A computational study on the decomposition of NH4ClO4 (AP) and its aggregates with nH2O (n 1, 2): Comparison of the gas-phase and condensed-phase results

The structures and energies of ammonium perchlorate (AP), chemically known as NH4ClO4, and different aggregates formed by AP with one and two water molecules in both gaseous and condensed phases have been investigated at the B3LYP/6-311G(3df,2p) level. The results show that for the gaseous dissociation processes, the main dissociation channel is NH4ClO4-nH2O NH3 nH2O-HClO4 (n 0, 1, 2) with dissociation energies of 14.7, 18.8, and 19.6 kcal/mol, respectively. The formation of nH2O-ClO4-(n 0, 1, 2) NH4 ion pairs is not favored energetically with 110.5, 114.4, and 116.9 kcal/mol dissociation energies. In solution, the dissociation energies predicted with the polarizable continuum model (PCM) become 30.8, 30.4, and 30.6 kcal/mol for the former process and 3.0, 5.5, and 7.2 kcal/mol for the latter case, respectively. These values show that in solution, AP and its aggregates mainly dissociate to ion pairs, reflecting the important effect of solvation. The calculated solvation free energies for NH3, NH4 and ClO4-are-4.3,-80.1 and-56.3 kcal/mol, respectively, which are in good agreement with the available experimental values. The predicted solvation free energy with PCM for the isolated AP,-30.0 kcal/mol is consistent with the average values determined from the aggregates of AP with one H2O (-30.2 kcal/mol) and 2H2O (-30.0 kcal/mol), respectively. These values clearly indicate that the effect of hydrogen bonding between the solvent (H2O) and the solvate (NH4ClO4) on the solvation energies appears to be fully incorporated in the PCM method.