O + NNH: A possible new route for NO_X formation in flames

We propose a new high temperature pathway for NO formation that involves the reaction of NNH with oxygen atoms. This reaction forms the HNNO* energized adduct via a rapid combination reaction; HNNO* then rapidly dissociates to NH + NO. The rate constant for O + NNH  NH + NO is calculated via a QRRK chemical activation analysis to be 3.3 × 10¹⁴ T^−0.23exp(+510/T) cm³ mol⁻¹ s⁻¹. This reaction sequence can be an important or even major route to NO formation under certain combustion conditions. The presence of significant quantities of NNH results from the reaction of H with N₂. The H + N₂  NNH reaction is only ca. 6 kcal/mol endothermic with a relatively low barrier. The reverse reaction, NNH dissociation, has been reported in the literature to be enhanced by tunneling. Our analysis of NNH dissociation indicates that tunneling dominates. We report a two‐term rate constant for NNH dissociation: 3.0 × 10⁸ + [M] {1.0 × 10¹³T^0.5exp(−1540/T)} s⁻¹. The first term accounts for pressure‐independent tunneling from the ground vibrational state, while the second term accounts for collisional activation to higher vibration states from which tunneling can also occur. ([M] is the total concentration in units of mol cm⁻³.) Use of this dissociation rate constant and microscopic reversibility results in a large rate constant for the H + N₂ reaction. As a result, we find that NNH  H + N₂ can be partially equilibrated under typical combustion conditions, resulting in NNH concentrations large enough for it to be important in bimolecular reactions. Our analysis of such reactions suggests that the reaction with oxygen atoms is especially important. © 1995 John Wiley & Sons, Inc.