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

Abstract

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.