Ab Inito Chemical Kinetics Modeling of Liquid-phase Reactions of Monomethylhydrazine and Nitrogen Tetroxide

Monomethylhydrazine (MMH) and nitrogen tetroxide (NTO) have been utilized as hypergolic bipropellants in spacecraft engines for many years, but the liquid-phase MMH/NTO reaction mechanism remains underexplored. This theoretical study investigates the liquid-phase reactions between MMH and NTO. By employing an implicit solvent model and conducting high-accuracy quantum chemical calculations at the CCSD(T)/CBS//M06-2<i>X</i>/6-311++G(d,p)/SMD theoretical level, we obtained the reaction paths for the isomerization reactions of NTO, for the reactions between MMH and <i>t</i>-ONONO₂, and for the reactions between MMH and NO₂. The corresponding reaction pathways in the gas phase were also computed for comparison at the CCSD(T)/CBS//M06-2<i>X</i>/6-311++G(d,p) theoretical level. The results indicate that the energy barriers for the liquid-phase reactions are lower than their gas-phase counterparts due to the solvent effects. The rate constants were calculated using the transition state theory. We established a kinetic model and applied it to simulate the early stage of the liquid-phase MMH/NTO reactions under premixed and adiabatic conditions. The results indicate that the required time for temperature rising from the initial temperature (-13 °C) to the boiling point temperature of MMH (87.5 °C) is approximately 0.05 μs, consistent with previous experimental observations in drop tests. In addition, the kinetic analysis revealed that the reaction steps N₂O₄ → <i>t</i>-ONONO₂ and CH₃NHNH₂ + <i>t</i>-ONONO₂ → CH₃NH₂ + N₂O + HNO₃ play critical roles in triggering the subsequent reactions, thereby dictating the rate of temperature rise. This work not only provides a new kinetic model for liquid-phase MMH/NTO reactions but also clarifies the role of liquid-phase reactions in the hypergolic ignition of MMH/NTO.