Towards hypergolic ignition modeling of monomethylhydrazine and nitrogen dioxide: Ab initio chemical kinetics of CH₃NH/CH₂NH₂/CH₂NH and nitrogen dioxide

The ab initio chemical kinetics of the reactions of CH₃NH/CH₂NH₂/CH₂NH and nitrogen dioxide (NO₂) and its application to modeling hypergolic ignition of monomethylhydrazine (MMH)/NO₂ were investigated in this work. The CCSD(T)/CBS//M062X/6–311G(d,p) single-reference method and the MRCI(9e,8o)/CBS//M062X/6–311++G(d,p) multi-reference method were employed for the kinetically important reaction pathways on the potential energy surfaces (PESs). The temperature- and pressure-dependent rate constants were determined by using the RRKM/Master Equation analysis combined with the transition state theory (TST) and the variable reaction coordinate transition state theory (VRC-TST). The formation of CH₂NH and HONO from CH₂NH₂ and NO₂ dominates in the temperature range of 300–2000K at 0.01 and 1 atm. The calculated rate constants and thermodynamics data were incorporated into a recently established MMH/NO₂ mechanism to investigate their influence on modeling MMH/NO₂ hypergolic ignition. The results show that with the updated rate constants, the consumptions of NO₂ and MMH are promoted, the conversion from CH₂NH₂ to CH₂NH is highly accelerated, and the mole fraction of CH₂NH increases. The rate of production indicates that 28 % of MMH decomposes to CH₃NH and NO₂, 3 % of MMH directly decomposes to CH₂NH and NH₃, and most CH₃NH radicals isomerize to CH₂NH₂ radicals and further form CH₂NH at 1200 K. The sensitivity analyses also substantiate the importance of the decomposition of MMH to CH₃NH/CH₂NH, the isomerization of CH₃NH, the beta-scission of CH₂NH₂, and the reaction of CH₂NH₂+NO₂→CH₂NH+HONO at 1200 K. This work provides not only new kinetic and thermochemical data but also a firm step toward understanding the hypergolic ignition of MMH/NO₂.