Large Intermediates in Hydrazine Decomposition: A Theoretical Study of the N₃H₅ and N₄H₆ Potential Energy Surfaces

Large complex formation involved in the thermal decomposition of hydrazine (N₂H₄) is studied using transition state theory-based theoretical kinetics. A comprehensive analysis of the N₃H₅ and N₄H₆ potential energy surfaces was performed at the CCSD(T)-F12a/aug-cc-pVTZ//ωB97x-D3/6-311++G(3df,3pd) level of theory, and pressure-dependent rate coefficients were determined. There are no low-barrier unimolecular decomposition pathways for triazane (n-N₃H₅), and its formation becomes more significant as the pressure increases; it is the primary product of N₂H₃ + NH₂ below 550, 800, 1150, and 1600 K at 0.1, 1, 10, and 100 bar, respectively. The N₄H₆ surface has two important entry channels, N₂H₄ + H₂NN and N₂H₃ + N₂H₃, each with different primary products. Interestingly, N₂H₄ + H₂NN primarily forms N₂H₃ + N₂H₃, while disproportionation of N₂H₃ + N₂H₃ predominantly leads to the other N₂H₂ isomer, HNNH. Stabilized tetrazane (n-N₄H₆) formation from N₂H₃ + N₂H₃ becomes significant only at relatively high pressures and low temperatures because of fall-off back into N₂H₃ + N₂H₃. Pressure-dependent rate coefficients for all considered reactions as well as thermodynamic properties of triazane and tetrazane, which should be considered for kinetic modeling of chemical processes involving nitrogen- and hydrogen-containing species, are reported.