A Computational Study on the Redox Reactions of Ammonia and Methylamine with Nitrogen Tetroxide

The redox reactions of NH₃ and CH₃NH₂ with N₂O₄ (NTO) have been studied by ab initio molecular orbital (MO) calculations at the UCCSD(T)∥UB3LYP/6-311+G(3df,2p) level of theory. These reactions are related to the well-known NTO-hydrazine(s) propellant systems. On the basis of the predicted potential energy surfaces, the mechanisms for these reactions were found to be similar to the hydrolysis of NTO and the hypergolic initiation reaction of the NTO-N₂H₄ mixture, primarily controlled by the conversion of NTO to ONONO₂ via very loose transition states (with NH₃ and CH₃NH₂ as spectators in the collision complexes) followed by the rapid attack of ONONO₂ at the spectating molecules producing HNO₃ and RNO (R = NH₂ and CH₃NH). The predicted mechanism for the NH₃ reaction compares closely with its isoelectronic process NTO + H₂O; similarly, the mechanism for the NTO + CH₃NH₂ reaction also compares closely with its isoelectronic NTO + NH₂NH₂ reaction. The kinetics for the formation of the final products, HNO₃ + RNO (R = NH₂, OH, CH₃NH, and N₂H₃), were found to be weakly pressure-dependent at low temperatures and affected by the strengths of H-NH₂ and H-OH but not in the RNH₂ case. We have also compared the predicted rate constant for the oxidation of NH₃ by N₂O₄ with that for the analogous NH₃ + N₂O₅ recently reported by Sarkar and Bandyopadhyay [J. Phys. Chem. A. 2020, 124, 3564-3572] under troposphere conditions. The rate of the latter reaction was estimated to be 2 orders of magnitude slower than that of the N₂O₄ reaction under troposphere conditions.