Branching fraction measurement of the prompt-NO switch reaction NCN + H

Abstract

The branching fraction ϕ of the prompt-NO switch reaction NCN + H (1), essentially yielding either CH + N2 (1a) or HCN + N (1b), is key for modeling prompt-NO formation in flames. Large discrepancies exist between available experimental data and flame modeling studies on the one hand and high-level theoretical predictions on the other hand, resulting in a factor of 5 uncertainty for ϕ1b at T=1500 K. By simultaneous monitoring of NCN and HCN concentration-time profiles during the reaction NCN + H at temperatures 1200K<T<2000K behind shock waves, ϕ1b has been directly measured for the first time. Thermal decomposition of cyanogen azide (NCN3) and ethyl iodide (C2H5I) served as sources for NCN radicals and H atoms. NCN has been detected by UV laser absorption at 329.130 nm and HCN detection was accomplished by mid-infrared frequency modulation spectroscopy at 3228.049cm−1. Kinetic simulations provided both the total rate constant k1 from the NCN and k1b from the HCN profiles. In order to account for a side reaction of residual BrCN from NCN3 synthesis, a rough estimate for the rate constant of the reaction BrCN + H was inferred from the experiments as well. The measured rate constants for k1 confirm earlier results (Faßheber et al., Phys. Chem. Chem. Phys. 16 (2014) 11647) and k1b follows the Arrhenius expression k1b/(cm3mol−1s−1)=4.2×1014exp(−38.2kJmol−1/RT) with moderate uncertainties of ±(37−49%). The branching fraction increases with temperature, yielding ϕ1b=0.22(±46%) at 1200 K, 0.47(±42%) at 1600 K, and 0.63(±53%) at 2000 K. The corresponding prompt-NO switch temperature of TS=1670K (ϕ1b=0.5) shows that the reaction NCN + H advances toward the Fenimore products HCN + N already at typical temperatures of hydrocarbon/air flames, in stark contrast to the most recent theoretical estimate reporting TS=3235K.