Formation and decomposition of chemically activated and stabilized hydrazine

Recombination of two amidogen radicals, NH₂ (X²B1), is relevant to hydrazine formation, ammonia oxidation and pyrolysis, nitrogen reduction (fixation), and a variety of other N/H/X combustion, environmental, and interstellar processes. We have performed a comprehensive analysis of the N₂H₄ potential energy surface, using a variety of theoretical methods, with thermochemical kinetic analysis and master equation simulations used to treat branching to different product sets in the chemically activated NH₂ + NH₂ process. For the first time, iminoammonium ylide (NH₃NH), the less stable isomer of hydrazine, is involved in the kinetic modeling of N₂H₄. A new, low-energy pathway is identified for the formation of NH₃ plus triplet NH, via initial production of NH₃ NH followed by singlet-triplet intersystem crossing. This new reaction channel results in the formation of dissociated products at a relatively rapid rate at even moderate temperatures and above. A further novel pathway is described for the decomposition of activated N₂H₄, which eventually leads to the formation of the simple products N₂ + 2H₂, via H ₂ elimination to cis-N₂H₂. This process, termed as "dihydrogen catalysis", may have significant implications in the formation and decomposition chemistry of hydrazine and ammonia in diverse environments. In this mechanism, stereoselective attack of cis-N ₂H₂ by molecular hydrogen results in decomposition to N₂ with a fairly low barrier. The reverse termolecular reaction leading to the gas-phase formation of cis-N₂H₂ + H ₂ achieves non-heterogeneous catalytic nitrogen fixation with a relatively low activation barrier (77 kcal mol^-1), much lower than the 125 kcal mol^-1 barrier recently reported for bimolecular addition of H₂ to N₂. This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N₂. We design heterogeneous analogues of this reaction using the model compound (CH₃)₂FeH₂ as a source of the H₂ catalyst and apply it to the decomposition of cis-diazene. The reaction is seen to proceed via a topologically similar transition state, suggesting that our newly described mechanism is general in nature.