Ultrafast Cold Reactions in the Bipropellant Monomethylhydrazine/Nitrogen Tetroxide: CPMD Simulations (2004)

Highly reactive intermediates, such as that shown, which arise under special conditions in hypergolic mixtures of monomethylhydrazine and nitrogen tetroxide, may trigger uncontrolled reaction behavior. Car–Parrinello molecular dynamics simulations provide an insight into the complex reactions on very short timescales. The bipropellant monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) has been employed in astronautics for a long time,1 for example, as fuel for the upper stage engine of the European launch vehicle Ariane 5 and in various U.S. spacecraft engines of the RS series (e.g. RS-21, RS-25, RS-28, RS-41, and RS-42). However, the mixture is unfortunately problematic in its handling and application. In 2001, flight 510 of the Ariane 5 project encountered serious problems due to complications within the upper stage engine.2 On contact the components of the mixture react violently, explosively emitting heat and gas, and therefore the mixture is known as hypergolic. However, the mixture does not heat up during the reaction that precedes ignition (“cold prereaction”). Such a fast process is not easy to examine experimentally in full detail, as it is especially difficult to create a well-defined mixture of the reactants. Here theory provides an attractive alternative in terms of molecular dynamics simulations5–8 within the Car–Parrinello approach,3, 4 where the extreme reactivity of the system is even advantageous. Car–Parrinello Molecular Dynamics (CPMD) allows the simulation of different types of reactions in a system through the description of the electronic structure with the density functional theory approach.9, 10 It is not necessary to predefine a reaction coordinate; rather, all degrees of freedom of the system can be considered. This study does not focus on highly precise computations of properties of isolated molecules. It aims rather at determining the relevant reaction schemes under varying reaction conditions like the ratio of starting materials, temperature, and impurities. As the first step, an MMH/NTO mixture with a ratio of 1:1 and a density of 0.9 g cm−3 was simulated (Scheme 1 and Figure 1). At temperatures above 300 K we initially observe electron transfer: an electron is transferred from an MMH molecule to an NTO molecule while the latter dissociates. The resulting ions are neutralized by a consecutive proton transfer. Afterwards a second electron is transferred, and again proton transfer yields neutral products (Scheme 1; details on the energetics are given in the Supporting Information). The oxidation state of the MMH molecule and its products during the molecular dynamics run can easily be determined from the NN distance (see Figure 1 and Scheme 2). Likewise the O-N-O angle of the NO2 fragment indicates whether it is reduced to an anion after the dissociation of NTO (Scheme 2). The protons may be transferred to an NO2 anion or to another MMH molecule, depending on the actual arrangement of the reactants. The reaction ultimately yields methyldiazene and nitrous acid. Simulation of a 1:1 mixture of MMH/NTO with a density of 0.9 g cm−3 (upper panel: NN distances in the MMH molecules; middle panel: NN distances in the NTO molecules; lower panel: temperature). Redox reaction leading from MMH to methyldiazene as observed in the CPMD simulations. Calculated structures (B3LYP/cc-pVQZ) of MMH, NTO, and their subsequent products; A: MMH, B: MMH radical cation, C: methylhydrazyl radical, D: protonated methyldiazene, E: trans-methyldiazene, F: NTO, G: NO2 radical, H: NO2 anion, I: HNO2. [a] Ref. 11. [b] Ref. 12. In the next step we checked the effect of impurities on the reaction. Typical by-products of the production process are methylamine, water, and nitric acid. However, simulations of mixtures containing these substances do not show essentially different reaction mechanisms. The impurities may act as proton acceptors, but in the end methyldiazene is formed anyway. Hence, it can be assumed to be the main product even under these conditions. In further CPMD simulations we investigated whether an ultrafast process may originate from methyldiazene to explain the unstable behavior. Methyldiazene was mixed with NTO, and the relevant impurities as well as the radical and ionic intermediates formed in the prereaction. Even at higher temperatures no reaction was observed. Indeed, studies of the first directly observed monosubstituted diazenes in 1965,13 and in particular of the isolatable pure methyldiazene,14 demonstrate a certain kinetic stability. Also the biological chemistry of methyldiazene as an intermediate and a metabolite in chemical carcinogenesis is well known. It reacts directly with hemoglobin15, 16 as does MMH. Furthermore, a reaction with cytochrome P450–FeIII was reported.17 We observe in simulations with an excess of MMH that the oxidation is not completed but stops after the oxidation to the methylhydrazyl radical. Partial disproportionation of two methylhydrazyl radicals follows, resulting in the formation of methyldiazene and MMH, as already suggested in experimental studies.18 However, this reaction sequence does not yield a new intermediate that might lead to a violent reaction, but a recombination of two methylhydrazyl radicals may yield dimethyltetrazane (Scheme 3), a very unstable molecule with a chain of four nitrogen atoms. This recombination reaction shows no or just a very small energy barrier (B3LYP/6-31G*(Gaussian)≈0.1 kcal mol−1, BLYP/6-31G*(Gaussian)=0.0 kcal mol−1). The formation of tetrazane has already been discussed several times in the literature in the context of the decay of hydrazyl radicals.19–21 A three-particle collision must occur to cause this exothermic addition reaction in order to dissipate the energy originating from bond formation. Presumably this reaction occurs mostly at the walls of the vessel. Reaction scheme for the formation of dimethyltetrazane isomers from an excess of MMH. Unsubstituted tetrazane is known to be very unstable in alkaline solutions.22 The perfluorated methyl-substituted tetrazane derivatives, for example, hexakis(trifluoromethyl)tetrazane, have been synthesized only recently due to this high reactivity.23, 24 A theoretical study of the thermochemical properties of tetrazane25 shows that the proton affinity of the nitrogen atoms of tetrazane is higher than that of ammonia or hydrazine. CPMD simulations of dimethyltetrazanes with nitrous acid show on the one hand a protonation of the terminal nitrogen atoms leading to detachment of either methylamine or ammonia, and on the other hand a protonation of a median nitrogen atom resulting in dissociation of the tetrazane derivative to MMH and protonated methyldiazene (Scheme 4). The protonation of a terminal nitrogen atom results in the decomposition reaction that was suggested in experimental studies of unsubstituted tetrazane.18, 20, 21 A further decomposition of the resulting methyltriazene was not observed. The molecular dynamics simulations mostly show a protonation at a terminal sp2-hybridized nitrogen atom. As for tetrazene,26 the protonation of the sp3-hybridized nitrogen might be inhibited because of the interaction of the free electron pair with the π bond of the two other nitrogen atoms. The decomposition in a protic environment to methylated amines or ammonia and nitrogen was proven experimentally.20, 21 The rate of the triazene formation in acidic solutions could not be determined in experiment since the reaction of protonated tetrazane to give triazene is much faster than the decay rate of triazene.20 Acid is also present in our simulations and results from the reduction of NTO. One-electron oxidation of hydrazine is described in textbooks as a method to form tetrazane, but the proof of dimethyltetrazane formation under these conditions is awkward: the by-product nitrous acid causes immediate decomposition. Experimental detection might be possible spectroscopically or by matrix isolation experiments. Reaction scheme for the decomposition of a dimethyltetrazane isomer in acid–base reactions. In conclusion with Car–Parrinello Molecular Dynamics simulations we have demonstrated that the cold prereaction in the mixture MMH/NTO can be assigned to an oxidation of monomethylhydrazine to methyldiazene. The mixture does not heat up until this point, and the succeeding reaction to nitrogen is kinetically inhibited. According to our simulations it is plausible to assume that this reaction takes place anyway prior to ignition and does not necessarily trigger an uncontrolled violent reaction. However, uncontrolled behavior could be explained by the reaction sequence involving dimethyltetrazane and methyltriazene. This would correspond to the experimental finding that violent decomposition occurs mainly at low temperatures and with an excess of MMH. With an excess of MMH the reaction can stop after the one-electron oxidation. If two of the resulting radicals meet, low temperatures favor a recombination. The energy released during tetrazane formation and the subsequent decomposition can finally trigger other decomposition reactions. We could identify the most relevant of the many possible reaction pathways in the MMH/NTO system with Car–Parrinello simulations. According to the simulations a violent reaction prior to ignition is possible by a combination of ultrafast redox, radical, and acid–base reactions. The static calculations of the relative energies and the geometries were performed with the Gaussian package27 using the hybrid functional B3LYP28, 29 (cc-pVQZ basis set) and with the CPMD package30 using the BLYP functional31, 29 in the local spin density approximation. A planewave basis set (energy cutoff: 70 Rydberg) and Troullier–Martins pseudopotentials32 were employed in the CPMD calculations. The molecules were placed in a box of dimensions 8.5×8.5×8.5 Å3 and were treated as isolated systems. The molecular dynamics simulations were performed with the CPMD package using periodic boundary conditions. A timestep of 4 a.u. (0.1 fs) and a fictitious electron mass of 400 a.u. were used. The starting geometries were optimized with molecular dynamics at finite temperatures and afterwards annealed to the desired temperatures. The size of the box was varied from 7.9×7.9×7.9 Å3 to 9.5×9.5×9.5 Å3 in order to get similar densities for the different ensembles of molecules. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2004/z54093_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.