Hildenbrand and coworkers 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 D298 = 78 ± 21 kJ mol−1 [accompanying paper in this journal by Hildenbrand, Lau, and Chandra]. 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-pVZ, using an anharmonic vibrational partition function and a variational treatment of the NH3 rotor. The corresponding enthalpy of formation is = 230.6 ± 3 kJ mol−1. The origin of the disagreement with experiment remains unexplained.
Citation: Journal of Physical Chemistry A
Pub Type: Journals
ab initio, ammonium nitrate, anharmonic, computational chemistry, quantum chemistry, thermochemistry