Quantum chemical modeling of spontaneous reactions of N₂O₄ with hydrazines in CCl₄ solution at low temperature

The spontaneous reactions of N₂O₄ (NTO) with hydrazines (XNH₂, X = NH₂, CH₃NH and (CH₃)₂N, denoted as HZs) have been studied by ab initio quantum chemical calculations at the PCM-CCSD(T)/6-311+G(3df,2p) level in CCl₄ solution at low temperature to elucidate the mechanisms and predict the kinetics of these well-known hypergolic processes experimentally studied by Saad et al. [AIAA J. 10 (1972) 1073; Ref. 1]. The key initiation reactions in these systems, similar to those unraveled recently by us for the gas-phase reactions, occur by the isomerization process of N₂O₄ → ONONO₂ via very loose transition states within the pre-reaction complexes (N₂O₄:XNH₂) with barriers from 7.2 to 8.7 kcal/mol, followed by concurrent ONONO₂ attack on XNH₂ through abstraction of one of H atoms by the NO₃ group to form HNO₃ + XN(H)NO. The predicted rate constants for these bimolecular reactions in CCl₄ at 253 K were primarily controlled by transformation of N₂O₄:XNH₂ to ONONO₂:XNH₂ within 4.7–278 ms. This result can satisfactorily account for the occurrence of the hypergolic reactions of NTO and HZs in CCl₄ solution at 253 K reported by Saad et al.