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 N 2 O 4 (NTO) with N 2 H 4 and CH 3 NHNH 2 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 N 2 O 4 -N 2 H 3 X (X= H, CH 3 ) propellant systems. The results of our calculations show that the hypergolic ignition reactions of NTO with N 2 H 4 and CH 3 NHNH 2 , producing N 2 H 3 NO + HNO 3 and CH 3 NHN(H)NO/CH 3 N(NO)NH 2 + HNO 3 , respectively, can be initiated by the rapid attack of hydrazine molecules by ONONO 2 , 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 N 2 O 4 at 300 K in a mixture containing an excess amount of N 2 H 4 at atmospheric pressure was predicted to be as short as 1.4 x 10 -5 s; similarly, a 50-50 mixture of N 2 O 4 and N 2 H 4 at atmospheric pressure decays by half within 4.1 x10 -5 s upon mixing at 300 K, both reflecting the very effective hypergolic initiation of these propellant systems.