Author:

Keywords:

nitrogen-containing heterocycles, ab-initio thermochemistry, potential-energy surface, nickel-iron hydrogenase, set model chemistry, electron correlation, gas-phase, thermal-decomposition, transition structures, pulse-radiolysis, Science & Technology, Physical Sciences, Chemistry, Physical, Physics, Atomic, Molecular & Chemical, Chemistry, Physics, NITROGEN-CONTAINING HETEROCYCLES, AB-INITIO THERMOCHEMISTRY, SET MODEL CHEMISTRY, ELECTRON CORRELATION, GAS-PHASE, THERMAL-DECOMPOSITION, TRANSITION STRUCTURES, PULSE-RADIOLYSIS, SHOCK-TUBE, KINETICS, 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics, 0306 Physical Chemistry (incl. Structural), 0307 Theoretical and Computational Chemistry, 3406 Physical chemistry, 3407 Theoretical and computational chemistry, 5102 Atomic, molecular and optical physics

Abstract:

Recombination of two amidogen radicals, NH2 (X(2)Bl), 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 NH3NH 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 N-2 + 2H(2), 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-N2H2 by molecular hydrogen results in decomposition to N-2 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 H-2 to N-2. This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N-2. We design heterogeneous analogues of this reaction using the model compound (CH3)(2)FeH2 as a source of the H-2 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.