Why mixtures of hydrazine and dinitrogen tetroxide are hypergolic?

Preparation and Characterization of Metastable trans-Dinitrogen Tetroxide

Under atmospheric conditions, NO₂ is in equilibrium with its dimers, N₂O₄, which can exist in the form of constitutional isomers and stereoisomers whose relative stabilities and reactivities are still being debated. Experimental limitations facing the spectroscopic characterization of the isomers of N₂O₄ prevent us from determining their relative contributions to reaction mechanisms possibly causing discrepancies in the reported reaction orders and rates. Using reflection-absorption infrared spectroscopy, molecular beam deposition, and matrix isolation techniques, it is shown that the relative abundances of NO₂ and its dimers can be controlled by heating or cooling the deposited gas. The comparison of spectra acquired from samples prepared using molecular beam deposition with those obtained using tube dosing deposition demonstrates how the N₂O₄ isomer distributions are sensitive to details of the experimental conditions and sample preparation protocols. These observations not only provide a better understanding of a possible source for the disagreements found in the literature, but also a methodology to control and quantify the chemical speciation in NO₂ vapors in terms of the relative abundances of NO₂ and of the various isomers of N₂O₄.

Experimental and Theoretical Investigations of the Radical-Radical Reaction: N2H3 + NO2

The N₂H₃ + NO₂ reaction plays a key role during the early stages of hypergolic ignition between N₂H₄ and N₂O₄. Here for the first time, the reaction kinetics of N₂H₃ in excess NO₂ was studied in 2.0 Torr of N₂ and in the narrow temperature range 298-348 K in a pulsed photolysis flow-tube reactor coupled to a mass spectrometer. The temporal profile of the product, HONO, was determined by direct detection of the m/z +47 amu ion signal. For each chosen [NO₂], the observed [HONO] trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, k', for the reaction of N₂H₃ with NO₂. The slope of the plot of k' versus [NO₂] yielded a value for the observed bimolecular rate coefficient, k_obs, which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10^-12 exp((520 ± 350)/T) cm³ molecule^-1 s^-1. The errors are 1σ and include estimated uncertainties in the NO₂ concentration. The potential energy surface of N₂H₃ + NO₂ was investigated by advanced ab initio quantum chemistry theories. It was found that the reaction occurs via a complex reaction mechanism, and all of the reaction channels have transition state energies below that of the entrance asymptote. The radical-radical addition forms the N₂H₃NO₂ adducts, while roaming-mediated isomerization reactions yield the N₂H₃ONO isomers, which undergo rapid dissociation reactions to several sets of distinct products. The RRKM multiwell master equation simulations revealed that the major product channel involves the formation of trans-HONO and trans-N₂H₂ below 500 K and the formation of NO + NH₂NHO above 500 K, which is nearly pressure independent. The pressure-dependent rate coefficients of the product channels were computed over a wide pressure-temperature range, which encompassed the experimental data.

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.

Ignition Delay Reduction with Sodium Addition to Imidazolium-Based Dicyanamide Ionic Liquid

A range of ionic liquids (ILs) have been synthesized and modeled to better understand the role of the cation in the ignition of hypergolic ionic liquids. Vogelhuber et al. have shown by density functional theory methods that the addition of sodium cations to an ionic liquid promotes ignition with white fuming nitric acid (WFNA) by lowering energy barriers. To validate this prediction, solid sodium dicyanamide (Na⁺DCA^-) was added at various weight percents to 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-). The ignition delay was measured for each mixture with WFNA. Overall, it was found that the Na⁺DCA^- lowered the ignition delay by 11 ms at 7 wt %. The calculations done by Vogelhuber et al. appear to be consistent with this observation. The sodium cation may play a role by orienting the anion with the WFNA resulting in the favorable reaction energetics observed.