Thermochemistry of Gaseous Ammonium Nitrate, NH4NO3(g)

Isomeric structures of isolated ammonium nitrate and its hydrogenated species identified through PES experiments and DFT calculations

Anion photoelectron spectroscopic (PES) experiments in conjunction with density functional theory (DFT) calculations shed light on the electronic and geometric structures of gas phase, isolated ammonium nitrate related anionic species, as well as their hydrogenated species with up to five added hydrogens. These species are directly generated by laser ablation and cooled in a supersonic expansion. Their vertical detachment energies (VDE: Eneutral - Eanion, both at the anionic geometry) are experimentally determined and the corresponding anionic structures are characterized and assigned through calculations. Based on the experimentally evaluated calculation algorithm, the corresponding neutral structures are also determined. The parent anionic species exists as (NH2OH·HONO)- in the gas phase with the extra electron valence bound. Crystal structure anion NH4NO3- is not present in our experiments, as within this structure the extra electron is dipole bound (electron affinity ∼ 0 eV). The isomerization must therefore occur for ammonium nitrate upon capturing an extra electron or during the laser ablation process itself. The ammonium nitrate anion is apparently a very reactive species. The calculated global minimum for the isolated parent neutral species has an HNO3·NH3 structure, different from the crystal structure in the bulk phase. The hydrogenated cluster anions can evolve from the parent (NH2OH·HONO)- species and exhibit moieties, which bind together as a single unit through interactions between noncovalently bonded species and are stable on the experimental timescale. The hydrogenation process forms stable moieties in the cluster anions, including water (H2O), nitroxyl (HNO), ammonia (NH3), or (HNOH). The calculated global minimum structures for hydrogenated cluster neutrals (NH4NO3 + nH, n = 1,…,5) contain ammonia and water, along with stable moieties (HONO, NO, and HNO). These stable moieties, along with intermediate species NO2H2 and ONH2, offer new insights into the behavior of ammonium nitrate energetic materials.

Comparative evaluation of thermal decomposition behavior and thermal stability of powdered ammonium nitrate under different atmosphere conditions

In order to analyze the thermal decomposition characteristics of ammonium nitrate (AN), its thermal behavior and stability under different conditions are studied, including different atmospheres, heating rates and gas flow rates. The evolved decomposition gases of AN in air and nitrogen are analyzed with a quadrupole mass spectrometer. Thermal stability of AN at different heating rates and gas flow rates are studied by differential scanning calorimetry, thermogravimetric analysis, paired comparison method and safety parameter evaluation. Experimental results show that the major evolved decomposition gases in air are H₂O, NH₃, N₂O, NO, NO₂ and HNO₃, while in nitrogen, H₂O, NH₃, NO and HNO₃ are major components. Compared with nitrogen atmosphere, lower initial and end temperatures, higher heat flux and broader reaction temperature range are obtained in air. Meanwhile, higher air gas flow rate tends to achieve lower reaction temperature and to reduce thermal stability of AN. Self-accelerating decomposition temperature of AN in air is much lower than that in nitrogen. It is considered that thermostability of AN is influenced by atmosphere, heating rate and gas flow rate, thus changes of boundary conditions will influence its thermostability, which is helpful to its safe production, storage, transportation and utilization.

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.