Ab Initio Chemical Kinetics for Nitrogen Tetroxide Reactions with 1,1- and 1,2-Dimethylhydrazines

This work employed the quantum-chemical method at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(3df,2p) level to study the mechanisms and kinetics of N₂O₄ (NTO) with H₂NN(CH₃)₂ and CH₃NHNHCH₃ hypergolic initiation reactions, the processes critical to the chemical rocket propulsion of the N₂O₄-hydrazine propellant systems. The reaction of N₂O₄ with the dimethylhydrazines (DMHZ's) can be started by the fast reaction of DMHZ's with ONONO₂, taking place after the novel N₂O₄→ONONO₂ transformation with each of DMHZ's as a spectator within the NTO-DMHZ collision complexes, through loose, roaming-like transition states during the bimolecular encounters. The barriers for such isomerization processes were found to be 7.2 and 9.9 kcal/mol for H₂NN(CH₃)₂ and CH₃NHNHCH₃, respectively. The kinetics of these reactions have been computed in the temperature range 200–2000 K; the results indicate that under the ambient temperature and pressure condition, the half-life of NTO in the presence of an excess amount of H₂NN(CH₃)₂ is predicted to be 3.3×10⁻⁵ s. The results of a similar estimate for CH₃NHNHCH₃ is about 2 orders of magnitude longer; both estimates indicate that very effective hypergolic reactions can occur upon mixing in these systems.