Ab initio kinetics of H-atom abstraction from monomethylhydrazine

Abstract

Monomethylhydrazine (CH₃NHNH₂, MMH) has been widely used as a propellant for attitude or trajectory control motors. Multi-channel reaction kinetics of H-atom abstraction in MMH may improve the understanding of combustion properties and the modeling of its chemical kinetics mechanisms. This study systematically investigated the ab initio kinetics of H-abstraction reactions of MMH with nine species: NO₂, OH, H, O₂, HO₂, NH₂, CH₃, CH₃O, and CH₃O₂. Four different H-atom abstraction were considered, resulting in the formation of corresponding CH₃NNH₂, t-CH₃NHNH, c-CH₃NHNH, and CH₂NHNH₂ radical. Their kinetic information was analyzed, including reaction enthalpy, barrier height, potential energy surface, and rate coefficient. An updated kinetics mechanism of the MMH system with 89 species and 547 reactions was applied for ignition delay time (IDT) simulations and sensitivity analyses. The results suggest that: (1) a distinction between t-CH₃NHNH₂ and c-CH₃NHNH₂ should be considered, (2) c-HONO and HNO₂ are the main products of H-abstraction reactions in MMH + NO₂, (3) MMH + CH₃O is a very fast reaction, which has previously been mechanistically neglected, (4) the IDTs of the MMH/NTO system had marked changes at low temperatures, which were mainly due to the influence of H-abstraction reactions from MMH by NO₂, and (5) the new MMH mechanism predicts IDTs for MMH in “air” that were noticeable effected at all conditions.