Initiation Step in the Condensed Phase Decomposition Process of Ammonium Perchlorate

Ammonium perchlorate (AP) is a commonly used oxidizer in solid propellant compositions. As a result, numerous experimental and theoretical studies have been carried out in order to better understand the behavior of its decomposition in both the liquid and gas phases. In this work, the first step of Ammonium perchlorate (AP) decomposition in the condensed phase has been investigated using quantum mechanics-based calculations. The process of proton transfer from ammonium ion (NH4+) to perchlorate ion (ClO4-) was investigated in-depth as it is thought to be the preferred reaction for the initiation of AP decomposition in the condensed phase. The calculations were carried out using density functional theory (DFT) at the B3LYP level in conjunction with the 6-311G++(d,p) basis set. The current study revealed that proton transfer from ammonium cation to perchlorate anion takes place in two steps. The first step involves the decomposition of ClO4- to ClO2- and O2 followed by the second step which is H+ transfer from NH4+ to ClO2- giving NH3 and HClO2. In order to better understand protonation and deprotonation, it is essential to know the proton affinity (PA) of the involved species. In this study, the CBS-QB3 method is used to determine the PA of various energetic ionic compounds. These compounds include guanidinium azotetrazolate, triaminoguanidinium azotetrazolate, guanidinium nitrate, ammonium nitrate, and ammonium perchlorate. Based on the results of the calculations, it is concluded that direct proton transfer process does not occur in the condensed phase because the proton affinities of all cations are significantly higher than those of the corresponding anions. In conjunction with experimental studies, this modelling study can help in the development of a better understanding of AP decomposition in the condensed phase. In the decomposition of ammonium perchlorate (AP), the widely believed initial step of direct proton transfer between NH4+ and ClO4- is not observed due to strong solvation effects. An alternate path involves two steps: first, ClO4- decomposes into ClO2- and O2; second, NH4+ transfers a proton to ClO2- forming NH3 and HClO2. Solvent effects lower proton affinities (PAs) in condensed phase compared to gas phase. Ab-initio calculations can predict proton transfer based on structural orientation, guiding the development of the reaction mechanism.image