatives. These constitute a small theoretical database for absolute N1s IPs with an average accuracy of ca. 0.3 eV. To understand the structure-IP relationship within this family, effects of side substituent and bridging groups, local bonding types (amine or imine N), charge and protonation states, and vibronic coupling were analyzed based on selected systems. This study in the gas phase collects inherent chemical shifts of nitrogen in high-energy NN and NC bonds, which provides an essential reference into XPS interpretations of more complex HEDCs in the solid state. We especially highlight the evident N1s chemical shifts induced by protonation for nitrogen in the five-membered ring (N₅H versus cyclo-N₅⁻, ca. 7 eV; NH₃⋯N₅H versus NH₄⁺⋯N₅⁻, ca. 3 eV; H₂O⋯N₅H versus H₃O⁺⋯N₅⁻, ca. 2 eV), and suggest XPS as a sensitive tool in determining the hydrogen positions in pentanitrogen-based HEDCs.