Initial Decomposition Pathways of Aqueous Hydroxylamine Solutions

Abstract

This work examined the reaction pathways involved in the initial decomposition of aqueous hydroxylamine solutions via the overall reaction, 2NH₂OH → NH₃ + HNO + H₂O, using quantum chemistry calculations incorporating solvent effects. Several possible decomposition mechanisms were identified and investigated: three neutral-neutral bimolecular, two water-catalyzed, one neutral trimolecular, two ion-neutral bimolecular, and one cation-catalyzed. Optimized structures for the reactants, products, and transition states were obtained at the ωB97XD/6-311++G(d,p)/SCRF = (solvent = water) level of theory, and the total electron energies of such structures were calculated at the CBS-QB3 level of theory. The cation-catalyzed reaction 2NH₂OH + NH₃OH⁺ → NH₄⁺ + HNO + H₂O + NH₂OH (maximum energy barrier (ΔE₀^‡) = 53.6 kJ/mol) and the anion-neutral bimolecular reaction NH₂OH + NH₂O^- → NH₃ + ¹NO^- + H₂O (ΔE₀^‡ = 79.0 kJ/mol) were both found to be plausible candidates for the dominant step in the initial decomposition. The results of this study indicate that both acidic and basic conditions can affect the thermal stability of hydroxylamine in water.