The state-of-the-art hypergol propellant combination currently used in the US for many space propulsion applications consists of monomethyl hydrazine, as the fuel, and nitrogen tetroxide, as the oxidizer. Here we report theoretical and experimental results obtained on a prototypical NO2-radical reaction with N2H3. The potential energy surface was investigated by ab initio multi-reference second-order perturbation, density functional, quadratic configuration interactions, and coupled-cluster theories. The reaction proceeds via a complex mechanism with submerged energy barriers and relatively large exothermicities. Direct NO2 addition to the NH site in N2H3 forms two isomeric adducts. The isomeric adducts undergo decomposition to trans-HONO + trans-NH=NH and trans-HONO + cis-NH=NH. Furthermore, the addition adducts undergo decomposition to NO, NNH and H2O via several steps involving isomerization and molecular elimination. Also, NO2 can add to the NH2 site in N2H3 to form an unstable adduct which undergoes isomerization to form the NH2NHONO isomer. Important reaction channels for the NH2NHONO isomers involve dissociation to NO + NH2NHO radical products. The reaction kinetics of N2H3 in excess NO2 was studied in 2 Torr of N2 and 298 K in a pulsed photolysis flow-tube reactor. Upon laser photolysis of N2H4, the reacting gas was mass spectrometrically sampled using a skimmer as the mixture escaped via a pin-hole inside the flow-tube reactor. The temporal profile of the product, HONO, was determined by direct detection of the m/z = 47 ion signal. For each chosen [NO2], the observed [HONO] trace could be fitted to a bi-exponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, k′, for the reaction of N2H3 with NO2. The slope of a plot of k′ versus [NO2] yielded a value for the bimolecular rate coefficient, k = (1.03 ± 0.29) × 10-11 cm3 molecule-1 s-1. Pressure-dependent rate coefficients of the N2H3 + NO2 reaction were determined by RRKM theory with multi-well master equation simulations. With △Edown = 100 × (T/300)0.85 cm-1 for the energy transfer probability, excellent agreement was observed between the measured value and the computed overall rate coefficient at 298 K and 2 Torr of N2. The negative energy barriers and large exothermicities for the various product channels suggest a significant role of this reaction in the early stages of hypergolic ignition of hydrazine and its derivatives.

Theoretical and experimental studies on the radical-radical reaction: NO2 + N2H3

249th National Meeting (2015)

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