Mechanism and Kinetics for Ammonium Dinitramide (ADN) Sublimation: A First-Principles Study

The mechanism for sublimation of NH4N(NO2)2 (ADN) has been investigated quantum-mechanically with generalized gradient approximation plane-wave density functional theory calculations; the solid surface is represented by a slab model and the periodic boundary conditions are applied. The calculated lattice constants for the bulk ADN, which were found to consist of NH4 +[ON(O)NNO2]− units, instead of NH4 +[N(NO2)2]−, agree quite well with experimental values. Results show that three steps are involved in the sublimation/decomposition of ADN. The first step is the relaxation of the surface layer with 1.6 kcal/mol energy per NH4ON(O)NNO2 unit; the second step is the sublimation of the surface layer to form a molecular [NH3]–[HON(O)NNO2] complex with a 29.4 kcal/mol sublimation energy, consistent with the experimental observation of Korobeinichev et al. The last step is the dissociation of the [H3N]–[HON(O)NNO2] complex to give NH3 and HON(O)NNO2 with the dissociation energy of 13.9 kcal/mol. Direct formation of NO2 (g) from solid ADN costs a much higher energy, 58.3 kcal/mol. Our calculated total sublimation enthalpy for ADN(s) → NH3(g) + HON(O)NNO2) (g), 44.9 kcal/mol via three steps, is in good agreement with the value, 42.1 kcal/mol predicted for the one-step sublimation process in this work and the value 44.0 kcal/mol computed by Politzer et al. using experimental thermochemical data. The sublimation rate constant for the rate-controlling step 2 can be represented as k sub = 2.18 × 1012 exp (−30.5 kcal/mol/RT) s–1, which agrees well with available experimental data within the temperature range studied. The high pressure limit decomposition rate constant for the molecular complex H3N···HON(O)NNO2 can be expressed by k dec = 3.18 × 1013 exp (−15.09 kcal/mol/RT) s–1. In addition, water molecules were found to increase the sublimation enthalpy of ADN, contrary to that found in the ammonium perchlorate system, in which water molecules were shown to reduce pronouncedly the enthalpy of sublimation.