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 (N 2 H 4 ) is studied using transition state theory-based theoretical kinetics. A comprehensive analysis of the N 3 H 5 and N 4 H 6 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-N 3 H 5 ), and its formation becomes more significant as the pressure increases; it is the primary product of N 2 H 3 + NH 2 below 550, 800, 1150, and 1600 K at 0.1, 1, 10, and 100 bar, respectively. The N 4 H 6 surface has two important entry channels, N 2 H 4 + H 2 NN and N 2 H 3 + N 2 H 3 , each with different primary products. Interestingly, N 2 H 4 + H 2 NN primarily forms N 2 H 3 + N 2 H 3 , while disproportionation of N 2 H 3 + N 2 H 3 predominantly leads to the other N 2 H 2 isomer, HNNH. Stabilized tetrazane (n-N 4 H 6 ) formation from N 2 H 3 + N 2 H 3 becomes significant only at relatively high pressures and low temperatures because of fall-off back into N 2 H 3 + N 2 H 3 . 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.