Towards reliable prediction of kinetics and mechanisms for elementary processes: Key combustion initiation reactions of ammonium perchlorate

A series of ClO_x reactions relevant to the combustion initiation of ammonium perchlorate has been studied. The reactions include the decomposition of ClO_x (x = 2 - 4) and HOClO_y (y = 2 - 3), the bimolecular reactions of HO + X (X = ClO, OClO, HOClO₃), ClO + Y (Y = O, ClO, OClO), H + HOClO₃ and some of their reverse processes. The geometries of the reactants, intermediates, transition states and products involved in these reactions were optimized at the B3LYP/6-311+G(3df, 2p) or PW91PW91/6-311+G(3df, 2p) level and final energies were refined at the modified Gaussian - 2 (G2M) level. Rate constants for major product channels have been predicted and compared with the available experimental data. Detailed mechanisms for these reactions have been discussed on the basis of predicted potential energy surfaces. The rate constant calculations were carried out by the conventional transition state theory (TST) or Rice-Ramsperger-Kassel-Marcus (RRKM) theory for reactions with well-defined transition states and variational TST or RRKM theory for those channels which occur barrierlessly. The results show that our predicted energies and rate constants are quite satisfactory and in good agreement with available experimental data. For practical applications, the molecular parameters and heats of formation of key species, and the rate constants for all the major reactions discussed under different conditions have been recommended.