Ab initio kinetic study of decomposition reactions of the unsym-dimethylhydrazine radical HNN(CH3)2

Unsym-dimethylhydrazine (UDMH) is an important nitrogen-containing liquid propellant fuel commonly combined with nitrogen tetroxide (NTO) in hypergolic systems. In this work, theoretical calculations were performed on the decomposition reactions of critical UDMH radical, HNN(CH3)2 to get insight into its combustion chemical kinetics. Specifically, reactions of its isomerization, β-scission, and H-abstraction reactions attacked by NO2 and NO, as well as the subsequent decomposition reactions of formed NN(CH3)2 were investigated. Potential energy surfaces (PESs) for these reactions were obtained by employing the CCSD(T)/CBS(D+T)//M06-2X/def2-TZVP method. The RRKM/Master Equation approach was used to calculate the temperature- and pressure-dependent rate constants. Comparisons with monomethylhydrazine (MMH) radicals were made to elucidate the effect of CH3-group substitution on the kinetic characteristics of UDMH-radical decomposition. For isomerization and β-scission reactions of HNN(CH3)2, it undergoes β-N-C scission reaction more readily than H-migration isomerization to H2NN(CH3)CH2. Additionally, the rate constants for β-scission reactions of UDMH radicals are close to those of MMH radicals. For the H-abstraction reactions of HNN(CH3)2 by NO2, the reaction forming NN(CH3)2 and cis-HONO from the decomposition of cis-ONON(H)N(CH3)2 has the highest rate constants, which differ significantly from the H-abstraction of MMH radical (HNN[H]CH3). HNN(CH3)2 primarily undergoes H-abstraction at the terminal nitrogen site, forming NN(CH3)2, whereas HNN(H)CH3 prefers H-abstraction from the central nitrogen atom. The H-abstraction reaction of HNN(CH3)2 by NO also easily produces NN(CH3)2. Subsequently, NN(CH3)2 dissociates via a low-barrier pathway to produce N2 and C2H6. The calculated rate constants provide important parameters for the kinetic modeling development of UDMH/NTO combustion.