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

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The redox reactions of NH 3 and CH 3 NH 2 with N 2 O 4 (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–N 2 H 4 mixture, primarily controlled by the conversion of NTO to ONONO 2 via very loose transition states (with NH 3 and CH 3 NH 2 as spectators in the collision complexes) followed by the rapid attack of ONONO 2 at the spectating molecules producing HNO 3 and RNO (R = NH 2 and CH 3 NH). The predicted mechanism for the NH 3 reaction compares closely with its isoelectronic process NTO + H 2 O; similarly, the mechanism for the NTO + CH 3 NH 2 reaction also compares closely with its isoelectronic NTO + NH 2 NH 2 reaction. The kinetics for the formation of the final products, HNO 3 + RNO (R = NH 2, OH, CH 3 NH, and N 2 H 3 ), were found to be weakly pressure-dependent at low temperatures and affected by the strengths of H–NH 2 and H–OH but not in the RNH 2 case. We have also compared the predicted rate constant for the oxidation of NH 3 by N 2 O 4 with that for the analogous NH 3 + N 2 O 5 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 N 2 O 4 reaction under troposphere conditions.