Shock Tube Study of the Reaction of CH with N₂:  Overall Rate and Branching Ratio

Abstract

We have studied the reaction between CH and N2, (1) CH + N2 --> products, in shock tube experiments using CH and NCN laser absorption. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1 nm. The overall rate coefficient of the CH + N2 reaction was measured between 1943 and 3543 K, in the 0.9-1.4 atm pressure range, using a CH perturbation approach. CH profiles recorded upon shock-heating dilute mixtures of ethane in argon and acetic anhydride in argon were perturbed by the addition of nitrogen. The perturbation in the CH concentration was principally due to the reaction between CH and N2. Rate coefficients for the overall reaction were inferred by kinetically modeling the perturbed CH profiles. A least-squares, two-parameter fit of the current overall rate coefficient measurements was k1 = 6.03 x 1012 exp(-11150/T [K]) (cm3 mol-1 s-1). The uncertainty in k1 was estimated to be approximately +/-25% and approximately +/-35% at approximately 3350 and approximately 2100 K, respectively. At high temperatures, there are two possible product channels for the reaction between CH and N2, (1a) CH + N2 --> HCN + N and (1b) CH + N2 --> H + NCN. The large difference in the rates of the reverse reactions enabled inference of the branching ratio of reaction 1, k1b/(k1b + k1a), in the 2228-2905 K temperature range by CH laser absorption in experiments in a nitrogen bath. The current CH measurements are consistent with a branching ratio of 1 and establish NCN and H as the primary products of the CH + N2 reaction. A detailed and systematic uncertainty analysis, taking into account experimental and mechanism-induced contributions, yields a conservative lower bound of 0.70 for the branching ratio. NCN was also detected by continuous-wave, narrow-line-width laser absorption at 329.13 nm. The measured NCN time histories were used to infer the rate coefficient of the reaction between H and NCN, H + NCN --> HCN + N, and to estimate an absorption coefficient for the NCN radical.

Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

“…The fast unimolecular decomposition of NCN 3 generates NCN in its singlet excited electronic state, which undergoes a collision-induced intersystem crossing (CIISC) process to the triplet ground state ( 3 NCN stated in the following as NCN). Our previous work showed that the CIISC process is the ratelimiting process for NCN formation at temperatures above 700 K, but is still fast compared to the reaction NCN + O 2 measured here [22].…”

“…5. The error arises from the statistical fit (±25%), the uncertainties of the absolute NCN 3 and O 2 concentrations (±5%), and of the rate constants assumed for the background mechanism. The two most important reactions for NCN consumption are NCN + NCN (8) and k 9 within error limits (k 9 ± 40%) add a ±25% uncertainty.…”

“…It is therefore unfeasible that NCN is formed through the simple reverse of reaction (6), which should be temperature dependent as well. Hence, it can be speculated that NCN reformation takes actually place through a reaction sequence according to 3 NCNOO → 1 NCNOO → 3 NCN + O 2 involving an intersystem crossing (ISC) process. To confirm this hypothesis, ab initio calculations of the triplet potential energy surface including accurate ISC probabilities to the singlet surface are needed.…”

Laminar hydrocarbon flame structure

“…First simulations with GDF-Kin 3.0 NCN and the rate constant calculated by Moskaleva Vasudevan et al [112] performed a new shocktube study of the reaction between CH and N 2 , with ethane as a CH source at high temperatures (T > 2500 K) and acetic anhydride for lower temperatures (1900 < T < 2500 K). From absorption measurements of CH and NCN concentrations and kinetic modeling of the influence of N 2 addition on the CH profile, an expression of the rate constant was derived:: k = 6.03 · 10 12 exp(−22,100/RT ) cm 3 · mole −1 · sec −1 .…”

Uncertainty analysis of NO production during methane combustion

“…Recently, Hanson and coworkers [41] studied the reaction between CH and N 2 in shock tube experiments using CH and NCN laser absorption. The CH measurements established NCN and H as the primary products of the CH + N 2 reaction.…”