Ab initio study of ammonium perchlorate combustion initiation processes: Unimolecular decomposition of perchloric acid and the related OH + ClO ₃ reaction

The unimolecular decomposition of HClO₄ has been investigated at the G2M//B3LYP/6-311 + G(3df, 2p) level of theory. Above 500 K, the decomposition process was found to depend strongly on pressure below 200 atm. The rate constants at the high- and low-pressure limits were predicted to be k₁ ^∞ = 1.5 × 10¹⁷ exp(-26500/T) s^-1 and k₁ ⁰ = 3.39 × 10³⁰ T^-10.9 exp(-29430/T) cm³ molecule^-1 s ^-1, respectively. Under the atmospheric-pressure condition, the predicted first-order rate coefficient for the temperature range 300-3000 K, k₁ = 5.0 × 10⁵¹ T^-11.64 exp(-30700/T) s^-1, agrees reasonably with experimental data obtained at 550-750 K by different experimental groups. We have also calculated the rate constants for the bimolecular reaction of OH with ClO₃ producing HClO₄ by association/stabilization and HO₂ + ClO₂ by association/fragmentation. At the high-pressure limit, the rate constants can be given as:k_-1 = 3.2 × 10^-10 T^0.07 exp(-25/T) cm³ molecule^-1 s^-1 and k₂ = 2.1 × 10^-10 T^0.09 exp(-18/T) cm³ molecule^-1 s^-1, respectively, for the temperature range 300-3000 K. Under the atmospheric pressure condition, the OH + ClO₃ reaction produces predominately the HO₂ + ClO₂ products.