Revised Thermochemistry of Gaseous Ammonium Nitrate, NH4NO3(g)

Theoretical and Experimental Insights into the Dissociation of 2-Hydroxyethylhydrazinium Nitrate Clusters Formed via Electrospray

Ionic liquids are used for myriad applications, including as catalysts, solvents, and propellants. Specifically, 2-hydroxyethylhydrazinium nitrate (HEHN) has been developed as a chemical propellant for space applications. The gas-phase behavior of HEHN ions and clusters is important in understanding its potential as an electrospray thruster propellant. Here, the unimolecular dissociation pathways of two clusters are experimentally observed, and theoretical modeling of hydrogen bonding and dissociation pathways is used to help rationalize those observations. The cation/deprotonated cation cluster [HEH₂ - H]⁺, which is observed from electrospray ionization, is calculated to be considerably more stable than the complementary cation/protonated anion adduct, [HEH + HNO₃]⁺, which is not observed experimentally. Upon collisional activation, a larger cluster [(HEHN)₂HEH]⁺ undergoes dissociation via loss of nitric acid at lower collision energies, as predicted theoretically. At higher collision energies, additional primary and secondary loss pathways open, including deprotonated cation loss, ion-pair loss, and double-nitric-acid loss. Taken together, these experimental and theoretical results contribute to a foundational understanding of the dissociation of protic ionic liquid clusters in the gas phase.

Thermochemistry of ammonium nitrate, NH₄NO₃, in the gas phase

Hildenbrand and co-workers have shown recently that the vapor above solid ammonium nitrate includes molecules of NH₄NO₃, not only NH₃ and HNO₃ as previously believed. Their measurements led to thermochemical values that imply an enthalpy change of D₂₉₈ = 98 ± 9 kJ mol⁻¹ for the gas-phase dissociation of ammonium nitrate into NH₃ and HNO₃. Using updated spectroscopic information for the partition function leads to the revised value of D₂₉₈ = 78 ± 21 kJ mol⁻¹ (accompanying paper in this journal, Hildenbrand, D. L., Lau, K. H., and Chandra, D. J. Phys. Chem. B 2010, DOI: 10.1021/jp105773q). In contrast, high-level ab initio calculations, detailed in the present report, predict a dissociation enthalpy half as large as the original result, 50 ± 3 kJ mol⁻¹. These are frozen-core CCSD(T) calculations extrapolated to the limiting basis set aug-cc-pV∞Z using an anharmonic vibrational partition function and a variational treatment of the NH₃ rotor. The corresponding enthalpy of formation is Δ(f)H₂₉₈°(NH₄NO₃,g) = −230.6 ± 3 kJ mol⁻¹. The origin of the disagreement with experiment remains unexplained.