Experimental and quantum mechanics investigations of early reactions of monomethylhydrazine with mixtures of NO₂ and N₂O₄

The gas-phase chemistry of the hypergolic system CH₃NHNH₂ - monomethylhydrazine (MMH), with oxidizers NO₂/N₂O₄ at room temperature and 1atm N₂ was investigated experimentally using a gold-coated chamber reactor, coupled with a Fourier transform infrared (FTIR) spectrometer. The IR-active species identified in the early reactions include HONO, monomethylhydrazinium nitrite (MMH·HONO), methyl diazene (CH₃NNH), methyl nitrate (CH₃ONO₂), methyl nitrite (CH₃ONO), nitromethane (CH₃NO₂), methyl azide (CH₃N₃), H₂O, N₂O and NO. In order to elucidate the mechanisms by which these observed products are formed, we carried out quantum mechanics calculations [CCSD(T)/M06-2X] for the possible reaction pathways. Based on these studies, we propose that the oxidation of MMH in an atmosphere of NO₂ occurs via two mechanisms: (1) sequential H-abstraction and HONO formation, and (2) reaction of MMH with asymmetric ONONO₂, leading to formation of methyl nitrate. These mechanisms successfully explain all intermediates observed experimentally. We conclude that the formation of asymmetric ONONO₂ is assisted by an aerosol formed by HONO and MMH that provides a large surface area for ONONO₂ to condense, leading to the generation of methyl nitrate. Thus we propose that the overall pre-ignition process involves both gas-phase and aerosol-phase reactions.