Decomposition of ammonium dinitramide (ADN) elucidated by quantum chemical computations

Ammonium dinitramide (ADN) is under examination to serve as a 'green' oxidizer achieving minimum signature of the exhaust gases of rocket propellants. However, stability issues and an erratic decomposition behavior limit its non-questionable general usability. This has initiated high research interest throughout the propellant community in order to optimize the ADN propellant system. Numerous experimental and theoretical investigations are revealing new aspects in the decomposition pathway up to this date. Here, we present a quantum chemical study based on density functional theory (DFT) exploring the initial decomposition reactions of ADN, its anion DN- and its acid HDN in vacuo as well as in aqueous solution. HDN exhibits several isomers with different protonation sites. These isomers lead to a multitude of transitions state structures towards the formation of HNO3 and N2O. We thereby identified a two-step reaction consisting of an isomerization with subsequent proton transfer due to a twist of the HDN molecule as the minimum energy barrier route towards HNO3 and N2O in vacuo. Alternative decomposition routes are presented and discussed. HNO3 is known to accelerate the decomposition of ADN autocatalytically. We investigate this effect by computing various clusters consisting of HNO3 and ADN related molecules ([HNO3_ADN], [HNO3_DN]- and [HNO3_HDN]) in order to establish an autocatalytic cycle. We complement our study by investigating the protonated, cationic species H[ADN]+ which may form in a low pH regime. We elucidate the influence of excess protons within the investigated molecule on the energy barriers of the decomposition pathways.