An Ab Initio Quantum Chemical and Kinetic Study of the NNH + O Reaction Potential Energy Surface: How Important Is This Route to NO in Combustion?

A detailed study of the NNH + O reaction potential energy surface using coupled cluster [CCSD(T)] and density functional (B3LYP) methods is reported. Three N 2 OH adducts have been located on this surface: cis- and trans- ONNH and ONHN. The product channels to NO + NH, N 2 O + H, N 2 + OH, and HNO + N have been characterized via the computation of minimum energy paths and of the appropriate transition states. Rate coefficients for the reaction of NNH + O to each of these reaction channels have been computed using RRKM techniques. As the reaction flux passing to these channels in combustion systems is very sensitive to the stability of NNH, the heats of formation of this species and of the transition state leading to its formation (NN−H) were also computed via complete basis estimates of the CCSD(T) energetics based on extrapolation of aug−cc−pV x Z results with x = 5, 6 obtaining a value of Δ f = 60.6 ± 0.5 kcal mol -1 . Additionally, a value of k 4 = 7.80 × 10 10 T 0.642 exp(1380 cal mol -1 / RT ) cm 3 mol -1 s -1 for the rate coefficient of the reaction NNH + O → NO + NH (4) between 1000 and 2600 K was obtained. This is approximately a factor of 4 less than the previous estimate of k 4 (Bozzelli, J. W.; Dean, A. M., Int. J. Chem. Kinet. 1995, 27, 1097). The new NNH rate data and thermochemistry are used to predict the level of NO produced in lean combustion in a completely stirred flow reactor. The overall conclusion arrived at on the basis of this work is that, in most combustion systems, the NNH + O pathway represents a very minor route to NO.