Ab initio chemical kinetics for hypergolic reactions of nitrogen tetroxide with hydrazine and methyl hydrazine

The kinetics and mechanisms for the hypergolic reactions of N₂O₄ (NTO) with N₂H₄ and CH₃NHNH₂ have been investigated by ab initio molecular orbital theory based on the UCCSD(T) method with the 6-311+G(3df, 2p) basis set. These reactions are important to the propulsion chemistry of the N₂O₄-N₂H₃X (X = H, CH₃) propellant systems. The results of our calculations show that the hypergolic ignition reactions of NTO with N₂H₄ and CH₃NHNH₂, producing N₂H₃NO + HNO₃ and CH₃NHN(H)NO/CH₃N(NO)NH₂ + HNO₃, respectively, can be initiated by the rapid attack of hydrazine molecules by ONONO₂, following the isomerization of NTO via loose, roaming-like transition states with 5.9 and 6.3 kcal/mol above the reactants in the presence of the hydrazine reactants as spectators. The rate constants for the reactions have been predicted in the temperature range 298–2000 K with the transition state theory; the result indicates, for example, that the half-life of N₂O₄ at 300 K in a mixture containing an excess amount of N₂H₄ at atmospheric pressure was predicted to be as short as 1.4 × 10⁻⁵ s; similarly, a 50–50 mixture of N₂O₄ and N₂H₄ at atmospheric pressure decays by half within 4.1 × 10⁻⁵ s upon mixing at 300 K, both reflecting the very effective hypergolic initiation of these propellant systems.