Structures, proton transfer and dissociation of hydroxylammonium nitrate (HAN) revealed by electrospray ionization...

Hydroxylammonium nitrate (HAN) is a potential propellant candidate for dual-mode propulsion systems that combine chemical and electrospray thrust capabilities for spacecraft applications. However, the electrospray dynamics of HAN is currently not well understood. Capitalizing on electrospray ionization guided-ion beam tandem mass spectrometry and collision-induced dissociation measurements, and augmented by extensive molecular dynamics simulations, this work characterized the structures and reaction dynamics of the species present in the electrosprays of HAN under different conditions, which mimic those possibly occurring in low earth orbit and outer space. While being ionic in nature, the HAN monomer, however, adopts a stable covalent structure HONH₂·HNO₃ in the gas phase. Spontaneous proton transfer between the HONH₂ and HNO₃ moieties within the HAN monomer can be induced in the presence of a NO₃^-, a water ligand or a second HAN monomer within 3-5 Å or a H⁺ within 8 Å, regardless of their collision impact parameters. These facts imply that HAN proton transfer is trigged by a charge and/or a dipole of the collision partner without the need of chemical interaction or physical contact. Moreover, the addition of NO₃^- to HAN leads to the formation of a stable ^-O₃N·HONH₃⁺·NO₃^- anion in negative electrosprays. In contrast, when a proton approaches the HONH₂·HNO₃ structure, dissociative reactions occur that lead to the H₂O, NO₂ and HONH₂ fragments (and their cations) but not intact HAN species in positive electrosprays.