A Theoretical Combustion Kinetic Study of Unsymmetrical Dimethylhydrazine: H-Atom Abstraction and Reactions on the Potential Energy Surface of C₂H₇N₂ Radicals

Unsymmetrical dimethylhydrazine (UDMH) is a widely used hypergolic rocket fuel. It is one of the most commonly used fuels for attitude control engines, such as those in missiles, satellites, spacecraft, and launch vehicles. We conducted a high-level theoretical study to develop a detailed combustion kinetic mechanism for UDMH, focusing on crucial elementary reactions. We analyzed 14 H atom abstraction reactions (HAA) of UDMH, four unimolecular reactions, and five chemical activation reactions on the potential energy surface (PES) of UDMH radicals (C₂H₇N₂). Seven reactive radicals Ḣ, ȮH, NO₂, HȮ₂, ĊH₃, CH₃Ȯ, and CH₃Ȯ₂ were involved. The M06-2X/6-311++G(d,p) method, along with the CCSD/cc-pVXZ (where X = T, Q) methods, was used for theoretical calculations. The rate coefficients and thermochemical property parameters for the 23 elementary reactions and related key species were calculated. Our results indicate that HAA by Ḣ, ȮH, and CH₃Ȯ dominate at low temperatures (300-700 K) compared to CH₃Ȯ₂, HȮ₂, and NO₂. Some of the calculated rate coefficients were compared with previous experimental and theoretical studies, and our computed values agreed well with reference data. For the reactions on the PES, the N-N breaking reaction and the CH₃NNH + ĊH₃ system show great importance in the initial reaction network. The pressure dependence of all the reactions on the PES was evaluated. Regarding the thermochemical properties of UDMH and its corresponding product radicals, our standard enthalpy values deviate by about 1 kcal·mol^-1 from database values.