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Ultrafast Cold Reactions in the Bipropellant MonomethylhydrazineNitrogen Tetroxide CPMD Simulations.

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The bipropellant monomethylhydrazine/nitrogen tetroxide is used as a
rocket fuel. Molecular dynamics simulations provide an explanation for
the “cold prereaction”, which yields methyldiazene and nitrous acid,
and for the subsequent explosive reaction via dimethyltetrazane.
I. Frank and co-workers give details in the following Communication.
Angew. Chem. Int. Ed. 2004, 43, 4585
DOI: 10.1002/anie.200454093
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Hypergolic Mixtures
Ultrafast Cold Reactions in the Bipropellant
Monomethylhydrazine/Nitrogen Tetroxide:
CPMD Simulations**
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
Christel Nonnenberg, Irmgard Frank,* and
Thomas M. Klaptke
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
simulations[5–8] 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
Scheme 1. Redox reaction leading from MMH to methyldiazene as
observed in the CPMD simulations.
[*] C. Nonnenberg, Dr. I. Frank, Prof. Dr. T. M. Klap4tke
Department Chemie und Biochemie
Ludwig-Maximilians-Universit9t M:nchen
81377 M:nchen (Germany)
Fax: (+ 49) 89-2180-77568
[**] We gratefully acknowledge the Deutsche Forschungsgemeinschaft
for financial funding and the Leibniz-Rechenzentrum M:nchen
(HLRB project h0621) for computational resources.
Supporting information for this article is available on the WWW
under or from the author.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. 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:
DOI: 10.1002/anie.200454093
Angew. Chem. Int. Ed. 2004, 43, 4586 –4589
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
Scheme 2. 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].
anion or to another MMH molecule, depending on the
actual arrangement of the reactants. The reaction ultimately
yields methyldiazene and nitrous acid.
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 hemoglobin[15, 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
Angew. Chem. Int. Ed. 2004, 43, 4586 –4589
Scheme 3. Reaction scheme for the formation of dimethyltetrazane isomers from an excess of MMH.
shows no or just a very small energy barrier (B3LYP/631G*(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.
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 tetrazane[25] 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
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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.
Computational Procedure
Scheme 4. Reaction scheme for the decomposition of a dimethyltetrazane isomer in acid–base reactions.
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 p 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. Oneelectron 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.
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
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The static calculations of the relative energies and the geometries
were performed with the Gaussian package[27] using the hybrid
functional B3LYP[28, 29] (cc-pVQZ basis set) and with the CPMD
package[30] using the BLYP functional[31, 29] in the local spin density
approximation. A planewave basis set (energy cutoff: 70 Rydberg)
and Troullier–Martins pseudopotentials[32] were employed in the
CPMD calculations. The molecules were placed in a box of
dimensions 8.5 K 8.5 K 8.5 L3 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 K 7.9 K 7.9 L3 to 9.5 K 9.5 K
9.5 L3 in order to get similar densities for the different ensembles of
Received: February 24, 2004
Revised: June 15, 2004 [Z54093]
Published Online: August 9, 2004
Keywords: density functional calculations · hypergolic mixtures ·
quantum chemistry · reactive intermediates
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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simulation, bipropellant, monomethylhydrazinenitrogen, reaction, cpmd, cold, tetroxide, ultrafast
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