Formation and decomposition of chemically activated and stabilized hydrazine

Document Type

Article

Publication Date

6-3-2010

Abstract

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. © 2010 American Chemical Society.

Identifier

77955556815 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp101640p

e-ISSN

15205215

ISSN

10895639

First Page

6235

Last Page

6249

Issue

21

Volume

114

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