An Ab initio chemical kinetic study on the reactions of H, OH, and Cl with HOClO₃

The mechanisms for reactions of H, HO, and Cl with HOClO₃, important elementary processes in the early stages of the ammonium perchlorate (AP) combustion reaction, have been investigated at the CCSD(T)/6-311+G(3df,2p)/ /PW91PW91/6-311+G(3df) level of theory. The rate constants for the low-energy channels have been calculated by statistical theory. For the reaction of H and HOClO₃, the main channels are the production of H₂ + ClO₄ (k_1a) and HO + HOClO₂ (k_1b); k_1a and k_1b can be represented as 1.07 × 10 ^-17 T ^1.97 exp(-7484/T ) and 6.08 × 10 ^-17T ^1.96 exp(-7729/T ) cm³ molecule ^-1 s^-1, respectively. For the HO + HOClO₃ reaction, the main pathway is the H₂O + ClO₄ (k _2a) production process, with the predicted rate constant k _2a = 1.24 × 10 ^-8 T ^-2.99 exp(1664/T ) for 300-500 K and k_2a = 1.27 × 10^-19 T ^2.12 exp(-1474/T) for 500-3000 K. For the Cl + HOClO₃ reaction, the formation of HOCl + ClO₃ (k_3a) andHCl + ClO₄ (k_3b) is dominant, with k_3a = 1.33 × 10^-12 T ^0.67 exp(-9658/T ) and k_3b = 1.75 × 10¹⁶ T ^1.63 exp(-11156/T ) cm³ molecules^-1 in the range of 300-3000 K. In addition, the heats of formation of ClO₃ and HOClO₃ have been predicted based on several isodesmic and/or isogyric reactions with Δ_fH ₀^o (ClO₃) = 47.0 ± 1.0 and Δ_fH₀^o (HOClO₃) = 5.5 ± 1.5 kcal/mol, respectively. These data may be used for kinetic simulation of the AP decomposition and combustion reaction.