O- A POSSIBLE NEW ROUTE FOR NOX FORMATION IN FLAMES(NNH )

We propose a new high temperature pathway for NO formation that involv es the reaction of NNH with oxygen atoms. This reaction forms the HNNO energized adduct via a rapid combination reaction; HNNO* then rapidl y dissociates to NH + NO. The rate constant for O + NNH = NH + NO is c alculated via a QRRK chemical activation analysis to be 3.3 x 10(14) T --0.23 exp(+510/T) cm(3) mol(-1) s(-1). This reaction sequence can be an important or even major route to NO formation under certain combust ion conditions. The presence of significant quantities of NNH results from the reaction of H with N-2. The H + N-2 = NNH reaction is only ca . 6 kcal/mol endothermic with a relatively low barrier. The reverse re action, NNH dissociation, has been reported in the literature to be en hanced by tunneling. Our analysis of NNH dissociation indicates that t unneling dominates. We report a two-term rate constant for NNH dissoci ation: 3.0 x 10(8) + [M]{1.0 x 10(13)T(0.5) exp(-1540/T)} s(-1). The f irst term accounts for pressure-independent tunneling from the ground vibrational state, while the second term accounts for collisional acti vation 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 + N-2 reaction. As a result, we find tha t NNH = H + N-2 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 su ggests that the reaction with oxygen atoms is especially important. (C ) 1995 John Wiley & Sons, Inc.